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+The Project Gutenberg EBook of Climatic Changes, by 
+Ellsworth Huntington and Stephen Sargent Visher
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: Climatic Changes
+       Their Nature and Causes
+
+Author: Ellsworth Huntington
+        Stephen Sargent Visher
+
+Release Date: October 26, 2011 [EBook #37855]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK CLIMATIC CHANGES ***
+
+
+
+
+Produced by Robin Monks, Wayne Hammond and the Online
+Distributed Proofreading Team at http://www.pgdp.net (This
+file was produced from images generously made available
+by The Internet Archive/Canadian Libraries)
+
+
+
+
+
+
+
+  [TRANSCRIBER'S NOTE: Spelling maintained as closely as possible to the
+  original document, while obvious typos have been corrected. Emdashes
+  in original text for negative temperatures changed to minus signs to
+  standardize temperatures.]
+
+
+
+
+  CLIMATIC CHANGES
+
+  THEIR NATURE AND CAUSES
+
+
+  PUBLISHED ON THE FOUNDATION
+  ESTABLISHED IN MEMORY OF
+  THEODORE L. GLASGOW
+
+
+
+
+  OTHER BOOKS BY THE SAME AUTHORS
+
+
+  ELLSWORTH HUNTINGTON
+
+  A. _Four books showing the development of knowledge as to Historical
+     Pulsations of Climate._
+
+      The Pulse of Asia. Boston, 1907. Explorations in Turkestan.
+      Expedition of 1903. Washington, 1905.
+      Palestine and Its Transformation. Boston, 1911.
+      The Climatic Factor, as Illustrated in Arid America. Washington,
+        1914.
+
+  B. _Two books illustrating the effect of climate on man._
+
+      Civilization and Climate. New Haven, 1915.
+      World Power and Evolution. New Haven, 1919.
+
+  C. _Four books illustrating the general principles of Geography._
+
+      Asia: A Geography Reader. Chicago, 1912.
+      The Red Man's Continent. New Haven, 1919.
+      Principles of Human Geography (with S. W. Cushing). New York,
+        1920.
+      Business Geography (with F. E. Williams). New York, 1922.
+
+  D. _A companion to the present volume._
+
+      Earth and Sun: An Hypothesis of Weather and Sunspots. New Haven.
+        In press.
+
+
+  STEPHEN SARGENT VISHER
+
+  Geography, Geology and Biology of Southern Dakota. Vermilion, 1912.
+  The Biology of Northwestern South Dakota. Vermilion, 1914.
+  The Geography of South Dakota. Vermilion, 1918.
+  Handbook of the Geology of Indiana (with others). Indianapolis, 1922.
+  Hurricanes of Australia and the South Pacific. Melbourne, 1922.
+
+
+
+
+  CLIMATIC CHANGES
+
+  THEIR NATURE AND CAUSES
+
+  BY
+
+  ELLSWORTH HUNTINGTON
+  Research Associate in Geography in Yale University
+
+  AND
+
+  STEPHEN SARGENT VISHER
+  Associate Professor of Geology in Indiana University
+
+  [Illustration]
+
+  NEW HAVEN
+  YALE UNIVERSITY PRESS
+  LONDON: HUMPHREY MILFORD: OXFORD UNIVERSITY PRESS
+  MDCCCCXXII
+
+
+
+  COPYRIGHT 1922 BY
+  YALE UNIVERSITY PRESS
+  Published 1922.
+
+
+
+  THE THEODORE L. GLASGOW MEMORIAL
+  PUBLICATION FUND
+
+
+  The present volume is the fifth work published by the Yale
+  University Press on the Theodore L. Glasgow Memorial Publication
+  Fund. This foundation was established September 17, 1918, by an
+  anonymous gift to Yale University in memory of Flight Sub-Lieutenant
+  Theodore L. Glasgow, R.N. He was born in Montreal, Canada, and was
+  educated at the University of Toronto Schools and at the Royal
+  Military College, Kingston. In August, 1916, he entered the Royal
+  Naval Air Service and in July, 1917, went to France with the Tenth
+  Squadron attached to the Twenty-second Wing of the Royal Flying
+  Corps. A month later, August 19, 1917, he was killed in action on
+  the Ypres front.
+
+
+
+  TO
+
+  THOMAS CHROWDER CHAMBERLIN
+
+  OF THE UNIVERSITY OF CHICAGO
+
+  WHOSE CLEAR AND MASTERLY DISCUSSION OF THE GREAT PROBLEMS
+  OF TERRESTRIAL EVOLUTION HAS BEEN ONE OF THE MOST INSPIRING
+  FACTORS IN THE WRITING OF THIS BOOK
+
+
+
+  _There is a toy, which I have heard, and I would not have it given
+  over, but waited upon a little. They say it is observed in the Low
+  Countries (I know not in what part), that every five and thirty
+  years the same kind and suit of years and weathers comes about
+  again; as great frosts, great wet, great droughts, warm winters,
+  summers with little heat, and the like, and they call it the prime;
+  it is a thing I do the rather mention, because, computing backwards,
+  I have found some concurrence._
+
+  FRANCIS BACON
+
+
+
+
+PREFACE
+
+
+Unity is perhaps the keynote of modern science. This means unity in
+time, for the present is but the outgrowth of the past, and the future
+of the present. It means unity of process, for there seems to be no
+sharp dividing line between organic and inorganic, physical and mental,
+mental and spiritual. And the unity of modern science means also a
+growing tendency toward co�peration, so that by working together
+scientists discover much that would else have remained hid.
+
+This book illustrates the modern trend toward unity in all of these
+ways. First, it is a companion volume to _Earth and Sun_. That volume is
+a discussion of the causes of weather, but a consideration of the
+weather of the present almost inevitably leads to a study of the climate
+of the past. Hence the two books were written originally as one, and
+were only separated from considerations of convenience. Second, the
+unity of nature is so great that when a subject such as climatic changes
+is considered, it is almost impossible to avoid other subjects, such as
+the movements of the earth's crust. Hence this book not only discusses
+climatic changes, but considers the causes of earthquakes and attempts
+to show how climatic changes may be related to great geological
+revolutions in the form, location, and altitude of the lands. Thus the
+book has a direct bearing on all the main physical factors which have
+molded the evolution of organic life, including man.
+
+In the third place, this volume illustrates the unity of modern science
+because it is pre�minently a co�perative product. Not only have the two
+authors shared in its production, but several of the Yale Faculty have
+also co�perated. From the geological standpoint, Professor Charles
+Schuchert has read the entire manuscript in its final form as well as
+parts at various stages. He has helped not only by criticisms,
+suggestions, and facts, but by paragraphs ready for the printer. In the
+same way in the domain of physics, Professor Leigh Page has repeatedly
+taken time to assist, and either in writing or by word of mouth has
+contributed many pages. In astronomy, the same cordial co�peration has
+come with equal readiness from Professor Frank Schlesinger. Professors
+Schuchert, Schlesinger, and Page have contributed so materially that
+they are almost co-authors of the volume. In mathematics, Professor
+Ernest W. Brown has been similarly helpful, having read and criticised
+the entire book. In certain chemical problems, Professor Harry W. Foote
+has been our main reliance. The advice and suggestions of these men have
+frequently prevented errors, and have again and again started new and
+profitable lines of thought. If we have made mistakes, it has been
+because we have not profited sufficiently by their co�peration. If the
+main hypothesis of this book proves sound, it is largely because it has
+been built up in constant consultation with men who look at the problem
+from different points of vision. Our appreciation of their generous and
+unstinted co�peration is much deeper than would appear from this brief
+paragraph.
+
+Outside the Yale Faculty we have received equally cordial assistance.
+Professor T. C. Chamberlin of the University of Chicago, to whom, with
+his permission, we take great pleasure in dedicating this volume, has
+read the entire proof and has made many helpful suggestions. We cannot
+speak too warmly of our appreciation not only of this, but of the way
+his work has served for years as an inspiration in the preliminary work
+of gathering data for this volume. Professor Harlow Shapley of Harvard
+University has contributed materially to the chapter on the sun and its
+journey through space; Professor Andrew E. Douglass of the University of
+Arizona has put at our disposal some of his unpublished results;
+Professors S. B. Woodworth and Reginald A. Daly, and Mr. Robert W.
+Sayles of Harvard, and Professor Henry F. Reid of Johns Hopkins have
+suggested new facts and sources of information; Professor E. R. Cumings
+of Indiana University has critically read the entire proof;
+conversations with Professor John P. Buwalda of the University of
+California while he was teaching at Yale make him another real
+contributor; and Mr. Wayland Williams has contributed the interesting
+quotation from Bacon on page x of this book. Miss Edith S. Russell has
+taken great pains in preparing the manuscript and in suggesting many
+changes that make for clearness. Many others have also helped, but it is
+impossible to make due acknowledgment because such contributions have
+become so thoroughly a part of the mental background of the book that
+their source is no longer distinct in the minds of the authors.
+
+The division of labor between the two authors has not followed any set
+rules. Both have had a hand in all parts of the book. The main draft of
+Chapters VII, VIII, IX, XI, and XIII was written by the junior author;
+his contributions are also especially numerous in Chapters X and XV; the
+rest of the book was written originally by the senior author.
+
+
+
+
+CONTENTS
+
+
+                                                                PAGE
+
+     I. The Uniformity of Climate                                  1
+
+    II. The Variability of Climate                                16
+
+   III. Hypotheses of Climatic Change                             33
+
+    IV. The Solar Cyclonic Hypothesis                             51
+
+     V. The Climate of History                                    64
+
+    VI. The Climatic Stress of the Fourteenth Century             98
+
+   VII. Glaciation According to the Solar Cyclonic Hypothesis    110
+
+  VIII. Some Problems of Glacial Periods                         130
+
+    IX. The Origin of Loess                                      155
+
+     X. Causes of Mild Geological Climates                       166
+
+    XI. Terrestrial Causes of Climatic Changes                   188
+
+   XII. Post-Glacial Crustal Movements and Climatic Changes      215
+
+  XIII. The Changing Composition of Oceans and Atmosphere        223
+
+   XIV. The Effect of Other Bodies on the Sun                    242
+
+    XV. The Sun's Journey through Space                          264
+
+   XVI. The Earth's Crust and the Sun                            285
+
+
+
+
+LIST OF ILLUSTRATIONS
+
+
+                                                                PAGE
+
+  Fig. 1. Climatic changes and mountain building                  25
+
+  Fig. 2. Storminess at sunspot maxima vs. minima                 54
+
+  Fig. 3. Relative rainfall at times of increasing and
+          decreasing sunspots                                 58, 59
+
+  Fig. 4. Changes of climate in California and in western
+          and central Asia                                        75
+
+  Fig. 5. Changes in California climate for 2000 years, as
+           measured by growth of Sequoia trees                    77
+
+  Fig. 6. Distribution of Pleistocene ice sheets                 123
+
+  Fig. 7. Permian geography and glaciation                       145
+
+  Fig. 8. Effect of diminution of storms on movement of water    175
+
+  Fig. 9. Cretaceous Paleogeography                              201
+
+  Fig. 10. Climatic changes of 140,000 years as inferred from
+           the stars                                             279
+
+  Fig. 11. Sunspot curve showing cycles, 1750 to 1920            283
+
+  Fig. 12. Seasonal distribution of earthquakes                  299
+
+  Fig. 13. Wandering of the pole from 1890 to 1898               303
+
+
+
+
+TABLES
+
+
+                                                                PAGE
+
+  1. The Geological Time Table                                     5
+
+  2. Types of Climatic Sequence                                   16
+
+  3. Correlation Coefficients between Rainfall and Growth of
+     Sequoias in California                                       80
+
+  4. Correlation Coefficients between Rainfall Records in
+     California and Jerusalem                                     84
+
+  5. Theoretical Probability of Stellar Approaches               260
+
+  6. Thirty-Eight Stars Having Largest Known Parallaxes     276, 277
+
+  7. Destructive Earthquakes from 1800 to 1899 Compared with
+     Sunspots                                                    289
+
+  8. Seasonal March of Earthquakes                               295
+
+  9. Deflection of Path of Pole Compared with Earthquakes        305
+
+  10. Earthquakes in 1903 to 1908 Compared with Departures of
+      the Projected Curve of the Earth's Axis from the
+      Eulerian Position                                          306
+
+
+
+
+CHAPTER I
+
+THE UNIFORMITY OF CLIMATE
+
+
+The r�le of climate in the life of today suggests its importance in the
+past and in the future. No human being can escape from the fact that his
+food, clothing, shelter, recreation, occupation, health, and energy are
+all profoundly influenced by his climatic surroundings. A change of
+season brings in its train some alteration in practically every phase of
+human activity. Animals are influenced by climate even more than man,
+for they have not developed artificial means of protecting themselves.
+Even so hardy a creature as the dog becomes notably different with a
+change of climate. The thick-haired "husky" of the Eskimos has outwardly
+little in common with the small and almost hairless canines that grovel
+under foot in Mexico. Plants are even more sensitive than animals and
+men. Scarcely a single species can flourish permanently in regions which
+differ more than 20�C. in average yearly temperature, and for most the
+limit of successful growth is 10�.[1] So far as we yet know every living
+species of plant and animal, including man, thrives best under definite
+and limited conditions of temperature, humidity, and sunshine, and of
+the composition and movement of the atmosphere or water in which it
+lives. Any departure beyond the limits means lessened efficiency, and in
+the long run a lower rate of reproduction and a tendency toward changes
+in specific characteristics. Any great departure means suffering or
+death for the individual and destruction for the species.
+
+Since climate has so profound an influence on life today, it has
+presumably been equally potent at other times. Therefore few scientific
+questions are more important than how and why the earth's climate has
+varied in the past, and what changes it is likely to undergo in the
+future. This book sets forth what appear to be the chief reasons for
+climatic variations during historic and geologic times. It assumes that
+causes which can now be observed in operation, as explained in a
+companion volume entitled _Earth and Sun_, and in such books as
+Humphreys' _Physics of the Air_, should be carefully studied before less
+obvious causes are appealed to. It also assumes that these same causes
+will continue to operate, and are the basis of all valid predictions as
+to the weather or climate of the future.
+
+In our analysis of climatic variations, we may well begin by inquiring
+how the earth's climate has varied during geological history. Such an
+inquiry discloses three great tendencies, which to the superficial view
+seem contradictory. All, however, have a similar effect in providing
+conditions under which organic evolution is able to make progress. The
+first tendency is toward uniformity, a uniformity so pronounced and of
+such vast duration as to stagger the imagination. Superposed upon this
+there seems to be a tendency toward complexity. During the greater part
+of geological history the earth's climate appears to have been
+relatively monotonous, both from place to place and from season to
+season; but since the Miocene the rule has been diversity and
+complexity, a condition highly favorable to organic evolution. Finally,
+the uniformity of the vast eons of the past and the tendency toward
+complexity are broken by pulsatory changes, first in one direction and
+then in another. To our limited human vision some of the changes, such
+as glacial periods, seem to be waves of enormous proportions, but
+compared with the possibilities of the universe they are merely as the
+ripples made by a summer zephyr.
+
+The uniformity of the earth's climate throughout the vast stretches of
+geological time can best be realized by comparing the range of
+temperature on the earth during that period with the possible range as
+shown in the entire solar system. As may be seen in Table 1, the
+geological record opens with the Archeozoic era, or "Age of Unicellular
+Life," as it is sometimes called, for the preceding cosmic time has left
+no record that can yet be read. Practically no geologists now believe
+that the beginning of the Archeozoic was less than one hundred million
+years ago; and since the discovery of the peculiar properties of radium
+many of the best students do not hesitate to say a billion or a billion
+and a half.[2] Even in the Archeozoic the rocks testify to a climate
+seemingly not greatly different from that of the average of geologic
+time. The earth's surface was then apparently cool enough so that it was
+covered with oceans and warm enough so that the water teemed with
+microscopic life. The air must have been charged with water vapor and
+with carbon dioxide, for otherwise there seems to be no possible way of
+explaining the formation of mudstones and sandstones, limestones of vast
+thickness, carbonaceous shales, graphites, and iron ores.[3] Although
+the Archeozoic has yielded no generally admitted fossils, yet what seem
+to be massive alg� and sponges have been found in Canada. On the other
+hand, abundant life is believed to have been present in the oceans, for
+by no other known means would it be possible to take from the air the
+vast quantities of carbon that now form carbonaceous shales and
+graphite.
+
+In the next geologic era, the Proterozoic, the researches of Walcott
+have shown that besides the marine alg� there must have been many other
+kinds of life. The Proterozoic fossils thus far discovered include not
+only microscopic radiolarians such as still form the red ooze of the
+deepest ocean floors, but the much more significant tubes of annelids or
+worms. The presence of the annelids, which are relatively high in the
+scale of organization, is generally taken to mean that more lowly forms
+of animals such as coelenterates and probably even the mollusca and
+primitive arthropods must already have been evolved. That there were
+many kinds of marine invertebrates living in the later Proterozoic is
+indicated by the highly varied life and more especially the trilobites
+found in the oldest Cambrian strata of the next succeeding period. In
+fact the Cambrian has sponges, primitive corals, a great variety of
+brachiopods, the beginnings of gastropods, a wonderful array of
+trilobites, and other lowly forms of arthropods. Since, under the
+postulate of evolution, the life of that time forms an unbroken sequence
+with that of the present, and since many of the early forms differ only
+in minor details from those of today, we infer that the climate then was
+not very different from that of today. The same line of reasoning leads
+to the conclusion that even in the middle of the Proterozoic, when
+multicellular marine animals must already have been common, the climate
+of the earth had already for an enormous period been such that all the
+lower types of oceanic invertebrates had already evolved.
+
+TABLE 1
+
+THE GEOLOGICAL TIME TABLE[4]
+
+  COSMIC TIME
+
+    FORMATIVE ERA. Birth and growth of the earth. Beginnings of the
+    atmosphere, hydrosphere, continental platforms, oceanic basins,
+    and possibly of life. No known geological record.
+
+  GEOLOGIC TIME
+
+    ARCHEOZOIC ERA. Origin of simplest life.
+
+    PROTEROZOIC ERA. Age of invertebrate origins. An early and a late
+    ice age, with one or more additional ones indicated.
+
+    PALEOZOIC ERA. Age of primitive vertebrate dominance.
+
+      _Cambrian Period._ First abundance of marine animals and dominance
+      of trilobites.
+
+      _Ordovician Period._ First known fresh-water fishes.
+
+      _Silurian Period._ First known land plants.
+
+      _Devonian Period._ First known amphibians. "Table Mountain" ice
+      age.
+
+      _Mississippian Period._ Rise of marine fishes (sharks).
+
+      _Pennsylvanian Period._ Rise of insects and first period of marked
+      coal accumulation.
+
+      _Permian Period._ Rise of reptiles. Another great ice age.
+
+    MESOZOIC ERA. Age of reptile dominance.
+
+      _Triassic Period._ Rise of dinosaurs. The period closes with a
+      cool climate.
+
+      _Jurassic Period._ Rise of birds and flying reptiles.
+
+      _Comanchean Period._ Rise of flowering plants and higher insects.
+
+      _Cretaceous Period._ Rise of archaic or primitive mammalia.
+
+    CENOZOIC ERA. Age of mammal dominance.
+
+      _Early Cenozoic or Eocene and Oligocene time._ Rise of higher
+      mammals. Glaciers in early Eocene of the Laramide Mountains.
+
+      _Late Cenozoic or Miocene and Pliocene time._ Transformation of
+      ape like animals into man.
+
+      _Glacial or Pleistocene time._ Last great ice age.
+
+  PRESENT TIME
+
+    PSYCHOZOIC ERA. Age of man or age of reason. Includes the present or
+    "Recent time," estimated to be probably less than 30,000 years.
+
+Moreover, they could live in most latitudes, for the indirect evidences
+of life in the Archeozoic and Proterozoic rocks are widely distributed.
+Thus it appears that at an almost incredibly early period, perhaps many
+hundred million years ago, the earth's climate differed only a little
+from that of the present.
+
+The extreme limits of temperature beyond which the climate of geological
+times cannot have departed can be approximately determined. Today the
+warmest parts of the ocean have an average temperature of about 30�C. on
+the surface. Only a few forms of life live where the average temperature
+is much higher than this. In deserts, to be sure, some highly organized
+plants and animals can for a short time endure a temperature as high as
+75�C. (167�F.). In certain hot springs, some of the lowest unicellular
+plant forms exist in water which is only a little below the boiling
+point. More complex forms, however, such as sponges, worms, and all the
+higher plants and animals, seem to be unable to live either in water or
+air where the temperature averages above 45�C. (113�F.) for any great
+length of time and it is doubtful whether they can thrive permanently
+even at that temperature. The obvious unity of life for hundreds of
+millions of years and its presence at all times in middle latitudes so
+far as we can tell seem to indicate that since the beginning of marine
+life the temperature of the oceans cannot have averaged much above 50�C.
+even in the warmest portions. This is putting the limit too high rather
+than too low, but even so the warmest parts of the earth can scarcely
+have averaged much more than 20� warmer than at present.
+
+Turning to the other extreme, we may inquire how much colder than now
+the earth's surface may have been since life first appeared. Proterozoic
+fossils have been found in places where the present average temperature
+approaches 0�C. If those places should be colder than now by 30�C., or
+more, the drop in temperature at the equator would almost certainly be
+still greater, and the seas everywhere would be permanently frozen. Thus
+life would be impossible. Since the contrasts between summer and winter,
+and between the poles and the equator seem generally to have been less
+in the past than at present, the range through which the mean
+temperature of the earth as a whole could vary without utterly
+destroying life was apparently less than would now be the case.
+
+These considerations make it fairly certain that for at least several
+hundred million years the average temperature of the earth's surface has
+never varied more than perhaps 30�C. above or below the present level.
+Even this range of 60�C. (108�F.) may be double or triple the range that
+has actually occurred. That the temperature has not passed beyond
+certain narrow limits, whatever their exact degree, is clear from the
+fact that if it had done so, all the higher forms of life would have
+been destroyed. Certain of the lowest unicellular forms might indeed
+have persisted, for when dormant they can stand great extremes of dry
+heat and of cold for a long time. Even so, evolution would have had to
+begin almost anew. The supposition that such a thing has happened is
+untenable, for there is no hint of any complete break in the record of
+life during geological times,--no sudden disappearance of the higher
+organisms followed by a long period with no signs of life other than
+indirect evidence such as occurs in the Archeozoic.
+
+A change of 60�C. or even of 20� in the average temperature of the
+earth's surface may seem large when viewed from the limited standpoint
+of terrestrial experience. Viewed, however, from the standpoint of
+cosmic evolution, or even of the solar system, it seems a mere trifle.
+Consider the possibilities. The temperature of empty space is the
+absolute zero, or -273�C. To this temperature all matter must fall,
+provided it exists long enough and is not appreciably heated by
+collisions or by radiation. At the other extreme lies the temperature of
+the stars. As stars go, our sun is only moderately hot, but the
+temperature of its surface is calculated to be nearly 7000�C., while
+thousands of miles in the interior it may rise to 20,000� or 100,000� or
+some other equally unknowable and incomprehensible figure. Between the
+limits of the absolute zero on the one hand, and the interior of a sun
+or star on the other, there is almost every conceivable possibility of
+temperature. Today the earth's surface averages not far from 14�C., or
+287� above the absolute zero. Toward the interior, the temperature in
+mines and deep wells rises about 1�C. for every 100 meters. At this rate
+it would be over 500�C. at a depth of ten miles, and over 5000� at 100
+miles.
+
+Let us confine ourselves to surface temperatures, which are all that
+concern us in discussing climate. It has been calculated by Poynting[5]
+that if a small sphere absorbed and re-radiated all the heat that fell
+upon it, its temperature at the distance of Mercury from the sun would
+average about 210�C.; at the distance of Venus, 85�; the earth 27�; Mars
+-30�; Neptune -219�. A planet much nearer the sun than is Mercury might
+be heated to a temperature of a thousand, or even several thousand,
+degrees, while one beyond Neptune would remain almost at absolute zero.
+It is well within the range of possibility that the temperature of a
+planet's surface should be anywhere from near -273�C. up to perhaps
+5000�C. or more, although the probability of low temperature is much
+greater than of high. Thus throughout the whole vast range of
+possibilities extending to perhaps 10,000�, the earth claims only 60� at
+most, or less than 1 per cent. This may be remarkable, but what is far
+more remarkable is that the earth's range of 60� includes what seem to
+be the two most critical of all possible temperatures, namely, the
+freezing point of water, 0�C., and the temperature where water can
+dissolve an amount of carbon dioxide equal to its own volume. The most
+remarkable fact of all is that the earth has preserved its temperature
+within these narrow limits for a hundred million years, or perchance a
+thousand million.
+
+To appreciate the extraordinary significance of this last fact, it is
+necessary to realize how extremely critical are the temperatures from
+about 0� to 40�C., and how difficult it is to find any good reason for a
+relatively uniform temperature through hundreds of millions of years.
+Since the dawn of geological time the earth's temperature has apparently
+always included the range from about the freezing point of water up to
+about the point where protoplasm begins to disintegrate. Henderson, in
+_The Fitness of the Environment_, rightly says that water is "the most
+familiar and the most important of all things." In many respects water
+and carbon dioxide form the most unique pair of substances in the whole
+realm of chemistry. Water has a greater tendency than any other known
+substance to remain within certain narrowly defined limits of
+temperature. Not only does it have a high specific heat, so that much
+heat is needed to raise its temperature, but on freezing it gives up
+more heat than any substance except ammonia, while none of the common
+liquids approach it in the amount of additional heat required for
+conversion into vapor after the temperature of vaporization has been
+reached. Again, water substance, as the physicists call all forms of
+H_{2}O, is unique in that it not only contracts on melting, but
+continues to contract until a temperature several degrees above its
+melting point is reached. That fact has a vast importance in helping to
+keep the earth's surface at a uniform temperature. If water were like
+most liquids, the bottoms of all the oceans and even the entire body of
+water in most cases would be permanently frozen.
+
+Again, as a solvent there is literally nothing to compare with water. As
+Henderson[6] puts it: "Nearly the whole science of chemistry has been
+built up around water and aqueous solution." One of the most significant
+evidences of this is the variety of elements whose presence can be
+detected in sea water. According to Henderson they include hydrogen,
+oxygen, nitrogen, carbon, chlorine, sodium, magnesium, sulphur,
+phosphorus, which are easily detected; and also arsenic, c�sium, gold,
+lithium, rubidium, barium, lead, boron, fluorine, iron, iodine, bromine,
+potassium, cobalt, copper, manganese, nickel, silver, silicon, zinc,
+aluminium, calcium, and strontium. Yet in spite of its marvelous power
+of solution, water is chemically rather inert and relatively stable. It
+dissolves all these elements and thousands of their compounds, but still
+remains water and can easily be separated and purified. Another unique
+property of water is its power of ionizing dissolved substances, a
+property which makes it possible to produce electric currents in
+batteries. This leads to an almost infinite array of electro-chemical
+reactions which play an almost dominant r�le in the processes of life.
+Finally, no common liquid except mercury equals water in its power of
+capillarity. This fact is of enormous moment in biology, most obviously
+in respect to the soil.
+
+Although carbon dioxide is far less familiar than water, it is almost as
+important. "These two simple substances," says Henderson, "are the
+common source of every one of the complicated substances which are
+produced by living beings, and they are the common end products of the
+wearing away of all the constituents of protoplasm, and of the
+destruction of those materials which yield energy to the body." One of
+the remarkable physical properties of carbon dioxide is its degree of
+solubility in water. This quality varies enormously in different
+substances. For example, at ordinary pressures and temperatures, water
+can absorb only about 5 per cent of its own volume of oxygen, while it
+can take up about 1300 times its own volume of ammonia. Now for carbon
+dioxide, unlike most gases, the volume that can be absorbed by water is
+nearly the same as the volume of the water. The volumes vary, however,
+according to temperature, being absolutely the same at a temperature of
+about 15�C. or 59�F., which is close to the ideal temperature for man's
+physical health and practically the same as the mean temperature of the
+earth's surface when all seasons are averaged together. "Hence, when
+water is in contact with air, and equilibrium has been established, the
+amount of free carbonic acid in a given volume of water is almost
+exactly equal to the amount in the adjacent air. Unlike oxygen,
+hydrogen, and nitrogen, carbonic acid enters water freely; unlike
+sulphurous oxide and ammonia, it escapes freely from water. Thus the
+waters can never wash carbonic acid completely out of the air, nor can
+the air keep it from the waters. It is the one substance which thus, in
+considerable quantities relative to its total amount, everywhere
+accompanies water. In earth, air, fire, and water alike these two
+substances are always associated.
+
+"Accordingly, if water be the first primary constituent of the
+environment, carbonic acid is inevitably the second,--because of its
+solubility possessing an equal mobility with water, because of the
+reservoir of the atmosphere never to be depleted by chemical action in
+the oceans, lakes, and streams. In truth, so close is the association
+between these two substances that it is scarcely correct logically to
+separate them at all; together they make up the real environment and
+they never part company."[7]
+
+The complementary qualities of carbon dioxide and water are of supreme
+importance because these two are the only known substances which are
+able to form a vast series of complex compounds with highly varying
+chemical formul�. No other known compounds can give off or take on atoms
+without being resolved back into their elements. No others can thus
+change their form freely without losing their identity. This power of
+change without destruction is the fundamental chemical characteristic of
+life, for life demands complexity, change, and growth.
+
+In order that water and carbon dioxide may combine to form the compounds
+on which life is based, the water must be in the liquid form, it must be
+able to dissolve carbon dioxide freely, and the temperature must not be
+high enough to break up the highly complex and delicate compounds as
+soon as they are formed. In other words, the temperature must be above
+freezing, while it must not rise higher than some rather indefinite
+point between 50�C. and the boiling point, where all water finally turns
+into vapor. In the whole range of temperature, so far as we know, there
+is no other interval where any such complex reactions take place. The
+temperature of the earth for hundreds of millions of years has remained
+firmly fixed within these limits.
+
+The astonishing quality of the earth's uniformity of temperature becomes
+still more apparent when we consider the origin of the sun's heat. What
+that origin is still remains a question of dispute. The old ideas of a
+burning sun, or of one that is simply losing an original supply of heat
+derived from some accident, such as collision with another body, were
+long ago abandoned. The impact of a constant supply of meteors affords
+an almost equally unsatisfactory explanation. Moulton[8] states that if
+the sun were struck by enough meteorites to keep up its heat, the earth
+would almost certainly be struck by enough so that it would receive
+about half of 1 per cent as much heat from them as from the sun. This is
+millions of times more heat than is now received from meteors. If the
+sun owes its heat to the impact of larger bodies at longer intervals,
+the geological record should show a series of interruptions far more
+drastic than is actually the case.
+
+It has also been supposed that the sun owes its heat to contraction. If
+a gaseous body contracts it becomes warmer. Finally, however, it must
+become so dense that its rate of contraction diminishes and the process
+ceases. Under the sun's present condition of size and density a radial
+contraction of 120 feet per year would be enough to supply all the
+energy now radiated by that body. This seems like a hopeful source of
+energy, but Kelvin calculated that twenty million years ago it was
+ineffective and ten million years hence it will be equally so. Moreover,
+if this is the source of heat, the amount of radiation from the sun
+would have to vary enormously. Twenty million years ago the sun would
+have extended nearly to the earth's orbit and would have been so tenuous
+that it would have emitted no more heat than some of the nebul� in
+space. Some millions of years later, when the sun's radius was twice as
+great as at present, that body would have emitted only one-fourth as
+much heat as now, which would mean that on the earth's surface the
+theoretical temperature would have been 200� below the present level.
+This is utterly out of accord with the uniformity of climate shown by
+the geological record. In the future, if the sun's contraction is the
+only source of heat, the sun can supply the present amount for only ten
+million years, which would mean a change utterly unlike anything of
+which the geological record holds even the faintest hint.[9]
+
+Altogether the problem of how the sun can have remained so uniform and
+how the earth's atmosphere and other conditions can also have remained
+so uniform throughout hundreds of millions of years is one of the most
+puzzling in the whole realm of nature. If appeal is taken to
+radioactivity and the breaking up of uranium into radium and helium,
+conditions can be postulated which will give the required amount of
+energy. Such is also the case if it be supposed that there is some
+unknown process which may induce an atomic change like radioactivity in
+bodies which are now supposed to be stable elements. In either case,
+however, there is as yet no satisfactory explanation of the _uniformity_
+of the earth's climate. A hundred million or a thousand million years
+ago the temperature of the earth's surface was very much the same as
+now. The earth had then presumably ceased to emit any great amount of
+heat, if we may judge from the fact that its surface was cool enough so
+that great ice sheets could accumulate on low lands within 40� of the
+equator. The atmosphere was apparently almost like that of today, and
+was almost certainly not different enough to make up for any great
+divergence of the sun from its present condition. We cannot escape the
+stupendous fact that in those remote times the sun must have been
+essentially the same as now, or else that some utterly unknown factor is
+at work.
+
+FOOTNOTES:
+
+[Footnote 1: W. A. Setchell: The Temperature Interval in the
+Geographical Distribution of Marine Alg�; Science, Vol. 52, 1920,
+p. 187.]
+
+[Footnote 2: J. Barrell: Rhythms and the Measurements of Geologic Time;
+Bull. Geol. Soc. Am., Vol. 28, Dec., 1917, pp. 745-904.]
+
+[Footnote 3: Pirsson and Schuchert: Textbook of Geology, 1915, pp.
+538-550.]
+
+[Footnote 4: From Charles Schuchert in The Evolution of the Earth and
+Its Inhabitants: Edited by R. S. Lull, New Haven, 1918, but with
+revisions by Professor Schuchert.]
+
+[Footnote 5: J. H. Poynting: Radiation in the Solar System; Phil. Trans.
+A, 1903, 202, p. 525.]
+
+[Footnote 6: L. J. Henderson: The Fitness of the Environment, 1913.]
+
+[Footnote 7: Henderson: _loc. cit._, p. 138.]
+
+[Footnote 8: F. R. Moulton: Introduction to Astronomy, 1916.]
+
+[Footnote 9: Moulton: _loc. cit._]
+
+
+
+
+CHAPTER II
+
+THE VARIABILITY OF CLIMATE
+
+
+The variability of the earth's climate is almost as extraordinary as its
+uniformity. This variability is made up partly of a long, slow tendency
+in one direction and partly of innumerable cycles of every conceivable
+duration from days, or even hours, up to millions of years. Perhaps the
+easiest way to grasp the full complexity of the matter is to put the
+chief types of climatic sequence in the form of a table.
+
+   +-----------------------------------------------------------+
+   |                                                           |
+   |                            TABLE 2                        |
+   |                                                           |
+   |                   TYPES OF CLIMATIC SEQUENCE              |
+   |                                                           |
+   |  1. Cosmic uniformity.         7. Br�ckner periods.       |
+   |  2. Secular progression.       8. Sunspot cycles.         |
+   |  3. Geologic oscillations.     9. Seasonal alternations.  |
+   |  4. Glacial fluctuations.     10. Pleionian migrations.   |
+   |  5. Orbital precessions.      11. Cyclonic vacillations.  |
+   |  6. Historical pulsations.    12. Daily vibrations.       |
+   |                                                           |
+   +-----------------------------------------------------------+
+
+In assigning names to the various types an attempt has been made to
+indicate something of the nature of the sequence so far as duration,
+periodicity, and general tendencies are concerned. Not even the rich
+English language of the twentieth century, however, furnishes words with
+enough shades of meaning to express all that is desired. Moreover,
+except in degree, there is no sharp distinction between some of the
+related types, such as glacial fluctuations and historic pulsations.
+Yet, taken as a whole, the table brings out the great contrast between
+two absolutely diverse extremes. At the one end lies well-nigh eternal
+uniformity, or an extremely slow progress in one direction throughout
+countless ages; at the other, rapid and regular vibrations from day to
+day, or else irregular and seemingly unsystematic vacillations due to
+cyclonic storms, both of which types are repeated millions of times
+during even a single glacial fluctuation.
+
+The meaning of cosmic uniformity has been explained in the preceding
+chapter. Its relation to the other types of climatic sequences seems to
+be that it sets sharply defined limits beyond which no changes of any
+kind have ever gone since life, as we know it, first began. Secular
+progression, on the other hand, means that in spite of all manner of
+variations, now this way and then the other, the normal climate of the
+earth, if there is such a thing, has on the whole probably changed a
+little, perhaps becoming more complex. After each period of continental
+uplift and glaciation--for such are pre�minently the times of
+complexity--it is doubtful whether the earth has ever returned to quite
+its former degree of monotony. Today the earth has swung away from the
+great diversity of the glacial period. Yet we still have contrasts of
+what seem to us great magnitude. In low depressions, such as Turfan in
+the central deserts of Eurasia, the thermometer sometimes ranges from
+0�F. in the morning to 60� in the shade at noon. On a cloudy day in the
+Amazon forest close to the seashore, on the contrary, the temperature
+for months may rise to 85� by day and sink no lower than 75� at night.
+
+The reasons for the secular progression of the earth's climate appear to
+be intimately connected with those which have caused the next, and, in
+many respects, more important type of climatic sequence, which consists
+of geological oscillations. Both the progression and the oscillations
+seem to depend largely on three purely terrestrial factors: first, the
+condition of the earth's interior, including both internal heat and
+contraction; second, the salinity and movement of the ocean; and third,
+the composition and amount of the atmosphere. To begin with the earth's
+interior--its loss of heat appears to be an almost negligible factor in
+explaining either secular progression or geologic oscillation. According
+to both the nebular and the planetesimal hypotheses, the earth's crust
+appears to be colder now than it was hundreds or thousands of millions
+of years ago. The emission of internal heat, however, had probably
+ceased to be of much climatic significance near the beginning of the
+geological record, for in southern Canada glaciation occurred very early
+in the Proterozoic era. On the other hand, the contraction of the earth
+has produced remarkable effects throughout the whole of geological time.
+It has lessened the earth's circumference by a thousand miles or more,
+as appears from the way in which the rocks have been folded and thrust
+bodily over one another. According to the laws of dynamics this must
+have increased the speed of the earth's rotation, thus shortening the
+day, and also having the more important effect of increasing the bulge
+at the equator. On the other hand, recent investigations indicate that
+tidal retardation has probably diminished the earth's rate of rotation
+more than seemed probable a few years ago, thus lengthening the day and
+diminishing the bulge at the equator. Thus two opposing forces have been
+at work, one causing acceleration and one retardation. Their combined
+effect may have been a factor in causing secular progression of climate.
+It almost certainly was of much importance in causing pronounced
+oscillations first one way and then the other. This matter, together
+with most of those touched in these first chapters, will be expanded in
+later parts of the book. On the whole the tendency appears to have been
+to create climatic diversity in place of uniformity.
+
+The increasing salinity of the oceans may have been another factor in
+producing secular progression, although of slight importance in respect
+to oscillations. While the oceans were still growing in volume, it is
+generally assumed that they must have been almost fresh for a vast
+period, although Chamberlin thinks that the change in salinity has been
+much less than is usually supposed. So far as the early oceans were
+fresher than those of today, their deep-sea circulation must have been
+less hampered than now by the heavy saline water which is produced by
+evaporation in warm regions. Although this saline water is warm, its
+weight causes it to descend, instead of moving poleward in a surface
+current; this descent slows up the rise of the cold water which has
+moved along in the depths of the ocean from high latitudes, and thus
+checks the general oceanic circulation. If the ancient oceans were
+fresher and hence had a freer circulation than now, a more rapid
+interchange of polar and equatorial water presumably tended to equalize
+the climate of all latitudes.
+
+Again, although the earth's atmosphere has probably changed far less
+during geological times than was formerly supposed, its composition has
+doubtless varied. The total volume of nitrogen has probably increased,
+for that gas is so inert that when it once becomes a part of the air it
+is almost sure to stay there. On the other hand, the proportions of
+oxygen, carbon dioxide, and water vapor must have fluctuated. Oxygen is
+taken out constantly by animals and by all the processes of rock
+weathering, but on the other hand the supply is increased when plants
+break up new carbon dioxide derived from volcanoes. As for the carbon
+dioxide, it appears probable that in spite of the increased supply
+furnished by volcanoes the great amounts of carbon which have gradually
+been locked up in coal and limestone have appreciably depleted the
+atmosphere. Water vapor also may be less abundant now than in the past,
+for the presence of carbon dioxide raises the temperature a little and
+thereby enables the air to hold more moisture. When the area of the
+oceans has diminished, and this has recurred very often, this likewise
+would tend to reduce the water vapor. Moreover, even a very slight
+diminution in the amount of heat given off by the earth, or a decrease
+in evaporation because of higher salinity in the oceans, would tend in
+the same direction. Now carbon dioxide and water vapor both have a
+strong blanketing effect whereby heat is prevented from leaving the
+earth. Therefore, the probable reduction in the carbon dioxide and water
+vapor of the earth's atmosphere has apparently tended to reduce the
+climatic monotony and create diversity and complexity. Hence, in spite
+of many reversals, the general tendency of changes, not only in the
+earth's interior and in the oceans, but also in the atmosphere, appears
+to be a secular progression from a relatively monotonous climate in
+which the evolution of higher organic forms would scarcely be rapid to
+an extremely diverse and complex climate highly favorable to progressive
+evolution. The importance of these purely terrestrial agencies must not
+be lost sight of when we come to discuss other agencies outside the
+earth.
+
+In Table 2 the next type of climatic sequence is geologic oscillation.
+This means slow swings that last millions of years. At one extreme of
+such an oscillation the climate all over the world is relatively
+monotonous; it returns, as it were, toward the primeval conditions at
+the beginning of the secular progression. At such times magnolias,
+sequoias, figs, tree ferns, and many other types of subtropical plants
+grew far north in places like Greenland, as is well known from their
+fossil remains of middle Cenozoic time, for example. At these same
+times, and also at many others before such high types of plants had
+evolved, reef-making corals throve in great abundance in seas which
+covered what is now Wisconsin, Michigan, Ontario, and other equally cool
+regions. Today these regions have an average temperature of only about
+70�F. in the warmest month, and average well below freezing in winter.
+No reef-making corals can now live where the temperature averages below
+68�F. The resemblance of the ancient corals to those of today makes it
+highly probable that they were equally sensitive to low temperature.
+Thus, in the mild portions of a geologic oscillation the climate seems
+to have been so equable and uniform that many plants and animals could
+live 1500 and at other times even 4000 miles farther from the equator
+than now.
+
+At such times the lands in middle and high latitudes were low and small,
+and the oceans extended widely over the continental platforms. Thus
+unhampered ocean currents had an opportunity to carry the heat of low
+latitudes far toward the poles. Under such conditions, especially if the
+conception of the great subequatorial continent of Gondwana land is
+correct, the trade winds and the westerlies must have been stronger and
+steadier than now. This would not only enable the westerlies, which are
+really southwesterlies, to carry more heat than now to high latitudes,
+but would still further strengthen the ocean currents. At the same time,
+the air presumably contained an abundance of water vapor derived from
+the broad oceans, and an abundance of atmospheric carbon dioxide
+inherited from a preceding time when volcanoes contributed much carbon
+dioxide to the air. These two constituents of the atmosphere may have
+exercised a pronounced blanketing effect whereby the heat of the earth
+with its long wave lengths was kept in, although the energy of the sun
+with its shorter wave lengths was not markedly kept out. Thus everything
+may have combined to produce mild conditions in high latitudes, and to
+diminish the contrast between equator and pole, and between summer and
+winter.
+
+Such conditions perhaps carry in themselves the seeds of decay. At any
+rate while the lands lie quiet during a period of mild climate great
+strains must accumulate in the crust because of the earth's contraction
+and tidal retardation. At the same time the great abundance of plants
+upon the lowlying plains with their mild climates, and the marine
+creatures upon the broad continental platforms, deplete the atmospheric
+carbon dioxide. Part of this is locked up as coal and part as limestone
+derived from marine plants as well as animals. Then something happens so
+that the strains and stresses of the crust are released. The sea floors
+sink; the continents become relatively high and large; mountain ranges
+are formed; and the former plains and emergent portions of the
+continental platforms are eroded into hills and valleys. The large size
+of the continents tends to create deserts and other types of climatic
+diversity; the presence of mountain ranges checks the free flow of winds
+and also creates diversity; the ocean currents are likewise checked,
+altered, and diverted so that the flow of heat from low to high
+latitudes is diminished. At the same time evaporation from the ocean
+diminishes so that a decrease in water vapor combines with the previous
+depletion of carbon dioxide to reduce the blanketing effect of the
+atmosphere. Thus upon periods of mild monotony there supervene periods
+of complexity, diversity, and severity. Turn to Table 1 and see how a
+glacial climate again and again succeeds a time when relative mildness
+prevailed almost everywhere. Or examine Fig. 1 and notice how the lines
+representing temperatures go up and down. In the figure Schuchert makes
+it clear that when the lands have been large and mountain-making has
+been important, as shown by the high parts of the lower shaded area, the
+climate has been severe, as shown by the descent of the snow line, the
+upper shaded area. In the diagram the climatic oscillations appear
+short, but this is merely because they have been crowded together,
+especially in the left hand or early part. There an inch in length may
+represent a hundred million years. Even at the right-hand end an inch is
+equivalent to several million years.
+
+The severe part of a climatic oscillation, as well as the mild part,
+will be shown in later chapters to bear in itself certain probable seeds
+of decay. While the lands are being uplifted, volcanic activity is
+likely to be vigorous and to add carbon dioxide to the air. Later, as
+the mountains are worn down by the many agencies of water, wind, ice,
+and chemical decay, although much carbon dioxide is locked up by the
+carbonation of the rocks, the carbon locked up in the coal is set free
+and increases the carbon dioxide of the air. At the same time the
+continents settle slowly downward, for the earth's crust though rigid as
+steel is nevertheless slightly viscous and will flow if subjected to
+sufficiently great and enduring pressure. The area from which
+evaporation can take place is thereby increased because of the spread of
+the oceans over the continents, and water vapor joins with the carbon
+dioxide to blanket the earth and thus tends to keep it uniformly warm.
+Moreover, the diminution of the lands frees the ocean currents from
+restraint and permits them to flow more freely from low latitudes to
+high. Thus in the course of millions of years there is a return toward
+monotony. Ultimately, however, new stresses accumulate in the earth's
+crust, and the way is prepared for another great oscillation. Perhaps
+the setting free of the stresses takes place simply because the strain
+at last becomes irresistible. It is also possible, as we shall see, that
+an external agency sometimes adds to the strain and thereby determines
+the time at which a new oscillation shall begin.
+
+In Table 2 the types of climatic sequences which follow "geologic
+oscillations" are "glacial fluctuations," "orbital precessions" and
+"historical pulsations." Glacial fluctuations and historical pulsations
+appear to be of the same type, except as to severity and duration, and
+hence may be considered together. They will be treated briefly here
+because the theories as to their causes are outlined in the next two
+chapters. Oddly enough, although the historic pulsations lie much closer
+to us than do the glacial fluctuations, they were not discovered until
+two or three generations later, and are still much less known. The most
+important feature of both sequences is the swing from a glacial to an
+inter-glacial epoch or from the arsis or accentuated part of an
+historical pulsation to the thesis or unaccented part. In a glacial
+epoch or in the arsis of an historic pulsation, storms are usually
+abundant and severe, the mean temperature is lower than usual, snow
+accumulates in high latitudes or upon lofty mountains. For example, in
+the last such period during the fourteenth century, great floods and
+droughts occurred alternately around the North Sea; it was several times
+possible to cross the Baltic Sea from Germany to Sweden on the ice, and
+the ice of Greenland advanced so much that shore ice caused the Norsemen
+to change their sailing route between Iceland and the Norse colonies in
+southern Greenland. At the same time in low latitudes and in parts of
+the continental interior there is a tendency toward diminished rainfall
+and even toward aridity and the formation of deserts. In Yucatan, for
+example, a diminution in tropical rainfall in the fourteenth century
+seems to have given the Mayas a last opportunity for a revival of their
+decaying civilization.
+
+[Illustration: _Fig. 1. Climatic changes and mountain building._
+
+(_After Schuchert, in The Evolution of the Earth and Its Inhabitants,
+edited by R. S. Lull._) Diagram showing the times and probable extent of
+the more or less marked climate changes in the geologic history of North
+America, and of its elevation into chains of mountains.]
+
+Among the climatic sequences, glacial fluctuations are perhaps of the
+most vital import from the standpoint of organic evolution; from the
+standpoint of human history the same is true of climatic pulsations.
+Glacial epochs have repeatedly wiped out thousands upon thousands of
+species and played a part in the origin of entirely new types of plants
+and animals. This is best seen when the life of the Pennsylvanian is
+contrasted with that of the Permian. An historic pulsation may wipe out
+an entire civilization and permit a new one to grow up with a radically
+different character. Hence it is not strange that the causes of such
+climatic phenomena have been discussed with extraordinary vigor. In few
+realms of science has there been a more imposing or more interesting
+array of theories. In this book we shall consider the more important of
+these theories. A new solar or cyclonic hypothesis and the hypothesis of
+changes in the form and altitude of the land will receive the most
+attention, but the other chief hypotheses are outlined in the next
+chapter, and are frequently referred to throughout the volume.
+
+Between glacial fluctuations and historical pulsations in duration, but
+probably less severe than either, come orbital precessions. These stand
+in a group by themselves and are more akin to seasonal alternations than
+to any other type of climatic sequence. They must have occurred with
+absolute regularity ever since the earth began to revolve around the sun
+in its present elliptical orbit. Since the orbit is elliptical and since
+the sun is in one of the two foci of the ellipse, the earth's distance
+from the sun varies. At present the earth is nearest the sun in the
+northern winter. Hence the rigor of winter in the northern hemisphere is
+mitigated, while that of the southern hemisphere is increased. In about
+ten thousand years this condition will be reversed, and in another ten
+thousand the present conditions will return once more. Such climatic
+precessions, as we may here call them, must have occurred unnumbered
+times in the past, but they do not appear to have been large enough to
+leave in the fossils of the rocks any traces that can be distinguished
+from those of other climatic sequences.
+
+We come now to Br�ckner periods and sunspot cycles. The Br�ckner periods
+have a length of about thirty-three years. Their existence was suggested
+at least as long ago as the days of Sir Francis Bacon, whose statement
+about them is quoted on the flyleaf of this book. They have since been
+detected by a careful study of the records of the time of harvest,
+vintage, the opening of rivers to navigation, and the rise or fall of
+lakes like the Caspian Sea. In his book on _Klimaschwankungen seit
+1700_, Br�ckner has collected an uncommonly interesting assortment of
+facts as to the climate of Europe for more than two centuries. More
+recently, by a study of the rate of growth of trees, Douglass, in his
+book on _Climatic Cycles and Tree Growth_, has carried the subject still
+further. In general the nature of the 33-year periods seems to be
+identical with that of the 11- or 12- year sunspot cycle, on the one
+hand, and of historic pulsations on the other. For a century observers
+have noted that the variations in the weather which everyone notices
+from year to year seem to have some relation to sunspots. For
+generations, however, the relationship was discussed without leading to
+any definite conclusion. The trouble was that the same change was
+supposed to take place in all parts of the world. Hence, when every sort
+of change was found somewhere at any given sunspot stage, it seemed as
+though there could not be a relationship. Of late years, however, the
+matter has become fairly clear. The chief conclusions are, first, that
+when sunspots are numerous the average temperature of the earth's
+surface is lower than normal. This does not mean that all parts are
+cooler, for while certain large areas grow cool, others of less extent
+become warm at times of many sunspots. Second, at times of many sunspots
+storms are more abundant than usual, but are also confined somewhat
+closely to certain limited tracks so that elsewhere a diminution of
+storminess may be noted. This whole question is discussed so fully in
+_Earth and Sun_ that it need not detain us further in this preliminary
+view of the whole problem of climate. Suffice it to say that a study of
+the sunspot cycle leads to the conclusion that it furnishes a clue to
+many of the unsolved problems of the climate of the past, as well as a
+key to prediction of the future.
+
+Passing by the seasonal alternations which are fully explained as the
+result of the revolution of the earth around the sun, we may merely
+point out that, like the daily vibrations which bring Table 2 to a
+close, they emphasize the outstanding fact that the main control of
+terrestrial climate is the amount of energy received from the sun. This
+same principle is illustrated by pleionian migrations. The term "pleion"
+comes from a Greek word meaning "more." It was taken by Arctowski to
+designate areas or periods where there is an excess of some climatic
+element, such as atmospheric pressure, rainfall, or temperature. Even if
+the effect of the seasons is eliminated, it appears that the course of
+these various elements does not run smoothly. As everyone knows, a
+period like the autumn of 1920 in the eastern United States may be
+unusually warm, while a succeeding period may be unseasonably cool.
+These departures from the normal show a certain rough periodicity. For
+example, there is evidence of a period of about twenty-seven days,
+corresponding to the sun's rotation and formerly supposed to be due to
+the moon's revolution which occupies almost the same length of time.
+Still other periods appear to have an average duration of about three
+months and of between two and three years. Two remarkable discoveries
+have recently been made in respect to such pleions. One is that a given
+type of change usually occurs simultaneously in a number of well-defined
+but widely separated centers, while a change of an opposite character
+arises in another equally well-defined, but quite different, set of
+centers. In general, areas of high pressure have one type of change and
+areas of low pressure the other type. So systematic are these
+relationships and so completely do they harmonize in widely separated
+parts of the earth, that it seems certain that they must be due to some
+outside cause, which in all probability can be only the sun. The second
+discovery is that pleions, when once formed, travel irregularly along
+the earth's surface. Their paths have not yet been worked out in detail,
+but a general migration seems well established. Because of this, it is
+probable that if unusually warm weather prevails in one part of a
+continent at a given time, the "thermo-pleion," or excess of heat, will
+not vanish but will gradually move away in some particular direction. If
+we knew the path that it would follow we might predict the general
+temperature along its course for some months in advance. The paths are
+often irregular, and the pleions frequently show a tendency to break up
+or suddenly revive. Probably this tendency is due to variations in the
+sun. When the sun is highly variable, the pleions are numerous and
+strong, and extremes of weather are frequent. Taken as a whole the
+pleions offer one of the most interesting and hopeful fields not only
+for the student of the causes of climatic variations, but for the man
+who is interested in the practical question of long-range weather
+forecasts. Like many other climatic phenomena they seem to represent the
+combined effect of conditions in the sun and upon the earth itself.
+
+The last of the climatic sequences which require explanation is the
+cyclonic vacillations. These are familiar to everyone, for they are the
+changes of weather which occur at intervals of a few days, or a week or
+two, at all seasons, in large parts of the United States, Europe, Japan,
+and some of the other progressive parts of the earth. They do not,
+however, occur with great frequency in equatorial regions, deserts, and
+many other regions. Up to the end of the last century, it was generally
+supposed that cyclonic storms were purely terrestrial in origin. Without
+any adequate investigation it was assumed that all irregularities in the
+planetary circulation of the winds arise from an irregular distribution
+of heat due to conditions within or upon the earth itself. These
+irregularities were supposed to produce cyclonic storms in certain
+limited belts, but not in most parts of the world. Today this view is
+being rapidly modified. Undoubtedly, the irregularities due to purely
+terrestrial conditions are one of the chief contributory causes of
+storms, but it begins to appear that solar variations also play a part.
+It has been found, for example, that not only the mean temperature of
+the earth's surface varies in harmony with the sunspot cycle, but that
+the frequency and severity of storms vary in the same way. Moreover, it
+has been demonstrated that the sun's radiation is not constant, but is
+subject to innumerable variations. This does not mean that the sun's
+general temperature varies, but merely that at some times heated gases
+are ejected rapidly to high levels so that a sudden wave of energy
+strikes the earth. Thus, the present tendency is to believe that the
+cyclonic variations, the changes of weather which come and go in such a
+haphazard, irresponsible way, are partly due to causes pertaining to the
+earth itself and partly to the sun.
+
+From this rapid survey of the types of climatic sequences, it is evident
+that they may be divided into four great groups. First comes cosmic
+uniformity, one of the most marvelous and incomprehensible of all known
+facts. We simply have no explanation which is in any respect adequate.
+Next come secular progression and geologic oscillations, two types of
+change which seem to be due mainly to purely terrestrial causes, that
+is, to changes in the lands, the oceans, and the air. The general
+tendency of these changes is toward complexity and diversity, thus
+producing progression, but they are subject to frequent reversals which
+give rise to oscillations lasting millions of years. The processes by
+which the oscillations take place are fully discussed in this book.
+Nevertheless, because they are fairly well understood, they are deferred
+until after the third group of sequences has been discussed. This group
+includes glacial fluctuations, historic pulsations, Br�ckner periods,
+sunspot cycles, pleionian migrations, and cyclonic vacillations. The
+outstanding fact in regard to all of these is that while they are
+greatly modified by purely terrestrial conditions, they seem to owe
+their origin to variations in the sun. They form the chief subject of
+_Earth and Sun_ and in their larger phases are the most important topic
+of this book also. The last group of sequences includes orbital
+precessions, seasonal alternations, and daily variations. These may be
+regarded as purely solar in origin. Yet their influence, like that of
+each of the other groups, is much modified by the earth's own
+conditions. Our main problem is to separate and explain the two great
+elements in climatic changes,--the effects of the sun, on the one hand,
+and of the earth on the other.
+
+
+
+
+CHAPTER III
+
+HYPOTHESES OF CLIMATIC CHANGE
+
+
+The next step in our study of climate is to review the main hypotheses
+as to the causes of glaciation. These hypotheses apply also to other
+types of climatic changes. We shall concentrate on glacial periods,
+however, not only because they are the most dramatic and well-known
+types of change, but because they have been more discussed than any
+other and have also had great influence on evolution. Moreover, they
+stand near the middle of the types of climatic sequences, and an
+understanding of them does much to explain the others. In reviewing the
+various theories we shall not attempt to cover all the ground, but shall
+merely state the main ideas of the few theories which have had an
+important influence upon scientific thought.
+
+The conditions which any satisfactory climatic hypothesis must satisfy
+are briefly as follows:
+
+(1) Due weight must be given to the fact that changes of climate are
+almost certainly due to the combined effect of a variety of causes, both
+terrestrial and solar or cosmic.
+
+(2) Attention must also be paid to both sides in the long controversy as
+to whether glaciation is due primarily to a diminution in the earth's
+supply of heat or to a _redistribution_ of the heat through changes in
+atmospheric and oceanic circulation. At present the great majority of
+authorities are on the side of a diminution of heat, but the other view
+also deserves study.
+
+(3) A satisfactory hypothesis must explain the frequent synchronism
+between two great types of phenomena; first, movements of the earth's
+crust whereby continents are uplifted and mountains upheaved; and,
+second, great changes of climate which are usually marked by relatively
+rapid oscillations from one extreme to another.
+
+(4) No hypothesis can find acceptance unless it satisfies the somewhat
+exacting requirements of the geological record, with its frequent but
+irregular repetition of long, mild periods, relatively cool or
+intermediate periods like the present, and glacial periods of more or
+less severity and perhaps accompanying the more or less widespread
+uplifting of continents. At least during the later glacial periods the
+hypothesis must explain numerous climatic epochs and stages superposed
+upon a single general period of continental upheaval. Moreover, although
+historical geology demands cycles of varied duration and magnitude, it
+does not furnish evidence of any rigid periodicity causing the cycles to
+be uniform in length or intensity.
+
+(5) Most important of all, a satisfactory explanation of climatic
+changes and crustal deformation must take account of all the agencies
+which are now causing similar phenomena. Whether any other agencies
+should be considered is open to question, although the relative
+importance of existing agencies may have varied.
+
+I. _Croll's Eccentricity Theory._ One of the most ingenious and most
+carefully elaborated scientific hypotheses is Croll's[10] precessional
+hypothesis as to the effect of the earth's own motions. So well was this
+worked out that it was widely accepted for a time and still finds a
+place in popular but unscientific books, such as Wells' _Outline of
+History_, and even in scientific works like Wright's _Quaternary Ice
+Age_. The gist of the hypothesis has already been given in connection
+with the type of climatic sequence known as orbital precessions. The
+earth is 93 million miles away from the sun in January and 97 million in
+July. The earth's axis "precesses," however, just as does that of a
+spinning top. Hence arises what is known as the precession of the
+equinoxes, that is, a steady change in the season at which the earth is
+in perihelion, or nearest to the sun. In the course of 21,000 years the
+time of perihelion varies from early in January through the entire
+twelve months and back to January. Moreover, the earth's orbit is
+slightly more elliptical at certain periods than at others, for the
+planets sometimes become bunched so that they all pull the earth in one
+direction. Hence, once in about one hundred thousand years the effect of
+the elliptical shape of the earth's orbit is at a maximum.
+
+Croll argued that these astronomical changes must alter the earth's
+climate, especially by their effect on winds and ocean currents. His
+elaborate argument contains a vast amount of valuable material. Later
+investigation, however, seems to have proven the inadequacy of his
+hypothesis. In the first place, the supposed cause does not seem nearly
+sufficient to produce the observed results. Second, Croll's hypothesis
+demands that glaciation in the northern and southern hemisphere take
+place alternately. A constantly growing collection of facts, however,
+indicates that glaciation does not occur in the two hemispheres
+alternately, but at the same time. Third, the hypothesis calls for the
+constant and frequent repetition of glaciation at absolutely regular
+intervals. The geological record shows no such regularity, for sometimes
+several glacial epochs follow in relatively close succession at
+irregular intervals of perhaps fifty to two hundred thousand years, and
+thus form a glacial period; and then for millions of years there are
+none. Fourth, the eccentricity hypothesis provides no adequate
+explanation for the glacial stages or subepochs, the historic
+pulsations, and the other smaller climatic variations which are
+superposed upon glacial epochs and upon one another in bewildering
+confusion. In spite of these objections, there can be little question
+that the eccentricity of the earth's orbit and the precession of the
+equinoxes with the resulting change in the season of perihelion must
+have some climatic effect. Hence Croll's theory deserves a permanent
+though minor place in any full discussion of the causes of climatic
+changes.
+
+II. _The Carbon Dioxide Theory._ At about the time that the eccentricity
+theory was being relegated to a minor niche, a new theory was being
+developed which soon exerted a profound influence upon geological
+thought. Chamberlin,[11] adopting an idea suggested by Tyndall, fired
+the imagination of geologists by his skillful exposition of the part
+played by carbon dioxide in causing climatic changes. Today this theory
+is probably more widely accepted than any other. We have already seen
+that the amount of carbon dioxide gas in the atmosphere has a decided
+climatic importance. Moreover, there can be little doubt that the amount
+of that gas in the atmosphere varies from age to age in response to the
+extent to which it is set free by volcanoes, consumed by plants,
+combined with rocks in the process of weathering, dissolved in the ocean
+or locked up in the form of coal and limestone. The main question is
+whether such variations can produce changes so rapid as glacial epochs
+and historical pulsations.
+
+Abundant evidence seems to show that the degree to which the air can be
+warmed by carbon dioxide is sharply limited. Humphreys, in his excellent
+book on the _Physics of the Air_, calculates that a layer of carbon
+dioxide forty centimeters thick has practically as much blanketing
+effect as a layer indefinitely thicker. In other words, forty
+centimeters of carbon dioxide, while having no appreciable effect on
+sunlight coming toward the earth, would filter out and thus retain in
+the atmosphere all the outgoing terrestrial heat that carbon dioxide is
+capable of absorbing. Adding more would be like adding another filter
+when the one in operation has already done all that that particular kind
+of filter is capable of doing. According to Humphreys' calculations, a
+doubling of the carbon dioxide in the air would in itself raise the
+average temperature about 1.3�C. and further carbon dioxide would have
+practically no effect. Reducing the present supply by half would reduce
+the temperature by essentially the same amount.
+
+The effect must be greater, however, than would appear from the figures
+given above, for any change in temperature has an effect on the amount
+of water vapor, which in turn causes further changes of temperature.
+Moreover, as Chamberlin points out, it is not clear whether Humphreys
+allows for the fact that when the 40 centimeters of CO_{2} nearest the
+earth has been heated by terrestrial radiation, it in turn radiates half
+its heat outward and half inward. The outward half is all absorbed in
+the next layer of carbon dioxide, and so on. The process is much more
+complex than this, but the end result is that even the last increment of
+CO_{2}, that is, the outermost portions in the upper atmosphere, must
+apparently absorb an infinitesimally small amount of heat. This fact,
+plus the effect of water vapor, would seem to indicate that a doubling
+or halving of the amount of CO_{2}, would have an effect of more than
+1.3�C. A change of even 2�C. above or below the present level of the
+earth's mean temperature would be of very appreciable climatic
+significance, for it is commonly believed that during the height of the
+glacial period the mean temperature was only 5� to 8�C. lower than now.
+
+Nevertheless, variations in atmospheric carbon dioxide do not
+necessarily seem competent to produce the relatively rapid climatic
+fluctuations of glacial epochs and historic pulsations as distinguished
+from the longer swings of glacial periods and geological eras. In
+Chamberlin's view, as in ours, the elevation of the land, the
+modification of the currents of the air and of the ocean, and all that
+goes with elevation as a topographic agency constitute a primary cause
+of climatic changes. A special effect of this is the removal of carbon
+dioxide from the air by the enhanced processes of weathering. This, as
+he carefully states, is a very slow process, and cannot of itself lead
+to anything so sudden as the oncoming of glaciation. But here comes
+Chamberlin's most distinctive contribution to the subject, namely, the
+hypothesis that changes in atmospheric temperature arising from
+variations in atmospheric carbon dioxide are able to cause a reversal of
+the deep-sea oceanic circulation.
+
+According to Chamberlin's view, the ordinary oceanic circulation of the
+greater part of geological time was the reverse of the present
+circulation. Warm water descended to the ocean depths in low latitudes,
+kept its heat while creeping slowly poleward, and rose in high latitudes
+producing the warm climate which enabled corals, for example, to grow in
+high latitudes. Chamberlin holds this opinion largely because there
+seems to him to be no other reasonable way to account for the enormously
+long warm periods when heat-loving forms of life lived in what are now
+polar regions of ice and snow. He explains this reversed circulation by
+supposing that an abundance of atmospheric carbon dioxide, together with
+a broad distribution of the oceans, made the atmosphere so warm that the
+evaporation in low latitudes was far more rapid than now. Hence the
+surface water of the ocean became a relatively concentrated brine. Such
+a brine is heavy and tends to sink, thereby setting up an oceanic
+circulation the reverse of that which now prevails. At present the polar
+waters sink because they are cold and hence contract. Moreover, when
+they freeze a certain amount of salt leaves the ice and thereby
+increases the salinity of the surrounding water. Thus the polar water
+sinks to the depths of the ocean, its place is taken by warmer and
+lighter water from low latitudes which moves poleward along the surface,
+and at the same time the cold water of the ocean depths is forced
+equatorward below the surface. But if the equatorial waters were so
+concentrated that a steady supply of highly saline water kept descending
+to low levels, the direction of the circulation would have to be
+reversed. The time when this would occur would depend upon the delicate
+balance between the downward tendencies of the cold polar water and of
+the warm saline equatorial water.
+
+Suppose that while such a reversed circulation prevailed, the
+atmospheric CO_{2} should be depleted, and the air cooled so much that
+the concentration of the equatorial waters by evaporation was no longer
+sufficient to cause them to sink. A reversal would take place, the
+present type of circulation would be inaugurated, and the whole earth
+would suffer a chill because the surface of the ocean would become cool.
+The cool surface-water would absorb carbon dioxide faster than the
+previous warm water had done, for heat drives off gases from water. This
+would hasten the cooling of the atmosphere still more, not only directly
+but by diminishing the supply of atmospheric moisture. The result would
+be glaciation. But ultimately the cold waters of the higher latitudes
+would absorb all the carbon dioxide they could hold, the slow
+equatorward creep would at length permit the cold water to rise to the
+surface in low latitudes. There the warmth of the equatorial sun and the
+depleted supply of carbon dioxide in the air would combine to cause the
+water to give up its carbon dioxide once more. If the atmosphere had
+been sufficiently depleted by that time, the rising waters in low
+latitudes might give up more carbon dioxide than the cold polar waters
+absorbed. Thus the atmospheric supply would increase, the air would
+again grow warm, and a tendency toward deglaciation, or toward an
+inter-glacial condition would arise. At such times the oceanic
+circulation is not supposed to have been reversed, but merely to have
+been checked and made slower by the increasing warmth. Thus
+inter-glacial conditions like those of today, or even considerably
+warmer, are supposed to have been produced with the present type of
+circulation.
+
+The emission of carbon dioxide in low latitudes could not permanently
+exceed the absorption in high latitudes. After the present type of
+circulation was finally established, which might take tens of thousands
+of years, the two would gradually become equal. Then the conditions
+which originally caused the oceanic circulation to be reversed would
+again destroy the balance; the atmospheric carbon dioxide would be
+depleted; the air would grow cooler; and the cycle of glaciation would
+be repeated. Each cycle would be shorter than the last, for not only
+would the swings diminish like those of a pendulum, but the agencies
+that were causing the main depletion of the atmospheric carbon dioxide
+would diminish in intensity. Finally as the lands became lower through
+erosion and submergence, and as the processes of weathering became
+correspondingly slow, the air would gradually be able to accumulate
+carbon dioxide; the temperature would increase; and at length the
+oceanic circulation would be reversed again. When the warm saline waters
+of low latitudes finally began to sink and to set up a flow of warm
+water poleward in the depths of the ocean, a glacial period would
+definitely come to an end.
+
+This hypothesis has been so skillfully elaborated, and contains so many
+important elements that one can scarcely study it without profound
+admiration. We believe that it is of the utmost value as a step toward
+the truth, and especially because it emphasizes the great function of
+oceanic circulation. Nevertheless, we are unable to accept it in full
+for several reasons, which may here be stated very briefly. Most of them
+will be discussed fully in later pages.
+
+(1) While a reversal of the deep-sea circulation would undoubtedly be of
+great climatic importance and would produce a warm climate in high
+latitudes, we see no direct evidence of such a reversal. It is equally
+true that there is no conclusive evidence against it, and the
+possibility of a reversal must not be overlooked. There seem, however,
+to be other modifications of atmospheric and oceanic circulation which
+are able to produce the observed results.
+
+(2) There is much, and we believe conclusive, evidence that a mere
+lowering of temperature would not produce glaciation. What seems to be
+needed is changes in atmospheric circulation and in precipitation. The
+carbon dioxide hypothesis has not been nearly so fully developed on the
+meteorological side as in other respects.
+
+(3) The carbon dioxide hypothesis seems to demand that the oceans should
+have been almost as saline as now in the Proterozoic era at the time of
+the first known glaciation. Chamberlin holds that such was the case, but
+the constant supply of saline material brought to the ocean by rivers
+and the relatively small deposition of such material on the sea floor
+seem to indicate that the early oceans must have been much fresher than
+those of today.
+
+(4) The carbon dioxide hypothesis does not attempt to explain minor
+climatic fluctuations such as post-glacial stages and historic
+pulsations, but these appear to be of the same nature as glacial epochs,
+differing only in degree.
+
+(5) Another reason for hesitation in accepting the carbon dioxide
+hypothesis as a full explanation of glacial fluctuations is the highly
+complex and non-observational character of the explanation of the
+alternation of glacial and inter-glacial epochs and of their constantly
+decreasing length.
+
+(6) Most important of all, a study of the variations of weather and of
+climate as they are disclosed by present records and by the historic
+past suggests that there are now in action certain other causes which
+are competent to explain glaciation without recourse to a process whose
+action is beyond the realm of observation.
+
+These considerations lead to the conclusion that the carbon dioxide
+hypothesis and the reversal of the oceanic circulation should be
+regarded as a tentative rather than a final explanation of glaciation.
+Nevertheless, the action of carbon dioxide seems to be an important
+factor in producing the longer oscillations of climate from one
+geological era to another. It probably plays a considerable part in
+preparing the way for glacial periods and in making it possible for
+other factors to produce the more rapid changes which have so deeply
+influenced organic evolution.
+
+III. _The Form of the Land._ Another great cause of climatic change
+consists of a group of connected phenomena dependent upon movements of
+the earth's crust. As to the climatic potency of changes in the lands
+there is practical agreement among students of climatology and
+glaciation. That the height and extent of the continents, the location,
+size, and orientation of mountain ranges, and the opening and closing of
+oceanic gateways at places like Panama, and the consequent diversion of
+oceanic currents, exert a profound effect upon climate can scarcely be
+questioned. Such changes may be introduced rapidly, but their
+disappearance is usually slow compared with the rapid pulsations to
+which climate has been subject during historic times and during stages
+of glacial retreat and advance, or even in comparison with the epochs
+into which the Pleistocene, Permian, and perhaps earlier glacial periods
+have been divided. Hence, while crustal movements appear to be more
+important than the eccentricity of the earth's orbit or the amount of
+carbon dioxide in the air, they do not satisfactorily explain glacial
+fluctuations, historic pulsations, and especially the present little
+cycles of climatic change. All these changes involve a relatively rapid
+swing from one extreme to another, while an upheaval of a continent,
+which is at best a slow geologic process, apparently cannot be undone
+for a long, long time. Hence such an upheaval, if acting alone, would
+lead to a relatively long-lived climate of a somewhat extreme type. It
+would help to explain the long swings, or geologic oscillations between
+a mild and uniform climate at one extreme, and a complex and varied
+climate at the other, but it would not explain the rapid climatic
+pulsations which are closely associated with great movements of the
+earth's crust. It might prepare the way for them, but could not cause
+them. That this conclusion is true is borne out by the fact that vast
+mountain ranges, like those at the close of the Jurassic and Cretaceous,
+are upheaved without bringing on glacial climates. Moreover, the marked
+Permian ice age follows long after the birth of the Hercynian Mountains
+and before the rise of others of later Permian origin.
+
+IV. _The Volcanic Hypothesis._ In the search for some cause of climatic
+change which is highly efficient and yet able to vary rapidly and
+independently, Abbot, Fowle, Humphreys, and others,[12] have concluded
+that volcanic eruptions are the missing agency. In _Physics of the Air_,
+Humphreys gives a careful study of the effect of volcanic dust upon
+terrestrial temperature. He begins with a mathematical investigation of
+the size of dust particles, and their quantity after certain eruptions.
+He demonstrates that the power of such particles to deflect light of
+short wave-lengths coming from the sun is perhaps thirty times more than
+their power to retain the heat radiated in long waves from the earth.
+Hence it is estimated that if a Krakatoa were to belch forth dust every
+year or two, the dust veil might cause a reduction of about 6�C. in the
+earth's surface temperature. As in every such complicated problem, some
+of the author's assumptions are open to question, but this touches their
+quantitative and not their qualitative value. It seems certain that if
+volcanic explosions were frequent enough and violent enough, the
+temperature of the earth's surface would be considerably lowered.
+
+Actual observation supports this theoretical conclusion. Humphreys
+gathers together and amplifies all that he and Abbot and Fowle have
+previously said as to observations of the sun's thermal radiation by
+means of the pyrheliometer. This summing up of the relations between the
+heat received from the sun, and the occurrence of explosive volcanic
+eruptions leaves little room for doubt that at frequent intervals during
+the last century and a half a slight lowering of terrestrial temperature
+has actually occurred after great eruptions. Nevertheless, it does not
+justify Humphreys' final conclusion that "phenomena within the earth
+itself suffice to modify its own climate, ... that these and these alone
+have actually caused great changes time and again in the geologic past."
+Humphreys sees so clearly the importance of the purely terrestrial point
+of view that he unconsciously slights the cosmic standpoint and ignores
+the important solar facts which he himself adduces elsewhere at
+considerable length.
+
+In addition to this the _degree_ to which the temperature of the earth
+as a whole is influenced by volcanic eruptions is by no means so clear
+as is the fact that there is some influence. Arctowski,[13] for example,
+has prepared numerous curves showing the march of temperature month
+after month for many years. During the period from 1909 to 1913, which
+includes the great eruption of Katmai in Alaska, low temperature is
+found to have prevailed at the time of the eruption, but, as Arctowski
+puts it, on the basis of the curves for 150 stations in all parts of the
+world: "The supposition that these abnormally low temperatures were due
+to the veil of volcanic dust produced by the Katmai eruption of June 6,
+1912, is completely out of the question. If that had been the case,
+temperature would have decreased from that date on, whereas it was
+decreasing for more than a year before that date."
+
+K�ppen,[14] in his comprehensive study of temperature for a hundred
+years, also presents a strong argument against the idea that volcanic
+eruptions have an important place in determining the present temperature
+of the earth. A volcanic eruption is a sudden occurrence. Whatever
+effect is produced by dust thrown into the air must occur within a few
+months, or as soon as the dust has had an opportunity to be wafted to
+the region in question. When the dust arrives, there will be a rapid
+drop through the few degrees of temperature which the dust is supposed
+to be able to account for, and thereafter a slow rise of temperature. If
+volcanic eruptions actually caused a frequent lowering of terrestrial
+temperature in the hundred years studied by K�ppen, there should be more
+cases where the annual temperature is decidedly below the normal than
+where it shows a large departure in the opposite direction. The contrary
+is actually the case.
+
+A still more important argument is the fact that the earth is now in an
+intermediate condition of climate. Throughout most of geologic time, as
+we shall see again and again, the climate of the earth has been milder
+than now. Regions like Greenland have not been the seat of glaciers, but
+have been the home of types of plants which now thrive in relatively low
+latitudes. In other words, the earth is today only part way from a
+glacial epoch to what may be called the normal, mild climate of the
+earth--a climate in which the contrast from zone to zone was much less
+than now, and the lower air averaged warmer. Hence it seems impossible
+to avoid the conclusion that the cause of glaciation is still operating
+with considerable although diminished efficiency. But volcanic dust is
+obviously not operating to any appreciable extent at present, for the
+upper air is almost free from dust a large part of the time.
+
+Again, as Chamberlin suggests, let it be supposed that a Krakatoan
+eruption every two years would produce a glacial period. Unless the most
+experienced field workers on the glacial formations are quite in error,
+the various glacial epochs of the Pleistocene glacial period had a joint
+duration of at least 150,000 years and perhaps twice as much. That would
+require 75,000 Krakatoan eruptions. But where are the pits and cones of
+such eruptions? There has not been time to erode them away since the
+Pleistocene glaciation. Their beds of volcanic ash would presumably be
+as voluminous as the glacial beds, but there do not seem to be
+accumulations of any such size. Even though the same volcano suffered
+repeated explosions, it seems impossible to find sufficient fresh
+volcanic debris. Moreover, the volcanic hypothesis has not yet offered
+any mechanism for systematic glacial variations. Hence, while the
+hypothesis is important, we must search further for the full explanation
+of glacial fluctuations, historic pulsations, and the earth's present
+quasi-glacial climate.
+
+V. _The Hypothesis of Polar Wandering._ Another hypothesis, which has
+some adherents, especially among geologists, holds that the position of
+the earth's axis has shifted repeatedly during geological times, thus
+causing glaciation in regions which are not now polar. Astrophysicists,
+however, are quite sure that no agency could radically change the
+relation between the earth and its axis without likewise altering the
+orbits of the planets to a degree that would be easily recognized.
+Moreover, the distribution of the centers of glaciation both in the
+Permian and Pleistocene periods does not seem to conform to this
+hypothesis.
+
+VI. _The Thermal Solar Hypothesis._ The only other explanations of the
+climatic changes of glacial and historic times which now seem to have
+much standing are two distinct and almost antagonistic solar hypotheses.
+One is the idea that changes in the earth's climate are due to
+variations in the heat emitted by the sun and hence in the temperature
+of the earth. The other is the entirely different idea that climatic
+changes arise from solar conditions which cause a _redistribution of the
+earth's atmospheric pressure_ and hence produce changes in winds, ocean
+currents, and especially storms. This second, or "cyclonic," hypothesis
+is the subject of a book entitled _Earth and Sun_, which is to be
+published as a companion to the present volume. It will be outlined in
+the next chapter. The other, or thermal, hypothesis may be dismissed
+briefly. Unquestionably a permanent change in the amount of heat emitted
+by the sun would permanently alter the earth's climate. There is
+absolutely no evidence, however, of any such change during geologic
+time. The evidence as to the earth's cosmic uniformity and as to secular
+progression is all against it. Suppose that for thirty or forty thousand
+years the sun cooled off enough so that the earth was as cool as during
+a glacial epoch. As glaciation is soon succeeded by a mild climate, some
+agency would then be needed to raise the sun's temperature. The impact
+of a shower of meteorites might accomplish this, but that would mean a
+very sudden heating, such as there is no evidence of in geological
+history. In fact, there is far more evidence of sudden cooling than of
+sudden heating. Moreover, it is far beyond the bounds of probability
+that such an impact should be repeated again and again with just such
+force as to bring the climate back almost to where it started and yet to
+allow for the slight changes which cause secular progression. Another
+and equally cogent objection to the thermal form of solar hypothesis is
+stated by Humphreys as follows: "A change of the solar constant
+obviously alters all surface temperatures by a roughly constant
+percentage. Hence a decrease of the heat from the sun would in general
+cause a decrease of the interzonal temperature gradients; and this in
+turn a less vigorous atmospheric circulation, and a less copious rain or
+snowfall--exactly the reverse of the condition, namely, abundant
+precipitation, most favorable to extensive glaciation."
+
+This brings us to the end of the main hypotheses as to climatic changes,
+aside from the solar cyclonic hypothesis which will be discussed in the
+next chapter. It appears that variations in the position of the earth at
+perihelion have a real though slight influence in causing cycles with a
+length of about 21,000 years. Changes in the carbon dioxide of the air
+probably have a more important but extremely slow influence upon
+geologic oscillations. Variations in the size, shape, and height of the
+continents are constantly causing all manner of climatic complications,
+but do not cause rapid fluctuations and pulsations. The eruption of
+volcanic dust appears occasionally to lower the temperature, but its
+potency to explain the complex climatic changes recorded in the rocks
+has probably been exaggerated. Finally, although minor changes in the
+amount of heat given out by the sun occur constantly and have been
+demonstrated to have a climatic effect, there is no evidence that such
+changes are the main cause of the climatic phenomena which we are trying
+to explain. Nevertheless, in connection with other solar changes they
+may be of high importance.
+
+FOOTNOTES:
+
+[Footnote 10: James Croll: Climate and Time, 1876.]
+
+[Footnote 11: T. C. Chamberlin: An attempt to frame a working hypothesis
+of the cause of glacial periods on an atmospheric basis; Jour. Geol.,
+Vol. VII, 1899, pp. 545-584, 667-685, 757-787.
+
+T. C. Chamberlin and R. D. Salisbury: Geology, Vol. II, 1906, pp.
+93-106, 655-677, and Vol. III, pp. 432-446.
+
+S. Arrhenius (Kosmische Physik, Vol. II, 1903, p. 503) carried out some
+investigations on carbon dioxide which have had a pronounced effect on
+later conclusions.
+
+F. Frech adopted Arrhenius' idea and developed it in a paper entitled
+Ueber die Klima-Aenderungen der Geologischen Vergangenheit. Compte
+Rendu, Tenth (Mexico) Congr. Geol. Intern., 1907 (=1908), pp. 299-325.
+
+The exact origin of the carbon dioxide theory has been stated so
+variously that it seems worth while to give the exact facts. Prompted by
+the suggestion, of Tyndall that glaciation might be due to depletion of
+atmospheric carbon dioxide, Chamberlin worked up the essentials of his
+early views before he saw any publication from Arrhenius, to whom the
+idea has often been attributed. In 1895 or earlier Chamberlin began to
+give the carbon dioxide hypothesis to his students and to discuss it
+before local scientific bodies. In 1897 he prepared a paper on "A Group
+of Hypotheses Bearing on Climatic Changes," Jour. Geol., Vol. V (1897),
+to be read at the meeting of the British Association at Toronto, basing
+his conclusions on Tyndall's determination of the competency of carbon
+dioxide as an absorber of heat radiated from the earth. He had
+essentially completed this when a paper by Arrhenius, "On the influence
+of carbonic acid in the air upon the temperature of the ground," Phil.
+Mag., 1896, pp. 237-276, first came to his attention. Chamberlin then
+changed his conservative, tentative statement of the functions of carbon
+dioxide to a more sweeping one based on Arrhenius' very definite
+quantitative deductions from Langley's experiments. Both Langley and
+Arrhenius were then in the ascendancy of their reputations and seemingly
+higher authorities could scarcely have been chosen, nor a finer
+combination than experiment and physico-mathematical development.
+Arrhenius' deductions were later proved to have been overstrained, while
+Langley's interpretation and even his observations were challenged.
+Chamberlin's latest views are more like his earlier and more
+conservative statement.]
+
+[Footnote 12: C. G. Abbot and F. E. Fowle: Volcanoes and Climate;
+Smiths. Misc. Coll., Vol. 60, 1913, 24 pp.
+
+W. J. Humphreys: Volcanic dust and other factors in the production of
+climatic and their possible relation to ice ages; Bull. Mount Weather
+Observatory, Vol. 6, Part 1, 1913, 26 pp. Also, Physics of the Air,
+1920.]
+
+[Footnote 13: H. Arctowski: The Pleonian Cycle of Climatic Fluctuations;
+Am. Jour. Sci., Vol. 42, 1916, pp. 27-33. See also Annals of the New
+York Academy of Sciences, Vol. 24, 1914.]
+
+[Footnote 14: W. K�ppen: �ber mehrj�hrige Perioden der Witterung
+ins besondere �zer die II-j�hrige Periode der Temperatur. Also,
+Lufttemperaturen Sonnenflecke und Vulcanausbr�che; Meteorologische
+Zeitschrift, Vol. 7, 1914, pp. 305-328.]
+
+
+
+
+CHAPTER IV
+
+THE SOLAR CYCLONIC HYPOTHESIS
+
+
+The progress of science is made up of a vast succession of hypotheses.
+The majority die in early infancy. A few live and are for a time widely
+accepted. Then some new hypothesis either destroys them completely or
+shows that, while they contain elements of truth, they are not the whole
+truth. In the previous chapter we have discussed a group of hypotheses
+of this kind, and have tried to point out fairly their degree of truth
+so far as it can yet be determined. In this chapter we shall outline
+still another hypothesis, the relation of which to present climatic
+conditions has been fully developed in _Earth and Sun_; while its
+relation to the past will be explained in the present volume. This
+hypothesis is not supposed to supersede the others, for so far as they
+are true they cannot be superseded. It merely seems to explain some of
+the many conditions which the other hypotheses apparently fail to
+explain. To suppose that it will suffer a fate more glorious than its
+predecessors would be presumptuous. The best that can be hoped is that
+after it has been pruned, enriched, and modified, it may take its place
+among the steps which finally lead to the goal of truth.
+
+In this chapter the new hypothesis will be sketched in broad outline in
+order that in the rest of this book the reader may appreciate the
+bearing of all that is said. Details of proof and methods of work will
+be omitted, since they are given in _Earth and Sun_. For the sake of
+brevity and clearness the main conclusions will be stated without the
+qualifications and exceptions which are fully explained in that volume.
+Here it will be necessary to pass quickly over points which depart
+radically from accepted ideas, and which therefore must arouse serious
+question in the minds of thoughtful readers. That, however, is a
+necessary consequence of the attempt which this book makes to put the
+problem of climate in such form that the argument can be followed by
+thoughtful students in any branch of knowledge and not merely by
+specialists. Therefore, the specialist can merely be asked to withhold
+judgment until he has read all the evidence as given in _Earth and Sun_,
+and then to condemn only those parts that are wrong and not the whole
+argument.
+
+Without further explanation let us turn to our main problem. In the
+realm of climatology the most important discovery of the last generation
+is that variations in the weather depend on variations in the activity
+of the sun's atmosphere. The work of the great astronomer, Newcomb, and
+that of the great climatologist, K�ppen, have shown beyond question that
+the temperature of the earth's surface varies in harmony with variations
+in the number and area of sunspots.[15] The work of Abbot has shown that
+the amount of heat radiated from the sun also varies, and that in
+general the variations correspond with those of the sunspots, although
+there are exceptions, especially when the spots are fewest. Here,
+however, there at once arises a puzzling paradox. The earth certainly
+owes its warmth to the sun. Yet when the sun emits the most energy, that
+is, when sunspots are most numerous, the earth's surface is coolest.
+Doubtless the earth receives more heat than usual at such times, and the
+upper air may be warmer than usual. Here we refer only to the air at the
+earth's surface.
+
+Another large group of investigators have shown that atmospheric
+pressure also varies in harmony with the number of sunspots. Some parts
+of the earth's surface have one kind of variation at times of many
+sunspots and other parts the reverse. These differences are systematic
+and depend largely on whether the region in question happens to have
+high atmospheric pressure or low. The net result is that when sunspots
+are numerous the earth's storminess increases, and the atmosphere is
+thrown into commotion. This interferes with the stable planetary winds,
+such as the trades of low latitudes and the prevailing westerlies of
+higher latitudes. Instead of these regular winds and the fair weather
+which they bring, there is a tendency toward frequent tropical
+hurricanes in the lower latitudes and toward more frequent and severe
+storms of the ordinary type in the latitudes where the world's most
+progressive nations now live. With the change in storminess there
+naturally goes a change in rainfall. Not all parts of the world,
+however, have increased storminess and more abundant rainfall when
+sunspots are numerous. Some parts change in the opposite way. Thus when
+the sun's atmosphere is particularly disturbed, the contrasts between
+different parts of the earth's surface are increased. For example, the
+northern United States and southern Canada become more stormy and rainy,
+as appears in Fig. 2, and the same is true of the Southwest and along
+the south Atlantic coast. In a crescent-shaped central area, however,
+extending from Wyoming through Missouri to Nova Scotia, the number of
+storms and the amount of rainfall decrease.
+
+[Illustration: _Fig. 2. Storminess at sunspot maxima vs. minima._
+(_After Kullmer._)
+
+Based on nine years' nearest sunspot minima and nine years' nearest
+sunspot maxima in the three sunspot cycles from 1888 to 1918. Heavy
+shading indicates excess of storminess when sunspots are numerous.
+Figures indicate average yearly number of storms by which years of
+maximum sunspots exceed those of minimum sunspots.]
+
+The two controlling factors of any climate are the temperature and the
+atmospheric pressure, for they determine the winds, the storms, and thus
+the rainfall. A study of the temperature seems to show that the peculiar
+paradox of a hot sun and a cool earth is due largely to the increased
+storminess during times of many sunspots. The earth's surface is heated
+by the rays of the sun, but most of the rays do not in themselves heat
+the air as they pass through it. The air gets its heat largely from the
+heat absorbed by the water vapor which is intimately mingled with its
+lower portions, or from the long heat waves sent out by the earth after
+it has been warmed by the sun. The faster the air moves along the
+earth's surface the less it becomes heated, and the more heat it takes
+away. This sounds like a contradiction, but not to anyone who has tried
+to heat a stove in the open air. If the air is still, the stove rapidly
+becomes warm and so does the air around it. If the wind is blowing, the
+cool air delays the heating of the stove and prevents the surface from
+ever becoming as hot as it would otherwise. That seems to be what
+happens on a large scale when sunspots are numerous. The sun actually
+sends to the earth more energy than usual, but the air moves with such
+unusual rapidity that it actually cools the earth's surface a trifle by
+carrying the extra heat to high levels where it is lost into space.
+
+There has been much discussion as to why storms are numerous when the
+sun's atmosphere is disturbed. Many investigators have supposed it was
+due entirely and directly to the heating of the earth's surface by the
+sun. This, however, needs modification for several reasons. In the first
+place, recent investigations show that in a great many cases changes in
+barometric pressure precede changes in temperature and apparently cause
+them by altering the winds and producing storms. This is the opposite of
+what would happen if the effect of solar heat upon the earth's surface
+were the only agency. In the second place, if storms were due
+exclusively to variations in the ordinary solar radiation which comes to
+the earth as light and is converted into heat, the solar effect ought to
+be most pronounced when the center of the sun's visible disk is most
+disturbed. As a matter of fact the storminess is notably greatest when
+the edges of the solar disk are most disturbed. These facts and others
+lead to the conclusion that some agency other than heat must also play
+some part in producing storminess.
+
+The search for this auxiliary agency raises many difficult questions
+which cannot yet be answered. On the whole the weight of evidence
+suggests that electrical phenomena of some kind are involved, although
+variations in the amount of ultra-violet light may also be important.
+Many investigators have shown that the sun emits electrons. Hale has
+proved that the sun, like the earth, is magnetized. Sunspots also have
+magnetic fields the strength of which is often fifty times as great as
+that of the sun as a whole. If electrons are sent to the earth, they
+must move in curved paths, for they are deflected by the sun's magnetic
+field and again by the earth's magnetic field. The solar deflection may
+cause their effects to be greatest when the spots are near the sun's
+margin; the terrestrial deflection may cause concentration in bands
+roughly concentric with the magnetic poles of the earth. These
+conditions correspond with the known facts.
+
+Farther than this we cannot yet go. The calculations of Humphreys seem
+to indicate that the direct electrical effect of the sun's electrons
+upon atmospheric pressure is too small to be of appreciable significance
+in intensifying storms. On the other hand the peculiar way in which
+activity upon the margins of the sun appears to be correlated not only
+with atmospheric electricity, but with barometric pressure, seems to be
+equally strong evidence in the other direction. Possibly the sun's
+electrons and its electrical waves produce indirect effects by being
+converted into heat, or by causing the formation of ozone and the
+condensation of water vapor in the upper air. Any one of these processes
+would raise the temperature of the upper air, for the ozone and the
+water vapor would be formed there and would tend to act as a blanket to
+hold in the earth's heat. But any such change in the temperature of the
+upper air would influence the lower air through changes in barometric
+pressure. These considerations are given here because the thoughtful
+reader is likely to inquire how solar activity can influence storminess.
+Moreover, at the end of this book we shall take up certain speculative
+questions in which an electrical hypothesis will be employed. For the
+main portions of this book it makes no difference how the sun's
+variations influence the earth's atmosphere. The only essential point is
+that when the solar atmosphere is active the storminess of the earth
+increases, and that is a matter of direct observation.
+
+Let us now inquire into the relation between the small cyclonic
+vacillations of the weather and the types of climatic changes known as
+historic pulsations and glacial fluctuations. One of the most
+interesting results of recent investigations is the evidence that
+sunspot cycles on a small scale present almost the same phenomena as do
+historic pulsations and glacial fluctuations. For instance, when
+sunspots are numerous, storminess increases markedly in a belt near the
+northern border of the area of greatest storminess, that is, in southern
+Canada and thence across the Atlantic to the North Sea and Scandinavia.
+(See Figs. 2 and 3.) Corresponding with this is the fact that the
+evidence as to climatic pulsations in historic times indicates that
+regions along this path, for instance Greenland, the North Sea region,
+and southern Scandinavia, were visited by especially frequent and severe
+storms at the climax of each pulsation. Moreover, the greatest
+accumulations of ice in the glacial period were on the poleward border
+of the general regions where now the storms appear to increase most at
+times of solar activity.
+
+[Illustration: _Fig. 3A. Relative rainfall at times of increasing and
+decreasing sunspots._
+
+Heavy shading, more rain with increasing spots. Light shading, more rain
+with decreasing spots. No data for unshaded areas.
+
+Figures indicate percentages of the average rainfall by which the
+rainfall during periods of increasing spots exceeds or falls short of
+rainfall during periods of decreasing spots. The excess or deficiency is
+stated in percentages of the average. Rainfall data from Walker:
+Sunspots and Rainfall.]
+
+[Illustration: _Fig. 3B. Relative rainfall at times of increasing and
+decreasing sunspots._
+
+Heavy shading, more rain with increasing spots. Light shading, more rain
+with decreasing spots. No data for unshaded areas. Figures indicate
+percentages of the average rainfall by which the rainfall during periods
+of increasing spots exceeds or falls short of rainfall during periods of
+decreasing spots. The excess or deficiency is stated in percentages of
+the average. Rainfall data from Walker: Sunspots and Rainfall.]
+
+Even more clear is the evidence from other regions where storms increase
+at times of many sunspots. One such region includes the southwestern
+United States, while another is the Mediterranean region and the
+semi-arid or desert parts of Asia farther east. In these regions
+innumerable ruins and other lines of evidence show that at the climax of
+each climatic pulsation there was more storminess and rainfall than at
+present, just as there now is when the sun is most active. In still
+earlier times, while ice was accumulating farther north, the basins of
+these semi-arid regions were filled with lakes whose strands still
+remain to tell the tale of much-increased rainfall and presumable
+storminess. If we go back still further in geological times to the
+Permian glaciation, the areas where ice accumulated most abundantly
+appear to be the regions where tropical hurricanes produce the greatest
+rainfall and the greatest lowering of temperature at times of many
+sunspots. From these and many other lines of evidence it seems probable
+that historic pulsations and glacial fluctuations are nothing more than
+sunspot cycles on a large scale. It is one of the fundamental rules of
+science to reason from the known to the unknown, from the near to the
+far, from the present to the past. Hence it seems advisable to
+investigate whether any of the climatic phenomena of the past may have
+arisen from an intensification of the solar conditions which now appear
+to give rise to similar phenomena on a small scale.
+
+The rest of this chapter will be devoted to a _r�sum�_ of certain
+tentative conclusions which have no bearing on the main part of this
+book, but which apply to the closing chapters. There we shall inquire
+into the periodicity of the climatic phenomena of geological times, and
+shall ask whether there is any reason to suppose that the sun's activity
+has exhibited similar periodicity. This leads to an investigation of the
+possible causes of disturbances in the sun's atmosphere. It is generally
+assumed that sunspots, solar prominences, the bright clouds known as
+facul�, and other phenomena denoting a perturbed state of the solar
+atmosphere, are due to some cause within the sun. Yet the limitation of
+these phenomena, especially the sunspots, to restricted latitudes, as
+has been shown in _Earth and Sun_, does not seem to be in harmony with
+an internal solar origin, even though a banded arrangement may be normal
+for a rotating globe. The fairly regular periodicity of the sunspots
+seems equally out of harmony with an internal origin. Again, the solar
+atmosphere has two kinds of circulation, one the so-called "rice
+grains," and the other the spots and their attendant phenomena. Now the
+rice grains present the appearance that would be expected in an
+atmospheric circulation arising from the loss of heat by the outer part
+of a gaseous body like the sun. For these reasons and others numerous
+good thinkers from Wolf to Schuster have held that sunspots owe their
+periodicity to causes outside the sun. The only possible cause seems to
+be the planets, acting either through gravitation, through forces of an
+electrical origin, or through some other agency. Various new
+investigations which are described in _Earth and Sun_ support this
+conclusion. The chief difficulty in accepting it hitherto has been that
+although Jupiter, because of its size, would be expected to dominate the
+sunspot cycle, its period of 11.86 years has not been detected. The
+sunspot cycle has appeared to average 11.2 years in length, and has been
+called the 11-year cycle. Nevertheless, a new analysis of the sunspot
+data shows that when attention is concentrated upon the major maxima,
+which are least subject to retardation or acceleration by other causes,
+a periodicity closely approaching that of Jupiter is evident. Moreover,
+when the effects of Jupiter, Saturn, and the other planets are combined,
+they produce a highly variable curve which has an extraordinary
+resemblance to the sunspot curve. The method by which the planets
+influence the sun's atmosphere is still open to question. It may be
+through tides, through the direct effect of gravitation, through
+electro-magnetic forces, or in some other way. Whichever it may be, the
+result may perhaps be slight differences of atmospheric pressure upon
+the sun. Such differences may set in motion slight whirling movements
+analogous to terrestrial storms, and these presumably gather momentum
+from the sun's own energy. Since the planetary influences vary in
+strength because of the continuous change in the relative distances and
+positions of the planets, the sun's atmosphere appears to be swayed by
+cyclonic disturbances of varying degrees of severity. The cyclonic
+disturbances known as sunspots have been proved by Hale to become more
+highly electrified as they increase in intensity. At the same time hot
+gases presumably well up from the lower parts of the solar atmosphere
+and thereby cause the sun to emit more heat. Thus by one means or
+another, the earth's atmosphere appears to be set in commotion and
+cycles of climate are inaugurated.
+
+If the preceding reasoning is correct, any disturbance of the solar
+atmosphere must have an effect upon the earth's climate. If the
+disturbance were great enough and of the right nature it might produce a
+glacial epoch. The planets are by no means the only bodies which act
+upon the sun, for that body sustains a constantly changing relation to
+millions of other celestial bodies of all sizes up to vast universes,
+and at all sorts of distances. If the sun and another star should
+approach near enough to one another, it is certain that the solar
+atmosphere would be disturbed much more than at present.
+
+Here we must leave the cyclonic hypothesis of climate and must refer the
+reader once more to _Earth and Sun_ for fuller details. In the rest of
+this book we shall discuss the nature of the climatic changes of past
+times and shall inquire into their relation to the various climatic
+hypotheses mentioned in the last two chapters. Then we shall inquire
+into the possibility that the solar system has ever been near enough to
+any of the stars to cause appreciable disturbances of the solar
+atmosphere. We shall complete our study by investigating the vexed
+question of why movements of the earth's crust, such as the uplifting of
+continents and mountain chains, have generally occurred at the same time
+as great climatic fluctuations. This would not be so surprising were it
+not that the climatic phenomena appear to have consisted of highly
+complex cycles while the uplift has been a relatively steady movement in
+one direction. We shall find some evidence that the solar disturbances
+which seem to cause climatic changes also have a relation to movements
+of the crust.
+
+FOOTNOTES:
+
+[Footnote 15: The so-called sunspot numbers to which reference is made
+again and again in this book are based on a system devised by Wolf and
+revised by A. Wolfer. The number and size of the spots are both taken
+into account. The numbers from 1749 to 1900 may be found in the Monthly
+Weather Review for April, 1902, and from 1901 to 1918 in the same
+journal for 1920.]
+
+
+
+
+CHAPTER V
+
+THE CLIMATE OF HISTORY[16]
+
+We are now prepared to consider the climate of the past. The first
+period to claim attention is the few thousand years covered by written
+history. Strangely enough, the conditions during this time are known
+with less accuracy than are those of geological periods hundreds of
+times more remote. Yet if pronounced changes have occurred since the
+days of the ancient Babylonians and since the last of the post-glacial
+stages, they are of great importance not only because of their possible
+historic effects, but because they bridge the gap between the little
+variations of climate which are observable during a single lifetime and
+the great changes known as glacial epochs. Only by bridging the gap can
+we determine whether there is any genetic relation between the great
+changes and the small. A full discussion of the climate of historic
+times is not here advisable, for it has been considered in detail in
+numerous other publications.[17] Our most profitable course would seem
+to be to consider first the general trend of opinion and then to take up
+the chief objections to each of the main hypotheses.
+
+In the hot debate over this problem during recent decades the ideas of
+geographers seem to have gone through much the same metamorphosis as
+have those of geologists in regard to the climate of far earlier times.
+
+As every geologist well knows, at the dawn of geology people believed in
+climatic uniformity--that is, it was supposed that since the completion
+of an original creative act there had been no important changes. This
+view quickly disappeared and was superseded by the hypothesis of
+progressive cooling and drying, an hypothesis which had much to do with
+the development of the nebular hypothesis, and which has in turn been
+greatly strengthened by that hypothesis. The discovery of evidence of
+widespread continental glaciation, however, necessitated a modification
+of this view, and succeeding years have brought to light a constantly
+increasing number of glacial, or at least cool, periods distributed
+throughout almost the whole of geological time. Moreover, each year,
+almost, brings new evidence of the great complexity of glacial periods,
+epochs, and stages. Thus, for many decades, geologists have more and
+more been led to believe that in spite of surprising uniformity, when
+viewed in comparison with the cosmic possibilities, the climate of the
+past has been highly unstable from the viewpoint of organic evolution,
+and its changes have been of all degrees of intensity.
+
+Geographers have lately been debating the reality of historic changes of
+climate in the same way in which geologists debated the reality of
+glacial epochs and stages. Several hypotheses present themselves but
+these may all be grouped under three headings; namely, the hypotheses of
+(1) progressive desiccation, (2) climatic uniformity, and (3)
+pulsations. The hypothesis of progressive desiccation has been widely
+advocated. In many of the drier portions of the world, especially
+between 30� and 40� from the equator, and pre�minently in western and
+central Asia and in the southwestern United States, almost innumerable
+facts seem to indicate that two or three thousand years ago the climate
+was distinctly moister than at present. The evidence includes old lake
+strands, the traces of desiccated springs, roads in places now too dry
+for caravans, other roads which make detours around dry lake beds where
+no lakes now exist, and fragments of dead forests extending over
+hundreds of square miles where trees cannot now grow for lack of water.
+Still stronger evidence is furnished by ancient ruins, hundreds of which
+are located in places which are now so dry that only the merest fraction
+of the former inhabitants could find water. The ruins of Palmyra, in the
+Syrian Desert, show that it must once have been a city like modern
+Damascus, with one or two hundred thousand inhabitants, but its water
+supply now suffices for only one or two thousand. All attempts to
+increase the water supply have had only a slight effect and the water is
+notoriously sulphurous, whereas in the former days, when it was
+abundant, it was renowned for its excellence. Hundreds of pages might be
+devoted to describing similar ruins. Some of them are even more
+remarkable for their dryness than is Niya, a site in the Tarim Desert of
+Chinese Turkestan. Yet there the evidence of desiccation within 2000
+years is so strong that even so careful and conservative a man as
+Hann,[18] pronounces it "�berzeugend."
+
+A single quotation from scores that might be used will illustrate the
+conclusions of some of the most careful arch�ologists.[19]
+
+     Among the regions which were once populous and highly civilized,
+     but which are now desert and deserted, there are few which were
+     more closely connected with the beginnings of our own civilization
+     than the desert parts of Syria and northern Arabia. It is only of
+     recent years that the vast extent and great importance of this lost
+     civilization has been fully recognized and that attempts have been
+     made to reduce the extent of the unexplored area and to discover
+     how much of the territory which has long been known as desert was
+     formerly habitable and inhabited. The results of the explorations
+     of the last twenty years have been most astonishing in this regard.
+     It has been found that practically all of the wide area lying
+     between the coast range of the eastern Mediterranean and the
+     Euphrates, appearing upon the maps as the Syrian Desert, an area
+     embracing somewhat more than 20,000 square miles, was more thickly
+     populated than any area of similar dimensions in England or in the
+     United States is today if one excludes the immediate vicinity of
+     the large modern cities. It has also been discovered that an
+     enormous desert tract lying to the east of Palestine, stretching
+     eastward and southward into the country which we know as Arabia,
+     was also a densely populated country. How far these settled regions
+     extended in antiquity is still unknown, but the most distant
+     explorations in these directions have failed to reach the end of
+     ruins and other signs of former occupation.
+
+     The traveler who has crossed the settled, and more or less
+     populous, coast range of northern Syria and descended into the
+     narrow fertile valley of the Orontes, encounters in any farther
+     journey toward the east an irregular range of limestone hills lying
+     north and south and stretching to the northeast almost halfway to
+     the Euphrates. These hills are about 2,500 feet high, rising in
+     occasional peaks from 3,000 to 3,500 feet above sea level. They are
+     gray and unrelieved by any visible vegetation. On ascending into
+     the hills the traveler is astonished to find at every turn remnants
+     of the work of men's hands, paved roads, walls which divided
+     fields, terrace walls of massive structure. Presently he comes upon
+     a small deserted and partly ruined town composed of buildings large
+     and small constructed of beautifully wrought blocks of limestone,
+     all rising out of the barren rock which forms the ribs of the
+     hills. If he mounts an eminence in the vicinity, he will be still
+     further astonished to behold similar ruins lying in all directions.
+     He may count ten or fifteen or twenty, according to the commanding
+     position of his lookout. From a distance it is often difficult to
+     believe that these are not inhabited places; but closer inspection
+     reveals that the gentle hand of time or the rude touch of
+     earthquake has been laid upon every building. Some of the towns are
+     better preserved than others; some buildings are quite perfect but
+     for their wooden roofs which time has removed, others stand in
+     picturesque ruins, while others still are level with the ground. On
+     a far-off hilltop stands the ruin of a pagan temple, and crowning
+     some lofty ridge lie the ruins of a great Christian monastery. Mile
+     after mile of this barren gray country may be traversed without
+     encountering a single human being. Day after day may be spent in
+     traveling from one ruined town to another without seeing any green
+     thing save a terebinth tree or two standing among the ruins, which
+     have sent their roots down into earth still preserved in the
+     foundations of some ancient building. No soil is visible anywhere
+     except in a few pockets in the rock from which it could not be
+     washed by the torrential rains of the wet season; yet every ruin is
+     surrounded with the remains of presses for the making of oil and
+     wine. Only one oasis has been discovered in these high plateaus.
+
+     Passing eastward from this range of hills, one descends into a
+     gently rolling country that stretches miles away toward the
+     Euphrates. At the eastern foot of the hills one finds oneself in a
+     totally different country, at first quite fertile and dotted with
+     frequent villages of flat-roofed houses. Here practically all the
+     remains of ancient times have been destroyed through ages of
+     building and rebuilding. Beyond this narrow fertile strip the soil
+     grows drier and more barren, until presently another kind of desert
+     is reached, an undulating waste of dead soil. Few walls or towers
+     or arches rise to break the monotony of the unbroken landscape; but
+     the careful explorer will find on closer examination that this
+     region was more thickly populated in antiquity even than the hill
+     country to the west. Every unevenness of the surface marks the site
+     of a town, some of them cities of considerable extent.
+
+     We may draw certain very definite conclusions as to the former
+     conditions of the country itself. There was soil upon the northern
+     hills where none now exists, for the buildings now show unfinished
+     foundation courses which were not intended to be seen; the soil in
+     depressions without outlets is deeper than it formerly was; there
+     are hundreds of olive and wine presses in localities where no tree
+     or vine could now find footing; and there are hillsides with ruined
+     terrace walls rising one above the other with no sign of earth near
+     them. There was also a large natural water supply. In the north as
+     well as in the south we find the dry beds of rivers, streams, and
+     brooks with sand and pebbles and well-worn rocks but no water in
+     them from one year's end to the other. We find bridges over these
+     dry streams and crudely made washing boards along their banks
+     directly below deserted towns. Many of the bridges span the beds of
+     streams that seldom or never have water in them and give clear
+     evidence of the great climatic changes that have taken place. There
+     are well heads and well houses, and inscriptions referring to
+     springs; but neither wells nor springs exist today except in the
+     rarest instances. Many of the houses had their rock-hewn cisterns,
+     never large enough to have supplied water for more than a brief
+     period, and corresponding to the cisterns which most of our recent
+     forefathers had which were for convenience rather than for
+     dependence. Some of the towns in southern Syria were provided with
+     large public reservoirs, but these are not large enough to have
+     supplied water to their original populations. The high plateaus
+     were of course without irrigation; but there are no signs, even in
+     the lower flatter country, that irrigation was ever practiced; and
+     canals for this purpose could not have completely disappeared.
+     There were forests in the immediate vicinity, forests producing
+     timbers of great length and thickness; for in the north and
+     northeast practically all the buildings had wooden roofs, wooden
+     intermediate floors, and other features of wood. Costly buildings,
+     such as temples and churches, employed large wooden beams; but wood
+     was used in much larger quantities in private dwellings, shops,
+     stables, and barns. If wood had not been plentiful and cheap--which
+     means grown near by--the builders would have adopted the building
+     methods of their neighbors in the south, who used very little wood
+     and developed the most perfect type of lithic architecture the
+     world has ever seen. And here there exists a strange anomaly:
+     Northern Syria, where so much wood was employed in antiquity, is
+     absolutely treeless now; while in the mountains of southern Syria,
+     where wood must have been scarce in antiquity to have forced upon
+     the inhabitants an almost exclusive use of stone, there are still
+     groves of scrub oak and pine, and travelers of half a century ago
+     reported large forests of chestnut trees.[20] It is perfectly
+     apparent that large parts of Syria once had soil and forests and
+     springs and rivers, while it has none of these now, and that it had
+     a much larger and better distributed rainfall in ancient times than
+     it has now.
+
+Professor Butler's careful work is especially interesting because of its
+contrast to the loose statements of those who believe in climatic
+uniformity. So far as I am aware, no opponent of the hypothesis of
+climatic changes has ever even attempted to show by careful statistical
+analysis that the ancient water supply of such ruins was no greater than
+that of the present. The most that has been done is to suggest that
+there may have been sources of water which are now unknown. Of course,
+this might be true in a single instance, but it could scarcely be the
+case in many hundreds or thousands of ruins.
+
+Although the arguments in favor of a change of climate during the last
+two thousand years seem too strong to be ignored, their very strength
+seems to have been a source of error. A large number of people have
+jumped to the conclusion that the change which appears to have occurred
+in certain regions occurred everywhere, and that it consisted of a
+gradual desiccation.
+
+Many observers, quite as careful as those who believe in progressive
+desiccation, point to evidences of aridity in past times in the very
+regions where the others find proof of moisture. Lakes such as the
+Caspian Sea fell to such a low level that parts of their present floors
+were exposed and were used as sites for buildings whose ruins are still
+extant. Elsewhere, for instance in the Tian-Shan Mountains, irrigation
+ditches are found in places where irrigation never seems to be necessary
+at present. In Syria and North Africa during the early centuries of the
+Christian era the Romans showed unparalleled activity in building great
+aqueducts and in watering land which then apparently needed water almost
+as much as it does today. Evidence of this sort is abundant and is as
+convincing as is the evidence of moister conditions in the past. It is
+admirably set forth, for example, in the comprehensive and ably written
+monograph of Leiter on the climate of North Africa.[21] The evidence
+cited there and elsewhere has led many authors strongly to advocate the
+hypothesis of climatic uniformity. They have done exactly as have the
+advocates of progressive change, and have extended their conclusions
+over the whole world and over the whole of historic times.
+
+The hypotheses of climatic uniformity and of progressive change both
+seem to be based on reliable evidence. They may seem to be diametrically
+opposed to one another, but this is only when there is a failure to
+group the various lines of evidence according to their dates, and
+according to the types of climate in which they happen to be located.
+When the facts are properly grouped in both time and space, it appears
+that evidence of moist conditions in the historic Mediterranean lands is
+found during certain periods; for instance, four or five hundred years
+before Christ, at the time of Christ, and 1000 A. D. The other kind of
+evidence, on the contrary, culminates at other epochs, such as about
+1200 B. C. and in the seventh and thirteenth centuries after Christ. It
+is also found during the interval from the culmination of a moist epoch
+to the culmination of a dry one, for at such times the climate was
+growing drier and the people were under stress. This was seemingly the
+case during the period from the second to the fourth centuries of our
+era. North Africa and Syria must then have been distinctly better
+watered than at present, as appears from Butler's vivid description; but
+they were gradually becoming drier, and the natural effect on a
+vigorous, competent people like the Romans was to cause them to
+construct numerous engineering works to provide the necessary water.
+
+The considerations which have just been set forth have led to a third
+hypothesis, that of pulsatory climatic changes. According to this, the
+earth's climate is not stable, nor does it change uniformly in one
+direction. It appears to fluctuate back and forth not only in the little
+waves which we see from year to year or decade to decade, but in much
+larger waves, which take hundreds of years or even a thousand. These in
+turn seem to merge into and be imposed on the greater waves which form
+glacial stages, glacial epochs, and glacial periods. At the present time
+there seems to be no way of determining whether the general tendency is
+toward aridity or toward glaciation. The seventh century of our era was
+apparently the driest time during the historic period--distinctly drier
+than the present--but the thirteenth century was almost equally dry, and
+the twelfth or thirteenth before Christ may have been very dry.
+
+The best test of an hypothesis is actual measurements. In the case of
+the pulsatory hypothesis we are fortunately able to apply this test by
+means of trees. The growth of vegetation depends on many factors--soil,
+exposure, wind, sun, temperature, rain, and so forth. In a dry region
+the most critical factor in determining how a tree's growth shall vary
+from year to year is the supply of moisture during the few months of
+most rapid growth.[22] The work of Douglass[23] and others has shown
+that in Arizona and California the thickness of the annual rings affords
+a reliable indication of the amount of moisture available during the
+period of growth. This is especially true when the growth of several
+years is taken as the unit and is compared with the growth of a similar
+number of years before or after. Where a long series of years is used,
+it is necessary to make corrections to eliminate the effects of age, but
+this can be done by mathematical methods of considerable accuracy. It is
+difficult to determine whether the climate at the beginning and end of a
+tree's life was the same, but it is easily possible to determine whether
+there have been pulsations while the tree was making its growth. If a
+large number of trees from various parts of a given district all formed
+thick rings at a certain period and then formed thin ones for a hundred
+years, after which the rings again become thick, we seem to be safe in
+concluding that the trees have lived through a long, dry period. The
+full reasons for this belief and details as to the methods of estimating
+climate from tree growth are given in _The Climatic Factor_.
+
+The results set forth in that volume may be summarized as follows:
+During the years 1911 and 1912, under the auspices of the Carnegie
+Institution of Washington, measurements were made of the thickness of
+the rings of growth on the stumps of about 450 sequoia trees in
+California. These trees varied in age from 250 to nearly 3250 years. The
+great majority were over 1000 years of age, seventy-nine were over 2000
+years, and three over 3000. Even where only a few trees are available
+the record is surprisingly reliable, except where occasional accidents
+occur. Where the number approximates 100, accidental variations are
+largely eliminated and we may accept the record with considerable
+confidence. Accordingly, we may say that in California we have a fairly
+accurate record of the climate for 2000 years and an approximate record
+for 1000 years more. The final results of the measurements of the
+California trees are shown in Fig. 4, where the climatic variations for
+3000 years in California are indicated by the solid line. The high parts
+of the line indicate rainy conditions, the low parts, dry. An
+examination of this curve shows that during 3000 years there have
+apparently been climatic variations more important than any which have
+taken place during the past century. In order to bring out the details
+more clearly, the more reliable part of the California curve, from 100
+B. C. to the present time, has been reproduced in Fig. 5. This is
+identical with the corresponding part of Fig. 4, except that the
+vertical scale is three times as great.
+
+[Illustration: _Fig. 4. Changes of climate in California (solid line)
+and in western and central Asia (dotted line)._
+
+Note. The curves of Figs. 4 and 5 are reproduced as published in _The
+Solar Hypothesis_ in 1914. Later work, however, has indicated that in
+the Asiatic curve the dash lines, which were tentatively inserted in
+1914, are probably more nearly correct than the dotted lines. Still
+further evidence indicates that the Asiatic curve is nearly like that of
+California in its main features.]
+
+The curve of tree growth in California seems to be a true representation
+of the general features of climatic pulsations in the Mediterranean
+region. This conclusion was originally based on the resemblance between
+the solid line of Fig. 4, representing tree growth, and the dotted line
+representing changes of climate in the eastern Mediterranean region as
+inferred from the study of ruins and of history before any work on this
+subject had been done in America.[24] The dotted line is here reproduced
+for its historical significance as a stage in the study of climatic
+changes. If it were to be redrawn today on the basis of the knowledge
+acquired in the last twelve years, it would be much more like the tree
+curve. For example, the period of aridity suggested by the dip of the
+dotted line about 300 A. D. was based largely on Professor Butler's data
+as to the paucity of inscriptions and ruins dating from that period in
+Syria. In the recent article, from which a long quotation has been
+given, he shows that later work proves that there is no such paucity. On
+the other hand, it has accentuated the marked and sudden decay in
+civilization and population which occurred shortly after 600 A. D. He
+reached the same conclusion to which the present authors had come on
+wholly different grounds, namely, that the dip in the dotted line about
+300 A. D. is not warranted, whereas the dip about 630 A. D. is extremely
+important. In similar fashion the work of Stein[25] in central Asia
+makes it clear that the contrast between the water supply about 200 B.C.
+and in the preceding and following centuries was greater than was
+supposed on the basis of the scanty evidence available when the dotted
+line of Fig. 4 was drawn in 1910.
+
+[Illustration: _Fig. 5. Changes in California climate for 2000 years,
+as measured by growth of Sequoia trees._
+
+Fig. 5 is the same as the later portion of Fig. 4, except that the
+vertical scale has been magnified threefold. It seems probable that the
+dotted line at the right is more nearly correct than the solid line.
+During the thirty years since the end of the curve the general tendency
+appears in general to have been somewhat upward.]
+
+Since the curve of the California trees is the only continuous and
+detailed record yet available for the climate of the last three thousand
+years, it deserves most careful study. It is especially necessary to
+determine the degree of accuracy with which the growth of the trees
+represents (1) the local rainfall and (2) the rainfall of remote regions
+such as Palestine. Perhaps the best way to determine these matters is
+the standard mathematical method of correlation coefficients. If two
+phenomena vary in perfect unison, as in the case of the turning of the
+wheels and the progress of an automobile when the brakes are not
+applied, the correlation coefficient is 1.00, being positive when the
+automobile goes forward and negative when it goes backward. If there is
+no relation between two phenomena, as in the case of the number of miles
+run by a given automobile each year and the number of chickens hatched
+in the same period, the coefficient is zero. A partial relationship
+where other factors enter into the matter is represented by a
+coefficient between zero and one, as in the case of the movement of the
+automobile and the consumption of gasoline. In this case the relation is
+very obvious, but is modified by other factors, including the roughness
+and grade of the road, the amount of traffic, the number of stops, the
+skill of the driver, the condition and load of the automobile, and the
+state of the weather. Such partial relationships are the kind for which
+correlation coefficients are most useful, for the size of the
+coefficients shows the relative importance of the various factors. A
+correlation coefficient four times the probable error, which can always
+be determined by a formula well known to mathematicians, is generally
+considered to afford evidence of some kind of relation between two
+phenomena. When the ratio between coefficient and error rises to six,
+the relationship is regarded as strong.
+
+Few people would question that there is a connection between tree growth
+and rainfall, especially in a climate with a long summer dry season like
+that of California. But the growth of the trees also depends on their
+position, the amount of shading, the temperature, insect pests, blights,
+the wind with its tendency to break the branches, and a number of other
+factors. Moreover, while rain commonly favors growth, great extremes are
+relatively less helpful than more moderate amounts. Again, the roots of
+a tree may tap such deep sources of water that neither drought nor
+excessive rain produces much effect for several years. Hence in
+comparing the growth of the huge sequoias with the rainfall we should
+expect a correlation coefficient high enough to be convincing, but
+decidedly below 1.00. Unfortunately there is no record of the rainfall
+where the sequoias grow, the nearest long record being that of
+Sacramento, nearly 200 miles to the northwest and close to sea level
+instead of at an altitude of about 6000 feet.
+
+Applying the method of correlation coefficients to the annual rainfall
+of Sacramento and the growth of the sequoias from 1863 to 1910, we
+obtain the results shown in Table 3. The trees of Section A of the table
+grew in moderately dry locations although the soil was fairly deep, a
+condition which seems to be essential to sequoias. In this case, as in
+all the others, the rainfall is reckoned from July to June, which
+practically means from October to May, since there is almost no summer
+rain. Thus the tree growth in 1861 is compared with the rainfall of the
+preceding rainy season, 1860-1861, or of several preceding rainy seasons
+as the table indicates.
+
+   +-------------------------------------------------------------------+
+   |                            TABLE 3                                |
+   |                                                                   |
+   |         CORRELATION COEFFICIENTS BETWEEN RAINFALL AND             |
+   |              GROWTH OF SEQUOIAS IN CALIFORNIA[26]                 |
+   |                                                                   |
+   |     (_r_) = _Correlation coefficient_                             |
+   |     (_e_) = _Probable error_                                      |
+   | (_r_/_e_) = _Ratio of coefficient to probable error_              |
+   |                                                                   |
+   | A. SACRAMENTO RAINFALL AND GROWTH OF 18 SEQUOIAS IN DRY           |
+   |                       LOCATIONS, 1861-1910                        |
+   |                                                                   |
+   |                                     (_r_)     (_e_)  (_r_/_e_)    |
+   |                                     ------    ------    -----     |
+   |   1 year of rainfall                -0.059    �0.096     0.6      |
+   |   2 years of rainfall               +0.288    �0.090     3.2      |
+   |   3 years of rainfall               +0.570    �0.066     8.7      |
+   |   4 years of rainfall               +0.470    �0.076     6.2      |
+   |                                                                   |
+   | B. SACRAMENTO RAINFALL AND GROWTH OF 112 SEQUOIAS MOSTLY IN       |
+   |                  MOIST LOCATIONS, 1861-1910                       |
+   |                                                                   |
+   |   3 years of rainfall               +0.340    �0.087     3.9      |
+   |   4 years of rainfall               +0.371    �0.084     4.5      |
+   |   5 years of rainfall               +0.398    �0.082     4.9      |
+   |   6 years of rainfall               +0.418    �0.079     5.3      |
+   |   7 years of rainfall               +0.471    �0.076     6.2      |
+   |   8 years of rainfall              (+0.520)   �0.071     7.3      |
+   |   9 years of rainfall               +0.575    �0.065     8.8      |
+   |  10 years of rainfall               +0.577    �0.065     8.8      |
+   |                                                                   |
+   | C. SACRAMENTO RAINFALL AND GROWTH OF 80 SEQUOIAS IN MOIST         |
+   |                  LOCATIONS, 1861-1910                             |
+   |                                                                   |
+   |  10 years of rainfall               +0.605    �0.062     9.8      |
+   |                                                                   |
+   | D. ANNUAL SEQUOIA GROWTH AND RAINFALL OF PRECEDING 5 YEARS        |
+   |          AT STATIONS ON SOUTHERN PACIFIC RAILROAD                 |
+   |                                                                   |
+   |        1 = _Years_                                                |
+   |        2 = _Altitude_ (_feet_)                                    |
+   |        3 = _Rainfall_ (_inches_)                                  |
+   |        4 = _Approximate distance from sequoias_ (_miles_)         |
+   |                                                                   |
+   |                 1        2     3     4   (_r_)   (_e_)  (_r_/_e_) |
+   |             ---------  ----  -----  ---  ------  ------ --------- |
+   | Sacramento, 1861-1910    70  19.40  200  +0.398  �0.081     4.9   |
+   | Colfax,     1871-1909  2400  48.94  200  +0.122  �0.113     1.1   |
+   | Summit,     1871-1909  7000  48.07  200  +0.148  �0.113     1.3   |
+   | Truckee,    1871-1909  5800  27.12  200  +0.300  �0.105     2.9   |
+   | Boca,       1871-1909  5500  20.34  200  +0.604  �0.076     8.0   |
+   | Winnemucca, 1871-1909  4300   8.65  300  +0.492  �0.089     5.5   |
+   |                                                                   |
+   +-------------------------------------------------------------------+
+
+In the first line of Section A a correlation coefficient of only -0.056,
+which is scarcely six-tenths of the probable error, means that there is
+no appreciable relation between the rainfall of a given season and the
+growth during the following spring and summer. The roots of the sequoias
+probably penetrate so deeply that the rain and melted snow of the spring
+months do not sink down rapidly enough to influence the trees before the
+growing season comes to an end. The precipitation of two preceding
+seasons, however, has some effect on the trees, as appears in the second
+line of Section A, where the correlation coefficient is +0.288, or 3.2
+times the probable error. When the rainfall of three seasons is taken
+into account the coefficient rises to +0.570, or 8.7 times the probable
+error, while with four years of rainfall the coefficient begins to fall
+off. Thus the growth of these eighteen sequoias on relatively dry slopes
+appears to have depended chiefly on the rainfall of the second and third
+preceding rainy seasons. The growth in 1900, for example, depended
+largely on the rainfall in the rainy seasons of 1897-1898 and 1898-1899.
+
+Section B of the table shows that with 112 trees, growing chiefly in
+moist depressions where the water supply is at a maximum, the
+correlation between growth and rainfall, +0.577 for ten years' rainfall,
+is even higher than with the dry trees. The seepage of the underground
+water is so slow that not until four years' rainfall is taken into
+account is the correlation coefficient more than four times the probable
+error. When only the trees growing in moist locations are employed, the
+coefficient between tree growth and the rainfall for ten years rises to
+the high figure of +0.605, or 9.8 times the probable error, as appears
+in Section C. These figures, as well as many others not here published,
+make it clear that the curve of sequoia growth from 1861 to 1910 affords
+a fairly close indication of the rainfall at Sacramento, provided
+allowance be made for a delay of three to ten years due to the fact that
+the moisture in the soil gradually seeps down the mountain-sides and
+only reaches the sequoias after a considerable interval.
+
+If a rainfall record were available for the place where the trees
+actually grow, the relationship would probably be still closer.
+
+The record at Fresno, for example, bears out this conclusion so far as
+it goes. But as Fresno lies at a low altitude and its rainfall is of
+essentially the Sacramento type, its short record is of less value than
+that of Sacramento. The only rainfall records among the Sierras at high
+levels, where the rainfall and temperature are approximately like those
+of the sequoia region, are found along the main line of the Southern
+Pacific railroad. This runs from Oakland northeastward seventy miles
+across the open plain to Sacramento, then another seventy miles, as the
+crow flies, through Colfax and over a high pass in the Sierras at
+Summit, next twenty miles or so down through Truckee to Boca, on the
+edge of the inland basin of Nevada, and on northeastward another 160
+miles to Winnemucca, where it turns east toward Ogden and Salt Lake
+City. Section D of Table 3 shows the correlation coefficients between
+the rainfall along the railroad and the growth of the sequoias. At
+Sacramento, which lies fairly open to winds from the Pacific and thus
+represents the general climate of central California, the coefficient is
+nearly five times the probable error, thus indicating a real relation to
+sequoia growth. Then among the foothills of the Sierras at Colfax, the
+coefficient drops till it is scarcely larger than the probable error. It
+rises rapidly, however, as one advances among the mountains, until at
+Boca it attains the high figure of +0.604 or eight times the probable
+error, and continues high in the dry area farther east. In other words
+the growth of the sequoias is a good indication of the rainfall where
+the trees grow and in the dry region farther east.
+
+In order to determine the degree to which the sequoia record represents
+the rainfall of other regions, let us select Jerusalem for comparison.
+The reasons for this selection are that Jerusalem furnishes the only
+available record that satisfies the following necessary conditions: (1)
+its record is long enough to be important; (2) it is located fairly near
+the latitude of the sequoias, 32�N versus 37�N; (3) it is located in a
+similar type of climate with winter rains and a long dry summer; (4) it
+lies well above sea level (2500 feet) and somewhat back from the
+seacoast, thus approximating although by no means duplicating the
+condition of the sequoias; and (5) it lies in a region where the
+evidence of climatic changes during historic times is strongest. The
+ideal place for comparison would be the valley in which grow the cedars
+of Lebanon. Those trees resemble the sequoias to an extraordinary
+degree, not only in their location, but in their great age. Some day it
+will be most interesting to compare the growth of these two famous
+groups of old trees.
+
+   +-------------------------------------------------------------------+
+   |                            TABLE 4                                |
+   |                                                                   |
+   |                CORRELATION COEFFICIENTS BETWEEN                   |
+   |                 RAINFALL RECORDS IN CALIFORNIA                    |
+   |                        AND JERUSALEM                              |
+   |                                                                   |
+   |     (_r_) = _Correlation coefficient_                             |
+   |     (_e_) = _Probable error_                                      |
+   | (_r_/_e_) = _Ratio of coefficient to probable error_              |
+   |                                                                   |
+   | A. JERUSALEM RAINFALL FOR 3 YEARS AND VARIOUS GROUPS OF           |
+   |                           SEQUOIAS[27]                            |
+   |                                                                   |
+   |                                          (_r_)    (_e_)  (_r_/_e_)|
+   |                                          ------   -----  ---------|
+   |  11 trees measured by Douglass           +0.453  �0.078    5.8    |
+   |  80 trees, moist locations, Groups IA,                            |
+   |       IIA, IIIA, VA                      +0.500  �0.073    6.8    |
+   | 101 trees, 69 in moist locations, 32 in                           |
+   |       dry, I, II, III                    +0.616  �0.061   10.1    |
+   | 112 trees, 80 in moist locations, 32 in                           |
+   |       dry, I, II, III, V                 +0.675  �0.053   12.7    |
+   |                                                                   |
+   | B. RAINFALL AT JERUSALEM AND AT STATIONS IN CALIFORNIA AND NEVADA |
+   |                                                                   |
+   |                   1 = _Altitude_ (_feet_)                         |
+   |                   2 = _Years_                                     |
+   |                                                                   |
+   |                                    -- 3 years --   -- 5 years --  |
+   |                    1       2      (_r_) (_r_/_e_) (_r_) (_r_/_e_) |
+   |                  ----  ---------  ------ -------  ------ -------  |
+   |  Sacramento,       70  1861-1910  +0.386   4.7    +0.352   4.2    |
+   |  Colfax,         2400  1871-1909  +0.311   3.1    +0.308   3.0    |
+   |  Summit,         7000  1871-1909  +0.099   0.9    +0.248   2.3    |
+   |  Truckee,        5800  1871-1909  +0.229   2.2    +0.337   3.3    |
+   |[A]Boca,          5500  1871-1909  +0.482   6.4    +0.617   8.6    |
+   |  Winnemucca,     4300  1871-1909  +0.235   2.2    +0.260   2.4    |
+   |  San Bernardino, 1050  1871-1909  +0.275   2.7    +0.177   1.8    |
+   |                                                                   |
+   | C. RAINFALL FOR 3 YEARS AT CALIFORNIA AND NEVADA STATIONS,        |
+   |                               1871-1909                           |
+   |                                                                   |
+   |                                                (_r_)  (_r_/_e_)   |
+   |                                                ------  -------    |
+   |  Sacramento and San Bernardino                 +0.663    10.7     |
+   |  San Bernardino and Winnemucca                 +0.291     2.8     |
+   |                                                                   |
+   +-------------------------------------------------------------------+
+
+The correlation coefficients for the sequoia growth and the rainfall at
+Jerusalem are given in Section A, Table 4. They are so high and so
+consistent that they scarcely leave room for doubt that where a hundred
+or more sequoias are employed, as in Fig. 5, their curve of growth
+affords a good indication of the fluctuations of climate in western
+Asia. The high coefficient for the eleven trees measured by Douglass
+suggests that where the number of trees falls as low as ten, as in the
+part of Fig. 4 from 710 to 840 B. C., the relation between tree growth
+and rainfall is still close even when only one year's growth is
+considered. Where the unit is ten years of growth, as in Figs. 4 and 5,
+the accuracy of the tree curve as a measure of rainfall is much greater
+than when a single year is used as in Table 4. When the unit is raised
+to thirty years, as in the smoothed part of Fig. 4 previous to
+240 B. C., even four trees, as from 960 to 1070, probably give a fair
+approximation to the general changes in rainfall, while a single tree
+prior to 1110 B. C. gives a rough indication.
+
+Table 4 shows a peculiar feature in the fact that the correlations of
+Section A between tree growth and the rainfall of Jerusalem are
+decidedly higher than those between the rainfall in the two regions.
+Only at Sacramento and Boca are the rainfall coefficients high enough to
+be conclusive. This, however, is not surprising, for even between
+Sacramento and San Bernardino, only 400 miles apart, the correlation
+coefficient for the rainfall by three-year periods is only 10.7 times
+the probable error, as appears in Section C of Table 4, while between
+San Bernardino and Winnemucca 500 miles away, the corresponding figure
+drops to 2.8. It must be remembered that in some respects the growth of
+the sequoias is a much better record of rainfall than are the records
+kept by man. The human record is based on the amount of water caught by
+a little gauge a few inches in diameter. Every gust of wind detracts
+from the accuracy of the record; a mile away the rainfall may be double
+what it is at the gauge. Each sequoia, on the other hand, draws its
+moisture from an area thousands of times as large as a rain gauge.
+Moreover, the trees on which Figs. 4 and 5 are based were scattered over
+an area fifty miles long and several hundred square miles in extent.
+Hence they represent the summation of the rainfall over an area millions
+of times as large as that of a rain gauge. This fact and the large
+correlation coefficients between sequoia growth and Jerusalem rainfall
+should be considered in connection with the fact that all the
+coefficients between the rainfall of California and Nevada and that of
+Jerusalem are positive. If full records of the complete rainfall of
+California and Nevada on the one hand and of the eastern Mediterranean
+region on the other were available for a long period, they would
+probably agree closely.
+
+Just how widely the sequoias can be used as a measure of the climate of
+the past is not yet certain. In some regions, as will shortly be
+explained, the climatic changes seem to have been of an opposite
+character from those of California. In others the Californian or eastern
+Mediterranean type of change seems sometimes to prevail but is not
+always evident. For example, at Malta the rainfall today shows a
+distinct relation to that of Jerusalem and to the growth of the
+sequoias. But the correlation coefficient between the rainfall of
+eight-year periods at Naples, a little farther north, and the growth of
+the sequoias at the end of the periods is -0.132, or only 1.4 times the
+probable error and much too small to be significant. This is in harmony
+with the fact that although Naples has summer droughts, they are not so
+pronounced as in California and Palestine, and the prevalence of storms
+is much greater. Jerusalem receives only 8 per cent of its rain in the
+seven months from April to October, and Sacramento 13, while Malta
+receives 31 per cent and Naples 43. Nevertheless, there is some evidence
+that in the past the climatic fluctuations of southern Italy followed
+nearly the same course as those of California and Palestine. This
+apparent discrepancy seems to be explained by our previous conclusion
+that changes of climate are due largely to a shifting of storm tracks.
+When sunspots are numerous the storms which now prevail in northern
+Italy seem to be shifted southward and traverse the Mediterranean to
+Palestine just as similar storms are shifted southward in the United
+States. This perhaps accounts for the agreement between the sequoia
+curve and the agricultural and social history of Rome from about
+400 B. C. to 100 A. D., as explained in _World Power and Evolution_. For
+our present purposes, however, the main point is that since rainfall
+records have been kept the fluctuations of climate indicated by the
+growth of the sequoias have agreed closely with fluctuations in the
+rainfall of the eastern Mediterranean region. Presumably the same was
+true in the past. In that case, the sequoia curve not only is a good
+indication of climatic changes or pulsations in regions of similar
+climate, but may serve as a guide to coincident but different changes in
+regions of other types.
+
+An enormous body of other evidence points to the same conclusion. It
+indicates that while the average climate of the present is drier than
+that of the past in regions having the Mediterranean type of winter
+rains and summer droughts, there have been pronounced pulsations during
+historic times so that at certain times there has actually been greater
+aridity than at present. This conclusion is so important that it seems
+advisable to examine the only important arguments that have been raised
+against it, especially against the idea that the general rainfall of the
+eastern Mediterranean was greater in the historic past than at present.
+The first objection is the unquestionable fact that droughts and famines
+have occurred at periods which seem on other evidence to have been
+moister than the present. This argument has been much used, but it seems
+to have little force. If the rainfall of a given region averages thirty
+inches and varies from fifteen to forty-five, a famine will ensue if the
+rainfall drops for a few years to the lower limit and does not rise much
+above twenty for a few years. If the climate of the place changes during
+the course of centuries, so that the rainfall averages only twenty
+inches, and ranges from seven to thirty-five, famine will again ensue if
+the rainfall remains near ten inches for a few years. The ravages of the
+first famine might be as bad as those of the second. They might even be
+worse, because when the rainfall is larger the population is likely to
+be greater and the distress due to scarcity of food would affect a
+larger number of people. Hence historic records of famines and droughts
+do not indicate that the climate was either drier or moister than at
+present. They merely show that at the time in question the climate was
+drier than the normal for that particular period.
+
+The second objection is that deserts existed in the past much as at
+present. This is not a real objection, however, for, as we shall see
+more fully, some parts of the world suffer one kind of change and others
+quite the opposite. Moreover, deserts have always existed, and when we
+talk of a change in their climate we merely mean that their boundaries
+have shifted. A concrete example of the mistaken use of ancient dryness
+as proof of climatic uniformity is illustrated by the march of Alexander
+from India to Mesopotamia. Hedin gives an excellent presentation of the
+case in the second volume of his _Overland to India_. He shows
+conclusively that Alexander's army suffered terribly from lack of water
+and provisions. This certainly proves that the climate was dry, but it
+by no means indicates that there has been no change from the past to the
+present. We do not know whether Alexander's march took place during an
+especially dry or an especially wet year. In a desert region like
+Makran, in southern Persia and Beluchistan, where the chief difficulties
+occurred, the rainfall varies greatly from year to year. We have no
+records from Makran, but the conditions there are closely similar to
+those of southern Arizona and New Mexico. In 1885 and 1905 the rainfall
+for five stations in that region was as follows:
+
+   +------------------------------------------------------------+
+   |                                                            |
+   |                                            _Mean rainfall  |
+   |                                            during period   |
+   |                        _1885_    _1905_    since           |
+   |                                            observations    |
+   |                                            began_          |
+   | Yuma, Arizona,          2.72     11.41        3.13         |
+   | Phoenix, Arizona,       3.77     19.73        7.27         |
+   | Tucson, Arizona,        5.26     24.17       11.66         |
+   | Lordsburg, New Mexico,  3.99     19.50        8.62         |
+   | El Paso, Texas (on New                                     |
+   |   Mexico border),       7.31     17.80        9.06         |
+   |                         ----     -----       -----         |
+   | Average,                4.61     18.52        7.95         |
+   |                                                            |
+   +------------------------------------------------------------+
+
+These stations are distributed over an area nearly 500 miles east and
+west. Manifestly a traveler who spent the year 1885 in that region would
+have had much more difficulty in finding water and forage than one who
+traveled in the same places in 1905. During 1885 the rainfall was 42 per
+cent less than the average, and during 1905 it was 134 per cent more
+than the average. Let us suppose, for the sake of argument, that the
+average rainfall of southeastern Persia is six inches today and was ten
+inches in the days of Alexander. If the rainfall from year to year
+varied as much in the past in Persia as it does now in New Mexico and
+Arizona, the rainfall during an ancient dry year, corresponding in
+character to 1885, would have been about 5.75 inches. On the other hand,
+if we suppose that the rainfall then averaged less than at present,--let
+us say four inches,--a wet year corresponding to 1905 in the American
+deserts might have had a rainfall of about ten inches. This being the
+case, it is clear that our estimate of what Alexander's march shows as
+to climate must depend largely on whether 325 B. C. was a wet year or a
+dry year. Inasmuch as we know nothing about this, we must fall back on
+the fact that a large army accomplished a journey in a place where today
+even a small caravan usually finds great difficulty in procuring forage
+and water. Moreover, elephants were taken 180 miles across what is now
+an almost waterless desert, and yet the old historians make no comment
+on such a feat which today would be practically impossible. These things
+seem more in harmony with a change of climate than with uniformity.
+Nevertheless, it is not safe to place much reliance on them except when
+they are taken in conjunction with other evidence, such as the numerous
+ruins, which show that Makran was once far more densely populated than
+now seems possible. Taken by itself, such incidents as Alexander's march
+cannot safely be used either as an argument for or against changes of
+climate.
+
+The third and strongest objection to any hypothesis of climatic changes
+during historic times is based on vegetation. The whole question is
+admirably set forth by J. W. Gregory,[28] who gives not only his own
+results, but those of the ablest scholars who have preceded him. His
+conclusions are important because they represent one of the few cases
+where a definite statistical attempt has been made to prove the exact
+condition of the climate of the past. After stating various less
+important reasons for believing that the climate of Palestine has not
+changed, he discusses vegetation. The following quotation indicates his
+line of thought. A sentence near the beginning is italicized in order to
+call attention to the importance which Gregory and others lay on this
+particular kind of evidence:
+
+    Some more certain test is necessary than the general conclusions
+    which can be based upon the historical and geographical evidence of
+    the Bible. In the absence of rain gauge and thermometric records,
+    _the most precise test of climate is given by the vegetation; and
+    fortunately the palm affords a very delicate test of the past
+    climate of Palestine and the eastern Mediterranean_.... The date
+    palm has three limits of growth which are determined by temperature;
+    thus it does not reach full maturity or produce ripe fruit of good
+    quality below the mean annual temperature of 69�F. The isothermal of
+    69� crosses southern Algeria near Biskra; it touches the northern
+    coasts of Cyrenaica near Derna and passes Egypt near the mouth of
+    the Nile, and then bends northward along the coast lands of
+    Palestine.
+
+    To the north of this line the date palm grows and produces fruit,
+    which only ripens occasionally, and its quality deteriorates as the
+    temperature falls below 69�. Between the isotherms of 68� and 64�,
+    limits which include northern Algeria, most of Sicily, Malta, the
+    southern parts of Greece and northern Syria, the dates produced are
+    so unripe that they are not edible. In the next cooler zone, north
+    of the isotherm of 62�, which enters Europe in southwestern
+    Portugal, passes through Sardinia, enters Italy near Naples, crosses
+    northern Greece and Asia Minor to the east of Smyrna, the date palm
+    is grown only for its foliage, since it does not fruit.
+
+    Hence at Benghazi, on the north African coast, the date palm is
+    fertile, but produces fruit of poor quality. In Sicily and at
+    Algiers the fruit ripens occasionally and at Rome and Nice the palm
+    is grown only as an ornamental tree.
+
+    The date palm therefore affords a test of variations in mean annual
+    temperature of three grades between 62� and 69�.
+
+    This test shows that the mean annual temperature of Palestine has
+    not altered since Old Testament times. The palm tree now grows dates
+    on the coast of Palestine and in the deep depression around the Dead
+    Sea, but it does not produce fruit on the highlands of Judea. Its
+    distribution in ancient times, as far as we can judge from the
+    Bible, was exactly the same. It grew at "Jericho, the city of palm
+    trees" (Deut. xxxiv: 3 and 2 Chron. xxviii: 15), and at Engedi, on
+    the western shore of the Dead Sea (2 Chron. xx: 2; Sirach xxiv: 14);
+    and though the palm does not still live at Jericho--the last
+    apparently died in 1838--its disappearance must be due to neglect,
+    for the only climatic change that would explain it would be an
+    increase in cold or moisture. In olden times the date palm certainly
+    grew on the highlands of Palestine; but apparently it never produced
+    fruit there, for the Bible references to the palm are to its beauty
+    and erect growth: "The righteous shall flourish like the palm" (Ps.
+    xcii: 12); "They are upright as the palm tree" (Jer. x: 5); "Thy
+    stature is like to a palm tree" (Cant. vii: 7). It is used as a
+    symbol of victory (Rev. vii: 9), but never praised as a source of
+    food.
+
+    Dates are not once referred to in the text of the Bible, but
+    according to the marginal notes the word translated "honey" in
+    2 Chron. xxxi: 5 may mean dates....
+
+    It appears, therefore, that the date palm had essentially the same
+    distribution in Palestine in Old Testament times as it has now; and
+    hence we may infer that the mean temperature was then the same as
+    now. If the climate had been moister and cooler, the date could not
+    have flourished at Jericho. If it had been warmer, the palms would
+    have grown freely at higher levels and Jericho would not have held
+    its distinction as _the_ city of palm trees.[29]
+
+In the main Gregory's conclusions seem to be well grounded, although
+even according to his data a change of 2� or 3� in mean temperature
+would be perfectly feasible. It will be noticed, however, that they
+apply to temperature and not to rainfall. They merely prove that two
+thousand years ago the mean temperature of Palestine and the neighboring
+regions was not appreciably different from what it is today. This,
+however, is in no sense out of harmony with the hypothesis of climatic
+pulsations. Students of glaciation believe that during the last glacial
+epoch the mean temperature of the earth as a whole was only 5� or 6�C.
+lower than at present. If the difference between the climate of today
+and of the time of Christ is a tenth as great as the difference between
+the climate of today and that which prevailed at the culmination of the
+last glacial epoch, the change in two thousand years has been of large
+dimensions. Yet this would require a rise of only half a degree
+Centigrade in the mean temperature of Palestine. Manifestly, so slight a
+change would scarcely be detectable in the vegetation.
+
+The slightness of changes in mean temperature as compared with changes
+in rainfall may be judged from a comparison of wet and dry years in
+various regions. For example, at Berlin between 1866 and 1905 the ten
+most rainy years had an average precipitation of 670 mm. and a mean
+temperature of 9.15�C. On the other hand, the ten years of least
+rainfall had an average of 483 mm. and a mean temperature of 9.35�. In
+other words, a difference of 137 mm., or 39 per cent, in rainfall was
+accompanied by a difference of only 0.2�C. in temperature. Such
+contrasts between the variability of mean rainfall and mean temperature
+are observable not only when individual years are selected, but when
+much longer periods are taken. For instance, in the western Gulf region
+of the United States the two inland stations of Vicksburg, Mississippi,
+and Shreveport, Louisiana, and the two maritime stations of New Orleans,
+Louisiana, and Galveston, Texas, lie at the margins of an area about 400
+miles long. During the ten years from 1875 to 1884 their rainfall
+averaged 59.4 inches,[30] while during the ten years from 1890 to 1899
+it averaged only 42.4 inches. Even in a region so well watered as the
+Gulf States, such a change--40 per cent more in the first decade than in
+the second--is important, and in drier regions it would have a great
+effect on habitability. Yet in spite of the magnitude of the change the
+mean temperature was not appreciably different, the average for the four
+stations being 67.36�F. during the more rainy decade and 66.94�F. during
+the less rainy decade--a difference of only 0.42�F. It is worth noticing
+that in this case the wetter period was also the warmer, whereas in
+Berlin it was the cooler. This is probably because a large part of the
+moisture of the Gulf States is brought by winds having a southerly
+component. Similar relationships are apparent in other places. We select
+Jerusalem because we have been discussing Palestine. At the time of
+writing, the data available in the _Quarterly Journal of the Palestine
+Exploration Fund_ cover the years from 1882-1899 and 1903-1909. Among
+these twenty-five years the thirteen which had most rain had an average
+of 34.1 inches and a temperature of 62.04�F. The twelve with least rain
+had 24.4 inches and a temperature of 62.44�. A difference of 40 per cent
+in rainfall was accompanied by a difference of only 0.4�F. in
+temperature.
+
+The facts set forth in the preceding paragraphs seem to show that
+extensive changes in precipitation and storminess can take place without
+appreciable changes of mean temperature. If such changed conditions can
+persist for ten years, as in one of our examples, there is no logical
+reason why they cannot persist for a hundred or a thousand. The evidence
+of changes in climate during the historic period seems to suggest
+changes in precipitation much more than in temperature. Hence the
+strongest of all the arguments against historic changes of climate seems
+to be of relatively little weight, and the pulsatory hypothesis seems to
+be in accord with all the known facts.
+
+Before the true nature of climatic changes, whether historic or
+geologic, can be rightly understood, another point needs emphasis. When
+the pulsatory hypothesis was first framed, it fell into the same error
+as the hypotheses of uniformity and of progressive change--that is, the
+assumption was made that the whole world is either growing drier or
+moister with each pulsation. A study of the ruins of Yucatan, in 1912,
+and of Guatemala, in 1913, as is explained in _The Climatic Factor_, has
+led to the conclusion that the climate of those regions has changed in
+the opposite way from the changes which appear to have taken place in
+the desert regions farther south. These Maya ruins in Central America
+are in many cases located in regions of such heavy rainfall, such dense
+forests, and such malignant fevers that habitation is now practically
+impossible. The land cannot be cultivated except in especially favorable
+places. The people are terribly weakened by disease and are among the
+lowest in Central America. Only a hundred miles from the unhealthful
+forests we find healthful areas, such as the coasts of Yucatan and the
+plateau of Guatemala. Here the vast majority of the population is
+gathered, the large towns are located, and the only progressive people
+are found. Nevertheless, in the past the region of the forests was the
+home of by far the most progressive people who are ever known to have
+lived in America previous to the days of Columbus. They alone brought to
+high perfection the art of sculpture; they were the only American people
+who invented the art of writing. It seems scarcely credible that such a
+people would have lived in the worst possible habitat when far more
+favored regions were close at hand. Therefore it seems as if the climate
+of eastern Guatemala and Yucatan must have been relatively dry at some
+past time. The Maya chronology and traditions indicate that this was
+probably at the same time when moister conditions apparently prevailed
+in the subarid or desert portions of the United States and Asia. Fig. 3
+shows that today at times of many sunspots there is a similar opposition
+between a tendency toward storminess and rain in subtropical regions and
+toward aridity in low latitudes near the heat equator.
+
+Thus our final conclusion is that during historic times there have been
+pulsatory changes of climate. These changes have been of the same type
+in regions having similar kinds of climate, but of different and
+sometimes opposite types in places having diverse climates. As to the
+cause of the pulsations, they cannot have been due to the precession of
+the equinoxes nor apparently to any allied astronomical cause, for the
+time intervals are too short and too irregular. They cannot have been
+due to changes in the percentage of carbon dioxide in the atmosphere,
+for not even the strongest believers in the climatic efficacy of that
+gas hold that its amount could fluctuate in any such violent way as
+would be necessary to explain the pulsations shown in the California
+curve of tree growth. Volcanic activity seems more probable as at least
+a partial cause, and it would be worth while to investigate the matter
+more fully. Nevertheless, it can apparently be only a minor cause. In
+the first place, the main effect of a cloud of dust is to alter the
+temperature, but Gregory's summary of the palm and the vine shows that
+variations in temperature are apparently of very slight importance
+during historic times. Again, ruins on the bottoms of enclosed salt
+lakes, old beaches now under the water, and signs of irrigation ditches
+where none are now needed indicate a climate drier than the present.
+Volcanic dust, however, cannot account for such a condition, for at
+present the air seems to be practically free from such dust for long
+periods. Thus we now experience the greatest extreme which the volcanic
+hypothesis permits in one direction, but there have been greater
+extremes in the same direction. The thermal solar hypothesis is likewise
+unable to explain the observed phenomena, for neither it nor the
+volcanic hypothesis offers any explanation of why the climate varies in
+one way in Mediterranean climates and in an opposite way in regions near
+the heat equator.
+
+This leaves the cyclonic hypothesis. It seems to fit the facts, for
+variations in cyclonic storms cause some regions to be moister and
+others drier than usual. At the same time the variations in temperature
+are slight, and are apparently different in different regions, some
+places growing warm when others grow cool. In the next chapter we shall
+study this matter more fully, for it can best be appreciated by
+examining the course of events in a specific century.
+
+FOOTNOTES:
+
+[Footnote 16: Much of this chapter is taken from The Solar Hypothesis of
+Climatic Changes; Bull. Geol. Soc. Am., Vol. 25, 1914.]
+
+[Footnote 17: Ellsworth Huntington: Explorations in Turkestan, 1905; The
+Pulse of Asia, 1907; Palestine and Its Transformation, 1911; The
+Climatic Factor, 1915; World Power and Evolution, 1919.]
+
+[Footnote 18: J. Hann: Klimatologie, Vol. 1, 1908, p. 352.]
+
+[Footnote 19: H. C. Butler: Desert Syria, the Land of a Lost
+Civilization; Geographical Review, Feb., 1920, pp. 77-108.]
+
+[Footnote 20: This is due to the fact that where these forests occur, in
+Gilead for example, the mountains to the west break down, so that the
+west winds with water from the Mediterranean are able to reach the inner
+range without having lost all their water. It is one of the misfortunes
+of Syria that its mountains generally rise so close to the sea that they
+shut off rainfall from the interior and cause the rain to fall on slopes
+too steep for easy cultivation.]
+
+[Footnote 21: H. Leiter: Die Frage der Klimaanderung waherend
+geschichtlicher Zeit in Nordafrika. Abhandl. K. K. Geographischen
+Gesellschaft, Wien, 1909, p. 143.]
+
+[Footnote 22: A most careful and convincing study of this problem is
+embodied in an article by J. W. Smith: The Effects of Weather upon the
+Yield of Corn; Monthly Weather Review, Vol. 42, 1914, pp. 78-92. On the
+basis of the yield of corn in Ohio for 60 years and in other states for
+shorter periods, he shows that the rainfall of July has almost as much
+influence on the crop as has the rainfall of all other months combined.
+See his Agricultural Meteorology, New York, 1920.]
+
+[Footnote 23: See chapter by A. E. Douglass in The Climatic Factor; and
+his book on Climatic Cycles and Tree-Growth; Carnegie Inst., 1919. Also
+article by M. N. Stewart: The Relation of Precipitation to Tree Growth,
+in the Monthly Weather Review, Vol. 41, 1913.]
+
+[Footnote 24: The dotted line is taken from Palestine and Its
+Transformation, pp. 327 and 403.]
+
+[Footnote 25: M. A. Stein: Ruins of Desert Cathay, London, 1912.]
+
+[Footnote 26: In the preparation and interpretation of this table the
+help of Mr. G. B. Cressey is gratefully acknowledged.]
+
+[Footnote 27: For the tree data used in these comparisons, see The
+Climatic Factor P. 328, and A. E. Douglass: Climatic Cycles and Tree
+Growth, p. 123.]
+
+[Footnote A: One year interpolated.]
+
+[Footnote 28: J. W. Gregory: Is the Earth Drying Up? Geog. Jour., Vol.
+43, 1914, pp. 148-172 and 293-318.]
+
+[Footnote 29: Geog. Jour., Vol. 43, pp. 159-161.]
+
+[Footnote 30: See A. J. Henry: Secular Variation of Precipitation in the
+United States; Bull. Am. Geog. Soc., Vol. 46, 1914, pp. 192-201.]
+
+
+
+
+CHAPTER VI
+
+THE CLIMATIC STRESS OF THE FOURTEENTH CENTURY
+
+
+In order to give concreteness to our picture of the climatic pulsations
+of historic times let us take a specific period and see how its changes
+of climate were distributed over the globe and how they are related to
+the little changes which now take place in the sunspot cycle. We will
+take the fourteenth century of the Christian era, especially the first
+half. This period is chosen because it is the last and hence the best
+known of the times when the climate of the earth seems to have taken a
+considerable swing toward the conditions which now prevail when the sun
+is most active, and which, if intensified, would apparently lead to
+glaciation. It has already been discussed in _World Power and
+Evolution_, but its importance and the fact that new evidence is
+constantly coming to light warrant a fuller discussion.
+
+To begin with Europe; according to the careful account of Pettersson[31]
+the fourteenth century shows
+
+    a record of extreme climatic variations. In the cold winters the
+    rivers Rhine, Danube, Thames, and Po were frozen for weeks and
+    months. On these cold winters there followed violent floods, so that
+    the rivers mentioned inundated their valleys. Such floods are
+    recorded in 55 summers in the 14th century. There is, of course,
+    nothing astonishing in the fact that the inundations of the great
+    rivers of Europe were more devastating 600 to 700 years ago than in
+    our days, when the flow of the rivers has been regulated by canals,
+    locks, etc.; but still the inundations in the 13th and 14th
+    centuries must have surpassed everything of that kind which has
+    occurred since then. In 1342 the waters of the Rhine rose so high
+    that they inundated the city of Mayence and the Cathedral "usque ad
+    cingulum hominis." The walls of Cologne were flooded so that they
+    could be passed by boats in July. This occurred also in 1374 in the
+    midst of the month of February, which is of course an unusual season
+    for disasters of the kind. Again in other years the drought was so
+    intense that the same rivers, the Danube, Rhine, and others, nearly
+    dried up, and the Rhine could be forded at Cologne. This happened at
+    least twice in the same century. There is one exceptional summer of
+    such evil record that centuries afterwards it was spoken of as "the
+    old hot summer of 1357."
+
+Pettersson goes on to speak of two oceanic phenomena on which the old
+chronicles lay greater stress than on all others:
+
+    The first [is] the great storm-floods on the coast of the North Sea
+    and the Baltic, which occurred so frequently that not less than
+    nineteen floods of a destructiveness unparalleled in later times are
+    recorded from the 14th century. The coastline of the North Sea was
+    completely altered by these floods. Thus on January 16, 1300, half
+    of the island Heligoland and many other islands were engulfed by the
+    sea. The same fate overtook the island of Borkum, torn into several
+    islands by the storm-flood of January 16, which remoulded the
+    Frisian Islands into their present shape, when also Wendingstadt, on
+    the island of Sylt, and Thiryu parishes were engulfed. This flood is
+    known under the name of "the great man-drowning." The coasts of the
+    Baltic also were exposed to storm-floods of unparalleled violence.
+    On November 1, 1304, the island of Ruden was torn asunder from Rugen
+    by the force of the waves. Time does not allow me to dwell upon
+    individual disasters of this kind, but it will be well to note that
+    of the nineteen great floods on record eighteen occurred in the cold
+    season between the autumnal and vernal equinoxes.
+
+    The second remarkable phenomenon mentioned by the chronicles is the
+    freezing of the entire Baltic, which occurred many times during the
+    cold winters of these centuries. On such occasions it was possible
+    to travel with carriages over the ice from Sweden to Bornholm and
+    from Denmark to the German coast (Lubeck), and in some cases even
+    from Gotland to the coast of Estland.
+
+Norlind[32] says that "the only authentic accounts" of the complete
+freezing of the Baltic in the neighborhood of the Kattegat are in the
+years 1296, 1306, 1323, and 1408. Of these 1296 is "much the most
+uncertain," while 1323 was the coldest year ever recorded, as appears
+from the fact that horses and sleighs crossed regularly from Sweden to
+Germany on the ice.
+
+Not only central Europe and the shores of the North Sea were marked by
+climatic stress during the fourteenth century, but Scandinavia also
+suffered. As Pettersson puts it:
+
+    On examining the historic (data) from the last centuries of the
+    Middle Ages, Dr. Bull of Christiania has come to the conclusion that
+    the decay of the Norwegian kingdom was not so much a consequence of
+    the political conditions at that time, as of the frequent failures
+    of the harvest so that corn [wheat] for bread had to be imported
+    from L�beck, Rostock, Wismar and so forth. The Hansa Union undertook
+    the importation and obtained political power by its economic
+    influence. The Norwegian land-owners were forced to lower their
+    rents. The population decreased and became impoverished. The revenue
+    sank 60 to 70 per cent. Even the income from Church property
+    decreased. In 1367 corn was imported from L�beck to a value of
+    one-half million kroner. The trade balance inclined to the
+    disadvantage of Norway whose sole article of export at that time
+    was dried fish. (The production of fish increased enormously in
+    the Baltic regions off south Sweden because of the same changes
+    which were influencing the lands, but this did not benefit
+    Norway.) Dr. Bull draws a comparison with the conditions described
+    in the Sagas when Nordland [at the Arctic Circle] produced enough
+    corn to feed the inhabitants of the country. At the time of
+    Asbj�rn Selsbane the chieftains in Trondhen�s [still farther north
+    in latitude 69�] grew so much corn that they did not need to go
+    southward to buy corn unless three successive years of dearth had
+    occurred. The province of Trondheim exported wheat to Iceland and
+    so forth. Probably the turbulent political state of Scandinavia at
+    the end of the Middle Ages was in a great measure due to
+    unfavorable climatic conditions, which lowered the standard of
+    life, and not entirely to misgovernment and political strife as
+    has hitherto been taken for granted.
+
+During this same unfortunate first half of the fourteenth century
+England also suffered from conditions which, if sufficiently
+intensified, might be those of a glacial period. According to Thorwald
+Rogers[33] the severest famine ever experienced in England was that of
+1315-1316, and the next worst was in 1321. In fact, from 1308 to 1322
+great scarcity of food prevailed most of the time. Other famines of less
+severity occurred in 1351 and 1369. "The same cause was at work in all
+these cases," says Rogers, "incessant rain, and cold, stormy summers. It
+is said that the inclemency of the seasons affected the cattle, and that
+numbers perished from disease and want." After the bad harvest of 1315
+the price of wheat, which was already high, rose rapidly, and in May,
+1316, was about five times the average. For a year or more thereafter it
+remained at three or four times the ordinary level. The severity of the
+famine may be judged from the fact that previous to the Great War the
+most notable scarcity of wheat in modern England and the highest
+relative price was in December, 1800. At that time wheat cost nearly
+three times the usual amount, instead of five as in 1316. During the
+famine of the early fourteenth century "it is said that people were
+reduced to subsist upon roots, upon horses and dogs, and stories are
+told of even more terrible acts by reason of the extreme famine." The
+number of deaths was so great that the price of labor suffered a
+permanent rise of at least 10 per cent. There simply were not people
+enough left among the peasants to do the work demanded by the more
+prosperous class who had not suffered so much.
+
+After the famine came drought. The year 1325 appears to have been
+peculiarly dry, and 1331, 1344, 1362, 1374, and 1377 were also dry. In
+general these conditions do little harm in England. They are of interest
+chiefly as showing how excessive rain and drought are apt to succeed one
+another.
+
+These facts regarding northern and central Europe during the fourteenth
+century are particularly significant when compared with the conclusions
+which we have drawn in _Earth and Sun_ from the growth of trees in
+Germany and from the distribution of storms. A careful study of all the
+facts shows that we are dealing with two distinct types of phenomena. In
+the first place, the climate of central Europe seems to have been
+peculiarly continental during the fourteenth century. The winters were
+so cold that the rivers froze, and the summers were so wet that there
+were floods every other year or oftener. This seems to be merely an
+intensification of the conditions which prevail at the present time
+during periods of many sunspots, as indicated by the growth of trees at
+Eberswalde in Germany and by the number of storms in winter as compared
+with summer. The prevalence of droughts, especially in the spring, is
+also not inconsistent with the existence of floods at other seasons, for
+one of the chief characteristics of a continental climate is that the
+variations from one season to another are more marked than in oceanic
+climates. Even the summer droughts are typically continental, for when
+continental conditions prevail, the difference between the same season
+in different years is extreme, as is well illustrated in Kansas. It must
+always be remembered that what causes famine is not so much absolute
+dryness as a temporary diminution of the rainfall.
+
+The second type of phenomena is peculiarly oceanic in character. It
+consists of two parts, both of which are precisely what would be
+expected if a highly continental climate prevailed over the land. In the
+first place, at certain times the cold area of high pressure, which is
+the predominating characteristic of a continent during the winter,
+apparently spread out over the neighboring oceans. Under such conditions
+an inland sea, such as the Baltic, would be frozen, so that horses could
+cross the ice even in the Far West. In the second place, because of the
+unusually high pressure over the continent, the barometric gradients
+apparently became intensified. Hence at the margin of the continental
+high-pressure area the winds were unusually strong and the storms of
+corresponding severity. Some of these storms may have passed entirely
+along oceanic tracks, while others invaded the borders of the land, and
+gave rise to the floods and to the wearing away of the coast described
+by Pettersson.
+
+Turning now to the east of Europe, Br�ckner's[34] study of the Caspian
+Sea shows that that region as well as western Europe was subject to
+great climatic vicissitudes in the first half of the fourteenth century.
+In 1306-1307 the Caspian Sea, after rising rapidly for several years,
+stood thirty-seven feet above the present level and it probably rose
+still higher during the succeeding decades. At least it remained at a
+high level, for Hamdulla, the Persian, tells us that in 1325 a place
+called Aboskun was under water.[35]
+
+Still further east the inland lake of Lop Nor also rose at about this
+time. According to a Chinese account the Dragon Town on the shore of Lop
+Nor was destroyed by a flood. From Himley's translation it appears that
+the level of the lake rose so as to overwhelm the city completely. This
+would necessitate the expansion of the lake to a point eighty miles east
+of Lulan, and fully fifty from the present eastern end of the Kara
+Koshun marsh. The water would have to rise nearly, or quite, to a strand
+which is now clearly visible at a height of twelve feet above the modern
+lake or marsh.
+
+In India the fourteenth century was characterized by what appears to
+have been the most disastrous drought in all history. Apparently the
+decrease in rainfall here was as striking as the increase in other parts
+of the world. No statistics are available but we are told that in the
+great famine which began in 1344 even the Mogul emperor was unable to
+obtain the necessaries of life for his household. No rain worth
+mentioning fell for years. In some places the famine lasted three or
+four years, and in some twelve, and entire cities were left without an
+inhabitant. In a later famine, 1769-1770, which occurred in Bengal
+shortly after the foundation of British rule in India, but while the
+native officials were still in power, a third of the population, or ten
+out of thirty millions, perished. The famine in the first half of the
+fourteenth century seems to have been far worse. These Indian famines
+were apparently due to weak summer monsoons caused presumably by the
+failure of central Asia to warm up as much as usual. The heavier
+snowfall, and the greater cloudiness of the summer there, which probably
+accompanied increased storminess, may have been the reason.
+
+The New World as well as the Old appears to have been in a state of
+climatic stress during the first half of the fourteenth century.
+According to Pettersson, Greenland furnishes an example of this. At
+first the inhabitants of that northland were fairly prosperous and were
+able to approach from Iceland without much hindrance from the ice. Today
+the North Atlantic Ocean northeast of Iceland is full of drift ice much
+of the time. The border of the ice varies from season to season, but in
+general it extends westward from Iceland not far from the Arctic circle
+and then follows the coast of Greenland southward to Cape Farewell at
+the southern tip and around to the western side for fifty miles or more.
+Except under exceptional circumstances a ship cannot approach the coast
+until well northward on the comparatively ice-free west coast. In the
+old Sagas, however, nothing is said of ice in this region. The route
+from Iceland to Greenland is carefully described. In the earliest times
+it went from Iceland a trifle north of west so as to approach the coast
+of Greenland after as short an ocean passage as possible. Then it went
+down the coast in a region where approach is now practically impossible
+because of the ice. At that time this coast was icy close to the shore,
+but there is no sign that navigation was rendered difficult as is now
+the case. Today no navigator would think of keeping close inland. The
+old route also went _north_ of the island on which Cape Farewell is
+located, although the narrow channel between the island and the mainland
+is now so blocked with ice that no modern vessel has ever penetrated it.
+By the thirteenth century, however, there appears to have been a change.
+In the Kungaspegel or _Kings' Mirror_, written at that time, navigators
+are warned not to make the east coast too soon on account of ice, but no
+new route is recommended in the neighborhood of Cape Farewell or
+elsewhere. Finally, however, at the end of the fourteenth century,
+nearly 150 years after the Kungaspegel, the old sailing route was
+abandoned, and ships from Iceland sailed directly southwest to avoid the
+ice. As Pettersson says:
+
+    ... At the end of the thirteenth and the beginning of the fourteenth
+    century the European civilization in Greenland was wiped out by an
+    invasion of the aboriginal population. The colonists in the
+    Vesterbygd were driven from their homes and probably migrated to
+    America leaving behind their cattle in the fields. So they were
+    found by Ivar Bardsson, steward to the Bishop of Gardar, in his
+    official journey thither in 1342.
+
+    The Eskimo invasion must not be regarded as a common raid. It was
+    the transmigration of a people, and like other big movements of this
+    kind [was] impelled by altered conditions of nature, in this case
+    the alterations of climate caused by [or which caused?] the advance
+    of the ice. For their hunting and fishing the Eskimos require an at
+    least partially open arctic sea. The seal, their principal prey,
+    cannot live where the surface of the sea is entirely frozen over.
+    The cause of the favorable conditions in the Viking-age was,
+    according to my hypothesis, that the ice then melted at a higher
+    latitude in the arctic seas.
+
+    The Eskimos then lived further north in Greenland and North America.
+    When the climate deteriorated and the sea which gave them their
+    living was closed by ice the Eskimos had to find a more suitable
+    neighborhood. This they found in the land colonized by the Norsemen
+    whom they attacked and finally annihilated.
+
+Finally, far to the south in Yucatan the ancient Maya civilization made
+its last flickering effort at about this time. Not much is known of this
+but in earlier periods the history of the Mayas seems to have agreed
+quite closely with the fluctuations in climate.[36] Among the Mayas, as
+we have seen, relatively dry periods were the times of greatest
+progress.
+
+Let us turn now to Fig. 3 once more and compare the climatic conditions
+of the fourteenth century with those of periods of increasing rainfall.
+Southern England, Ireland, and Scandinavia, where the crops were ruined
+by extensive rain and storms in summer, are places where storminess and
+rainfall now increase when sunspots are numerous. Central Europe and the
+coasts of the North Sea, where flood and drought alternated, are regions
+which now have relatively less rain when sunspots increase than when
+they diminish. However, as appears from the trees measured by Douglass,
+the winters become more continental and hence cooler, thus corresponding
+to the cold winters of the fourteenth century when people walked on the
+ice from Scandinavia to Denmark. When such high pressure prevails in the
+winter, the total rainfall is diminished, but nevertheless the storms
+are more severe than usual, especially in the spring. In southeastern
+Europe, the part of the area whence the Caspian derives its water,
+appears to have less rainfall during times of increasing sunspots than
+when sunspots are few, but in an equally large area to the south, where
+the mountains are higher and the run-off of the rain is more rapid, the
+reverse is the case. This seems to mean that a slight diminution in the
+water poured in by the Volga would be more than compensated by the water
+derived from Persia and from the Oxus and Jaxartes rivers, which in the
+fourteenth century appear to have filled the Sea of Aral and overflowed
+in a large stream to the Caspian. Still farther east in central Asia, so
+far as the records go, most of the country receives more rain when
+sunspots are many than when they are few, which would agree with what
+happened when the Dragon Town was inundated. In India, on the contrary,
+there is a large area where the rainfall diminishes at times of many
+sunspots, thus agreeing with the terrible famine from which the Moguls
+suffered so severely. In the western hemisphere, Greenland, Arizona, and
+California are all parts of the area where the rain increases with many
+sunspots, while Yucatan seems to lie in an area of the opposite type.
+Thus all the evidence seems to show that at times of climatic stress,
+such as the fourteenth century, the conditions are essentially the same
+as those which now prevail at times of increasing sunspots.
+
+As to the number of sunspots, there is little evidence previous to about
+1750. Yet that little is both interesting and important. Although
+sunspots have been observed with care in Europe only a little more than
+three centuries, the Chinese have records which go back nearly to the
+beginning of the Christian era. Of course the records are far from
+perfect, for the work was done by individuals and not by any great
+organization which continued the same methods from generation to
+generation. The mere fact that a good observer happened to use his
+smoked glass to advantage may cause a particular period to appear to
+have an unusual number of spots. On the other hand, the fact that such
+an observer finds spots at some times and not at others tends to give a
+valuable check on his results, as does the comparison of one observer's
+work with that of another. Hence, in spite of many and obvious defects,
+most students of the problem agree that the Chinese record possesses
+much value, and that for a thousand years or more it gives a fairly true
+idea of the general aspect of the sun. In the Chinese records the years
+with many spots fall in groups, as would be expected, and are sometimes
+separated by long intervals. Certain centuries appear to have been
+marked by unusual spottedness. The most conspicuous of these is the
+fourteenth, when the years 1370 to 1385 were particularly noteworthy,
+for spots large enough to be visible to the naked eye covered the sun
+much of the time. Hence Wolf,[37] who has made an exhaustive study of
+the matter, concludes that there was an absolute maximum of spots about
+1372. While this date is avowedly open to question, the great abundance
+of sunspots at that time makes it probable that it cannot be far wrong.
+If this is so, it seems that the great climatic disturbances of which we
+have seen evidence in the fourteenth century occurred at a time when
+sunspots were increasing, or at least when solar activity was under some
+profoundly disturbing influence. Thus the evidence seems to show not
+merely that the climate of historic times has been subject to important
+pulsations, but that those pulsations were magnifications of the little
+climatic changes which now take place in sunspot cycles. The past and
+the present are apparently a unit except as to the intensity of the
+changes.
+
+FOOTNOTES:
+
+[Footnote 31: O. Pettersson: The connection between hydrographical and
+meteorological phenomena; Quarterly Journal of the Royal Meteorological
+Society, Vol. 38, pp. 174-175.]
+
+[Footnote 32: A. Norlind: Einige Bemerkungen �ber das Klima der
+historischen Zeit nebst einem Verzeichnis mittelaltlicher Witterungs
+erscheinungen; Lunds Univ. Arsskrift, N. F., Vol. 10, 1914, 53 pp.]
+
+[Footnote 33: Thorwald Rogers: A History of Agriculture and Prices in
+England.]
+
+[Footnote 34: E. Br�ckner: Klimaschwankungen seit 1700, Vienna, 1891.]
+
+[Footnote 35: For a full discussion of the changes in the Caspian Sea,
+see The Pulse of Asia, pp. 329-358.]
+
+[Footnote 36: S. Q. Morley: The Inscriptions at Cop�n; Carnegie Inst. of
+Wash., No. 219, 1920.
+
+Ellsworth Huntington: The Red Man's Continent, 1919.]
+
+[Footnote 37: See summary of Wolf's work with additional information by
+H. Fritz; Z�rich Vierteljahrschrift, Vol. 38, 1893, pp. 77-107.]
+
+
+
+
+CHAPTER VII
+
+GLACIATION ACCORDING TO THE SOLAR-CYCLONIC HYPOTHESIS[38]
+
+
+The remarkable phenomena of glacial periods afford perhaps the best
+available test to which any climatic hypothesis can be subjected. In
+this chapter and the two that follow, we shall apply this test. Since
+much more is known about the recent Great Ice Age, or Pleistocene
+glaciation, than about the more ancient glaciations, the problems of the
+Pleistocene will receive especial attention. In the present chapter the
+oncoming of glaciation and the subsequent disappearance of the ice will
+be outlined in the light of what would be expected according to the
+solar-cyclonic hypothesis. Then in the next chapter several problems of
+especial climatic significance will be considered, such as the
+localization of ice sheets, the succession of severe glacial and mild
+inter-glacial epochs, the sudden commencement of glaciation and the
+peculiar variations in the height of the snow line. Other topics to be
+considered are the occurrence of pluvial or rainy climates in
+non-glaciated regions, and glaciation near sea level in subtropical
+latitudes during the Permian and Proterozoic. Then in Chapter IX we
+shall consider the development and distribution of the remarkable
+deposits of wind-blown material known as loess.
+
+Facts not considered at the time of framing an hypothesis are especially
+significant in testing it. In this particular case, the cyclonic
+hypothesis was framed to explain the historic changes of climate
+revealed by a study of ruins, tree rings, and the terraces of streams
+and lakes, without special thought of glaciation or other geologic
+changes. Indeed, the hypothesis had reached nearly its present form
+before much attention was given to geological phases of the problem.
+Nevertheless, it appears to meet even this severe test.
+
+According to the solar-cyclonic hypothesis, the Pleistocene glacial
+period was inaugurated at a time when certain terrestrial conditions
+tended to make the earth especially favorable for glaciation. How these
+conditions arose will be considered later. Here it is enough to state
+what they were. Chief among them was the fact that the continents stood
+unusually high and were unusually large. This, however, was not the
+primary cause of glaciation, for many of the areas which were soon to be
+glaciated were little above sea level. For example, it seems clear that
+New England stood less than a thousand feet higher than now. Indeed,
+Salisbury[39] estimates that eastern North America in general stood not
+more than a few hundred feet higher than now, and W. B. Wright[40]
+reaches the same conclusion in respect to the British Isles.
+Nevertheless, widespread lands, even if they are not all high, lead to
+climatic conditions which favor glaciation. For example, enlarged
+continents cause low temperature in high latitudes because they
+interfere with the ocean currents that carry heat polewards. Such
+continents also cause relatively cold winters, for lands cool much
+sooner than does the ocean. Another result is a diminution of water
+vapor, not only because cold air cannot hold much vapor, but also
+because the oceanic area from which evaporation takes place is reduced
+by the emergence of the continents. Again, when the continents are
+extensive the amount of carbonic acid gas in the atmosphere probably
+decreases, for the augmented erosion due to uplift exposes much igneous
+rock to the air, and weathering consumes the atmospheric carbon dioxide.
+When the supply of water vapor and of atmospheric carbon dioxide is
+small, an extreme type of climate usually prevails. The combined result
+of all these conditions is that continental emergence causes the climate
+to be somewhat cool and to be marked by relatively great contrasts from
+season to season and from latitude to latitude.
+
+When the terrestrial conditions thus permitted glaciation, unusual solar
+activity is supposed to have greatly increased the number and severity
+of storms and to have altered their location, just as now happens at
+times of many sunspots. If such a change in storminess had occurred when
+terrestrial conditions were unfavorable for glaciation, as, for example,
+when the lands were low and there were widespread epicontinental seas in
+middle and high latitudes, glaciation might not have resulted. In the
+Pleistocene, however, terrestrial conditions permitted glaciation, and
+therefore the supposed increase in storminess caused great ice sheets.
+
+The conditions which prevail at times of increased storminess have been
+discussed in detail in _Earth and Sun_. Those which apparently brought
+on glaciation seem to have acted as follows: In the first place the
+storminess lowered the temperature of the earth's surface in several
+ways. The most important of these was the rapid upward convection in the
+centers of cyclonic storms whereby abundant heat was carried to high
+levels where most of it was radiated away into space. The marked
+increase in the number of tropical cyclones which accompanies increased
+solar activity was probably important in this respect. Such cyclones
+carry vast quantities of heat and moisture out of the tropics. The
+moisture, to be sure, liberates heat upon condensing, but as
+condensation occurs above the earth's surface, much of the heat escapes
+into space. Another reason for low temperature was that under the
+influence of the supposedly numerous storms of Pleistocene times
+evaporation over the oceans must have increased. This is largely because
+the velocity of the winds is relatively great when storms are strong and
+such winds are powerful agents of evaporation. But evaporation requires
+heat, and hence the strong winds lower the temperature.[B]
+
+The second great condition which enabled increased storminess to bring
+on glaciation was the location of the storm tracks. Kullmer's maps, as
+illustrated in Fig. 2, suggest that a great increase in solar activity,
+such as is postulated in the Pleistocene, might shift the main storm
+track poleward even more than it is shifted by the milder solar changes
+during the twelve-year sunspot cycle. If this is so, the main track
+would tend to cross North America through the middle of Canada instead
+of near the southern border. Thus there would be an increase in
+precipitation in about the latitude of the Keewatin and Labradorean
+centers of glaciation. From what is known of storm tracks in Europe, the
+main increase in the intensity of storms would probably center in
+Scandinavia. Fig. 3 in Chapter V bears this out. That figure, it will be
+recalled, shows what happens to precipitation when solar activity is
+increasing. A high rate of precipitation is especially marked in the
+boreal storm track, that is, in the northern United States, southern
+Canada, and northwestern Europe.
+
+Another important condition in bringing on glaciation would be the fact
+that when storms are numerous the total precipitation appears to
+increase in spite of the slightly lower temperature. This is largely
+because of the greater evaporation. The excessive evaporation arises
+partly from the rapidity of the winds, as already stated, and partly
+from the fact that in areas where the air is clear the sun would
+presumably be able to act more effectively than now. It would do so
+because at times of abundant sunspots the sun in our own day has a
+higher solar constant than at times of milder activity. Our whole
+hypothesis is based on the supposition that what now happens at times of
+many sunspots was intensified in glacial periods.
+
+A fourth condition which would cause glaciation to result from great
+solar activity would be the fact that the portion of the yearly
+precipitation falling as snow would increase, while the proportion of
+rain would diminish in the main storm track. This would arise partly
+because the storms would be located farther north than now, and partly
+because of the diminution in temperature due to the increased
+convection. The snow in itself would still further lower the
+temperature, for snow is an excellent reflector of sunlight. The
+increased cloudiness which would accompany the more abundant storms
+would also cause an unusually great reflection of the sunlight and still
+further lower the temperature. Thus at times of many sunspots a strong
+tendency toward the accumulation of snow would arise from the rapid
+convection and consequent low temperature, from the northern location of
+storms, from the increased evaporation and precipitation, from the
+larger percentage of snowy rather than rainy precipitation, and from the
+great loss of heat due to reflection from clouds and snow.
+
+If events at the beginning of the last glacial period took place in
+accordance with the cyclonic hypothesis, as outlined above, one of the
+inevitable results would be the production of snowfields. The places
+where snow would accumulate in special quantities would be central
+Canada, the Labrador plateau, and Scandinavia, as well as certain
+mountain regions. As soon as a snowfield became somewhat extensive, it
+would begin to produce striking climatic alterations in addition to
+those to which it owed its origin.[41] For example, within a snowfield
+the summers remain relatively cold. Hence such a field is likely to be
+an area of high pressure at all seasons. The fact that the snowfield is
+always a place of relatively high pressure results in outblowing surface
+winds except when these are temporarily overcome by the passage of
+strong cyclonic storms. The storms, however, tend to be concentrated
+near the margins of the ice throughout the year instead of following
+different paths in each of the four seasons. This is partly because
+cyclonic lows always avoid places of high pressure and are thus pushed
+out of the areas where permanent snow has accumulated. On the other
+hand, at times of many sunspots, as Kullmer has shown, the main storm
+track tends to be drawn poleward, perhaps by electrical conditions.
+Hence when a snowfield is present in the north, the lows, instead of
+migrating much farther north in summer than in winter, as they now do,
+would merely crowd on to the snowfield a little farther in summer than
+in winter. Thus the heavy precipitation which is usual in humid climates
+near the centers of lows would take place near the advancing margin of
+the snowfield and cause the field to expand still farther southward.
+
+The tendency toward the accumulation of snow on the margins of the
+snowfields would be intensified not only by the actual storms
+themselves, but by other conditions. For example, the coldness of the
+snow would tend to cause prompt condensation of the moisture brought by
+the winds that blow toward the storm centers from low latitudes. Again,
+in spite of the general dryness of the air over a snowfield, the lower
+air contains some moisture due to evaporation from the snow by day
+during the clear sunny weather of anti-cyclones or highs. Where this is
+sufficient, the cold surface of the snowfields tends to produce a frozen
+fog whenever the snowfield is cooled by radiation, as happens at night
+and during the passage of highs. Such a frozen fog is an effective
+reflector of solar radiation. Moreover, because ice has only half the
+specific heat of water, and is much more transparent to heat, such a
+"radiation fog" composed of ice crystals is a much less effective
+retainer of heat than clouds or fog made of unfrozen water particles.
+Shallow fogs of this type are described by several polar expeditions.
+They clearly retard the melting of the snow and thus help the icefield
+to grow.
+
+For all these reasons, so long as storminess remained great, the
+Pleistocene snowfields, according to the solar hypothesis, must have
+deepened and expanded. In due time some of the snow was converted into
+glacial ice. When that occurred, the growth of the snowfield as well as
+of the ice cap must have been accelerated by glacial movement. Under
+such circumstances, as the ice crowded southward toward the source of
+the moisture by which it grew, the area of high pressure produced by its
+low temperature would expand. This would force the storm track southward
+in spite of the contrary tendency due to the sun. When the ice sheet had
+become very extensive, the track would be crowded relatively near to the
+northern margin of the trade-wind belt. Indeed, the Pleistocene ice
+sheets, at the time of their maximum extension, reached almost as far
+south as the latitude now marking the northern limit of the trade-wind
+belt in summer. As the storm track with its frequent low pressure and
+the subtropical belt with its high pressure were forced nearer and
+nearer together, the barometric gradient between the two presumably
+became greater, winds became stronger, and the storms more intense.
+
+This zonal crowding would be of special importance in summer, at which
+time it would also be most pronounced. In the first place, the storms
+would be crowded far upon the ice cap which would then be protected from
+the sun by a cover of fog and cloud more fully than at any other season.
+Furthermore, the close approach of the trade-wind belt to the storm belt
+would result in a great increase in the amount of moisture drawn from
+the belt of evaporation which the trade winds dominate. In the
+trade-wind belt, clear skies and high temperature make evaporation
+especially rapid. Indeed, in spite of the vast deserts it is probable
+that more than three-fourths of the total evaporation now taking place
+on the earth occurs in the belt of trades, an area which includes about
+one-half of the earth's surface.
+
+The agency which could produce this increased drawing northward of
+moisture from the trade-wind belt would be the winds blowing into the
+lows. According to the cyclonic hypothesis, many of these lows would be
+so strong that they would temporarily break down the subtropical belt of
+high pressure which now usually prevails between the trades and the zone
+of westerly winds. This belt is even now often broken by tropical
+cyclones. If the storms of more northerly regions temporarily destroyed
+the subtropical high-pressure belt, even though they still remained on
+its northern side, they would divert part of the trade winds. Hence the
+air which now is carried obliquely equatorward by those winds would be
+carried spirally northward into the cyclonic lows. Precipitation in the
+storm track on the margin of the relatively cold ice sheet would thus be
+much increased, for most winds from low latitudes carry abundant
+moisture. Such a diversion of moisture from low latitudes probably
+explains the deficiency of precipitation along the heat equator at times
+of solar activity, as shown in Fig. 3. Taken as a whole, the summer
+conditions, according to the cyclonic hypothesis, would be such that
+increased evaporation in low latitudes would co�perate with increased
+storminess, cloudiness, and fog in higher latitudes to preserve and
+increase the accumulation of ice upon the borders of the ice sheet. The
+greater the storminess, the more this would be true and the more the ice
+sheet would be able to hold its own against melting in summer. Such a
+combination of precipitation and of protection from the sun is
+especially important if an ice sheet is to grow.
+
+The meteorologist needs no geologic evidence that the storm track was
+shoved equatorward by the growth of the ice sheet, for he observes a
+similar shifting whenever a winter's snow cap occupies part of the
+normal storm tract. The geologist, however, may welcome geologic
+evidence that such an extreme shift of the storm track actually occurred
+during the Pleistocene. Harmer, in 1901, first pointed out the evidence
+which was repeated with approval by Wright of the Ireland Geological
+Survey in 1914.[42] According to these authorities, numerous boulders of
+a distinctive chalk were deposited by Pleistocene icebergs along the
+coast of Ireland. Their distribution shows that at the time of maximum
+glaciation the strong winds along the south coast of Ireland were from
+the northeast while today they are from the southwest. Such a reversal
+could apparently be produced only by a southward shift of the center of
+the main storm track from its present position in northern Ireland,
+Scotland, and Norway to a position across northern France, central
+Germany, and middle Russia. This would mean that while now the centers
+of the lows commonly move northeastward a short distance north of
+southern Ireland, they formerly moved eastward a short distance south of
+Ireland. It will be recalled that in the northern hemisphere the winds
+spiral into a low counter-clockwise and that they are strongest near the
+center. When the centers pass not far north of a given point, the strong
+winds therefore blow from the west or southwest, while when the centers
+pass just south of that point, the strong winds come from the east or
+northeast.
+
+In addition to the consequences of the crowding of the storm track
+toward the trade-wind belt, several other conditions presumably operated
+to favor the growth of the ice sheet. For example, the lowering of the
+sea level by the removal of water to form the snowfields and glaciers
+interfered with warm currents. It also increased the rate of erosion,
+for it was equivalent to an uplift of all the land. One consequence of
+erosion and weathering was presumably a diminution of the carbon dioxide
+in the atmosphere, for although the ice covered perhaps a tenth of the
+lands and interfered with carbonation to that extent, the removal of
+large quantities of soil by accelerated erosion on the other nine-tenths
+perhaps more than counterbalanced the protective effect of the ice. At
+the same time, the general lowering of the temperature of the ocean as
+well as the lands increased the ocean's capacity for carbon dioxide and
+thus facilitated absorption. At a temperature of 50�F. water absorbs 32
+per cent more carbon dioxide than at 68�. The high waves produced by the
+severe storms must have had a similar effect on a small scale. Thus the
+percentage of carbon dioxide in the atmosphere was presumably
+diminished. Of less significance than these changes in the lands and the
+air, but perhaps not negligible, was the increased salinity of the ocean
+which accompanied the removal of water to form snow, and the increase of
+the dissolved mineral load of the rejuvenated streams. Increased
+salinity slows up the deep-sea circulation, as we shall see in a later
+chapter. This increases the contrasts from zone to zone.
+
+At times of great solar activity the agencies mentioned above would
+apparently co�perate to cause an advance of ice sheets into lower
+latitudes. The degree of solar activity would have much to do with the
+final extent of the ice sheets. Nevertheless, certain terrestrial
+conditions would tend to set limits beyond which the ice would not
+greatly advance unless the storminess were extraordinarily severe. The
+most obvious of these conditions is the location of oceans and of
+deserts or semi-arid regions. The southwestward advance of the European
+ice sheet and the southeastward advance of the Labradorean sheet in
+America were stopped by the Atlantic. The semi-aridity of the Great
+Plains, produced by their position in the lee of the Rocky Mountains,
+stopped the advance of the Keewatin ice sheet toward the southwest. The
+advance of the European ice sheet southeast seems to have been stopped
+for similar reasons. The cessation of the advance would be brought about
+in such an area not alone by the light precipitation and abundant
+sunshine, but by the dryness of the air, and also by the power of dust
+to absorb the sun's heat. Much dust would presumably be drawn in from
+the dry regions by passing cyclonic storms and would be scattered over
+the ice.
+
+The advance of the ice is also slowed up by a rugged topography, as
+among the Appalachians in northern Pennsylvania. Such a topography
+besides opposing a physical obstruction to the movement of the ice
+provides bare south-facing slopes which the sun warms effectively. Such
+warm slopes are unfavorable to glacial advance. The rugged topography
+was perhaps quite as effective as the altitude of the Appalachians in
+causing the conspicuous northward dent in the glacial margin in
+Pennsylvania. Where glaciers lie in mountain valleys the advance beyond
+a certain point is often interfered with by the deployment of the ice at
+the mouths of gorges. Evaporation and melting are more rapid where a
+glacier is broad and thin than where it is narrow and thick, as in a
+gorge. Again, where the topography or the location of oceans or dry
+areas causes the glacial lobes to be long and narrow, the elongation of
+the lobe is apparently checked in several ways. Toward the end of the
+lobe, melting and evaporation increase rapidly because the planetary
+westerly winds are more likely to overcome the glacial winds and sweep
+across a long, narrow lobe than across a broad one. As they cross the
+lobe, they accelerate evaporation, and probably lessen cloudiness, with
+a consequent augmentation of melting. Moreover, although lows rarely
+cross a broad ice sheet, they do cross a narrow lobe. For example,
+Nansen records that strong lows occasionally cross the narrow southern
+part of the Greenland ice sheet. The longer the lobe, the more likely it
+is that lows will cross it, instead of following its margin. Lows which
+cross a lobe do not yield so much snow to the tip as do those which
+follow the margin. Hence elongation is retarded and finally stopped even
+without a change in the earth's general climate.
+
+Because of these various reasons the advances of the ice during the
+several epochs of a glacial period might be approximately equal, even if
+the durations of the periods of storminess and low temperature were
+different. Indeed, they might be sub-equal, even if the periods differed
+in intensity as well as length. Differences in the periods would
+apparently be manifested less in the extent of the ice than in the depth
+of glacial erosion and in the thickness of the terminal moraines,
+outwash plains, and other glacial or glacio-fluvial formations.
+
+Having completed the consideration of the conditions leading to the
+advance of the ice, let us now consider the condition of North America
+at the time of maximum glaciation.[43] Over an area of nearly four
+million square miles, occupying practically all the northern half of the
+continent and part of the southern half, as appears in Fig. 6, the
+surface was a monotonous and almost level plain of ice covered with
+snow. When viewed from a high altitude, all parts except the margins
+must have presented a uniformly white and sparkling appearance. Along
+the margins, however, except to the north, the whiteness was irregular,
+for the view must have included not only fresh snow, but moving clouds
+and dirty snow or ice. Along the borders where melting was in progress
+there was presumably more or less spottedness due to morainal material
+or glacial d�bris brought to the surface by ice shearage and wastage.
+Along the dry southwestern border it is also possible that there were
+numerous dark spots due to dust blown onto the ice by the wind.
+
+[Illustration: _Fig. 6. Distribution of Pleistocene ice sheets._
+(_After Schuchert._)]
+
+The great white sheet with its ragged border was roughly circular in
+form, with its center in central Canada. Yet there were many departures
+from a perfectly circular form. Some were due to the oceans, for, except
+in northern Alaska, the ice extended into the ocean all the way from New
+Jersey around by the north to Washington. On the south, topographic
+conditions made the margin depart from a simple arc. From New Jersey to
+Ohio it swung northward. In the Mississippi Valley it reached far south;
+indeed most of the broad wedge between the Ohio and the Missouri rivers
+was occupied by ice. From latitude 37� near the junction of the Missouri
+and the Mississippi, however, the ice margin extended almost due north
+along the Missouri to central North Dakota. It then stretched westward
+to the Rockies. Farther west lowland glaciation was abundant as far
+south as western Washington. In the Rockies, the Cascades, and the
+Sierra Nevadas glaciation was common as far south as Colorado and
+southern California, respectively, and snowfields were doubtless
+extensive enough to make these ranges ribbons of white. Between these
+lofty ranges lay a great unglaciated region, but even in the Great Basin
+itself, in spite of its present aridity, certain ranges carried
+glaciers, while great lakes expanded widely.
+
+In this vast field of snow the glacial ice slowly crept outward,
+possibly at an average speed of half a foot a day, but varying from
+almost nothing in winter at the north, to several feet a day in summer
+at the south.[44] The force which caused the movement was the presence
+of the ice piled up not far from the margins. Almost certainly, however,
+there was no great dome from the center in Canada outward, as some early
+writers assumed. Such a dome would require that the ice be many
+thousands of feet thick near its center. This is impossible because of
+the fact that ice is more voluminous than water (about 9 per cent near
+the freezing point). Hence when subjected to sufficient pressure it
+changes to the liquid form. As friction and internal heat tend to keep
+the bottom of a glacier warm, even in cold regions, the probabilities
+are that only under very special conditions was a continental ice sheet
+much thicker than about 2500 feet. In Antarctica, where the temperature
+is much lower than was probably attained in the United States, the ice
+sheet is nearly level, several expeditions having traveled hundreds of
+miles with practically no change in altitude. In Shackleton's trip
+almost to the South Pole, he encountered a general rise of 3000 feet in
+1200 miles. Mountains, however, projected through the ice even near the
+pole and the geologists conclude that the ice is not very thick even at
+the world's coldest point, the South Pole.
+
+Along the margin of the ice there were two sorts of movement, much more
+rapid than the slow creep of the ice. One was produced by the outward
+drift of snow carried by the outblowing dry winds and the other and more
+important was due to the passage of cyclonic storms. Along the border of
+the ice sheet, except at the north, storm presumably closely followed
+storm. Their movement, we judge, was relatively slow until near the
+southern end of the Mississippi lobe, but when this point was passed
+they moved much more rapidly, for then they could go toward instead of
+away from the far northern path which the sun prescribes when solar
+activity is great. The storms brought much snow to the icefield, perhaps
+sometimes in favored places as much as the hundred feet a year which is
+recorded for some winters in the Sierras at present. Even the
+unglaciated intermontane Great Basin presumably received considerable
+precipitation, perhaps twice as much as its present scanty supply. The
+rainfall was enough to support many lakes, one of which was ten times as
+large as Great Salt Lake; and grass was doubtless abundant upon many
+slopes which are now dry and barren. The relatively heavy precipitation
+in the Great Basin was probably due primarily to the increased number of
+storms, but may also have been much influenced by their slow eastward
+movement. The lows presumably moved slowly in that general region not
+only because they were retarded and turned from their normal path by the
+cold ice to the east, but because during the summer the area between the
+Sierra snowfields on the west and the Rocky Mountain and Mississippi
+Valley snowfields on the east was relatively warm. Hence it was normally
+a place of low pressure and therefore of inblowing winds. Slow-moving
+lows are much more effective than fast-moving ones in drawing moisture
+northwestward from the Gulf of Mexico, for they give the moisture more
+time to move spirally first northeast, under the influence of the normal
+southwesterly winds, then northwest and finally southwest as it
+approaches the storm center. In the case of the present lows, before
+much moisture-laden air can describe such a circuit, first eastward and
+then westward, the storm center has nearly always moved eastward across
+the Rockies and even across the Great Plains. A result of this is the
+regular decrease in precipitation northward, northwestward, and westward
+from the Gulf of Mexico.
+
+Along the part of the glacial margins where for more than 3000 miles the
+North American ice entered the Atlantic and the Pacific oceans, myriads
+of great blocks broke off and floated away as stately icebergs, to
+scatter boulders far over the ocean floor and to melt in warmer climes.
+Where the margin lay upon the lands numerous streams issued from beneath
+the ice, milk-white with rock flour, and built up great outwash plains
+and valley trains of gravel and sand. Here and there, just beyond the
+ice, marginal lakes of strange shapes occupied valleys which had been
+dammed by the advancing ice. In many of them the water level rose until
+it reached some low point in the divide and then overflowed, forming
+rapids and waterfalls. Indeed, many of the waterfalls of the eastern
+United States and Canada were formed in just this way and not a few
+streams now occupy courses through ridges instead of parallel to them,
+as in pre-glacial times.
+
+In the zone to the south of the continental ice sheet, the plant and
+animal life of boreal, cool temperate, and warm temperate regions
+commingled curiously. Heather and Arctic willow crowded out elm and oak;
+musk ox, hairy mammoth, and marmot contested with deer, chipmunk, and
+skunk for a chance to live. Near the ice on slopes exposed to the cold
+glacial gales, the immigrant boreal species were dominant, but not far
+away in more protected areas the species that had formerly lived there
+held their own. In Europe during the last two advances of the great ice
+sheet the caveman also struggled with fierce animals and a fiercer
+climate to maintain life in an area whose habitability had long been
+decreasing.
+
+The next step in our history of glaciation is to outline the
+disappearance of the ice sheets. When a decrease in solar activity
+produced a corresponding decrease in storminess, several influences
+presumably combined to cause the disappearance of the ice. Most of their
+results are the reverse of those which brought on glaciation. A few
+special aspects, however, some of which have been discussed in _Earth
+and Sun_, ought to be brought to mind. A diminution in storminess
+lessens upward convection, wind velocity, and evaporation, and these
+changes, if they occurred, must have united to raise the temperature of
+the lower air by reducing the escape of heat. Again a decrease in the
+number and intensity of tropical cyclones presumably lessened the amount
+of moisture carried into mid-latitudes, and thus diminished the
+precipitation. The diminution of snowfall on the ice sheets when
+storminess diminished was probably highly important. The amount of
+precipitation on the sheets was presumably lessened still further by
+changes in the storminess of middle latitudes. When storminess
+diminishes, the lows follow a less definite path, as Kullmer's maps
+show, and on the average a more southerly path. Thus, instead of all the
+lows contributing snow to the ice sheet, a large fraction of the
+relatively few remaining lows would bring rain to areas south of the ice
+sheet. As storminess decreased, the trades and westerlies probably
+became steadier, and thus carried to high latitudes more warm water than
+when often interrupted by storms. Steadier southwesterly winds must have
+produced a greater movement of atmospheric as well as oceanic heat to
+high latitudes. The warming due to these two causes was probably the
+chief reason for the disappearance of the European ice sheet and of
+those on the Pacific coast of North America. The two greater American
+ice sheets, however, and the glaciers elsewhere in the lee of high
+mountain ranges, probably disappeared chiefly because of lessened
+precipitation. If there were no cyclonic storms to draw moisture
+northward from the Gulf of Mexico, most of North America east of the
+Rocky Mountain barrier would be arid. Therefore a diminution of
+storminess would be particularly effective in causing the disappearance
+of ice sheets in these regions.
+
+That evaporation was an especially important factor in causing the ice
+from the Keewatin center to disappear, is suggested by the relatively
+small amount of water-sorted material in its drift. In South Dakota, for
+example, less than 10 per cent of the drift is stratified.[45] On the
+other hand, Salisbury estimates that perhaps a third of the Labradorean
+drift in eastern Wisconsin is crudely stratified, about half of that in
+New Jersey, and more than half of the drift in western Europe.
+
+When the sun's activity began to diminish, all these conditions, as well
+as several others, would co�perate to cause the ice sheets to disappear.
+Step by step with their disappearance, the amelioration of the climate
+would progress so long as the period of solar inactivity continued and
+storms were rare. If the inactivity continued long enough, it would
+result in a fairly mild climate in high latitudes, though so long as the
+continents were emergent this mildness would not be of the extreme type.
+The inauguration of another cycle of increased disturbance of the sun,
+with a marked increase in storminess, would inaugurate another glacial
+epoch. Thus a succession of glacial and inter-glacial epochs might
+continue so long as the sun was repeatedly disturbed.
+
+FOOTNOTES:
+
+[Footnote 38: This chapter is an amplification and revision of the
+sketch of the glacial period contained in The Solar Hypothesis of
+Climatic Changes; Bull. Geol. Soc. Am., Vol. 25, 1914.]
+
+[Footnote 39: R. D. Salisbury: Physical Geography of the Pleistocene, in
+Outlines of Geologic History, by Willis, Salisbury, and others, 1910, p.
+265.]
+
+[Footnote 40: The Quaternary Ice Age, 1914, p. 364.]
+
+[Footnote B: For fuller discussion of climatic controls see S. S.
+Visher: Seventy Laws of Climate, Annals Assoc. Am. Geographers, 1922.]
+
+[Footnote 41: Many of these alterations are implied or discussed in the
+following papers:
+
+1. F. W. Harmer: Influence of Winds upon the Climate of the Pleistocene;
+Quart. Jour. Geol. Soc., Vol. 57, 1901, p. 405.
+
+2. C. E. P. Brooks: Meteorological Conditions of an Ice Sheet; Quart.
+Jour. Royal Meteorol. Soc., Vol. 40, 1914, pp. 53-70, and The Evolution
+of Climate in Northwest Europe; _op. cit._, Vol. 47, 1921, pp. 173-194.
+
+3. W. H. Hobbs: The R�le of the Glacial Anticyclone in the Air
+Circulation of the Globe; Proc. Am. Phil. Soc., Vol. 54, 1915, pp.
+185-225.]
+
+[Footnote 42: W. B. Wright: The Quaternary Ice Age, 1914, p. 100.]
+
+[Footnote 43: The description of the distribution of the ice sheet is
+based on T. C. Chamberlin's wall map of North America at the maximum of
+glaciation, 1913.]
+
+[Footnote 44: Chamberlin and Salisbury: Geology, 1906, Vol. 3, and W. H.
+Hobbs: Characteristics of Existing Glaciers, 1911.]
+
+[Footnote 45: S. S. Visher: The Geography of South Dakota; S. D. Geol.
+Surv., 1918.]
+
+
+
+
+CHAPTER VIII
+
+SOME PROBLEMS OF GLACIAL PERIODS
+
+
+Having outlined in general terms the coming of the ice sheets and their
+disappearance, we are now ready to discuss certain problems of
+compelling climatic interest. The discussion will be grouped under five
+heads: (I) the localization of glaciation; (II) the sudden coming of
+glaciation; (III) peculiar variations in the height of the snow line and
+of glaciation; (IV) lakes and other evidences of humidity in unglaciated
+regions during the glacial epochs; (V) glaciation at sea level and in
+low latitudes in the Permian and Proterozoic eras. The discussion of
+perhaps the most difficult of all climatic problems of glaciation, that
+of the succession of cold glacial and mild inter-glacial epochs, has
+been postponed to the next to the final chapter of this book. It cannot
+be properly considered until we take up the history of solar
+disturbances.
+
+I. The first problem, the localization of the ice sheets, arises from
+the fact that in both the Pleistocene and the Permian periods glaciation
+was remarkably limited. In neither period were all parts of high
+latitudes glaciated; yet in both cases glaciation occurred in large
+regions in lower latitudes. Many explanations of this localization have
+been offered, but most are entirely inadequate. Even hypotheses with
+something of proven worth, such as those of variations in volcanic dust
+and in atmospheric carbon dioxide, fail to account for localization. The
+cyclonic form of the solar hypothesis, however, seems to afford a
+satisfactory explanation.
+
+The distribution of the ice in the last glacial period is well known,
+and is shown in Fig. 6. Four-fifths of the ice-covered area, which was
+eight million square miles, more or less, was near the borders of the
+North Atlantic in eastern North America and northwestern Europe. The ice
+spread out from two great centers in North America, the Labradorean east
+of Hudson Bay, and the Keewatin west of the bay. There were also many
+glaciers in the western mountains, especially in Canada, while
+subordinate centers occurred in Newfoundland, the Adirondacks, and the
+White Mountains. The main ice sheet at its maximum extension reached as
+far south as latitude 39� in Kansas and Kentucky, and 37� in Illinois.
+Huge boulders were transferred more than one thousand miles from their
+source in Canada. The northward extension was somewhat less. Indeed, the
+northern margin of the continent was apparently relatively little
+glaciated and much of Alaska unglaciated. Why should northern Kentucky
+be glaciated when northern Alaska was not?
+
+In Europe the chief center from which the continental glacier moved was
+the Scandinavian highlands. It pushed across the depression now occupied
+by the Baltic to southern Russia and across the North Sea depression to
+England and Belgium. The Alps formed a center of considerable
+importance, and there were minor centers in Scotland, Ireland, the
+Pyrenees, Apennines, Caucasus, and Urals. In Asia numerous ranges also
+contained large glaciers, but practically all the glaciation was of the
+alpine type and very little of the vast northern lowland was covered
+with ice.
+
+In the southern hemisphere glaciation at low latitudes was less striking
+than in the northern hemisphere. Most of the increase in the areas of
+ice was confined to mountains which today receive heavy precipitation
+and still contain small glaciers. Indeed, except for relatively slight
+glaciation in the Australian Alps and in Tasmania, most of the
+Pleistocene glaciation in the southern hemisphere was merely an
+extension of existing glaciers, such as those of south Chile, New
+Zealand, and the Andes. Nevertheless, fairly extensive glaciation
+existed much nearer the equator than is now the case.
+
+In considering the localization of Pleistocene glaciation, three main
+factors must be taken into account, namely, temperature, topography, and
+precipitation. The absence of glaciation in large parts of the Arctic
+regions of North America and of Asia makes it certain that low
+temperature was not the controlling factor. Aside from Antarctica, the
+coldest place in the world is northeastern Siberia. There for seven
+months the average temperature is below 0�C., while the mean for the
+whole year is below -10�C. If the temperature during a glacial period
+averaged 6�C. lower than now, as is commonly supposed, this part of
+Siberia would have had a temperature below freezing for at least nine
+months out of the twelve even if there were no snowfield to keep the
+summers cold. Yet even under such conditions no glaciation occurred,
+although in other places, such as parts of Canada and northwestern
+Europe, intense glaciation occurred where the mean temperature is much
+higher.
+
+The topography of the lands apparently had much more influence upon the
+localization of glaciation than did temperature. Its effect, however,
+was always to cause glaciation exactly where it would be expected and
+not in unexpected places as actually occurred. For example, in North
+America the western side of the Canadian Rockies suffered intense
+glaciation, for there precipitation was heavy because the westerly winds
+from the Pacific are forced to give up their moisture as they rise. In
+the same way the western side of the Sierra Nevadas was much more
+heavily glaciated than the eastern side. In similar fashion the windward
+slopes of the Alps, the Caucasus, the Himalayas, and many other mountain
+ranges suffered extensive glaciation. Low temperature does not seem to
+have been the cause of this glaciation, for in that case it is hard to
+see why both sides of the various ranges did not show an equal
+percentage of increase in the size of their icefields.
+
+From what has been said as to temperature and topography, it is evident
+that variations in precipitation have had much more to do with
+glaciation than have variations in temperature. In the Arctic lowlands
+and on the leeward side of mountains, the slight development of
+glaciation appears to have been due to scarcity of precipitation. On the
+windward side of mountains, on the other hand, a notable increase in
+precipitation seems to have led to abundant glaciation. Such an increase
+in precipitation must be dependent on increased evaporation and this
+could arise either from relatively high temperature or strong winds.
+Since the temperature in the glacial period was lower than now, we seem
+forced to attribute the increased precipitation to a strengthening of
+the winds. If the westerly winds from the Pacific should increase in
+strength and waft more moisture to the western side of the Canadian
+Rockies, or if similar winds increased the snowfall on the upper slopes
+of the Alps or the Tian-Shan Mountains, the glaciers would extend lower
+than now without any change in temperature.
+
+Although the incompetence of low temperature to cause glaciation, and
+the relative unimportance of the mountains in northeastern Canada and
+northwestern Europe throw most glacial hypotheses out of court, they are
+in harmony with the cyclonic hypothesis. The answer of that hypothesis
+to the problem of the localization of ice sheets seems to be found in
+certain maps of storminess and rainfall in relation to solar activity.
+In Fig. 2 a marked belt of increased storminess at times of many
+sunspots is seen in southern Canada. A comparison of this with a series
+of maps given in _Earth and Sun_ shows that the stormy belt tends to
+migrate northward in harmony with an increase in the activity of the
+sun's atmosphere. If the sun were sufficiently active the belt of
+maximum storminess would apparently pass through the Keewatin and
+Labradorean centers of glaciation instead of well to the south of them,
+as at present. It would presumably cross another center in Greenland,
+and then would traverse the fourth of the great centers of Pleistocene
+glaciation in Scandinavia. It would not succeed in traversing northern
+Asia, however, any more than it does now, because of the great
+high-pressure area which develops there in winter. When the ice sheets
+expanded from the main centers of glaciation, the belt of storms would
+be pushed southward and outward. Thus it might give rise to minor
+centers of glaciers such as the Patrician between Hudson Bay and Lake
+Superior, or the centers in Ireland, Cornwall, Wales, and the northern
+Ural Mountains. As the main ice sheets advanced, however, the minor
+centers would be overridden and the entire mass of ice would be merged
+into one vast expanse in the Atlantic portion of each of the two
+continents.
+
+In this connection it may be well to consider briefly the most recent
+hypothesis as to the growth and hence the localization of glaciation. In
+1911 and more fully in 1915, Hobbs,[46] advanced the anti-cyclonic
+hypothesis of the origin of ice sheets. This hypothesis has the great
+merit of focusing attention upon the fact that ice sheets are pronounced
+anti-cyclonic regions of high pressure. This is proved by the strong
+outblowing winds which prevail along their margins. Such winds must, of
+course, be balanced by inward-moving winds at high levels. Abundant
+observations prove that such is the case. For example, balloons sent up
+by Barkow near the margin of the Antarctic ice sheet reveal the
+occurrence of inblowing winds, although they rarely occur below a height
+of 9000 meters. The abundant data gathered by Guervain on the coast of
+Greenland indicate that outblowing winds prevail up to a height of about
+4000 meters. At that height inblowing winds commence and increase in
+frequency until at an altitude of over 5000 meters they become more
+common than outblowing winds. It should be noted, however, that in both
+Antarctica and Greenland, although the winds at an elevation of less
+than a thousand meters generally blow outward, there are frequent and
+decided departures from this rule, so that "variable winds" are quite
+commonly mentioned in the reports of expeditions and balloon soundings.
+
+The undoubted anti-cyclonic conditions which Hobbs thus calls to the
+attention of scientists seem to him to necessitate a peculiar mechanism
+in order to produce the snow which feeds the glaciers. He assumes that
+the winds which blow toward the centers of the ice sheets at high levels
+carry the necessary moisture by which the glaciers grow. When the air
+descends in the centers of the highs, it is supposed to be chilled on
+reaching the surface of the ice, and hence to give up its moisture in
+the form of minute crystals. This conclusion is doubtful for several
+reasons. In the first place, Hobbs does not seem to appreciate the
+importance of the variable winds which he quotes Arctic and Antarctic
+explorers as describing quite frequently on the edges of the ice sheets.
+They are one of many signs that cyclonic storms are fairly frequent on
+the borders of the ice though not in its interior. Thus there is a
+distinct and sufficient form of precipitation actually at work near the
+margin of the ice, or exactly where the thickness of the ice sheet would
+lead us to expect.
+
+Another consideration which throws grave doubt on the anti-cyclonic
+hypothesis of ice sheets is the small amount of moisture possible in the
+highs because of their low temperature. Suppose, for the sake of
+argument, that the temperature in the middle of an ice sheet averages
+20�F. This is probably much higher than the actual fact and therefore
+unduly favorable to the anti-cyclonic hypothesis. Suppose also that the
+decrease in temperature from the earth's surface upward proceeds at the
+rate of 1�F. for each 300 feet, which is 50 per cent less than the
+actual rate for air with only a slight amount of moisture, such as is
+found in cold regions. Then at a height of 10,000 feet, where the
+inblowing winds begin to be felt, the temperature would be -20�F. At
+that temperature the air is able to hold approximately 0.166 grain of
+moisture per cubic foot when fully saturated. This is an exceedingly
+small amount of moisture and even if it were all precipitated could
+scarcely build a glacier. However, it apparently would not be
+precipitated because when such air descends in the center of the
+anti-cyclone it is warmed adiabatically, that is, by compression. On
+reaching the surface it would have a temperature of 20� and would be
+able to hold 0.898 grain of water vapor per cubic foot; in other words,
+it would have a relative humidity of about 18 per cent. Under no
+reasonable assumption does the upper air at the center of an ice sheet
+appear to reach the surface with a relative humidity of more than 20 or
+25 per cent. Such air cannot give up moisture. On the contrary, it
+absorbs it and tends to diminish rather than increase the thickness of
+the sheet of ice and snow. But after the surplus heat gained by descent
+has been lost by radiation, conduction, and evaporation, the air may
+become super-saturated with the moisture picked up while warm. Hobbs
+reports that explorers in Antarctica and Greenland have frequently
+observed condensation on their clothing. If such moisture is not derived
+directly from the men's own bodies, it is apparently picked up from the
+ice sheet by the descending air, and not added to the ice sheet by air
+from aloft.
+
+The relation of all this to the localization of ice sheets is this. If
+Hobbs' anti-cyclonic hypothesis of glacial growth is correct, it would
+appear that ice sheets should grow up where the temperature is lowest
+and the high-pressure areas most persistent; for instance, in northern
+Siberia. It would also appear that so far as the topography permitted,
+the ice sheets ought to move out uniformly in all directions; hence the
+ice sheet ought to be as prominent to the north of the Keewatin and
+Labradorean centers as to the south, which is by no means the case.
+Again, in mountainous regions, such as the glacial areas of Alaska and
+Chile, the glaciation ought not to be confined to the windward slope of
+the mountains so closely as is actually the fact. In each of these cases
+the glaciated region was large enough so that there was probably a true
+anti-cyclonic area comparable with that now prevailing over southern
+Greenland. In both places the correlation between glaciation and
+mountain ranges seems much too close to support the anti-cyclonic
+hypothesis, for the inblowing winds which on that hypothesis bring the
+moisture are shown by observation to occur at heights far greater than
+that of all but the loftiest ranges.
+
+II. The sudden coming of glaciation is another problem which has been a
+stumbling-block in the way of every glacial hypothesis. In his _Climates
+of Geologic Times_, Schuchert states that the fossils give almost no
+warning of an approaching catastrophe. If glaciation were solely due to
+uplift, or other terrestrial changes aside from vulcanism, Schuchert
+holds that it would have come slowly and the stages preceding glaciation
+would have affected life sufficiently to be recorded in the rocks. He
+considers that the suddenness of the coming of glaciation is one of the
+strongest arguments against the carbon dioxide hypothesis of glaciation.
+
+According to the cyclonic hypothesis, however, the suddenness of the
+oncoming of glaciation is merely what would be expected on the basis of
+what happens today. Changes in the sun occur suddenly. The sunspot cycle
+is only eleven or twelve years long, and even this short period of
+activity is inaugurated more suddenly than it declines. Again the
+climatic record derived from the growth of trees, as given in Figs. 4
+and 5, also shows that marked changes in climate are initiated more
+rapidly than they disappear. In this connection, however, it must be
+remembered that solar activity may arise in various ways, as will appear
+more fully later. Under certain conditions storminess may increase and
+decrease slowly.
+
+III. The height of the snow line and of glaciation furnishes another
+means of testing glacial hypotheses. It is well established that in
+times of glaciation the snow line was depressed everywhere, but least
+near the equator. For example, according to Penck, permanent snow
+extended 4000 feet lower than now in the Alps, whereas it stood only
+1500 feet below the present level near the equator in Venezuela. This
+unequal depression is not readily accounted for by any hypothesis
+depending solely upon the lowering of temperature. By the carbon dioxide
+and the volcanic dust hypotheses, the temperature presumably was lowered
+almost equally in all latitudes, but a little more at the equator than
+elsewhere. If glaciation were due to a temporary lessening of the
+radiation received from the sun, such as is demanded by the thermal
+solar hypothesis, and by the longer periods of Croll's hypothesis, the
+lowering would be distinctly greatest at the equator. Thus, according to
+all these hypotheses, the snow line should have been depressed most at
+the equator, instead of least.
+
+The cyclonic hypothesis explains the lesser depression of the snow line
+at the equator as due to a diminution of precipitation. The
+effectiveness of precipitation in this respect is illustrated by the
+present great difference in the height of the snow line on the humid and
+dry sides of mountains. On the wet eastern side of the Andes near the
+equator, the snow line lies at 16,000 feet; on the dry western side, at
+18,500 feet. Again, although the humid side of the Himalayas lies toward
+the south, the snow line has a level of 15,000 feet, while farther
+north, on the dry side, it is 16,700 feet.[47] The fact that the snow
+line is lower near the margin of the Alps than toward the center points
+in the same direction. The bearing of all this on the glacial period may
+be judged by looking again at Fig. 3 in Chapter V. This shows that at
+times of sunspot activity and hence of augmented storminess, the
+precipitation diminishes near the heat equator, that is, where the
+average temperature for the whole year is highest. At present the great
+size of the northern continents and their consequent high temperature in
+summer, cause the heat equator to lie north of the "real" equator,
+except where Australia draws it to the southward.[48] When large parts
+of the northern continents were covered with ice, however, the heat
+equator and the true equator were probably much closer than now, for the
+continents could not become so hot. If so, the diminution in equatorial
+precipitation, which accompanies increased storminess throughout the
+world as a whole, would take place more nearly along the true equator
+than appears in Fig. 3. Hence so far as precipitation alone is
+concerned, we should actually expect that the snow line near the equator
+would rise a little during glacial periods. Another factor, however,
+must be considered. K�ppen's data, it will be remembered, show that at
+times of solar activity the earth's temperature falls more at the
+equator than in higher latitudes. If this effect were magnified it would
+lower the snow line. The actual position of the snow line at the equator
+during glacial periods thus appears to be the combined effect of
+diminished precipitation, which would raise the line, and of lower
+temperature, which would bring it down.
+
+Before leaving this subject it may be well to recall that the relative
+lessening of precipitation in equatorial latitudes during the glacial
+epochs was probably caused by the diversion of moisture from the
+trade-wind belt. This diversion was presumably due to the great number
+of tropical cyclones and to the fact that the cyclonic storms of middle
+latitudes also drew much moisture from the trade-wind belt in summer
+when the northern position of the sun drew that belt near the storm
+track which was forced to remain south of the ice sheet. Such diversion
+of moisture out of the trade-wind belt must diminish the amount of water
+vapor that is carried by the trades to equatorial regions; hence it
+would lessen precipitation in the belt of so-called equatorial calms,
+which lies along the heat equator rather than along the geographical
+equator.
+
+Another phase of the vertical distribution of glaciation has been the
+subject of considerable discussion. In the Alps and in many other
+mountains the glaciation of the Pleistocene period appears to have had
+its upper limit no higher than today. This has been variously
+interpreted. It seems, however, to be adequately explained as due to
+decreased precipitation at high altitudes during the cold periods. This
+is in spite of the fact that precipitation in general increased with
+increased storminess. The low temperature of glacial times presumably
+induced condensation at lower altitudes than now, and most of the
+precipitation occurred upon the lower slopes of the mountains,
+contributing to the lower glaciers, while little of it fell upon the
+highest glaciers. Above a moderate altitude in all lofty mountains the
+decrease in the amount of precipitation is rapid. In most cases the
+decrease begins at a height of less than 3000 feet above the base of the
+main slope, provided the slope is steep. The colder the air, the lower
+the altitude at which this occurs. For example, it is much lower in
+winter than in summer. Indeed, the higher altitudes in the Alps are
+sunny in winter even where there are abundant clouds lower down.
+
+IV. The presence of extensive lakes and other evidences of a pluvial
+climate during glacial periods in non-glaciated regions which are
+normally dry is another of the facts which most glacial hypotheses fail
+to explain satisfactorily. Beyond the ice sheets many regions appear to
+have enjoyed an unusually heavy precipitation during the glacial epochs.
+The evidence of this is abundant, including numerous abandoned strand
+lines of salt lakes and an abundance of coarse material in deltas and
+flood plains. J. D. Whitney,[49] in an interesting but neglected volume,
+was one of the first to marshal the evidence of this sort. More recently
+Free[50] has amplified this. According to him in the Great Basin region
+of the United States sixty-two basins either contain unmistakable
+evidence of lakes, or belong to one of the three great lake groups named
+below. Two of these, the Lake Lahontan and the Lake Bonneville groups,
+comprise twenty-nine present basins, while the third, the Owens-Searles
+chain, contained at least five large lakes, the lowest being in Death
+Valley. In western and central Asia a far greater series of salt lakes
+is found and most of these are surrounded by strands at high levels.
+Many of these are described in _Explorations in Turkestan_, _The Pulse
+of Asia_, and _Palestine and Its Transformation_. There has been a good
+deal of debate as to whether these lakes actually date from the glacial
+period, as is claimed by C. E. P. Brooks, for example, or from some
+other period. The evidence, however, seems to be convincing that the
+lakes expanded when the ice also expanded.
+
+According to the older glacial hypotheses the lower temperature which is
+postulated as the cause of glaciation would almost certainly mean less
+evaporation over the oceans and hence less precipitation during glacial
+periods. To counteract this the only way in which the level of the lakes
+could be raised would be because the lower temperature would cause less
+evaporation from their surfaces. It seems quite impossible, however,
+that the lowering of temperature, which is commonly taken to have been
+not more than 10�C., could counteract the lessened precipitation and
+also cause an enormous expansion of most of the lakes. For example,
+ancient Lake Bonneville was more than ten times as large as its modern
+remnant, Great Salt Lake, and its average depth more than forty times as
+great.[51] Many small lakes in the Old World expanded still more.[52]
+For example, in eastern Persia many basins which now contain no lake
+whatever are floored with vast deposits of lacustrine salt and are
+surrounded by old lake bluffs and beaches. In northern Africa similar
+conditions prevail.[53] Other, but less obvious, evidence of more
+abundant rainfall in regions that are now dry is found in thick strata
+of gravel, sand, and fine silt in the alluvial deposits of flood plains
+and deltas.[54]
+
+The cyclonic hypothesis supposes that increased storminess accounts for
+pluvial climates in regions that are now dry just as it accounts for
+glaciation in the regions of the ice sheets. Figs. 2 and 3, it will be
+remembered, illustrate what happens when the sun is active. Solar
+activity is accompanied by an increase in storminess in the southwestern
+United States in exactly the region where elevated strands of diminished
+salt lakes are most numerous. In Fig. 3, the same condition is seen in
+the region of salt lakes in the Old World. Judging by these maps, which
+illustrate what has happened since careful meteorological records were
+kept, an increase in solar activity is accompanied by increased rainfall
+in large parts of what are now semi-arid and desert regions. Such
+precipitation would at once cause the level of the lakes to rise. Later,
+when ice sheets had developed in Europe and America, the high-pressure
+areas thus caused might force the main storm belt so far south that it
+would lie over these same arid regions. The increase in tropical
+hurricanes at times of abundant sunspots may also have a bearing on the
+climate of regions that are now arid. During the glacial period some of
+the hurricanes probably swept far over the lands. The numerous tropical
+cyclones of Australia, for example, are the chief source of
+precipitation for that continent.[55] Some of the stronger cyclones
+locally yield more rain in a day or two than other sources yield in a
+year.
+
+V. The occurrence of widespread glaciation near the tropics during the
+Permian, as shown in Fig. 7, has given rise to much discussion. The
+recent discovery of glaciation in latitudes as low as 30� in the
+Proterozoic is correspondingly significant. In all cases the occurrence
+of glaciation in low and middle latitudes is probably due to the same
+general causes. Doubtless the position and altitude of the mountains had
+something to do with the matter. Yet taken by itself this seems
+insufficient. Today the loftiest range in the world, the Himalayas, is
+almost unglaciated, although its southern slope may seem at first
+thought to be almost ideally located in this respect. Some parts rise
+over 20,000 feet and certain lower slopes receive 400 inches of rain per
+year. The small size of the Himalayan glaciers in spite of these
+favorable conditions is apparently due largely to the seasonal character
+of the monsoon winds. The strong outblowing monsoons of winter cause
+about half the year to be very dry with clear skies and dry winds from
+the interior of Asia. In all low latitudes the sun rides high in the
+heavens at midday, even in winter, and thus melts snow fairly
+effectively in clear weather. This is highly unfavorable to glaciation.
+The inblowing southern monsoons bring all their moisture in midsummer at
+just the time when it is least effective in producing snow. Conditions
+similar to those now prevailing in the Himalayas must accompany any
+great uplift of the lands which produces high mountains and large
+continents in subtropical and middle latitudes. Hence, uplift alone
+cannot account for extensive glaciation in subtropical latitudes during
+the Permian and Proterozoic.
+
+[Illustration: _Fig. 7. Permian geography and glaciation._
+(_After Schuchert._)]
+
+The assumption of a great general lowering of temperature is also not
+adequate to explain glaciation in subtropical latitudes. In the first
+place this would require a lowering of many degrees,--far more than in
+the Pleistocene glacial period. The marine fossils of the Permian,
+however, do not indicate any such condition. In the second place, if the
+lands were widespread as they appear to have been in the Permian, a
+general lowering of temperature would diminish rather than increase the
+present slight efficiency of the monsoons in producing glaciation.
+Monsoons depend upon the difference between the temperatures of land and
+water. If the general temperature were lowered, the reduction would be
+much less pronounced on the oceans than on the lands, for water tends to
+preserve a uniform temperature, not only because of its mobility, but
+because of the large amount of heat given out when freezing takes place,
+or consumed in evaporation. Hence the general lowering of temperature
+would make the contrast between continents and oceans less than at
+present in summer, for the land temperature would be brought toward that
+of the ocean. This would diminish the strength of the inblowing summer
+monsoons and thus cut off part of the supply of moisture. Evidence that
+this actually happened in the cold fourteenth century has already been
+given in Chapter VI. On the other hand, in winter the lands would be
+much colder than now and the oceans only a little colder, so that the
+dry outblowing monsoons of the cold season would increase in strength
+and would also last longer than at present. In addition to all this, the
+mere fact of low temperature would mean a general reduction in the
+amount of water vapor in the air. Thus, from almost every point of view
+a mere lowering of temperature seems to be ruled out as a cause of
+Permian glaciation. Moreover, if the Permian or Proterozoic glacial
+periods were so cold that the lands above latitude 30� were snow-covered
+most of the time, the normal surface winds in subtropical latitudes
+would be largely equatorward, just as the winter monsoons now are. Hence
+little or no moisture would be available to feed the snowfields which
+give rise to the glaciers.
+
+It has been assumed by Marsden Manson and others that increased general
+cloudiness would account for the subtropical glaciation of the Permian
+and Proterozoic. Granting for the moment that there could be universal
+persistent cloudiness, this would not prevent or counteract the
+outblowing anti-cyclonic winds so characteristic of great snowfields.
+Therefore, under the hypothesis of general cloudiness there would be no
+supply of moisture to cause glaciation in low latitudes. Indeed,
+persistent cloudiness in all higher latitudes would apparently deprive
+the Himalayas of most of their present moisture, for the interior of
+Asia would not become hot in summer and no inblowing monsoons would
+develop. In fact, winds of all kinds would seemingly be scarce, for they
+arise almost wholly from contrasts of temperature and hence of
+atmospheric pressure. The only way to get winds and hence precipitation
+would be to invoke some other agency, such as cyclonic storms, but that
+would be a departure from the supposition that glaciation arose from
+cloudiness.
+
+Let us now inquire how the cyclonic hypothesis accounts for glaciation
+in low latitudes. We will first consider the terrestrial conditions in
+the early Permian, the last period of glaciation in such latitudes.
+Geologists are almost universally agreed that the lands were
+exceptionally extensive and also high, especially in low latitudes. One
+evidence of this is the presence of abundant conglomerates composed of
+great boulders. It is also probable that the carbon dioxide in the air
+during the early Permian had been reduced to a minimum by the
+extraordinary amount of coal formed during the preceding period. This
+would tend to produce low temperature and thus make the conditions
+favorable for glaciation as soon as an accentuation of solar activity
+caused unusual storminess. If the storminess became extreme when
+terrestrial conditions were thus universally favorable to glaciation, it
+would presumably produce glaciation in low latitudes. Numerous and
+intense tropical cyclones would carry a vast amount of moisture out of
+the tropics, just as now happens when the sun is active, but on a far
+larger scale. The moisture would be precipitated on the equatorward
+slopes of the subtropical mountain ranges. At high elevations this
+precipitation would be in the form of snow even in summer. Tropical
+cyclones, however, as is shown in _Earth and Sun_, occur in the autumn
+and winter as well as in summer. For example, in the Bay of Bengal the
+number recorded in October is fifty, the largest for any month; while in
+November it is thirty-four, and December fourteen as compared with an
+average of forty-two for the months of July to September. From January
+to March, when sunspot numbers averaged more than forty, the number of
+tropical hurricanes was 143 per cent greater than when the sunspot
+numbers averaged below forty. During the months from April to June,
+which also would be times of considerable snowy precipitation, tropical
+hurricanes averaged 58 per cent more numerous with sunspot numbers above
+forty than with numbers below forty, while from July to September the
+difference amounted to 23 per cent. Even at this season some snow falls
+on the higher slopes, while the increased cloudiness due to numerous
+storms also tends to preserve the snow. Thus a great increase in the
+frequency of sunspots is accompanied by increased intensity of tropical
+hurricanes, especially in the cooler autumn and spring months, and
+results not only in a greater accumulation of snow but in a decrease in
+the melting of the snow because of more abundant clouds. At such times
+as the Permian, the general low temperature due to rapid convection and
+to the scarcity of carbon dioxide presumably joined with the extension
+of the lands in producing great high-pressure areas over the lands in
+middle latitudes during the winters, and thus caused the more northern,
+or mid-latitude type of cyclonic storms to be shifted to the equatorward
+side of the continents at that season. This would cause an increase of
+precipitation in winter as well as during the months when tropical
+hurricanes abound. Many other circumstances would co�perate to produce a
+similar result. For example, the general low temperature would cause the
+sea to be covered with ice in lower latitudes than now, and would help
+to create high-pressure areas in middle latitudes, thus driving the
+storms far south. If the sea water were fresher than now, as it probably
+was to a notable extent in the Proterozoic and perhaps to some slight
+extent in the Permian, the higher freezing point would also further the
+extension of the ice and help to keep the storms away from high
+latitudes. If to this there is added a distribution of land and sea such
+that the volume of the warm ocean currents flowing from low to high
+latitudes was diminished, as appears to have been the case, there seems
+to be no difficulty in explaining the subtropical location of the main
+glaciation in both the Permian and the Proterozoic. An increase of
+storminess seems to be the key to the whole situation.
+
+One other possibility may be mentioned, although little stress should be
+laid on it. In _Earth and Sun_ it has been shown that the main storm
+track in both the northern and southern hemispheres is not concentric
+with the geographical poles. Both tracks are roughly concentric with the
+corresponding magnetic poles, a fact which may be important in
+connection with the hypothesis of an electrical effect of the sun upon
+terrestrial storminess. The magnetic poles are known to wander
+considerably. Such wandering gives rise to variations in the direction
+of the magnetic needle from year to year. In 1815 the compass in England
+pointed 24-1/2� W. of N. and in 1906 17� 45' W. Such a variation seems
+to mean a change of many miles in the location of the north magnetic
+pole. Certain changes in the daily march of electromagnetic phenomena
+over the oceans have led Bauer and his associates to suggest that the
+magnetic poles may even be subject to a slight daily movement in
+response to the changes in the relative positions of the earth and sun.
+Thus there seems to be a possibility that a pronounced change in the
+location of the magnetic pole in Permian times, for example, may have
+had some connection with a shifting in the location of the belt of
+storms. It must be clearly understood that there is as yet no evidence
+of any such change, and the matter is introduced merely to call
+attention to a possible line of investigation.
+
+Any hypothesis of Permian and Proterozoic glaciation must explain not
+only the glaciation of low latitudes but the lack of glaciation and the
+accumulation of red desert beds in high latitudes. The facts already
+presented seem to explain this. Glaciation could not occur extensively
+in high latitudes partly because during most of the year the air was too
+cold to hold much moisture, but still more because the winds for the
+most part must have blown outward from the cold northern areas and the
+cyclonic storm belt was pushed out of high latitudes. Because of these
+conditions precipitation was apparently limited to a relatively small
+number of storms during the summer. Hence great desert areas must have
+prevailed at high latitudes. Great aridity now prevails north of the
+Himalayas and related ranges, and red beds are accumulating in the
+centers of the great deserts, such as those of the Tarim Basin and the
+Transcaspian. The redness is not due to the original character of the
+rock, but to intense oxidation, as appears from the fact that along the
+edges of the desert and wherever occasional floods carry sediment far
+out into the midst of the sand, the material has the ordinary brownish
+shades. As soon as one goes out into the places where the sand has been
+exposed to the air for a long time, however, it becomes pink, and then
+red. Such conditions may have given rise to the high degree of oxidation
+in the famous Permian red beds. If the air of the early Permian
+contained an unusual percentage of oxygen because of the release of that
+gas by the great plant beds which formed coal in the preceding era, as
+Chamberlin has thought probable, the tendency to produce red beds would
+be still further increased.
+
+It must not be supposed, however, that these conditions would absolutely
+limit glaciation to subtropical latitudes. The presence of early Permian
+glaciation in North America at Boston and in Alaska and in the Falkland
+Islands of the South Atlantic Ocean proves that at least locally there
+was sufficient moisture to form glaciers near the coast in relatively
+high latitudes. The possibility of this would depend entirely upon the
+form of the lands and the consequent course of ocean currents. Even in
+those high latitudes cyclonic storms would occur unless they were kept
+out by conditions of pressure such as have been described above.
+
+The marine faunas of Permian age in high latitudes have been interpreted
+as indicating mild oceanic temperatures. This is a point which requires
+further investigation. Warm oceans during times of slight solar activity
+are a necessary consequence of the cyclonic hypothesis, as will appear
+later. The present cold oceans seem to be the expectable result of the
+Pleistocene glaciation and of the present relatively disturbed condition
+of the sun. If a sudden disturbance threw the solar atmosphere into
+violent commotion within a few thousand years during Permian times,
+glaciation might occur as described above, while the oceans were still
+warm. In fact their warmth would increase evaporation while the violent
+cyclonic storms and high winds would cause heavy rain and keep the air
+cool by constantly raising it to high levels where it would rapidly
+radiate its heat into space.
+
+Nevertheless it is not yet possible to determine how warm the oceans
+were at the actual time of the Permian glaciation. Some faunas formerly
+reported as Permian are now known to be considerably older. Moreover,
+others of undoubted Permian age are probably not strictly
+contemporaneous with the glaciation. So far back in the geological
+record it is very doubtful whether we can date fossils within the limits
+of say 100,000 years. Yet a difference of 100,000 years would be more
+than enough to allow the fossils to have lived either before or after
+the glaciation, or in an inter-glacial epoch. One such epoch is known to
+have occurred and nine others are suggested by the inter-stratification
+of glacial till and marine sediments in eastern Australia. The warm
+currents which would flow poleward in inter-glacial epochs must have
+favored a prompt reintroduction of marine faunas driven out during times
+of glaciation. Taken all and all, the Permian glaciation seems to be
+accounted for by the cyclonic hypothesis quite as well as does the
+Pleistocene. In both these cases, as well as in the various pulsations
+of historic times, it seems to be necessary merely to magnify what is
+happening today in order to reproduce the conditions which prevailed in
+the past. If the conditions which now prevail at times of sunspot minima
+were magnified, they would give the mild conditions of inter-glacial
+epochs and similar periods. If the conditions which now prevail at times
+of sunspot maxima are magnified a little they seem to produce periods of
+climatic stress such as those of the fourteenth century. If they are
+magnified still more the result is apparently glacial epochs like those
+of the Pleistocene, and if they are magnified to a still greater extent,
+the result is Permian or Proterozoic glaciation. Other factors must
+indeed be favorable, for climatic changes are highly complex and are
+unquestionably due to a combination of circumstances. The point which is
+chiefly emphasized in this book is that among those several
+circumstances, changes in cyclonic storms due apparently to activity of
+the sun's atmosphere must always be reckoned.
+
+FOOTNOTES:
+
+[Footnote 46: W. H. Hobbs: Characteristics of Existing Glaciers, 1911.
+The R�le of the Glacial Anticyclones in the Air Circulation of the
+Globe; Proc. Am. Phil. Soc., Vol. 54, 1915, pp. 185-225.]
+
+[Footnote 47: R. D. Salisbury: Physiography, 1919.]
+
+[Footnote 48: Griffith Taylor: Australian Meteorology, 1920, p. 283.]
+
+[Footnote 49: J. D. Whitney: Climatic Changes of the Later Geological
+Times, 1882.]
+
+[Footnote 50: E. E. Free: U. S. Dept. of Agriculture, Bull. 54, 1914.
+Mr. Free has prepared a summary of this Bulletin which appears in The
+Solar Hypothesis, Bull. Geol. Sec. of Am., Vol. 25, pp. 559-562.]
+
+[Footnote 51: G. K. Gilbert: Lake Bonneville; Monograph 1, U. S. Geol.
+Surv.]
+
+[Footnote 52: C. E. P. Brooks: Quart. Jour. Royal Meteorol. Soc., 1914,
+pp. 63-66.]
+
+[Footnote 53: H. J. L. Beadnell: A. Egyptian Oasis, London, 1909.
+Ellsworth Huntington: The Libyan Oasis of Kharga; Bull. Am. Geog. Soc.,
+Vol. 42, Sept., 1910, pp. 641-661.]
+
+[Footnote 54: S. S. Visher: The Bajada of the Tucson Bolson of Southern
+Arizona; Science, N. S., Mar. 23, 1913.
+
+Ellsworth Huntington: The Basins of Eastern Persia and Seistan, in
+Explorations in Turkestan.]
+
+[Footnote 55: Griffith Taylor: Australian Meteorology, 1920, p. 189.]
+
+
+
+
+CHAPTER IX
+
+THE ORIGIN OF LOESS
+
+
+One of the most remarkable formations associated with glacial deposits
+consists of vast sheets of the fine-grained, yellowish, wind-blown
+material called loess. Somewhat peculiar climatic conditions evidently
+prevailed when it was formed. At present similar deposits are being laid
+down only near the leeward margin of great deserts. The famous loess
+deposits of China in the lee of the Desert of Gobi are examples. During
+the Pleistocene period, however, loess accumulated in a broad zone along
+the margin of the ice sheet at its maximum extent. In the Old World it
+extended from France across Germany and through the Black Earth region
+of Russia into Siberia. In the New World a still larger area is
+loess-covered. In the Mississippi Valley, tens of thousands of square
+miles are mantled by a layer exceeding twenty feet in thickness and in
+many places approaching a hundred feet. Neither the North American nor
+the European deposits are associated with a desert. Indeed, loess is
+lacking in the western and drier parts of the great plains and is best
+developed in the well-watered states of Iowa, Illinois, and Missouri.
+Part of the loess overlies the non-glacial materials of the great
+central plain, but the northern portions overlie the drift deposits of
+the first three glaciations. A few traces of loess are associated with
+the Kansan and Illinoian, the second and third glaciations, but most of
+the America loess appears to have been formed at approximately the time
+of the Iowan or fourth glaciation, while only a little overlies the
+drift sheets of the Wisconsin age. The loess is thickest near the margin
+of the Iowan till sheet and thins progressively both north and south.
+The thinning southward is abrupt along the stream divides, but very
+gradual along the larger valleys. Indeed, loess is abundant along the
+bluffs of the Mississippi, especially the east bluff, almost to the Gulf
+of Mexico.[56]
+
+It is now generally agreed that all typical loess is wind blown. There
+is still much question, however, as to its time of origin, and thus
+indirectly as to its climatic implications. Several American and
+European students have thought that the loess dates from inter-glacial
+times. On the other hand, Penck has concluded that the loess was formed
+shortly before the commencement of the glacial epochs; while many
+American geologists hold that the loess accumulated while the ice sheets
+were at approximately their maximum size. W. J. McGee, Chamberlin and
+Salisbury, Keyes, and others lean toward this view. In this chapter the
+hypothesis is advanced that it was formed at the one other possible
+time, namely, immediately following the retreat of the ice.
+
+These four hypotheses as to the time of origin of loess imply the
+following differences in its climatic relations. If loess was formed
+during typical inter-glacial epochs, or toward the close of such epochs,
+profound general aridity must seemingly have prevailed in order to kill
+off the vegetation and thus enable the wind to pick up sufficient dust.
+If the loess was formed during times of extreme glaciation when the
+glaciers were supplying large quantities of fine material to outflowing
+streams, less aridity would be required, but there must have been sharp
+contrasts between wet seasons in summer when the snow was melting and
+dry seasons in winter when the storms were forced far south by the
+glacial high pressure. Alternate floods and droughts would thus affect
+broad areas along the streams. Hence arises the hypothesis that the wind
+obtained the loess from the flood plains of streams at times of maximum
+glaciation. If the loess was formed during the rapid retreat of the ice,
+alternate summer floods and winter droughts would still prevail, but
+much material could also be obtained by the winds not only from flood
+plains, but also from the deposits exposed by the melting of the ice and
+not yet covered by vegetation.
+
+The evidence for and against the several hypotheses may be stated
+briefly. In support of the hypothesis of the inter-glacial origin of
+loess, Shimek and others state that the glacial drift which lies beneath
+the loess commonly gives evidence that some time elapsed between the
+disappearance of the ice and the deposition of the loess. For example,
+abundant shells of land snails in the loess are not of the sort now
+found in colder regions, but resemble those found in the drier regions.
+It is probable that if they represented a glacial epoch they would be
+depauperated by the cold as are the snails of far northern regions. The
+gravel pavement discussed below seems to be strong evidence of erosion
+between the retreat of the ice and the deposition of the loess.
+
+Turning to the second hypothesis, namely, that the loess accumulated
+near the close of the inter-glacial epoch rather than in the midst of
+it, we may follow Penck. The mammalian fossils seem to him to prove that
+the loess was formed while boreal animals occupied the region, for they
+include remains of the hairy mammoth, woolly rhinoceros, and reindeer.
+On the other hand, the typical inter-glacial beds not far away yield
+remains of species characteristic of milder climates, such as the
+elephant, the smaller rhinoceros, and the deer. In connection with these
+facts it should be noted that occasional remains of tundra vegetation
+and of trees are found beneath the loess, while in the loess itself
+certain steppe animals, such as the common gopher or spermaphyl, are
+found. Penck interprets this as indicating a progressive desiccation
+culminating just before the oncoming of the next ice sheet.
+
+The evidence advanced in favor of the hypothesis that the loess was
+formed when glaciation was near its maximum includes the fact that if
+the loess does not represent the outwash from the Iowan ice, there is
+little else that does, and presumably there must have been outwash. Also
+the distribution of loess along the margins of streams suggests that
+much of the material came from the flood plains of overloaded streams
+flowing from the melting ice.
+
+Although there are some points in favor of the hypothesis that the loess
+originated (1) in strictly inter-glacial times, (2) at the end of
+inter-glacial epochs, and (3) at times of full glaciation, each
+hypothesis is much weakened by evidence that supports the others. The
+evidence of boreal animals seems to disprove the hypothesis that the
+loess was formed in the middle of a mild inter-glacial epoch. On the
+other hand, Penck's hypothesis as to loess at the end of inter-glacial
+times fails to account for certain characteristics of the lowest part of
+the loess deposits and of the underlying topography. Instead of normal
+valleys and consequent prompt drainage such as ought to have developed
+before the end of a long inter-glacial epoch, the surface on which the
+loess lies shows many undrained depressions. Some of these can be seen
+in exposed banks, while many more are inferred from the presence of
+shells of pond snails here and there in the overlying loess. The pond
+snails presumably lived in shallow pools occupying depressions in the
+uneven surface left by the ice. Another reason for questioning whether
+the loess was formed at the end of an inter-glacial epoch is that this
+hypothesis does not provide a reasonable origin for the material which
+composes the loess. Near the Alps where the loess deposits are small and
+where glaciers probably persisted in the inter-glacial epochs and thus
+supplied flood plain material in large quantities, this does not appear
+important. In the broad upper Mississippi Basin, however, and also in
+the Black Earth region of Russia there seems to be no way to get the
+large body of material composing the loess except by assuming the
+existence of great deserts to windward. But there seems to be little or
+no evidence of such deserts where they could be effective. The
+mineralogical character of the loess of Iowan age proves that the
+material came from granitic rocks, such as formed a large part of the
+drift. The nearest extensive outcrops of granite are in the southwestern
+part of the United States, nearly a thousand miles from Iowa and
+Illinois. But the loess is thickest near the ice margin and thins toward
+the southwest and in other directions, whereas if its source were the
+southwestern desert, its maximum thickness would probably be near the
+margin of the desert.
+
+The evidence cited above seems inconsistent not only with the hypothesis
+that the loess was formed at the end of an inter-glacial epoch, but also
+with the idea that it originated at times of maximum glaciation either
+from river-borne sediments or from any other source. A further and more
+convincing reason for this last conclusion is the probability and almost
+the certainty that when the ice advanced, its front lay close to areas
+where the vegetation was not much thinner than that which today prevails
+under similar climatic conditions. If the average temperature of glacial
+maxima was only 6�C. lower than that of today, the conditions just
+beyond the ice front when it was in the loess region from southern
+Illinois to Minnesota would have been like those now prevailing in
+Canada from New Brunswick to Winnipeg. The vegetation there is quite
+different from the grassy, semi-arid vegetation of which evidence is
+found in the loess. The roots and stalks of such grassy vegetation are
+generally agreed to have helped produce the columnar structure which
+enables the loess to stand with almost vertical surfaces.
+
+We are now ready to consider the probability that loess accumulated
+mainly during the retreat of the ice. Such a retreat exposed a zone of
+drift to the outflowing glacial winds. Most glacial hypotheses, such as
+that of uplift, or depleted carbon dioxide, call for a gradual retreat
+of the ice scarcely faster than the vegetation could advance into the
+abandoned area. Under the solar-cyclonic hypothesis, on the other hand,
+the climatic changes may have been sudden and hence the retreat of the
+ice may have been much more rapid than the advance of vegetation. Now
+wind-blown materials are derived from places where vegetation is scanty.
+Scanty vegetation on good soil, it is true, is usually due to aridity,
+but may also result because the time since the soil was exposed to the
+air has not been long enough for the soil to be sufficiently weathered
+to support vegetation. Even when weathering has had full opportunity, as
+when sand bars, mud flats, and flood plains are exposed, vegetation
+takes root only slowly. Moreover, storms and violent winds may prevent
+the spread of vegetation, as is seen on sandy beaches even in distinctly
+humid regions like New Jersey and Denmark. Thus it appears that unless
+the retreat of the ice were as slow as the advance of vegetation, a
+barren area of more or less width must have bordered the retreating ice
+and formed an ideal source of loess.
+
+Several other lines of evidence seemingly support the conclusion that
+the loess was formed during the retreat of the ice. For example, Shimek,
+who has made almost a lifelong study of the Iowan loess, emphasizes the
+fact that there is often an accumulation of stones and pebbles at its
+base. This suggests that the underlying till was eroded before the loess
+was deposited upon it. The first reaction of most students is to assume
+that of course this was due to running water. That is possible in many
+cases, but by no means in all. So widespread a sheet of gravel could not
+be deposited by streams without destroying the irregular basins and
+hollows of which we have seen evidence where the loess lies on glacial
+deposits. On the other hand, the wind is competent to produce a similar
+gravel pavement without disturbing the old topography. "Desert
+pavements" are a notable feature in most deserts. On the edges of an ice
+sheet, as Hobbs has made us realize, the commonest winds are outward.
+They often attain a velocity of eighty miles an hour in Antarctica and
+Greenland. Such winds, however, usually decline rapidly in velocity only
+a few score miles from the ice. Thus their effect would be to produce
+rapid erosion of the freshly bared surface near the retreating ice. The
+pebbles would be left behind as a pavement, while sand and then loess
+would be deposited farther from the ice where the winds were weaker and
+where vegetation was beginning to take root. Such a decrease in wind
+velocity may explain the occasional vertical gradation from gravel
+through sand to coarse loess and then to normal fine loess. As the ice
+sheet retreated the wind in any given place would gradually become less
+violent. As the ice continued to retreat the area where loess was
+deposited would follow at a distance, and thus each part of the gravel
+pavement would in turn be covered with the loess.
+
+The hypothesis that loess is deposited while the ice is retreating is in
+accord with many other lines of evidence. For example, it accords with
+the boreal character of the mammal remains as described above. Again,
+the advance of vegetation into the barren zone along the front of the
+ice would be delayed by the strong outblowing winds. The common pioneer
+plants depend largely on the wind for the distribution of their seeds,
+but the glacial winds would carry them away from the ice rather than
+toward it. The glacial winds discourage the advance of vegetation in
+another way, for they are drying winds, as are almost all winds blowing
+from a colder to a warmer region. The fact that remains of trees
+sometimes occur at the bottom of the loess probably means that the
+deposition of loess extended into the forests which almost certainly
+persisted not far from the ice. This seems more likely than that a
+period of severe aridity before the advance of the ice killed the trees
+and made a steppe or desert. Penck's chief argument in favor of the
+formation of loess before the advance of the ice rather than after, is
+that since loess is lacking upon the youngest drift sheet in Europe it
+must have been formed before rather than after the last or W�rm advance
+of the ice. This breaks down on two counts. First, on the corresponding
+(Wisconsin) drift sheet in America, loess is present,--in small
+quantities to be sure, but unmistakably present. Second, there is no
+reason to assume that conditions were identical at each advance and
+retreat of the ice. Indeed, the fact that in Europe, as in the United
+States, nearly all the loess was formed at one time, and only a little
+is associated with the other ice advances, points clearly against
+Penck's fundamental assumption that the accumulation of loess was due to
+the approach of a cold climate.
+
+Having seen that the loess was probably formed during the retreat of the
+ice, we are now ready to inquire what conditions the cyclonic hypothesis
+would postulate in the loess areas during the various stages of a
+glacial cycle. Fig. 2, in Chapter IV, gives the best idea of what would
+apparently happen in North America, and events in Europe would
+presumably be similar. During the nine maximum years on which Fig. 2 is
+based the sunspot numbers averaged seventy, while during the nine
+minimum years they averaged less than five. It seems fair to suppose
+that the maximum years represent the average conditions which prevailed
+in the past at times when the sun was in a median stage between the full
+activity which led to glaciation and the mild activity of the minimum
+years which appear to represent inter-glacial conditions. This would
+mean that when a glacial period was approaching, but before an ice sheet
+had accumulated to any great extent, a crescent-shaped strip from
+Montana through Illinois to Maine would suffer a diminution in
+storminess ranging up to 60 per cent as compared with inter-glacial
+conditions. This is in strong contrast with an increase in storminess
+amounting to 75 or even 100 per cent both in the boreal storm belt in
+Canada and in the subtropical belt in the Southwest. Such a decrease in
+storminess in the central United States would apparently be most
+noticeable in summer, as is shown in _Earth and Sun_. Hence it would
+have a maximum effect in producing aridity. This would favor the
+formation of loess, but it is doubtful whether the aridity would become
+extreme enough to explain such vast deposits as are found throughout
+large parts of the Mississippi Basin. That would demand that hundreds of
+thousands of square miles should become almost absolute desert, and it
+is not probable that any such thing occurred. Nevertheless, according to
+the cyclonic hypothesis the period immediately before the advent of the
+ice would be relatively dry in the central United States, and to that
+extent favorable to the work of the wind.
+
+As the climatic conditions became more severe and the ice sheet
+expanded, the dryness and lack of storms would apparently diminish. The
+reason, as has been explained, would be the gradual pushing of the
+storms southward by the high-pressure area which would develop over the
+ice sheet. Thus at the height of a glacial epoch there would apparently
+be great storminess in the area where the loess is found, especially in
+summer. Hence the cyclonic hypothesis does not accord with the idea of
+great deposition of loess at the time of maximum glaciation.
+
+Finally we come to the time when the ice was retreating. We have already
+seen that not only the river flood plains, but also vast areas of fresh
+glacial deposits would be exposed to the winds, and would remain without
+vegetation for a long time. At that very time the retreat of the ice
+sheet would tend to permit the storms to follow paths determined by the
+degree of solar activity, in place of the far southerly paths to which
+the high atmospheric pressure over the expanded ice sheet had previously
+forced them. In other words, the conditions shown in Fig. 2 would tend
+to reappear when the sun's activity was diminishing and the ice sheet
+was retreating, just as they had appeared when the sun was becoming more
+active and the ice sheet was advancing. This time, however, the
+semi-arid conditions arising from the scarcity of storms would prevail
+in a region of glacial deposits and widely spreading river deposits, few
+or none of which would be covered with vegetation. The conditions would
+be almost ideal for eolian erosion and for the transportation of loess
+by the wind to areas a little more remote from the ice where grassy
+vegetation had made a start.
+
+The cyclonic hypothesis also seems to offer a satisfactory explanation
+of variations in the amount of loess associated with the several glacial
+epochs. It attributes these to differences in the rate of disappearance
+of the ice, which in turn varied with the rate of decline of solar
+activity and storminess. This is supposed to be the reason why the Iowan
+loess deposits are much more extensive than those of the other epochs,
+for the Iowan ice sheet presumably accomplished part of its retreat much
+more suddenly than the other ice sheets.[57] The more sudden the
+retreat, the greater the barren area where the winds could gather fine
+bits of dust. Temporary readvances may also have been so distributed and
+of such intensity that they frequently accentuated the condition shown
+in Fig. 2, thus making the central United States dry soon after the
+exposure of great amounts of glacial d�bris. The closeness with which
+the cyclonic hypothesis accords with the facts as to the loess is one of
+the pleasant surprises of the hypothesis. The first draft of Fig. 2 and
+the first outlines of the hypothesis were framed without thought of the
+loess. Yet so far as can now be seen, both agree closely with the
+conditions of loess formation.
+
+FOOTNOTES:
+
+[Footnote 56: Chamberlin and Salisbury: Geology, 1906, Vol. III, pp.
+405-412.]
+
+[Footnote 57: It may have retreated soon after reaching its maximum. If
+so, the general lack of thick terminal moraines would be explained. See
+page 122.]
+
+
+
+
+CHAPTER X
+
+CAUSES OF MILD GEOLOGICAL CLIMATES
+
+
+In discussions of climate, as of most subjects, a peculiar psychological
+phenomenon is observable. Everyone sees the necessity of explaining
+conditions different from those that now exist, but few realize that
+present conditions may be abnormal, and that they need explanation just
+as much as do others. Because of this tendency glaciation has been
+discussed with the greatest fullness, while there has been much neglect
+not only of the periods when the climate of the earth resembled that of
+the present, but also of the vastly longer periods when it was even
+milder than now.
+
+How important the periods of mild climate have been in geological times
+may be judged from the relative length of glacial compared with
+inter-glacial epochs, and still more from the far greater relative
+length of the mild parts of periods and eras when compared with the
+severe parts. Recent estimates by R. T. Chamberlin[58] indicate that
+according to the consensus of opinion among geologists the average
+inter-glacial epoch during the Pleistocene was about five times as long
+as the average glacial epoch, while the whole of a given glacial epoch
+averaged five times as long as the period when the ice was at a maximum.
+Climatic periods far milder, longer, and more monotonous than any
+inter-glacial epoch appear repeatedly during the course of geological
+history. Our task in this chapter is to explain them.
+
+Knowlton[59] has done geology a great service by collecting the evidence
+as to the mild type of climate which has again and again prevailed in
+the past. He lays special stress on botanical evidence since that
+pertains to the variable atmosphere of the lands, and hence furnishes a
+better guide than does the evidence of animals that lived in the
+relatively unchanging water of the oceans. The nature of the evidence
+has already been indicated in various parts of this book. It includes
+palms, tree ferns, and a host of other plants which once grew in regions
+which are now much too cold to support them. With this must be placed
+the abundant reef-building corals and other warmth-loving marine
+creatures in latitudes now much too cold for them. Of a piece with this
+are the conditions of inter-glacial epochs in Europe, for example, when
+elephants and hippopotamuses, as well as many species of plants from low
+latitudes, were abundant. These conditions indicate not only that the
+climate was warmer than now, but that the contrast from season to season
+was much less. Indeed, Knowlton goes so far as to say that "relative
+uniformity, mildness, and comparative equability of climate, accompanied
+by high humidity, have prevailed over the greater part of the earth,
+extending to, or into, polar circles, during the greater part of
+geologic time--since, at least, the Middle Paleozoic. This is the
+regular, the ordinary, the normal condition." ... "By many it is thought
+that one of the strongest arguments against a gradually cooling globe
+and a humid, non-zonally disposed climate in the ages before the
+Pleistocene is the discovery of evidences of glacial action practically
+throughout the entire geologic column. Hardly less than a dozen of these
+are now known, ranging in age from Huronian to Eocene. It seems to be a
+very general assumption by those who hold this view that these evidences
+of glacial activities are to be classed as ice ages, largely comparable
+in effect and extent to the Pleistocene refrigeration, but as a matter
+of fact only three are apparently of a magnitude to warrant such
+designation. These are the Huronian glaciation, that of the
+'Permo-Carboniferous,' and that of the Pleistocene. The others, so far
+as available data go, appear to be explainable as more or less local
+manifestations that had no widespread effect on, for instance, ocean
+temperatures, distribution of life, et cetera. They might well have been
+of the type of ordinary mountain glaciers, due entirely to local
+elevation and precipitation." ... "If the sun had been the principal
+source of heat in pre-Pleistocene time, terrestrial temperatures would
+of necessity have been disposed in zones, whereas the whole trend of
+this paper has been the presentation of proof that these temperatures
+were distinctly non-zonal. Therefore it seems to follow that the sun--at
+least the present small-angle sun--could not have been the sole or even
+the principal source of heat that warmed the early oceans."
+
+Knowlton is so strongly impressed by the widespread fossil floras that
+usually occur in the middle parts of the geological periods, that as
+Schuchert[3] puts it, he neglects the evidence of other kinds. In the
+middle of the periods and eras the expansion of the warm oceans over the
+continents was greatest, while the lands were small and hence had more
+or less insular climates of the oceanic type. At such times, the marine
+fauna agrees with the flora in indicating a mild climate. Large
+colony-forming foraminifera, stony corals, shelled cephalopods,
+gastropods and thick-shelled bivalves, generally the cemented forms,
+were common in the Far North and even in the Arctic. This occurred in
+the Silurian, Devonian, Pennsylvanian, and Jurassic periods, yet at
+other times, such as the Cretaceous and Eocene, such forms were very
+greatly reduced in variety in the northern regions or else wholly
+absent. These things, as Schuchert[60] says, can only mean that Knowlton
+is right when he states that "climatic zoning such as we have had since
+the beginning of the Pleistocene did not obtain in the geologic ages
+prior to the Pleistocene." It does not mean, however, that there was a
+"non-zonal arrangement" and that the temperature of the oceans was
+everywhere the same and "without widespread effect on the distribution
+of life."
+
+Students of paleontology hold that as far back as we can go in the study
+of plants, there are evidences of seasons and of relatively cool
+climates in high latitudes. The cycads, for instance, are one of the
+types most often used as evidence of a warm climate. Yet Wieland,[61]
+who has made a lifelong study of these plants, says that many of them
+"might well grow in temperate to cool climates. Until far more is
+learned about them they should at least be held as valueless as indices
+of tropic climates." The inference is "that either they or their close
+relatives had the capacity to live in every clime. There is also a
+suspicion that study of the associated ferns may compel revision of the
+long-accepted view of the universality of tropic climates throughout the
+Mesozoic." Nathorst is quoted by Wieland as saying, "I think ... that
+during the time when the Gingkophytes and Cycadophytes dominated, many
+of them must have adapted themselves for living in cold climates also.
+Of this I have not the least doubt."
+
+Another important line of evidence which Knowlton and others have cited
+as a proof of the non-zonal arrangement of climate in the past, is the
+vast red beds which are found in the Proterozoic, late Silurian,
+Devonian, Permian, and Triassic, and in some Tertiary formations. These
+are believed to resemble laterite, a red and highly oxidized soil which
+is found in great abundance in equatorial regions. Knowlton does not
+attempt to show that the red beds present equatorial characteristics in
+other respects, but bases his conclusion on the statement that "red beds
+are not being formed at the present time in any desert region." This is
+certainly an error. As has already been said, in both the Transcaspian
+and Takla Makan deserts, the color of the sand regularly changes from
+brown on the borders to pale red far out in the desert. Kuzzil Kum, or
+Red Sand, is the native name. The sands in the center of the desert
+apparently were originally washed down from the same mountains as those
+on the borders, and time has turned them red. Since the same condition
+is reported from the Arabian Desert, it seems that redness is
+characteristic of some of the world's greatest deserts. Moreover, beds
+of salt and gypsum are regularly found in red beds, and they can
+scarcely originate except in deserts, or in shallow almost landlocked
+bays on the coasts of deserts, as appears to have happened in the
+Silurian where marine fossils are found interbedded with gypsum.
+
+Again, Knowlton says that red beds cannot indicate deserts because the
+plants found in them are not "pinched or depauperate, nor do they
+indicate xerophytic adaptations. Moreover, very considerable deposits of
+coal are found in red beds in many parts of the world, which implies the
+presence of swamps but little above sea-level."
+
+Students of desert botany are likely to doubt the force of these
+considerations. As MacDougal[62] has shown, the variety of plants in
+deserts is greater than in moist regions. Not only do xerophytic desert
+species prevail, but halophytes are present in the salty areas, and
+hygrophytes in the wet swampy areas, while ordinary mesophytes prevail
+along the water courses and are washed down from the mountains. The
+ordinary plants, not the xerophytes, are the ones that are chiefly
+preserved since they occur in most abundance near streams where
+deposition is taking place. So far as swamps are concerned, few are of
+larger size than those of Seistan in Persia, Lop Nor in Chinese
+Turkestan, and certain others in the midst of the Asiatic deserts.
+Streams flowing from the mountains into deserts are almost sure to form
+large swamps, such as those along the Tarim River in central Asia. Lake
+Chad in Africa is another example. In it, too, reeds are very numerous.
+
+Putting together the evidence on both sides in this disputed question,
+it appears that throughout most of geological time there is some
+evidence of a zonal arrangement of climate. The evidence takes the form
+of traces of cool climates, of seasons, and of deserts. Nevertheless,
+there is also strong evidence that these conditions were in general less
+intense than at present and that times of relatively warm, moist climate
+without great seasonal extremes have prevailed very widely during
+periods much longer than those when a zonal arrangement as marked as
+that of today prevailed. As Schuchert[63] puts it: "Today the variation
+on land between the tropics and the poles is roughly between 110� and
+-60�F., in the oceans between 85� and 31�F. In the geologic past the
+temperature of the oceans for the greater parts of the periods probably
+was most often between 85� and 55�F., while on land it may have varied
+between 90� and 0�F. At rare intervals the extremes were undoubtedly as
+great as they are today. The conclusion is therefore that at all times
+the earth had temperature zones, varying between the present-day
+intensity and times which were almost without such belts, and at these
+latter times the greater part of the earth had an almost uniformly mild
+climate, without winters."
+
+It is these mild climates which we must now attempt to explain. This
+leads us to inquire what would happen to the climate of the earth as a
+whole if the conditions which now prevail at times of few sunspots were
+to become intensified. That they could become greatly intensified seems
+highly probable, for there is good reason to think that aside from the
+sunspot cycle the sun's atmosphere is in a disturbed condition. The
+prominences which sometimes shoot out hundreds of thousands of miles
+seem to be good evidence of this. Suppose that the sun's atmosphere
+should become very quiet. This would apparently mean that cyclonic
+storms would be much less numerous and less severe than during the
+present times of sunspot minima. The storms would also apparently follow
+paths in middle latitudes somewhat as they do now when sunspots are
+fewest. The first effect of such a condition, if we can judge from what
+happens at present, would be a rise in the general temperature of the
+earth, because less heat would be carried aloft by storms. Today, as is
+shown in _Earth and Sun_, a difference of perhaps 10 per cent in the
+average storminess during periods of sunspot maxima and minima is
+correlated with a difference of 3�C. in the temperature at the earth's
+surface. This includes not only an actual lowering of 0.6�C. at times of
+sunspot maxima, but the overcoming of the effect of increased insolation
+at such times, an effect which Abbot calculates as about 2.5�C. If the
+storminess were to be reduced to one-half or one-quarter its present
+amount at sunspot minima, not only would the loss of heat by upward
+convection in storms be diminished, but the area covered by clouds would
+diminish so that the sun would have more chance to warm the lower air.
+Hence the average rise of temperature might amount to as much at 5� or
+10�C.
+
+Another effect of the decrease in storminess would be to make the
+so-called westerly winds, which are chiefly southwesterly in the
+northern hemisphere and northwesterly in the southern hemisphere, more
+strong and steady than at present. They would not continually suffer
+interruption by cyclonic winds from other directions, as is now the
+case, and would have a regularity like that of the trades. This
+conclusion is strongly re�nforced in a paper by Clayton[64] which came
+to hand after this chapter had been completed. From his studies of the
+solar constant and the temperature of the earth which are described in
+_Earth and Sun_, he reaches the following conclusion: "The results of
+these researches have led me to believe: 1. That if there were no
+variation in solar radiation the atmospheric motions would establish a
+stable system with exchanges of air between equator and pole and between
+ocean and land, in which the only variations would be daily and annual
+changes set in operation by the relative motions of the earth and sun.
+2. The existing abnormal changes, which we call weather, have their
+origins chiefly, if not entirely, in the variations of solar radiation."
+
+If cyclonic storms and "weather" were largely eliminated and if the
+planetary system of winds with its steady trades and southwesterlies
+became everywhere dominant, the regularity and volume of the
+poleward-flowing currents, such as the Gulf Stream and the Atlantic
+Drift in one ocean, and the Japanese Current in another, would be
+greatly increased. How important this is may be judged from the work of
+Helland-Hansen and Nansen.[65] These authors find that with the passage
+of each cyclonic storm there is a change in the temperature of the
+surface water of the Atlantic Ocean. Winds at right angles to the course
+of the Drift drive the water first in one direction and then in the
+other but do not advance it in its course. Winds with an easterly
+component, on the other hand, not only check the Drift but reverse it,
+driving the warm water back toward the southwest and allowing cold water
+to well up in its stead. The driving force in the Atlantic Drift is
+merely the excess of the winds with a westerly component over those with
+an easterly component.
+
+Suppose that the numbers in Fig. 8 represent the strength of the winds
+in a certain part of the North Atlantic or North Pacific, that is, the
+total number of miles moved by the air per year. In quadrant A of the
+left-hand part all the winds move from a more or less southwesterly
+direction and produce a total movement of the air amounting to thirty
+units per year. Those coming from points between north and west move
+twenty-five units; those between north and east, twenty units; and those
+between east and south, twenty-five units. Since the movement of the
+winds in quadrants B and D is the same, these winds have no effect in
+producing currents. They merely move the water back and forth, and thus
+give it time to lose whatever heat it has brought from more southerly
+latitudes. On the other hand, since the easterly winds in quadrant C do
+not wholly check the currents caused by the westerly winds of quadrant
+A, the effective force of the westerly winds amounts to ten, or the
+difference between a force of thirty in quadrant A and of twenty in
+quadrant C. Hence the water is moved forward toward the northeast, as
+shown by the thick part of arrow A.
+
+[Illustration: _Fig. 8. Effect of diminution of storms on movement of
+water._]
+
+Now suppose that cyclonic storms should be greatly reduced in number so
+that in the zone of prevailing westerlies they were scarcely more
+numerous than tropical hurricanes now are in the trade-wind belt. Then
+the more or less southwesterly winds in quadrant A� in the right-hand
+part of Fig. 8 would not only become more frequent but would be stronger
+than at present. The total movement from that quarter might rise to
+sixty units, as indicated in the figure. In quadrants B� and D� the
+movement would fall to fifteen and in quadrant C� to ten. B� and D�
+would balance one another as before. The movement in A�, however, would
+exceed that in C� by fifty instead of ten. In other words, the
+current-making force would become five times as great as now. The actual
+effect would be increased still more, for the winds from the southwest
+would be stronger as well as steadier if there were no storms. A strong
+wind which causes whitecaps has much more power to drive the water
+forward than a weaker wind which does not cause whitecaps. In a wave
+without a whitecap the water returns to practically the original point
+after completing a circle beneath the surface. In a wave with a
+whitecap, however, the cap moves forward. Any increase in velocity
+beyond the rate at which whitecaps are formed has a great influence upon
+the amount of water which is blown forward. Several times as much water
+is drifted forward by a persistent wind of twenty miles an hour as by a
+ten-mile wind.[66]
+
+In this connection a suggestion which is elaborated in Chapter XIII may
+be mentioned. At present the salinity of the oceans checks the general
+deep-sea circulation and thereby increases the contrasts from zone to
+zone. In the past, however, the ocean must have been fresher than now.
+Hence the circulation was presumably less impeded, and the transfer of
+heat from low latitudes to high was facilitated.
+
+Consider now the magnitude of the probable effect of a diminution in
+storms. Today off the coast of Norway in latitude 65�N. and longitude
+10�E., the mean temperature in January is 2�C. and in July 12�C. This
+represents a plus anomaly of about 22� in January and 2� in July; that
+is, the Norwegian coast is warmer than the normal for its latitude by
+these amounts. Suppose that in some past time the present distribution
+of lands and seas prevailed, but Norway was a lowland where extensive
+deposits could accumulate in great flood plains. Suppose, also, that the
+sun's atmosphere was so inactive that few cyclonic storms occurred,
+steady winds from the west-southwest prevailed, and strong,
+uninterrupted ocean currents brought from the Caribbean Sea and Gulf of
+Mexico much greater supplies of warm water than at present. The
+Norwegian winters would then be warmer than now not only because of the
+general increase in temperature which the earth regularly experiences at
+sunspot minima, but because the currents would accentuate this
+condition. In summer similar conditions would prevail except that the
+warming effect of the winds and currents would presumably be less than
+in winter, but this might be more than balanced by the increased heat of
+the sun during the long summer days, for storms and clouds would be
+rare.
+
+If such conditions raised the winter temperature only 8�C. and the
+summer temperature 4�C., the climate would be as warm as that of the
+northern island of New Zealand (latitude 35�-43�S.). The flora of that
+part of New Zealand is subtropical and includes not only pines and
+beeches, but palms and tree ferns. A climate scarcely warmer than that
+of New Zealand would foster a flora like that which existed in far
+northern latitudes during some of the milder geological periods. If,
+however, the general temperature of the earth's surface were raised 5�
+because of the scarcity of storms, if the currents were strong enough so
+that they increased the present anomaly by 50 per cent, and if more
+persistent sunshine in summer raised the temperature at that season
+about 4�C., the January temperature would be 18�C. and the July
+temperature 22�C. These figures perhaps make summer and winter more
+nearly alike than was ever really the case in such latitudes.
+Nevertheless, they show that a diminution of storms and a consequent
+strengthening and steadying of the southwesterlies might easily raise
+the temperature of the Norwegian coast so high that corals could
+flourish within the Arctic Circle.
+
+Another factor would co�perate in producing mild temperatures in high
+latitudes during the winter, namely, the fogs which would presumably
+accumulate. It is well known that when saturated air from a warm ocean
+is blown over the lands in winter, as happens so often in the British
+Islands and around the North Sea, fog is formed. The effect of such a
+fog is indeed to shut out the sun's radiation, but in high latitudes
+during the winter when the sun is low, this is of little importance.
+Another effect is to retain the heat of the earth itself. When a
+constant supply of warm water is being brought from low latitudes this
+blanketing of the heat by the fog becomes of great importance. In the
+past, whenever cyclonic storms were weak and westerly winds were
+correspondingly strong, winter fogs in high latitudes must have been
+much more widespread and persistent than now.
+
+The bearing of fogs on vegetation is another interesting point. If a
+region in high latitudes is constantly protected by fog in winter, it
+can support types of vegetation characteristic of fairly low latitudes,
+for plants are oftener killed by dry cold than by moist cold. Indeed,
+excessive evaporation from the plant induced by dry cold when the
+evaporated water cannot be rapidly replaced by the movement of sap is a
+chief reason why large plants are winterkilled. The growing of
+transplanted palms on the coast of southwestern Ireland, in spite of its
+location in latitude 50�N., is possible only because of the great
+fogginess in winter due to the marine climate. The fogs prevent the
+escape of heat and ward off killing frosts. The tree ferns in latitude
+46�S. in New Zealand, already referred to, are often similarly protected
+in winter. Therefore, the relative frequency of fogs in high latitudes
+when storms were at a minimum would apparently tend not merely to
+produce mild winters but to promote tropical vegetation.
+
+The strong steady trades and southwesterlies which would prevail at
+times of slight solar activity, according to our hypothesis, would have
+a pronounced effect on the water of the deep seas as well as upon that
+of the surface. In the first place, the deep-sea circulation would be
+hastened. For convenience let us speak of the northern hemisphere. In
+the past, whenever the southwesterly winds were steadier than now, as
+was probably the case when cyclonic storms were relatively rare, more
+surface water than at present was presumably driven from low latitudes
+and carried to high latitudes. This, of course, means that a greater
+volume of water had to flow back toward the equator in the lower parts
+of the ocean, or else as a cool surface current. The steady
+southwesterly winds, however, would interfere with south-flowing surface
+currents, thus compelling the polar waters to find their way equatorward
+beneath the surface. In low latitudes the polar waters would rise and
+their tendency would be to lower the temperature. Hence steadier
+westerlies would make for lessened latitudinal contrasts in climate not
+only by driving more warm water poleward but by causing more polar water
+to reach low latitudes.
+
+At this point a second important consideration must be faced. Not only
+would the deep-sea circulation be hastened, but the ocean depths might
+be warmed. The deep parts of the ocean are today cold because they
+receive their water from high latitudes where it sinks because of low
+temperature. Suppose, however, that a diminution in storminess combined
+with other conditions should permit corals to grow in latitude 70�N. The
+ocean temperature would then have to average scarcely lower than 20�C.
+and even in the coldest month the water could scarcely fall below about
+15�C. Under such conditions, if the polar ocean were freely connected
+with the rest of the oceans, no part of it would probably have a
+temperature much below 10�C., for there would be no such thing as ice
+caps and snowfields to reflect the scanty sunlight and radiate into
+space what little heat there was. On the contrary, during the winter an
+almost constant state of dense fogginess would prevail. So great would
+be the blanketing effect of this that a minimum monthly temperature of
+10�C. for the coldest part of the ocean may perhaps be too low for a
+time when corals thrived in latitude 70�.
+
+The temperature of the ocean depths cannot permanently remain lower than
+that of the coldest parts of the surface. Temporarily this might indeed
+happen when a solar change first reduced the storminess and strengthened
+the westerlies and the surface currents. Gradually, however, the
+persistent deep-sea circulation would bring up the colder water in low
+latitudes and carry downward the water of medium temperature at the
+coldest part of the surface. Thus in time the whole body of the ocean
+would become warm. The heat which at present is carried away from the
+earth's surface in storms would slowly accumulate in the oceans. As the
+process went on, all parts of the ocean's surface would become warmer,
+for equatorial latitudes would be less and less cooled by cold water
+from below, while the water blown from low latitudes to high would be
+correspondingly warmer. The warming of the ocean would come to an end
+only with the attainment of a state of equilibrium in which the loss of
+heat by radiation and evaporation from the ocean's surface equaled the
+loss which under other circumstances would arise from the rise of warm
+air in cyclonic storms. When once the oceans were warmed, they would
+form an extremely strong conservative force tending to preserve an
+equable climate in all latitudes and at all seasons. According to the
+solar cyclonic hypothesis such conditions ought to have prevailed
+throughout most of geological time. Only after a strong and prolonged
+solar disturbance with its consequent storminess would conditions like
+those of today be expected.
+
+In this connection another possibility may be mentioned. It is commonly
+assumed that the earth's axis is held steadily in one direction by the
+fact that the rotating earth is a great gyroscope. Having been tilted to
+a certain position, perhaps by some extraneous force, the axis is
+supposed to maintain that position until some other force intervenes.
+Cordeiro,[67] however, maintains that this is true only of an absolutely
+rigid gyroscope. He believes that it is mathematically demonstrable that
+if an elastic gyroscope be gradually tilted by some extraneous force,
+and if that force then ceases to act, the gyroscope as a whole will
+oscillate back and forth. The earth appears to be slightly elastic.
+Cordeiro therefore applies his formul� to it, on the following
+assumptions: (1) That the original position of the axis was nearly
+vertical to the plane of the ecliptic in which the earth revolves around
+the sun; (2) that at certain times the inclination has been even greater
+than now; and (3) that the position of the axis with reference to the
+earth has not changed to any great extent, that is, the earth's poles
+have remained essentially stationary with reference to the earth,
+although the whole earth has been gyroscopically tilted back and forth
+repeatedly.
+
+With a vertical axis the daylight and darkness in all parts of the earth
+would be of equal duration, being always twelve hours. There would be no
+seasons, and the climate would approach the average condition now
+experienced at the two equinoxes. On the whole the climate of high
+latitudes would give the impression of being milder than now, for there
+would be less opportunity for the accumulation of snow and ice with
+their strong cooling effect. On the other hand, if the axis were tilted
+more than now, the winter nights would be longer and the winters more
+severe than at present, and there would be a tendency toward glaciation.
+Thus Cordeiro accounts for alternating mild and glacial epochs. The
+entire swing from the vertical position to the maximum inclination and
+back to the vertical may last millions of years depending on the earth's
+degree of elasticity. The swing beyond the vertical position in the
+other direction would be equally prolonged. Since the axis is now
+supposed to be much nearer its maximum than its minimum degree of
+tilting, the duration of epochs having a climate more severe than that
+of the present would be relatively short, while the mild epochs would be
+long.
+
+Cordeiro's hypothesis has been almost completely ignored. One reason is
+that his treatment of geological facts, and especially his method of
+riding rough-shod over widely accepted conclusions, has not commended
+his work to geologists. Therefore they have not deemed it worth while to
+urge mathematicians to test the assumptions and methods by which he
+reached his results. It is perhaps unfair to test Cordeiro by geology,
+for he lays no claim to being a geologist. In mathematics he labors
+under the disadvantage of having worked outside the usual professional
+channels, so that his work does not seem to have been subjected to
+sufficiently critical analysis.
+
+Without expressing any opinion as to the value of Cordeiro's results we
+feel that the subject of the earth's gyroscopic motion and of a possible
+secular change in the direction of the axis deserves investigation for
+two chief reasons. In the first place, evidences of seasonal changes and
+of seasonal uniformity seem to occur more or less alternately in the
+geological record. Second, the remarkable discoveries of Garner and
+Allard[68] show that the duration of daylight has a pronounced effect
+upon the reproduction of plants. We have referred repeatedly to the tree
+ferns, corals, and other forms of life which now live in relatively low
+latitudes and which cannot endure strong seasonal contrasts, but which
+once lived far to the north. On the other hand, Sayles,[69] for example,
+finds that microscopical examination of the banding of ancient shales
+and slates indicates distinct seasonal banding like that of recent
+Pleistocene clays or of the Squantum slate formed during or near the
+Permian glacial period. Such seasonal banding is found in rocks of
+various ages: (a) Huronian, in cobalt shales previously reported by
+Coleman; (b) late Proterozoic or early Cambrian in Hiwassee slate; (c)
+lower Cambrian, in Georgian slates of Vermont; (d) lower Ordovician, in
+Georgia (Rockmart slate), Tennessee (Athens shale), Vermont (slates),
+and Quebec (Beekmantown formation); and (e) Permian in Massachusetts
+(Squantum slate). How far the periods during which such evidence of
+seasons was recorded really alternated with mild periods, when tropical
+species lived in high latitudes and the contrast of seasons was almost
+or wholly lacking, we have as yet no means of knowing. If periods
+characterized by marked seasonal changes should be found to have
+alternated with those when the seasons were of little importance, the
+fact would be of great geological significance.
+
+The discoveries of Garner and Allard as to the effect of light on
+reproduction began with a peculiar tobacco plant which appeared in some
+experiments at Washington. The plant grew to unusual size, and seemed to
+promise a valuable new variety. It formed no seeds, however, before the
+approach of cold weather. It was therefore removed to a greenhouse where
+it flowered and produced seed. In succeeding years the flowering was
+likewise delayed till early winter, but finally it was discovered that
+if small plants were started in the greenhouse in the early fall they
+flowered at the same time as the large ones. Experiments soon
+demonstrated that the time of flowering depends largely upon the length
+of the daily period when the plants are exposed to light. The same is
+true of many other plants, and there is great variety in the conditions
+which lead to flowering. Some plants, such as witch hazel, appear to be
+stimulated to bloom by very short days, while others, such as evening
+primrose, appear to require relatively long days. So sensitive are
+plants in this respect that Garner and Allard, by changing the length of
+the period of light, have caused a flowerbud in its early stages not
+only to stop developing but to return once more to a vegetative shoot.
+
+    Common iris, which flowers in May and June, will not blossom under
+    ordinary conditions when grown in the greenhouse in winter, even
+    under the same temperature conditions that prevail in early summer.
+    Again, one variety of soy beans will regularly begin to flower in
+    June of each year, a second variety in July, and a third in August,
+    when all are planted on the same date. There are no temperature
+    differences during the summer months which could explain these
+    differences in time of flowering; and, since "internal causes" alone
+    cannot be accepted as furnishing a satisfactory explanation, some
+    external factor other than temperature must be responsible.
+
+    The ordinary varieties of cosmos regularly flower in the fall in
+    northern latitudes if they are planted in the spring or summer. If
+    grown in a warm greenhouse during the winter months the plants also
+    flower readily, so that the cooler weather of fall is not a
+    necessary condition. If successive plantings of cosmos are made in
+    the greenhouse during the late winter and early spring months,
+    maintaining a uniform temperature throughout, the plantings made
+    after a certain date will fail to blossom promptly, but, on the
+    contrary, will continue to grow till the following fall, thus
+    flowering at the usual season for this species. This curious
+    reversal of behavior with advance of the season cannot be attributed
+    to change in temperature. Some other factor is responsible for the
+    failure of cosmos to blossom during the summer months. In this
+    respect the behavior of cosmos is just the opposite of that observed
+    in iris.
+
+    Certain varieties of soy beans change their behavior in a peculiar
+    manner with advance of the summer season. The variety known as
+    Biloxi, for example, when planted early in the spring in the
+    latitude of Washington, D. C., continues to grow throughout the
+    summer, flowering in September. The plants maintain growth without
+    flowering for fifteen to eighteen weeks, attaining a height of five
+    feet or more. As the dates of successive plantings are moved forward
+    through the months of June and July, however there is a marked
+    tendency for the plants to cut short the period of growth which
+    precedes flowering. This means, of course, that there is a tendency
+    to flower at approximately the same time of year regardless of the
+    date of planting. As a necessary consequence, the size of the plants
+    at the time of flowering is reduced in proportion to the delay in
+    planting.
+
+The bearing of this on geological problems lies in a query which it
+raises as to the ability of a genus or family of plants to adapt itself
+to days of very different length from those to which it is wonted. Could
+tree ferns, ginkgos, cycads, and other plants whose usual range of
+location never subjects them to daylight for more than perhaps fourteen
+hours or less than ten, thrive and reproduce themselves if subjected to
+periods of daylight ranging all the way from nothing up to about
+twenty-four hours? No answer to this is yet possible, but the question
+raises most interesting opportunities of investigation. If Cordeiro is
+right as to the earth's elastic gyroscopic motion, there may have been
+certain periods when a vertical or almost vertical axis permitted the
+days to be of almost equal length at all seasons in all latitudes. If
+such an absence of seasons occurred when the lands were low, when the
+oceans were extensive and widely open toward the poles, and when storms
+were relatively inactive, the result might be great mildness of climate
+such as appears sometimes to have prevailed in the middle of geological
+eras. Suppose on the other hand that the axis should be tilted more than
+now, and that the lands should be widely emergent and the storm belt
+highly active in low latitudes, perhaps because of the activity of the
+sun. The conditions might be favorable for glaciation at latitudes as
+low as those where the Permo-Carboniferous ice sheets appear to have
+centered. The possibilities thus suggested by Cordeiro's hypothesis are
+so interesting that the gyroscopic motion of the earth ought to be
+investigated more thoroughly. Even if no such gyroscopic motion takes
+place, however, the other causes of mild climate discussed in this
+chapter may be enough to explain all the observed phenomena.
+
+Many important biological consequences might be drawn from this study of
+mild geological climates, but this book is not the place for them. In
+the first chapter we saw that one of the most remarkable features of the
+climate of the earth is its wonderful uniformity through hundreds of
+millions of years. As we come down through the vista of years the mild
+geological periods appear to represent a return as nearly as possible to
+this standard condition of uniformity. Certain changes of the earth
+itself, as we shall see in the next chapter, may in the long run tend
+slightly to change the exact conditions of this climatic standard, as we
+might perhaps call it. Yet they act so slowly that their effect during
+hundreds of millions of years is still open to question. At most they
+seem merely to have produced a slight increase in diversity from season
+to season and from zone to zone. The normal climate appears still to be
+of a milder type than that which happens to prevail at present. Some
+solar condition, whose possible nature will be discussed later, seems
+even now to cause the number of cyclonic storms to be greater than
+normal. Hence the earth's climate still shows something of the great
+diversity of seasons and of zones which is so marked a characteristic of
+glacial epochs.
+
+FOOTNOTES:
+
+[Footnote 58: Rollin T. Chamberlin: Personal Communication.]
+
+[Footnote 59: F. H. Knowlton: Evolution of Geologic Climates; Bull.
+Geol. Soc. Am., Vol. 30, 1919, pp. 499-566.]
+
+[Footnote 60: Chas. Schuchert: Review of Knowlton's Evolution of
+Geological Climates, in Am. Jour. Sci., 1921.]
+
+[Footnote 61: G. R. Wieland: Distribution and Relationships of the
+Cycadeoids; Am. Jour. Bot., Vol. 7, 1920, pp. 125-145.]
+
+[Footnote 62: D. T. MacDougal: Botanical Features of North American
+Deserts; Carnegie Instit. of Wash., No. 99, 1908.]
+
+[Footnote 63: _Loc. cit._]
+
+[Footnote 64: H. H. Clayton: Variation in Solar Radiation and the
+Weather; Smiths. Misc. Coll., Vol. 71, No. 3, Washington, 1920.]
+
+[Footnote 65: B. Helland Hansen and F. Nansen: Temperature Variations in
+the North Atlantic Ocean and in the Atmosphere; Misc. Coll., Smiths.
+Inst., Vol. 70, No. 4, Washington, 1920.]
+
+[Footnote 66: The climatic significance of ocean currents is well
+discussed in Croll's Climate and Time, 1875, and his Climate and
+Cosmogony, 1889.]
+
+[Footnote 67: F. J. B. Cordeiro: The Gyroscope, 1913.]
+
+[Footnote 68: W. W. Garner and H. A. Allard: Flowering and Fruition of
+Plants as Controlled by Length of Day; Yearbook Dept. Agri., 1920, pp.
+377-400.]
+
+[Footnote 69: Report of Committee on Sedimentation, National Research
+Council, April, 1922.]
+
+
+
+
+CHAPTER XI
+
+TERRESTRIAL CAUSES OF CLIMATIC CHANGES
+
+
+The major portion of this book has been concerned with the explanation
+of the more abrupt and extreme changes of climate. This chapter and the
+next consider two other sorts of climatic changes, the slight secular
+progression during the hundreds of millions of years of recorded earth
+history, and especially the long slow geologic oscillations of millions
+or tens of millions of years. It is generally agreed among geologists
+that the progressive change has tended toward greater extremes of
+climate; that is, greater seasonal contrasts, and greater contrasts from
+place to place and from zone to zone.[70] The slow cyclic changes have
+been those that favored widespread glaciation at one extreme near the
+ends of geologic periods and eras, and mild temperatures even in
+subpolar regions at the other extreme during the medial portions of the
+periods.
+
+As has been pointed out in an earlier chapter, it has often been assumed
+that all climatic changes are due to terrestrial causes. We have seen,
+however, that there is strong evidence that solar variations play a
+large part in modifying the earth's climate. We have also seen that no
+known terrestrial agency appears to be able to produce the abrupt
+changes noted in recent years, the longer cycles of historical times, or
+geological changes of the shorter type, such as glaciation.
+Nevertheless, terrestrial changes doubtless have assisted in producing
+both the progressive change and the slow cyclic changes recorded in the
+rocks, and it is the purpose of this chapter and the two that follow to
+consider what terrestrial changes have taken place and the probable
+effect of such changes.
+
+The terrestrial changes that have a climatic significance are numerous.
+Some, such as variations in the amount of volcanic dust in the higher
+air, have been considered in an earlier chapter. Others are too
+imperfectly known to warrant discussion, and in addition there are
+presumably others which are entirely unknown. Doubtless some of these
+little known or unknown changes have been of importance in modifying
+climate. For example, the climatic influence of vegetation, animals, and
+man may be appreciable. Here, however, we shall confine ourselves to
+purely physical causes, which will be treated in the following order:
+First, those concerned with the solid parts of the earth, namely: (I)
+amount of land; (II) distribution of land; (III) height of land; (IV)
+lava flows; and (V) internal heat. Second, those which arise from the
+salinity of oceans, and third, those depending on the composition and
+amount of atmosphere.
+
+The terrestrial change which appears indirectly to have caused the
+greatest change in climate is the contraction of the earth. The problem
+of contraction is highly complex and is as yet only imperfectly
+understood. Since only its results and not its processes influence
+climate, the following section as far as page 196 is not necessary to
+the general reader. It is inserted in order to explain why we assume
+that there have been oscillations between certain types of distribution
+of the lands.
+
+The extent of the earth's contraction may be judged from the shrinkage
+indicated by the shortening of the rock formations in folded mountains
+such as the Alps, Juras, Appalachians, and Caucasus. Geologists are
+continually discovering new evidence of thrust faults of great magnitude
+where masses of rock are thrust bodily over other rocks, sometimes for
+many miles. Therefore, the estimates of the amount of shrinkage based on
+the measurements of folds and faults need constant revision upward.
+Nevertheless, they have already reached a considerable figure. For
+example, in 1919, Professor A. Heim estimated the shortening of the
+meridian passing through the modern Alps and the ancient Hercynian and
+Caledonian mountains as fully a thousand miles in Europe, and over five
+hundred miles for the rest of this meridian.[71] This is a radial
+shortening of about 250 miles. Possibly the shrinkage has been even
+greater than this. Chamberlin[72] has compared the density of the earth,
+moon, Mars, and Venus with one another, and found it probable that the
+radial shrinkage of the earth may be as much as 570 miles. This result
+is not so different from Heim's as appears at first sight, for Heim made
+no allowance for unrecognized thrust faults and for the contraction
+incident to metamorphism. Moreover, Heim did not include shrinkage
+during the first half of geological time before the above-mentioned
+mountain systems were upheaved.
+
+According to a well-established law of physics, contraction of a
+rotating body results in more rapid rotation and greater centrifugal
+force. These conditions must increase the earth's equatorial bulge and
+thereby cause changes in the distribution of land and water. Opposed to
+the rearrangement of the land due to increased rotation caused by
+contraction, there has presumably been another rearrangement due to
+tidal retardation of the earth's rotation and a consequent lessening of
+the equatorial bulge. G. H. Darwin long ago deduced a relatively large
+retardation due to lunar tides. A few years ago W. D. MacMillan, on
+other assumptions, deduced only a negligible retardation. Still more
+recently Taylor[73] has studied the tides of the Irish Sea, and his work
+has led Jeffreys[74] and Brown[75] to conclude that there has been
+considerable retardation, perhaps enough, according to Brown, to equal
+the acceleration due to the earth's contraction. From a prolonged and
+exhaustive study of the motions of the moon Brown concludes that tidal
+friction or some other cause is now lengthening the day at the rate of
+one second per thousand years, or an hour in almost four million years
+if the present rate continues. He makes it clear that the retardation
+due to tides would not correspond in point of time with the acceleration
+due to contraction. The retardation would occur slowly, and would take
+place chiefly during the long quiet periods of geologic history, while
+the acceleration would occur rapidly at times of diastrophic
+deformation. As a consequence, the equatorial bulge would alternately be
+reduced at a slow rate, and then somewhat suddenly augmented.
+
+The less rigid any part of the earth is, the more quickly it responds to
+the forces which lead to bulging or which tend to lessen the bulge.
+Since water is more fluid than land, the contraction of the earth and
+the tidal retardation presumably tend alternately to increase and
+decrease the amount of water near the equator more than the amount of
+land. Thus, throughout geological history we should look for cyclic
+changes in the relative area of the lands within the tropics and similar
+changes of opposite phase in higher latitudes. The extent of the change
+would depend upon (a) the amount of alteration in the speed of rotation,
+and (b) the extent of low land in low latitudes and of shallow sea in
+high latitudes. According to Slichter's tables, if the earth should
+rotate in twenty-three hours instead of twenty-four, the great Amazon
+lowland would be submerged by the inflow of oceanic water, while wide
+areas in Hudson Bay, the North Sea, and other northern regions, would
+become land because the ocean water would flow away from them.[76]
+
+Following the prompt equatorward movement of water which would occur as
+the speed of rotation increased, there must also be a gradual movement
+or creepage of the solid rocks toward the equator, that is, a bulging of
+the ocean floor and of the lands in low latitudes, with a consequent
+emergence of the lands there and a relative rise of sea level in higher
+latitudes. Tidal retardation would have a similar effect. Suess[77] has
+described widespread elevated strand lines in the tropics which he
+interprets as indicating a relatively sudden change in sea level, though
+he does not suggest a cause of the change. However, in speaking of
+recent geological times, Suess reports that a movement more recent than
+the old strands "was an accumulation of water toward the equator, a
+diminution toward the poles, and (it appears) as though this last
+movement were only one of the many oscillations which succeed each other
+with the same tendency, i.e., with a positive excess at the equator, a
+negative excess at the poles." (Vol. II, p. 551.) This creepage of the
+rocks equatorward seemingly might favor the growth of mountains in
+tropical and subtropical regions, because it is highly improbable that
+the increase in the bulge would go on in all longitudes with perfect
+uniformity. Where it went on most rapidly mountains would arise. That
+such irregularity of movement has actually occurred is suggested not
+only by the fact that many Cenozoic and older mountain ranges extend
+east and west, but by the further fact that these include some of our
+greatest ranges, many of which are in fairly low latitudes. The
+Himalayas, the Javanese ranges, and the half-submerged Caribbean chains
+are examples. Such mountains suggest a thrust in a north and south
+direction which is just what would happen if the solid mass of the earth
+were creeping first equatorward and then poleward.
+
+A fact which is in accord with the idea of a periodic increase in the
+oceans in low latitudes because of renewed bulging at the equator is the
+exposure in moderately high latitudes of the greatest extent of ancient
+rocks. This seems to mean that in low latitudes the frequent deepening
+of the oceans has caused the old rocks to be largely covered by
+sediments, while the old lands in higher latitudes have been left more
+fully exposed to erosion.
+
+Another suggestion of such periodic equatorward movements of the ocean
+water is found in the reported contrast between the relative stability
+with which the northern part of North America has remained slightly
+above sea level except at times of widespread submergence, while the
+southern parts have suffered repeated submergence alternating with great
+emergence.[78] Furthermore, although the northern part of North America
+has been generally exposed to erosion since the Proterozoic, it has
+supplied much less sediment than have the more southern land areas.[79]
+This apparently means that much of Canada has stood relatively low,
+while repeated and profound uplift alternating with depression has
+occurred in subtropical latitudes, apparently in adjustment to changes
+in the earth's speed of rotation. The uplifts generally followed the
+times of submergence due to equatorward movement of the water, though
+the buckling of the crust which accompanies shrinkage doubtless caused
+some of the submergence. The evidence that northern North America stood
+relatively low throughout much of geological time depends not only on
+the fact that little sediment came to the south from the north, but also
+on the fact that at times of especially widespread epicontinental seas,
+the submergence was initiated at the north.[80] This is especially true
+for Ordovician, Silurian, Devonian, and Jurassic times in North America.
+General submergence of this kind is supposed to be due chiefly to the
+overflowing of the ocean when its level is slowly raised by the
+deposition of sediment derived from the erosion of what once were
+continental highlands but later are peneplains. The fact that such
+submergence began in high latitudes, however, seems to need a further
+explanation. The bulging of the rock sphere at the equator and the
+consequent displacement of some of the water in low latitudes would
+furnish such an explanation, as would also a decrease in the speed of
+rotation induced by tidal retardation, if that retardation were great
+enough and rapid enough to be geologically effective.
+
+The climatic effects of the earth's contraction, which we shall shortly
+discuss, are greatly complicated by the fact that contraction has taken
+place irregularly. Such irregularity has occurred in spite of the fact
+that the processes which cause contraction have probably gone on quite
+steadily throughout geological history. These processes include the
+chemical reorganization of the minerals of the crust, a process which is
+illustrated by the metamorphism of sedimentary rocks into crystalline
+forms. The escape of gases through volcanic action or otherwise has been
+another important process.
+
+Although the processes which cause contraction probably go on steadily,
+their effect, as Chamberlin[81] and others have pointed out, is probably
+delayed by inertia. Thus the settling of the crust or its movement on a
+large scale is delayed. Perhaps the delay continues until the stresses
+become so great that of themselves they overcome the inertia, or
+possibly some outside agency, whose nature we shall consider later,
+re�nforces the stresses and gives the slight impulse which is enough to
+release them and allow the earth's crust to settle into a new state of
+equilibrium. When contraction proceeds actively, the ocean segments,
+being largest and heaviest, are likely to settle most, resulting in a
+deepening of the oceans and an emergence of the lands. Following each
+considerable contraction there would be an increase in the speed of
+rotation. The repeated contractions with consequent growth of the
+equatorial bulge would alternate with long quiet periods during which
+tidal retardation would again decrease the speed of rotation and hence
+lessen the bulge. The result would be repeated changes of distribution
+of land and water, with consequent changes in climate.
+
+I. We shall now consider the climatic effect of the repeated changes in
+the relative amounts of land and water which appear to have resulted
+from the earth's contraction and from changes in its speed of rotation.
+During many geologic epochs a larger portion of the earth was covered
+with water than at present. For example, during at least twelve out of
+about twenty epochs, North America has suffered extensive
+inundations,[82] and in general the extensive submergence of Europe, the
+other area well known geologically, has coincided with that of North
+America. At other times, the ocean has been less extensive than now, as
+for example during the recent glacial period, and probably during
+several of the glacial periods of earlier date. Each of the numerous
+changes in the relative extent of the lands must have resulted in a
+modification of climate.[83] This modification would occur chiefly
+because water becomes warm far more slowly than land, and cools off far
+more slowly.
+
+An increase in the lands would cause changes in several climatic
+conditions. (a) The range of temperature between day and night and
+between summer and winter would increase, for lands become warmer by day
+and in summer than do oceans, and cooler at night and in winter. The
+higher summer temperature when the lands are widespread is due chiefly
+to the fact that the land, if not snow-covered, absorbs more of the
+sun's radiant energy than does the ocean, for its reflecting power is
+low. The lower winter temperature when lands are widespread occurs not
+only because they cool off rapidly but because the reduced oceans cannot
+give them so much heat. Moreover, the larger the land, the more
+generally do the winds blow outward from it in winter and thus prevent
+the ocean heat from being carried inland. So long as the ocean is not
+frozen in high latitudes, it is generally the chief source of heat in
+winter, for the nights are several months long near the poles, and even
+when the sun does shine its angle is so low that reflection from the
+snow is very great. Furthermore, although on the average there is more
+reflection from water than from land, the opposite is true in high
+latitudes in winter when the land is snow-covered while the ocean is
+relatively dark and is roughened by the waves. Another factor in causing
+large lands to have extremely low temperature in winter is the fact that
+in proportion to their size they are less protected by fog and cloud
+than are smaller areas. The belt of cloud and fog which is usually
+formed when the wind blows from the ocean to the relatively cold land is
+restricted to the coastal zone. Thus the larger the land, the smaller
+the fraction in which loss of heat by radiation is reduced by clouds and
+fogs. Hence an increase in the land area is accompanied by an increase
+in the contrasts in temperature between land and water.
+
+(b) The contrasts in temperature thus produced must cause similar
+contrasts in atmospheric pressure, and hence stronger barometric
+gradients. (c) The strong gradients would mean strong winds, flowing
+from land to sea or from sea to land. (d) Local convection would also be
+strengthened in harmony with the expansion of the lands, for the more
+rapid heating of land than of water favors active convection.
+
+(e) As the extent of the ocean diminished, there would normally be a
+decrease in the amount of water vapor for three reasons: (1) Evaporation
+from the ocean is the great source of water vapor. Other conditions
+being equal, the smaller the ocean becomes, the less the evaporation.
+(2) The amount of water vapor in the air diminishes as convection
+increases, since upward convection is a chief method by which
+condensation and precipitation are produced, and water vapor removed
+from the atmosphere. (3) Nocturnal cooling sufficient to produce dew and
+frost is very much more common upon land than upon the ocean. The
+formation of dew and frost diminishes the amount of water vapor at least
+temporarily. (f) Any diminution in water vapor produced in these ways,
+or otherwise, is significant because water vapor is the most essential
+part of the atmosphere so far as regulation of temperature is concerned.
+It tends to keep the days from becoming hot or the nights cold.
+Therefore any decrease in water vapor would increase the diurnal and
+seasonal range of temperature, making the climate more extreme and
+severe. Thus a periodic increase in the area of the continents would
+clearly make for periodic increased climatic contrasts, with great
+extremes, a type of climatic change which has recurred again and again.
+Indeed, each great glaciation accompanied or followed extensive
+emergence of the lands.[84]
+
+Whether or not there has been a _progressive_ increase from era to era
+in the area of the lands is uncertain. Good authorities disagree widely.
+There is no doubt, however, that at present the lands are more extensive
+than at most times in the past, though smaller, perhaps, than at certain
+periods. The wide expanse of lands helps explain the prominence of
+seasons at present as compared with the past.
+
+II. The contraction of the earth, as we have seen, has produced great
+changes in the distribution as well as in the extent of land and water.
+Large parts of the present continents have been covered repeatedly by
+the sea, and extensive areas now covered with water have been land. In
+recent geological times, that is, during the Pliocene and Pleistocene,
+much of the present continental shelf, the zone less than 600 feet below
+sea level, was land. If the whole shelf had been exposed, the lands
+would have been greater than at present by an area larger than North
+America. When the lands were most elevated, or a little earlier, North
+America was probably connected with Asia and almost with Europe. Asia in
+turn was apparently connected with the larger East Indian islands. In
+much earlier times land occupied regions where now the ocean is fairly
+deep. Groups of islands, such as the East Indies and Malaysia and
+perhaps the West Indies, were united into widespreading land masses.
+Figs. 7 and 9, illustrating the paleography of the Permian and the
+Cretaceous periods, respectively, indicate a land distribution radically
+different from that of today.
+
+So far as appears from the scattered facts of geological history, the
+changes in the distribution of land seem to have been marked by the
+following characteristics: (1) Accompanying the differentiation of
+continental and oceanic segments of the earth's crust, the oceans have
+become somewhat deeper, and their basins perhaps larger, while the
+continents, on the average, have been more elevated and less subject to
+submergence. Hence there have been less radical departures from the
+present distribution during the relatively recent Cenozoic era than in
+the ancient Paleozoic because the submergence of continental areas has
+become less general and less frequent. For example, the last extensive
+epeiric or interior sea in North America was in the Cretaceous, at least
+ten million years ago, and according to Barrell perhaps fifty million,
+while in Europe, according to de Lapparent,[85] a smaller share of the
+present continent has been submerged since the Cretaceous than before.
+Indeed, as in North America, the submergence has decreased on the
+average since the Paleozoic era. (2) The changes in distribution of land
+which have taken place during earth history have been cyclic.
+Repeatedly, at the close of each of the score or so of geologic periods,
+the continents emerged more or less, while at the close of the groups of
+periods known as eras, the lands were especially large and emergent.
+After each emergence, a gradual encroachment of the sea took place, and
+toward the close of several of the earlier periods, the sea appears to
+have covered a large fraction of the present land areas. (3) On the
+whole, the amount of land in the middle and high latitudes of the
+northern hemisphere appears to have increased during geologic time. Such
+an increase does not require a growth of the continents, however, in the
+broader sense of the term, but merely that a smaller fraction of the
+continent and its shelf should be submerged. (4) In tropical latitudes,
+on the other hand, the extent of the lands seems to have decreased,
+apparently by the growth of the ocean basins. South America and Africa
+are thought by many students to have been connected, and Africa was
+united with India via Madagascar, as is suggested in Fig. 9. The most
+radical cyclic as well as the most radical progressive changes in land
+distribution also seem to have taken place in tropical regions.[86]
+
+[Illustration: _Fig. 9. Cretaceous Paleogeography._
+(_After Schuchert._)]
+
+Although there is much evidence of periodic increase of the sea in
+equatorial latitudes and of land in high latitudes, it has remained for
+the zo�logist Metcalf to present a very pretty bit of evidence that at
+certain times submergence along the equator coincided with emergence in
+high latitudes, and vice versa. Certain fresh water frogs which carry
+the same internal parasite are confined to two widely separated areas in
+tropical and south temperate America and in Australia. The extreme
+improbability that both the frogs and the parasites could have
+originated independently in two unconnected areas and could have
+developed by convergent evolution so that they are almost identical in
+the two continents makes it almost certain that there must have been a
+land connection between South America and Australia, presumably by way
+of Antarctica. The facts as to the parasites seem also to prove that
+while the land connection existed there was a sea across South America
+in equatorial latitudes. The parasite infests not only the frogs but the
+American toads known as Bufo. Now Bufo originated north of the equator
+in America and differs from the frogs which originated in southern South
+America in not being found in Australia. This raises the question of how
+the frogs could go to Australia via Antarctica carrying the parasite
+with them, while the toads could not go. Metcalf's answer is that the
+toads were cut off from the southern part of South America by an
+equatorial sea until after the Antarctic connection between the Old
+World and the New was severed.
+
+    As Patagonia let go of Antarctica by subsidence of the intervening
+    land area, there was a probable concomitant rise of land through
+    what is now middle South America and the northern and southern
+    portions of this continent came together.[87]
+
+These various changes in the earth's crust have given rise to certain
+specific types of distribution of the lands, which will now be
+considered. We shall inquire what climatic conditions would arise from
+changes in (a) the continuity of the lands from north to south, (b) the
+amount of land in tropical latitudes, and (c) the amount of land in
+middle and high latitudes.
+
+(a) At present the westward drift of warm waters, set in motion by the
+trade winds, is interrupted by land masses and turned poleward,
+producing the important Gulf Stream Drift and Japan Current in the
+northern hemisphere, and corresponding, though less important, currents
+in the southern hemisphere. During the past, quite different sets of
+ocean currents doubtless have existed in response to a different
+distribution of land. Repeatedly, in the mid-Cretaceous (Fig. 9) and
+several other periods, the present American barrier to the westward
+moving tropical current was broken in Central America. Even if the
+supposed continent of "Gondwana Land" extended from Africa to South
+America in equatorial latitudes, strong currents must still have flowed
+westward along its northern shore under the impulse of the peculiarly
+strong trade winds which the equatorial land would create. Nevertheless
+at such times relatively little warm tropical water presumably entered
+the North Atlantic, for it escaped into the Pacific. At several other
+times, such as the late Ordovician and mid-Devonian, when the isthmian
+barrier existed, it probably turned an important current northward into
+what is now the Mississippi Basin instead of into the Atlantic. There it
+traversed an epeiric, or mid-continental sea open to both north and
+south. Hence its effectiveness in warming Arctic regions must have been
+quite different from that of the present Gulf Stream.
+
+(b) We will next consider the influences of changes in the amount of
+equatorial and tropical land. As such lands are much hotter than the
+corresponding seas, the intensity and width of the equatorial belt of
+low pressure must be great when they are extensive. Hence the trade
+winds must have been stronger than now whenever tropical lands were more
+extensive than at present. This is because the trades are produced by
+the convection due to excessive heat along the heat equator. There the
+air expands upward and flows poleward at high altitudes. The trade wind
+consists of air moving toward the heat equator to take the place of the
+air which there rises. When the lands in low latitudes were wide the
+trade winds must also have dominated a wide belt. The greater width of
+the trade-wind belt today over Africa than over the Atlantic illustrates
+the matter. The belt must have been still wider when Gondwana Land was
+large, as it is believed to have been during the Paleozoic era and the
+early Mesozoic.
+
+An increase in the width of the equatorial belt of low pressure under
+the influence of broad tropical lands would be accompanied not only by
+stronger and more widespread trade winds, but by a corresponding
+strengthening of the subtropical belts of high pressure. The chief
+reason would be the greater expansion of the air in the equatorial low
+pressure belt and the consequent more abundant outflow of air at high
+altitudes in the form of anti-trades or winds returning poleward above
+the trades. Such winds would pile up the air in the region of the
+high-pressure belt. Moreover, since the meridians converge as one
+proceeds away from the equator, the air of the poleward-moving
+anti-trades tends to be crowded as it reaches higher latitudes, thus
+increasing the pressure. Unless there were a corresponding increase in
+tropical cyclones, one of the most prominent results of the strengthened
+trades and the intensified subtropical high-pressure belt at times of
+broad lands in low latitudes would be great deserts. It will be recalled
+that the trade-wind lowlands and the extra-tropical belt of highs are
+the great desert belts at present. The trade-wind lowlands are desert
+because air moving into warmer latitudes takes up water except where it
+is cooled by rising on mountain-sides. The belt of highs is arid because
+there, too, air is being warmed, but in this case by descending from
+aloft.
+
+Again, if the atmospheric pressure in the subtropical belt should be
+intensified, the winds flowing poleward from this belt would necessarily
+become stronger. These would begin as southwesterlies in the northern
+hemisphere and northwesterlies in the southern. In the preceding chapter
+we have seen that such winds, especially when cyclonic storms are few
+and mild, are a powerful agent in transferring subtropical heat
+poleward. If the strength of the westerlies were increased because of
+broad lands in low latitudes, their efficacy in transferring heat would
+be correspondingly augmented. It is thus evident that any change in the
+extent of tropical lands during the geologic past must have had
+important climatic consequences in changing the velocity of the
+atmospheric circulation and in altering the transfer of heat from low
+latitudes to high. When the equatorial and tropical lands were broad the
+winds and currents must have been strong, much heat must have been
+carried away from low latitudes, and the contrast between low and high
+latitudes must have been relatively slight. As we have already remarked,
+leading paleogeographers believe that changes in the extent of the lands
+have been especially marked in low latitudes, and that on the average
+there has been a decrease in the extent of land within the tropics.
+Gondwana Land is the greatest illustration of this. In the same way, on
+the numerous paleogeographic maps of North America, most
+paleogeographers have shown fairly extensive lands south of the latitude
+of the United States during most of the geologic epochs.[88]
+
+(c) There is evidence that during geologic history the area of the lands
+in middle and high latitudes, as well as in low latitudes, has changed
+radically. An increase in such lands would cause the winters to grow
+colder. This would be partly because of the loss of heat by radiation
+into the cold dry air over the continents in winter, and partly because
+of increased reflection from snow and frost, which gather much more
+widely upon the land than upon the ocean. Furthermore, in winter when
+the continents are relatively cold, there is a strong tendency for winds
+to blow out from the continent toward the ocean. The larger the land the
+stronger this tendency. In Asia it gives rise to strong winter monsoons.
+The effect of such winds is illustrated by the way in which the
+westerlies prevent the Gulf Stream from warming the eastern United
+States in winter. The Gulf Stream warms northwestern Europe much more
+than the United States because, in Europe, the prevailing winds are
+onshore.
+
+Another effect of an increase in the area of the lands in middle and
+high latitudes would be to interpose barriers to oceanic circulation and
+thus lower the temperature of polar regions. This would not mean
+glaciation in high latitudes, however, even when the lands were
+widespread as in the Mesozoic and early Tertiary. Students of glaciology
+are more and more thoroughly convinced that glaciation depends on the
+availability of moisture even more than upon low temperature.
+
+In conclusion it may be noted that each of the several climatic
+influences of increased land area in the high latitudes would tend to
+increase the contrasts between land and sea, between winter and summer,
+and between low latitudes and high. In other words, so far as the effect
+upon high latitudes themselves is concerned, an expansion of the lands
+there would tend in the same direction as a diminution in low latitudes.
+In so far as the general trend of geological evolution has been toward
+more land in high latitudes and less in low, it would help to produce a
+progressive increase in climatic diversity such as is faintly indicated
+in the rock strata. On the other hand, the oscillations in the
+distribution of the lands, of which geology affords so much evidence,
+must certainly have played an important part in producing the periodic
+changes of climate which the earth has undergone.
+
+III. Throughout geological history there is abundant evidence that the
+process of contraction has led to marked differences not only in the
+distribution and area of the lands, but in their height. On the whole
+the lands have presumably increased in height since the Proterozoic,
+somewhat in proportion to the increased differentiation of continents
+and oceans.[89] If there has been such an increase, the contrast between
+the climate of ocean and land must have been accentuated, for highlands
+have a greater diurnal and seasonal range of temperature than do
+lowlands. The ocean has very little range of either sort. The large
+range at high altitudes is due chiefly to the small quantity of water
+vapor, for this declines steadily with increased altitude. A diminution
+in the density of the other constituents of the air also decreases the
+blanketing effect of the atmosphere. In conformity with the great
+seasonal range in temperature at times when the lands stand high, the
+direction of the wind would be altered. When the lands are notably
+warmer than the oceans, the winds commonly flow from land to sea, and
+when the continents are much colder than the oceans, the direction is
+reversed. The monsoons of Asia are examples. Strong seasonal winds
+disturb the normal planetary circulation of the trade winds in low
+latitudes and of the westerlies in middle latitudes. They also interfere
+with the ocean currents set in motion by the planetary winds. The net
+result is to hinder the transfer of heat from low latitudes to high, and
+thus to increase the contrasts between the zones. Local as well as zonal
+contrasts are also intensified. The higher the land, the greater,
+relatively speaking, are the cloudiness and precipitation on seaward
+slopes, and the drier the interior. Indeed, most highlands are arid.
+Henry's[90] recent study of the vertical distribution of rainfall on
+mountain-sides indicates that a decrease sets in at about 3500 feet in
+the tropics and only a little higher in mid-latitudes.
+
+In addition to the main effects upon atmospheric circulation and
+precipitation, each of the many upheavals of the lands must have been
+accompanied by many minor conditions which tended toward diversity. For
+example, the streams were rejuvenated, and instead of meandering perhaps
+over vast flood plains they intrenched their channels and in many cases
+dug deep gorges. The water table was lowered, soil was removed from
+considerable areas, the bare rock was exposed, and the type of dominant
+vegetation altered in many places. An almost barren ridge may represent
+all that remains of what was once a vast forested flood plain. Thus,
+increased elevation of the land produces contrasted conditions of slope,
+vegetation, availability of ground water, exposure to wind and so forth,
+and these unite in diversifying climate. Where mountains are formed,
+strong contrasts are sure to occur. The windward slopes may be very
+rainy, while neighboring leeward slopes are parched by a dry foehn wind.
+At the same time the tops may be snow-covered. Increased local contrasts
+in climatic conditions are known to influence the intensity of cyclonic
+storms,[91] and these affect the climatic conditions of all middle and
+high latitudes, if not of the entire earth. The paths followed by
+cyclonic storms are also altered by increased contrast between land and
+water. When the continents are notably colder than the neighboring
+oceans, high atmospheric pressure develops on the lands and interferes
+with the passage of lows, which are therefore either deflected around
+the continent or forced to move slowly.
+
+The distribution of lofty mountains has an even more striking climatic
+effect than the general uplift of a region. In Proterozoic times there
+was a great range in the Lake Superior region; in the late Devonian the
+Acadian mountains of New England and the Maritime Provinces of Canada
+possibly attained a height equal to the present Rockies. Subsequently,
+in the late Paleozoic a significant range stood where the Ouachitas now
+are. Accompanying the uplift of each of these ranges, and all others,
+the climate of the surrounding area, especially to leeward, must have
+been altered greatly. Many extensive salt deposits found now in fairly
+humid regions, for example, the Pennsylvanian and Permian deposits of
+Kansas and Oklahoma, were probably laid down in times of local aridity
+due to the cutting off of moisture-bearing winds by the mountains of
+Llanoria in Louisiana and Texas. Hence such deposits do not necessarily
+indicate periods of widespread and profound aridity.
+
+When the causes of ancient glaciation were first considered by
+geologists, about the middle of the nineteenth century, it was usually
+assumed that the glaciated areas had been elevated to great heights, and
+thus rendered cold enough to permit the accumulation of glaciers. The
+many glaciers occurring in the Alps of central Europe where glaciology
+arose doubtless suggested this explanation. However, it is now known
+that most of the ancient glaciation was not of the alpine type, and
+there is adequate proof that the glacial periods cannot be explained as
+due directly and solely to uplift. Nevertheless, upheavals of the lands
+are among the most important factors in controlling climate, and
+variations in the height of the lands have doubtless assisted in
+producing climate oscillations, especially those of long duration.
+Moreover, the progressive increase in the height of the lands has
+presumably played a part in fostering local and zonal diversity in
+contrast with the relative uniformity of earlier geological times.
+
+IV. The contraction of the earth has been accompanied by volcanic
+activity as well as by changes in the extent, distribution, and altitude
+of the lands. The probable part played by volcanic dust as a
+contributory factor in producing short sudden climatic variations has
+already been discussed. There is, however, another though probably less
+important respect in which volcanic activity may have had at least a
+slight climatic significance. The oldest known rocks, those of the
+Archean era, contain so much igneous matter that many students have
+assumed that they show that the entire earth was once liquid. It is now
+considered that they merely indicate igneous activity of great
+magnitude. In the later part of Proterozoic time, during the second
+quarter of the earth's history according to Schuchert's estimate, there
+were again vast outflowings of lava. In the Lake Superior district, for
+example, a thickness of more than a mile accumulated over a large area,
+and lavas are common in many areas where rocks of this age are known.
+The next quarter of the earth's history elapsed without any
+correspondingly great outflows so far as is known, though several lesser
+ones occurred. Toward the end of the last quarter, and hence quite
+recently from the geological standpoint, another period of outflows,
+perhaps as noteworthy as that of the Proterozoic, occurred in the
+Cretaceous and Tertiary.
+
+The climatic effects of such extensive lava flows would be essentially
+as follows: In the first place so long as the lavas were hot they would
+set up a local system of convection with inflowing winds. This would
+interfere at least a little with the general winds of the area. Again,
+where the lava flowed out into water, or where rain fell upon hot lava,
+there would be rapid evaporation which would increase the rainfall. Then
+after the lava had cooled, it would still influence climate a trifle in
+so far as its color was notably darker or lighter than that of the
+average surface. Dark surfaces absorb solar heat and become relatively
+warm when the sun shines upon them. Dark objects likewise radiate heat
+more rapidly than light-colored objects. Hence they cool more rapidly at
+night, and in the winter. As most lavas are relatively dark they
+increase the average diurnal range of temperature. Hence even after they
+are cool they increase the climatic diversity of the land.
+
+The amount of heat given to the atmosphere by an extensive lava flow,
+though large according to human standards, is small compared with the
+amount received from the sun by a like area, except during the first few
+weeks or months before the lava has formed a thick crust. Furthermore,
+probably only a small fraction of any large series of flows occurred in
+a given century or millennium. Moreover, even the largest lava flows
+covered an area of only a few hundredths of one per cent of the earth's
+surface. Nevertheless, the conditions which modify climate are so
+complicated that it would be rash to state that this amount of
+additional heat has been of no climatic significance. Like the
+proverbial "straw that broke the camel's back," the changes it would
+surely produce in local convection, atmospheric pressure, and the
+direction of the wind may have helped to shift the paths of storms and
+to produce other complications which were of appreciable climatic
+significance.
+
+V. The last point which we shall consider in connection with the effect
+of the earth's interior upon climate is internal heat. The heat given
+off by lavas is merely a small part of that which is emitted by the
+earth as a whole. In the earliest part of geological history enough heat
+may have escaped from the interior of the earth to exert a profound
+influence on the climate. Knowlton,[92] as we have seen, has recently
+built up an elaborate theory on this assumption. At present, however,
+accurate measurements show that the escape of heat is so slight that it
+has no appreciable influence except in a few volcanic areas. It is
+estimated to raise the average temperature of the earth's surface less
+than 0.1�C.[93]
+
+In order to contribute enough heat to raise the surface temperature
+1�C., the temperature gradient from the interior of the earth to the
+surface would need to be ten times as great as now, for the rate of
+conduction varies directly with the gradient. If the gradient were ten
+times as great as now, the rocks at a depth of two and one-half miles
+would be so hot as to be almost liquid according to Barrell's[94]
+estimates. The thick strata of unmetamorphosed Paleozoic rocks indicate
+that such high temperatures have not prevailed at such slight depths
+since the Proterozoic. Furthermore, the fact that the climate was cold
+enough to permit glaciation early in the Proterozoic era and at from one
+to three other times before the opening of the Paleozoic suggests that
+the rate of escape of heat was not rapid even in the first half of the
+earth's recorded history. Yet even if the general escape of heat has
+never been large since the beginning of the better-known part of
+geological history, it was presumably greater in early times than at
+present.
+
+If there actually has been an appreciable decrease in the amount of heat
+given out by the earth's interior, its effects would agree with the
+observed conditions of the geological record. It would help to explain
+the relative mildness of zonal, seasonal, and local contrasts of climate
+in early geological times, but it would not help to explain the long
+oscillations from era to era which appear to have been of much greater
+importance. Those oscillations, so far as we can yet judge, may have
+been due in part to solar changes, but in large measure they seem to be
+explained by variations in the extent, distribution, and altitude of the
+lands. Such variations appear to be the inevitable result of the earth's
+contraction.
+
+FOOTNOTES:
+
+[Footnote 70: Chas. Schuchert: The Earth's Changing Surface and Climate
+during Geologic Time; in Lull: The Evolution of the Earth and Its
+Inhabitants, 1918, p. 55.]
+
+[Footnote 71: Quoted by J. Cornet: Cours de G�ologie, 1920, p. 330.]
+
+[Footnote 72: T. C. Chamberlin: The Order of Magnitude of the Shrinkage
+of the Earth; Jour. Geol., Vol. 28, 1920, pp. 1-17, 126-157.]
+
+[Footnote 73: G. I. Taylor: Philosophical Transactions, A. 220, 1919,
+pp. 1-33; Monthly Notices Royal Astron. Soc., Jan., 1920, Vol. 80,
+p. 308.]
+
+[Footnote 74: J. Jeffreys: Monthly Notices Royal Astron. Soc., Jan.,
+1920, Vol. 80, p. 309.]
+
+[Footnote 75: E. W. Brown: personal communication.]
+
+[Footnote 76: C. S. Slichter: The Rotational Period of a Heterogeneous
+Spheroid; in Contributions to the Fundamental Problems of Geology, by
+T. C. Chamberlin, _et al._, Carnegie Inst. of Wash., No. 107, 1909.]
+
+[Footnote 77: E. Suess: The Face of the Earth, Vol. II, p. 553, 1901.]
+
+[Footnote 78: Chas. Schuchert: The Earth's Changing Surface and Climate;
+in Lull: The Evolution of the Earth and Its Inhabitants, 1918, p. 78.]
+
+[Footnote 79: J. Barren: Rhythms and the Measurement of Geologic Time;
+Bull. Geol. Soc. Am., Vol. 28, 1917, p. 838.]
+
+[Footnote 80: Chas. Schuchert: _loc. cit._, p. 78.]
+
+[Footnote 81: T. C. Chamberlin: Diastrophism, the Ultimate Basis of
+Correlation; Jour. Geol., Vol. 16, 1909; Chas. Schuchert: _loc. cit._]
+
+[Footnote 82: Pirsson-Schuchert: Textbook of Geology, 1915, Vol. II, p.
+982; Chas. Schuchert: Paleogeography of North America; Bull. Geol. Soc.
+Am., Vol. 20, pp. 427-606; reference on p. 499.]
+
+[Footnote 83: The general subject of the climatic significance of
+continentality is discussed by C. E. P. Brooks: continentality and
+Temperature; Quart. Jour. Royal Meteorol. Soc., April, 1917, and Oct.,
+1918.]
+
+[Footnote 84: Chas. Schuchert: Climates of Geologic Time; in The
+Climatic Factor; Carnegie Institution, 1914, p. 286.]
+
+[Footnote 85: A. de Lapparent: Trait� de G�ologie, 1906.]
+
+[Footnote 86: Chas. Schuchert: Historical Geology, 1915, p. 464.]
+
+[Footnote 87: M. M. Metcalf: Upon an important method of studying
+problems of relationship and of geographical distribution; Proceedings
+National Academy of Sciences, Vol. 6, July, 1920, pp. 432-433.]
+
+[Footnote 88: Chas. Schuchert: Paleogeography of North America; Bull.
+Geol. Soc. Am., Vol. 20, 1910; and Willis, Salisbury, and others:
+Outlines of Geologic History, 1910.]
+
+[Footnote 89: Chas. Schuchert: The Earth's Changing Surface and Climate;
+in Lull: The Evolution of the Earth and Its Inhabitants, 1918, p. 50.]
+
+[Footnote 90: A. J. Henry: The Decrease of Precipitation with Altitude;
+Monthly Weather Review, Vol. 47, 1919, pp. 33-41.]
+
+[Footnote 91: Chas. F. Brooks: Monthly Weather Review, Vol. 46, 1918, p.
+511; and also A. J. Henry and others: Weather Forecasting in the United
+States, 1913.]
+
+[Footnote 92: F. H. Knowlton: Evolution of Geologic Climates; Bull.
+Geol. Soc. Am., Vol. 30, Dec., 1919, pp. 499-566.]
+
+[Footnote 93: Talbert, quoted by I. Bowman: Forest Physiography, 1911,
+p. 63.]
+
+[Footnote 94: J. Barrell: Rhythms and the Measurement of Geologic Time;
+Bull. Geol. Soc. Am., Vol. 28, 1917, pp. 745-904.]
+
+
+
+
+CHAPTER XII
+
+POST-GLACIAL CRUSTAL MOVEMENTS AND CLIMATIC CHANGES
+
+
+An interesting practical application of some of the preceding
+generalizations is found in an attempt by C. E. P. Brooks[95] to
+interpret post-glacial climatic changes almost entirely in terms of
+crustal movement. We believe that he carries the matter much too far,
+but his discussion is worthy of rather full recapitulation, not only
+for its theoretical value but because it gives a good summary of
+post-glacial changes. His climatic table for northwest Europe as
+reprinted from the annual report of the Smithsonian Institution for
+1917, p. 366, is as follows:
+
+      _Phase_                      _Climate_                _Date_
+
+  1. The Last Great            Arctic climate.      30,000-18,000 B. C.
+       Glaciation.
+  2. The Retreat of the        Severe continental   18,000-6000 B. C.
+       Glaciers.                 climate.
+  3. The Continental Phase.    Continental climate.  6000-4000 B. C.
+  4. The Maritime Phase.       Warm and moist.       4000-3000 B. C.
+  5. The Later Forest Phase.   Warm and dry.         3000-1800 B. C.
+  6. The Peat-Bog Phase.       Cooler and moister.  1800 B. C.-300 A. D.
+  7. The Recent Phase.         Becoming drier.       300 A. D.-
+
+Brooks bases his chronology largely on De Geer's measurements of the
+annual layers of clay in lake bottoms but makes much use of other
+evidence. According to Brooks the last glacial epoch lasted roughly from
+30,000 to 18,000 B. C., but this includes a slight amelioration of
+climate followed by a readvance of the ice, known as the Buhl stage.
+During the time of maximum glaciation the British Isles stood twenty or
+thirty feet higher than now and Scandinavia was "considerably" more
+elevated. The author believes that this caused a fall of 1�C. in the
+temperature of the British Isles and of 2�C. in Scandinavia. By an
+ingenious though not wholly convincing method of calculation he
+concludes that this lowering of temperature, aided by an increase in the
+area of the lands, sufficed to start an ice sheet in Scandinavia. The
+relatively small area of ice cooled the air and gave rise to an area of
+high barometric pressure. This in turn is supposed to have caused
+further expansion of the ice and to have led to full-fledged glaciation.
+
+About 18,000 B. C. the retreat of the ice began in good earnest. Even
+though no evidence has yet been found, Brooks believes there must have
+been a change in the distribution of land and sea to account for the
+diminution of the ice. The ensuing millenniums formed the Magdalenian
+period in human history, the last stage of the Paleolithic, when man
+lived in caves and reindeer were abundant in central Europe.[96] At
+first the ice retreated very slowly and there were periods when for
+scores of years the ice edge remained stationary or even readvanced.
+About 10,000 B. C. the edge of the ice lay along the southern coast of
+Sweden. During the next 2000 years it withdrew more rapidly to about
+59�N. Then came the Fennoscandian pause, or Gschnitz stage, when for
+about 200 years the ice edge remained in one position, forming a great
+moraine. Brooks suggests that this pause about 8000 B. C. was due to the
+closing of the connection between the Atlantic Ocean and the Baltic Sea
+and the synchronous opening of a connection between the Baltic and the
+White Seas, whereby cold Arctic waters replaced the warmer Atlantic
+waters. He notes, however, that about 7500 B. C. the obliquity of the
+ecliptic was probably nearly 1� greater than at present. This he
+calculates to have caused the climate of Germany and Sweden to be 1�F.
+colder than at present in winter and 1�F. warmer in summer.
+
+The next climatic stage was marked by a rise of temperature till about
+6000 B. C. During this period the ice at first retreated, presumably
+because the climate was ameliorating, although no cause of such
+amelioration is assigned. At length the ice lay far enough north to
+allow a connection between the Baltic and the Atlantic by way of Lakes
+Wener and Wetter in southern Sweden. This is supposed to have warmed the
+Baltic Sea and to have caused the climate to become distinctly milder.
+Next the land rose once more so that the Baltic was separated from the
+Atlantic and was converted into the Ancylus lake of fresh water. The
+southwest Baltic region then stood 400 feet higher than now. The result
+was the Daun stage, about 5000 B. C., when the ice halted or perhaps
+readvanced a little, its front being then near Ragunda in about latitude
+63�. Why such an elevation did not cause renewed glaciation instead of
+merely the slight Daun pause, Brooks does not explain, although his
+calculations as to the effect of a slight elevation of the land during
+the main period of glaciation from 30,000 to 18,000 B. C. would seem to
+demand a marked readvance.
+
+After 5000 B. C. there ensued a period when the climate, although still
+distinctly continental, was relatively mild. The winters, to be sure,
+were still cold but the summers were increasingly warm. In Sweden, for
+example, the types of vegetation indicate that the summer temperature
+was 7�F. higher than now. Storms, Brooks assumes, were comparatively
+rare except on the outer fringe of Great Britain. There they were
+sufficiently abundant so that in the Northwest they gave rise to the
+first Peat-Bog period, during which swamps replaced forests of birch and
+pine. Southern and eastern England, however, probably had a dry
+continental climate. Even in northwest Norway storms were rare as is
+indicated by remains of forests on islands now barren because of the
+strong winds and fierce storms. Farther east most parts of central and
+northern Europe were relatively dry. This was the early Neolithic period
+when man advanced from the use of unpolished to polished stone
+implements.
+
+Not far from 4000 B. C. the period of continental climate was replaced
+by a comparatively moist maritime climate. Brooks believes that this was
+because submergence opened the mouth of the Baltic and caused the fresh
+Ancylus lake to give place to the so-called Litorina sea. The
+temperature in Sweden averaged about 3�F. higher than at present and in
+southwestern Norway 2�. More important than this was the small annual
+range of temperature due to the fact that the summers were cool while
+the winters were mild. Because of the presence of a large expanse of
+water in the Baltic region, storms, as our author states, then crossed
+Great Britain and followed the Baltic depression, carrying the moisture
+far inland. In spite of the additional moisture thus available the snow
+line in southern Norway was higher than now.
+
+At this point Brooks turns to other parts of the world. He states that
+not far from 4000 B. C., a submergence of the lands, rarely amounting to
+more than twenty-five feet, took place not only in the Baltic region but
+in Ireland, Iceland, Spitzbergen, and other parts of the Arctic Ocean,
+as well as in the White Sea, Greenland, and the eastern part of North
+America. Evidences of a mild climate are found in all those places.
+Similar evidence of a mild warm climate is found in East Africa, East
+Australia, Tierra del Fuego, and Antarctica. The dates are not
+established with certainty but they at least fall in the period
+immediately preceding the present epoch. In explanation of these
+conditions Brooks assumes a universal change of sea level. He suggests
+with some hesitation that this may have been due to one of Pettersson's
+periods of maximum "tide-generating force." According to Pettersson the
+varying positions of the moon, earth, and sun cause the tides to vary in
+cycles of about 9, 90, and 1800 years, though the length of the periods
+is not constant. When tides are high there is great movement of ocean
+waters and hence a great mixture of the water at different latitudes.
+This is supposed to cause an amelioration of climate. The periods of
+maximum and minimum tide-generating force are as follows:
+
+  Maxima 3500 B. C.--------2100 B. C.--------350 B. C.-------A. D. 1434
+  Minima ---------2800 B. C.--------1200 B. C.-------A. D. 530---------
+
+Brooks thinks that the big trees in California and the Norse sagas and
+Germanic myths indicate a rough agreement of climatic phenomena with
+Pettersson's last three dates, while the mild climate of 4000 B. C. may
+really belong to 3500 B. C. He gives no evidence confirming Pettersson's
+view at the other three dates.
+
+To return to Brooks' sketch of the relation of climatic pulsations to
+the altitude of the lands, by 3000 B. C., that is, toward the close of
+the Neolithic period, further elevation is supposed to have taken place
+over the central latitudes of western Europe. Southern Britain, which
+had remained constantly above its present level ever since 30,000 B. C.,
+was perhaps ninety feet higher than now. Ireland was somewhat enlarged
+by elevation, the Straits of Dover were almost closed, and parts of the
+present North Sea were land. To these conditions Brooks ascribes the
+prevalence of a dry continental climate. The storms shifted northward
+once more, the winds were mild, as seems to be proved by remains of
+trees in exposed places; and forests replaced fields of peat and heath
+in Britain and Germany. The summers were perhaps warmer than now but the
+winters were severe. The relatively dry climate prevailed as far west as
+Ireland. For example, in Drumkelin Bog in Donegal County a corded oak
+road and a two-story log cabin appear to belong to this time. Fourteen
+feet of bog lie below the floor and twenty-six above. This period,
+perhaps 3000-2000 B. C., was the legendary heroic age of Ireland when
+"the vigour of the Irish reached a level not since attained." This, as
+Brooks points out, may have been a result of the relatively dry climate,
+for today the extreme moisture of Ireland seems to be a distinct
+handicap. In Scandinavia, civilization, or at least the stage of
+relative progress, was also high at this time.
+
+By 1600 B. C. the land had assumed nearly its present level in the
+British Isles and the southern Baltic region, while northern Scandinavia
+still stood lower than now. The climate of Britain and Germany was so
+humid that there was an extensive formation of peat even on high ground
+not before covered. This moist stage seems to have lasted almost to the
+time of Christ, and may have been the reason why the Romans described
+Britain as peculiarly wet and damp. At this point Brooks again departs
+from northwest Europe to a wider field:
+
+    It is possible that we have to attribute this damp period in
+    Northwest Europe to some more general cause, for Ellsworth
+    Huntington's curves of tree-growth in California and climate in
+    Western Asia both show moister conditions from about 1000 B. C. to
+    A. D. 200, and the same author believes that the Mediterranean lands
+    had a heavier rainfall about 500 B. C. to A. D. 200. It seems that
+    the phase was marked by a general increase of the storminess of the
+    temperate regions of the northern hemisphere at least, with a
+    maximum between Ireland and North Germany, indicating probably that
+    the Baltic again became the favourite track of depressions from the
+    Atlantic.
+
+Brooks ends his paper with a brief r�sum� of glacial changes in North
+America, but as the means of dating events are unreliable the degree of
+synchronism with Europe is not clear. He sums up his conclusions as
+follows:
+
+    On the whole it appears that though there is a general similarity in
+    the climatic history of the two sides of the North Atlantic, the
+    changes are not really contemporaneous, and such relationship as
+    appears is due mainly to the natural similarity in the geographical
+    history of two regions both recovering from an Ice Age, and only
+    very partially to world-wide pulsations of climate. Additional
+    evidence on this head will be available when Baron de Geer publishes
+    the results of his recent investigations of the seasonal glacial
+    clays of North America, especially if, as he hopes, he is able to
+    correlate the banding of these clays with the growth-rings of the
+    big trees.
+
+    When we turn to the northwest of North America, this is brought out
+    very markedly. For in Yukon and Alaska the Ice Age was a very mild
+    affair compared with its severity in eastern America and
+    Scandinavia. As the land had not a heavy ice-load to recover from,
+    there were no complicated geographical changes. Also, there were no
+    fluctuations of climate, but simply a gradual passage to present
+    conditions. The latter circumstance especially seems to show that
+    the emphasis laid on geographical rather than astronomical factors
+    of _great_ climatic changes is not misplaced.
+
+Brooks' painstaking discussion of post-glacial climatic changes is of
+great value because of the large body of material which he has so
+carefully wrought together. His strong belief in the importance of
+changes in the level of the lands deserves serious consideration. It is
+difficult, however, to accept his final conclusion that such changes are
+the main factors in recent climatic changes. It is almost impossible,
+for example, to believe that movements of the land could produce almost
+the same series of climatic changes in Europe, Central Asia, the western
+and eastern parts of North America, and the southern hemisphere. Yet
+such changes appear to have occurred during and since the glacial
+period. Again there is no evidence whatever that movements of the land
+have anything to do with the historic cycles of climate or with the
+cycles of weather in our own day, which seem to be the same as glacial
+cycles on a small scale. Also, as Dr. Simpson points out in discussing
+Brooks' paper, there appears "no solution along these lines of the
+problem connected with rich vegetation in both polar circles and the
+ice-age which produced the ice-sheet at sea-level in Northern India."
+Nevertheless, we may well believe that Brooks is right in holding that
+changes in the relative level and relative area of land and sea have had
+important local effects. While they are only one of the factors involved
+in climatic changes, they are certainly one that must constantly be kept
+in mind.
+
+FOOTNOTES:
+
+[Footnote 95: C. E. P. Brooks: The Evolution of Climate in Northwest
+Europe. Quart. Jour. Royal Meteorol. Soc., Vol. 47, 1921, pp. 173-194.]
+
+[Footnote 96: H. F. Osborn: Men of the Old Stone Age, N. Y., 1915; J. M.
+Tyler: The New Stone Age in Northwestern Europe, N. Y., 1920.]
+
+
+
+
+CHAPTER XIII
+
+THE CHANGING COMPOSITION OF OCEANS AND ATMOSPHERE
+
+
+Having discussed the climatic effect of movements of the earth's crust
+during the course of geological time, we are now ready to consider the
+corresponding effects due to changes in the movable envelopes--the
+oceans and the atmosphere. Variations in the composition of sea water
+and of air and in the amount of air must almost certainly have occurred,
+and must have produced at least slight climatic consequences. It should
+be pointed out at once that such variations appear to be far less
+important climatically than do movements of the earth's crust and
+changes in the activity of the sun. Moreover, in most cases, they are
+not reversible as are the crustal and solar phenomena. Hence, while most
+of them appear to have been unimportant so far as climatic oscillations
+and fluctuations are concerned, they seemingly have aided in producing
+the slight secular progression to which we have so often referred.
+
+There is general agreement among geologists that the ocean has become
+increasingly saline throughout the ages. Indeed, calculations of the
+rate of accumulation of salt have been a favorite method of arriving at
+estimates of the age of the ocean, and hence of the earliest marine
+sediments. So far as known, however, no geologist or climatologist has
+discussed the probable climatic effects of increased salinity. Yet it
+seems clear that an increase in salinity must have a slight effect upon
+climate.
+
+Salinity affects climate in four ways: (1) It appreciably influences the
+rate of evaporation; (2) it alters the freezing point; (3) it produces
+certain indirect effects through changes in the absorption of carbon
+dioxide; and (4) it has an effect on oceanic circulation.
+
+(1) According to the experiments of Mazelle and Okada, as reported by
+Kr�mmel,[97] evaporation from ordinary sea water is from 9 to 30 per
+cent less rapid than from fresh water under similar conditions. The
+variation from 9 to 30 per cent found in the experiments depends,
+perhaps, upon the wind velocity. When salt water is stagnant, rapid
+evaporation tends to result in the development of a film of salt on the
+top of the water, especially where it is sheltered from the wind. Such a
+film necessarily reduces evaporation. Hence the relatively low salinity
+of the oceans in the past probably had a tendency to increase the amount
+of water vapor in the air. Even a little water vapor augments slightly
+the blanketing effect of the air and to that extent diminishes the
+diurnal and seasonal range of temperature and the contrast from zone to
+zone.
+
+(2) Increased salinity means a lower freezing temperature of the oceans
+and hence would have an effect during cold periods such as the present
+and the Pleistocene ice age. It would not, however, be of importance
+during the long warm periods which form most of geologic time. A
+salinity of about 3.5 per cent at present lowers the freezing point of
+the ocean roughly 2�C. below that of fresh water. If the ocean were
+fresh and our winters as cold as now, all the harbors of New England and
+the Middle Atlantic States would be icebound. The Baltic Sea would also
+be frozen each winter, and even the eastern harbors of the British Isles
+would be frequently locked in ice. At high latitudes the area of
+permanently frozen oceans would be much enlarged. The effect of such a
+condition upon marine life in high latitudes would be like that of a
+change to a warmer climate. It would protect the life on the continental
+shelf from the severe battering of winter storms. It would also lessen
+the severity of the winter temperature in the water for when water
+freezes it gives up much latent heat,--eighty calories per cubic
+centimeter. Part of this raises the temperature of the underlying water.
+
+The expansion of the ice near northern shores would influence the life
+of the lands quite differently from that of the oceans. It would act
+like an addition of land to the continents and would, therefore,
+increase the atmospheric contrasts from zone to zone and from
+continental interior to ocean. In summer the ice upon the sea would tend
+to keep the coastal lands cool, very much as happens now near the Arctic
+Ocean, where the ice floes have a great effect through their reflection
+of light and their absorption of heat in melting. In winter the virtual
+enlargement of the continents by the addition of an ice fringe would
+decrease the snowfall upon the lands. Still more important would be the
+effect in intensifying the anti-cyclonic conditions which normally
+prevail in winter not only over continents but over ice-covered oceans.
+Hence the outblowing cold winds would he strengthened.[98] The net
+effect of all these conditions would apparently be a diminution of
+snowfall in high latitudes upon the lands even though the summer
+snowfall upon the ocean and the coasts may have increased. This
+condition may have been one reason why widespread glaciation does not
+appear to have prevailed in high latitudes during the Proterozoic and
+Permian glaciations, even though it occurred farther south. If the ocean
+during those early glacial epochs were ice-covered down to middle
+latitudes, a lack of extensive glaciation in high latitudes would be no
+more surprising than is the lack of Pleistocene glaciation in the
+northern parts of Alaska and Asia. Great ice sheets are impossible
+without a large supply of moisture.
+
+(3) Among the indirect effects of salinity one of the chief appears to
+be that the low salinity of the water in the past and the greater ease
+with which it froze presumably allowed the temperature of the entire
+ocean to be slightly higher than now. This is because ice serves as a
+blanket and hinders the radiation of heat from the underlying water. The
+temperature of the ocean has a climatic significance not only directly,
+but indirectly through its influence on the amount of carbon dioxide
+held by the oceans. A change of even 1�C. from the present mean
+temperature of 2�C. would alter the ability of the entire ocean to
+absorb carbon dioxide by about 4 per cent. This, according to F. W.
+Clarke,[99] is because the oceans contain from eighteen to twenty-seven
+times as much carbon dioxide as the air when only the free carbon
+dioxide is considered, and about seventy times as much according to
+Johnson and Williamson[100] when the partially combined carbon dioxide
+is also considered. Moreover, the capacity of water for carbon dioxide
+varies sharply with the temperature.[101] Hence a rise in temperature of
+only 1�C. would theoretically cause the oceans to give up from 30 to 280
+times as much carbon dioxide as the air now holds. This, however, is on
+the unfounded assumption that the oceans are completely saturated. The
+important point is merely that a slight change in ocean temperature
+would cause a disproportionately large change in the amount of carbon
+dioxide in the air with all that this implies in respect to blanketing
+the earth, and thus altering temperature.
+
+(4) Another and perhaps the most important effect of salinity upon
+climate depends upon the rapidity of the deep-sea circulation. The
+circulation is induced by differences of temperature, but its speed is
+affected at least slightly by salinity. The vertical circulation is now
+dominated by cold water from subpolar latitudes. Except in closed seas
+like the Mediterranean the lower portions of the ocean are near the
+freezing point. This is because cold water sinks in high latitudes by
+reason of its superior density, and then "creeps" to low latitudes.
+There it finally rises and replaces either the water driven poleward by
+the winds, or that which has evaporated from the Surface.[102]
+
+During past ages, when the sea water was less salty, the circulation was
+presumably more rapid than now. This was because, in tropical regions,
+the rise of cold water is hindered by the sinking of warm surface water
+which is relatively dense because evaporation has removed part of the
+water and caused an accumulation of salt. According to Kr�mmel and
+Mill,[103] the surface salinity of the subtropical belt of the North
+Atlantic commonly exceeds 3.7 per cent and sometimes reaches 3.77 per
+cent, whereas the underlying waters have a salinity of less than 3.5 per
+cent and locally as little as 3.44 per cent. The other oceans are
+slightly less saline than the North Atlantic at all depths, but the
+vertical salinity gradients along the tropics are similar. According to
+the Smithsonian Physical Tables, the difference in salinity between the
+surface water and that lying below is equivalent to a difference of .003
+in density, where the density of fresh water is taken as 1.000. Since
+the decrease in density produced by warming water from the temperature
+of its greatest density (4�C.) to the highest temperatures which ever
+prevail in the ocean (30�C. or 86�F.) is only .004, the more saline
+surface waters of the dry tropics are at most times almost as dense as
+the less saline but colder waters beneath the surface, which have come
+from higher latitudes. During days of especially great evaporation,
+however, the most saline portions of the surface waters in the dry
+tropics are denser than the underlying waters and therefore sink, and
+produce a temporary local stagnation in the general circulation. Such a
+sinking of the warm surface waters is reported by Kr�mmel, who detected
+it by means of the rise in temperature which it produces at considerable
+depths. If such a hindrance to the circulation did not exist, the
+velocity of the deep-sea movements would be greater.
+
+If in earlier times a more rapid circulation occurred, low latitudes
+must have been cooled more than now by the rise of cold waters. At the
+same time higher latitudes were presumably warmed by a greater flow of
+warm water from tropical regions because less of the surface heat sank
+in low latitudes. Such conditions would tend to lessen the climatic
+contrast between the different latitudes. Hence, in so far as the rate
+of deep-sea circulation depends upon salinity, the slowly increasing
+amount of salt in the oceans must have tended to increase the contrasts
+between low and high latitudes. Thus for several reasons, the increase
+of salinity during geologic history seems to deserve a place among the
+minor agencies which help to explain the apparent tendency toward a
+secular progression of climate in the direction of greater contrasts
+between tropical and subpolar latitudes.
+
+Changes in the composition and amount of the atmosphere have presumably
+had a climatic importance greater than that of changes in the salinity
+of the oceans. The atmospheric changes may have been either progressive
+or cyclic, or both. In early times, according to the nebular hypothesis,
+the atmosphere was much more dense than now and contained a larger
+percentage of certain constituents, notably carbon dioxide and water.
+The planetesimal hypothesis, on the other hand, postulates an increase
+in the density of the atmosphere, for according to this hypothesis the
+density of the atmosphere depends upon the power of the earth to hold
+gases, and this power increases as the earth grows bigger with the
+infall of material from without.[104]
+
+Whichever hypothesis may be correct, it seems probable that when life
+first appeared on the land the atmosphere resembled that of today in
+certain fundamental respects. It contained the elements essential to
+life, and its blanketing effect was such as to maintain temperatures not
+greatly different from those of the present. The evidence of this
+depends largely upon the narrow limits of temperature within which the
+activities of modern life are possible, and upon the cumulative evidence
+that ancient life was essentially similar to the types now living. The
+resemblance between some of the oldest forms and those of today is
+striking. For example, according to Professor Schuchert:[105] "Many of
+the living genera of forest trees had their origin in the Cretaceous,
+and the giant sequoias of California go back to the Triassic, while
+Ginkgo is known in the Permian. Some of the fresh-water molluscs
+certainly were living in the early periods of the Mesozoic, and the
+lung-fish of today (Ceratodus) is known as far back as the Triassic and
+is not very unlike other lung-fishes of the Devonian. The higher
+vertebrates and insects, on the other hand, are very sensitive to their
+environment, and therefore do not extend back generically beyond the
+Cenozoic, and only in a few instances even as far as the Oligocene. Of
+marine invertebrates the story is very different, for it is well known
+that the horseshoe crab (Limulus) lived in the Upper Jurassic, and
+Nautilus in the Triassic, with forms in the Devonian not far removed
+from this genus. Still longer-ranging genera occur among the
+brachiopods, for living Lingula and Crania have specific representatives
+as far back as the early Ordovician. Among living foraminifers, Lagena,
+Globigerina, and Nodosaria are known in the later Cambrian or early
+Ordovician. In the Middle Cambrian near Field, British Columbia, Walcott
+has found a most varied array of invertebrates among which are
+crustaceans not far removed from living forms. Zo�logists who see these
+wonderful fossils are at once struck with their modernity and the little
+change that has taken place in certain stocks since that far remote
+time. Back of the Paleozoic, little can be said of life from the generic
+standpoint, since so few fossils have been recovered, but what is at
+hand suggests that the marine environment was similar to that of today."
+
+At present, as we have repeatedly seen, little growth takes place either
+among animals or plants at temperatures below 0�C. or above 40�C., and
+for most species the limiting temperatures are about 10� and 30�. The
+maintenance of so narrow a scale of temperature is a function of the
+atmosphere, as well as of the sun. Without an atmosphere, the
+temperature by day would mount fatally wherever the sun rides high in
+the sky. By night it would fall everywhere to a temperature approaching
+absolute zero, that is -273�C. Some such temperature prevails a few
+miles above the earth's surface, beyond the effective atmosphere.
+Indeed, even if the atmosphere were almost as it is now, but only lacked
+one of the minor constituents, a constituent which is often actually
+ignored in statements of the composition of the air, life would be
+impossible. Tyndall concludes that if water vapor were entirely removed
+from the atmosphere for a single day and night, all life--except that
+which is dormant in the form of seeds, eggs, or spores--would be
+exterminated. Part would be killed by the high temperature developed by
+day when the sun was high, and part, by the cold night.
+
+The testimony of ancient glaciation as to the slight difference in the
+climate and therefore in the atmosphere of early and late geological
+times is almost as clear as that of life. Just as life proves that the
+earth can never have been extremely cold during hundreds of millions of
+years, so glaciation in moderately low latitudes near the dawn of earth
+history and at several later times, proves that the earth was not
+particularly hot even in those early days. The gentle progressive change
+of climate which is recorded in the rocks appears to have been only in
+slight measure a change in the mean temperature of the earth as a whole,
+and almost entirely a change in the distribution of temperature from
+place to place and season to season. Hence it seems probable that
+neither the earth's own emission of heat, nor the supply of solar heat,
+nor the power of the atmosphere to retain heat can have been much
+greater a few hundred million years ago than now. It is indeed possible
+that these three factors may have varied in such a way that any
+variation in one has been offset by variations of the others in the
+opposite direction. This, however, is so highly improbable that it seems
+advisable to assume that all three have remained relatively constant.
+This conclusion together with a realization of the climatic significance
+of carbon dioxide has forced most of the adherents of the nebular
+hypothesis to abandon their assumption that carbon dioxide, the heaviest
+gas in the air, was very abundant until taken out by coal-forming plants
+or combined with the calcium oxide of igneous rocks to form the
+limestone secreted by animals. In the same way the presence of sun
+cracks in sedimentary rocks of all ages suggests that the air cannot
+have contained vast quantities of water vapor such as have been assumed
+by Knowlton and others in order to account for the former lack of sharp
+climatic contrast between the zones. Such a large amount of water vapor
+would almost certainly be accompanied by well-nigh universal and
+continual cloudiness so that there would be little chance for the pools
+on the earth's water-soaked surface to dry up. Furthermore, there is
+only one way in which such cloudiness could be maintained and that is by
+keeping the air at an almost constant temperature night and day. This
+would require that the chief source of warmth be the interior of the
+earth, a condition which the Proterozoic, Permian, and other widespread
+glaciations seem to disprove.
+
+Thus there appears to be strong evidence against the radical changes in
+the atmosphere which are sometimes postulated. Yet some changes must
+have taken place, and even minor changes would be accompanied by some
+sort of climatic effect. The changes would take the form of either an
+increase or a decrease in the atmosphere as a whole, or in its
+constituent elements. The chief means by which the atmosphere has
+increased appear to be as follows: (a) By contributions from the
+interior of the earth via volcanoes and springs and by the weathering of
+igneous rocks with the consequent release of their enclosed gases;[106]
+(b) by the escape of some of the abundant gases which the ocean holds in
+solution; (c) by the arrival on the earth of gases from space, either
+enclosed in meteors or as free-flying molecules; (d) by the release of
+gases from organic compounds by oxidation, or by exhalation from animals
+and plants. On the other hand, one or another of the constituents of the
+atmosphere has presumably decreased (a) by being locked up in newly
+formed rocks or organic compounds; (b) by being dissolved in the ocean;
+(c) by the escape of molecules into space; and (d) by the condensation
+of water vapor.
+
+The combined effect of the various means of increase and decrease
+depends partly on the amount of each constituent received from the
+earth's interior or from space, and partly on the fact that the agencies
+which tend to deplete the atmosphere are highly selective in their
+action. Our knowledge of how large a quantity of new gases the air has
+received is very scanty, but judging by present conditions the general
+tendency is toward a slow increase chiefly because of meteorites,
+volcanic action, and the work of deep-seated springs. As to decrease,
+the case is clearer. This is because the chemically active gases,
+oxygen, CO_{2}, and water vapor, tend to be locked up in the rocks,
+while the chemically inert gases, nitrogen and argon, show almost no
+such tendency. Though oxygen is by far the most abundant element in the
+earth's crust, making up more than 50 per cent of the total, it forms
+only about one-fifth of the air. Nitrogen, on the other hand, is very
+rare in the rocks, but makes up nearly four-fifths of the air. It would,
+therefore, seem probable that throughout the earth's history, there has
+been a progressive increase in the amount of atmospheric nitrogen, and
+presumably a somewhat corresponding increase in the mass of the air. On
+the other hand, it is not clear what changes have occurred in the amount
+of atmospheric oxygen. It may have increased somewhat or perhaps even
+notably. Nevertheless, because of the greater increase in nitrogen, it
+may form no greater percentage of the air now than in the distant past.
+
+As to the absolute amounts of oxygen, Barrell[107] thought that
+atmospheric oxygen began to be present only after plants had appeared.
+It will be recalled that plants absorb carbon dioxide and separate the
+carbon from the oxygen, using the carbon in their tissues and setting
+free the oxygen. As evidence of a paucity of oxygen in the air in early
+Proterozoic times, Barrell cites the fact that the sedimentary rocks of
+that remote time commonly are somewhat greyish or greenish-grey wackes,
+or other types, indicating incomplete oxidation. He admits, however,
+that the stupendous thicknesses of red sandstones, quartzite, and
+hematitic iron ores of the later Proterozoic prove that by that date
+there was an abundance of atmospheric oxygen. If so, the change from
+paucity to abundance must have occurred before fossils were numerous
+enough to give much clue to climate. However, Barrell's evidence as to
+a former paucity of atmospheric oxygen is not altogether convincing. In
+the first place, it does not seem justifiable to assume that there could
+be no oxygen until plants appeared to break down the carbon dioxide, for
+some oxygen is contributed by volcanoes,[108] and lightning decomposes
+water into its elements. Part of the hydrogen thus set free escapes into
+space, for the earth's gravitative force does not appear great enough to
+hold this lightest of gases, but the oxygen remains. Thus electrolysis
+of water results in the accumulation of oxygen. In the second place,
+there is no proof that the ancient greywackes are not deoxidized
+sediments. Light colored rock formations do not necessarily indicate
+a paucity of atmospheric oxygen, for such rocks are abundant
+even in recent times. For example, the Tertiary formations are
+characteristically light colored, a result, however, of deoxidation.
+Finally, the fact that sedimentary rocks, irrespective of their age,
+contain an average of about 1.5 per cent more oxygen than do igneous
+rocks,[109] suggests that oxygen was present in the air in quantity even
+when the earliest shales and sandstones were formed, for atmospheric
+oxygen seems to be the probable source of the extra oxygen they contain.
+The formation of these particular sedimentary rocks by weathering of
+igneous rocks involves only a little carbon dioxide and water. Although
+it seems probable that oxygen was present in the atmosphere even at the
+beginning of the geological record, it may have been far less abundant
+then than now. It may have been removed from the atmosphere by animals
+or by the oxidation of the rocks almost as rapidly as it was added by
+volcanoes, plants, and other agencies.
+
+After this chapter was in type, St. John[C] announced his interesting
+discovery that oxygen is apparently lacking in the atmosphere of Venus.
+He considers that this proves that Venus has no life. Furthermore he
+concludes that so active an element as oxygen cannot be abundant in the
+atmosphere of a planet unless plants continually supply large quantities
+by breaking down carbon dioxide.
+
+But even if the earth has experienced a notable increase in atmospheric
+oxygen since the appearance of life, this does not necessarily involve
+important climatic changes except those due to increased atmospheric
+density. This is because oxygen has very little effect upon the passage
+of light or heat, being transparent to all but a few wave lengths. Those
+absorbed are chiefly in the ultra violet.
+
+The distinct possibility that oxygen has increased in amount, makes it
+the more likely that there has been an increase in the total atmosphere,
+for the oxygen would supplement the increase in the relatively inert
+nitrogen and argon, which has presumably taken place. The climatic
+effects of an increase in the atmosphere include, in the first place, an
+increased scattering of light as it approaches the earth. Nitrogen,
+argon, and oxygen all scatter the short waves of light and thus
+interfere with their reaching the earth. Abbot and Fowle,[110] who have
+carefully studied the matter, believe that at present the scattering is
+quantitatively important in lessening insolation. Hence our supposed
+general increase in the volume of the air during part of geological
+times would tend to reduce the amount of solar energy reaching the
+earth's surface. On the other hand, nitrogen and argon do not appear to
+absorb the long wave lengths known as heat, and oxygen absorbs so little
+as to be almost a non-absorber. Therefore the reduced penetration of the
+air by solar radiation due to the scattering of light would apparently
+not be neutralized by any direct increase in the blanketing effect of
+the atmosphere, and the temperature near the earth's surface would be
+slightly lowered by a thicker atmosphere. This would diminish the amount
+of water vapor which would be held in the air, and thereby lower the
+temperature a trifle more.
+
+In the second place, the higher atmospheric pressure which would result
+from the addition of gases to the air would cause a lessening of the
+rate of evaporation, for that rate declines as pressure increases.
+Decreased evaporation would presumably still further diminish the vapor
+content of the atmosphere. This would mean a greater daily and seasonal
+range of temperature, as is very obvious when we compare clear weather
+with cloudy. Cloudy nights are relatively warm while clear nights are
+cool, because water vapor is an almost perfect absorber of radiant heat,
+and there is enough of it in the air on moist nights to interfere
+greatly with the escape of the heat accumulated during the day.
+Therefore, if atmospheric moisture were formerly much more abundant than
+now, the temperature must have been much more uniform. The tendency
+toward climatic severity as time went on would be still further
+increased by the cooling which would result from the increased wind
+velocity discussed below; for cooling by convection increases with the
+velocity of the wind, as does cooling by conduction.
+
+Any persistent lowering of the general temperature of the air would
+affect not only its ability to hold water vapor, but would produce a
+lessening in the amount of atmospheric carbon dioxide, for the colder
+the ocean becomes the more carbon dioxide it can hold in solution. When
+the oceanic temperature falls, part of the atmospheric carbon dioxide is
+dissolved in the ocean. This minor constituent of the air is important
+because although it forms only 0.003 per cent of the earth's atmosphere,
+Abbot and Fowle's[111] calculations indicate that it absorbs over 10 per
+cent of the heat radiated outward from the earth. Hence variations in
+the amount of carbon dioxide may have caused an appreciable variation in
+temperature and thus in other climatic conditions. Humphreys, as we have
+seen, has calculated that a doubling of the carbon dioxide in the air
+would directly raise the earth's temperature to the extent of 1.3�C.,
+and a halving would lower it a like amount. The indirect results of such
+an increase or decrease might be greater than the direct results, for
+the change in temperature due to variations in carbon dioxide would
+alter the capacity of the air to hold moisture.
+
+Two conditions would especially help in this respect; first, changes in
+nocturnal cooling, and second, changes in local convection. The presence
+of carbon dioxide diminishes nocturnal cooling because it absorbs the
+heat radiated by the earth, and re-radiates part of it back again. Hence
+with increased carbon dioxide and with the consequent warmer nights
+there would be less nocturnal condensation of water vapor to form dew
+and frost. Local convection is influenced by carbon dioxide because this
+gas lessens the temperature gradient. In general, the less the gradient,
+that is, the less the contrast between the temperature at the surface
+and higher up, the less convection takes place. This is illustrated by
+the seasonal variation in convection. In summer, when the gradient is
+steepest, convection reaches its maximum. It will be recalled that when
+air rises it is cooled by expansion, and if it ascends far the moisture
+is soon condensed and precipitated. Indeed, local convection is
+considered by C. P. Day to be the chief agency which keeps the lower air
+from being continually saturated with moisture. The presence of carbon
+dioxide lessens convection because it increases the absorption of heat
+in the zone above the level in which water vapor is abundant, thus
+warming these higher layers. The lower air may not be warmed
+correspondingly by an increase in carbon dioxide if Abbot and Fowle are
+right in stating that near the earth's surface there is enough water
+vapor to absorb practically all the wave lengths which carbon dioxide is
+capable of absorbing. Hence carbon dioxide is chiefly effective at
+heights to which the low temperature prevents water vapor from
+ascending. Carbon dioxide is also effective in cold winters and in high
+latitudes when even the lower air is too cold to contain much water
+vapor. Moreover, carbon dioxide, by altering the amount of atmospheric
+water vapor, exerts an indirect as well as a direct effect upon
+temperature.
+
+Other effects of the increase in air pressure which we are here assuming
+during at least the early part of geological times are corresponding
+changes in barometric contrasts, in the strength of winds, and in the
+mass of air carried by the winds along the earth's surface. The increase
+in the mass of the air would re�nforce the greater velocity of the winds
+in their action as eroding and transporting agencies. Because of the
+greater weight of the air, the winds would be capable of picking up more
+dust and of carrying it farther and higher; while the increased
+atmospheric friction would keep it aloft a longer time. The significance
+of dust at high levels and its relation to solar radiation have already
+been discussed in connection with volcanoes. It will be recalled that on
+the average it lowers the surface temperature. At lower levels, since
+dust absorbs heat quickly and gives it out quickly, its presence raises
+the temperature of the air by day and lowers it by night. Hence an
+increase in dustiness tends toward greater extremes.
+
+From all these considerations it appears that if the atmosphere has
+actually evolved according to the supposition which is here tentatively
+entertained, the general tendency of the resultant climatic changes must
+have been partly toward long geological oscillations and partly toward a
+general though very slight increase in climatic severity and in the
+contrasts between the zones. This seems to agree with the geological
+record, although the fact that we are living in an age of relative
+climatic severity may lead us astray.
+
+The significant fact about the whole matter is that the three great
+types of terrestrial agencies, namely, those of the earth's interior,
+those of the oceans, and those of the air, all seem to have suffered
+changes which lead to slow variations of climate. Many reversals have
+doubtless taken place, and the geologic oscillations thus induced are
+presumably of much greater importance than the progressive change, yet
+so far as we can tell the purely terrestrial changes throughout the
+hundreds of millions of years of geological time have tended toward
+complexity and toward increased contrasts from continent to ocean, from
+latitude to latitude, from season to season, and from day to night.
+
+Throughout geological history the slow and almost imperceptible
+differentiation of the earth's surface has been one of the most
+noteworthy of all changes. It has been opposed by the extraordinary
+conservatism of the universe which causes the average temperature today
+to be so like that of hundreds of millions of years ago that many types
+of life are almost identical. Nevertheless, the differentiation has gone
+on. Often, to be sure, it has presumably been completely masked by the
+disturbances of the solar atmosphere which appear to have been the cause
+of the sharper, shorter climatic pulsations. But regardless of cosmic
+conservatism and of solar impulses toward change, the slow
+differentiation of the earth's surface has apparently given to the world
+of today much of the geographical complexity which is so stimulating a
+factor in organic evolution. Such complexity--such diversity from place
+to place--appears to be largely accounted for by purely terrestrial
+causes. It may be regarded as the great terrestrial contribution to the
+climatic environment which guides the development of life.
+
+FOOTNOTES:
+
+[Footnote 97: Encyclop�dia Britannica, 11th edition: article "Ocean."]
+
+[Footnote 98: C. E. P. Brooks: The Meteorological Conditions of an Ice
+sheet and Their Bearing on the Desiccation of the Globe; Quart. Jour.
+Royal Meteorol. Soc., Vol. 40, 1914, pp. 53-70.]
+
+[Footnote 99: Data of Geochemistry, Fourth Ed., 1920; Bull. No. 695, U.
+S. Geol. Survey.]
+
+[Footnote 100: Quoted by Schuchert in The Evolution of the Earth.]
+
+[Footnote 101: Smithsonian Physical Tables, Sixth Revision, 1914, p.
+142.]
+
+[Footnote 102: Chamberlin, in a very suggestive article "On a possible
+reversal of oceanic circulation" (Jour. of Geol., Vol. 14, pp. 363-373,
+1906), discusses the probable climatic consequences of a reversal in the
+direction of deep-sea circulation. It is not wholly beyond the bounds of
+possibility that, in the course of ages the increasing drainage of salt
+from the lands not only by nature but by man's activities in agriculture
+and drainage, may ultimately cause such a reversal by increasing the
+ocean's salinity until the more saline tropical portion is heavier than
+the cooler but fresher subpolar waters. If that should happen,
+Greenland, Antarctica, and the northern shores of America and Asia would
+be warmed by the tropical heat which had been transferred poleward
+beneath the surface of the ocean, without loss _en route_. Subpolar
+regions, under such a condition of reversed deep-sea circulation, might
+have a mild climate. Indeed, they might be among the world's most
+favorable regions climatically.]
+
+[Footnote 103: Encyclop�dia Britannica: article "Ocean."]
+
+[Footnote 104: Chamberlin and Salisbury: Geology, Vol. II, pp. 1-132,
+1906; and T. C. Chamberlin: The Origin of the Earth, 1916.]
+
+[Footnote 105: Personal communication.]
+
+[Footnote 106: R. T. Chamberlin: Gases in Rocks, Carnegie Inst. of
+Wash., No. 106, 1908.]
+
+[Footnote 107: J. Barrell: The Origin of the Earth, in Evolution of the
+Earth and Its Inhabitants, 1918, p. 44, and more fully in an unpublished
+manuscript.]
+
+[Footnote 108: F. W. Clarke: Data of Geochemistry, Fourth Ed., 1920,
+Bull. No. 695, U. S. Geol. Survey, p. 256.]
+
+[Footnote 109: F. W. Clarke: _loc. cit._, pp. 27-34 et al.]
+
+[Footnote C: Chas. E. St. John: Science Service Press Reports from the
+Mt. Wilson Observatory, May, 1922.]
+
+[Footnote 110: Abbot and Fowle: Annals Astrophysical Observatory;
+Smiths. Inst., Vol. II, 1908, p. 163.
+
+F. E. Fowle: Atmospheric Scattering of Light; Misc. Coll. Smiths. Inst.,
+Vol. 69, 1918.]
+
+[Footnote 111: Abbot and Fowle: _loc. cit._, p. 172.]
+
+
+
+
+CHAPTER XIV
+
+THE EFFECT OF OTHER BODIES ON THE SUN
+
+
+If solar activity is really an important factor in causing climatic
+changes, it behooves us to subject the sun to the same kind of inquiry
+to which we have subjected the earth. We have inquired into the nature
+of the changes through which the earth's crust, the oceans, and the
+atmosphere have influenced the climate of geological times. It has not
+been necessary, however, to study the origin of the earth, nor to trace
+its earlier stages. Our study of the geological record begins only when
+the earth had attained practically its present mass, essentially its
+present shape, and a climate so similar to that of today that life as we
+know it was possible. In other words, the earth had passed the stages of
+infancy, childhood, youth, and early maturity, and had reached full
+maturity. As it still seems to be indefinitely far from old age, we
+infer that during geological times its relative changes have been no
+greater than those which a man experiences between the ages of perhaps
+twenty-five and forty.
+
+Similar reasoning applies with equal or greater force to the sun.
+Because of its vast size it presumably passes through its stages of
+development much more slowly than the earth. In the first chapter of
+this book we saw that the earth's relative uniformity of climate for
+hundreds of millions of years seems to imply a similar uniformity in
+solar activity. This accords with a recent tendency among astronomers
+who are more and more recognizing that the stars and the solar system
+possess an extraordinary degree of conservatism. Changes that once were
+supposed to take place in thousands of years are now thought to have
+required millions. Hence in this chapter we shall assume that throughout
+geological times the condition of the sun has been almost as at present.
+It may have been somewhat larger, or different in other ways, but it was
+essentially a hot, gaseous body such as we see today and it gave out
+essentially the same amount of energy. This assumption will affect the
+general validity of what follows only if it departs widely from the
+truth. With this assumption, then, let us inquire into the degree to
+which the sun's atmosphere has probably been disturbed throughout
+geological times.
+
+In _Earth and Sun_, as already explained, a detailed study has led to
+the conclusion that cyclonic storms are influenced by the electrical
+action of the sun. Such action appears to be most intense in sunspots,
+but apparently pertains also to other disturbed areas in the sun's
+atmosphere. A study of sunspots suggests that their true periodicity is
+almost if not exactly identical with that of the orbital revolution of
+Jupiter, 11.8 years. Other investigations show numerous remarkable
+coincidences between sunspots and the orbital revolution of the other
+planets, including especially Saturn and Mercury. This seems to indicate
+that there is some truth in the hypothesis that sunspots and other
+related disturbances of the solar atmosphere owe their periodicity to
+the varying effects of the planets as they approach and recede from the
+sun in their eccentric orbits and as they combine or oppose their
+effects according to their relative positions. This does not mean that
+the energy of the solar disturbances is supposed to come from the
+planets, but merely that their variations act like the turning of a
+switch to determine when and how violently the internal forces of the
+sun shall throw the solar atmosphere into commotion. This hypothesis is
+by no means new, for in one form or another it has been advocated by
+Wolfer, Birkeland, E. W. Brown, Schuster, Arctowski, and others.
+
+The agency through which the planets influence the solar atmosphere is
+not yet clear. The suggested agencies are the direct pull of
+gravitation, the tidal effect of the planets, and an electro-magnetic
+effect. In _Earth and Sun_ the conclusion is reached that the first two
+are out of the question, a conclusion in which E. W. Brown acquiesces.
+Unless some unknown cause is appealed to, this leaves an
+electro-magnetic hypothesis as the only one which has a reasonable
+foundation. Schuster inclines to this view. The conclusions set forth in
+_Earth and Sun_ as to the electrical nature of the sun's influence on
+the earth point somewhat in the same direction. Hence in this chapter we
+shall inquire what would happen to the sun, and hence to the earth, on
+their journey through space, if the solar atmosphere is actually subject
+to disturbance by the electrical or other effects of other heavenly
+bodies. It need hardly be pointed out that we are here venturing into
+highly speculative ground, and that the verity or falsity of the
+conclusions reached in this chapter has nothing to do with the validity
+of the reasoning in previous chapters. Those chapters are based on the
+assumption that terrestrial causes of climatic changes are supplemented
+by solar disturbances which produce their effect partly through
+variations in temperature but also through variations in the intensity
+and paths of cyclonic storms. The present chapter seeks to shed some
+light on the possible causes and sequence of solar disturbances.
+
+Let us begin by scanning the available evidence as to solar disturbances
+previous to the time when accurate sunspot records are available. Two
+rather slender bits of evidence point to cycles of solar activity
+lasting hundreds of years. One of these has already been discussed in
+Chapter VI, where the climatic stress of the fourteenth century was
+described. At that time sunspots are known to have been unusually
+numerous, and there were great climatic extremes. Lakes overflowed in
+Central Asia; storms, droughts, floods, and cold winters were unusually
+severe in Europe; the Caspian Sea rose with great rapidity; the trees of
+California grew with a vigor unknown for centuries; the most terrible of
+recorded famines occurred in England and India; the Eskimos were
+probably driven south by increasing snowiness in Greenland; and the
+Mayas of Yucatan appear to have made their last weak attempt at a
+revival of civilization under the stimulus of greater storminess and
+less constant rainfall.
+
+The second bit of evidence is found in recent exhaustive studies of
+periodicities by Turner[112] and other astronomers. They have sought
+every possible natural occurrence for which a numerical record is
+available for a long period. The most valuable records appear to be
+those of tree growth, Nile floods, Chinese earthquakes, and sunspots.
+Turner reaches the conclusion that all four types of phenomena show the
+same periodicity, namely, cycles with an average length of about 260 to
+280 years. He suggests that if this is true, the cycles in tree growth
+and in floods, both of which are climatic, are probably due to a
+non-terrestrial cause. The fact that the sunspots show similar cycles
+suggests that the sun's variations are the cause.
+
+These two bits of evidence are far too slight to form the foundation of
+any theory as to changes in solar activity in the geological past.
+Nevertheless it may be helpful to set forth certain possibilities as a
+stimulus to further research. For example, it has been suggested that
+meteoric bodies may have fallen into the sun and caused it suddenly to
+flare up, as it were. This is not impossible, although it does not
+appear to have taken place since men became advanced enough to make
+careful observations. Moreover, the meteorites which now fall on the
+earth are extremely small, the average size being computed as no larger
+than a grain of wheat. The largest ever found on the earth's surface, at
+Bacubirito in Mexico, weighs only about fifty tons, while within the
+rocks the evidences of meteorites are extremely scanty and
+insignificant. If meteorites had fallen into the sun often enough and of
+sufficient size to cause glacial fluctuations and historic pulsations of
+climate, it seems highly probable that the earth would show much more
+evidence of having been similarly disturbed. And even if the sun should
+be bombarded by large meteors the result would probably not be sudden
+cold periods, which are the most notable phenomena of the earth's
+climatic history, but sudden warm periods followed by slow cooling.
+Nevertheless, the disturbance of the sun by collision with meteoric
+matter can by no means be excluded as a possible cause of climatic
+variations.
+
+Allied to the preceding hypothesis is Shapley's[113] nebular hypothesis.
+At frequent intervals, averaging about once a year during the last
+thirty years, astronomers have discovered what are known as nov�. These
+are stars which were previously faint or even invisible, but which flash
+suddenly into brilliancy. Often their light-giving power rises seven or
+eight magnitudes--a thousand-fold. In addition to the spectacular nov�
+there are numerous irregular variables whose brilliancy changes in every
+ratio from a few per cent up to several magnitudes. Most of them are
+located in the vicinity of nebul�, as is also the case with nov�. This,
+as well as other facts, makes it probable that all these stars are
+"friction variables," as Shapley calls them. Apparently as they pass
+through the nebul� they come in contact with its highly diffuse matter
+and thereby become bright much as the earth would become bright if its
+atmosphere were filled with millions of almost infinitesimally small
+meteorites. A star may also lose brilliancy if nebulous matter
+intervenes between it and the observer. If our sun has been subjected to
+any of these changes some sort of climatic effect must have been
+produced.
+
+In a personal communication Shapley amplifies the nebular climatic
+hypothesis as follows:
+
+    Within 700 light years of the sun in many directions (Taurus,
+    Cygnus, Ophiuchus, Scorpio) are great diffuse clouds of nebulosity,
+    some bright, most of them dark. The probability that stars moving in
+    the general region of such clouds will encounter this material is
+    very high, for the clouds fill enormous volumes of space,--e.g.,
+    probably more than a hundred thousand cubic light years in the Orion
+    region, and are presumably composed of rarefied gases or of dust
+    particles. Probably throughout all our part of space such nebulosity
+    exists (it is all around us, we are sure), but only in certain
+    regions is it dense enough to affect conspicuously the stars
+    involved in it. If a star moving at high velocity should collide
+    with a dense part of such a nebulous cloud, we should probably have
+    a typical nova. If the relative velocity of nebulous material and
+    star were low or moderate, or if the material were rare, we should
+    not expect a conspicuous effect on the star's light.
+
+    In the nebulous region of Orion, which is probably of unusually high
+    density, there are about 100 known stars, varying between 20% and
+    80% of their total light--all of them irregularly--some slowly, some
+    suddenly. Apparently they are "friction variables." Some of the
+    variables suddenly lose 40% of their light as if blanketed by
+    nebulous matter. In the Trifid Nebula there are variables like those
+    of Orion, in Messier 8 also, and probably many of the 100 or so
+    around the Rho Ophiuchi region belong to this kind.
+
+    I believe that our sun could not have been a typical nova, at least
+    not since the Archeozoic, that is for perhaps a billion years. I
+    believe we have in geological climates final proof of this, because
+    an increase in the amount of solar radiation by 1000 times as in the
+    typical nova, would certainly punctuate emphatically the life cycle
+    on the earth, even if the cause of the nova would not at the same
+    time eliminate the smaller planets. But the sun may have been one of
+    these miniature nov� or friction variables; and I believe it very
+    probable that its wanderings through this part of space could not
+    long leave its mean temperature unaffected to the amount of a few
+    per cent.
+
+    One reason we have not had this proposal insisted upon before is
+    that the data back of it are mostly new--the Orion variables have
+    been only recently discovered and studied, the distribution and
+    content of the dark nebul� are hardly as yet generally known.
+
+This interesting hypothesis cannot be hastily dismissed. If the sun
+should pass through a nebula it seems inevitable that there would be at
+least slight climatic effects and perhaps catastrophic effects through
+the action of the gaseous matter not only on the sun but on the earth's
+own atmosphere. As an explanation of the general climatic conditions of
+the past, however, Shapley points out that the hypothesis has the
+objection of being vague, and that nebulosity should not be regarded as
+more than "a possible factor." One of the chief difficulties seems to be
+the enormously wide distribution of as yet undiscovered nebulous matter
+which must be assumed if any large share of the earth's repeated
+climatic changes is to be ascribed to such matter. If such matter is
+actually abundant in space, it is hard to see how any but the nearest
+stars would be visible. Another objection is that there is no known
+nebulosity near at hand with which to connect the climatic vicissitudes
+of the last glacial period. Moreover, the known nebul� are so much less
+numerous than stars that the chances that the sun will encounter one of
+them are extremely slight. This, however, is not an objection, for
+Shapley points out that during geological times the sun can never have
+varied as much as do the nov�, or even as most of the friction
+variables. Thus the hypothesis stands as one that is worth
+investigating, but that cannot be finally rejected or accepted until it
+is made more definite and until more information is available.
+
+Another suggested cause of solar variations is the relatively sudden
+contraction of the sun such as that which sometimes occurs on the earth
+when continents are uplifted and mountains upheaved. It seems improbable
+that this could have occurred in a gaseous body like the sun. Lacking,
+as it does, any solid crust which resists a change of form, the sun
+probably shrinks steadily. Hence any climatic effects thus produced must
+be extremely gradual and must tend steadily in one direction for
+millions of years.
+
+Still another suggestion is that the tidal action of the stars and other
+bodies which may chance to approach the sun's path may cause
+disturbances of the solar atmosphere. The vast kaleidoscope of space is
+never quiet. The sun, the stars, and all the other heavenly bodies are
+moving, often with enormous speed. Hence the effect of gravitation upon
+the sun must vary constantly and irregularly, as befits the geological
+requirements. In the case of the planets, however, the tidal effect does
+not seem competent to produce the movements of the solar atmosphere
+which appear to be concerned in the inception of sunspots. Moreover,
+there is only the most remote probability that a star and the sun will
+approach near enough to one another to produce a pronounced
+gravitational disturbance in the solar atmosphere. For instance, if it
+be assumed that changes in Jupiter's tidal effect on the sun are the
+main factor in regulating the present difference between sunspot maxima
+and sunspot minima, the chances that a star or some non-luminous body of
+similar mass will approach near enough to stimulate solar activity and
+thereby bring on glaciation are only one in twelve billion years, as
+will be explained below. This seems to make a gravitational hypothesis
+impossible.
+
+Another possible cause of solar disturbances is that the stars in their
+flight through space may exert an electrical influence which upsets the
+equilibrium of the solar atmosphere. At first thought this seems even
+more impossible than a gravitational effect. Electrostatic effects,
+however, differ greatly from those of tides. They vary as the diameter
+of a body instead of as its mass; their differentials also vary
+inversely as the square of the distance instead of as the cube.
+Electrostatic effects also increase as the fourth power of the
+temperature or at least would do so if they followed the law of black
+bodies; they are stimulated by the approach of one body to another; and
+they are cumulative, for if ions arrive from space they must accumulate
+until the body to which they have come begins to discharge them. Hence,
+on the basis of assumptions such as those used in the preceding
+paragraph, the chances of an electrical disturbance of the solar
+atmosphere sufficient to cause glaciation on the earth may be as high as
+one in twenty or thirty million years. This seems to put an electrical
+hypothesis within the bounds of possibility. Further than that we cannot
+now go. There may be other hypotheses which fit the facts much better,
+but none seems yet to have been suggested.
+
+In the rest of this chapter the tidal and electrical hypotheses of
+stellar action on the sun will be taken up in detail. The tidal
+hypothesis is considered because in discussions of the effect of the
+planets it has hitherto held almost the entire field. The electrical
+hypothesis will be considered because it appears to be the best yet
+suggested, although it still seems doubtful whether electrical effects
+can be of appreciable importance over such vast distances as are
+inevitably involved. The discussion of both hypotheses will necessarily
+be somewhat technical, and will appeal to the astronomer more than to
+the layman. It does not form a necessary part of this book, for it has
+no bearing on our main thesis of the effect of the sun on the earth. It
+is given here because ultimately the question of changes in solar
+activity during geological times must be faced.
+
+In the astronomical portion of the following discussion we shall follow
+Jeans[114] in his admirable attempt at a mathematical analysis of the
+motions of the universe. Jeans divides the heavenly bodies into five
+main types. (1) Spiral nebul�, which are thought by some astronomers to
+be systems like our own in the making, and by others to be independent
+universes lying at vast distances beyond the limits of our Galactic
+universe, as it is called from the Galaxy or Milky Way. (2) Nebul� of a
+smaller type, called planetary. These lie within the Galactic portion of
+the universe and seem to be early stages of what may some day be stars
+or solar systems. (3) Binary or multiple stars, which are
+extraordinarily numerous. In some parts of the heavens they form 50 or
+even 60 per cent of the stars and in the galaxy as a whole they seem to
+form "fully one third." (4) Star clusters. These consist of about a
+hundred groups of stars in each of which the stars move together in the
+same direction with approximately the same velocity. These, like the
+spiral nebul�, are thought by some astronomers to lie outside the limits
+of the galaxy, but this is far from certain. (5) The solar system.
+According to Jeans this seems to be unique. It does not fit into the
+general mathematical theory by which he explains spiral nebul�,
+planetary nebul�, binary stars, and star clusters. It seems to demand a
+special explanation, such as is furnished by tidal disruption due to the
+passage of the sun close to another star.
+
+The part of Jeans' work which specially concerns us is his study of the
+probability that some other star will approach the sun closely enough to
+have an appreciable gravitative or electrical effect, and thus cause
+disturbances in the solar atmosphere. Of course both the star and the
+sun are moving, but to avoid circumlocution we shall speak of such
+mutual approaches simply as approaches of the sun. For our present
+purpose the most fundamental fact may be summed up in a quotation from
+Jeans in which he says that most stars "show evidence of having
+experienced considerable disturbance by other systems; there is no
+reason why our solar system should be expected to have escaped the
+common fate." Jeans gives a careful calculation from which it is
+possible to derive some idea of the probability of any given degree of
+approach of the sun and some other star. Of course all such calculations
+must be based on certain assumptions. The assumptions made by Jeans are
+such as to make the probability of close approaches as great as
+possible. For example, he allows only 560 million years for the entire
+evolution of the sun, whereas some astronomers and geologists would put
+the figure ten or more times as high. Nevertheless, Jeans' assumptions
+at least show the order of magnitude which we may expect on the basis of
+reasonable astronomical conclusions.
+
+According to the planetary hypothesis of sunspots, the difference in the
+effect of Jupiter when it is nearest and farthest from the sun is the
+main factor in starting the sunspot cycle and hence the corresponding
+terrestrial cycle. The climatic difference between sunspot maxima and
+minima, as measured by temperature, apparently amounts to at least a
+twentieth and perhaps a tenth of the difference between the climate of
+the last glacial epoch and the present. We may suppose, then, that a
+body which introduced a gravitative or electrical factor twenty times as
+great as the difference in Jupiter's effect at its maximum and minimum
+distances from the sun would cause a glacial epoch if the effect lasted
+long enough. Of course the other planets combine their effects with that
+of Jupiter, but for the sake of simplicity we will leave the others out
+of account. The difference between Jupiter's maximum and minimum tidal
+effect on the sun amounts to 29 per cent of the planet's average effect.
+The corresponding difference, according to the electrical hypothesis, is
+about 19 per cent, for electrostatic action varies as the square of the
+distance instead of as the cube. Let us assume that a body exerting four
+times Jupiter's present tidal effect and placed at the average distance
+of Jupiter from the sun would disturb the sun's atmosphere twenty times
+as much as the present difference between sunspot maxima and minima, and
+thus, perhaps, cause a glacial period on the earth.
+
+On the basis of this assumption our first problem is to estimate the
+frequency with which a star, visible or dark, is likely to approach near
+enough to the sun to produce a _tidal_ effect four times that of
+Jupiter. The number of visible stars is known or at least well
+estimated. As to dark stars, which have grown cool, Arrhenius believed
+that they are a hundred times as numerous as bright stars; few
+astronomers believe that there are less than three or four times as
+many. Dr. Shapley of the Harvard Observatory states that a new
+investigation of the matter suggests that eight or ten is probably a
+maximum figure. Let us assume that nine is correct. The average visible
+star, so far as measured, has a mass about twice that of the sun, or
+about 2100 times that of Jupiter. The distances of the stars have been
+measured in hundreds of cases and thus we can estimate how many stars,
+both visible and invisible, are on an average contained in a given
+volume of space. On this basis Jeans estimates that there is only one
+chance in thirty billion years that a visible star will approach within
+2.8 times the distance of Neptune from the sun, that is, within about
+eight billion miles. If we include the invisible stars the chances
+become one in three billion years. In order to produce four times the
+tidal effect of Jupiter, however, the average star would have to
+approach within about four billion miles of the sun, and the chances of
+that are only one in twelve billion years. The disturbing star would be
+only 40 per cent farther from the sun than Neptune, and would almost
+pass within the solar system.
+
+Even though Jeans holds that the frequency of the mutual approach of the
+sun and a star was probably much greater in the distant past than at
+present, the figures just given lend little support to the tidal
+hypothesis. In fact, they apparently throw it out of court. It will be
+remembered that Jeans has made assumptions which give as high a
+frequency of stellar encounters as is consistent with the astronomical
+facts. We have assumed nine dark stars for every bright one, which may
+be a liberal estimate. Also, although we have assumed that a disturbance
+of the sun's atmosphere sufficient to cause a glacial period would arise
+from a tidal effect only twenty times as great as the difference in
+Jupiter's effect when nearest the sun and farthest away, in our
+computations this has actually been reduced to thirteen. With all these
+favorable assumptions the chances of a stellar approach of the sort here
+described are now only one in twelve billion years. Yet within a hundred
+million years, according to many estimates of geological time, and
+almost certainly within a billion, there have been at least half a dozen
+glaciations.
+
+Our use of Jeans' data interposes another and equally insuperable
+difficulty to any tidal hypothesis. Four billion miles is a very short
+distance in the eyes of an astronomer. At that distance a star twice the
+size of the sun would attract the outer planets more strongly than the
+sun itself, and might capture them. If a star should come within four
+billion miles of the sun, its effect in distorting the orbits of all the
+planets would be great. If this had happened often enough to cause all
+the glaciations known to geologists, the planetary orbits would be
+strongly elliptical instead of almost circular. The consideration here
+advanced militate so strongly against the tidal hypothesis of solar
+disturbances that it seems scarcely worth while to consider it further.
+
+Let us turn now to the electrical hypothesis. Here the conditions are
+fundamentally different from those of the tidal hypothesis. In the first
+place the electrostatic effect of a body has nothing to do with its
+mass, but depends on the area of its surface; that is, it varies as the
+square of the radius. Second, the emission of electrons varies
+exponentially. If hot glowing stars follow the same law as black bodies
+at lower temperatures, the emission of electrons, like the emission of
+other kinds of energy, varies as the fourth power of the absolute
+temperature. In other words, suppose there are two black bodies,
+otherwise alike, but one with a temperature of 27� C. or 300� on the
+absolute scale, and the other with 600� on the absolute scale. The
+temperature of one is twice as high as that of the other, but the
+electrostatic effect will be sixteen times as great.[115] Third, the
+number of electrons that reach a given body varies inversely as the
+square of the distance, instead of as the cube which is the case with
+tide-making forces.
+
+In order to use these three principles in calculating the effect of the
+stars we must know the diameters, distances, temperature, and number of
+the stars. The distances and number may safely be taken as given by
+Jeans in the calculations already cited. As to the diameters, the
+measurements of the stars thus far made indicate that the average mass
+is about twice that of the sun. The average density, as deduced by
+Shapley[116] from the movements of double stars, is about one-eighth the
+solar density. This would give an average diameter about two and a half
+times that of the sun. For the dark stars, we shall assume for
+convenience that they are ten times as numerous as the bright ones. We
+shall also assume that their diameter is half that of the sun, for being
+cool they must be relatively dense, and that their temperature is the
+same as that which we shall assume for Jupiter.
+
+As to Jupiter we shall continue our former assumption that a body with
+four times the effectiveness of that planet, which here means with twice
+as great a radius, would disturb the sun enough to cause glaciation. It
+would produce about twenty times the electrostatic effect which now
+appears to be associated with the difference in Jupiter's effect at
+maximum and minimum. The temperature of Jupiter must also be taken into
+account. The planet is supposed to be hot because its density is low,
+being only about 1.25 that of water. Nevertheless, it is probably not
+luminous, for as Moulton[117] puts it, shadows upon it are black and its
+moons show no sign of illumination except from the sun. Hence a
+temperature of about 600�C., or approximately 900� on the absolute
+scale, seems to be the highest that can reasonably be assigned to the
+cold outer layer whence electrons are emitted. As to the temperature of
+the sun, we shall adopt the common estimate of about 6300�C. on the
+absolute scale. The other stars will be taken as averaging the same,
+although of course they vary greatly.
+
+When Jeans' method of calculating the probability of a mutual approach
+of the sun and a star is applied to the assumptions given above, the
+results are as shown in Table 5. On that basis the dark stars seem to be
+of negligible importance so far as the electrical hypothesis is
+concerned. Even though they may be ten times as numerous as the bright
+ones there appears to be only one chance in 130 billion years that one
+of them will approach the sun closely enough to cause the assumed
+disturbance of the solar atmosphere. On the other hand, if all the
+visible stars were the size of the sun, and as hot as that body, their
+electrical effect would be fourfold that of our assumed dark star
+because of their size, and 2401 times as great because of their
+temperature, or approximately 10,000 times as great. Under such
+conditions the theoretical chance of an approach that would cause
+glaciation is one in 130 million years. If the average visible star is
+somewhat cooler than the sun and has a radius about two and one-half
+times as great, as appears to be the fact, the chances rise to one in
+thirty-eight million years. A slight and wholly reasonable change in our
+assumptions would reduce this last figure to only five or ten million.
+For instance, the earth's mean temperature during the glacial period has
+been assumed as 10�C. lower than now, but the difference may have been
+only 6�. Again, the temperature of the outer atmosphere of Jupiter where
+the electrons are shot out may be only 500� or 700� absolute, instead of
+900�. Or the diameter of the average star may be five or ten times that
+of the sun, instead of only two and one-half times as great. All this,
+however, may for the present be disregarded. The essential point is that
+even when the assumptions err on the side of conservatism, the results
+are of an order of magnitude which puts the electrical hypothesis within
+the bounds of possibility, whereas similar assumptions put the tidal
+hypothesis, with its single approach in twelve billion years, far beyond
+those limits.
+
+The figures for Betelgeuse in Table 5 are interesting. At a meeting of
+the American Association for the Advancement of Science in December,
+1920, Michelson reported that by measurements of the interference of
+light coming from the two sides of that bright star in Orion, the
+observers at Mount Wilson had confirmed the recent estimates of three
+other authorities that the star's diameter is about 218 million miles,
+or 250 times that of the sun. If other stars so much surpass the
+estimates of only a decade or two ago, the average diameter of all the
+visible stars must be many times that of the sun. The low figure for
+Betelgeuse in section D of the table means that if all the stars were as
+large as Betelgeuse, several might often be near enough to cause
+profound disturbances of the solar atmosphere. Nevertheless, because of
+the low temperature of the giant red stars of the Betelgeuse type, the
+distance at which one of them would produce a given electrical effect is
+only about five times the distance at which our assumed average star
+would produce the same effect. This, to be sure, is on the assumption
+that the radiation of energy from incandescent bodies varies according
+to temperature in the same ratio as the radiation from black bodies.
+Even if this assumption departs somewhat from the truth, it still seems
+almost certain that the lower temperature of the red compared with the
+high temperature of the white stars must to a considerable degree reduce
+the difference in electrical effect which would otherwise arise from
+their size.
+
+TABLE 5
+
+THEORETICAL PROBABILITY OF STELLAR APPROACHES
+
+  ---------------------------------------------------------------------
+                   |      1        |     2     |    3     |     4      |
+                   |               |           |_Average  |            |
+                   |  _Dark Stars_ |   _Sun_   |  Star_   |_Betelgeuse_|
+  ---------------------------------------------------------------------
+  A. Approximate   |               |           |          |            |
+  radius in miles  |    430,000    |  860,000  | 2,150,000|218,000,000 |
+                   |               |           |          |            |
+  B. Assumed       |               |           |          |            |
+  temperature above|               |           |          |            |
+  absolute zero.   |    900� C.    |   6300� C.|  5400� C.|  3150� C.  |
+                   |               |           |          |            |
+  C. Approximate   |               |           |          |            |
+  theoretical      |               |           |          |            |
+  distance at which|               |           |          |            |
+  star would cause |               |           |          |            |
+  solar disturbance|               |           |          |            |
+  great enough to  |               |           |          |            |
+  cause glaciation |               |           |          |            |
+  (billions[118]   |               |           |          |            |
+  of miles).       |      1.2      |    120    |    220   |   3200     |
+                   |               |           |          |            |
+  D. Average       |               |           |          |            |
+  interval between |               |           |          |            |
+  approaches       |               |           |          |            |
+  close enough to  |               |           |          |            |
+  cause glaciation |               |           |          |            |
+  if all stars     |130,000,000,000|           |          |            |
+  were of given    |[119]          |           |          |            |
+  type. Years.     |               |130,000,000|38,000,000|  700,000   |
+  ---------------------------------------------------------------------
+
+Thus far in our attempt to estimate the distance at which a star might
+disturb the sun enough to cause glaciation on the earth, we have
+considered only the star's size and temperature. No account has been
+taken of the degree to which its atmosphere is disturbed. Yet in the
+case of the sun this seems to be one of the most important factors. The
+magnetic field of sunspots is sometimes 50 or 100 times as strong as
+that of the sun in general. The strength of the magnetic field appears
+to depend on the strength of the electrical currents in the solar
+atmosphere. But the intensity of the sunspots and, by inference, of the
+electrical currents, may depend on the electrical action of Jupiter and
+the other planets. If we apply a similar line of reasoning to the stars,
+we are at once led to question whether the electrical activity of double
+stars may not be enormously greater than that of isolated stars like the
+sun.
+
+If this line of reasoning is correct, the atmosphere of every double
+star must be in a state of commotion vastly greater than that of the
+sun's atmosphere even when it is most disturbed. For example, suppose
+the sun were accompanied by a companion of equal size at a distance of
+one million miles, which would make it much like many known double
+stars. Suppose also that in accordance with the general laws of physics
+the electrical effect of the two suns upon one another is proportional
+to the fourth power of the temperature, the square of the radius, and
+the inverse square of the distance. Then the effect of each sun upon the
+other would be sixty billion (6 � 10^{10}) times as great as the present
+electrical effect of Jupiter upon the sun. Just what this would mean as
+to the net effect of a pair of such suns upon the electrical potential
+of other bodies at a distance we can only conjecture. The outstanding
+fact is that the electrical conditions of a double star must be
+radically different and vastly more intense than those of a single star
+like the sun.
+
+This conclusion carries weighty consequences. At present twenty or more
+stars are known to be located within about 100 trillion miles of the sun
+(five parsecs, as the astronomers say), or 16.5 light years. According
+to the assumptions employed in Table 5 an average single star would
+influence the sun enough to cause glaciation if it came within
+approximately 200 billion miles. If the star were double, however, it
+might have an electrical capacity enormously greater than that of the
+sun. Then it would be able to cause glaciation at a correspondingly
+great distance. Today Alpha Centauri, the nearest known star about
+twenty-five trillion miles, or 4.3 light years from the sun, and Sirius,
+the brightest star in the heavens, is about fifty trillion miles away,
+or 8.5 light years. If these stars were single and had a diameter three
+times that of the sun, and if they were of the same temperature as has
+been assumed for Betelgeuse, which is about fifty times as far away as
+Alpha Centauri, the relative effects of the three stars upon the sun
+would be, approximately, Betelgeuse 700, Alpha Centauri 250, Sirius 1.
+But Alpha Centauri is triple and Sirius double, and both are much hotter
+than Betelgeuse. Hence Alpha Centauri and even Sirius may be far more
+effective than Betelgeuse.
+
+The two main components of Alpha Centauri are separated by an average
+distance of about 2,200,000,000 miles, or somewhat less than that of
+Neptune from the sun. A third and far fainter star, one of the faintest
+yet measured, revolves around them at a great distance. In mass and
+brightness the two main components are about like the sun, and we will
+assume that the same is true of their radius. Then, according to the
+assumptions made above, their effect in disturbing one another
+electrically would be about 10,000 times the total effect of Jupiter
+upon the sun, or 2500 times the effect that we have assumed to be
+necessary to produce a glacial period. We have already seen in Table 5
+that, according to our assumptions, a single star like the sun would
+have to approach within 120 billion miles of the solar system, or within
+2 per cent of a light year, in order to cause glaciation. By a similar
+process of reasoning it appears that if the mutual electrical excitation
+of the two main parts of Alpha Centauri, regardless of the third part,
+is proportional to the apparent excitation of the sun by Jupiter, Alpha
+Centauri would be 5000 times as effective as the sun. In other words, if
+it came within 8,500,000,000,000 miles of the sun, or 1.4 light years,
+it would so change the electrical conditions as to produce a glacial
+epoch. In that case Alpha Centauri is now so near that it introduces a
+disturbing effect equal to about one-sixth of the effect needed to cause
+glaciation on the earth. Sirius and perhaps others of the nearer and
+brighter or larger stars may also create appreciable disturbances in the
+electrical condition of the sun's atmosphere, and may have done so to a
+much greater degree in the past, or be destined to do so in the future.
+Thus an electrical hypothesis of solar disturbances seems to indicate
+that the position of the sun in respect to other stars may be a factor
+of great importance in determining the earth's climate.
+
+FOOTNOTES:
+
+[Footnote 112: H. H. Turner: On a Long Period in Chinese Earthquake
+Records; Mon. Not. Royal Astron. Soc., Vol. 79, 1919, pp. 531-539; Vol.
+80, 1920, pp. 617-619; Long Period Terms in the Growth of Trees; _idem_,
+pp.793-808.]
+
+[Footnote 113: Harlow Shapley: Note on a Possible Factor in Geologic
+Climates; Jour. Geol., Vol. 29, No. 4, May, 1921; Nov� and Variable
+Stars, Pub. Astron. Soc. Pac., No. 194, Aug., 1921.]
+
+[Footnote 114: J. H. Jeans: Problems of Cosmogony and Stellar Dynamics,
+Cambridge, 1919.]
+
+[Footnote 115: This fact is so important and at the same time so
+surprising to the layman, that a quotation from The Electron Theory of
+Matter by O. W. Richardson, 1914, pp. 326 and 334 is here added.
+
+"It is a very familiar fact that when material bodies are heated they
+emit electromagnetic radiations, in the form of thermal, luminous, and
+actinic rays, in appreciable quantities. Such an effect is a natural
+consequence of the electron and kinetic theories of matter. On the
+kinetic theory, temperature is a measure of the violence of the motion
+of the ultimate particles; and we have seen that on the electron theory,
+electromagnetic radiation is a consequence of their acceleration. The
+calculation of this emission from the standpoint of the electron theory
+alone is a very complex problem which takes us deeply into the structure
+of matter and which has probably not yet been satisfactorily resolved.
+Fortunately, we can find out a great deal about these phenomena by the
+application of general principles like the conservation of energy and
+the second law of thermodynamics without considering special assumptions
+about the ultimate constitution of matter. It is to be borne in mind
+that the emission under consideration occurs at all temperatures
+although it is more marked the higher the temperature.... The energy per
+unit volume, _in vacuo_, of the radiation in equilibrium in an enclosure
+at the absolute temperature, T, is equal to a universal constant, A,
+multiplied by the fourth power of the absolute temperature. Since the
+intensity of the radiation is equal to the energy per unit volume
+multiplied by the velocity of light, it follows that the former must
+also be proportional to the fourth power of the absolute temperature.
+Moreover, if E is the total emission from unit area of a perfectly black
+body, we see from p. 330 that E=A�T^{4}, where A� is a new universal
+constant. This result is usually known as Stefan's Law. It was suggested
+by Stefan in the inaccurate form that the total radiant energy of
+emission from bodies varies as the fourth power of the absolute
+temperature, as a generalization from the results of experiments. The
+credit for showing that it is a consequence of the existence of
+radiation pressure combined with the principles of thermodynamics is due
+to Bartoli and Boltzmann."]
+
+[Footnote 116: Quoted by Moulton in his Introduction to Astronomy.]
+
+[Footnote 117: Introduction to Astronomy.]
+
+[Footnote 118: The term billions, here and elsewhere, is used in the
+American sense, 10^{9}.]
+
+[Footnote 119: The assumed number of stars here is ten times as great as
+in the other parts of this line.]
+
+
+
+
+CHAPTER XV
+
+THE SUN'S JOURNEY THROUGH SPACE
+
+
+Having gained some idea of the nature of the electrical hypothesis of
+solar disturbances and of the possible effect of other bodies upon the
+sun's atmosphere, let us now compare the astronomical data with those of
+geology. Let us take up five chief points for which the geologist
+demands an explanation, and which any hypothesis must meet if it is to
+be permanently accepted. These are (1) the irregular intervals at which
+glacial periods occur; (2) the division of glacial periods into epochs
+separated sometimes by hundreds of thousands of years; (3) the length of
+glacial periods and epochs; (4) the occurrence of glacial stages and
+historic pulsations in the form of small climatic waves superposed upon
+the larger waves of glacial epochs; (5) the occurrence of climatic
+conditions much milder than those of today, not only in the middle
+portion of the great geological eras, but even in some of the recent
+inter-glacial epochs.
+
+1. The irregular duration of the interval from one glacial epoch to
+another corresponds with the irregular distribution of the stars. If
+glaciation is indirectly due to stellar influences, the epochs might
+fall close together, or might be far apart. If the average interval were
+ten million years, one interval might be thirty million or more and the
+next only one or two hundred thousand. According to Schuchert, the known
+periods of glacial or semi-glacial climate have been approximately as
+follows:
+
+LIST OF GLACIAL PERIODS
+
+  1. Archeozoic.
+     (1/4 of geological time or perhaps much more)
+
+      No known glacial periods.
+
+  2. Proterozoic.
+     (1/4 of geological time)
+
+      a. Oldest known glacial period near base of Proterozoic in
+         Canada. Evidence widely distributed.
+
+      b. Indian glacial period; time unknown.
+
+      c. African glacial period; time unknown.
+
+      d. Glaciation near end of Proterozoic in Australia, Norway,
+         and China.
+
+  3. Paleozoic.
+     (1/4 of geological time)
+
+      a. Late Ordovician(?). Local in Arctic Norway.
+
+      b. Silurian. Local in Alaska.
+
+      c. Early Devonian. Local in South Africa.
+
+      d. Early Permian. World-wide and very severe.
+
+  4. Mesozoic and Cenozoic.
+     (1/4 of geological time)
+
+      a-b. None definitely determined during Mesozoic, although
+        there appears to have been periods of cooling (a) in the
+        late Triassic, and (b) in the late Cretacic, with at least
+        local glaciation in early Eocene.
+
+      c. Severe glacial period during Pleistocene.
+
+This table suggests an interesting inquiry. During the last few decades
+there has been great interest in ancient glaciation and geologists have
+carefully examined rocks of all ages for signs of glacial deposits. In
+spite of the large parts of the earth which are covered with deposits
+belonging to the Mesozoic and Cenozoic, which form the last quarter of
+geological time, the only signs of actual glaciation are those of the
+great Pleistocene period and a few local occurrences at the end of the
+Mesozoic or beginning of the Cenozoic. Late in the Triassic and early in
+the Jurassic, the climate appears to have been rigorous, although no
+tillites have been found to demonstrate glaciation. In the preceding
+quarter, that is, the Paleozoic, the Permian glaciation was more severe
+than that of the Pleistocene, and the Devonian than that of the Eocene,
+while the Ordovician evidences of low temperature are stronger than
+those at the end of the Triassic. In view of the fact that rocks of
+Paleozoic age cover much smaller areas than do those of later age, the
+three Paleozoic glaciations seem to indicate a relative frequency of
+glaciation. Going back to the Proterozoic, it is astonishing to find
+that evidence of two highly developed glacial periods, and possibly
+four, has been discovered. Since the Indian and the African glaciations
+of Proterozoic times are as yet undated, we cannot be sure that they are
+not of the same date as the others. Nevertheless, even two is a
+surprising number, for not only are most Proterozoic rocks so
+metamorphosed that possible evidences of glacial origin are destroyed,
+but rocks of that age occupy far smaller areas than either those of
+Paleozoic or, still more, Mesozoic and Cenozoic age. Thus the record of
+the last three-quarters of geological time suggests that if rocks of all
+ages were as abundant and as easily studied as those of the later
+periods, the frequency of glacial periods would be found to increase as
+one goes backward toward the beginnings of the earth's history. This is
+interesting, for Jeans holds that the chances that the stars would
+approach one another were probably greater in the past than at present.
+This conclusion is based on the assumption that our universe is like the
+spiral nebul� in which the orbits of the various members are nearly
+circular during the younger stages. Jeans considers it certain that in
+such cases the orbits will gradually become larger and more elliptical
+because of the attraction of one body for another. Thus as time goes on
+the stars will be more widely distributed and the chances of approach
+will diminish. If this is correct, the agreement between astronomical
+theory and geological conclusions suggests that the two are at least not
+in opposition.
+
+The first quarter of geological time as well as the last three must be
+considered in this connection. During the Archeozoic, no evidence of
+glaciation has yet been discovered. This suggests that the geological
+facts disprove the astronomical theory. But our knowledge of early
+geological times is extremely limited, so limited that lack of evidence
+of glaciation in the Archeozoic may have no significance. Archeozoic
+rocks have been studied minutely over a very small percentage of the
+earth's land surface. Moreover, they are highly metamorphosed so that,
+even if glacial tills existed, it would be hard to recognize them.
+Third, according to both the nebular and the planetesimal hypotheses, it
+seems possible that during the earliest stages of geological history the
+earth's interior was somewhat warmer than now, and the surface may have
+been warmed more than at present by conduction, by lava flows, and by
+the fall of meteorites. If the earth during the Archeozoic period
+emitted enough heat to raise its surface temperature a few degrees, the
+heat would not prevent the development of low forms of life but might
+effectively prevent all glaciation. This does not mean that it would
+prevent changes of climate, but merely changes so extreme that their
+record would be preserved by means of ice. It will be most interesting
+to see whether future investigations in geology and astronomy indicate
+either a semi-uniform distribution of glacial periods throughout the
+past, or a more or less regular decrease in frequency from early times
+down to the present.
+
+2. The Pleistocene glacial period was divided into at least four epochs,
+while in the Permian at least one inter-glacial epoch seems certain, and
+in some places the alternation between glacial and non-glacial beds
+suggests no less than nine. In the other glaciations the evidence is not
+yet clear. The question of periodicity is so important that it
+overthrows most glacial hypotheses. Indeed, had their authors known the
+facts as established in recent years, most of the hypotheses would never
+have been advanced. The carbon dioxide hypothesis is the only one which
+was framed with geologically rapid climatic alternations in mind. It
+certainly explains the facts of periodicity better than does any of its
+predecessors, but even so it does not account for the intimate way in
+which variations of all degrees from those of the weather up to glacial
+epochs seem to grade into one another.
+
+According to our stellar hypothesis, occasional groups of glacial epochs
+would be expected to occur close together and to form long glacial
+periods. This is because many of the stars belong to groups or clusters
+in which the stars move in parallel paths. A good example is the cluster
+in the Hyades, where Boss has studied thirty-nine stars with special
+care.[120] The stars are grouped about a center about 130 light years
+from the sun. The stars themselves are scattered over an area about
+thirty light years in diameter. They average about the same distance
+apart as do those near the sun, but toward the center of the group they
+are somewhat closer together. The whole thirty-nine sweep forward in
+essentially parallel paths. Boss estimates that 800,000 years ago the
+cluster was only half as far from the sun as at present, but probably
+that was as near as it has been during recent geological times. All of
+the thirty-nine stars of this cluster, as Moulton[121] puts it, "are
+much greater in light-giving power than the sun. The luminosities of
+even the five smallest are from five to ten times that of the sun, while
+the largest are one hundred times greater in light-giving power than our
+own luminary. Their masses are probably much greater than that of the
+sun." If the sun were to pass through such a cluster, first one star and
+then another might come so near as to cause a profound disturbance in
+the sun's atmosphere.
+
+3. Another important point upon which a glacial hypothesis may come to
+grief is the length of the periods or rather of the epochs which compose
+the periods. During the last or Pleistocene glacial period the evidence
+in America and Europe indicates that the inter-glacial epochs varied in
+length and that the later ones were shorter than the earlier. Chamberlin
+and Salisbury, from a comparison of various authorities, estimate that
+the intervals from one glacial epoch to another form a declining series,
+which may be roughly expressed as follows: 16-8-4-2-1, where unity is
+the interval from the climax of the late Wisconsin, or last glacial
+epoch, to the present. Most authorities estimate the culmination of the
+late Wisconsin glaciation as twenty or thirty thousand years ago. Penck
+estimates the length of the last inter-glacial period as 60,000 years
+and the preceding one as 240,000.[122] R. T. Chamberlin, as already
+stated, finds that the consensus of opinion is that inter-glacial epochs
+have averaged five times as long as glacial epochs. The actual duration
+of the various glaciations probably did not vary in so great a ratio as
+did the intervals from one glaciation to another. The main point,
+however, is the irregularity of the various periods.
+
+The relation of the stellar electrical hypothesis to the length of
+glacial epochs may be estimated from column C, in Table 5. There we see
+that the distances at which a star might possibly disturb the sun enough
+to cause glaciation range all the way from 120 billion miles in the case
+of a small star like the sun, to 3200 billion in the case of Betelgeuse,
+while for double stars the figure may rise a hundred times higher. From
+this we can calculate how long it would take a star to pass from a point
+where its influence would first amount to a quarter of the assumed
+maximum to a similar point on the other side of the sun. In making these
+calculations we will assume that the relative rate at which the star and
+the sun approach each other is about twenty-two miles per second, or 700
+million miles per year, which is the average rate of motion of all the
+known stars. According to the distances in Table 5 this gives a range
+from about 500 years up to about 10,000, which might rise to a million
+in the case of double stars. Of course the time might be relatively
+short if the sun and a rapidly moving star were approaching one another
+almost directly, or extremely long if the sun and the star were moving
+in almost the same direction and at somewhat similar rates,--a condition
+more common than the other. Here, as in so many other cases, the
+essential point is that the figures which we thus obtain seem to be of
+the right order of magnitude.
+
+4. Post-glacial climatic stages are so well known that in Europe they
+have definite names. Their sequence has already been discussed in
+Chapter XII. Fossils found in the peat bogs of Denmark and Scandinavia,
+for example, prove that since the final disappearance of the continental
+ice cap at the close of the Wisconsin there has been at least one period
+when the climate of Europe was distinctly milder than now. Directly
+overlying the sheets of glacial drift laid down by the ice there is a
+flora corresponding to that of the present tundras. Next come remains of
+a forest vegetation dominated by birches and poplars, showing that the
+climate was growing a little warmer. Third, there follow evidences of a
+still more favorable climate in the form of a forest dominated by pines;
+fourth, one where oak predominates; and fifth, a flora similar to that
+of the Black Forest of Germany, indicating that in Scandinavia the
+temperature was then decidedly higher than today. This fifth flora has
+retreated southward once more, having been driven back to its present
+latitude by a slight recurrence of a cool stormy climate.[123] In
+central Asia evidence of post-glacial stages is found not only in five
+distinct moraines but in a corresponding series of elevated strands
+surrounding salt lakes and of river terraces in non-glaciated arid
+regions.[124]
+
+In historic as well as prehistoric times, as we have already seen, there
+have been climatic fluctuations. For instance, the twelfth or thirteenth
+century B. C. appears to have been almost as mild as now, as does the
+seventh century B. C. On the other hand about 1000 B. C., at the time of
+Christ, and in the fourteenth century there were times of relative
+severity. Thus it appears that both on a large and on a small scale
+pulsations of climate are the rule. Any hypothesis of climatic changes
+must satisfy the periods of these pulsations. These conditions furnish a
+problem which makes difficulty for almost all hypotheses of climatic
+change. According to the present hypothesis, earth movements such as are
+discussed in Chapter XII may co�perate with two astronomical factors.
+One is the constant change in the positions of the stars, a change which
+we have already called kaleidoscopic, and the other is the fact that a
+large proportion of the stars are double or multiple. When one star in a
+group approaches the sun closely enough to cause a great solar
+disturbance, numerous others may approach or recede and have a minor
+effect. Thus, whenever the sun is near groups of stars we should expect
+that the earth would show many minor climatic pulsations and stages
+which might or might not be connected with glaciation. The historic
+pulsations shown in the curve of tree growth in California, Fig. 4, are
+the sort of changes that would be expected if movements of the stars
+have an effect on the solar atmosphere.
+
+Not only are fully a third of all the visible stars double, as we have
+already seen, but at least a tenth of these are known to be triple or
+multiple. In many of the double stars the two bodies are close together
+and revolve so rapidly that whatever periodicity they might create in
+the sun's atmosphere would be very short. In the triplets, however, the
+third star is ordinarily at least ten times as far from the other two as
+they are from each other, and its period of rotation sometimes runs into
+hundreds or thousands of years. An actual multiple star in the
+constellation Polaris will serve as an example. The main star is
+believed by Jeans to consist of two parts which are almost in contact
+and whirl around each other with extraordinary speed in four days. If
+this is true they must keep each other's atmospheres in a state of
+intense commotion. Much farther away a third star revolves around this
+pair in twelve years. At a much greater distance a fourth star revolves
+around the common center of gravity of itself and the other three in a
+period which may be 20,000 years. Still more complicated cases probably
+exist. Suppose such a system were to traverse a path where it would
+exert a perceptible influence on the sun for thirty or forty thousand
+years. The varying movements of its members would produce an intricate
+series of cycles which might show all sorts of major and minor
+variations in length and intensity. Thus the varied and irregular stages
+of glaciation and the pulsations of historic times might be accounted
+for on the hypothesis of the proximity of the sun to a multiple star, as
+well as on that of the less pronounced approach and recession of a
+number of stars. In addition to all this, an almost infinitely complex
+series of climatic changes of long and short duration might arise if the
+sun passed through a nebula.
+
+5. We have seen in Chapter VIII that the contrast between the somewhat
+severe climate of the present and the generally mild climate of the past
+is one of the great geological problems. The glacial period is not a
+thing of the distant past. Geologists generally recognize that it is
+still with us. Greenland and Antarctica are both shrouded in ice sheets
+in latitudes where fossil floras prove that at other periods the climate
+was as mild as in England or even New Zealand. The present glaciated
+regions, be it noted, are on the polar borders of the world's two most
+stormy oceanic areas, just where ice would be expected to last longest
+according to the solar cyclonic hypothesis. In contrast with the
+semi-glacial conditions of the present, the last inter-glacial epoch was
+so mild that not only men but elephants and hippopotamuses flourished in
+central Europe, while at earlier times in the middle of long eras, such
+as the Paleozoic and Mesozoic, corals, cycads, and tree ferns flourished
+within the Arctic circle.
+
+If the electro-stellar hypothesis of solar disturbances proves well
+founded, it may explain these peculiarities. Periods of mild climate
+would represent a return of the sun and the earth to their normal
+conditions of quiet. At such times the atmosphere of the sun is assumed
+to be little disturbed by sunspots, facul�, prominences, and other
+allied evidences of movements; and the rice-grain structure is perhaps
+the most prominent of the solar markings. The earth at such times is
+supposed to be correspondingly free from cyclonic storms. Its winds are
+then largely of the purely planetary type, such as trade winds and
+westerlies. Its rainfall also is largely planetary rather than cyclonic.
+It falls in places such as the heat equator where the air rises under
+the influence of heat, or on the windward slopes of mountains, or in
+regions where warm winds blow from the ocean over cold lands.
+
+According to the electro-stellar hypothesis, the conditions which
+prevailed during hundreds of millions of years of mild climate mean
+merely that the solar system was then in parts of the heavens where
+stars--especially double stars--were rare or small, and electrical
+disturbances correspondingly weak. Today, on the other hand, the sun is
+fairly near a number of stars, many of which are large doubles. Hence it
+is supposed to be disturbed, although not so much as at the height of
+the last glacial epoch.
+
+After the preceding parts of this book had been written, the assistance
+of Dr. Schlesinger made it possible to test the electro-stellar
+hypothesis by comparing actual astronomical dates with the dates of
+climatic or solar phenomena. In order to make this possible, Dr.
+Schlesinger and his assistants have prepared Table 6, giving the
+position, magnitude, and motions of the thirty-eight nearest stars, and
+especially the date at which each was nearest the sun. In column 10
+where the dates are given, a minus sign indicates the past and a plus
+sign the future. Dr. Shapley has kindly added column 12, giving the
+absolute magnitudes of the stars, that of the sun being 4.8, and column
+13, showing their luminosity or absolute radiation, that of the sun
+being unity. Finally, column 14 shows the effective radiation received
+by the sun from each star when the star is at a minimum distance. Unity
+in this case is the effect of a star like the sun at a distance of one
+light year.
+
+It is well known that radiation of all kinds, including light, heat, and
+electrical emissions, varies in direct proportion to the exposed
+surface, that is, as the square of the radius of a sphere, and inversely
+as the square of the distance. From black bodies, as we have seen, the
+total radiation varies as the fourth power of the absolute temperature.
+It is not certain that either light or electrical emissions from
+incandescent bodies vary in quite this same proportion, nor is it yet
+certain whether luminous and electrical emissions vary exactly together.
+Nevertheless they are closely related. Since the light coming from each
+star is accurately measured, while no information is available as to
+electrical emissions, we have followed Dr. Shapley's suggestion and used
+the luminosity of the stars as the best available measure of total
+radiation. This is presumably an approximate measure of electrical
+activity, provided some allowance be made for disturbances by outside
+bodies such as companion stars. Hence the inclusion of column 14.
+
+TABLE 6
+
+THIRTY-EIGHT STARS HAVING LARGEST KNOWN PARALLAXES
+
+   Star
+   Code
+   1 Groombr. 34
+   2 ++[Greek: �] Cassiop.
+   3
+   4 ++[Greek: k] Tucan�
+   5 [Greek: t] Ceti
+   6 [Greek: d]_2 Eridani
+   7 ++[Greek: e] Eridani
+   8 ++40(0)^2 Eridani
+   9  Cordoba Z. 243
+   10 Weisse 592
+   11 ++[Greek: a] Can. Maj. (Sirius)
+   12 ++[Greek: a] Can. Min. (Procyon)
+   13 ++Fedorenko 1457-8
+   14  Groombr. 1618
+   15  Weisse 234
+   16  Lalande 21185
+   17  Lalande 21258
+   18
+   19  Lalande 25372
+   20 ++[Greek: a] Centauri
+   21 ++[Greek: x] Bootes
+   22 ++Lalande 27173
+   23  Weisse 1259
+   24  Lacaille 7194
+   25 ++[Greek: b] 416
+   26 Argel -0.17415-6
+   27 Barnard's star
+   28 ++70p Ophiuchi
+   29 ++[Greek: S] 2398
+   30 [Greek: s] Draconis
+   31 ++[Greek: a] Aquil� (Altair)
+   32 ++61 Cygni
+   33 Lacaille 8760
+   34 [Greek: e] Indi
+   35 ++Kr�ger 60
+   36 Lacaille 9352
+   37 Lalande 46650
+   38 C. G. A. 32416
+
+    (++ Double star.)
+
+            (1)          (2)        (3)      (4)      (5)        (6)
+           Right     Declination  Visual  Spectrum  Proper   Radial
+  Star   Ascension   [Greek: d]   Mag. m            Motion  Velocity
+  code   [Greek: a]       1900                              km. per
+               1900                                           sec.
+   ------------------------------------------------------------------
+   1     0^h 12^m.7    +43�27'       8.1      Ma      2".89     +  3
+   2         43 .0     +57 17        3.6      F8      1 .24     + 10
+   3         43 .9      +4 55       12.3      F0      3 .01     .....
+   4       1 12 .4     -69 24        5.0      F8        .39      + 12
+   5         39 .4     -16 28        3.6      K0      1 .92      - 16
+   ------------------------------------------------------------------
+   6       3 15 .9     -43 27        4.3      G5      3 .16      + 87
+   7         28 .2     - 9 48        3.8      K0        .97      + 16
+   8       4 10 .7     - 7 49        4.5      G5      4 .08      - 42
+   9       5  7 .7     -44 59        9.2      K2      8 .75      +242
+   10        26 .4     - 3 42        8.8      K2      2 .22     .....
+   ------------------------------------------------------------------
+   11     6 40 .7      -16 35      -1.6       A0      1 .32      -  8
+   12     7 34 .1      + 5 29       0.5       F5      1 .24      -  4
+   13     9  7 .6      +53  7       7.9       Ma      1 .68      + 10
+   14    10  5 .3      +49 58       6.8       K5p     1 .45      - 30
+   15       14 .2      +20 22       9.0       ...       .49     .....
+   ------------------------------------------------------------------
+   16       57 .9      +36 38       7.6       Mb      4 .78      - 87
+   17    11  0 .5      +44  2       8.5       K5      4 .52      + 65
+   18       12 .0      -57  2      12.0       ...     2 .69     .....
+   19    13 40 .7      +15 26       8.5       K5      2 .30     .....
+   20    14 32 .8      -60 25       0.2       G       3 .68      + 22
+   ------------------------------------------------------------------
+   21    14 46 .8      +19 31       4.6       K5p       .17      +  4
+   22       51 .6      -20 58       5.8       Kp      1 .96      + 20
+   23    16 41 .4      +33 41       8.4       ...       .37     .....
+   24    17 11 .5      -46 32       5.7       K         .97     .....
+   25       12 .1      -34 53       5.9       K5      1 .19      -  4
+   ------------------------------------------------------------------
+   26       37 .0      +68 26       9.1       K       1 .33     .....
+   27       52 .9      + 4 25       9.7       Mb     10 .30      - 80
+   28    18  0 .4      + 2 31       4.3       K       1 .13     .....
+   29       41 .7      +59 29       8.8       K       2 .31     .....
+   30    19 32 .5      +69 29       4.8       G5      1 .84      + 26
+   ------------------------------------------------------------------
+   31       45 .9      + 8 36       1.2       A5        .66      - 33
+   32    21  2 .4      +38 15       5.6       K5      5 .20      - 64
+   33       11 .4      -39 15       6.6       G       3 .53      + 13
+   34       55 .7      -57 12       4.8       K5      4 .70      - 39
+   35    22 24 .4      +57 12       9.2       ...       .87     .....
+   ------------------------------------------------------------------
+   36       59 .4      -36 26       7.1       K       6 .90      + 12
+   37    23 44 .0      + 1 52       8.7       Ma      1 .39     .....
+   38       59 .5      -37 51       8.2       G       6 .05      + 26
+
+
+         (7)          (9)           (11)             (13)       (14)
+       Present      Minimum       Magnitude       Luminosity  Effective
+       Parallax    Distance      at Min. Dist.          |     radiation
+      [Greek: p]   Light Yrs.          |                |        at
+         |            |                |                |      minimum
+         |     (8)    |       (10)     |     (12)       |     distance
+ Star    |   Maximum  |     Time of    |   Absolute     |     from sun
+ Code    |  Parallax  |     Minimum    |   Magnitude    |         |
+         |     |      |     Distance   |      |         |         |
+ -----------------------------------------------------------------------
+ 1   ".28    ".28    11.6     -4000   8.1   10.3      0.0063    0.000051
+ 2    .18     .19    17.1    -47000   3.5    4.9      0.91      0.003110
+ 3    .24    ....    ....    ......  ....   14.2      0.00017   ........
+ 4    .16     .23    14.2   -264000   4.2    6.0      0.33      0.001610
+ 5    .32     .37     8.8    +46000   3.3    6.1      0.30      0.003840
+ -----------------------------------------------------------------------
+ 6    .16     .22    14.8    -33000   3.6    5.3      0.63      0.002960
+ 7    .31     .46     7.1   -106000   3.0    6.3      0.25      0.004970
+ 8    .21     .23    14.2    +19000   4.3    6.1      0.30      0.001470
+ 9    .32     .68     4.8    -10000   7.6   11.7      0.0017    0.000074
+ 10   .17    ....    ....    ......   ....   9.9      0.009     ........
+ -----------------------------------------------------------------------
+ 11   .37     .41     8.0    +65000  -1.8    1.2     27.50      0.429000
+ 12   .31     .32    10.2    +34000   0.5    3.0      5.25      0.051300
+ 13   .16     .16    20.4    -24000   7.9    8.9      0.023     0.000055
+ 14   .18     .23    14.2    +69000   6.3    8.1      0.048     0.000238
+ 15   .19    ....    ....    ......   ....  10.4      0.0057    ........
+ -----------------------------------------------------------------------
+ 16   .41     .76     4.3    +20000   6.2   10.7      0.0044    0.000238
+ 17   .19     .22    14.8    -20000   8.2    9.9      0.009     0.000041
+ 18   .34    ....    ....    ......   ....  14.7      0.00011   ........
+ 19   .19    ....    ....    ......   ....   9.9      0.009     ........
+ 20   .76    1.03     3.2    -28000  -0.5    4.6      1.20      0.117500
+ -----------------------------------------------------------------------
+ 21   .17     .22    14.8   -598000   4.0    5.8      0.40      0.001815
+ 22   .18     .19    17.1    -36000   5.6    7.1      0.12      0.000412
+ 23   .18    ....    ....    ......   ....   9.7      0.011     ........
+ 24   .19    ....    ....    ......   ....   7.1      0.12      ........
+ 25   .17     .17    19.2    +21000   5.7    7.1      0.12      0.000329
+ -----------------------------------------------------------------------
+ 26   .22    ....    ....    ......   ....  10.8      0.004     ........
+ 27   .53     .70     4.7    +10000   9.1   13.3      0.0025    0.000114
+ 28   .19    ....    ....    ......   ....   5.7      0.44      ........
+ 29   .29    ....    ....    ......   ....  11.1      0.0030    ........
+ 30   .20     .23    14.2    -49000   4.5    6.3      0.25      0.001238
+ -----------------------------------------------------------------------
+ 31   .21     .51     6.4   +117000  -0.7    2.8      6.30      0.153600
+ 32   .30     .38     8.6    +19000   5.1    8.0      0.053     0.000715
+ 33   .25     .26    12.6    -11000   6.6    8.6      0.030     0.000189
+ 34   .28     .31    10.5    +17000   4.6    7.0      0.13      0.001230
+ 35   .26    ....    ....   .......  ....   11.3      0.0025    ........
+ -----------------------------------------------------------------------
+ 36   .29     .29    11.2     -3000   7.1    9.4      0.014     0.000111
+ 37   .17    ....    ....   .......  ....    9.9      0.009     ........
+ 38   .22     .22    14.8     -7000   8.2    9.9      0.009     0.000041
+ -----------------------------------------------------------------------
+
+On the basis of column 14 and of the movements and distances of the
+stars as given in the other columns Fig. 10 has been prepared. This
+gives an estimate of the approximate electrical energy received by the
+sun from the nearest stars for 70,000 years before and after the
+present. It is based on the twenty-six stars for which complete data are
+available in Table 6. The inclusion of the other twelve would not alter
+the form of the curve, for even the largest of them would not change any
+part by more than about half of 1 per cent, if as much. Nor would the
+curve be visibly altered by the omission of all except four of the
+twenty-six stars actually used. The four that are important, and their
+relative luminosity when nearest the sun, are Sirius 429,000, Altair
+153,000, Alpha Centauri 117,500, and Procyon 51,300. The figure for the
+next star is only 4970, while for this star combined with the other
+twenty-one that are unimportant it is only 24,850.
+
+Figure 10 is not carried more than 70,000 years into the past or into
+the future because the stars near the sun at more remote times are not
+included among the thirty-eight having the largest known parallaxes.
+That is, they have either moved away or are not yet near enough to be
+included. Indeed, as Dr. Schlesinger strongly emphasizes, there may be
+swiftly moving, bright or gigantic stars which are now quite far away,
+but whose inclusion would alter Fig. 10 even within the limits of the
+140,000 years there shown. It is almost certain, however, that the most
+that these would do would be to raise, but not obliterate, the minima on
+either side of the main maximum.
+
+[Illustration: _Fig. 10. Climatic changes of 140,000 years as inferred
+from the stars._]
+
+In preparing Fig. 10 it has been necessary to make allowance for double
+stars. Passing by the twenty-two unimportant stars, it appears that the
+companion of Sirius is eight or ten magnitudes smaller than that star,
+while the companions of Procyon and Altair are five or more magnitudes
+smaller than their bright comrades. This means that the luminosity of
+the faint components is at most only 1 per cent of that of their bright
+companions and in the case of Sirius not a hundredth of 1 per cent.
+Hence their inclusion would have no visible effect on Fig. 10. In Alpha
+Centauri, on the other hand, the two components are of almost the same
+magnitude. For this reason the effective radiation of that star as given
+in column 14 is doubled in Fig. 10, while for another reason it is
+raised still more. The other reason is that if our inferences as to the
+electrical effect of the sun on the earth and of the planets on the sun
+are correct, double stars, as we have seen, must be much more effective
+electrically than single stars. By the same reasoning two bright stars
+close together must excite one another much more than a bright star and
+a very faint one, even if the distances in both cases are the same. So,
+too, other things being equal, a triple star must be more excited
+electrically than a double star. Hence in preparing Fig. 10 all double
+stars receive double weight and each part of Alpha Centauri receives an
+additional 50 per cent because both parts are bright and because they
+have a third companion to help in exciting them.
+
+According to the electro-stellar hypothesis, Alpha Centauri is more
+important climatically than any other star in the heavens not only
+because it is triple and bright, but because it is the nearest of all
+stars, and moves fairly rapidly. Sirius and Procyon move slowly in
+respect to the sun, only about eleven and eight kilometers per second
+respectively, and their distances at minimum are fairly large, that is,
+8 and 10.2 light years. Hence their effect on the sun changes slowly.
+Altair moves faster, about twenty-six kilometers per second, and its
+minimum distance is 6.4 light years, so that its effect changes fairly
+rapidly. Alpha Centauri moves about twenty-four kilometers per second,
+and its minimum distance is only 3.2 light years. Hence its effect
+changes very rapidly, the change in its apparent luminosity as seen from
+the sun amounting at maximum to about 30 per cent in 10,000 years
+against 14 per cent for Altair, 4 for Sirius, and 2 for Procyon. The
+vast majority of the stars change so much more slowly than even Procyon
+that their effect is almost uniform. All the stars at a distance of more
+than perhaps twenty or thirty light years may be regarded as sending to
+the sun a practically unchanging amount of radiation. It is the bright
+stars within this limit which are important, and their importance
+increases with their proximity, their speed of motion, and the
+brightness and number of their companions. Hence Alpha Centauri causes
+the main maximum in Fig. 10, while Sirius, Altair, and Procyon combine
+to cause a general rise of the curve from the past to the future.
+
+Let us now interpret Fig. 10 geologically. The low position of the curve
+fifty to seventy thousand years ago suggests a mild inter-glacial
+climate distinctly less severe than that of the present. Geologists say
+that such was the case. The curve suggests a glacial epoch culminating
+about 28,000 years ago. The best authorities put the climax of the last
+glacial epoch between twenty-five and thirty thousand years ago. The
+curve shows an amelioration of climate since that time, although it
+suggests that there is still considerable severity. The retreat of the
+ice from North America and Europe, and its persistence in Greenland and
+Antarctica agree with this. And the curve indicates that the change of
+climate is still persisting, a conclusion in harmony with the evidence
+as to historic changes.
+
+If Alpha Centauri is really so important, the effect of its variations,
+provided it has any, ought perhaps to be evident in the sun. The
+activity of the star's atmosphere presumably varies, for the orbits of
+the two components have an eccentricity of 0.51. Hence during their
+period of revolution, 81.2 years, the distance between them ranges from
+1,100,000,000 to 3,300,000,000 miles. They were at a minimum distance in
+1388, 1459, 1550, 1631, 1713, 1794, 1875, and will be again in 1956. In
+Fig. 11, showing sunspot variations, it is noticeable that the years
+1794 and 1875 come just at the ends of periods of unusual solar
+activity, as indicated by the heavy horizontal line. A similar period of
+great activity seems to have begun about 1914. If its duration equals
+the average of its two predecessors, it will end about 1950. Back in the
+fourteenth century a period of excessive solar activity, which has
+already been described, culminated from 1370 to 1385, or just before the
+two parts of Alpha Centauri were at a minimum distance. Thus in three
+and perhaps four cases the sun has been unusually active during a time
+when the two parts of the star were most rapidly approaching each other
+and when their atmospheres were presumably most disturbed and their
+electrical emanations strongest.
+
+[Illustration: _Fig. 11. Sunspot curve showing cycles, 1750 to 1920._
+
+_Note._ The asterisks indicate two absolute minima of sunspots in 1810
+and 1913, and the middle years (1780 and 1854) of two periods when the
+sunspot maxima never fell below 95. If Alpha Centauri has an effect on
+the sun's atmosphere, the end of another such period would be expected
+not far from 1957.]
+
+The fact that Alpha Centauri, the star which would be expected most
+strongly to influence the sun, and hence the earth, was nearest the sun
+at the climax of the last glacial epoch, and that today the solar
+atmosphere is most active when the star is presumably most disturbed may
+be of no significance. It is given for what it is worth. Its importance
+lies not in the fact that it proves anything but that no contradiction
+is found when we test the electro-stellar hypothesis by facts which were
+not thought of when the hypothesis was framed. A vast amount of
+astronomical work is still needed before the matter can be brought to
+any definite conclusion. In case the hypothesis stands firm, it may be
+possible to use the stars as a help in determining the exact chronology
+of the later part of geological times. If the hypothesis is disproved,
+it will merely leave the question of solar variations where it is today.
+It will not influence the main conclusions of this book as to the causes
+and nature of climatic changes. Its value lies in the fact that it calls
+attention to new lines of research.
+
+FOOTNOTES:
+
+[Footnote 120: Lewis Boss: Convergent of a Moving Cluster in Taurus;
+Astronom. Jour., Vol. 26, No. 4, 1908, pp. 31-36.]
+
+[Footnote 121: F. R. Moulton: in Introduction to Astronomy, 1916.]
+
+[Footnote 122: A. Penck: Die Alpen im Eiszeitalter, Leipzig, 1909.]
+
+[Footnote 123: R. D. Salisbury: Physical Geography of the Pleistocene,
+in Outlines of Geologic History, by Willis and Salisbury, 1910, pp.
+273-274.]
+
+[Footnote 124: Davis, Pumpelly, and Huntington: Explorations in
+Turkestan, Carnegie Inst. of Wash., No. 26, 1905.
+
+In North America the stages have been the subject of intensive studies
+on the part of Taylor, Leverett, Goldthwait, and many others.]
+
+
+
+
+CHAPTER XVI
+
+THE EARTH'S CRUST AND THE SUN
+
+
+Although the problems of this book may lead far afield, they ultimately
+bring us back to the earth and to the present. Several times in the
+preceding pages there has been mention of the fact that periods of
+extreme climatic fluctuations are closely associated with great
+movements of the earth's crust whereby mountains are uplifted and
+continents upheaved. In attempting to explain this association the
+general tendency has been to look largely at the past instead of the
+present. Hence it has been almost impossible to choose among three
+possibilities, all beset with difficulties. First, the movements of the
+crust may have caused the climatic fluctuations; second, climatic
+changes may cause crustal movements; and third, variations in solar
+activity or in some other outside agency may give rise to both types of
+terrestrial phenomena.
+
+The idea that movements of the earth's crust are the main cause of
+geological changes of climate is becoming increasingly untenable as the
+complexity and rapidity of climatic changes become more clear,
+especially during post-glacial times. It implies that the earth's
+surface moves up and down with a speed and facility which appear to be
+out of the question. If volcanic activity be invoked the problem becomes
+no clearer. Even if volcanic dust should fill the air frequently and
+completely, neither its presence nor absence would produce such peculiar
+features as the localization of glaciers, the distribution of loess, and
+the mild climate of most parts of geological time. Nevertheless, because
+of the great difficulties presented by the other two possibilities many
+geologists still hold that directly or indirectly the greater climatic
+changes have been mainly due to movements of the earth's crust and to
+the reaction of the crustal movements on the atmosphere.
+
+The possibility that climatic changes are in themselves a cause of
+movements of the earth's crust seems so improbable that no one appears
+to have investigated it with any seriousness. Nevertheless, it is worth
+while to raise the question whether climatic extremes may co�perate with
+other agencies in setting the time when the earth's crust shall be
+deformed.
+
+As to the third possibility, it is perfectly logical to ascribe both
+climatic changes and crustal deformation to some outside agency, solar
+or otherwise, but hitherto there has been so little evidence on this
+point that such an ascription has merely begged the question. If
+heavenly bodies should approach the earth closely enough so that their
+gravitational stresses caused crustal deformation, all life would
+presumably be destroyed. As to the sun, there has hitherto been no
+conclusive evidence that it is related to crustal movements, although
+various writers have made suggestions along this line. In this chapter
+we shall carry these suggestions further and shall see that they are at
+least worthy of study.
+
+As a preliminary to this study it may be well to note that the
+coincidence between movements of the earth's crust and climatic changes
+is not so absolute as is sometimes supposed. For example, the profound
+crustal changes at the end of the Mesozoic were not accompanied by
+widespread glaciation so far as is yet known, although the temperature
+appears to have been lowered. Nor was the violent volcanic and
+diastrophic activity in the Miocene associated with extreme climates.
+Indeed, there appears to have been little contrast from zone to zone,
+for figs, bread fruit trees, tree ferns, and other plants of low
+latitudes grew in Greenland. Nevertheless, both at the end of the
+Mesozoic and in the Miocene the climate may possibly have been severe
+for a time, although the record is lost. On the other hand, Kirk's
+recent discovery of glacial till in Alaska between beds carrying an
+undoubted Middle Silurian fauna indicates glaciation at a time when
+there was little movement of the crust so far as yet appears.[125] Thus
+we conclude that while climatic changes and crustal movements usually
+occur together, they may occur separately.
+
+According to the solar-cyclonic hypothesis such a condition is to be
+expected. If the sun were especially active when the terrestrial
+conditions prohibited glaciation, changes of climate would still occur,
+but they would be milder than under other circumstances, and would leave
+little record in the rocks. Or there might be glaciation in high
+latitudes, such as that of southern Alaska in the Middle Silurian, and
+none elsewhere. On the other hand, when the sun was so inactive that no
+great storminess occurred, the upheaval of continents and the building
+of mountains might go on without the formation of ice sheets, as
+apparently happened at the end of the Mesozoic. The lack of absolute
+coincidence between glaciation and periods of widespread emergence of
+the lands is evident even today, for there is no reason to suppose that
+the lands are notably lower or less extensive now than they were during
+the Pleistocene glaciation. In fact, there is much evidence that many
+areas have risen since that time. Yet glaciation is now far less
+extensive than in the Pleistocene. Any attempt to explain this
+difference on the basis of terrestrial changes is extremely difficult,
+for the shape and altitude of continents and mountains have not changed
+much in twenty or thirty thousand years. Yet the present moderately mild
+epoch, like the puzzling inter-glacial epochs of earlier times, is
+easily explicable on the assumption that the sun's atmosphere may
+sometimes vary in harmony with crustal activity, but does not
+necessarily do so at all times.
+
+Turning now to the main problem of how climatic changes may be connected
+with movements of the earth's crust, let us follow our usual method and
+examine what is happening today. Let us first inquire whether
+earthquakes, which are one of the chief evidences that crustal movements
+are actually taking place in our own times, show any connection with
+sunspots. In order to test this, we have compared _Milne's Catalogue of
+Destructive Earthquakes_ from 1800 to 1899, with Wolf's sunspot numbers
+for the same period month by month. The earthquake catalogue, as its
+compiler describes it, "is an attempt to give a list of earthquakes
+which have announced changes of geological importance in the earth's
+crust; movements which have probably resulted in the creation or the
+extension of a line of fault, the vibrations accompanying which could,
+with proper instruments, have been recorded over a continent or the
+whole surface of our world. Small earthquakes have been excluded, while
+the number of large earthquakes both for ancient and modern times has
+been extended. As an illustration of exclusion, I may mention that
+between 1800 and 1808, which are years taken at random, I find in
+Mallet's catalogue 407 entries. Only thirty-seven of these, which were
+accompanied by structural damage, have been retained. Other catalogues
+such as those of Perry and Fuchs have been treated similarly."[126]
+
+If the earthquakes in such a carefully selected list bear a distinct
+relation to sunspots, it is at least possible and perhaps probable that
+a similar relation may exist between solar activity and geological
+changes in the earth's crust. The result of the comparison of
+earthquakes and sunspots is shown in Table 7. The first column gives the
+sunspot numbers; the second, the number of months that had the
+respective spot numbers during the century from 1800 to 1899. Column C
+shows the total number of earthquakes during the months having any
+particular degree of spottedness; while D, which is the significant
+column, gives the average number of destructive earthquakes per month
+under each of the six conditions of solar spottedness. The regularity of
+column D is so great as to make it almost certain that we are here
+dealing with a real relationship. Column F, which shows the average
+number of earthquakes in the month succeeding any given condition of the
+sun, is still more regular except for the last entry.
+
+TABLE 7
+
+DESTRUCTIVE EARTHQUAKES FROM 1800 TO 1899 COMPARED WITH SUNSPOTS
+
+  A: _Sunspot numbers_
+  B: _Number of months per Wolf's Table_
+  C: _Number of earthquakes_
+  D: _Average number of earthquakes per month_
+  E: _Number of earthquakes in succeeding month_
+  F: _Average number of earthquakes in succeeding month_
+
+        A       B          C            D            E           F
+
+       0-15   344        522          1.52         512          1.49
+      15-30   194        306          1.58         310          1.60
+      30-50   237        433          1.83         439          1.85
+      50-70   195        402          2.06         390          2.00
+     70-100   135        286          2.12         310          2.30
+   over 100    95        218          2.30         175          1.84
+
+The chance that six numbers taken at random will arrange themselves in
+any given order is one in 720. In other words, there is one chance in
+720 that the regularity of column D is accidental. But column F is as
+regular as column D except for the last entry. If columns D and E were
+independent there would be one chance in about 500,000 that the six
+numbers in both columns would fall in the same order, and one chance in
+14,400 that five numbers in each would fall in the same order. But the
+two columns are somewhat related, for although the after-shocks of a
+great earthquake are never included in Milne's table, a world-shaking
+earthquake in one region during a given month probably creates
+conditions that favor similar earthquakes elsewhere during the next
+month. Hence the probability that we are dealing with a purely
+accidental arrangement in Table 7 is less than one in 14,400 and greater
+than one in 500,000. It may be one in 20,000 or 100,000. In any event it
+is so slight that there is high probability that directly or indirectly
+sunspots and earthquakes are somehow connected.
+
+In ascertaining the relation between sunspots and earthquakes it would
+be well if we could employ the strict method of correlation
+coefficients. This, however, is impossible for the entire century, for
+the record is by no means homogeneous. The earlier decades are
+represented by only about one-fourth as many earthquakes as the later
+ones, a condition which is presumably due to lack of information. This
+makes no difference with the method employed in Table 7, since years
+with many and few sunspots are distributed almost equally throughout the
+entire nineteenth century, but it renders the method of correlation
+coefficients inapplicable. During the period from 1850 onward the record
+is much more nearly homogeneous, though not completely so. Even in these
+later decades, however, allowance must be made for the fact that there
+are more earthquakes in winter than in summer, the average number per
+month for the fifty years being as follows:
+
+    Jan. 2.8              May  2.4              Sept. 2.5
+    Feb. 2.4              June 2.3              Oct.  2.6
+    Mar. 2.5              July 2.4              Nov.  2.7
+    Apr. 2.4              Aug. 2.4              Dec.  2.8
+
+The correlation coefficient between the departures from these monthly
+averages and the corresponding departures from the monthly averages of
+the sunspots for the same period, 1850-1899, are as follows:
+
+    Sunspots and earthquakes of same month: +0.042, or 1.5 times the
+    probable error.
+
+    Sunspots of a given month and earthquakes of that month and the
+    next: +0.084, or 3.1 times the probable error.
+
+    Sunspots of three consecutive months and earthquakes of three
+    consecutive months allowing a lag of one month, i.e., sunspots of
+    January, February, and March compared with earthquakes of February,
+    March, and April; sunspots of February, March, and April with
+    earthquakes of March, April, and May, etc.; +0.112, or 4.1 times the
+    probable error.
+
+These coefficients are all small, but the number of individual cases,
+600 months, is so large that the probable error is greatly reduced,
+being only �0.027 or �0.028. Moreover, the nature of our data is such
+that even if there is a strong connection between solar changes and
+earth movements, we should not expect a large correlation coefficient.
+In the first place, as already mentioned, the earthquake data are not
+strictly homogeneous. Second, an average of about two and one-half
+strong earthquakes per month is at best only a most imperfect indication
+of the actual movement of the earth's crust. Third, the sunspots are
+only a partial and imperfect measure of the activity of the sun's
+atmosphere. Fourth, the relation between solar activity and earthquakes
+is almost certainly indirect. In view of all these conditions, the
+regularity of Table 7 and the fact that the most important correlation
+coefficient rises to more than four times the probable error makes it
+almost certain that the solar and terrestrial phenomena are really
+connected.
+
+We are now confronted by the perplexing question of how this connection
+can take place. Thus far only three possibilities present themselves,
+and each is open to objections. The chief agencies concerned in these
+three possibilities are heat, electricity, and atmospheric pressure.
+Heat may be dismissed very briefly. We have seen that the earth's
+surface becomes relatively cool when the sun is active. Theoretically
+even the slightest change in the temperature of the earth's surface must
+influence the thermal gradient far into the interior and hence cause a
+change of volume which might cause movements of the crust. Practically
+the heat of the surface ceases to be of appreciable importance at a
+depth of perhaps twenty feet, and even at that depth it does not act
+quickly enough to cause the relatively prompt response which seems to be
+characteristic of earthquakes in respect to the sun.
+
+The second possibility is based on the relationship between solar and
+terrestrial electricity. When the sun is active the earth's atmospheric
+electrical potential is subject to slight variations. It is well known
+that when two opposing points of an ionized solution are oppositely
+charged electrically, a current passes through the liquid and sets up
+electrolysis whereby there is a segregation of materials, and a
+consequent change in the volume of the parts near the respective
+electrical poles. The same process takes place, although less freely, in
+a hot mass such as forms the interior of the earth. The question arises
+whether internal electrical currents may not pass between the two
+oppositely charged poles of the earth, or even between the great
+continental masses and the regions of heavier rock which underlie the
+oceans. Could this lead to electrolysis, hence to differentiation in
+volume, and thus to movements of the earth's crust? Could the results
+vary in harmony with the sun? Bowie[127] has shown that numerous
+measurements of the strength and direction of the earth's gravitative
+pull are explicable only on the assumption that the upheaval of a
+continent or a mountain range is due in part not merely to pressure, or
+even to flowage of the rocks beneath the crust, but also to an actual
+change in volume whereby the rocks beneath the continent attain
+relatively great volume and those under the oceans a small volume in
+proportion to their weight. The query arises whether this change of
+volume may be related to electrical currents at some depth below the
+earth's surface.
+
+The objections to this hypothesis are numerous. First, there is little
+evidence of electrolytic differentiation in the rocks. Second, the outer
+part of the earth's crust is a very poor conductor so that it is
+doubtful whether even a high degree of electrification of the surface
+would have much effect on the interior. Third, electrolysis due to any
+such mild causes as we have here postulated must be an extremely slow
+process, too slow, presumably, to have any appreciable result within a
+month or two. Other objections join with these three in making it seem
+improbable that the sun's electrical activity has any direct effect upon
+movements of the earth's crust.
+
+The third, or meteorological hypothesis, which makes barometric pressure
+the main intermediary between solar activity and earthquakes, seems at
+first sight almost as improbable as the thermal and electrical
+hypotheses. Nevertheless, it has a certain degree of observational
+support of a kind which is wholly lacking in the other two cases. Among
+the extensive writings on the periodicity of earthquakes one main fact
+stands out with great distinctness: earthquakes vary in number according
+to the season. This fact has already been shown incidentally in the
+table of earthquake frequency by months. If allowance is made for the
+fact that February is a short month, there is a regular decrease in the
+frequency of severe earthquakes from December and January to June. Since
+most of Milne's earthquakes occurred in the northern hemisphere, this
+means that severe earthquakes occur in winter about 20 per cent oftener
+than in summer.
+
+The most thorough investigation of this subject seems to have been that
+of Davisson.[128] His results have been worked over and amplified by
+Knott,[129] who has tested them by Schuster's exact mathematical
+methods. His results are given in Table 8.[130] Here the northern
+hemisphere is placed first; then come the East Indies and the Malay
+Archipelago lying close to the equator; and finally the southern
+hemisphere. In the northern hemisphere practically all the maxima come
+in the winter, for the month of December appears in fifteen cases out of
+the twenty-five in column D, while January, February, or November
+appears in six others. It is also noticeable that in sixteen cases out
+of twenty-five the ratio of the actual to the expected amplitude in
+column G is four or more, so that a real relationship is indicated,
+while the ratio falls below three only in Japan and Zante. The
+equatorial data, unlike those of the northern hemisphere, are
+indefinite, for in the East Indies no month shows a marked maximum and
+the expected amplitude exceeds the actual amplitude. Even in the Malay
+Archipelago, which shows a maximum in May, the ratio of actual to
+expected amplitude is only 2.6. Turning to the southern hemisphere, the
+winter months of that hemisphere are as strongly marked by a maximum as
+are the winter months of the northern hemisphere. July or August appears
+in five out of six cases. Here the ratio between the actual and expected
+amplitudes is not so great as in the northern hemisphere. Nevertheless,
+it is practically four in Chile, and exceeds five in Peru and Bolivia,
+and in the data for the entire southern hemisphere.
+
+TABLE 8
+
+SEASONAL MARCH OF EARTHQUAKES
+
+AFTER DAVISSON AND KNOTT
+
+  A: _Region_
+  B: _Limiting Dates_
+  C: _Number of Shocks_
+  D: _Maximum Month_
+  E: _Amplitude_
+  F: _Expected Amplitude_
+  G: _Ratio of Actual to Expected Amplitude_
+
+           A              B       C        D        E      F        G
+
+  Northern Hemisphere  223-1850  5879     Dec.    0.110   0.023    4.8
+  Northern Hemisphere 1865-1884  8133     Dec.    0.290   0.020   14.5
+  Europe              1865-1884  5499     Dec.    0.350   0.024   14.6
+  Europe               306-1843  1961     Dec.    0.220   0.040    5.5
+  Southeast Europe    1859-1887  3470     Dec.    0.210   0.030    7.0
+  Vesuvius District   1865-1883   513     Dec.    0.250   0.078    3.2
+  Italy:
+    Old Tromometre    1872-1887 61732     Dec.    0.490   0.007   70.0
+    Old Tromometre    1876-1887 38546     Dec.    0.460   0.009   49.5
+    Normal Tromometre 1876-1887 38546     Dec.    0.490   0.009   52.8
+  Balkan, etc.        1865-1884   624     Dec.    0.270   0.071    3.8
+  Hungary, etc.       1865-1884   384     Dec.    0.310   0.090    3.4
+  Italy               1865-1883  2350  Dec.(Sept.)0.140   0.037    3.8
+  Grecian Archip.     1859-1881  3578   Dec.-Jan. 0.164   0.030    5.5
+  Austria             1865-1884   461     Jan.    0.370   0.083    4.4
+  Switzerland, etc.   1865-1883   524     Jan.    0.560   0.077    7.3
+  Asia                1865-1884   458     Feb.    0.330   0.083    4.0
+  North America       1865-1884   552     Nov.    0.350   0.075    4.7
+  California          1850-1886   949     Oct.    0.300   0.058    5.2
+  Japan               1878-1881   246     Dec.    0.460   0.113    4.1
+  Japan               1872-1880   367  Dec.-Jan.  0.256   0.093    2.8
+  Japan               1876-1891  1104     Feb.    0.190   0.053    3.6
+  Japan               1885-1889  2997     Oct.    0.080   0.032    2.5
+  Zante               1825-1863  1326     Aug.    0.100   0.049    2.0
+  Italy, North        1865-1883  1513 Sept.(Nov.) 0.210   0.046    4.6
+        of Naples
+  East Indies         1873-1881   515 Aug., Oct., 0.071?  0.078    0.9
+                                        or Dec.?
+  Malay Archip.       1865-1884   598     May     0.190   0.072    2.6
+  New Zealand         1869-1879   585  Aug.-Sept. 0.203   0.073    2.8
+  Chile               1873-1881   212     July    0.480   0.122    3.9
+  Southern Hemisphere 1865-1884   751     July    0.370   0.065    5.7
+  New Zealand         1868-1890   641  March, May 0.050   0.070    0.7
+  Chile               1865-1883?  316  July, Dec. 0.270   0.100    2.7
+  Peru, Bolivia       1865-1884   350     July    0.480   0.095    5.1
+
+The whole relationship between earthquakes and the seasons in the
+northern and southern hemispheres is summed up in Fig. 12 taken from
+Knott. The northern hemisphere shows a regular diminution in earthquake
+frequency from December until June, and an increase the rest of the
+year. In the southern hemisphere the course of events is the same so far
+as summer and winter are concerned, for August with its maximum comes in
+winter, while February with its minimum comes in summer. In the southern
+hemisphere the winter month of greatest seismic activity has over 100
+per cent more earthquakes than the summer month of least activity. In
+the northern hemisphere this difference is about 80 per cent, but this
+smaller figure occurs partly because the northern data include certain
+interesting and significant regions like Japan and China where the usual
+conditions are reversed.[131] If equatorial regions were included in
+Fig. 12, they would give an almost straight line.
+
+The connection between earthquakes and the seasons is so strong that
+almost no students of seismology question it, although they do not agree
+as to its cause. A meteorological hypothesis seems to be the only
+logical explanation.[132] Wherever sufficient data are available,
+earthquakes appear to be most numerous when climatic conditions cause
+the earth's surface to be most heavily loaded or to change its load most
+rapidly. The main factor in the loading is apparently atmospheric
+pressure. This acts in two ways. First, when the continents become cold
+in winter the pressure increases. On an average the air at sea level
+presses upon the earth's surface at the rate of 14.7 pounds per square
+inch, or over a ton per square foot, and only a little short of thirty
+million tons per square mile. An average difference of one inch between
+the atmospheric pressure of summer and winter over ten million square
+miles of the continent of Asia, for example, means that the continent's
+load in winter is about ten million million tons heavier than in summer.
+Second, the changes in atmospheric pressure due to the passage of storms
+are relatively sharp and sudden. Hence they are probably more effective
+than the variations in the load from season to season. This is suggested
+by the rapidity with which the terrestrial response seems to follow the
+supposed solar cause of earthquakes. It is also suggested by the fact
+that violent storms are frequently followed by violent earthquakes.
+"Earthquake weather," as Dr. Schlesinger suggests, is a common phrase in
+the typhoon region of Japan, China, and the East Indies. During tropical
+hurricanes a change of pressure amounting to half an inch in two hours
+is common. On September 22, 1885, at False Point Lighthouse on the Bay
+of Bengal, the barometer fell about an inch in six hours, then nearly an
+inch and a half in not much over two hours, and finally rose fully two
+inches inside of two hours. A drop of two inches in barometric pressure
+means that a load of about two million tons is removed from each square
+mile of land; the corresponding rise of pressure means the addition of a
+similar load. Such a storm, and to a less degree every other storm,
+strikes a blow upon the earth's surface, first by removing millions of
+tons of pressure and then by putting them on again.[133] Such storms, as
+we have seen, are much more frequent and severe when sunspots are
+numerous than at other times. Moreover, as Veeder[134] long ago showed,
+one of the most noteworthy evidences of a connection between sunspots
+and the weather is a sudden increase of pressure in certain widely
+separated high pressure areas. In most parts of the world winter is not
+only the season of highest pressure and of most frequent changes of
+Veeder's type, but also of severest storms. Hence a meteorological
+hypothesis would lead to the expectation that earthquakes would occur
+more frequently in winter than in summer. On the Chinese coast, however,
+and also on the oceanic side of Japan, as well as in some more tropical
+regions, the chief storms come in summer in the form of typhoons. These
+are the places where earthquakes also are most abundant in summer. Thus,
+wherever we turn, storms and the related barometric changes seem to be
+most frequent and severe at the very times when earthquakes are also
+most frequent.
+
+[Illustration: _Fig. 12. Seasonal distribution of earthquakes. (After
+Davisson and Knott.)_
+
+  solid line   ----  Northern Hemisphere.
+  dashed line  ....  Southern Hemisphere.]
+
+Other meteorological factors, such as rain, snow, winds, and currents,
+probably have some effect on earthquakes through their ability to load
+the earth's crust. The coming of vegetation may also help. These
+agencies, however, appear to be of small importance compared with the
+storms. In high latitudes and in regions of abundant storminess most of
+these factors generally combine with barometric pressure to produce
+frequent changes in the load of the earth's crust, especially in winter.
+In low latitudes, on the other hand, there are few severe storms, and
+relatively little contrast in pressure and vegetation from season to
+season; there is no snow; and the amount of ground water changes little.
+With this goes the twofold fact that there is no marked seasonal
+distribution of earthquakes, and that except in certain local volcanic
+areas, earthquakes appear to be rare. In proportion to the areas
+concerned, for example, there is little evidence of earthquakes in
+equatorial Africa and South America.
+
+The question of the reality of the connection between meteorological
+conditions and crustal movements is so important that every possible
+test should be applied. At the suggestion of Professor Schlesinger we
+have looked up a very ingenious line of inquiry. During the last decades
+of the nineteenth century, a long series of extremely accurate
+observations of latitude disclosed a fact which had previously been
+suspected but not demonstrated, namely, that the earth wabbles a little
+about its axis. The axis itself always points in the same direction, and
+since the earth slides irregularly around it the latitude of all parts
+of the earth keeps changing. Chandler has shown that the wabbling thus
+induced consists of two parts. The first is a movement in a circle with
+a radius of about fifteen feet which is described in approximately 430
+days. This so-called Eulerian movement is a normal gyroscopic motion
+like the slow gyration of a spinning top. This depends on purely
+astronomical causes, and no terrestrial cause can stop it or eliminate
+it. The period appears to be constant, but there are certain puzzling
+irregularities. The usual amplitude of this movement, as
+Schlesinger[135] puts it, "is about 0".27, but twice in recent years it
+has jumped to 0".40. Such a change could be accounted for by supposing
+that the earth had received a severe blow or a series of milder blows
+tending in the same direction." These blows, which were originally
+suggested by Helmert are most interesting in view of our suggestion as
+to the blows struck by storms.
+
+The second movement of the pole has a period of a year, and is roughly
+an ellipse whose longest radius is fourteen feet and the shortest, four
+feet; or, to put it technically, there is an annual term with a maximum
+amplitude of about 0".20. This, however, varies irregularly. The result
+is that the pole seems to wander over the earth's surface in the spiral
+fashion illustrated in Fig. 13. It was early suggested that this
+peculiar wandering of the pole in an annual period must be due to
+meteorological causes. Jeffreys[136] has investigated the matter
+exhaustively. He assumes certain reasonable values for the weight of air
+added or subtracted from different parts of the earth's surface
+according to the seasons. He also considers the effect of precipitation,
+vegetation, and polar ice, and of variations of temperature and
+atmospheric pressure in their relation to movements of the ocean. Then
+he proceeds to compare all these with the actual wandering of the pole
+from 1907 to 1913. While it is as yet too early to say that any special
+movement of the pole was due to the specific meteorological conditions
+of any particular year, Jeffreys' work makes it clear that
+meteorological causes, especially atmospheric pressure, are sufficient
+to cause the observed irregular wanderings. Slight wanderings may arise
+from various other sources such as movements of the rocks when
+geological faults occur or the rush of a great wave due to a submarine
+earthquake. So far as known, however, all these other agencies cause
+insignificant displacements compared with those arising from movements
+of the air. This fact coupled with the mathematical certainty that
+meteorological phenomena must produce some wandering of the pole, has
+caused most astronomers to accept Jeffreys' conclusion. If we follow
+their example we are led to conclude that changes in atmospheric
+pressure and in the other meteorological conditions strike blows which
+sometimes shift the earth several feet from its normal position in
+respect to the axis.
+
+[Illustration: _Fig. 13. Wandering of the pole from 1890 to 1898._
+(_After Moulton._)]
+
+If the foregoing reasoning is correct, the great and especially the
+sudden departures from the smooth gyroscopic circle described by the
+pole in the Eulerian motion would be expected to occur at about the same
+time as unusual earthquake activity. This brings us to an interesting
+inquiry carried out by Milne[137] and amplified by Knott.[138] Taking
+Albrecht's representation of the irregular spiral-like motion of the
+pole, as given in Fig. 13, they show that there is a preponderance of
+severe earthquakes at times when the direction of motion of the earth in
+reference to its axis departs from the smooth Eulerian curve. A summary
+of their results is given in Table 9. The table indicates that during
+the period from 1892 to 1905 there were nine different times when the
+curve of Fig. 13 changed its direction or was deflected by less than 10�
+during a tenth of a year. In other words, during those periods it did
+not curve as much as it ought according to the Eulerian movement. At
+such times there were 179 world-shaking earthquakes, or an average of
+about 19.9 per tenth of a year. According to the other lines of Table 9,
+in thirty-two cases the deflection during a tenth of a year was between
+10� and 25�, while in fifty-six cases it was from 25� to 40�. During
+these periods the curve remained close to the Eulerian path and the
+world-shaking earthquakes averaged only 8.2 and 12.9. Then, when the
+deflection was high, that is, when meteorological conditions threw the
+earth far out of its Eulerian course, the earthquakes were again
+numerous, the number rising to 23.4 when the deflection amounted to more
+than 55�.
+
+TABLE 9
+
+DEFLECTION OF PATH OF POLE COMPARED WITH EARTHQUAKES
+
+                   _No. of      _No. of        _Average No.
+    _Deflection_  Deflections_  Earthquakes_  of Earthquakes_
+       0-10�          9          179            19.9
+      10-25�         32          263             8.2
+      25-40�         56          722            12.9
+      40-55�         19          366            19.3
+    over 55�          7          164            23.4
+
+In order to test this conclusion in another way we have followed a
+suggestion of Professor Schlesinger. Under his advice the Eulerian
+motion has been eliminated and a new series of earthquake records has
+been compared with the remaining motions of the poles which presumably
+arise largely from meteorological causes. For this purpose use has been
+made of the very full records of earthquakes published under the
+auspices of the International Seismological Commission for the years
+1903 to 1908, the only years for which they are available. These include
+every known shock of every description which was either recorded by
+seismographs or by direct observation in any part of the world. Each
+shock is given the same weight, no matter what its violence or how
+closely it follows another. The angle of deflection has been measured as
+Milne measured it, but since the Eulerian motion is eliminated, our zero
+is approximately the normal condition which would prevail if there were
+no meteorological complications. Dividing the deflections into six equal
+groups according to the size of the angle, we get the result shown in
+Table 10.
+
+TABLE 10
+
+EARTHQUAKES IN 1903-1908 COMPARED WITH DEPARTURES OF THE PROJECTED
+CURVE OF THE EARTH'S AXIS FROM THE EULERIAN POSITION
+
+    _Average angle of deflection_     _Average daily number
+   (_10 periods of 1/10 year each_)      of earthquakes_
+              -10.5�                         8.31
+               11.5�                         8.35
+               25.8�                         8.23
+               40.2�                         8.14
+               54.7�                         8.86
+               90.3�                        11.81
+
+Here where some twenty thousand earthquakes are employed the result
+agrees closely with that of Milne for a different series of years and
+for a much smaller number of earthquakes. So long as the path of the
+pole departs less than about 45� from the smooth gyroscopic Eulerian
+path, the number of earthquakes is almost constant, about eight and a
+quarter per day. When the angle becomes large, however, the number
+increases by nearly 50 per cent. Thus the work of Milne, Knott, and
+Jeffreys is confirmed by a new investigation. Apparently earthquakes and
+crustal movements are somehow related to sudden changes in the load
+imposed on the earth's crust by meteorological conditions.
+
+This conclusion is quite as surprising to the authors as to the
+reader--perhaps more so. At the beginning of this investigation we had
+no faith whatever in any important relation between climate and
+earthquakes. At its end we are inclined to believe that the relation is
+close and important.
+
+It must not be supposed, however, that meteorological conditions are the
+_cause_ of earthquakes and of movements of the earth's crust. Even
+though the load that the climatic agencies can impose upon the earth's
+crust runs into millions of tons per square mile, it is a trifle
+compared with what the crust is able to support. There is, however, a
+great difference between the cause and the occasion of a phenomenon.
+Suppose that a thick sheet of glass is placed under an increasing
+strain. If the strain is applied slowly enough, even so rigid a material
+as glass will ultimately bend rather than break. But suppose that while
+the tension is high the glass is tapped. A gentle tap may be followed by
+a tiny crack. A series of little taps may be the signal for small cracks
+to spread in every direction. A few slightly harder taps may cause the
+whole sheet to break suddenly into many pieces. Yet even the hardest tap
+may be the merest trifle compared with the strong force which is keeping
+the glass in a state of strain and which would ultimately bend it if
+given time.
+
+The earth as a whole appears to stand between steel and glass in
+rigidity. It is a matter of common observation that rocks stand high in
+this respect and in the consequent difficulty with which they can be
+bent without breaking. Because of the earth's contraction the crust
+endures a constant strain, which must gradually become enormous. This
+strain is increased by the fact that sediment is transferred from the
+lands to the borders of the sea and there forms areas of thick
+accumulation. From this has arisen the doctrine of isostasy, or of the
+equalization of crustal pressure. An important illustration of this is
+the oceanward and equatorial creep which has been described in Chapter
+XI. There we saw that when the lands have once been raised to high
+levels or when a shortening of the earth's axis by contraction has
+increased the oceanic bulge at the equator, or when the reverse has
+happened because of tidal retardation, the outer part of the earth
+appears to creep slowly back toward a position of perfect isostatic
+adjustment. If the sun had no influence upon the earth, either direct or
+indirect, isostasy and other terrestrial processes might flex the
+earth's crust so gradually that changes in the form and height of the
+lands would always take place slowly, even from the geological point of
+view. Thus erosion would usually be able to remove the rocks as rapidly
+as they were domed above the general level. If this happened, mountains
+would be rare or unknown, and hence climatic contrasts would be far less
+marked than is actually the case on our earth where crustal movements
+have repeatedly been rapid enough to produce mountains.
+
+Nature's methods rarely allow so gradual an adjustment to the forces of
+isostasy. While the crust is under a strain, not only because of
+contraction, but because of changes in its load through the transference
+of sediments and the slow increase or decrease in the bulge at the
+equator, the atmosphere more or less persistently carries on the tapping
+process. The violence of that process varies greatly, and the variations
+depend largely on the severity of the climatic contrasts. If the main
+outlines of the cyclonic hypothesis are reliable, one of the first
+effects of a disturbance of the sun's atmosphere is increased storminess
+upon the earth. This is accompanied by increased intensity in almost
+every meteorological process. The most important effect, however, so far
+as the earth's crust is concerned would apparently be the rapid and
+intense changes of atmospheric pressure which would arise from the swift
+passage of one severe storm after another. Each storm would be a little
+tap on the tensely strained crust. Any single tap might be of little
+consequence, even though it involved a change of a billion tons in the
+pressure on an area no larger than the state of Rhode Island. Yet a
+rapid and irregular succession of such taps might possibly cause the
+crust to crack, and finally to collapse in response to stresses arising
+from the shrinkage of the earth.
+
+Another and perhaps more important effect of variations in storminess
+and especially in the location of the stormy areas would be an
+acceleration of erosion in some places and a retardation elsewhere. A
+great increase in rainfall may almost denude the slopes of soil, while a
+diminution to the point where much of the vegetation dies off has a
+similar effect. If such changes should take place rapidly, great
+thicknesses of sediment might be concentrated in certain areas in a
+short time, thus disturbing the isostatic adjustment of the earth's
+crust. This might set up a state of strain which would ultimately have
+to be relieved, thus perhaps initiating profound crustal movements.
+Changes in the load of the earth's crust due to erosion and the
+deposition of sediment, no matter how rapid they may be from the
+geological standpoint, are slow compared with those due to changes in
+barometric pressure. A drop of an inch in barometric pressure is
+equivalent to the removal of about five inches of solid rock. Even under
+the most favorable circumstances, the removal of an average depth of
+five inches of rock or its equivalent in soil over millions of square
+miles would probably take several hundred years, while the removal of a
+similar load of air might occur in half a day or even a few hours. Thus
+the erosion and deposition due to climatic variations presumably play
+their part in crustal deformation chiefly by producing crustal stresses,
+while the storms, as it were, strike sharp, sudden blows.
+
+Suppose now that a prolonged period of world-wide mild climate, such as
+is described in Chapter X, should permit an enormous accumulation of
+stresses due to contraction and tidal retardation. Suppose that then a
+sudden change of climate should produce a rapid shifting of the deep
+soil that had accumulated on the lands, with a corresponding
+localization and increase in strains. Suppose also that frequent and
+severe storms play their part, whether great or small, by producing an
+intensive tapping of the crust. In such a case the ultimate collapse
+would be correspondingly great, as would be evident in the succeeding
+geological epoch. The sea floor might sink lower, the continents might
+be elevated, and mountain ranges might be shoved up along lines of
+special weakness. This is the story of the geological period as known to
+historical geology. The force that causes such movements would be the
+pull of gravity upon the crust surrounding the earth's shrinking
+interior. Nevertheless climatic changes might occasionally set the date
+when the gravitative pull would finally overcome inertia, and thus usher
+in the crustal movements that close old geologic periods and inaugurate
+new ones. This, however, could occur only if the crust were under
+sufficient strain. As Lawson[139] says in his discussion of the "elastic
+rebound theory," the sudden shifts of the crust which seem to be the
+underlying cause of earthquakes "can occur only after the accumulation
+of strain to a limit and ... this accumulation involves a slow creep of
+the region affected. In the long periods between great earthquakes the
+energy necessary for such shocks is being stored up in the rocks as
+elastic compression."
+
+If a period of intense storminess should occur when the earth as a whole
+was in such a state of strain, the sudden release of the strains might
+lead to terrestrial changes which would alter the climate still further,
+making it more extreme, and perhaps permitting the storminess due to the
+solar disturbances to bring about glaciation. At the same time if
+volcanic activity should increase it would add its quota to the tendency
+toward glaciation. Nevertheless, it might easily happen that a very
+considerable amount of crustal movement would take place without causing
+a continental ice sheet or even a marked alpine ice sheet. Or again, if
+the strains in the earth's crust had already been largely released
+through other agencies before the stormy period began, the climate might
+become severe enough to cause glaciation in high latitudes without
+leading to any very marked movements of the earth's crust, as apparently
+happened in the Mid-Silurian period.
+
+FOOTNOTES:
+
+[Footnote 125: E. Kirk: Paleozoic Glaciation in Alaska; Am. Jour. Sci.,
+1918, p. 511.]
+
+[Footnote 126: J. Milne: Catalogue of Destructive Earthquakes; Rep.
+Brit. Asso. Adv. Sci., 1911.]
+
+[Footnote 127: Wm. Bowie: Lecture before the Geological Club of Yale
+University. See Am. Jour. Sci., 1921.]
+
+[Footnote 128: Chas. Davisson: On the Annual and Semi-annual Seismic
+Periods; Roy. Soc. of London, Philosophical Transactions, Vol. 184,
+1893, 1107 _ff._]
+
+[Footnote 129: C. G. Knott: The Physics of Earthquake Phenomena, Oxford,
+1908.]
+
+[Footnote 130: In Table 8 the first column indicates the region; the
+second, the dates; and the third, the number of shocks. The fourth
+column gives the month in which the annual maximum occurs when the crude
+figures are smoothed by the use of overlapping six-monthly means. In
+other words, the average for each successive six months has been placed
+in the middle of the period. Thus the average of January to June,
+inclusive, is placed between March and April, that for February to July
+between April and May, and so on. This method eliminates the minor
+fluctuations and also all periodicities having a duration of less than a
+year. If there were no annual periodicity the smoothing would result in
+practically the same figure for each month. The column marked
+"Amplitude" gives the range from the highest month to the lowest divided
+by the number of earthquakes and then corrected according to Schuster's
+method which is well known to mathematicians, but which is so confusing
+to the layman that it will not be described. Next, in the column marked
+"Expected Amplitude," we have the amplitude that would be expected if a
+series of numbers corresponding to the earthquake numbers and having a
+similar range were arranged in accidental order throughout the year.
+This also is calculated by Schuster's method in which the expected
+amplitude is equal to the square root of "pi" divided by the number of
+shocks. When the actual amplitude is four or more times the expected
+amplitude, the probability that there is a real periodicity in the
+observed phenomena becomes so great that we may regard it as practically
+certain. If there is no periodicity the two are equal. The last column
+gives the number of times by which the actual exceeds the expected
+amplitude, and thus is a measure of the probability that earthquakes
+vary systematically in a period of a year.]
+
+[Footnote 131: N. F. Drake: Destructive Earthquakes in China; Bull.
+Seism. Soc. Am., Vol. 2, 1912, pp. 40-91, 124-133.]
+
+[Footnote 132: The only other explanation that seems to have any
+standing is the psychological hypothesis of Montessus de Ballore as
+given in Les Tremblements de Terre. He attributes the apparent seasonal
+variation in earthquakes to the fact that in winter people are within
+doors, and hence notice movements of the earth much more than in summer
+when they are out of doors. There is a similar difference between
+people's habits in high latitudes and low. Undoubtedly this does have a
+marked effect upon the degree to which minor earthquake shocks are
+noticed. Nevertheless, de Ballore's contention, as well as any other
+psychological explanation, is completely upset by two facts: First,
+instrumental records show the same seasonal distribution as do records
+based on direct observation, and instruments certainly are not
+influenced by the seasons. Second, in some places, notably China, as
+Drake has shown, the summer rather than the winter is very decidedly the
+time when earthquakes are most frequent.]
+
+[Footnote 133: A comparison of tropical hurricanes with earthquakes is
+interesting. Taking all the hurricanes recorded in August, September,
+and October, from 1880 to 1899, and the corresponding earthquakes in
+Milne's catalogue, the correlation coefficient between hurricanes and
+earthquakes is +0.236, with a probable error of �0.082, the month being
+used as the unit. This is not a large correlation, yet when it is
+remembered that the hurricanes represent only a small part of the
+atmospheric disturbances in any given month, it suggests that with
+fuller data the correlation might be large.]
+
+[Footnote 134: Ellsworth Huntington: The Geographic Work of Dr. M. A.
+Veeder; Geog. Rev., Vol. 3, March and April, 1917, Nos. 3 and 4.]
+
+[Footnote 135: Frank Schlesinger: Variations of Latitude; Their Bearing
+upon Our Knowledge of the Interior of the Earth; Proc. Am. Phil. Soc.,
+Vol. 54, 1915, pp. 351-358. Also Smithsonian Report for 1916, pp.
+248-254.]
+
+[Footnote 136: Harold Jeffreys: Causes Contributory to the Annual
+Variations of Latitude; Monthly Notices, Royal Astronomical Soc., Vol.
+76, 1916, pp. 499-525.]
+
+[Footnote 137: John Milne: British Association Reports for 1903 and
+1906.]
+
+[Footnote 138: C. G. Knott: The Physics of Earthquake Phenomena, Oxford,
+1908.]
+
+[Footnote 139: A. C. Lawson: The Mobility of the Coast Ranges of
+California; Univ. of Calif. Pub., Geology, Vol. 12, No. 7, pp. 431-473.]
+
+
+
+
+CONCLUSION
+
+
+Here we must bring this study of the earth's evolution to a close. Its
+fundamental principle has been that the present, if rightly understood,
+affords a full key to the past. With this as a guide we have touched on
+many hypotheses, some essential and some unessential to the general line
+of thought. The first main hypothesis is that the earth's present
+climatic variations are correlated with changes in the solar atmosphere.
+This is the keynote of the whole book. It is so well established,
+however, that it ranks as a theory rather than as an hypothesis. Next
+comes the hypothesis that variations in the solar atmosphere influence
+the earth's climate chiefly by causing variations not only in
+temperature but also in atmospheric pressure and thus in storminess,
+wind, and rainfall. This, too, is one of the essential foundations on
+which the rest of the book is built, but though this cyclonic hypothesis
+is still a matter of discussion, it seems to be based on strong
+evidence. These two hypotheses might lead us astray were they not
+balanced by another. This other is that many climatic conditions are due
+to purely terrestrial causes, such as the form and altitude of the
+lands, the degree to which the continents are united, the movement of
+ocean currents, the activity of volcanoes, and the composition of the
+atmosphere and the ocean. Only by combining the solar and the
+terrestrial can the truth be perceived. Finally, the last main
+hypothesis of this book holds that if the climatic conditions which now
+prevail at times of solar activity were magnified sufficiently and if
+they occurred in conjunction with certain important terrestrial
+conditions of which there is good evidence, they would produce most of
+the notable phenomena of glacial periods. For example, they would
+explain such puzzling conditions as the localization and periodicity of
+glaciation, the formation of loess, and the occurrence of glaciation in
+low latitudes during Permian and Proterozoic times. The converse of this
+is that if the conditions which now prevail at times when the sun is
+relatively inactive should be intensified, that is, if the sun's
+atmosphere should become calmer than now, and if the proper terrestrial
+conditions of topographic form and atmospheric composition should
+prevail, there would arise the mild climatic conditions which appear to
+have prevailed during the greater part of geological time. In short,
+there seems thus far to be no phase of the climate of the past which is
+not in harmony with an hypothesis which combines into a single unit the
+three main hypotheses of this book, solar, cyclonic, and terrestrial.
+
+Outside the main line of thought lie several other hypotheses. Several
+of these, as well as some of the main hypotheses, are discussed chiefly
+in _Earth and Sun_, but as they are given a practical application in
+this book they deserve a place in this final summary. Each of these
+secondary hypotheses is in its way important. Yet any or all may prove
+untrue without altering our main conclusions. This point cannot be too
+strongly emphasized, for there is always danger that differences of
+opinion as to minor hypotheses and even as to details may divert
+attention from the main point. Among the non-essential hypotheses is the
+idea that the sun's atmosphere influences that of the earth electrically
+as well as thermally. This idea is still so new that it has only just
+entered the stage of active discussion, and naturally the weight of
+opinion is against it. Although not necessary to the main purpose of
+this book, it plays a minor r�le in the chapter dealing with the
+relation of the sun to other astronomical bodies. It also has a vital
+bearing on the further advance of the science of meteorology and the art
+of weather forecasting. Another secondary hypothesis holds that sunspots
+are set in motion by the planets. Whether the effect is gravitational or
+more probably electrical, or perhaps of some other sort, does not
+concern us at present, although the weight of evidence seems to point
+toward electronic emissions. This question, like that of the relative
+parts played by heat and electricity in terrestrial climatic changes,
+can be set aside for the moment. What does concern us is a third
+hypothesis, namely, that if the planets really determine the periodicity
+of sunspots, even though not supplying the energy, the sun in its flight
+through space must have been repeatedly and more strongly influenced in
+the same way by many other heavenly bodies. In that case, climatic
+changes like those of the present, but sometimes greatly magnified, have
+presumably arisen because of the constantly changing position of the
+solar system in respect to other parts of the universe. Finally, the
+fourth of our secondary hypotheses postulates that at present the date
+of movements of the earth's crust is often determined by the fact that
+storms and other meteorological conditions keep changing the load upon
+first one part of the earth's surface and then upon another. Thus
+stresses that have accumulated in the earth's isostatic shell during the
+preceding months are released. In somewhat the same way epochs of
+extreme storminess and rapid erosion in the past may possibly have set
+the date for great movements of the earth's crust. This hypothesis, like
+the other three in our secondary or non-essential group, is still so new
+that only the first steps have been taken in testing it. Yet it seems to
+deserve careful study.
+
+In testing all the hypotheses here discussed, primary and secondary
+alike, the first necessity is a far greater amount of quantitative work.
+In this book there has been a constant attempt to subject every
+hypothesis to the test of statistical facts of observation.
+Nevertheless, we have been breaking so much new ground that in many
+cases exact facts are not yet available, while in others they can be
+properly investigated only by specialists in physics, astronomy, or
+mathematics. In most cases the next great step is to ascertain whether
+the forces here called upon are actually great enough to produce the
+observed results. Even though they act only as a means of releasing the
+far greater forces due to the contraction of the earth and the sun, they
+need to be rigidly tested as to their ability to play even this minor
+r�le. Still another line of study that cries aloud for research is a
+fuller comparison between earthquakes on the one hand and meteorological
+conditions and the wandering of the poles on the other. Finally, an
+extremely interesting and hopeful quest is the determination of the
+positions and movements of additional stars and other celestial bodies,
+the faint and invisible as well as the bright, in order to ascertain the
+probable magnitude of their influence upon the sun and thus upon the
+earth at various times in the past and in the future. Perhaps we are
+even now approaching some star that will some day give rise to a period
+of climatic stress like that of the fourteenth century, or possibly to a
+glacial epoch. Or perhaps the variations in others of the nearer stars
+as well as Alpha Centauri may show a close relation to changes in the
+sun.
+
+Throughout this volume we have endeavored to discover new truth
+concerning the physical environment that has molded the evolution of all
+life. We have seen how delicate is the balance among the forces of
+nature, even though they be of the most stupendous magnitude. We have
+seen that a disturbance of this balance in one of the heavenly bodies
+may lead to profound changes in another far away. Yet during the billion
+years, more or less, of which we have knowledge, there appears never to
+have been a complete cataclysm involving the destruction of all life.
+One star after another, if our hypothesis is correct, has approached the
+solar system closely enough to set the atmosphere of the sun in such
+commotion that great changes of climate have occurred upon the earth.
+Yet never has the solar system passed so close to any other body or
+changed in any other way sufficiently to blot out all living things. The
+effect of climatic changes has always been to alter the environment and
+therefore to destroy part of the life of a given time, but with this
+there has invariably gone a stimulus to other organic types. New
+adaptations have occurred, new lines of evolutionary progress have been
+initiated, and the net result has been greater organic diversity and
+richness. Temporarily a great change of climate may seem to retard
+evolution, but only for a moment as the geologist counts time. Then it
+becomes evident that the march of progress has actually been more rapid
+than usual. Thus the main periods of climatic stress are the most
+conspicuous milestones upon the upward path toward more varied
+adaptation. The end of each such period of stress has found the life of
+the world nearer to the high mentality which reaches out to the utmost
+limits of space, of time, and of thought in the search for some
+explanation of the meaning of the universe. Each approach of the sun to
+other bodies, if such be the cause of the major climatic changes, has
+brought the organic world one step nearer to the solution of the
+greatest of all problems,--the problem of whether there is a psychic
+goal beyond the mental goal toward which we are moving with ever
+accelerating speed. Throughout the vast eons of geological time the
+adjustment of force to force, of one body of matter to another, and of
+the physical environment to the organic response has been so delicate,
+and has tended so steadily toward the one main line of mental progress
+that there seems to be a purpose in it all. If the cosmic uniformity of
+climate continues to prevail and if the uniformity is varied by changes
+as stimulating as those of the past, the imagination can scarcely
+picture the wonders of the future. In the course of millions or even
+billions of years the development of mind, and perhaps of soul, may
+excel that of today as far as the highest known type of mentality excels
+the primitive plasma from which all life appears to have arisen.
+
+
+
+
+INDEX
+
+* Indicates illustrations.
+
+
+  Abbot, C. G., cited, 45, 52, 237, 238, 239.
+
+  Aboskun, 104.
+
+  Africa, earthquakes, 301;
+    East, _see_ East Africa;
+    lakes, 143;
+    North, _see_ North Africa.
+
+  African glaciation, 266.
+
+  Air, _see_ Atmosphere.
+
+  Alaska, glacial till in, 287;
+    Ice Age in, 221.
+
+  Albrecht, cited, 304.
+
+  Alexander, march of, 88 f.
+
+  Allard, H. A., cited, 183, 184.
+
+  Alpha Centauri, companion of, 280;
+    distance from sun, 262;
+    luminosity, 278;
+    speed of, 281;
+    variations, 282.
+
+  Alps, loess in, 159;
+    precipitation in, 141;
+    snow level in, 139.
+
+  Altair, companion of, 280;
+    luminosity, 278;
+    speed of, 281.
+
+  Amazon forest, temperature, 17.
+
+  Ancylus lake, 217.
+
+  Andes, snow line, 139.
+
+  Animals, climate and, 1.
+
+  Antarctica, mild climate, 219;
+    thickness of ice in, 125;
+    winds, 135, 161.
+
+  Anti-cyclonic hypothesis, 135 ff.
+
+  Appalachians, effect on ice sheet, 121.
+
+  Arabia, civilization in, 67.
+
+  Aral, Sea of, 108.
+
+  Archean rocks, 211.
+
+  Archeozoic, 3 f.;
+    climate of, 267.
+
+  Arctic Ocean, submergence, 219.
+
+  Arctowski, H., cited, 29, 46, 244.
+
+  Argon, increase of, 236.
+
+  Arizona, rainfall, 89, 108;
+    trees measured in, 73.
+
+  Arrhenius, S., cited, 36, 254.
+
+  Arsis, of pulsation, 24.
+
+  Asbj�rn Selsbane, corn of, 101.
+
+  Asia, atmospheric pressure, 298;
+    central, changes of climate, *75;
+    central, post-glacial climate, 271;
+    climate, 66;
+    glaciation in, 131;
+    storminess in, 60;
+    western, climate in, 84 f.
+
+  Atlantic Ocean, storminess, 57.
+
+  Atmosphere, changes, 19 f., 229;
+    composition of, 223-241;
+    effect on temperature, 231.
+
+  Atmospheric circulation, glaciation and, 42.
+
+  Atmospheric electricity, solar relations of, 56.
+
+  Atmospheric pressure, earthquakes and, 298;
+    evaporation and, 237;
+    increase in, 239;
+    redistribution of, 49;
+    variation, 53.
+
+  Australia, East, mild climate, 219;
+    precipitation, 144.
+
+  Axis, earth's, 48;
+    wabbling of, 301.
+
+
+  Bacon, Sir Francis, cited, 27.
+
+  Bacubirito, meteor at, 246.
+
+  Baltic Sea, as lake, 217;
+    freezing of, 100;
+    ice, 26;
+    storm-floods, 99;
+    submergence, 219.
+
+  Bardsson, Ivar, 106.
+
+  Barkow, cited, 135.
+
+  Barometric pressure, solar relations of, 56.
+
+  Barrell, J., cited, 3, 200, 213, 234.
+
+  Bartoli, A. G., cited, 257.
+
+  Bauer, L. A., cited, 150.
+
+  Beaches, under water, 97.
+
+  Beadnell, H. J. L., cited, 143.
+
+  Beluchistan, rainfall, 89.
+
+  Bengal, Bay of, cyclones in, 149.
+
+  Bengal, famine in, 104 f.
+
+  Berlin, rainfall and temperature, 93.
+
+  Betelgeuse, 259 f.;
+    distance from sun, 262.
+
+  Bible, climatic evidence in, 91 f.;
+    palms in, 92.
+
+  Binary stars, 252.
+
+  Birkeland, K., cited, 244.
+
+  Black Earth region, loess in, 159.
+
+  Boca, Cal., correlation coefficients, 83, 85.
+
+  Boltzmann, L., cited, 257.
+
+  Bonneville, Lake, 142, 143.
+
+  Borkum, storm-flood in, 99.
+
+  Boss, L. cited, 268, 269.
+
+  Botanical evidence of mild climates, 167 ff.
+
+  Boulders, on Irish coast, 119.
+
+  Bowie, W., cited, 293.
+
+  Bowman, I., cited, 213.
+
+  Britain, forests, 220;
+    level of land, 220.
+
+  British Isles, height of land, 111;
+    temperature, 216.
+
+  Brooks, C. E. P., cited, 115, 143, 196, 215, 225.
+
+  Brooks, C. F., cited, 209.
+
+  Brown, E. W., cited, 191, 244.
+
+  Br�ckner, E., cited, 27.
+
+  Br�ckner periods, 27 f.
+
+  Bufo, habitat of, 202.
+
+  Buhl stage, 216.
+
+  Bull, Dr., cited, 100, 101.
+
+  Butler, H. C., cited, 66, 67 ff., 70, 76.
+
+
+  California, changes of climate, *75;
+    correlations of rainfall, 86;
+    measurements of sequoias in, 73, 74 ff.;
+    rainfall, 108.
+
+  Cambrian period, 4 f.
+
+  Canada, storminess, 53 f., 57;
+    storm tracks in, 113.
+
+  Cape Farewell, shore ice at, 105.
+
+  Carbon dioxide, erosion and, 119 f.;
+    from volcanoes, 23;
+    hypothesis, 139;
+    importance of, 9, 11 f.;
+    in Permian, 148;
+    in atmosphere, 20, 96, 238;
+    in ocean, 226;
+    nebular hypothesis and, 232;
+    theory of glaciation, 36 ff.
+
+  Caribbean mountains, origin of, 193.
+
+  Carnegie Institution of Washington, 74.
+
+  Caspian Sea, climatic stress, 104;
+    rainfall, 107 f.;
+    rise and fall, 27;
+    ruins in, 71.
+
+  Cenozoic, climate, 266;
+    fossils, 21.
+
+  Central America, Maya ruins, 95.
+
+  Chad, Lake, swamps of, 171.
+
+  Chamberlin, R. T., cited, 166, 233, 269.
+
+  Chamberlin, T. C., cited, 19, 36, 38, 39, 42 f., 48, 122, 125,
+     152, 156, 190, 195, 227, 269.
+
+  Chandler, S. C., cited, 301.
+
+  Chinese earthquakes, periodicity of, 245.
+
+  Chinese, sunspot observations, 108 f.
+
+  Chinese Turkestan, desiccation in, 66.
+
+  Chronology, glacial, 215.
+
+  Clarke, F. W., cited, 226, 235.
+
+  Clayton, H. H., cited, 173 f.
+
+  Climate, effect of contraction, 189 ff.;
+    affect of salinity, 224;
+    in history, 64-97;
+    uniformity, 1-15;
+    variability, 16-32.
+
+  Climates, mild, causes of, 166-187;
+    mild, periods of, 274.
+
+  Climatic changes, and crustal movements, 285 ff.;
+    hypotheses of, 33-50;
+    mountain-building and, *25;
+    post-glacial crustal movements and, 215-222;
+    terrestrial causes of, 188-214.
+
+  Climatic sequence, 16 f.
+
+  Climatic stages, post-glacial, 270.
+
+  Climatic stress, in fourteenth century, 98-109.
+
+  Climatic uniformity, hypothesis of, 65, 71 f.
+
+  Climatic zoning, 169.
+
+  Cloudiness, glaciation and, 114, 147.
+
+  Clouds, as protection, 197.
+
+  Colfax, Cal., correlation coefficients, 83.
+
+  Cologne, flood at, 99.
+
+  Compass, variations, 150.
+
+  Continental climate, variations, 103.
+
+  Continents, effect on climate, 111 f.
+
+  Contraction, effect on climate, 189 ff., 199, 207;
+    effect on lands, 207;
+    heat of sun and, 13 f.;
+    irregular, 195;
+    of the earth, 18;
+    of the sun, 249;
+    stresses caused by, 310.
+
+  Convection, carbon dioxide and, 239.
+
+  Corals, in high latitudes, 21, 39, 167, 178.
+
+  Cordeiro, F. J. B., cited, 181, 183, 186.
+
+  Correlation coefficients, earthquakes and sunspots, 291;
+    Jerusalem rainfall and sequoia growth, 83 ff.;
+    rainfall and tree growth, 79 ff.
+
+  Cosmos, effect of light, 185.
+
+  Cressey, G. B., cited, 80.
+
+  Cretaceous, lava, 211;
+    mountain ranges, 44;
+    paleogeography, *201;
+    submergence of North America, 200.
+
+  Croll, J., cited, 34 ff., 176.
+
+  Croll's hypothesis, snow line, 139.
+
+  Crust, climate and movements of, 63, 287, 310;
+    movements of, 43;
+    strains in, 22.
+
+  Currents and planetary winds, 174.
+
+  Cycads, 169.
+
+  Cyclonic hypothesis, 97;
+    loess and, 163;
+    Permian glaciation and, 148;
+    snow line, 139.
+
+  Cyclonic storms, in glacial epochs, 140 f.;
+    solar electricity and, 243 (_see_ Storms, Storminess).
+
+  Cyclonic vacillations, 30 f.;
+    nature of, 57 ff.
+
+
+  Daily vibrations, 28 f.
+
+  Danube, frozen, 98.
+
+  Darwin, G. H., cited, 191.
+
+  Daun stage, 217.
+
+  Davis, W. M., cited, 271.
+
+  Davisson, C., cited, 294, 295, 299.
+
+  Day, C. P., cited, 239.
+
+  Day, length of, 18, 191.
+
+  Dead Sea, palms near, 92.
+
+  Death Valley, 142.
+
+  De Ballore, M., cited, 297, 298.
+
+  Deep-sea circulation, rapidity, 227;
+    salinity and, 176;
+    solar activity and, 179.
+
+  De Geer, S., cited, 215, 221.
+
+  De Lapparent, A., cited, 200.
+
+  Denmark, fossils, 271.
+
+  "Desert pavements," 161.
+
+  Deserts, abundant flora of, 171;
+    and pulsations theory, 88 ff.;
+    red beds of, 170.
+
+  Devonian, climate, 266;
+    mountains, 209.
+
+  Dog, climate and, 1.
+
+  Donegal County, Ireland, 220.
+
+  Double stars, 272, 280;
+    electrical effect of, 261.
+
+  Douglass, A. E., cited, 28, 73, 74 f., 84, 85, 107.
+
+  Dragon Town, destruction of, 104, 108.
+
+  Drake, N. F., cited, 297, 298.
+
+  Droughts, and pulsations theory, 87 f.;
+    in England, 102;
+    in India, 104 f.
+
+  Drumkelin Bog, Ireland, log cabin in, 220.
+
+  Dust, at high levels, 240.
+
+
+  Earth, crust of and the sun, 285-317;
+    internal heat, 212;
+    nature of mild climate, 274;
+    position of axis, 181;
+    rigidity of, 307;
+    temperature gradient, 213;
+    temperature of surface, 8.
+
+  Earthquakes, and seasons, 294, 297;
+    and sunspots, 288 f.;
+    and tropical hurricanes, 300;
+    and wandering of pole, 304 f.;
+    cause of, 307;
+    compared with departures from Eulerian position, 306;
+    seasonal distribution of, 299;
+    seasonal march, 295.
+
+  "Earthquake weather," 298.
+
+  East Africa, mild climate, 219.
+
+  East Indies, earthquakes of, 296.
+
+  Eberswalde, tree growth at, 102 f.
+
+  Ecliptic, obliquity of, 217.
+
+  Electrical currents, in solar atmosphere, 261.
+
+  Electrical emissions, variation of, 275.
+
+  Electrical hypothesis, 150, 250 f., 256 ff.
+
+  Electrical phenomena, storminess and, 56.
+
+  Electricity, and earthquakes, 292;
+    solar, 243.
+
+  Electro-magnetic hypothesis, 244.
+
+  Electrons, solar, 56;
+    variation of, 256.
+
+  Electro-stellar hypothesis, 274.
+
+  Elevation, climatic changes and, 39.
+
+  Engedi, palms in, 92.
+
+  England, climatic stress, 101 f.;
+    storminess and rainfall, 107.
+
+  Eocene, climate, 266.
+
+  Equinoxes, precession of, 96.
+
+  Erosion, storminess and, 309.
+
+  Eskimo, in Greenland, 106.
+
+  Eulerian movement, 301, 304.
+
+  Euphrates, 67.
+
+  Europe, climatic stress, 98 ff., 102 f.;
+    climatic table, 215;
+    glaciation in, 131;
+    ice sheet, 121;
+    inundations of rivers, 99;
+    post-glacial climate, 271;
+    rainfall, 107;
+    submergence, 196, 200.
+
+  Evaporation, and glaciation, 112, 114;
+    atmospheric pressure and, 237;
+    from plants, 179;
+    importance, 129;
+    in trade-wind belt, 117;
+    rapidity of, 224.
+
+  Evening primrose, effect of light, 184.
+
+  Evolution, climate and, 20;
+    geographical complexity and, 241;
+    glaciation and, 33;
+    of the earth, 311.
+
+
+  Facul�, cause of, 61.
+
+  False Point Lighthouse, barometric pressure at, 299.
+
+  Famine, cause of, 103;
+    in England, 101 f.;
+    in India, 104 f.;
+    pulsations theory and, 87 f.
+
+  Faunas, and mild climates, 168 f.;
+    in Permian, 152 f.
+
+  Fennoscandian pause, 216.
+
+  Flowering, light and, 184.
+
+  Fog, and glaciation, 116;
+    as protection, 197;
+    temperature and, 178.
+
+  Forests, climate and, 66.
+
+  Form of the land, 43 ff.
+
+  Fossil floras, and mild climates, 168;
+    in Antarctica, 273;
+    in Greenland, 273.
+
+  Fossils, 169, 230;
+    and loess, 158;
+    Archeozoic, 3 f.;
+    Cenozoic, 21;
+    dating of, 153;
+    glaciation and, 138;
+    in peat bogs, 271;
+    mild climate, 167;
+    Proterozoic, 4, 6 f.
+
+  Fourteenth century, climatic stress in, 98-109.
+
+  Fowle, F. E., cited, 45, 237, 238, 239.
+
+  Frech, F., cited, 36.
+
+  Free, E. E., cited, 142.
+
+  Freezing, salinity and, 224.
+
+  Fresno, rainfall record, 82.
+
+  "Friction variables," 247.
+
+  Frisian Islands, storm-flood, 99.
+
+  Fritz, H., cited, 109.
+
+  Frogs, distribution of, 202.
+
+  Fuchs, cited, 289.
+
+
+  Galaxy, 252.
+
+  Galveston, Tex., rainfall and temperature, 94.
+
+  Garner, W. W., cited, 183, 184.
+
+  Gasses, in air, 233.
+
+  Geographers, and climatic changes, 65 ff.
+
+  Geological time table, *5.
+
+  Geologic oscillations, 18 f., 21 ff., 188, 240.
+
+  Geologists, changes in ideas of, 64 f.
+
+  Germanic myths, 219.
+
+  Germany, forests, 220;
+    growth of trees in, 102;
+    storms in, 102.
+
+  Gilbert, G. K., cited, 143.
+
+  Glacial epochs, causes of, 268;
+    dates of, 216;
+    intervals between, 264 f.;
+    length of, 166 f.
+
+  Glacial fluctuations, 24 ff.;
+    nature of, 57 ff.
+
+  Glacial period, at present, 272;
+    ice in, 57 f.;
+    length of, 269;
+    list, 265;
+    temperature, 38.
+
+  Glaciation, and loess, 155 f.;
+    and movement of crust, 287;
+    conditions favorable for, 111;
+    extent of, 124;
+    hypotheses of, 33 ff.;
+    in southern Canada, 18;
+    localization of, 130 ff.;
+    Permian, *145;
+    solar-cyclonic hypothesis of, 110-129;
+    suddenness of, 138;
+    upper limit of, 141.
+
+  Goldthwait, J. W., cited, 271.
+
+  Gondwana land, 21, 204.
+
+  Gravitation, effect on sun, 250;
+    pull of, 244.
+
+  Great Basin, in glacial period, 126;
+    salt lakes in, 142.
+
+  Great Ice Age, see Pleistocene.
+
+  Great Plains, effect on ice sheet, 120.
+
+  Greenland, climatic stress, 105 ff.;
+    ice, 26;
+    rainfall, 108;
+    storminess, 57;
+    submergence, 219;
+    vegetation, 21, 37, 287;
+    winds, 135, 161.
+
+  Gregory, J. W., cited, 90 ff., 97.
+
+  Gschnitz stage, 216.
+
+  Guatemala, ruins in, 95.
+
+  Guervain, cited, 135.
+
+  Gyroscope, earth as, 181.
+
+
+  Hale, G. E., cited, 56, 62.
+
+  Hamdulla, cited, 104.
+
+  Hann, J., cited, 66.
+
+  Hansa Union, operations of, 100.
+
+  Harmer, F. W., cited, 115, 119.
+
+  Heat, and earthquakes, 292;
+    earth's internal, 18.
+
+  Hedin, S., cited, 88.
+
+  Heim, A., cited, 190.
+
+  Heligoland, flood in, 99.
+
+  Helland-Hansen, B., cited, 174.
+
+  Helmert, F. R., cited, 302.
+
+  Henderson, L. J., cited, 9, 10, 11, 12.
+
+  Henry, A. J., cited, 94, 208.
+
+  Hercynian Mountains, 45.
+
+  High pressure and glaciation, 115, 135.
+
+  Himalayas, glaciation, 144;
+    origin of, 193;
+    snow line, 139.
+
+  Himley, cited, 104.
+
+  Historic pulsations, 24 f.;
+    nature of, 57 ff.
+
+  History, climate of, 64-97;
+    climatic pulsations and, 26.
+
+  Hobbs, W. H., cited, 115, 125, 135, 161.
+
+  Hot springs, temperature of, 6.
+
+  Humphreys, W. J., cited, 2, 37 f., 45, 46, 50, 56, 238.
+
+  Hurricanes, in arid regions, 144;
+    sunspots and, 53.
+
+  Hyades, cluster in, 268.
+
+
+  Ice, accumulations, 57 f.;
+    advances of, 122;
+    distribution of, 131;
+    drift, 105.
+
+  Ice sheets, disappearance, 128;
+    limits, 120;
+    localization, 130 ff.;
+    rate of retreat, 165;
+    thickness, 125.
+
+  Iceland, submergence, 219.
+
+  Iowan ice sheet, rapid retreat, 165.
+
+  Iowan loess, 158.
+
+  India, drought, 104 f.;
+    famine, 104 f.;
+    rainfall, 108.
+
+  Indian glaciation, 266.
+
+  Inter-glacial epoch, Permian, 153.
+
+  Internal heat of earth, 212.
+
+  Ireland, Drumkelin Bog, 220;
+    in glacial period, 119;
+    level of land, 220;
+    storminess and rainfall, 107;
+    submergence, 219.
+
+  Irish Sea, tides, 191.
+
+  Irrigation ditches, abandoned, 97.
+
+  Isostasy, 307 ff.
+
+  Italy, southern, climate of, 86 f.
+
+
+  Japan, earthquakes of, 296.
+
+  Javanese mountains, origin of, 193.
+
+  Jaxartes, 108.
+
+  Jeans, J. H., cited, 251, 252, 253, 266, 272.
+
+  Jeffreys, H., cited, 302, 303, 306.
+
+  Jeffreys, J., cited, 191.
+
+  Jericho, palms in, 92.
+
+  Jerusalem, rainfall, 86;
+    rainfall and temperature, 94;
+    rainfall in, and sequoia growth, 83 ff.
+
+  Johnson, cited, 226.
+
+  Judea, palms in, 92.
+
+  Jupiter, and sunspots, 243;
+    effect of, 253;
+    periodicity of, 61 f.;
+    temperature of, 258;
+    tidal effect of, 250.
+
+  Jurassic, climate, 266;
+    mountain ranges, 44.
+
+
+  Kansas, variations of seasons, 103.
+
+  Kara Koshun marsh, Lop Nor, 104.
+
+  Keewatin center, 113;
+    evaporation in, 129.
+
+  Keewatin ice sheet, 121.
+
+  Kelvin, Lord, cited, 13 f.
+
+  Keyes, C. R., cited, 156.
+
+  Kirk, E., cited, 287.
+
+  Knott, C. G., cited, 294, 295, 297, 299, 304, 306.
+
+  Knowlton, F. H., cited 167, 169, 170, 212, 232.
+
+  K�ppen, W., 47, 52, 140.
+
+  Krakatoa, glaciation and, 48;
+    volcanic hypothesis and, 45.
+
+  Kr�mmel, O., cited, 224, 228.
+
+  Kullmer, C. J., cited, 113, 115, 128;
+    map of storminess, *54.
+
+  _Kungaspegel_, sea routes described, 106.
+
+
+  Labor, price in England, 102.
+
+  Labradorean center of glaciation, 113.
+
+  Lahontan, Lake, 142.
+
+  Lake strands, _see_ Strands.
+
+  Lake Superior, lava, 211.
+
+  Lakes, during glacial periods, 141 f.;
+    in semi-arid regions, 60;
+    of Great Basin, 126;
+    ruins in, 97.
+
+  Land, and water, climatic effect of, 196 ff.;
+    distribution of, 200, form of, 43 ff.;
+    range of temperature and, 196.
+
+  Lavas, climatic effect of, 211.
+
+  Lawson, A. C., cited, 310.
+
+  Lebanon, cedars of, 83.
+
+  Leiter, H., cited, 71.
+
+  Leverett, F., cited, 271.
+
+  Life, atmosphere and, 229 f.;
+    chemical characteristic of, 12;
+    effect of salinity, 225;
+    of glacial period, 127;
+    persistence of forms, 230.
+
+  Light, effect of atmosphere on, 236;
+    effect on plants, 184 ff.;
+    ultra-violet, storminess and, 56;
+    variation of, 275.
+
+  Litorina sea, 218.
+
+  Loess, date of, 156 ff.;
+    origin of, 155, 165.
+
+  Lop Nor, rise of, 104;
+    swamps, 171.
+
+  Lows, and glacial lobes, 122;
+    movements of, 126;
+    see Storms and Cyclones.
+
+  Lulan, 104.
+
+  Lull, R. S., cited, 5, 188.
+
+
+  MacDougal, D. T., cited, 171.
+
+  McGee, W. J., cited, 156.
+
+  Macmillan, W. D., cited, 191.
+
+  Magdalenian period, 216.
+
+  Magnetic fields of sunspots, 56.
+
+  Magnetic poles, relation to storm tracks, 150.
+
+  Makran, climate, 89;
+    rainfall, 89.
+
+  Malay Archipelago, earthquakes of, 296.
+
+  Mallet, R., cited, 288.
+
+  Malta, rainfall, 86.
+
+  Manson, M., cited, 147.
+
+  Mayas, civilization, 26;
+    ruins, 95.
+
+  Mayence, flood at, 99.
+
+  Mazelle, E., cited, 224.
+
+  Mediterranean, climate of, 72;
+    rainfall records, 86;
+    storminess in, 60.
+
+  Mercury, and sunspots, 243.
+
+  Mesozoic, climate, 266;
+    crustal changes, 286;
+    emergence of lands, 287.
+
+  Messier, 8;
+    variables, 248.
+
+  Metcalf, M. M., cited, 202.
+
+  Meteorological factors and earthquakes, 300 f.
+
+  Meteorological hypothesis of crustal movements, 294.
+
+  Meteors, and sun's heat, 13, 246.
+
+  Michelson, A. A., cited, 259.
+
+  Middle Silurian, fauna in Alaska, 287.
+
+  Mild climates, _see_ Climates, mild.
+
+  Milky Way, 252.
+
+  Mill, H. R., cited, 228.
+
+  Milne, J., cited, 288, 290, 294, 304, 306.
+
+  Miocene, crustal changes, 287.
+
+  Mississippi Basin, loess in, 159.
+
+  Mogul emperor, and famine, 104.
+
+  Monsoons, character of, 146;
+    direction of, 208;
+    Indian famines and, 105.
+
+  Moulton, F. R., cited, 13, 258, 269.
+
+  Mountain building, climatic changes and, *25.
+
+  Mountains, folding of, 190;
+    rainfall, on, 208.
+
+  Multiple stars, 252.
+
+
+  Nansen, F., cited, 122, 174.
+
+  Naples, rainfall, 86.
+
+  Nathorst, cited, 169.
+
+  Nebul�, 247.
+
+  Nebular hypothesis, 232, 267.
+
+  Neolithic period, 218.
+
+  Nevada, correlations of rainfall, 86.
+
+  New England, height of land, 111.
+
+  New Mexico, rainfall, 89.
+
+  New Orleans, La., rainfall and temperature, 94.
+
+  New Zealand, climate, 177;
+    tree ferns, 179.
+
+  Newcomb, S., cited, 52.
+
+  Nile floods, periodicity in, 245.
+
+  Nitrogen, in atmosphere, 19.
+
+  Niya, Chinese Turkestan, desiccation at, 66.
+
+  Nocturnal cooling, changes in, 238 f.
+
+  Norlind, A., cited, 100.
+
+  Norsemen, route to Greenland, 26.
+
+  Norse sagas, 219.
+
+  North Africa, climate of, 71;
+    Roman aqueducts in, 71.
+
+  North America, at maximum glaciation, 122 ff.;
+    emergence of lands, 193;
+    glaciation in, 131;
+    height of land, 111;
+    interior sea in, 200;
+    inundations, 196;
+    loess in, 155;
+    submergence of lands, 19, 21.
+
+  North Atlantic Ocean, salinity, 228.
+
+  North Sea, climatic stress, 98 ff.;
+    floods around, 26, 99;
+    rainfall, 107;
+    storminess, 57.
+
+  Northern hemisphere, earthquakes of, 294.
+
+  Norway, decay, 100;
+    temperature, 177.
+
+  Nov�, 247.
+
+
+  Oceanic circulation, carbon dioxide and, 39 ff.
+
+  Oceanic climate, characteristics, 103.
+
+  Oceanic currents, diversion, 44;
+    influence of land distribution, 203.
+
+  Oceans, age of, 223;
+    composition of, 223-241;
+    deepening of, 199;
+    salinity, 19, 223;
+    temperature, 6, 152, 180, 226.
+
+  Okada, T., cited, 224.
+
+  Old Testament, temperature, 92.
+
+  Orbital precessions, 27.
+
+  Ordovician, climate, 266.
+
+  Organic evolution, glacial fluctuations and, 26.
+
+  Orion, nebulosity near, 247;
+    stars near, 248.
+
+  Orontes, 67.
+
+  Osborn, H. F., cited, 216.
+
+  Owens-Searles, lakes, 142.
+
+  Oxus, 108.
+
+  Oxygen, in atmosphere, 20, 234;
+    in Permian, 152.
+
+  Ozone, cause of, 56.
+
+
+  Paleolithic, 216.
+
+  Paleozoic, climate, 266;
+    mountains in, 209.
+
+  Palestine, change of climate, 91 f.
+
+  Palms, climatic change and, 91 f.;
+    in Ireland, 179.
+
+  Palmyra, ruins of, 66.
+
+  Parallaxes of stars, 276 f.
+
+  Patrician center, 134.
+
+  Peat-bog period, first, 218.
+
+  Penck, A., cited, 139, 156, 157, 158, 269.
+
+  Pennsylvanian, life of, 26.
+
+  Periodicities, 245 f.
+
+  Periodicity, of climatic phenomena, 60 f.;
+    of glaciation, 268;
+    of sunspots, 243.
+
+  Permian, climate, 266;
+    distribution of glaciation, 152;
+    glaciation, 60, 144, *145, 226;
+    glaciation and mountains, 45;
+    life of, 26;
+    red beds, 151;
+    temperature, 146 f.
+
+  Perry, cited, 289.
+
+  Persia, lakes, 143;
+    rainfall, 89.
+
+  Pettersson, O., cited, 98 ff., 100 f., 103, 106, 219.
+
+  Pirsson, L. V., cited, 3, 196.
+
+  Planetary hypothesis, 253, 267.
+
+  Planetary nebul�, 252.
+
+  Planets, and sunspots, 243;
+    effect of star on, 255;
+    sunspot cycle and, 62;
+    temperatures, 8 f.
+
+  Plants, climate and, 1 f.;
+    effect of light, 184 ff.
+
+  Pleion, defined, 29.
+
+  Pleionian migrations, 29 f.
+
+  Pleistocene, climate, 266;
+    duration of, 48;
+    glaciation, 110 ff.;
+    ice sheets, *123.
+
+  Pluvial climate, causes of, 143;
+    during glacial periods, 141.
+
+  Po, frozen, 98.
+
+  Polaris, 272.
+
+  Polar wandering, hypothesis of, 48 f.
+
+  Pole and earthquakes, 305.
+
+  Post-glacial crustal movements and climatic changes, 215-222.
+
+  Poynting, J. H., cited, 8.
+
+  Precessional hypothesis, 34 f.
+
+  Precipitation, and glaciation, 114, 133;
+    during glacial period, 118;
+    snow line and, 139;
+    temperature and, 94.
+
+  Procyon, companion of, 280;
+    luminosity, 278;
+    speed of, 281.
+
+  Progressive change, 241.
+
+  Progressive desiccation, hypothesis of, 65 ff.
+
+  Proterozoic, 4 f.;
+    fossils, 6 f.;
+    glaciation, 18, 144, 226, 266;
+    lava, 211;
+    mountains in, 209;
+    oceanic salinity, 42 f.;
+    oxygen in air, 234;
+    red beds, 151;
+    temperature, 146 f.
+
+  Pulsations, hypothesis of, 65, 72 ff.
+
+  Pulsatory climatic changes, 72 ff.
+
+  Pulsatory hypothesis, 272.
+
+  Pumpelly, R., cited, 271.
+
+
+  Radiation, variation of, 275.
+
+  Radioactivity, heat of sun and, 14 f.
+
+  Rainfall, changes in, 93 f.;
+    glaciation and, 50;
+    sunspots and, 53, *58, 59;
+    tree growth and, 79.
+
+  Red beds, 151, 170.
+
+  Rhine, flood, 99;
+    frozen, 98.
+
+  Rho Ophiuchi, variables, 248.
+
+  "Rice grains," 61.
+
+  Richardson, O. W., cited, 256.
+
+  Rigidity, of earth, 307.
+
+  Roads, climate and, 66.
+
+  Rogers, Thorwald, cited, 101.
+
+  Romans, aqueduct of, 71.
+
+  Rome, history of, 87.
+
+  Rotation, of earth, 18 f.
+
+  Ruden, storm-flood, 99.
+
+  Rugen, storm-flood, 99.
+
+  Ruins, as climatic evidence, 66;
+    rainfall and, 60.
+
+
+  Sacramento, correlation coefficients, 82 f., 85;
+    rainfall, 86;
+    rainfall record, 79.
+
+  Sagas, cited, 105 f.
+
+  St. John, C. E., cited, 236.
+
+  Salinity, deep-sea circulation and, 176;
+    effect on climate, 224;
+    in North Atlantic, 228;
+    ocean temperature and, 226;
+    of ocean, 19, 120.
+
+  Salisbury, R. D., cited, 111, 125, 129, 139, 156, 206, 269, 271.
+
+  Salt, in ocean, 223.
+
+  San Bernardino, correlation of rainfall, 85.
+
+  Saturn, and sunspots, 243;
+    sunspot cycle and, 62.
+
+  Sayles, R. W., cited, 183.
+
+  Scandinavia, climatic stress, 100 f.;
+    fossils, 271;
+    post-glacial climate, 271;
+    rainfall, 107;
+    storminess, 57, 107;
+    temperature, 216.
+
+  Scandinavian center of glaciation, 113.
+
+  Schlesinger, F., cited, 275, 278, 298, 301, 305.
+
+  Schuchert, C., cited, 3, 5, 23, *25, *123, 138, *145, 168, 169,
+     172, 188, 193, 196, 198, 200, *201, 206, 211, 230, 265.
+
+  Schuster, A., cited, 61, 244, 294, 296.
+
+  Sculpture, Maya, 96.
+
+  Sea level and glaciation, 119.
+
+  Seasonal alternations, 28 f.
+
+  Seasonal banding, 183 f.
+
+  Seasonal changes, geological, 183.
+
+  Seasons, and earthquakes, 294, 295, 297, 299;
+    evidences of, 169.
+
+  Secular progression, 17 ff., 188.
+
+  Seistan, swamps, 171.
+
+  Sequoias, measurements of, 74 ff.;
+    rainfall record, 79.
+
+  Setchell, W. A., cited, 1.
+
+  Shackleton, E., cited, 125.
+
+  Shapley, H., cited, 246, 247, 254, 256, 275.
+
+  Shimek, E., cited, 157, 161.
+
+  Shreveport, La., rainfall and temperature, 93 f.
+
+  Shrinkage of the earth, 190.
+
+  Siberia, and glaciation, 132.
+
+  Sierras, rainfall records, 82.
+
+  Simpson, G. C., cited, 222.
+
+  Sirius, companion of, 280;
+    distance from sun, 262;
+    luminosity, 278;
+    speed of, 281.
+
+  Slichter, C. S., cited, 192.
+
+  Smith, J. W., cited, 73.
+
+  Snowfall, glaciation and, 50, 114.
+
+  Snowfield, climatic effects of, 115.
+
+  Snow line, height of, 138;
+    in Andes, 139;
+    in Himalayas, 139.
+
+  Solar activity, cycles of, 245;
+    deep-sea circulation and, 179;
+    ice and, 134.
+
+  Solar constant, 114.
+
+  Solar-cyclonic hypothesis, 51-63, 287;
+    glaciation and, 110-129.
+
+  Solar prominences, cause of, 61.
+
+  Solar system, 252;
+    conservation of, 243;
+    proximity to stars, 63.
+
+  Solar variations, storms and, 31.
+
+  South America, earthquakes, 301.
+
+  South Pole, thickness of ice at, 125.
+
+  Southern hemisphere, earthquakes, 296;
+    glaciation in, 131 f.
+
+  Southern Pacific railroad, rainfall records along, 82.
+
+  Soy beans, effect of light, 185 f.
+
+  Space, sun's journey through, 264-284.
+
+  Spiral nebul�, 251 f.;
+    universe of, 267.
+
+  Spitzbergen, submergence, 219.
+
+  Springs, climate and, 66.
+
+  Stars, approach to sun, 253;
+    binary, 252;
+    clusters, 252, 268;
+    effect on solar atmosphere, 63;
+    dark, 254;
+    parallaxes of, 276 f.;
+    tidal action of, 249.
+
+  Stefan's Law, 257.
+
+  Stein, M. A., cited, 78.
+
+  Stellar approaches, probability of, 260.
+
+  Storm belt in arid regions, 144.
+
+  Storm-floods, in fourteenth century, 99.
+
+  Storminess, and erosion, 309;
+    and ice, 134;
+    effect on glaciation, 112;
+    sunspots and, 163;
+    temperature and, 94, 173.
+
+  Storms, blows of, 300, 302;
+    increase, 60;
+    movement of, 125 f.;
+    movement of water and, *175;
+    origin of, 30 f.;
+    sunspots and, 28, 53;
+    _see_ Cyclones and Lows.
+
+  Storm tracks, during glacial period, 117;
+    location, 113;
+    relation to magnetic poles, 150;
+    shifting of, 119.
+
+  Strands, climate and, 66;
+    in semi-arid regions, 60;
+    of salt lakes, 142.
+
+  Suess, E., cited, 192.
+
+  Sun, and the earth's crust, 285-317;
+    approach to star, 253;
+    atmosphere of, 61, 274;
+    atmosphere of, and weather, 52;
+    cooling of, 49;
+    contraction of, 249;
+    disturbances of, 172;
+    effect of other bodies on, 242-263;
+    heat, 13;
+    journey through space, 264-284;
+    Knowlton's hypothesis of, 168.
+
+  Suncracks, 232.
+
+  Sunspot cycles, 27 f.
+
+  Sunspots, and earthquakes, 289;
+    causes of, 61;
+    magnetic field of, 261;
+    maximum of, 109;
+    mild climates and, 172;
+    number, 108 f.;
+    periodicity, 243;
+    planetary hypothesis of, 253;
+    records, 245;
+    storminess and, 163;
+    storms and, 300;
+    temperature of earth and, 52, 173.
+
+  Sunspot variations, 282.
+
+  Swamps, as desert phenomena, 171.
+
+  Sylt, storm-flood, 99.
+
+  Syria, civilization in, 67;
+    inscriptions in, 76;
+    Roman aqueducts in, 71.
+
+  Syrian Desert, ruins in, 66.
+
+
+  Talbert, cited, 213.
+
+  Tarim Basin, red beds, 151.
+
+  Tarim Desert, desiccation, 66.
+
+  Tarim River, swamps, 171.
+
+  Taylor, G., cited, 140, 144, 191, 271.
+
+  Temperature, change of in Atlantic, 174;
+    changes in, 93;
+    climatic change and, 49;
+    critical, 9;
+    geological time and, 3;
+    glacial period, 38;
+    glaciation and, 42, 132, 139;
+    gradient of earth, 213;
+    of ocean, 180;
+    in Norway, 177;
+    in Permian, 146 f.;
+    in Proterozoic, 146 f.;
+    limits, 6 ff.;
+    precipitation and, 94;
+    range of, 3, 8;
+    solar activity and, 140;
+    storminess and, 94, 112, 173;
+    sunspots and, 28, 173;
+    volcanic eruptions and, 46;
+    zones, 172.
+
+  Terrestial causes of climatic changes, 188-214.
+
+  Tertiary, lava, 211.
+
+  Thames, frozen, 98.
+
+  Thermal solar hypothesis, 49 f., 97.
+
+  Thermo-pleion, movements of, 30.
+
+  Thesis, of pulsations, 24.
+
+  Thiryu, storm-flood, 99.
+
+  Tian-Shan Mountains, irrigation in, 71.
+
+  Tidal action of stars, 249.
+
+  Tidal effect, of Jupiter, 253;
+    of planets, 244.
+
+  Tidal hypothesis, 251.
+
+  Tidal retardation, effect on land and sea, 191;
+    rotation of earth and, 18 f.;
+    stress caused by, 310.
+
+  Tides, cycles of, 219.
+
+  Time, geological, _see_ Geological time.
+
+  Toads, distribution of, 202.
+
+  Tobacco plant, effect of light, 184.
+
+  Topography, and glaciation, 132.
+
+  Transcaspian Basin, red beds, 151.
+
+  Tree ferns, in New Zealand, 179.
+
+  Tree growth, periodicity in, 245;
+    rainfall and, 79.
+
+  Trees, in California, 219;
+    measurement of, 73 ff.
+
+  Triassic, climate, 266.
+
+  Trifid Nebula, variables, 248.
+
+  Trondheim, wheat in, 101.
+
+  Trondhen�s, corn in, 101.
+
+  Tropical cyclones, in glacial epochs, 140 f.;
+    occurrence, 148;
+    solar activity and, 113.
+
+  Tropical hurricanes, earthquakes and, 300;
+    sunspots and, 149.
+
+  Turfan, temperature, 17.
+
+  Turner, H. H., cited, 245.
+
+  Tyler, J. M., cited, 216.
+
+  Tyndall, J., cited, 36, 37.
+
+  Typhoon region, "earthquake weather," 298.
+
+  Typhoons, occurrence, 300.
+
+
+  United States, rainfall and temperature in Gulf region, 93 f.;
+    salt lakes in, 142;
+    southwestern, climate, 66;
+    storminess, 53 f., 60.
+
+
+  Variables, 247.
+
+  Veeder, M. A., cited, 300.
+
+  Vegetation, theory of pulsations and, 90.
+
+  Venus, atmosphere of, 236.
+
+  Vesterbygd, invasion of, 106.
+
+  Vicksburg, Miss., rainfall and temperature, 93 f.
+
+  Volcanic activity, climate and, 210;
+    movement of the earth's crust and, 285;
+    times of uplifting lands and, 23.
+
+  Volcanic dust, climatic changes and, 97.
+
+  Volcanic hypothesis, climatic change and, 45 ff.;
+    snow line, 139.
+
+  Volcanoes, activity of, 96.
+
+  Volga, 108.
+
+
+  Walcott, C. D., cited, 4, 230.
+
+  Wandering of the pole, 302.
+
+  Water, importance, 9.
+
+  Water vapor, condensation of, 56;
+    effect on life, 231;
+    in atmosphere, 19.
+
+  Wave, effect on movement of water, 176.
+
+  Weather, changes of, 31 f.;
+    origin of, 174;
+    variations, 52.
+
+  Wells, H. G., cited, 35.
+
+  Wendingstadt, storm-flood, 99.
+
+  Westerlies, 21 f.
+
+  Wheat, price in England, 102.
+
+  White Sea, submergence, 219.
+
+  Whitney, J. D., cited, 142.
+
+  Wieland, G. R., cited, 169.
+
+  Williamson, E. D., cited, 226.
+
+  Willis, B., cited, 206.
+
+  Winds, at ice front, 162;
+    effect on currents, 174;
+    glaciation and, 133;
+    in Antarctica, 161;
+    in glacial period, 119;
+    in Greenland, 161;
+    planetary system of, 174;
+    velocity, 240.
+
+  Witch hazel, effect of light, 184.
+
+  Wolf, J. R., cited, 61, 109, 288.
+
+  Wolfer, cited, 244.
+
+  Wright, W. B., cited, 35, 111, 119.
+
+  Writing, among Mayas, 96.
+
+
+  Yucatan, Maya civilization, 26, 107;
+    rainfall, 108;
+    ruins, 95.
+
+  Yukon, Ice Age in, 221.
+
+
+  Zante, earthquakes of, 296.
+
+  Zonal crowding, 117.
+
+
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+<pre>
+
+The Project Gutenberg EBook of Climatic Changes, by 
+Ellsworth Huntington and Stephen Sargent Visher
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: Climatic Changes
+       Their Nature and Causes
+
+Author: Ellsworth Huntington
+        Stephen Sargent Visher
+
+Release Date: October 26, 2011 [EBook #37855]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK CLIMATIC CHANGES ***
+
+
+
+
+Produced by Robin Monks, Wayne Hammond and the Online
+Distributed Proofreading Team at http://www.pgdp.net (This
+file was produced from images generously made available
+by The Internet Archive/Canadian Libraries)
+
+
+
+
+
+
+</pre>
+
+
+<div class="book"><!-- Begin Book -->
+<div class="transnote">
+<p><b>TRANSCRIBER'S NOTE:</b> Spelling maintained as closely as possible to the
+original document, while obvious typos have been corrected. Emdashes
+in original text for negative temperatures changed to minus signs to
+standardize temperatures.</p>
+</div>
+
+<!--File: cc007.png-->
+<a class="pagenum break" id="Page_vi" title="pg vi"></a>
+
+
+
+<h1>CLIMATIC CHANGES</h1>
+
+<h2>THEIR NATURE AND CAUSES</h2>
+
+
+<!--File: cc008.png-->
+<a class="pagenum break" id="Page_vii" title="pg vii"></a>
+<!--[Blank Page]-->
+<!--File: cc009.png Page -->
+<a class="pagenum break" id="Page_viii" title="pg viii"></a>
+
+
+
+<h3>PUBLISHED ON THE FOUNDATION<br />
+ESTABLISHED IN MEMORY OF<br />
+THEODORE L. GLASGOW</h3>
+<hr />
+<!--File: cc010.png-->
+<a class="pagenum break" id="Page_ix" title="pg 1x"></a>
+
+
+
+<h3>OTHER BOOKS BY THE SAME AUTHORS</h3>
+
+
+<h3>ELLSWORTH HUNTINGTON</h3>
+
+<ol style="list-style-type:upper-alpha">
+
+<li><em>Four books showing the development of knowledge as to Historical Pulsations of Climate.</em>
+ <ul class="none">
+  <li>The Pulse of Asia. Boston, 1907.</li>
+  <li>Explorations in Turkestan. Expedition of 1903. Washington, 1905.</li>
+  <li>Palestine and Its Transformation. Boston, 1911.</li>
+  <li>The Climatic Factor, as Illustrated in Arid America. Washington, 1914.</li>
+ </ul>
+</li>
+
+<li><em>Two books illustrating the effect of climate on man.</em>
+<ul class="none">
+  <li>Civilization and Climate. New Haven, 1915.</li>
+  <li>World Power and Evolution. New Haven, 1919.</li>
+ </ul>
+</li>
+
+<li><em>Four books illustrating the general principles of Geography.</em>
+ <ul class="none">
+  <li>Asia: A Geography Reader. Chicago, 1912.</li>
+  <li>The Red Man's Continent. New Haven, 1919.</li>
+  <li>Principles of Human Geography (with S. W. Cushing). New York, 1920.</li>
+  <li>Business Geography (with F. E. Williams). New York, 1922.</li>
+ </ul>
+</li>
+
+<li><em>A companion to the present volume.</em>
+ <ul class="none">
+  <li>Earth and Sun: An Hypothesis of Weather and Sunspots. New Haven. In press.</li>
+ </ul>
+</li>
+</ol>
+
+
+<h3>STEPHEN SARGENT VISHER</h3>
+
+
+<p>Geography, Geology and Biology of Southern Dakota. Vermilion, 1912.<br />
+The Biology of Northwestern South Dakota. Vermilion, 1914.<br />
+The Geography of South Dakota. Vermilion, 1918.<br />
+Handbook of the Geology of Indiana (with others). Indianapolis, 1922.<br />
+Hurricanes of Australia and the South Pacific. Melbourne, 1922.</p>
+
+<!--File: cc011.png-->
+<a class="pagenum break" id="Page_x" title="pg x"></a>
+
+
+<hr />
+
+<h1>CLIMATIC CHANGES</h1>
+
+<h3>THEIR NATURE AND CAUSES<br /><br />
+
+BY<br /><br />
+
+ELLSWORTH HUNTINGTON</h3>
+<h5>Research Associate in Geography in Yale University</h5>
+
+<h3>AND<br /><br />
+
+STEPHEN SARGENT VISHER</h3>
+<h5>Associate Professor of Geology<br />
+in Indiana University</h5>
+
+<div class="figcenter">
+<img  src="images/front.png" width="170" height="179" alt="" title="Shield with motto &quot;Lux et Veritas&quot;" />
+</div>
+<h4>NEW HAVEN<br />
+YALE UNIVERSITY PRESS<br />
+LONDON: HUMPHREY MILFORD: OXFORD UNIVERSITY PRESS<br />
+MDCCCCXXII</h4>
+
+<!--File: cc012.png-->
+
+
+
+<hr />
+
+
+<h3>COPYRIGHT 1922 BY</h3>
+<h3>YALE UNIVERSITY PRESS</h3>
+
+<h3>Published 1922.</h3>
+<hr />
+<!--File: cc013.png-->
+
+
+
+
+<h3>THE THEODORE L. GLASGOW MEMORIAL</h3>
+<h3>PUBLICATION FUND</h3>
+
+
+<blockquote><p>The present volume is the fifth work published by the Yale
+University Press on the Theodore L. Glasgow Memorial Publication
+Fund. This foundation was established September 17, 1918,
+by an anonymous gift to Yale University in memory of Flight
+Sub-Lieutenant Theodore L. Glasgow, R.N. He was born in
+Montreal, Canada, and was educated at the University of Toronto
+Schools and at the Royal Military College, Kingston. In August,
+1916, he entered the Royal Naval Air Service and in July, 1917,
+went to France with the Tenth Squadron attached to the Twenty-second
+Wing of the Royal Flying Corps. A month later, August
+19, 1917, he was killed in action on the Ypres front.</p></blockquote>
+<!--File: cc014.png-->
+
+<!--[Blank Page]-->
+<!--File: cc015.png-->
+
+
+
+
+<h3>TO<br /><br />
+
+THOMAS CHROWDER CHAMBERLIN<br /><br />
+
+OF THE UNIVERSITY OF CHICAGO<br /><br />
+
+WHOSE CLEAR AND MASTERLY DISCUSSION
+OF THE GREAT PROBLEMS OF TERRESTRIAL EVOLUTION
+HAS BEEN ONE OF THE MOST INSPIRING FACTORS
+IN THE WRITING OF THIS BOOK</h3>
+
+<!--File: cc016.png-->
+<blockquote>
+<p><span class="smcap">There</span> <em>is a toy, which I have heard, and I would not have
+it given over, but waited upon a little. They say it is observed
+in the Low Countries (I know not in what part),
+that every five and thirty years the same kind and suit
+of years and weathers comes about again; as great frosts,
+great wet, great droughts, warm winters, summers with
+little heat, and the like, and they call it the prime; it is a
+thing I do the rather mention, because, computing backwards,
+I have found some concurrence.</em></p>
+
+<p style="text-align:right;">FRANCIS BACON</p>
+</blockquote>
+
+<!--File: cc017.png-->
+
+
+
+<h2 id="preface">PREFACE</h2>
+
+
+<p>Unity is perhaps the keynote of modern science.
+This means unity in time, for the present is but
+the outgrowth of the past, and the future of the
+present. It means unity of process, for there seems to be
+no sharp dividing line between organic and inorganic,
+physical and mental, mental and spiritual. And the unity
+of modern science means also a growing tendency toward
+co&ouml;peration, so that by working together scientists discover
+much that would else have remained hid.</p>
+
+<p>This book illustrates the modern trend toward unity in
+all of these ways. First, it is a companion volume to
+<em>Earth and Sun</em>. That volume is a discussion of the causes
+of weather, but a consideration of the weather of the
+present almost inevitably leads to a study of the climate
+of the past. Hence the two books were written originally
+as one, and were only separated from considerations of
+convenience. Second, the unity of nature is so great that
+when a subject such as climatic changes is considered, it
+is almost impossible to avoid other subjects, such as the
+movements of the earth's crust. Hence this book not only
+discusses climatic changes, but considers the causes of
+earthquakes and attempts to show how climatic changes
+may be related to great geological revolutions in the
+form, location, and altitude of the lands. Thus the book
+has a direct bearing on all the main physical factors
+which have molded the evolution of organic life, including
+man.</p>
+<!--File: cc018.png-->
+<p>In the third place, this volume illustrates the unity of
+modern science because it is pre&euml;minently a co&ouml;perative
+product. Not only have the two authors shared in its
+production, but several of the Yale Faculty have also
+co&ouml;perated. From the geological standpoint, Professor
+Charles Schuchert has read the entire manuscript in its
+final form as well as parts at various stages. He has
+helped not only by criticisms, suggestions, and facts, but
+by paragraphs ready for the printer. In the same way
+in the domain of physics, Professor Leigh Page has repeatedly
+taken time to assist, and either in writing or by
+word of mouth has contributed many pages. In astronomy,
+the same cordial co&ouml;peration has come with equal
+readiness from Professor Frank Schlesinger. Professors
+Schuchert, Schlesinger, and Page have contributed so
+materially that they are almost co-authors of the volume.
+In mathematics, Professor Ernest W. Brown has been
+similarly helpful, having read and criticised the entire
+book. In certain chemical problems, Professor Harry W.
+Foote has been our main reliance. The advice and suggestions
+of these men have frequently prevented errors, and
+have again and again started new and profitable lines of
+thought. If we have made mistakes, it has been because
+we have not profited sufficiently by their co&ouml;peration. If
+the main hypothesis of this book proves sound, it is
+largely because it has been built up in constant consultation
+with men who look at the problem from different
+points of vision. Our appreciation of their generous and
+unstinted co&ouml;peration is much deeper than would appear
+from this brief paragraph.</p>
+
+<p>Outside the Yale Faculty we have received equally
+cordial assistance. Professor T. C. Chamberlin of the University
+of Chicago, to whom, with his permission, we take
+great pleasure in dedicating this volume, has read the
+<!--File: cc019.png-->
+entire proof and has made many helpful suggestions.
+We cannot speak too warmly of our appreciation not
+only of this, but of the way his work has served for years
+as an inspiration in the preliminary work of gathering
+data for this volume. Professor Harlow Shapley of Harvard
+University has contributed materially to the chapter
+on the sun and its journey through space; Professor
+Andrew E. Douglass of the University of Arizona has
+put at our disposal some of his unpublished results;
+Professors S. B. Woodworth and Reginald A. Daly, and
+Mr. Robert W. Sayles of Harvard, and Professor Henry
+F. Reid of Johns Hopkins have suggested new facts and
+sources of information; Professor E. R. Cumings of
+Indiana University has critically read the entire proof;
+conversations with Professor John P. Buwalda of the
+University of California while he was teaching at Yale
+make him another real contributor; and Mr. Wayland
+Williams has contributed the interesting quotation from
+Bacon on page x of this book. Miss Edith S. Russell has
+taken great pains in preparing the manuscript and in
+suggesting many changes that make for clearness. Many
+others have also helped, but it is impossible to make due
+acknowledgment because such contributions have become
+so thoroughly a part of the mental background of the
+book that their source is no longer distinct in the minds
+of the authors.</p>
+
+<p>The division of labor between the two authors has not
+followed any set rules. Both have had a hand in all parts
+of the book. The main draft of Chapters VII, VIII, IX,
+XI, and XIII was written by the junior author; his contributions
+are also especially numerous in Chapters X
+and XV; the rest of the book was written originally by
+the senior author.</p>
+
+<!--File: cc020.png-->
+<!--[Blank Page]-->
+<!--File: cc021.png-->
+
+
+<p>&nbsp;</p>
+<table>
+<tr><th colspan="3"><a id="content"></a>CONTENTS</th></tr>
+<tr>
+<td class="tdl">I</td>
+<td class="tdl"><a href="#Chapt_I">The Uniformity of Climate</a></td>
+<td class="tdl">1</td>
+</tr>
+<tr>
+<td class="tdl">II.</td>
+<td class="tdl"><a href="#Chapt_II">The Variability of Climate</a></td>
+<td class="tdl">16</td>
+</tr>
+<tr>
+<td class="tdl">III.</td>
+<td class="tdl"><a href="#Chapt_III">Hypotheses of Climatic Change</a></td>
+<td class="tdl">33</td>
+</tr>
+<tr>
+<td class="tdl">IV.</td>
+<td class="tdl"><a href="#Chapt_IV">The Solar Cyclonic Hypothesis</a></td>
+<td class="tdl">51</td>
+</tr>
+<tr>
+<td class="tdl">V.</td>
+<td class="tdl"><a href="#Chapt_V">The Climate of History</a></td>
+<td class="tdl">64</td>
+</tr>
+<tr>
+<td class="tdl">VI.</td>
+<td class="tdl"><a href="#Chapt_IV">The Climatic Stress of the Fourteenth Century</a></td>
+<td class="tdl">98</td>
+</tr>
+<tr>
+<td class="tdl">VII.</td>
+<td class="tdl"><a href="#Chapt_VII">Glaciation According to the Solar Cyclonic Hypothesis</a></td>
+<td class="tdl">110</td>
+</tr>
+<tr>
+<td class="tdl">VIII.</td>
+<td class="tdl"><a href="#Chapt_VIII">Some Problems of Glacial Periods </a></td>
+<td class="tdl">130</td>
+</tr>
+<tr>
+<td class="tdl">IX.</td>
+<td class="tdl"><a href="#Chapt_IX">The Origin of Loess</a></td>
+<td class="tdl">155</td>
+</tr>
+<tr>
+<td class="tdl">X.</td>
+<td class="tdl"><a href="#Chapt_X">Causes of Mild Geological Climates </a></td>
+<td class="tdl">166</td>
+</tr>
+<tr>
+<td class="tdl">XI.</td>
+<td class="tdl"><a href="#Chapt_XI">Terrestrial Causes of Climatic Changes</a></td>
+<td class="tdl">188</td>
+</tr>
+<tr>
+<td class="tdl">XII.</td>
+<td class="tdl"><a href="#Chapt_XII">Post-Glacial Crustal Movements and Climatic Changes</a></td>
+<td class="tdl">215</td>
+</tr>
+<tr>
+<td class="tdl">XIII.</td>
+<td class="tdl"><a href="#Chapt_XIII">The Changing Composition of Oceans and Atmosphere</a></td>
+<td class="tdl">223</td>
+</tr>
+<tr>
+<td class="tdl">XIV.</td>
+<td class="tdl"><a href="#Chapt_XIV">The Effect of Other Bodies on the Sun</a></td>
+<td class="tdl">242</td>
+</tr>
+<tr>
+<td class="tdl">XV.</td>
+<td class="tdl"><a href="#Chapt_XV">The Sun's Journey through Space</a></td>
+<td class="tdl">264</td>
+</tr>
+<tr>
+<td class="tdl">XVI.</td>
+<td class="tdl"><a href="#Chapt_XVI">The Earth's Crust and the Sun</a></td>
+<td class="tdl">285</td>
+</tr>
+</table>
+
+<!--File: cc022.png-->
+
+
+<p>&nbsp;</p>
+<table>
+<tr>
+<th colspan="3">LIST OF ILLUSTRATIONS</th>
+</tr>
+<tr>
+<td class="tdl">Fig. 1.</td>
+<td class="tdl"><a href="#Fig_1">Climatic changes and mountain building</a></td>
+<td class="tdl">25</td>
+</tr>
+<tr>
+<td class="tdl">Fig. 2.</td>
+<td class="tdl"><a href="#Fig_2">Storminess at sunspot maxima vs. minima</a></td>
+<td class="tdl">54</td>
+</tr>
+<tr>
+<td class="tdl">Fig. 3.</td>
+<td class="tdl"><a href="#Fig_3a">Relative rainfall at times of increasing and decreasing sunspots</a></td>
+<td class="tdl">58, 59</td>
+</tr>
+<tr>
+<td class="tdl">Fig. 4.</td>
+<td class="tdl"><a href="#Fig_4">Changes of climate in California and in western and central Asia</a></td>
+<td class="tdl">75</td>
+</tr>
+<tr>
+<td class="tdl">Fig. 5.</td>
+<td class="tdl"><a href="#Fig_5">Changes in California climate for 2000 years, as measured by growth of Sequoia trees</a></td>
+<td class="tdl">77</td>
+</tr>
+<tr>
+<td class="tdl">Fig. 6.</td>
+<td class="tdl"><a href="#Fig_6">Distribution of Pleistocene ice sheets</a></td>
+<td class="tdl">123</td>
+</tr>
+<tr>
+<td class="tdl">Fig. 7.</td>
+<td class="tdl"><a href="#Fig_7">Permian geography and glaciation </a></td>
+<td class="tdl">145</td>
+</tr>
+<tr>
+<td class="tdl">Fig. 8.</td>
+<td class="tdl"><a href="#Fig_8">Effect of diminution of storms on movement of water</a></td>
+<td class="tdl">175</td>
+</tr>
+<tr>
+<td class="tdl">Fig. 9.</td>
+<td class="tdl"><a href="#Fig_9">Cretaceous Paleogeography</a></td>
+<td class="tdl">201</td>
+</tr>
+<tr><td class="tdl">Fig. 10.</td>
+<td class="tdl"><a href="#Fig_10">Climatic changes of 140,000 years as inferred from the stars</a></td>
+<td class="tdl">279</td>
+</tr>
+<tr><td class="tdl">Fig. 11.</td>
+<td class="tdl"><a href="#Fig_11">Sunspot curve showing cycles, 1750 to 1920</a></td>
+<td class="tdl">283</td>
+</tr>
+<tr><td class="tdl">Fig. 12.</td>
+<td class="tdl"><a href="#Fig_12">Seasonal distribution of earthquakes</a></td>
+<td class="tdl">299</td>
+</tr>
+<tr><td class="tdl">Fig. 13.</td>
+<td class="tdl"><a href="#Fig_13">Wandering of the pole from 1890 to 1898</a></td>
+<td class="tdl">303</td>
+</tr>
+</table>
+
+
+<!--File: cc023.png-->
+
+
+
+<p>&nbsp;</p>
+<table>
+<tr><th id="tables" colspan="3">TABLES</th></tr>
+<tr>
+<td class="tdl">1.</td>
+<td class="tdl"><a href="#table_1">The Geological Time Table</a></td>
+<td class="tdl">5</td>
+</tr>
+<tr>
+<td class="tdl">2.</td>
+<td class="tdl"><a href="#table_2">Types of Climatic Sequence</a></td>
+<td class="tdl">16</td>
+</tr>
+<tr>
+<td class="tdl">3.</td>
+<td class="tdl"><a href="#table_3">Correlation Coefficients between Rainfall and Growth of Sequoias in California</a></td>
+<td class="tdl">80</td>
+</tr>
+<tr>
+<td class="tdl">4.</td>
+<td class="tdl"><a href="#table_4">Correlation Coefficients between Rainfall Records in California and Jerusalem</a></td>
+<td class="tdl">84</td>
+</tr>
+<tr>
+<td class="tdl">5.</td>
+<td class="tdl"><a href="#table_5">Theoretical Probability of Stellar Approaches</a></td>
+<td class="tdl"> 260</td>
+</tr>
+<tr>
+<td class="tdl">6.</td>
+<td class="tdl"><a href="#table_6">Thirty-Eight Stars Having Largest Known Parallaxes</a></td>
+<td class="tdl">276, 277</td>
+</tr>
+<tr><td class="tdl">7.</td>
+<td class="tdl"><a href="#table_7">Destructive Earthquakes from 1800 to 1899 Compared with Sunspots</a></td>
+<td class="tdl">289</td>
+</tr>
+<tr>
+<td class="tdl">8.</td>
+<td class="tdl"><a href="#table_8">Seasonal March of Earthquakes</a></td>
+<td class="tdl">295</td>
+</tr>
+<tr>
+<td class="tdl">9.</td>
+<td class="tdl"><a href="#table_9">Deflection of Path of Pole Compared with Earthquakes</a></td>
+<td class="tdl">305</td>
+</tr>
+<tr>
+<td class="tdl">10.</td>
+<td class="tdl"><a href="#table_10">Earthquakes in 1903 to 1908 Compared with Departures of the Projected Curve of the Earth's Axis from the Eulerian Position</a></td>
+<td class="tdl">306</td>
+</tr>
+</table>
+
+<!--File: cc024.png-->
+
+<!--[Blank Page]-->
+
+<!--File: cc025.png Page 1-->
+<a class="pagenum break" id="Page_1" title="pg 1"></a>
+
+<h2 id="Chapt_I"><a href="#content">CHAPTER I</a></h2>
+
+<h3>THE UNIFORMITY OF CLIMATE</h3>
+
+
+<p>The r&ocirc;le of climate in the life of today suggests its
+importance in the past and in the future. No human
+being can escape from the fact that his food,
+clothing, shelter, recreation, occupation, health, and
+energy are all profoundly influenced by his climatic surroundings.
+A change of season brings in its train some
+alteration in practically every phase of human activity.
+Animals are influenced by climate even more than man,
+for they have not developed artificial means of protecting
+themselves. Even so hardy a creature as the dog
+becomes notably different with a change of climate. The
+thick-haired "husky" of the Eskimos has outwardly
+little in common with the small and almost hairless
+canines that grovel under foot in Mexico. Plants are even
+more sensitive than animals and men. Scarcely a single
+species can flourish permanently in regions which differ
+more than 20&deg;C. in average yearly temperature, and for
+most the limit of successful growth is 10&deg;.<sup><a href="#fn1" id="r1">[1]</a></sup>
+So far as we
+yet know every living species of plant and animal, including
+man, thrives best under definite and limited conditions
+of temperature, humidity, and sunshine, and of the composition
+and movement of the atmosphere or water in
+which it lives. Any departure beyond the limits means
+lessened efficiency, and in the long run a lower rate of
+
+<!--File: cc026.png Page_2-->
+<a class="pagenum break" id="Page_2" title="pg 2"></a>
+
+reproduction and a tendency toward changes in specific
+characteristics. Any great departure means suffering or
+death for the individual and destruction for the species.</p>
+
+<p>Since climate has so profound an influence on life
+today, it has presumably been equally potent at other
+times. Therefore few scientific questions are more important
+than how and why the earth's climate has varied
+in the past, and what changes it is likely to undergo in
+the future. This book sets forth what appear to be the
+chief reasons for climatic variations during historic and
+geologic times. It assumes that causes which can now be
+observed in operation, as explained in a companion
+volume entitled <em>Earth and Sun</em>, and in such books as
+Humphreys' <em>Physics of the Air</em>, should be carefully
+studied before less obvious causes are appealed to. It
+also assumes that these same causes will continue to
+operate, and are the basis of all valid predictions as to
+the weather or climate of the future.</p>
+
+<p>In our analysis of climatic variations, we may well
+begin by inquiring how the earth's climate has varied
+during geological history. Such an inquiry discloses three
+great tendencies, which to the superficial view seem contradictory.
+All, however, have a similar effect in providing
+conditions under which organic evolution is able to
+make progress. The first tendency is toward uniformity,
+a uniformity so pronounced and of such vast duration
+as to stagger the imagination. Superposed upon this
+there seems to be a tendency toward complexity. During
+the greater part of geological history the earth's climate
+appears to have been relatively monotonous, both from
+place to place and from season to season; but since the
+Miocene the rule has been diversity and complexity, a
+condition highly favorable to organic evolution. Finally,
+the uniformity of the vast eons of the past and the
+
+<!--File: cc027.png Page_3-->
+<a class="pagenum break" id="Page_3" title="pg 3"></a>
+
+tendency toward complexity are broken by  pulsatory
+changes, first in one direction and then in another. To
+our limited human vision some of the changes, such as
+glacial periods, seem to be waves of enormous proportions,
+but compared with the possibilities of the universe
+they are merely as the ripples made by a summer zephyr.</p>
+
+<p>The uniformity of the earth's climate throughout the
+vast stretches of geological time can best be realized by
+comparing the range of temperature on the earth during
+that period with the possible range as shown in the entire
+solar system. As may be seen in Table 1, the geological
+record opens with the Archeozoic era, or "Age of Unicellular
+Life," as it is sometimes called, for the preceding
+cosmic time has left no record that can yet be read.
+Practically no geologists now believe that the beginning
+of the Archeozoic was less than one hundred million
+years ago; and since the discovery of the peculiar properties
+of radium many of the best students do not hesitate
+to say a billion or a billion and a half.<sup><a href="#fn2" id="r2">[2]</a></sup>
+Even in the
+Archeozoic the rocks testify to a climate seemingly not
+greatly different from that of the average of geologic
+time. The earth's surface was then apparently cool
+enough so that it was covered with oceans and warm
+enough so that the water teemed with microscopic life.
+The air must have been charged with water vapor and
+with carbon dioxide, for otherwise there seems to be no
+possible way of explaining the formation of mudstones
+and sandstones, limestones of vast thickness, carbonaceous
+shales, graphites, and iron ores.<sup><a href="#fn3" id="r3">[3]</a></sup> Although the
+Archeozoic has yielded no generally admitted fossils, yet
+what seem to be massive alg&aelig; and sponges have been
+
+<!--File: cc028.png Page_4-->
+<a class="pagenum break" id="Page_4" title="pg 4"></a>
+
+found in Canada. On the other hand, abundant life is
+believed to have been present in the oceans, for by no
+other known means would it be possible to take from the
+air the vast quantities of carbon that now form carbonaceous
+shales and graphite.</p>
+
+<p>In the next geologic era, the Proterozoic, the researches
+of Walcott have shown that besides the marine
+alg&aelig; there must have been many other kinds of life. The
+Proterozoic fossils thus far discovered include not only
+microscopic radiolarians such as still form the red ooze
+of the deepest ocean floors, but the much more significant
+tubes of annelids or worms. The presence of the
+annelids, which are relatively high in the scale of organization,
+is generally taken to mean that more lowly forms
+of animals such as coelenterates and probably even the
+mollusca and primitive arthropods must already have
+been evolved. That there were many kinds of marine
+invertebrates living in the later Proterozoic is indicated
+by the highly varied life and more especially the trilobites
+found in the oldest Cambrian strata of the next
+succeeding period. In fact the Cambrian has sponges,
+primitive corals, a great variety of brachiopods, the
+beginnings of gastropods, a wonderful array of trilobites,
+and other lowly forms of arthropods. Since, under the
+postulate of evolution, the life of that time forms an unbroken
+sequence with that of the present, and since many
+of the early forms differ only in minor details from those
+of today, we infer that the climate then was not very
+different from that of today. The same line of reasoning
+leads to the conclusion that even in the middle of the
+Proterozoic, when multicellular marine animals must
+already have been common, the climate of the earth had
+already for an enormous period been such that all the
+lower types of oceanic invertebrates had already evolved.</p>
+
+<!--File: cc029.png Page_5-->
+<a class="pagenum break" id="Page_5" title="pg 5"></a>
+
+<table id="table_1" class="borders">
+
+<tr><th><a href="#tables">TABLE 1</a></th></tr>
+
+<tr><th>THE GEOLOGICAL TIME TABLE<sup><a href="#fn4" id="r4">[4]</a></sup></th></tr>
+
+
+<tr><th>COSMIC TIME</th></tr>
+<tr><td class="tdtxt"><span class="smcap">Formative Era.</span> Birth and growth of the earth. Beginnings of
+the atmosphere, hydrosphere, continental platforms, oceanic
+basins, and possibly of life. No known geological record.</td></tr>
+<tr><th>GEOLOGIC TIME</th></tr>
+<tr><td class="tdtxt"><span class="smcap">Archeozoic Era.</span> Origin of simplest life.</td></tr>
+<tr><td class="tdtxt"><span class="smcap">Proterozoic Era.</span> Age of invertebrate origins. An early and a late
+ice age, with one or more additional ones indicated.</td></tr>
+<tr><td class="tdtxt"><span class="smcap">Paleozoic Era.</span> Age of primitive vertebrate dominance.</td></tr>
+<tr><td class="tdi2"><em>Cambrian Period.</em> First abundance of marine animals and dominance
+of trilobites.</td></tr>
+<tr><td class="tdi2"><em>Ordovician Period.</em> First known fresh-water fishes.</td></tr>
+<tr><td class="tdi2"><em>Silurian Period.</em> First known land plants.</td></tr>
+<tr><td class="tdi2"><em>Devonian Period.</em> First known amphibians. "Table Mountain" ice age.</td></tr>
+<tr><td class="tdi2"><em>Mississippian Period.</em> Rise of marine fishes (sharks).</td></tr>
+<tr><td class="tdi2"><em>Pennsylvanian Period.</em> Rise of insects and first period of marked coal accumulation.</td></tr>
+<tr><td class="tdi2"><em>Permian Period.</em> Rise of reptiles. Another great ice age.</td></tr>
+<tr><td class="tdtxt"><span class="smcap">Mesozoic Era.</span> Age of reptile dominance.</td></tr>
+<tr><td class="tdi2"><em>Triassic Period.</em> Rise of dinosaurs. The period closes with a cool
+climate.</td></tr>
+<tr><td class="tdi2"><em>Jurassic Period.</em> Rise of birds and flying reptiles.</td></tr>
+<tr><td class="tdi2"><em>Comanchean Period.</em> Rise of flowering plants and higher insects.</td></tr>
+<tr><td class="tdi2"><em>Cretaceous Period.</em> Rise of archaic or primitive mammalia.</td></tr>
+<tr><td class="tdtxt"><span class="smcap">Cenozoic Era.</span> Age of mammal dominance.</td></tr>
+<tr><td class="tdi2"><em>Early Cenozoic or Eocene and Oligocene time.</em> Rise of higher
+mammals. Glaciers in early Eocene of the Laramide Mountains.</td></tr>
+<tr><td class="tdi2"><em>Late Cenozoic or Miocene and Pliocene time.</em> Transformation of
+ape like animals into man.</td></tr>
+<tr><td class="tdi2"><em>Glacial or Pleistocene time.</em> Last great ice age.</td></tr>
+<tr><th>PRESENT TIME</th></tr>
+<tr><td class="tdtxt"><span class="smcap">Psychozoic Era</span>. Age of man or age of reason. Includes the
+present or "Recent time," estimated to be probably less than
+30,000 years.</td></tr>
+</table>
+
+<!--File: cc030.png Page_6-->
+<a class="pagenum break" id="Page_6" title="pg 6"></a>
+
+<p>Moreover, they could live in most latitudes, for the indirect
+evidences of life in the Archeozoic and Proterozoic
+rocks are widely distributed. Thus it appears that
+at an almost incredibly early period, perhaps many hundred
+million years ago, the earth's climate differed only
+a little from that of the present.</p>
+
+<p>The extreme limits of temperature beyond which the
+climate of geological times cannot have departed can be
+approximately determined. Today the warmest parts of
+the ocean have an average temperature of about 30&deg;C.
+on the surface. Only a few forms of life live where the
+average temperature is much higher than this. In deserts,
+to be sure, some highly organized plants and animals can
+for a short time endure a temperature as high as 75&deg;C.
+(167&deg;F.). In certain hot springs, some of the lowest unicellular
+plant forms exist in water which is only a little
+below the boiling point. More complex forms, however,
+such as sponges, worms, and all the higher plants and
+animals, seem to be unable to live either in water or air
+where the temperature averages above 45&deg;C. (113&deg;F.)
+for any great length of time and it is doubtful whether
+they can thrive permanently even at that temperature.
+The obvious unity of life for hundreds of millions of
+years and its presence at all times in middle latitudes so
+far as we can tell seem to indicate that since the beginning
+of marine life the temperature of the oceans
+cannot have averaged much above 50&deg;C. even in the
+warmest portions. This is putting the limit too high
+rather than too low, but even so the warmest parts of
+the earth can scarcely have averaged much more than
+20&deg; warmer than at present.</p>
+
+<p>Turning to the other extreme, we may inquire how
+much colder than now the earth's surface may have been
+since life first appeared. Proterozoic fossils have been
+
+<!--File: cc031.png Page _7-->
+<a class="pagenum break" id="Page_7" title="pg 7"></a>
+
+found in places where the present average temperature
+approaches 0&deg;C. If those places should be colder than
+now by 30&deg;C., or more, the drop in temperature at the
+equator would almost certainly be still greater, and the
+seas everywhere would be permanently frozen. Thus
+life would be impossible. Since the contrasts between
+summer and winter, and between the poles and the
+equator seem generally to have been less in the past than
+at present, the range through which the mean temperature
+of the earth as a whole could vary without utterly
+destroying life was apparently less than would now be
+the case.</p>
+
+<p>These considerations make it fairly certain that for at
+least several hundred million years the average temperature
+of the earth's surface has never varied more than
+perhaps 30&deg;C. above or below the present level. Even this
+range of 60&deg;C. (108&deg;F.) may be double or triple the range
+that has actually occurred. That the temperature has not
+passed beyond certain narrow limits, whatever their
+exact degree, is clear from the fact that if it had done so,
+all the higher forms of life would have been destroyed.
+Certain of the lowest unicellular forms might indeed have
+persisted, for when dormant they can stand great extremes
+of dry heat and of cold for a long time. Even
+so, evolution would have had to begin almost anew. The
+supposition that such a thing has happened is untenable,
+for there is no hint of any complete break in the record
+of life during geological times,&mdash;no sudden disappearance
+of the higher organisms followed by a long period
+with no signs of life other than indirect evidence such as
+occurs in the Archeozoic.</p>
+
+<p>A change of 60&deg;C. or even of 20&deg; in the average temperature
+of the earth's surface may seem large when
+viewed from the limited standpoint of terrestrial experience.
+<!--File: cc032.png Page_8-->
+<a class="pagenum break" id="Page_8" title="pg 8"></a>
+Viewed, however, from the standpoint of
+cosmic evolution, or even of the solar system, it seems
+a mere trifle. Consider the possibilities. The temperature
+of empty space is the absolute zero, or -273&deg;C.
+To this temperature all matter must fall, provided it
+exists long enough and is not appreciably heated by collisions
+or by radiation. At the other extreme lies the
+temperature of the stars. As stars go, our sun is only
+moderately hot, but the temperature of its surface is
+calculated to be nearly 7000&deg;C., while thousands of miles
+in the interior it may rise to 20,000&deg; or 100,000&deg; or some
+other equally unknowable and incomprehensible figure.
+Between the limits of the absolute zero on the one hand,
+and the interior of a sun or star on the other, there is
+almost every conceivable possibility of temperature.
+Today the earth's surface averages not far from 14&deg;C.,
+or 287&deg; above the absolute zero. Toward the interior,
+the temperature in mines and deep wells rises about 1&deg;C.
+for every 100 meters. At this rate it would be over 500&deg;C.
+at a depth of ten miles, and over 5000&deg; at 100 miles.</p>
+
+<p>Let us confine ourselves to surface temperatures,
+which are all that concern us in discussing climate. It
+has been calculated by Poynting<sup><a href="#fn5" id="r5">[5]</a></sup> that if a small sphere
+absorbed and re-radiated all the heat that fell upon it,
+its temperature at the distance of Mercury from the sun
+would average about 210&deg;C.; at the distance of Venus,
+85&deg;; the earth 27&deg;; Mars -30&deg;; Neptune 219&deg;. A planet
+much nearer the sun than is Mercury might be heated to
+a temperature of a thousand, or even several thousand,
+degrees, while one beyond Neptune would remain almost
+at absolute zero. It is well within the range of possibility
+that the temperature of a planet's surface should be
+<!--File: cc033.png Page 9-->
+<a class="pagenum break" id="Page_9" title="pg 9"></a>
+anywhere from near -273&deg;C. up to perhaps 5000&deg;C. or
+more, although the probability of low temperature is
+much greater than of high. Thus throughout the whole
+vast range of possibilities extending to perhaps 10,000&deg;,
+the earth claims only 60&deg; at most, or less than 1 per cent.
+This may be remarkable, but what is far more remarkable
+is that the earth's range of 60&deg; includes what seem
+to be the two most critical of all possible temperatures,
+namely, the freezing point of water, 0&deg;C., and the temperature
+where water can dissolve an amount of carbon
+dioxide equal to its own volume. The most remarkable
+fact of all is that the earth has preserved its temperature
+within these narrow limits for a hundred million years,
+or perchance a thousand million.</p>
+
+<p>To appreciate the extraordinary significance of this
+last fact, it is necessary to realize how extremely critical
+are the temperatures from about 0&deg; to 40&deg;C., and how
+difficult it is to find any good reason for a relatively
+uniform temperature through hundreds of millions of
+years. Since the dawn of geological time the earth's
+temperature has apparently always included the range
+from about the freezing point of water up to about the
+point where protoplasm begins to disintegrate. Henderson,
+in <em>The Fitness of the Environment</em>, rightly says that
+water is "the most familiar and the most important of
+all things." In many respects water and carbon dioxide
+form the most unique pair of substances in the whole
+realm of chemistry. Water has a greater tendency than
+any other known substance to remain within certain
+narrowly defined limits of temperature. Not only does it
+have a high specific heat, so that much heat is needed to
+raise its temperature, but on freezing it gives up more
+heat than any substance except ammonia, while none of
+the common liquids approach it in the amount of additional
+<!--File: cc034.png Page_10-->
+<a class="pagenum break" id="Page_10" title="pg 10"></a>
+heat required for conversion into vapor after the
+temperature of vaporization has been reached. Again,
+water substance, as the physicists call all forms of H<sub>2</sub>O,
+is unique in that it not only contracts on melting, but
+continues to contract until a temperature several degrees
+above its melting point is reached. That fact has a vast
+importance in helping to keep the earth's surface at a
+uniform temperature. If water were like most liquids,
+the bottoms of all the oceans and even the entire body of
+water in most cases would be permanently frozen.</p>
+
+<p>Again, as a solvent there is literally nothing to compare
+with water. As Henderson<sup><a href="#fn6" id="r6">[6]</a></sup> puts it: "Nearly the
+whole science of chemistry has been built up around
+water and aqueous solution." One of the most significant
+evidences of this is the variety of elements whose presence
+can be detected in sea water. According to Henderson
+they include hydrogen, oxygen, nitrogen, carbon,
+chlorine, sodium, magnesium, sulphur, phosphorus, which
+are easily detected; and also arsenic, c&aelig;sium, gold,
+lithium, rubidium, barium, lead, boron, fluorine, iron,
+iodine, bromine, potassium, cobalt, copper, manganese,
+nickel, silver, silicon, zinc, aluminium, calcium, and
+strontium. Yet in spite of its marvelous power of solution,
+water is chemically rather inert and relatively
+stable. It dissolves all these elements and thousands of
+their compounds, but still remains water and can easily
+be separated and purified. Another unique property of
+water is its power of ionizing dissolved substances, a
+property which makes it possible to produce electric
+currents in batteries. This leads to an almost infinite
+array of electro-chemical reactions which play an almost
+dominant r&ocirc;le in the processes of life. Finally, no
+common liquid except mercury equals water in its power
+<!--File: cc035.png Page_11-->
+<a class="pagenum break" id="Page_11" title="pg 11"></a>
+of capillarity. This fact is of enormous moment in
+biology, most obviously in respect to the soil.</p>
+
+<p>Although carbon dioxide is far less familiar than
+water, it is almost as important. "These two simple substances,"
+says Henderson, "are the common source of
+every one of the complicated substances which are produced
+by living beings, and they are the common end
+products of the wearing away of all the constituents of
+protoplasm, and of the destruction of those materials
+which yield energy to the body." One of the remarkable
+physical properties of carbon dioxide is its degree of
+solubility in water. This quality varies enormously in
+different substances. For example, at ordinary pressures
+and temperatures, water can absorb only about 5 per
+cent of its own volume of oxygen, while it can take up
+about 1300 times its own volume of ammonia. Now for
+carbon dioxide, unlike most gases, the volume that can
+be absorbed by water is nearly the same as the volume
+of the water. The volumes vary, however, according to
+temperature, being absolutely the same at a temperature
+of about 15&deg;C. or 59&deg;F., which is close to the ideal temperature
+for man's physical health and practically the
+same as the mean temperature of the earth's surface
+when all seasons are averaged together. "Hence, when
+water is in contact with air, and equilibrium has been
+established, the amount of free carbonic acid in a given
+volume of water is almost exactly equal to the amount
+in the adjacent air. Unlike oxygen, hydrogen, and nitrogen,
+carbonic acid enters water freely; unlike sulphurous
+oxide and ammonia, it escapes freely from water. Thus
+the waters can never wash carbonic acid completely out
+of the air, nor can the air keep it from the waters. It is
+the one substance which thus, in considerable quantities
+relative to its total amount, everywhere accompanies
+<!--File: cc036.png Page_12-->
+<a class="pagenum break" id="Page_12" title="pg 12"></a>
+water. In earth, air, fire, and water alike these two substances
+are always associated.</p>
+
+<p>"Accordingly, if water be the first primary constituent
+of the environment, carbonic acid is inevitably
+the second,&mdash;because of its solubility possessing an
+equal mobility with water, because of the reservoir of the
+atmosphere never to be depleted by chemical action in
+the oceans, lakes, and streams. In truth, so close is the
+association between these two substances that it is
+scarcely correct logically to separate them at all; together
+they make up the real environment and they never
+part company."<sup><a href="#fn7" id="r7">[7]</a></sup></p>
+
+<p>The complementary qualities of carbon dioxide and
+water are of supreme importance because these two are
+the only known substances which are able to form a vast
+series of complex compounds with highly varying chemical
+formul&aelig;. No other known compounds can give off
+or take on atoms without being resolved back into their
+elements. No others can thus change their form freely
+without losing their identity. This power of change without
+destruction is the fundamental chemical characteristic
+of life, for life demands complexity, change, and
+growth.</p>
+
+<p>In order that water and carbon dioxide may combine
+to form the compounds on which life is based, the water
+must be in the liquid form, it must be able to dissolve
+carbon dioxide freely, and the temperature must not be
+high enough to break up the highly complex and delicate
+compounds as soon as they are formed. In other words,
+the temperature must be above freezing, while it must
+not rise higher than some rather indefinite point between
+50&deg;C. and the boiling point, where all water finally turns
+into vapor. In the whole range of temperature, so far as
+<!--File: cc037.png Page_13-->
+<a class="pagenum break" id="Page_13" title="pg 13"></a>
+we know, there is no other interval where any such complex
+reactions take place. The temperature of the earth
+for hundreds of millions of years has remained firmly
+fixed within these limits.</p>
+
+<p>The astonishing quality of the earth's uniformity of
+temperature becomes still more apparent when we consider
+the origin of the sun's heat. What that origin is
+still remains a question of dispute. The old ideas of a
+burning sun, or of one that is simply losing an original
+supply of heat derived from some accident, such as collision
+with another body, were long ago abandoned. The
+impact of a constant supply of meteors affords an almost
+equally unsatisfactory explanation. Moulton<sup><a href="#fn8" id="r8">[8]</a></sup> states that
+if the sun were struck by enough meteorites to keep up
+its heat, the earth would almost certainly be struck by
+enough so that it would receive about half of 1 per cent
+as much heat from them as from the sun. This is millions
+of times more heat than is now received from meteors.
+If the sun owes its heat to the impact of larger bodies at
+longer intervals, the geological record should show a
+series of interruptions far more drastic than is actually
+the case.</p>
+
+<p>It has also been supposed that the sun owes its heat
+to contraction. If a gaseous body contracts it becomes
+warmer. Finally, however, it must become so dense that
+its rate of contraction diminishes and the process ceases.
+Under the sun's present condition of size and density a
+radial contraction of 120 feet per year would be enough
+to supply all the energy now radiated by that body. This
+seems like a hopeful source of energy, but Kelvin calculated
+that twenty million years ago it was ineffective
+and ten million years hence it will be equally so. Moreover,
+if this is the source of heat, the amount of radiation
+<!--File: cc038.png Page_14-->
+<a class="pagenum break" id="Page_14" title="pg 14"></a>
+from the sun would have to vary enormously.
+Twenty million years ago the sun would have extended
+nearly to the earth's orbit and would have been so tenuous
+that it would have emitted no more heat than some
+of the nebul&aelig; in space. Some millions of years later,
+when the sun's radius was twice as great as at present,
+that body would have emitted only one-fourth as much
+heat as now, which would mean that on the earth's surface
+the theoretical temperature would have been 200&deg;
+below the present level. This is utterly out of accord with
+the uniformity of climate shown by the geological record.
+In the future, if the sun's contraction is the only source
+of heat, the sun can supply the present amount for only
+ten million years, which would mean a change utterly
+unlike anything of which the geological record holds
+even the faintest hint.<sup><a href="#fn9" id="r9">[9]</a></sup></p>
+
+<p>Altogether the problem of how the sun can have remained
+so uniform and how the earth's atmosphere and
+other conditions can also have remained so uniform
+throughout hundreds of millions of years is one of the
+most puzzling in the whole realm of nature. If appeal is
+taken to radioactivity and the breaking up of uranium
+into radium and helium, conditions can be postulated
+which will give the required amount of energy. Such is
+also the case if it be supposed that there is some unknown
+process which may induce an atomic change like radioactivity
+in bodies which are now supposed to be stable
+elements. In either case, however, there is as yet no
+satisfactory explanation of the <em>uniformity</em> of the earth's
+climate. A hundred million or a thousand million years
+ago the temperature of the earth's surface was very
+much the same as now. The earth had then presumably
+ceased to emit any great amount of heat, if we may judge
+<!--File: cc039.png Page_15-->
+<a class="pagenum break" id="Page_15" title="pg 15"></a>
+from the fact that its surface was cool enough so that
+great ice sheets could accumulate on low lands within 40&deg;
+of the equator. The atmosphere was apparently almost
+like that of today, and was almost certainly not different
+enough to make up for any great divergence of the sun
+from its present condition. We cannot escape the stupendous
+fact that in those remote times the sun must
+have been essentially the same as now, or else that some
+utterly unknown factor is at work.</p>
+
+<!--File: cc040.png Page_16-->
+<a class="pagenum break" id="Page_16" title="pg 16"></a>
+
+
+
+<h2 id="Chapt_II"><a href="#content">CHAPTER II</a></h2>
+
+<h3>THE VARIABILITY OF CLIMATE</h3>
+
+
+<p>The variability of the earth's climate is almost as
+extraordinary as its uniformity. This variability
+is made up partly of a long, slow tendency in one
+direction and partly of innumerable cycles of every conceivable
+duration from days, or even hours, up to millions
+of years. Perhaps the easiest way to grasp the full complexity
+of the matter is to put the chief types of climatic
+sequence in the form of a table.</p>
+
+
+<table id="table_2" class="borders">
+<tr><th colspan="4"><a href="#tables">TABLE 2</a></th></tr>
+<tr><th colspan="4">TYPES OF CLIMATIC SEQUENCE</th> </tr>
+<tr><td class="tdl">1.</td><td class="tdl">Cosmic uniformity. </td><td class="tdl">7.</td><td class="tdl">Br&uuml;ckner periods.</td></tr>
+<tr><td class="tdl">2.</td><td class="tdl">Secular progression.</td><td class="tdl">8.</td><td class="tdl">Sunspot cycles.</td></tr>
+<tr><td class="tdl">3.</td><td class="tdl">Geologic oscillations.</td><td class="tdl">9.</td><td class="tdl">Seasonal alternations.</td></tr>
+<tr><td class="tdl">4.</td><td class="tdl">Glacial fluctuations.</td><td class="tdl">10.</td><td class="tdl">Pleionian migrations. </td></tr>
+<tr><td class="tdl">5.</td><td class="tdl">Orbital precessions.</td><td class="tdl">11.</td><td class="tdl">Cyclonic vacillations.</td></tr>
+<tr><td class="tdl">6.</td><td class="tdl">Historical pulsations.</td><td class="tdl">12.</td><td class="tdl">Daily vibrations.</td></tr>
+</table>
+
+<p>In assigning names to the various types an attempt
+has been made to indicate something of the nature of the
+sequence so far as duration, periodicity, and general
+tendencies are concerned. Not even the rich English
+language of the twentieth century, however, furnishes
+words with enough shades of meaning to express all that
+<!--File: cc041.png Page 17-->
+<a class="pagenum break" id="Page_17" title="pg 17"></a>
+is desired. Moreover, except in degree, there is no sharp
+distinction between some of the related types, such as
+glacial fluctuations and historic pulsations. Yet, taken as
+a whole, the table brings out the great contrast between
+two absolutely diverse extremes. At the one end lies well-nigh
+eternal uniformity, or an extremely slow progress in
+one direction throughout countless ages; at the other,
+rapid and regular vibrations from day to day, or else
+irregular and seemingly unsystematic vacillations due to
+cyclonic storms, both of which types are repeated millions
+of times during even a single glacial fluctuation.</p>
+
+<p>The meaning of cosmic uniformity has been explained
+in the preceding chapter. Its relation to the other types
+of climatic sequences seems to be that it sets sharply
+defined limits beyond which no changes of any kind have
+ever gone since life, as we know it, first began. Secular
+progression, on the other hand, means that in spite of all
+manner of variations, now this way and then the other,
+the normal climate of the earth, if there is such a thing,
+has on the whole probably changed a little, perhaps becoming
+more complex. After each period of continental
+uplift and glaciation&mdash;for such are pre&euml;minently the
+times of complexity&mdash;it is doubtful whether the earth has
+ever returned to quite its former degree of monotony.
+Today the earth has swung away from the great diversity
+of the glacial period. Yet we still have contrasts of what
+seem to us great magnitude. In low depressions, such as
+Turfan in the central deserts of Eurasia, the thermometer
+sometimes ranges from 0&deg;F. in the morning to 60&deg;
+in the shade at noon. On a cloudy day in the Amazon
+forest close to the seashore, on the contrary, the temperature
+for months may rise to 85&deg; by day and sink no lower
+than 75&deg; at night.</p>
+
+<p>The reasons for the secular progression of the earth's
+<!--File: cc042.png Page 18-->
+<a class="pagenum break" id="Page_18" title="pg 18"></a>
+climate appear to be intimately connected with those
+which have caused the next, and, in many respects, more
+important type of climatic sequence, which consists of
+geological oscillations. Both the progression and the
+oscillations seem to depend largely on three purely terrestrial
+factors: first, the condition of the earth's interior,
+including both internal heat and contraction;
+second, the salinity and movement of the ocean; and
+third, the composition and amount of the atmosphere.
+To begin with the earth's interior&mdash;its loss of heat appears
+to be an almost negligible factor in explaining
+either secular progression or geologic oscillation. According
+to both the nebular and the planetesimal hypotheses,
+the earth's crust appears to be colder now than it was
+hundreds or thousands of millions of years ago. The
+emission of internal heat, however, had probably ceased
+to be of much climatic significance near the beginning of
+the geological record, for in southern Canada glaciation
+occurred very early in the Proterozoic era. On the other
+hand, the contraction of the earth has produced remarkable
+effects throughout the whole of geological time. It
+has lessened the earth's circumference by a thousand
+miles or more, as appears from the way in which the
+rocks have been folded and thrust bodily over one
+another. According to the laws of dynamics this must
+have increased the speed of the earth's rotation, thus
+shortening the day, and also having the more important
+effect of increasing the bulge at the equator. On the other
+hand, recent investigations indicate that tidal retardation
+has probably diminished the earth's rate of rotation
+more than seemed probable a few years ago, thus lengthening
+the day and diminishing the bulge at the equator.
+Thus two opposing forces have been at work, one causing
+acceleration and one retardation. Their combined
+<!--File: cc043.png Page 19-->
+<a class="pagenum break" id="Page_19" title="pg 19"></a>
+effect may have been a factor in causing secular progression
+of climate. It almost certainly was of much importance
+in causing pronounced oscillations first one way
+and then the other. This matter, together with most of
+those touched in these first chapters, will be expanded in
+later parts of the book. On the whole the tendency appears
+to have been to create climatic diversity in place
+of uniformity.</p>
+
+<p>The increasing salinity of the oceans may have been
+another factor in producing secular progression, although
+of slight importance in respect to oscillations.
+While the oceans were still growing in volume, it is generally
+assumed that they must have been almost fresh
+for a vast period, although Chamberlin thinks that the
+change in salinity has been much less than is usually
+supposed. So far as the early oceans were fresher than
+those of today, their deep-sea circulation must have been
+less hampered than now by the heavy saline water which
+is produced by evaporation in warm regions. Although
+this saline water is warm, its weight causes it to descend,
+instead of moving poleward in a surface current; this
+descent slows up the rise of the cold water which has
+moved along in the depths of the ocean from high latitudes,
+and thus checks the general oceanic circulation.
+If the ancient oceans were fresher and hence had a freer
+circulation than now, a more rapid interchange of polar
+and equatorial water presumably tended to equalize the
+climate of all latitudes.</p>
+
+<p>Again, although the earth's atmosphere has probably
+changed far less during geological times than was
+formerly supposed, its composition has doubtless varied.
+The total volume of nitrogen has probably increased, for
+that gas is so inert that when it once becomes a part of
+the air it is almost sure to stay there. On the other hand,
+<!--File: cc044.png Page 20-->
+<a class="pagenum break" id="Page_20" title="pg 20"></a>
+the proportions of oxygen, carbon dioxide, and water
+vapor must have fluctuated. Oxygen is taken out constantly
+by animals and by all the processes of rock
+weathering, but on the other hand the supply is increased
+when plants break up new carbon dioxide derived from
+volcanoes. As for the carbon dioxide, it appears probable
+that in spite of the increased supply furnished by
+volcanoes the great amounts of carbon which have gradually
+been locked up in coal and limestone have appreciably
+depleted the atmosphere. Water vapor also may
+be less abundant now than in the past, for the presence
+of carbon dioxide raises the temperature a little and
+thereby enables the air to hold more moisture. When the
+area of the oceans has diminished, and this has recurred
+very often, this likewise would tend to reduce the water
+vapor. Moreover, even a very slight diminution in the
+amount of heat given off by the earth, or a decrease in
+evaporation because of higher salinity in the oceans,
+would tend in the same direction. Now carbon dioxide and
+water vapor both have a strong blanketing effect whereby
+heat is prevented from leaving the earth. Therefore, the
+probable reduction in the carbon dioxide and water
+vapor of the earth's atmosphere has apparently tended
+to reduce the climatic monotony and create diversity and
+complexity. Hence, in spite of many reversals, the general
+tendency of changes, not only in the earth's interior
+and in the oceans, but also in the atmosphere, appears to
+be a secular progression from a relatively monotonous
+climate in which the evolution of higher organic forms
+would scarcely be rapid to an extremely diverse and
+complex climate highly favorable to progressive evolution.
+The importance of these purely terrestrial agencies
+must not be lost sight of when we come to discuss other
+agencies outside the earth.</p>
+
+<!--File: cc045.png Page 21-->
+<a class="pagenum break" id="Page_21" title="pg 21"></a>
+
+<p>In Table 2 the next type of climatic sequence is geologic
+oscillation. This means slow swings that last
+millions of years. At one extreme of such an oscillation
+the climate all over the world is relatively monotonous;
+it returns, as it were, toward the primeval conditions at
+the beginning of the secular progression. At such times
+magnolias, sequoias, figs, tree ferns, and many other
+types of subtropical plants grew far north in places like
+Greenland, as is well known from their fossil remains of
+middle Cenozoic time, for example. At these same times,
+and also at many others before such high types of plants
+had evolved, reef-making corals throve in great abundance
+in seas which covered what is now Wisconsin,
+Michigan, Ontario, and other equally cool regions. Today
+these regions have an average temperature of only about
+70&deg;F. in the warmest month, and average well below
+freezing in winter. No reef-making corals can now live
+where the temperature averages below 68&deg;F. The resemblance
+of the ancient corals to those of today makes
+it highly probable that they were equally sensitive to low
+temperature. Thus, in the mild portions of a geologic
+oscillation the climate seems to have been so equable and
+uniform that many plants and animals could live 1500
+and at other times even 4000 miles farther from the
+equator than now.</p>
+
+<p>At such times the lands in middle and high latitudes
+were low and small, and the oceans extended widely over
+the continental platforms. Thus unhampered ocean currents
+had an opportunity to carry the heat of low latitudes
+far toward the poles. Under such conditions, especially
+if the conception of the great subequatorial
+continent of Gondwana land is correct, the trade winds
+and the westerlies must have been stronger and steadier
+than now. This would not only enable the westerlies,
+<!--File: cc046.png Page 22-->
+<a class="pagenum break" id="Page_22" title="pg 22"></a>
+which are really southwesterlies, to carry more heat than
+now to high latitudes, but would still further strengthen
+the ocean currents. At the same time, the air presumably
+contained an abundance of water vapor derived from
+the broad oceans, and an abundance of atmospheric
+carbon dioxide inherited from a preceding time when
+volcanoes contributed much carbon dioxide to the air.
+These two constituents of the atmosphere may have
+exercised a pronounced blanketing effect whereby the
+heat of the earth with its long wave lengths was kept in,
+although the energy of the sun with its shorter wave
+lengths was not markedly kept out. Thus everything may
+have combined to produce mild conditions in high latitudes,
+and to diminish the contrast between equator and
+pole, and between summer and winter.</p>
+
+<p>Such conditions perhaps carry in themselves the seeds
+of decay. At any rate while the lands lie quiet during a
+period of mild climate great strains must accumulate in
+the crust because of the earth's contraction and tidal
+retardation. At the same time the great abundance of
+plants upon the lowlying plains with their mild climates,
+and the marine creatures upon the broad continental
+platforms, deplete the atmospheric carbon dioxide. Part
+of this is locked up as coal and part as limestone derived
+from marine plants as well as animals. Then something
+happens so that the strains and stresses of the crust are
+released. The sea floors sink; the continents become
+relatively high and large; mountain ranges are formed;
+and the former plains and emergent portions of the
+continental platforms are eroded into hills and valleys.
+The large size of the continents tends to create deserts
+and other types of climatic diversity; the presence of
+mountain ranges checks the free flow of winds and also
+creates diversity; the ocean currents are likewise
+<!--File: cc047.png Page 23-->
+<a class="pagenum break" id="Page_23" title="pg 23"></a>
+checked, altered, and diverted so that the flow of heat
+from low to high latitudes is diminished. At the same
+time evaporation from the ocean diminishes so that a
+decrease in water vapor combines with the previous depletion
+of carbon dioxide to reduce the blanketing effect
+of the atmosphere. Thus upon periods of mild monotony
+there supervene periods of complexity, diversity, and
+severity. Turn to Table 1 and see how a glacial climate
+again and again succeeds a time when relative mildness
+prevailed almost everywhere. Or examine Fig. 1 and
+notice how the lines representing temperatures go up and
+down. In the figure Schuchert makes it clear that when
+the lands have been large and mountain-making has been
+important, as shown by the high parts of the lower shaded
+area, the climate has been severe, as shown by the descent
+of the snow line, the upper shaded area. In the diagram
+the climatic oscillations appear short, but this is merely
+because they have been crowded together, especially in
+the left hand or early part. There an inch in length may
+represent a hundred million years. Even at the right-hand
+end an inch is equivalent to several million years.</p>
+
+<p>The severe part of a climatic oscillation, as well as the
+mild part, will be shown in later chapters to bear in itself
+certain probable seeds of decay. While the lands are
+being uplifted, volcanic activity is likely to be vigorous
+and to add carbon dioxide to the air. Later, as the mountains
+are worn down by the many agencies of water,
+wind, ice, and chemical decay, although much carbon
+dioxide is locked up by the carbonation of the rocks, the
+carbon locked up in the coal is set free and increases the
+carbon dioxide of the air. At the same time the continents
+settle slowly downward, for the earth's crust though
+rigid as steel is nevertheless slightly viscous and will
+flow if subjected to sufficiently great and enduring pressure.
+<!--File: cc048.png Pae 24-->
+<a class="pagenum break" id="Page_24" title="pg 24"></a>
+The area from which evaporation can take place
+is thereby increased because of the spread of the oceans
+over the continents, and water vapor joins with the carbon
+dioxide to blanket the earth and thus tends to keep it
+uniformly warm. Moreover, the diminution of the lands
+frees the ocean currents from restraint and permits them
+to flow more freely from low latitudes to high. Thus in
+the course of millions of years there is a return toward
+monotony. Ultimately, however, new stresses accumulate
+in the earth's crust, and the way is prepared for another
+great oscillation. Perhaps the setting free of the stresses
+takes place simply because the strain at last becomes
+irresistible. It is also possible, as we shall see, that an
+external agency sometimes adds to the strain and thereby
+determines the time at which a new oscillation shall
+begin.</p>
+
+<p>In Table 2 the types of climatic sequences which follow
+"geologic oscillations" are "glacial fluctuations,"
+"orbital precessions" and "historical pulsations."
+Glacial fluctuations and historical pulsations appear to
+be of the same type, except as to severity and duration,
+and hence may be considered together. They will be
+treated briefly here because the theories as to their
+causes are outlined in the next two chapters. Oddly
+enough, although the historic pulsations lie much closer
+to us than do the glacial fluctuations, they were not
+discovered until two or three generations later, and are
+still much less known. The most important feature of
+both sequences is the swing from a glacial to an inter-glacial
+epoch or from the arsis or accentuated part of an
+historical pulsation to the thesis or unaccented part. In a
+glacial epoch or in the arsis of an historic pulsation,
+storms are usually abundant and severe, the mean temperature
+is lower than usual, snow accumulates in high
+<!--File: cc049.png Page 25-->
+<a class="pagenum break" id="Page_25" title="pg 25"></a>
+
+<!--File: cc050.png Page 26-->
+<a class="pagenum break" id="Page_26" title="pg 26"></a>
+
+latitudes or upon lofty mountains. For example, in the
+last such period during the fourteenth century, great
+floods and droughts occurred alternately around the
+North Sea; it was several times possible to cross the
+Baltic Sea from Germany to Sweden on the ice, and the
+ice of Greenland advanced so much that shore ice caused
+the Norsemen to change their sailing route between Iceland
+and the Norse colonies in southern Greenland. At
+the same time in low latitudes and in parts of the continental
+interior there is a tendency toward diminished
+rainfall and even toward aridity and the formation of
+deserts. In Yucatan, for example, a diminution in tropical
+rainfall in the fourteenth century seems to have given
+the Mayas a last opportunity for a revival of their decaying
+civilization.</p>
+<img src="images/fig1.png" width="800" height="284" alt="Fig. 1" id="Fig_1" />
+<p class="center"><em>Fig. 1. Climatic changes and mountain building.<br />
+(After Schuchert, in The Evolution of the Earth and Its Inhabitants, edited by R. S. Lull.)</em></p>
+
+<p class="blockquot">Diagram showing the times and probable extent of the more or less marked climate changes in
+the geologic history of North America, and of its elevation into chains of mountains.</p>
+
+<p>Among the climatic sequences, glacial fluctuations are
+perhaps of the most vital import from the standpoint of
+organic evolution; from the standpoint of human history
+the same is true of climatic pulsations. Glacial epochs
+have repeatedly wiped out thousands upon thousands of
+species and played a part in the origin of entirely new
+types of plants and animals. This is best seen when the
+life of the Pennsylvanian is contrasted with that of the
+Permian. An historic pulsation may wipe out an entire
+civilization and permit a new one to grow up with a radically
+different character. Hence it is not strange that the
+causes of such climatic phenomena have been discussed
+with extraordinary vigor. In few realms of science has
+there been a more imposing or more interesting array of
+theories. In this book we shall consider the more important
+of these theories. A new solar or cyclonic hypothesis
+and the hypothesis of changes in the form and altitude of
+the land will receive the most attention, but the other
+
+<!--File: cc051.png Page 27-->
+<a class="pagenum break" id="Page_27" title="pg 27"></a>
+
+chief hypotheses are outlined in the next chapter, and are
+frequently referred to throughout the volume.</p>
+
+<p>Between glacial fluctuations and historical pulsations
+in duration, but probably less severe than either, come
+orbital precessions. These stand in a group by themselves
+and are more akin to seasonal alternations than
+to any other type of climatic sequence. They must have
+occurred with absolute regularity ever since the earth
+began to revolve around the sun in its present elliptical
+orbit. Since the orbit is elliptical and since the sun is in
+one of the two foci of the ellipse, the earth's distance
+from the sun varies. At present the earth is nearest the
+sun in the northern winter. Hence the rigor of winter in
+the northern hemisphere is mitigated, while that of the
+southern hemisphere is increased. In about ten thousand
+years this condition will be reversed, and in another ten
+thousand the present conditions will return once more.
+Such climatic precessions, as we may here call them,
+must have occurred unnumbered times in the past, but
+they do not appear to have been large enough to leave in
+the fossils of the rocks any traces that can be distinguished
+from those of other climatic sequences.</p>
+
+<p>We come now to Br&uuml;ckner periods and sunspot cycles.
+The Br&uuml;ckner periods have a length of about thirty-three
+years. Their existence was suggested at least as long ago
+as the days of Sir Francis Bacon, whose statement about
+them is quoted on the flyleaf of this book. They have
+since been detected by a careful study of the records of
+the time of harvest, vintage, the opening of rivers to
+navigation, and the rise or fall of lakes like the Caspian
+Sea. In his book on <em>Klimaschwankungen seit 1700</em>,
+Br&uuml;ckner has collected an uncommonly interesting assortment
+of facts as to the climate of Europe for more than
+two centuries. More recently, by a study of the rate of
+
+<!--File: cc052.png Page 28-->
+<a class="pagenum break" id="Page_28" title="pg 28"></a>
+
+growth of trees, Douglass, in his book on <em>Climatic Cycles
+and Tree Growth</em>, has carried the subject still further.
+In general the nature of the 33-year periods seems to be
+identical with that of the 11- or 12- year sunspot cycle,
+on the one hand, and of historic pulsations on the other.
+For a century observers have noted that the variations
+in the weather which everyone notices from year to year
+seem to have some relation to sunspots. For generations,
+however, the relationship was discussed without leading
+to any definite conclusion. The trouble was that the same
+change was supposed to take place in all parts of the
+world. Hence, when every sort of change was found
+somewhere at any given sunspot stage, it seemed as
+though there could not be a relationship. Of late years,
+however, the matter has become fairly clear. The chief
+conclusions are, first, that when sunspots are numerous
+the average temperature of the earth's surface is lower
+than normal. This does not mean that all parts are cooler,
+for while certain large areas grow cool, others of less
+extent become warm at times of many sunspots. Second,
+at times of many sunspots storms are more abundant
+than usual, but are also confined somewhat closely to
+certain limited tracks so that elsewhere a diminution of
+storminess may be noted. This whole question is discussed
+so fully in <em>Earth and Sun</em> that it need not detain
+us further in this preliminary view of the whole problem
+of climate. Suffice it to say that a study of the sunspot
+cycle leads to the conclusion that it furnishes a clue to
+many of the unsolved problems of the climate of the
+past, as well as a key to prediction of the future.</p>
+
+<p>Passing by the seasonal alternations which are fully
+explained as the result of the revolution of the earth
+around the sun, we may merely point out that, like the
+daily vibrations which bring Table 2 to a close, they
+<!--File: cc053.png Page 29-->
+<a class="pagenum break" id="Page_29" title="pg 29"></a>
+emphasize the outstanding fact that the main control of
+terrestrial climate is the amount of energy received from
+the sun. This same principle is illustrated by pleionian
+migrations. The term "pleion" comes from a Greek word
+meaning "more." It was taken by Arctowski to designate
+areas or periods where there is an excess of some
+climatic element, such as atmospheric pressure, rainfall,
+or temperature. Even if the effect of the seasons is eliminated,
+it appears that the course of these various elements
+does not run smoothly. As everyone knows, a period
+like the autumn of 1920 in the eastern United States may
+be unusually warm, while a succeeding period may be
+unseasonably cool. These departures from the normal
+show a certain rough periodicity. For example, there is
+evidence of a period of about twenty-seven days, corresponding
+to the sun's rotation and formerly supposed to
+be due to the moon's revolution which occupies almost
+the same length of time. Still other periods appear to
+have an average duration of about three months and of
+between two and three years. Two remarkable discoveries
+have recently been made in respect to such pleions. One
+is that a given type of change usually occurs simultaneously
+in a number of well-defined but widely separated
+centers, while a change of an opposite character arises
+in another equally well-defined, but quite different, set
+of centers. In general, areas of high pressure have one
+type of change and areas of low pressure the other type.
+So systematic are these relationships and so completely
+do they harmonize in widely separated parts of the earth,
+that it seems certain that they must be due to some outside
+cause, which in all probability can be only the sun.
+The second discovery is that pleions, when once formed,
+travel irregularly along the earth's surface. Their paths
+have not yet been worked out in detail, but a general
+<!--File: cc054.png Page 30-->
+<a class="pagenum break" id="Page_30" title="pg 30"></a>
+migration seems well established. Because of this, it is
+probable that if unusually warm weather prevails in one
+part of a continent at a given time, the "thermo-pleion,"
+or excess of heat, will not vanish but will gradually move
+away in some particular direction. If we knew the path
+that it would follow we might predict the general temperature
+along its course for some months in advance.
+The paths are often irregular, and the pleions frequently
+show a tendency to break up or suddenly revive. Probably
+this tendency is due to variations in the sun. When
+the sun is highly variable, the pleions are numerous and
+strong, and extremes of weather are frequent. Taken as
+a whole the pleions offer one of the most interesting and
+hopeful fields not only for the student of the causes of
+climatic variations, but for the man who is interested in
+the practical question of long-range weather forecasts.
+Like many other climatic phenomena they seem to represent
+the combined effect of conditions in the sun and
+upon the earth itself.</p>
+
+<p>The last of the climatic sequences which require explanation
+is the cyclonic vacillations. These are familiar
+to everyone, for they are the changes of weather which
+occur at intervals of a few days, or a week or two, at all
+seasons, in large parts of the United States, Europe,
+Japan, and some of the other progressive parts of the
+earth. They do not, however, occur with great frequency
+in equatorial regions, deserts, and many other regions.
+Up to the end of the last century, it was generally supposed
+that cyclonic storms were purely terrestrial in
+origin. Without any adequate investigation it was assumed
+that all irregularities in the planetary circulation
+of the winds arise from an irregular distribution of heat
+due to conditions within or upon the earth itself. These
+irregularities were supposed to produce cyclonic storms
+<!--File: cc055.png Page 31-->
+<a class="pagenum break" id="Page_31" title="pg 31"></a>
+in certain limited belts, but not in most parts of the
+world. Today this view is being rapidly modified. Undoubtedly,
+the irregularities due to purely terrestrial
+conditions are one of the chief contributory causes of
+storms, but it begins to appear that solar variations also
+play a part. It has been found, for example, that not
+only the mean temperature of the earth's surface varies
+in harmony with the sunspot cycle, but that the frequency
+and severity of storms vary in the same way. Moreover,
+it has been demonstrated that the sun's radiation is not
+constant, but is subject to innumerable variations. This
+does not mean that the sun's general temperature varies,
+but merely that at some times heated gases are ejected
+rapidly to high levels so that a sudden wave of energy
+strikes the earth. Thus, the present tendency is to believe
+that the cyclonic variations, the changes of weather
+which come and go in such a haphazard, irresponsible
+way, are partly due to causes pertaining to the earth
+itself and partly to the sun.</p>
+
+<p>From this rapid survey of the types of climatic sequences,
+it is evident that they may be divided into four
+great groups. First comes cosmic uniformity, one of the
+most marvelous and incomprehensible of all known facts.
+We simply have no explanation which is in any respect
+adequate. Next come secular progression and geologic
+oscillations, two types of change which seem to be due
+mainly to purely terrestrial causes, that is, to changes in
+the lands, the oceans, and the air. The general tendency
+of these changes is toward complexity and diversity, thus
+producing progression, but they are subject to frequent
+reversals which give rise to oscillations lasting millions
+of years. The processes by which the oscillations take
+place are fully discussed in this book. Nevertheless, because
+they are fairly well understood, they are deferred
+<!--File: cc056.png Page 32-->
+<a class="pagenum break" id="Page_32" title="pg 32"></a>
+until after the third group of sequences has been discussed.
+This group includes glacial fluctuations, historic
+pulsations, Br&uuml;ckner periods, sunspot cycles, pleionian
+migrations, and cyclonic vacillations. The outstanding
+fact in regard to all of these is that while they are greatly
+modified by purely terrestrial conditions, they seem to
+owe their origin to variations in the sun. They form the
+chief subject of <em>Earth and Sun</em> and in their larger phases
+are the most important topic of this book also. The last
+group of sequences includes orbital precessions, seasonal
+alternations, and daily variations. These may be regarded
+as purely solar in origin. Yet their influence, like
+that of each of the other groups, is much modified by the
+earth's own conditions. Our main problem is to separate
+and explain the two great elements in climatic changes,&mdash;the
+effects of the sun, on the one hand, and of the earth
+on the other.</p>
+
+<!--File: cc057.png Page 33-->
+<a class="pagenum break" id="Page_33" title="pg 33"></a>
+
+
+
+
+
+<h2 id="Chapt_III"><a href="#content">CHAPTER III</a></h2>
+
+<h3>HYPOTHESES OF CLIMATIC CHANGE</h3>
+
+
+<p>The next step in our study of climate is to review
+the main hypotheses as to the causes of glaciation.
+These hypotheses apply also to other types
+of climatic changes. We shall concentrate on glacial
+periods, however, not only because they are the most
+dramatic and well-known types of change, but because
+they have been more discussed than any other and have
+also had great influence on evolution. Moreover, they
+stand near the middle of the types of climatic sequences,
+and an understanding of them does much to explain the
+others. In reviewing the various theories we shall not
+attempt to cover all the ground, but shall merely state
+the main ideas of the few theories which have had an
+important influence upon scientific thought.</p>
+
+<p>The conditions which any satisfactory climatic hypothesis
+must satisfy are briefly as follows:</p>
+
+
+<ol>
+<li>Due weight must be given to the fact that changes
+of climate are almost certainly due to the combined effect
+of a variety of causes, both terrestrial and solar or
+cosmic.</li>
+
+<li>Attention must also be paid to both sides in the
+long controversy as to whether glaciation is due primarily
+to a diminution in the earth's supply of heat or to
+a <em>redistribution</em> of the heat through changes in atmospheric
+and oceanic circulation. At present the great
+<!--File: cc058.png Page 34-->
+<a class="pagenum break" id="Page_34" title="pg 34"></a>
+majority of authorities are on the side of a diminution of
+heat, but the other view also deserves study.</li>
+
+<li>A satisfactory hypothesis must explain the frequent
+synchronism between two great types of phenomena;
+first, movements of the earth's crust whereby
+continents are uplifted and mountains upheaved; and,
+second, great changes of climate which are usually
+marked by relatively rapid oscillations from one extreme
+to another.</li>
+
+<li>No hypothesis can find acceptance unless it satisfies
+the somewhat exacting requirements of the geological
+record, with its frequent but irregular repetition of long,
+mild periods, relatively cool or intermediate periods like
+the present, and glacial periods of more or less severity
+and perhaps accompanying the more or less widespread
+uplifting of continents. At least during the later glacial
+periods the hypothesis must explain numerous climatic
+epochs and stages superposed upon a single general
+period of continental upheaval. Moreover, although historical
+geology demands cycles of varied duration and
+magnitude, it does not furnish evidence of any rigid
+periodicity causing the cycles to be uniform in length or
+intensity.</li>
+
+<li>Most important of all, a satisfactory explanation
+of climatic changes and crustal deformation must take
+account of all the agencies which are now causing similar
+phenomena. Whether any other agencies should be considered
+is open to question, although the relative importance
+of existing agencies may have varied.
+</li>
+</ol>
+
+
+<p>I. <em>Croll's Eccentricity Theory.</em> One of the most ingenious
+and most carefully elaborated scientific hypotheses
+is Croll's<sup><a href="#fn10" id="r10">[10]</a></sup> precessional hypothesis as to the
+effect of the earth's own motions. So well was this worked
+out that it was widely accepted for a time and still finds a
+<!--File: cc059.png Page 35-->
+<a class="pagenum break" id="Page_35" title="pg 35"></a>
+place in popular but unscientific books, such as Wells'
+<em>Outline of History</em>, and even in scientific works like
+Wright's <em>Quaternary Ice Age</em>. The gist of the hypothesis
+has already been given in connection with the type of
+climatic sequence known as orbital precessions. The earth
+is 93 million miles away from the sun in January and 97
+million in July. The earth's axis "precesses," however,
+just as does that of a spinning top. Hence arises what is
+known as the precession of the equinoxes, that is, a
+steady change in the season at which the earth is in perihelion,
+or nearest to the sun. In the course of 21,000 years
+the time of perihelion varies from early in January
+through the entire twelve months and back to January.
+Moreover, the earth's orbit is slightly more elliptical at
+certain periods than at others, for the planets sometimes
+become bunched so that they all pull the earth in one
+direction. Hence, once in about one hundred thousand
+years the effect of the elliptical shape of the earth's orbit
+is at a maximum.</p>
+
+<p>Croll argued that these astronomical changes must
+alter the earth's climate, especially by their effect on
+winds and ocean currents. His elaborate argument contains
+a vast amount of valuable material. Later investigation,
+however, seems to have proven the inadequacy of
+his hypothesis. In the first place, the supposed cause does
+not seem nearly sufficient to produce the observed results.
+Second, Croll's hypothesis demands that glaciation in the
+northern and southern hemisphere take place alternately.
+A constantly growing collection of facts, however, indicates
+that glaciation does not occur in the two hemispheres
+alternately, but at the same time. Third, the
+hypothesis calls for the constant and frequent repetition
+of glaciation at absolutely regular intervals. The geological
+<!--File: cc060.png Page 36-->
+<a class="pagenum break" id="Page_36" title="pg 36"></a>
+record shows no such regularity, for sometimes
+several glacial epochs follow in relatively close succession
+at irregular intervals of perhaps fifty to two hundred
+thousand years, and thus form a glacial period; and
+then for millions of years there are none. Fourth, the
+eccentricity hypothesis provides no adequate explanation
+for the glacial stages or subepochs, the historic pulsations,
+and the other smaller climatic variations which are
+superposed upon glacial epochs and upon one another in
+bewildering confusion. In spite of these objections, there
+can be little question that the eccentricity of the earth's
+orbit and the precession of the equinoxes with the resulting
+change in the season of perihelion must have some
+climatic effect. Hence Croll's theory deserves a permanent
+though minor place in any full discussion of the
+causes of climatic changes.</p>
+
+<p>II. <em>The Carbon Dioxide Theory.</em> At about the time
+that the eccentricity theory was being relegated to a
+minor niche, a new theory was being developed which
+soon exerted a profound influence upon geological
+thought. Chamberlin,<sup><a href="#fn11" id="r11">[11]</a></sup> adopting an idea suggested by
+<!--File: cc061.png Page 37-->
+<a class="pagenum break" id="Page_37" title="pg 37"></a>
+Tyndall, fired the imagination of geologists by his skillful
+exposition of the part played by carbon dioxide in
+causing climatic changes. Today this theory is probably
+more widely accepted than any other. We have already
+seen that the amount of carbon dioxide gas in the atmosphere
+has a decided climatic importance. Moreover,
+there can be little doubt that the amount of that gas in
+the atmosphere varies from age to age in response to the
+extent to which it is set free by volcanoes, consumed by
+plants, combined with rocks in the process of weathering,
+dissolved in the ocean or locked up in the form of coal
+and limestone. The main question is whether such variations
+can produce changes so rapid as glacial epochs and
+historical pulsations.</p>
+
+<p>Abundant evidence seems to show that the degree to
+which the air can be warmed by carbon dioxide is sharply
+limited. Humphreys, in his excellent book on the <em>Physics
+of the Air</em>, calculates that a layer of carbon dioxide forty
+centimeters thick has practically as much blanketing
+effect as a layer indefinitely thicker. In other words, forty
+centimeters of carbon dioxide, while having no appreciable
+<!--File: cc062.png Page 38-->
+<a class="pagenum break" id="Page_38" title="pg 38"></a>
+effect on sunlight coming toward the earth, would
+filter out and thus retain in the atmosphere all the outgoing
+terrestrial heat that carbon dioxide is capable of
+absorbing. Adding more would be like adding another
+filter when the one in operation has already done all that
+that particular kind of filter is capable of doing. According
+to Humphreys' calculations, a doubling of the carbon
+dioxide in the air would in itself raise the average temperature
+about 1.3&deg;C. and further carbon dioxide would
+have practically no effect. Reducing the present supply
+by half would reduce the temperature by essentially the
+same amount.</p>
+
+<p>The effect must be greater, however, than would appear
+from the figures given above, for any change in
+temperature has an effect on the amount of water vapor,
+which in turn causes further changes of temperature.
+Moreover, as Chamberlin points out, it is not clear
+whether Humphreys allows for the fact that when the
+40 centimeters of CO<sub>2</sub> nearest the earth has been heated
+by terrestrial radiation, it in turn radiates half its heat
+outward and half inward. The outward half is all absorbed
+in the next layer of carbon dioxide, and so on.
+The process is much more complex than this, but the end
+result is that even the last increment of CO<sub>2</sub>, that is, the
+outermost portions in the upper atmosphere, must apparently
+absorb an infinitesimally small amount of heat.
+This fact, plus the effect of water vapor, would seem to
+indicate that a doubling or halving of the amount of CO<sub>2</sub>,
+would have an effect of more than 1.3&deg;C. A change of
+even 2&deg;C. above or below the present level of the earth's
+mean temperature would be of very appreciable climatic
+significance, for it is commonly believed that during the
+height of the glacial period the mean temperature was
+only 5&deg; to 8&deg;C. lower than now.</p>
+<!--File: cc063.png Page 39-->
+<a class="pagenum break" id="Page_39" title="pg 37"></a>
+
+<p>Nevertheless, variations in atmospheric carbon dioxide
+do not necessarily seem competent to produce the relatively
+rapid climatic fluctuations of glacial epochs and
+historic pulsations as distinguished from the longer
+swings of glacial periods and geological eras. In Chamberlin's
+view, as in ours, the elevation of the land, the
+modification of the currents of the air and of the ocean,
+and all that goes with elevation as a topographic agency
+constitute a primary cause of climatic changes. A special
+effect of this is the removal of carbon dioxide from the
+air by the enhanced processes of weathering. This, as he
+carefully states, is a very slow process, and cannot of
+itself lead to anything so sudden as the oncoming of
+glaciation. But here comes Chamberlin's most distinctive
+contribution to the subject, namely, the hypothesis that
+changes in atmospheric temperature arising from variations
+in atmospheric carbon dioxide are able to cause a
+reversal of the deep-sea oceanic circulation.</p>
+
+<p>According to Chamberlin's view, the ordinary oceanic
+circulation of the greater part of geological time was
+the reverse of the present circulation. Warm water descended
+to the ocean depths in low latitudes, kept its heat
+while creeping slowly poleward, and rose in high latitudes
+producing the warm climate which enabled corals,
+for example, to grow in high latitudes. Chamberlin holds
+this opinion largely because there seems to him to be no
+other reasonable way to account for the enormously long
+warm periods when heat-loving forms of life lived in
+what are now polar regions of ice and snow. He explains
+this reversed circulation by supposing that an abundance
+of atmospheric carbon dioxide, together with a broad
+distribution of the oceans, made the atmosphere so warm
+that the evaporation in low latitudes was far more rapid
+than now. Hence the surface water of the ocean became
+<!--File: cc064.png Page 40-->
+<a class="pagenum break" id="Page_40" title="pg 40"></a>
+a relatively concentrated brine. Such a brine is heavy
+and tends to sink, thereby setting up an oceanic circulation
+the reverse of that which now prevails. At present
+the polar waters sink because they are cold and hence
+contract. Moreover, when they freeze a certain amount
+of salt leaves the ice and thereby increases the salinity
+of the surrounding water. Thus the polar water sinks
+to the depths of the ocean, its place is taken by warmer
+and lighter water from low latitudes which moves poleward
+along the surface, and at the same time the cold
+water of the ocean depths is forced equatorward below
+the surface. But if the equatorial waters were so concentrated
+that a steady supply of highly saline water kept
+descending to low levels, the direction of the circulation
+would have to be reversed. The time when this would
+occur would depend upon the delicate balance between
+the downward tendencies of the cold polar water and of
+the warm saline equatorial water.</p>
+
+<p>Suppose that while such a reversed circulation prevailed,
+the atmospheric CO<sub>2</sub> should be depleted, and the
+air cooled so much that the concentration of the equatorial
+waters by evaporation was no longer sufficient to
+cause them to sink. A reversal would take place, the
+present type of circulation would be inaugurated, and
+the whole earth would suffer a chill because the surface
+of the ocean would become cool. The cool surface-water
+would absorb carbon dioxide faster than the previous
+warm water had done, for heat drives off gases
+from water. This would hasten the cooling of the atmosphere
+still more, not only directly but by diminishing
+the supply of atmospheric moisture. The result would be
+glaciation. But ultimately the cold waters of the higher
+latitudes would absorb all the carbon dioxide they could
+hold, the slow equatorward creep would at length permit
+<!--File: cc065.png Page 41-->
+<a class="pagenum break" id="Page_41" title="pg 41"></a>
+the cold water to rise to the surface in low latitudes.
+There the warmth of the equatorial sun and the depleted
+supply of carbon dioxide in the air would combine to
+cause the water to give up its carbon dioxide once more.
+If the atmosphere had been sufficiently depleted by that
+time, the rising waters in low latitudes might give up
+more carbon dioxide than the cold polar waters absorbed.
+Thus the atmospheric supply would increase, the air
+would again grow warm, and a tendency toward deglaciation,
+or toward an inter-glacial condition would
+arise. At such times the oceanic circulation is not supposed
+to have been reversed, but merely to have been
+checked and made slower by the increasing warmth.
+Thus inter-glacial conditions like those of today, or even
+considerably warmer, are supposed to have been produced
+with the present type of circulation.</p>
+
+<p>The emission of carbon dioxide in low latitudes could
+not permanently exceed the absorption in high latitudes.
+After the present type of circulation was finally established,
+which might take tens of thousands of years, the
+two would gradually become equal. Then the conditions
+which originally caused the oceanic circulation to be
+reversed would again destroy the balance; the atmospheric
+carbon dioxide would be depleted; the air would
+grow cooler; and the cycle of glaciation would be repeated.
+Each cycle would be shorter than the last, for not
+only would the swings diminish like those of a pendulum,
+but the agencies that were causing the main depletion of
+the atmospheric carbon dioxide would diminish in intensity.
+Finally as the lands became lower through erosion
+and submergence, and as the processes of weathering
+became correspondingly slow, the air would gradually be
+able to accumulate carbon dioxide; the temperature would
+increase; and at length the oceanic circulation would be
+<!--File: cc066.png Page 42-->
+<a class="pagenum break" id="Page_42" title="pg 42"></a>
+reversed again. When the warm saline waters of low latitudes
+finally began to sink and to set up a flow of warm
+water poleward in the depths of the ocean, a glacial
+period would definitely come to an end.</p>
+
+<p>This hypothesis has been so skillfully elaborated, and
+contains so many important elements that one can
+scarcely study it without profound admiration. We believe
+that it is of the utmost value as a step toward the
+truth, and especially because it emphasizes the great
+function of oceanic circulation. Nevertheless, we are
+unable to accept it in full for several reasons, which
+may here be stated very briefly. Most of them will be discussed
+fully in later pages.</p>
+
+<p>(1) While a reversal of the deep-sea circulation would
+undoubtedly be of great climatic importance and would
+produce a warm climate in high latitudes, we see no
+direct evidence of such a reversal. It is equally true that
+there is no conclusive evidence against it, and the possibility
+of a reversal must not be overlooked. There seem,
+however, to be other modifications of atmospheric and
+oceanic circulation which are able to produce the observed
+results.</p>
+
+<p>(2) There is much, and we believe conclusive, evidence
+that a mere lowering of temperature would not produce
+glaciation. What seems to be needed is changes in atmospheric
+circulation and in precipitation. The carbon
+dioxide hypothesis has not been nearly so fully developed
+on the meteorological side as in other respects.</p>
+
+<p>(3) The carbon dioxide hypothesis seems to demand
+that the oceans should have been almost as saline as now
+in the Proterozoic era at the time of the first known
+glaciation. Chamberlin holds that such was the case, but
+the constant supply of saline material brought to the
+ocean by rivers and the relatively small deposition of
+<!--File: cc067.png Page 43-->
+<a class="pagenum break" id="Page_43" title="pg 43"></a>
+such material on the sea floor seem to indicate that the
+early oceans must have been much fresher than those of
+today.</p>
+
+<p>(4) The carbon dioxide hypothesis does not attempt
+to explain minor climatic fluctuations such as post-glacial
+stages and historic pulsations, but these appear to be of
+the same nature as glacial epochs, differing only in
+degree.</p>
+
+<p>(5) Another reason for hesitation in accepting the
+carbon dioxide hypothesis as a full explanation of glacial
+fluctuations is the highly complex and non-observational
+character of the explanation of the alternation of glacial
+and inter-glacial epochs and of their constantly decreasing
+length.</p>
+
+<p>(6) Most important of all, a study of the variations of
+weather and of climate as they are disclosed by present
+records and by the historic past suggests that there are
+now in action certain other causes which are competent
+to explain glaciation without recourse to a process whose
+action is beyond the realm of observation.</p>
+
+<p>These considerations lead to the conclusion that the
+carbon dioxide hypothesis and the reversal of the oceanic
+circulation should be regarded as a tentative rather than
+a final explanation of glaciation. Nevertheless, the action
+of carbon dioxide seems to be an important factor in producing
+the longer oscillations of climate from one geological
+era to another. It probably plays a considerable
+part in preparing the way for glacial periods and in
+making it possible for other factors to produce the more
+rapid changes which have so deeply influenced organic
+evolution.</p>
+
+<p>III. <em>The Form of the Land.</em> Another great cause of
+climatic change consists of a group of connected phenomena
+dependent upon movements of the earth's crust.
+<!--File: cc068.png Page 44-->
+<a class="pagenum break" id="Page_44" title="pg 44"></a>
+As to the climatic potency of changes in the lands there
+is practical agreement among students of climatology
+and glaciation. That the height and extent of the continents,
+the location, size, and orientation of mountain
+ranges, and the opening and closing of oceanic gateways
+at places like Panama, and the consequent diversion of
+oceanic currents, exert a profound effect upon climate
+can scarcely be questioned. Such changes may be introduced
+rapidly, but their disappearance is usually slow
+compared with the rapid pulsations to which climate has
+been subject during historic times and during stages of
+glacial retreat and advance, or even in comparison with
+the epochs into which the Pleistocene, Permian, and
+perhaps earlier glacial periods have been divided. Hence,
+while crustal movements appear to be more important
+than the eccentricity of the earth's orbit or the amount of
+carbon dioxide in the air, they do not satisfactorily explain
+glacial fluctuations, historic pulsations, and especially
+the present little cycles of climatic change. All
+these changes involve a relatively rapid swing from one
+extreme to another, while an upheaval of a continent,
+which is at best a slow geologic process, apparently
+cannot be undone for a long, long time. Hence such an
+upheaval, if acting alone, would lead to a relatively long-lived
+climate of a somewhat extreme type. It would help
+to explain the long swings, or geologic oscillations between
+a mild and uniform climate at one extreme, and a
+complex and varied climate at the other, but it would not
+explain the rapid climatic pulsations which are closely
+associated with great movements of the earth's crust. It
+might prepare the way for them, but could not cause
+them. That this conclusion is true is borne out by the fact
+that vast mountain ranges, like those at the close of the
+Jurassic and Cretaceous, are upheaved without bringing
+<!--File: cc069.png Page 45-->
+<a class="pagenum break" id="Page_45" title="pg 45"></a>
+on glacial climates. Moreover, the marked Permian ice
+age follows long after the birth of the Hercynian Mountains
+and before the rise of others of later Permian
+origin.</p>
+
+<p>IV. <em>The Volcanic Hypothesis.</em> In the search for some
+cause of climatic change which is highly efficient and yet
+able to vary rapidly and independently, Abbot, Fowle,
+Humphreys, and others,<sup><a href="#fn12" id="r12">[12]</a></sup> have concluded that volcanic
+eruptions are the missing agency. In <em>Physics of the Air</em>,
+Humphreys gives a careful study of the effect of volcanic
+dust upon terrestrial temperature. He begins with
+a mathematical investigation of the size of dust particles,
+and their quantity after certain eruptions. He demonstrates
+that the power of such particles to deflect light of
+short wave-lengths coming from the sun is perhaps thirty
+times more than their power to retain the heat radiated
+in long waves from the earth. Hence it is estimated that
+if a Krakatoa were to belch forth dust every year or
+two, the dust veil might cause a reduction of about 6&deg;C.
+in the earth's surface temperature. As in every such complicated
+problem, some of the author's assumptions are
+open to question, but this touches their quantitative and
+not their qualitative value. It seems certain that if volcanic
+explosions were frequent enough and violent
+enough, the temperature of the earth's surface would be
+considerably lowered.</p>
+
+<p>Actual observation supports this theoretical conclusion.
+Humphreys gathers together and amplifies all that
+he and Abbot and Fowle have previously said as to observations
+of the sun's thermal radiation by means of the
+<!--File: cc070.png Page 46-->
+<a class="pagenum break" id="Page_46" title="pg 46"></a>
+pyrheliometer. This summing up of the relations between
+the heat received from the sun, and the occurrence of
+explosive volcanic eruptions leaves little room for doubt
+that at frequent intervals during the last century and a
+half a slight lowering of terrestrial temperature has
+actually occurred after great eruptions. Nevertheless, it
+does not justify Humphreys' final conclusion that "phenomena
+within the earth itself suffice to modify its own
+climate,... that these and these alone have actually
+caused great changes time and again in the geologic
+past." Humphreys sees so clearly the importance of the
+purely terrestrial point of view that he unconsciously
+slights the cosmic standpoint and ignores the important
+solar facts which he himself adduces elsewhere at considerable
+length.</p>
+
+<p>In addition to this the <em>degree</em> to which the temperature
+of the earth as a whole is influenced by volcanic eruptions
+is by no means so clear as is the fact that there is some
+influence. Arctowski,<sup><a href="#fn13" id="r13">[13]</a></sup> for example, has prepared numerous
+curves showing the march of temperature month
+after month for many years. During the period from
+1909 to 1913, which includes the great eruption of Katmai
+in Alaska, low temperature is found to have prevailed at
+the time of the eruption, but, as Arctowski puts it, on the
+basis of the curves for 150 stations in all parts of the
+world: "The supposition that these abnormally low temperatures
+were due to the veil of volcanic dust produced
+by the Katmai eruption of June 6, 1912, is completely out
+of the question. If that had been the case, temperature
+would have decreased from that date on, whereas it was
+decreasing for more than a year before that date."</p>
+<!--File: cc071.png Page 47-->
+<a class="pagenum break" id="Page_47" title="pg 47"></a>
+
+<p>K&ouml;ppen,<sup><a href="#fn14" id="r14">[14]</a></sup> in his comprehensive study of temperature
+for a hundred years, also presents a strong argument
+against the idea that volcanic eruptions have an important
+place in determining the present temperature of
+the earth. A volcanic eruption is a sudden occurrence.
+Whatever effect is produced by dust thrown into the air
+must occur within a few months, or as soon as the dust
+has had an opportunity to be wafted to the region in
+question. When the dust arrives, there will be a rapid
+drop through the few degrees of temperature which the
+dust is supposed to be able to account for, and thereafter
+a slow rise of temperature. If volcanic eruptions actually
+caused a frequent lowering of terrestrial temperature in
+the hundred years studied by K&ouml;ppen, there should be
+more cases where the annual temperature is decidedly
+below the normal than where it shows a large departure
+in the opposite direction. The contrary is actually the
+case.</p>
+
+<p>A still more important argument is the fact that the
+earth is now in an intermediate condition of climate.
+Throughout most of geologic time, as we shall see again
+and again, the climate of the earth has been milder than
+now. Regions like Greenland have not been the seat of
+glaciers, but have been the home of types of plants which
+now thrive in relatively low latitudes. In other words, the
+earth is today only part way from a glacial epoch to what
+may be called the normal, mild climate of the earth&mdash;a
+climate in which the contrast from zone to zone was much
+less than now, and the lower air averaged warmer. Hence
+it seems impossible to avoid the conclusion that the
+cause of glaciation is still operating with considerable
+<!--File: cc072.png Page 48-->
+<a class="pagenum break" id="Page_48" title="pg 48"></a>
+although diminished efficiency. But volcanic dust is
+obviously not operating to any appreciable extent at
+present, for the upper air is almost free from dust a large
+part of the time.</p>
+
+<p>Again, as Chamberlin suggests, let it be supposed that
+a Krakatoan eruption every two years would produce a
+glacial period. Unless the most experienced field workers
+on the glacial formations are quite in error, the various
+glacial epochs of the Pleistocene glacial period had a
+joint duration of at least 150,000 years and perhaps twice
+as much. That would require 75,000 Krakatoan eruptions.
+But where are the pits and cones of such eruptions?
+There has not been time to erode them away since the
+Pleistocene glaciation. Their beds of volcanic ash would
+presumably be as voluminous as the glacial beds, but
+there do not seem to be accumulations of any such size.
+Even though the same volcano suffered repeated explosions,
+it seems impossible to find sufficient fresh volcanic
+debris. Moreover, the volcanic hypothesis has not yet
+offered any mechanism for systematic glacial variations.
+Hence, while the hypothesis is important, we must search
+further for the full explanation of glacial fluctuations,
+historic pulsations, and the earth's present quasi-glacial
+climate.</p>
+
+<p>V. <em>The Hypothesis of Polar Wandering.</em> Another hypothesis,
+which has some adherents, especially among
+geologists, holds that the position of the earth's axis has
+shifted repeatedly during geological times, thus causing
+glaciation in regions which are not now polar. Astrophysicists,
+however, are quite sure that no agency could
+radically change the relation between the earth and its
+axis without likewise altering the orbits of the planets to
+a degree that would be easily recognized. Moreover, the
+distribution of the centers of glaciation both in the Permian
+<!--File: cc073.png Page 49-->
+<a class="pagenum break" id="Page_49" title="pg 49"></a>
+and Pleistocene periods does not seem to conform
+to this hypothesis.</p>
+
+<p>VI. <em>The Thermal Solar Hypothesis.</em> The only other
+explanations of the climatic changes of glacial and historic
+times which now seem to have much standing are
+two distinct and almost antagonistic solar hypotheses.
+One is the idea that changes in the earth's climate are
+due to variations in the heat emitted by the sun and
+hence in the temperature of the earth. The other is the
+entirely different idea that climatic changes arise from
+solar conditions which cause a <em>redistribution of the
+earth's atmospheric pressure</em> and hence produce changes
+in winds, ocean currents, and especially storms. This
+second, or "cyclonic," hypothesis is the subject of a book
+entitled <em>Earth and Sun</em>, which is to be published as a
+companion to the present volume. It will be outlined in
+the next chapter. The other, or thermal, hypothesis may
+be dismissed briefly. Unquestionably a permanent change
+in the amount of heat emitted by the sun would permanently
+alter the earth's climate. There is absolutely no
+evidence, however, of any such change during geologic
+time. The evidence as to the earth's cosmic uniformity
+and as to secular progression is all against it. Suppose
+that for thirty or forty thousand years the sun cooled off
+enough so that the earth was as cool as during a glacial
+epoch. As glaciation is soon succeeded by a mild climate,
+some agency would then be needed to raise the sun's
+temperature. The impact of a shower of meteorites might
+accomplish this, but that would mean a very sudden heating,
+such as there is no evidence of in geological history.
+In fact, there is far more evidence of sudden cooling than
+of sudden heating. Moreover, it is far beyond the bounds
+of probability that such an impact should be repeated
+again and again with just such force as to bring the climate
+<!--File: cc074.png Page 50-->
+<a class="pagenum break" id="Page_50" title="pg 50"></a>
+back almost to where it started and yet to allow for
+the slight changes which cause secular progression.
+Another and equally cogent objection to the thermal form
+of solar hypothesis is stated by Humphreys as follows:
+"A change of the solar constant obviously alters all surface
+temperatures by a roughly constant percentage.
+Hence a decrease of the heat from the sun would in general
+cause a decrease of the interzonal temperature
+gradients; and this in turn a less vigorous atmospheric
+circulation, and a less copious rain or snowfall&mdash;exactly
+the reverse of the condition, namely, abundant precipitation,
+most favorable to extensive glaciation."</p>
+
+<p>This brings us to the end of the main hypotheses as to
+climatic changes, aside from the solar cyclonic hypothesis
+which will be discussed in the next chapter. It appears
+that variations in the position of the earth at perihelion
+have a real though slight influence in causing cycles with
+a length of about 21,000 years. Changes in the carbon
+dioxide of the air probably have a more important but
+extremely slow influence upon geologic oscillations.
+Variations in the size, shape, and height of the continents
+are constantly causing all manner of climatic complications,
+but do not cause rapid fluctuations and pulsations.
+The eruption of volcanic dust appears occasionally to
+lower the temperature, but its potency to explain the
+complex climatic changes recorded in the rocks has probably
+been exaggerated. Finally, although minor changes
+in the amount of heat given out by the sun occur constantly
+and have been demonstrated to have a climatic
+effect, there is no evidence that such changes are the main
+cause of the climatic phenomena which we are trying to
+explain. Nevertheless, in connection with other solar
+changes they may be of high importance.</p>
+<!--File: cc075.png Page 51-->
+<a class="pagenum break" id="Page_51" title="pg 51"></a>
+
+
+
+
+<h2 id="Chapt_IV"><a href="#content">CHAPTER IV</a></h2>
+
+<h3>THE SOLAR CYCLONIC HYPOTHESIS</h3>
+
+
+<p>The progress of science is made up of a vast succession
+of hypotheses. The majority die in early
+infancy. A few live and are for a time widely
+accepted. Then some new hypothesis either destroys them
+completely or shows that, while they contain elements of
+truth, they are not the whole truth. In the previous chapter
+we have discussed a group of hypotheses of this kind,
+and have tried to point out fairly their degree of truth so
+far as it can yet be determined. In this chapter we shall
+outline still another hypothesis, the relation of which to
+present climatic conditions has been fully developed in
+<em>Earth and Sun</em>; while its relation to the past will be explained
+in the present volume. This hypothesis is not
+supposed to supersede the others, for so far as they are
+true they cannot be superseded. It merely seems to explain
+some of the many conditions which the other
+hypotheses apparently fail to explain. To suppose that
+it will suffer a fate more glorious than its predecessors
+would be presumptuous. The best that can be hoped is
+that after it has been pruned, enriched, and modified, it
+may take its place among the steps which finally lead to
+the goal of truth.</p>
+
+<p>In this chapter the new hypothesis will be sketched in
+broad outline in order that in the rest of this book the
+reader may appreciate the bearing of all that is said.
+Details of proof and methods of work will be omitted,
+<!--File: cc076.png Page 52-->
+<a class="pagenum break" id="Page_52" title="pg 52"></a>
+since they are given in <em>Earth and Sun</em>. For the sake of
+brevity and clearness the main conclusions will be stated
+without the qualifications and exceptions which are fully
+explained in that volume. Here it will be necessary to
+pass quickly over points which depart radically from accepted
+ideas, and which therefore must arouse serious
+question in the minds of thoughtful readers. That, however,
+is a necessary consequence of the attempt which
+this book makes to put the problem of climate in such
+form that the argument can be followed by thoughtful
+students in any branch of knowledge and not merely by
+specialists. Therefore, the specialist can merely be asked
+to withhold judgment until he has read all the evidence
+as given in <em>Earth and Sun</em>, and then to condemn only
+those parts that are wrong and not the whole argument.</p>
+
+<p>Without further explanation let us turn to our main
+problem. In the realm of climatology the most important
+discovery of the last generation is that variations in the
+weather depend on variations in the activity of the sun's
+atmosphere. The work of the great astronomer, Newcomb,
+and that of the great climatologist, K&ouml;ppen, have
+shown beyond question that the temperature of the
+earth's surface varies in harmony with variations in the
+number and area of sunspots.<sup><a href="#fn15" id="r15">[15]</a></sup> The work of Abbot has
+shown that the amount of heat radiated from the sun also
+varies, and that in general the variations correspond with
+those of the sunspots, although there are exceptions,
+especially when the spots are fewest. Here, however,
+there at once arises a puzzling paradox. The earth certainly
+<!--File: cc077.png Page 53-->
+<a class="pagenum break" id="Page_53" title="pg 53"></a>
+owes its warmth to the sun. Yet when the sun emits
+the most energy, that is, when sunspots are most numerous,
+the earth's surface is coolest. Doubtless the earth
+receives more heat than usual at such times, and the
+upper air may be warmer than usual. Here we refer only
+to the air at the earth's surface.</p>
+
+<p>Another large group of investigators have shown that
+atmospheric pressure also varies in harmony with the
+number of sunspots. Some parts of the earth's surface
+have one kind of variation at times of many sunspots and
+other parts the reverse. These differences are systematic
+and depend largely on whether the region in question
+happens to have high atmospheric pressure or low. The
+net result is that when sunspots are numerous the
+earth's storminess increases, and the atmosphere is
+thrown into commotion. This interferes with the stable
+planetary winds, such as the trades of low latitudes and
+the prevailing westerlies of higher latitudes. Instead of
+these regular winds and the fair weather which they
+bring, there is a tendency toward frequent tropical hurricanes
+in the lower latitudes and toward more frequent
+and severe storms of the ordinary type in the latitudes
+where the world's most progressive nations now live.
+With the change in storminess there naturally goes a
+change in rainfall. Not all parts of the world, however,
+have increased storminess and more abundant rainfall
+when sunspots are numerous. Some parts change in the
+opposite way. Thus when the sun's atmosphere is particularly
+disturbed, the contrasts between different parts
+of the earth's surface are increased. For example, the
+northern United States and southern Canada become
+more stormy and rainy, as appears in Fig. 2, and the
+same is true of the Southwest and along the south Atlantic
+coast. In a crescent-shaped central area, however,
+<!--File: cc078.png Page 54-->
+<a class="pagenum break" id="Page_54" title="pg 54"></a>
+extending from Wyoming through Missouri to Nova
+Scotia, the number of storms and the amount of rainfall
+decrease.</p>
+
+
+<img id="Fig_2" src="images/fig2.png" width="806" height="567" alt="Fig. 2"  />
+<p class="center"><em>Fig. 2. Storminess of sunspot maxima vs. minima.</em><br />
+(<em>After Kullmer.</em>)</p>
+
+<p class="blockquot">Based on nine years' nearest sunspot minima and nine years' nearest sunspot
+maxima in the three sunspot cycles from 1888 to 1918. Heavy shading
+indicates excess of storminess when sunspots are numerous. Figures indicate
+average yearly number of storms by which years of maximum sunspots
+exceed those of minimum sunspots.</p>
+
+<p>The two controlling factors of any climate are the
+temperature and the atmospheric pressure, for they determine
+the winds, the storms, and thus the rainfall. A
+study of the temperature seems to show that the peculiar
+paradox of a hot sun and a cool earth is due largely to
+the increased storminess during times of many sunspots.
+The earth's surface is heated by the rays of the sun, but
+<!--File: cc079.png Page 55-->
+<a class="pagenum break" id="Page_55" title="pg 55"></a>
+most of the rays do not in themselves heat the air as
+they pass through it. The air gets its heat largely from
+the heat absorbed by the water vapor which is intimately
+mingled with its lower portions, or from the long heat
+waves sent out by the earth after it has been warmed by
+the sun. The faster the air moves along the earth's surface
+the less it becomes heated, and the more heat it takes
+away. This sounds like a contradiction, but not to anyone
+who has tried to heat a stove in the open air. If the air
+is still, the stove rapidly becomes warm and so does the
+air around it. If the wind is blowing, the cool air delays
+the heating of the stove and prevents the surface from
+ever becoming as hot as it would otherwise. That seems
+to be what happens on a large scale when sunspots are
+numerous. The sun actually sends to the earth more
+energy than usual, but the air moves with such unusual
+rapidity that it actually cools the earth's surface a trifle
+by carrying the extra heat to high levels where it is lost
+into space.</p>
+
+<p>There has been much discussion as to why storms are
+numerous when the sun's atmosphere is disturbed. Many
+investigators have supposed it was due entirely and
+directly to the heating of the earth's surface by the sun.
+This, however, needs modification for several reasons.
+In the first place, recent investigations show that in a
+great many cases changes in barometric pressure precede
+changes in temperature and apparently cause them by
+altering the winds and producing storms. This is the
+opposite of what would happen if the effect of solar heat
+upon the earth's surface were the only agency. In the
+second place, if storms were due exclusively to variations
+in the ordinary solar radiation which comes to the earth
+as light and is converted into heat, the solar effect ought
+<!--File: cc080.png  Page 56-->
+<a class="pagenum break" id="Page_56" title="pg 56"></a>
+to be most pronounced when the center of the sun's
+visible disk is most disturbed. As a matter of fact the
+storminess is notably greatest when the edges of the
+solar disk are most disturbed. These facts and others lead
+to the conclusion that some agency other than heat must
+also play some part in producing storminess.</p>
+
+<p>The search for this auxiliary agency raises many difficult
+questions which cannot yet be answered. On the
+whole the weight of evidence suggests that electrical
+phenomena of some kind are involved, although variations
+in the amount of ultra-violet light may also be
+important. Many investigators have shown that the sun
+emits electrons. Hale has proved that the sun, like the
+earth, is magnetized. Sunspots also have magnetic fields
+the strength of which is often fifty times as great as that
+of the sun as a whole. If electrons are sent to the earth,
+they must move in curved paths, for they are deflected
+by the sun's magnetic field and again by the earth's
+magnetic field. The solar deflection may cause their
+effects to be greatest when the spots are near the sun's
+margin; the terrestrial deflection may cause concentration
+in bands roughly concentric with the magnetic poles
+of the earth. These conditions correspond with the known
+facts.</p>
+
+<p>Farther than this we cannot yet go. The calculations of
+Humphreys seem to indicate that the direct electrical
+effect of the sun's electrons upon atmospheric pressure
+is too small to be of appreciable significance in intensifying
+storms. On the other hand the peculiar way in which
+activity upon the margins of the sun appears to be correlated
+not only with atmospheric electricity, but with
+barometric pressure, seems to be equally strong evidence
+in the other direction. Possibly the sun's electrons and
+its electrical waves produce indirect effects by being
+<!--File: cc081.png Page 57-->
+<a class="pagenum break" id="Page_57" title="pg 57"></a>
+converted into heat, or by causing the formation of ozone
+and the condensation of water vapor in the upper air.
+Any one of these processes would raise the temperature
+of the upper air, for the ozone and the water vapor would
+be formed there and would tend to act as a blanket to
+hold in the earth's heat. But any such change in the temperature
+of the upper air would influence the lower air
+through changes in barometric pressure. These considerations
+are given here because the thoughtful reader
+is likely to inquire how solar activity can influence
+storminess. Moreover, at the end of this book we shall
+take up certain speculative questions in which an electrical
+hypothesis will be employed. For the main portions
+of this book it makes no difference how the sun's
+variations influence the earth's atmosphere. The only table 6
+essential point is that when the solar atmosphere is active
+the storminess of the earth increases, and that is a matter
+of direct observation.</p>
+
+<p>Let us now inquire into the relation between the small
+cyclonic vacillations of the weather and the types of
+climatic changes known as historic pulsations and glacial
+fluctuations. One of the most interesting results of recent
+investigations is the evidence that sunspot cycles on a
+small scale present almost the same phenomena as do
+historic pulsations and glacial fluctuations. For instance,
+when sunspots are numerous, storminess increases
+markedly in a belt near the northern border of the area
+of greatest storminess, that is, in southern Canada and
+thence across the Atlantic to the North Sea and Scandinavia.
+(See Figs. 2 and 3.) Corresponding with this is the
+fact that the evidence as to climatic pulsations in historic
+times indicates that regions along this path, for instance
+Greenland, the North Sea region, and southern Scandinavia,
+<!--File: cc082.png Page 58-->
+<a class="pagenum break" id="Page_58" title="pg 58"></a>
+
+
+<!--File: cc083.png Page 59-->
+<a class="pagenum break" id="Page_59" title="pg 59"></a>
+
+
+
+<!--File: cc084.png Page 60-->
+<a class="pagenum break" id="Page_60" title="pg 60"></a>
+
+were visited by especially frequent and severe
+storms at the climax of each pulsation. Moreover, the
+greatest accumulations of ice in the glacial period were
+on the poleward border of the general regions where now
+the storms appear to increase most at times of solar
+activity.</p>
+<img id="Fig_3a" src="images/fig3a.png" width="924" height="922" alt="Fig 3a"  />
+
+<p class="center"><em>Fig. 3.a Relative rainfall at times of increasing and decreasing sunspots</em></p>
+<p class="blockquot">Heavy shading, more rain with increasing spots. Light shading, more rain with decreasing
+spots. No data for unshaded areas.</p>
+
+<p class="blockquot">Figures indicate percentages of the average rainfall by which the rainfall during
+periods of increasing spots exceeds or falls short of rainfall during periods of decreasing
+spots. The excess or deficiency is stated in percentages of the average. Rainfall
+data from Walker: Sunspots and Rainfall.</p>
+
+<img id="Fig_3b" src="images/fig3b.png" width="919" height="909" alt="Fig.3b"  />
+<p class="center"> <em>Fig. 3.b Relative rainfall at times of increasing and decreasing
+sunspots.</em></p>
+
+<p class="blockquot">Heavy shading, more rain with increasing spots. Light shading, more rain with decreasing
+spots. No data for unshaded areas.</p>
+
+<p class="blockquot">Figures indicate percentages of the average rainfall by which the rainfall during
+periods of increasing spots exceeds or falls short of rainfall during periods of decreasing
+spots. The excess or deficiency is stated in percentages of the average. Rainfall
+data from Walker: Sunspots and Rainfall.</p>
+
+<p>Even more clear is the evidence from other regions
+where storms increase at times of many sunspots. One
+such region includes the southwestern United States,
+while another is the Mediterranean region and the semi-arid
+or desert parts of Asia farther east. In these regions
+innumerable ruins and other lines of evidence show that
+at the climax of each climatic pulsation there was more
+storminess and rainfall than at present, just as there
+now is when the sun is most active. In still earlier times,
+while ice was accumulating farther north, the basins of
+these semi-arid regions were filled with lakes whose
+strands still remain to tell the tale of much-increased
+rainfall and presumable storminess. If we go back still
+further in geological times to the Permian glaciation, the
+areas where ice accumulated most abundantly appear to
+be the regions where tropical hurricanes produce the
+greatest rainfall and the greatest lowering of temperature
+at times of many sunspots. From these and many
+other lines of evidence it seems probable that historic
+pulsations and glacial fluctuations are nothing more than
+sunspot cycles on a large scale. It is one of the fundamental
+rules of science to reason from the known to the
+unknown, from the near to the far, from the present to
+the past. Hence it seems advisable to investigate whether
+any of the climatic phenomena of the past may have
+arisen from an intensification of the solar conditions
+which now appear to give rise to similar phenomena on
+a small scale.</p>
+<p><!--File: cc085.png Page 61-->
+<a class="pagenum break" id="Page_61" title="pg 61"></a>
+The rest of this chapter will be devoted to a <em>r&eacute;sum&eacute;</em>
+of certain tentative conclusions which have no bearing
+on the main part of this book, but which apply to the
+closing chapters. There we shall inquire into the periodicity
+of the climatic phenomena of geological times, and
+shall ask whether there is any reason to suppose that the
+sun's activity has exhibited similar periodicity. This
+leads to an investigation of the possible causes of disturbances
+in the sun's atmosphere. It is generally assumed
+that sunspots, solar prominences, the bright clouds
+known as facul&aelig;, and other phenomena denoting a perturbed
+state of the solar atmosphere, are due to some
+cause within the sun. Yet the limitation of these phenomena,
+especially the sunspots, to restricted latitudes,
+as has been shown in <em>Earth and Sun</em>, does not seem to be
+in harmony with an internal solar origin, even though
+a banded arrangement may be normal for a rotating
+globe. The fairly regular periodicity of the sunspots
+seems equally out of harmony with an internal origin.
+Again, the solar atmosphere has two kinds of circulation,
+one the so-called "rice grains," and the other
+the spots and their attendant phenomena. Now the rice
+grains present the appearance that would be expected in
+an atmospheric circulation arising from the loss of heat
+by the outer part of a gaseous body like the sun. For
+these reasons and others numerous good thinkers from
+Wolf to Schuster have held that sunspots owe their
+periodicity to causes outside the sun. The only possible
+cause seems to be the planets, acting either through
+gravitation, through forces of an electrical origin, or
+through some other agency. Various new investigations
+which are described in <em>Earth and Sun</em> support this conclusion.
+The chief difficulty in accepting it hitherto has
+been that although Jupiter, because of its size, would be
+<!--File: cc086.png Page 62-->
+<a class="pagenum break" id="Page_62" title="pg 62"></a>
+expected to dominate the sunspot cycle, its period of
+11.86 years has not been detected. The sunspot cycle has
+appeared to average 11.2 years in length, and has been
+called the 11-year cycle. Nevertheless, a new analysis of
+the sunspot data shows that when attention is concentrated
+upon the major maxima, which are least subject to
+retardation or acceleration by other causes, a periodicity
+closely approaching that of Jupiter is evident. Moreover,
+when the effects of Jupiter, Saturn, and the other planets
+are combined, they produce a highly variable curve which
+has an extraordinary resemblance to the sunspot curve.
+The method by which the planets influence the sun's
+atmosphere is still open to question. It may be through
+tides, through the direct effect of gravitation, through
+electro-magnetic forces, or in some other way. Whichever
+it may be, the result may perhaps be slight differences of
+atmospheric pressure upon the sun. Such differences
+may set in motion slight whirling movements analogous
+to terrestrial storms, and these presumably gather momentum
+from the sun's own energy. Since the planetary
+influences vary in strength because of the continuous
+change in the relative distances and positions of the
+planets, the sun's atmosphere appears to be swayed by
+cyclonic disturbances of varying degrees of severity. The
+cyclonic disturbances known as sunspots have been
+proved by Hale to become more highly electrified as they
+increase in intensity. At the same time hot gases presumably
+well up from the lower parts of the solar atmosphere
+and thereby cause the sun to emit more heat. Thus
+by one means or another, the earth's atmosphere appears
+to be set in commotion and cycles of climate are inaugurated.</p>
+
+<p>If the preceding reasoning is correct, any disturbance
+of the solar atmosphere must have an effect upon the
+<!--File: cc087.png Page 63-->
+<a class="pagenum break" id="Page_63" title="pg 63"></a>
+earth's climate. If the disturbance were great enough and
+of the right nature it might produce a glacial epoch. The
+planets are by no means the only bodies which act upon
+the sun, for that body sustains a constantly changing
+relation to millions of other celestial bodies of all sizes
+up to vast universes, and at all sorts of distances. If the
+sun and another star should approach near enough to one
+another, it is certain that the solar atmosphere would be
+disturbed much more than at present.</p>
+
+<p>Here we must leave the cyclonic hypothesis of climate
+and must refer the reader once more to <em>Earth and Sun</em>
+for fuller details. In the rest of this book we shall discuss
+the nature of the climatic changes of past times and shall
+inquire into their relation to the various climatic hypotheses
+mentioned in the last two chapters. Then we shall
+inquire into the possibility that the solar system has ever
+been near enough to any of the stars to cause appreciable
+disturbances of the solar atmosphere. We shall complete
+our study by investigating the vexed question of why
+movements of the earth's crust, such as the uplifting of
+continents and mountain chains, have generally occurred
+at the same time as great climatic fluctuations. This
+would not be so surprising were it not that the climatic
+phenomena appear to have consisted of highly complex
+cycles while the uplift has been a relatively steady movement
+in one direction. We shall find some evidence that
+the solar disturbances which seem to cause climatic
+changes also have a relation to movements of the crust.</p>
+<!--File: cc088.png Page 64-->
+<a class="pagenum break" id="Page_64" title="pg 64"></a>
+
+
+
+
+<h2 id="Chapt_V"><a href="#content">CHAPTER V</a></h2>
+
+<h3>THE CLIMATE OF HISTORY<sup><a href="#fn16" id="r16">[16]</a></sup></h3>
+
+
+<p>We are now prepared to consider the climate of
+the past. The first period to claim attention is
+the few thousand years covered by written
+history. Strangely enough, the conditions during this
+time are known with less accuracy than are those of
+geological periods hundreds of times more remote. Yet
+if pronounced changes have occurred since the days of
+the ancient Babylonians and since the last of the post-glacial
+stages, they are of great importance not only
+because of their possible historic effects, but because they
+bridge the gap between the little variations of climate
+which are observable during a single lifetime and the
+great changes known as glacial epochs. Only by bridging
+the gap can we determine whether there is any genetic
+relation between the great changes and the small. A full
+discussion of the climate of historic times is not here
+advisable, for it has been considered in detail in numerous
+other publications.<sup><a href="#fn17" id="r17">[17]</a></sup> Our most profitable course would
+seem to be to consider first the general trend of opinion
+and then to take up the chief objections to each of the
+main hypotheses.</p>
+
+<p>In the hot debate over this problem during recent
+<!--File: cc089.png Page 65-->
+<a class="pagenum break" id="Page_65" title="pg 65"></a>
+decades the ideas of geographers seem to have gone
+through much the same metamorphosis as have those of
+geologists in regard to the climate of far earlier times.</p>
+
+<p>As every geologist well knows, at the dawn of geology
+people believed in climatic uniformity&mdash;that is, it was
+supposed that since the completion of an original creative
+act there had been no important changes. This view
+quickly disappeared and was superseded by the hypothesis
+of progressive cooling and drying, an hypothesis
+which had much to do with the development of the nebular
+hypothesis, and which has in turn been greatly
+strengthened by that hypothesis. The discovery of evidence
+of widespread continental glaciation, however,
+necessitated a modification of this view, and succeeding
+years have brought to light a constantly increasing number
+of glacial, or at least cool, periods distributed
+throughout almost the whole of geological time. Moreover,
+each year, almost, brings new evidence of the great
+complexity of glacial periods, epochs, and stages. Thus,
+for many decades, geologists have more and more been
+led to believe that in spite of surprising uniformity, when
+viewed in comparison with the cosmic possibilities, the
+climate of the past has been highly unstable from the
+viewpoint of organic evolution, and its changes have been
+of all degrees of intensity.</p>
+
+<p>Geographers have lately been debating the reality of
+historic changes of climate in the same way in which
+geologists debated the reality of glacial epochs and
+stages. Several hypotheses present themselves but these
+may all be grouped under three headings; namely, the
+hypotheses of (1) progressive desiccation, (2) climatic
+uniformity, and (3) pulsations. The hypothesis of progressive
+desiccation has been widely advocated. In many
+of the drier portions of the world, especially between 30&deg;
+<!--File: cc090.png Page 66-->
+<a class="pagenum break" id="Page_66" title="pg 66"></a>
+and 40&deg; from the equator, and pre&euml;minently in western
+and central Asia and in the southwestern United States,
+almost innumerable facts seem to indicate that two or
+three thousand years ago the climate was distinctly
+moister than at present. The evidence includes old lake
+strands, the traces of desiccated springs, roads in places
+now too dry for caravans, other roads which make detours
+around dry lake beds where no lakes now exist, and
+fragments of dead forests extending over hundreds of
+square miles where trees cannot now grow for lack of
+water. Still stronger evidence is furnished by ancient
+ruins, hundreds of which are located in places which are
+now so dry that only the merest fraction of the former
+inhabitants could find water. The ruins of Palmyra, in
+the Syrian Desert, show that it must once have been a
+city like modern Damascus, with one or two hundred
+thousand inhabitants, but its water supply now suffices
+for only one or two thousand. All attempts to increase the
+water supply have had only a slight effect and the water
+is notoriously sulphurous, whereas in the former days,
+when it was abundant, it was renowned for its excellence.
+Hundreds of pages might be devoted to describing similar
+ruins. Some of them are even more remarkable for
+their dryness than is Niya, a site in the Tarim Desert of
+Chinese Turkestan. Yet there the evidence of desiccation
+within 2000 years is so strong that even so careful and
+conservative a man as Hann,<sup><a href="#fn18" id="r18">[18]</a></sup> pronounces it "&uuml;berzeugend."</p>
+
+<p>A single quotation from scores that might be used will
+illustrate the conclusions of some of the most careful
+arch&aelig;ologists.<sup><a href="#fn19" id="r19">[19]</a></sup></p>
+<!--File: cc091.png Page 67-->
+<a class="pagenum break" id="Page_67" title="pg 67"></a>
+
+<blockquote>
+<p>Among the regions which were once populous and highly
+civilized, but which are now desert and deserted, there are few
+which were more closely connected with the beginnings of our
+own civilization than the desert parts of Syria and northern
+Arabia. It is only of recent years that the vast extent and great
+importance of this lost civilization has been fully recognized and
+that attempts have been made to reduce the extent of the unexplored
+area and to discover how much of the territory which has
+long been known as desert was formerly habitable and inhabited.
+The results of the explorations of the last twenty years have been
+most astonishing in this regard. It has been found that practically
+all of the wide area lying between the coast range of the
+eastern Mediterranean and the Euphrates, appearing upon the
+maps as the Syrian Desert, an area embracing somewhat more
+than 20,000 square miles, was more thickly populated than any
+area of similar dimensions in England or in the United States
+is today if one excludes the immediate vicinity of the large
+modern cities. It has also been discovered that an enormous
+desert tract lying to the east of Palestine, stretching eastward
+and southward into the country which we know as Arabia, was
+also a densely populated country. How far these settled regions
+extended in antiquity is still unknown, but the most distant
+explorations in these directions have failed to reach the end of
+ruins and other signs of former occupation.</p>
+
+<p>The traveler who has crossed the settled, and more or less
+populous, coast range of northern Syria and descended into the
+narrow fertile valley of the Orontes, encounters in any farther
+journey toward the east an irregular range of limestone hills
+lying north and south and stretching to the northeast almost
+halfway to the Euphrates. These hills are about 2,500 feet high,
+rising in occasional peaks from 3,000 to 3,500 feet above sea level.
+They are gray and unrelieved by any visible vegetation. On
+ascending into the hills the traveler is astonished to find at every
+turn remnants of the work of men's hands, paved roads, walls
+which divided fields, terrace walls of massive structure. Presently
+he comes upon a small deserted and partly ruined town
+<!--File: cc092.png Page 68-->
+<a class="pagenum break" id="Page_68" title="pg 68"></a>
+composed of buildings large and small constructed of beautifully
+wrought blocks of limestone, all rising out of the barren
+rock which forms the ribs of the hills. If he mounts an eminence
+in the vicinity, he will be still further astonished to behold
+similar ruins lying in all directions. He may count ten or fifteen
+or twenty, according to the commanding position of his lookout.
+From a distance it is often difficult to believe that these are not
+inhabited places; but closer inspection reveals that the gentle
+hand of time or the rude touch of earthquake has been laid upon
+every building. Some of the towns are better preserved than
+others; some buildings are quite perfect but for their wooden
+roofs which time has removed, others stand in picturesque ruins,
+while others still are level with the ground. On a far-off hilltop
+stands the ruin of a pagan temple, and crowning some lofty ridge
+lie the ruins of a great Christian monastery. Mile after mile of
+this barren gray country may be traversed without encountering
+a single human being. Day after day may be spent in traveling
+from one ruined town to another without seeing any green
+thing save a terebinth tree or two standing among the ruins,
+which have sent their roots down into earth still preserved in
+the foundations of some ancient building. No soil is visible
+anywhere except in a few pockets in the rock from which it
+could not be washed by the torrential rains of the wet season;
+yet every ruin is surrounded with the remains of presses for the
+making of oil and wine. Only one oasis has been discovered in
+these high plateaus.</p>
+
+<p>Passing eastward from this range of hills, one descends into a
+gently rolling country that stretches miles away toward the
+Euphrates. At the eastern foot of the hills one finds oneself in a
+totally different country, at first quite fertile and dotted with
+frequent villages of flat-roofed houses. Here practically all the
+remains of ancient times have been destroyed through ages of
+building and rebuilding. Beyond this narrow fertile strip the
+soil grows drier and more barren, until presently another kind
+of desert is reached, an undulating waste of dead soil. Few walls
+or towers or arches rise to break the monotony of the unbroken
+<!--File: cc093.png Page 69-->
+<a class="pagenum break" id="Page_69" title="pg 69"></a>
+landscape; but the careful explorer will find on closer examination
+that this region was more thickly populated in antiquity
+even than the hill country to the west. Every unevenness of the
+surface marks the site of a town, some of them cities of considerable
+extent.</p>
+
+<p>We may draw certain very definite conclusions as to the
+former conditions of the country itself. There was soil upon the
+northern hills where none now exists, for the buildings now show
+unfinished foundation courses which were not intended to be
+seen; the soil in depressions without outlets is deeper than it
+formerly was; there are hundreds of olive and wine presses in
+localities where no tree or vine could now find footing; and
+there are hillsides with ruined terrace walls rising one above
+the other with no sign of earth near them. There was also a large
+natural water supply. In the north as well as in the south we
+find the dry beds of rivers, streams, and brooks with sand and
+pebbles and well-worn rocks but no water in them from one
+year's end to the other. We find bridges over these dry streams
+and crudely made washing boards along their banks directly
+below deserted towns. Many of the bridges span the beds of
+streams that seldom or never have water in them and give
+clear evidence of the great climatic changes that have taken
+place. There are well heads and well houses, and inscriptions
+referring to springs; but neither wells nor springs exist today
+except in the rarest instances. Many of the houses had their
+rock-hewn cisterns, never large enough to have supplied water
+for more than a brief period, and corresponding to the cisterns
+which most of our recent forefathers had which were for convenience
+rather than for dependence. Some of the towns in
+southern Syria were provided with large public reservoirs, but
+these are not large enough to have supplied water to their
+original populations. The high plateaus were of course without
+irrigation; but there are no signs, even in the lower flatter
+country, that irrigation was ever practiced; and canals for this
+purpose could not have completely disappeared. There were
+forests in the immediate vicinity, forests producing timbers of
+great length and thickness; for in the north and northeast practically
+<!--File: cc094.png-->
+<a class="pagenum break" id="Page_70" title="pg 70"></a>
+ all the buildings had wooden roofs, wooden intermediate
+floors, and other features of wood. Costly buildings, such as
+temples and churches, employed large wooden beams; but wood
+was used in much larger quantities in private dwellings, shops,
+stables, and barns. If wood had not been plentiful and cheap&mdash;which
+means grown near by&mdash;the builders would have adopted
+the building methods of their neighbors in the south, who used
+very little wood and developed the most perfect type of lithic
+architecture the world has ever seen. And here there exists a
+strange anomaly: Northern Syria, where so much wood was
+employed in antiquity, is absolutely treeless now; while in the
+mountains of southern Syria, where wood must have been
+scarce in antiquity to have forced upon the inhabitants an almost
+exclusive use of stone, there are still groves of scrub oak and
+pine, and travelers of half a century ago reported large forests
+of chestnut trees.<sup><a href="#fn20" id="r20">[20]</a></sup> It is perfectly apparent that large parts of
+Syria once had soil and forests and springs and rivers, while it
+has none of these now, and that it had a much larger and better
+distributed rainfall in ancient times than it has now.</p>
+</blockquote>
+
+<p>Professor Butler's careful work is especially interesting
+because of its contrast to the loose statements of
+those who believe in climatic uniformity. So far as I am
+aware, no opponent of the hypothesis of climatic changes
+has ever even attempted to show by careful statistical
+analysis that the ancient water supply of such ruins was
+no greater than that of the present. The most that has
+been done is to suggest that there may have been sources
+of water which are now unknown. Of course, this might
+be true in a single instance, but it could scarcely be the
+case in many hundreds or thousands of ruins.</p>
+
+<!--File: cc095.png Page 71-->
+<a class="pagenum break" id="Page_71" title="pg 71"></a>
+
+<p>Although the arguments in favor of a change of climate
+during the last two thousand years seem too strong
+to be ignored, their very strength seems to have been a
+source of error. A large number of people have jumped
+to the conclusion that the change which appears to have
+occurred in certain regions occurred everywhere, and
+that it consisted of a gradual desiccation.</p>
+
+<p>Many observers, quite as careful as those who believe
+in progressive desiccation, point to evidences of aridity
+in past times in the very regions where the others find
+proof of moisture. Lakes such as the Caspian Sea fell to
+such a low level that parts of their present floors were
+exposed and were used as sites for buildings whose ruins
+are still extant. Elsewhere, for instance in the Tian-Shan
+Mountains, irrigation ditches are found in places where
+irrigation never seems to be necessary at present. In
+Syria and North Africa during the early centuries of the
+Christian era the Romans showed unparalleled activity
+in building great aqueducts and in watering land which
+then apparently needed water almost as much as it does
+today. Evidence of this sort is abundant and is as convincing
+as is the evidence of moister conditions in the
+past. It is admirably set forth, for example, in the comprehensive
+and ably written monograph of Leiter on the
+climate of North Africa.<sup><a href="#fn21" id="r21">[21]</a></sup> The evidence cited there and
+elsewhere has led many authors strongly to advocate the
+hypothesis of climatic uniformity. They have done exactly
+as have the advocates of progressive change, and
+have extended their conclusions over the whole world and
+over the whole of historic times.</p>
+
+<p>The hypotheses of climatic uniformity and of progressive
+<!--File: cc096.png Pag2 72-->
+<a class="pagenum break" id="Page_72" title="pg 72"></a>
+change both seem to be based on reliable evidence.
+They may seem to be diametrically opposed to one
+another, but this is only when there is a failure to group
+the various lines of evidence according to their dates, and
+according to the types of climate in which they happen
+to be located. When the facts are properly grouped in
+both time and space, it appears that evidence of moist
+conditions in the historic Mediterranean lands is found
+during certain periods; for instance, four or five hundred
+years before Christ, at the time of Christ, and 1000 A. D.
+The other kind of evidence, on the contrary, culminates
+at other epochs, such as about 1200 B. C. and in the
+seventh and thirteenth centuries after Christ. It is also
+found during the interval from the culmination of a moist
+epoch to the culmination of a dry one, for at such times
+the climate was growing drier and the people were under
+stress. This was seemingly the case during the period
+from the second to the fourth centuries of our era. North
+Africa and Syria must then have been distinctly better
+watered than at present, as appears from Butler's vivid
+description; but they were gradually becoming drier, and
+the natural effect on a vigorous, competent people like the
+Romans was to cause them to construct numerous engineering
+works to provide the necessary water.</p>
+
+<p>The considerations which have just been set forth have
+led to a third hypothesis, that of pulsatory climatic
+changes. According to this, the earth's climate is not
+stable, nor does it change uniformly in one direction. It
+appears to fluctuate back and forth not only in the little
+waves which we see from year to year or decade to
+decade, but in much larger waves, which take hundreds of
+years or even a thousand. These in turn seem to merge
+into and be imposed on the greater waves which form
+glacial stages, glacial epochs, and glacial periods. At the
+<!--File: cc097.png Page 73-->
+<a class="pagenum break" id="Page_73" title="pg 73"></a>
+present time there seems to be no way of determining
+whether the general tendency is toward aridity or toward
+glaciation. The seventh century of our era was apparently
+the driest time during the historic period&mdash;distinctly
+drier than the present&mdash;but the thirteenth century was
+almost equally dry, and the twelfth or thirteenth before
+Christ may have been very dry.</p>
+
+<p>The best test of an hypothesis is actual measurements.
+In the case of the pulsatory hypothesis we are fortunately
+able to apply this test by means of trees. The
+growth of vegetation depends on many factors&mdash;soil, exposure,
+wind, sun, temperature, rain, and so forth. In a
+dry region the most critical factor in determining how a
+tree's growth shall vary from year to year is the supply
+of moisture during the few months of most rapid
+growth.<sup><a href="#fn22" id="r22">[22]</a></sup>
+The work of Douglass<sup><a href="#fn23" id="r23">[23]
+</a></sup> and others has shown
+that in Arizona and California the thickness of the
+annual rings affords a reliable indication of the amount
+of moisture available during the period of growth. This
+is especially true when the growth of several years is
+taken as the unit and is compared with the growth of
+a similar number of years before or after. Where a long
+series of years is used, it is necessary to make corrections
+to eliminate the effects of age, but this can be done by
+mathematical methods of considerable accuracy. It is
+difficult to determine whether the climate at the beginning
+<!--File: cc098.png Page 74-->
+<a class="pagenum break" id="Page_74" title="pg 74"></a>
+and end of a tree's life was the same, but it is easily possible
+to determine whether there have been pulsations
+while the tree was making its growth. If a large number
+of trees from various parts of a given district all formed
+thick rings at a certain period and then formed thin ones
+for a hundred years, after which the rings again become
+thick, we seem to be safe in concluding that the trees have
+lived through a long, dry period. The full reasons for this
+belief and details as to the methods of estimating climate
+from tree growth are given in <em>The Climatic Factor</em>.</p>
+
+<p>The results set forth in that volume may be summarized
+as follows: During the years 1911 and 1912, under
+the auspices of the Carnegie Institution of Washington,
+measurements were made of the thickness of the rings of
+growth on the stumps of about 450 sequoia trees in California.
+These trees varied in age from 250 to nearly
+3250 years. The great majority were over 1000 years of
+age, seventy-nine were over 2000 years, and three over
+3000. Even where only a few trees are available the
+record is surprisingly reliable, except where occasional
+accidents occur. Where the number approximates 100,
+accidental variations are largely eliminated and we may
+accept the record with considerable confidence. Accordingly,
+we may say that in California we have a fairly
+accurate record of the climate for 2000 years and an
+approximate record for 1000 years more. The final results
+of the measurements of the California trees are
+shown in Fig. 4, where the climatic variations for 3000
+years in California are indicated by the solid line. The
+high parts of the line indicate rainy conditions, the low
+parts, dry. An examination of this curve shows that
+during 3000 years there have apparently been climatic
+variations more important than any which have taken
+place during the past century. In order to bring out the
+<!--File: cc099.png Page 75-->
+<a class="pagenum break" id="Page_75" title="pg 75"></a>
+
+<!--File: cc100.png Page 76-->
+<a class="pagenum break" id="Page_76" title="pg 76"></a>
+details more clearly, the more reliable part of the California
+curve, from 100 B. C. to the present time, has been
+reproduced in Fig. 5. This is identical with the corresponding
+part of Fig. 4, except that the vertical scale is
+three times as great.</p>
+<img src="images/fig4.png" width="800" height="192" alt="Fig. 4"  id="Fig_4" />
+<p class="center"><em>Fig. 4. Changes of climate in California (solid line) and in western and
+central Asia (dotted line).</em></p>
+
+<p class="blockquot">Note. The curves of Figs. 4 and 5 are reproduced as published in <em>The Solar Hypothesis</em> in 1914. Later work,
+however, has indicated that in the Asiatic curve the dash lines, which were tentatively inserted in 1914, are
+probably more nearly correct than the dotted lines. Still further evidence indicates that the Asiatic curve is
+nearly like that of California in its main features.</p>
+
+<p>The curve of tree growth in California seems to be a
+true representation of the general features of climatic
+pulsations in the Mediterranean region. This conclusion
+was originally based on the resemblance between the
+solid line of Fig. 4, representing tree growth, and the
+dotted line representing changes of climate in the eastern
+Mediterranean region as inferred from the study of ruins
+and of history before any work on this subject had been
+done in America.<sup><a href="#fn24" id="r24">[24]</a></sup> The dotted line is here reproduced for
+its historical significance as a stage in the study of climatic
+changes. If it were to be redrawn today on the
+basis of the knowledge acquired in the last twelve years,
+it would be much more like the tree curve. For example,
+the period of aridity suggested by the dip of the dotted
+line about 300 A. D. was based largely on Professor
+Butler's data as to the paucity of inscriptions and ruins
+dating from that period in Syria. In the recent article,
+from which a long quotation has been given, he shows
+that later work proves that there is no such paucity. On
+the other hand, it has accentuated the marked and sudden
+decay in civilization and population which occurred
+shortly after 600 A. D. He reached the same conclusion
+to which the present authors had come on wholly different
+grounds, namely, that the dip in the dotted line about 300
+A. D. is not warranted, whereas the dip about 630 A. D.
+is extremely important. In similar fashion the work of
+<!--File: cc101.png Page 77-->
+<a class="pagenum break" id="Page_77" title="pg 77"></a>
+
+<!--File: cc102.pngPage 78-->
+<a class="pagenum break" id="Page_78" title="pg 78"></a>
+Stein<sup><a href="#fn25" id="r25">[25]</a></sup> in central Asia makes it clear that the contrast
+between the water supply about 200 B. C. and in the preceding
+and following centuries was greater than was
+supposed on the basis of the scanty evidence available
+when the dotted line of Fig. 4 was drawn in 1910.</p>
+<img src="images/fig5.png" width="800" height="294" alt="Fig. 5"  id="Fig_5" />
+<p class="center"><em>Fig. 5. Changes in California climate for 2000 years, as measured by
+growth of Sequoia trees.</em></p>
+
+<p class="blockquot">Fig. 5 is the same as the later portion of Fig. 4, except that the vertical scale has been magnified threefold.
+It seems probable that the dotted line at the right is more nearly correct than the solid line. During the thirty
+years since the end of the curve the general tendency appears in general to have been somewhat upward.</p>
+
+<p>Since the curve of the California trees is the only continuous
+and detailed record yet available for the climate
+of the last three thousand years, it deserves most careful
+study. It is especially necessary to determine the degree
+of accuracy with which the growth of the trees represents
+(1) the local rainfall and (2) the rainfall of remote
+regions such as Palestine. Perhaps the best way to determine
+these matters is the standard mathematical method
+of correlation coefficients. If two phenomena vary in
+perfect unison, as in the case of the turning of the wheels
+and the progress of an automobile when the brakes are
+not applied, the correlation coefficient is 1.00, being positive
+when the automobile goes forward and negative
+when it goes backward. If there is no relation between
+two phenomena, as in the case of the number of miles run
+by a given automobile each year and the number of
+chickens hatched in the same period, the coefficient is
+zero. A partial relationship where other factors enter
+into the matter is represented by a coefficient between
+zero and one, as in the case of the movement of the automobile
+and the consumption of gasoline. In this case the
+relation is very obvious, but is modified by other factors,
+including the roughness and grade of the road, the
+amount of traffic, the number of stops, the skill of the
+driver, the condition and load of the automobile, and the
+state of the weather. Such partial relationships are the
+kind for which correlation coefficients are most useful,
+for the size of the coefficients shows the relative importance
+<!--File: cc103.png Page 79-->
+<a class="pagenum break" id="Page_79" title="pg 79"></a>
+of the various factors. A correlation coefficient
+four times the probable error, which can always be determined
+by a formula well known to mathematicians, is
+generally considered to afford evidence of some kind of
+relation between two phenomena. When the ratio between
+coefficient and error rises to six, the relationship is regarded
+as strong.</p>
+
+<p>Few people would question that there is a connection
+between tree growth and rainfall, especially in a climate
+with a long summer dry season like that of California.
+But the growth of the trees also depends on their position,
+the amount of shading, the temperature, insect pests,
+blights, the wind with its tendency to break the branches,
+and a number of other factors. Moreover, while rain
+commonly favors growth, great extremes are relatively
+less helpful than more moderate amounts. Again, the
+roots of a tree may tap such deep sources of water that
+neither drought nor excessive rain produces much effect
+for several years. Hence in comparing the growth of the
+huge sequoias with the rainfall we should expect a correlation
+coefficient high enough to be convincing, but decidedly
+below 1.00. Unfortunately there is no record of
+the rainfall where the sequoias grow, the nearest long
+record being that of Sacramento, nearly 200 miles to the
+northwest and close to sea level instead of at an altitude
+of about 6000 feet.</p>
+
+<p>Applying the method of correlation coefficients to the
+annual rainfall of Sacramento and the growth of the
+sequoias from 1863 to 1910, we obtain the results shown
+in Table 3. The trees of Section A of the table grew in
+moderately dry locations although the soil was fairly
+deep, a condition which seems to be essential to sequoias.
+In this case, as in all the others, the rainfall is reckoned
+from July to June, which practically means from October
+to May, since there is almost no summer rain. Thus the
+tree growth in 1861 is compared with the rainfall of the
+preceding rainy season, 1860-1861, or of several preceding
+rainy seasons as the table indicates.</p>
+<!--File: cc104.png Page 80-->
+<a class="pagenum break" id="Page_80" title="pg 80"></a>
+
+<table id="table_3" class="borders">
+<tr><th colspan="8"><a href="#tables">TABLE 3</a></th></tr>
+<tr><th colspan="8">CORRELATION COEFFICIENTS BETWEEN RAINFALL<br />
+ AND GROWTH OF SEQUOIAS IN CALIFORNIA<sup><a href="#fn26" id="r26">[26]</a></sup></th></tr>
+<tr><td colspan="8">&nbsp;</td></tr>
+<tr><td colspan="8" class="tdi2">
+(<i>r</i>)=<i>Correlation coefficient</i><br />
+(<i>e</i>)=<i>Probable error</i><br />
+(<i>r/e</i>)=<i>Ratio of coefficient to probable error</i>
+</td></tr>
+<tr><td colspan="8">&nbsp;</td></tr>
+<tr><th colspan="8"> <span class="smcap">A. Sacramento Rainfall and Growth of 18 Sequoias in Dry
+Locations, 1861-1910</span> </th></tr>
+<tr>
+<th colspan="5"></th>
+<th>(<em>r</em>)</th>
+<th>(<em>e</em>)</th>
+<th>(<em>r/e</em>)</th>
+</tr>
+<tr>
+<td class="tdl" colspan="5">1 year of rainfall</td>
+<td>&minus;0.059</td>
+<td>&plusmn;0.096 </td>
+<td>0.6</td>
+</tr>
+<tr>
+<td class="tdl" colspan="5">2 years of rainfall</td>
+<td>+0.288</td>
+<td>&plusmn;0.090</td>
+<td>3.2</td>
+</tr>
+<tr>
+<td class="tdl" colspan="5">3 years of rainfall</td>
+<td>+0.570</td>
+<td>&plusmn;0.066</td>
+<td>8.7</td>
+</tr>
+<tr>
+<td class="tdl" colspan="5">4 years of rainfall</td>
+<td>+0.470</td>
+<td>&plusmn;0.076</td>
+<td>6.2</td>
+</tr>
+<tr><td colspan="8">&nbsp;</td></tr>
+<tr>
+<th colspan="8"><span class="smcap">B. Sacramento Rainfall and Growth of 112 Sequoias Mostly in
+Moist Locations, 1861-1910</span> </th>
+</tr>
+<tr>
+<td class="tdl" colspan="5">3 years of rainfall</td>
+<td>+0.340</td>
+<td>&plusmn;0.087</td>
+<td>3.9</td></tr>
+<tr>
+<td class="tdl" colspan="5">4 years of rainfall</td>
+<td>+0.371</td>
+<td>&plusmn;0.084</td>
+<td>4.5</td>
+</tr>
+<tr><td class="tdl" colspan="5">5 years of rainfall</td>
+<td>+0.398</td>
+<td>&plusmn;0.082</td>
+<td>4.9</td>
+</tr>
+<tr>
+<td class="tdl" colspan="5">6 years of rainfall</td>
+<td>+0.418</td>
+<td>&plusmn;0.079</td>
+<td>5.3</td>
+</tr>
+<tr>
+<td class="tdl" colspan="5">7 years of rainfall</td>
+<td>+0.471</td>
+<td>&plusmn;0.076</td>
+<td>6.2</td>
+</tr>
+<tr>
+<td class="tdl" colspan="5">8 years of rainfall </td>
+<td>(+0.520)</td>
+<td>&plusmn;0.071</td>
+<td>7.3</td>
+</tr>
+<tr>
+<td class="tdl" colspan="5">9 years of rainfall</td>
+<td>+0.575</td>
+<td>&plusmn;0.065</td>
+<td>8.8</td>
+</tr>
+<tr>
+<td class="tdl" colspan="5">10 years of rainfall</td>
+<td>+0.577</td>
+<td>&plusmn;0.065</td>
+<td>8.8</td>
+</tr>
+<tr><td colspan="8">&nbsp;</td></tr>
+<tr>
+<th colspan="8"><span class="smcap">C. Sacramento Rainfall and Growth of 80 Sequoias in Moist
+Locations, 1861-1910</span> </th>
+</tr>
+<tr>
+<td class="tdl" colspan="5">10 years of rainfall</td>
+<td>+0.605</td>
+<td>&plusmn;0.062</td>
+<td>9.8</td>
+</tr>
+<tr><td colspan="8">&nbsp;</td></tr>
+<tr>
+<th colspan="8"><span class="smcap">D. Annual Sequoia Growth and Rainfall of Preceding 5 Years
+At Stations on Southern Pacific Railroad</span></th>
+</tr>
+<tr>
+<th>&nbsp;</th>
+<th><em>Years</em></th>
+<th><em>Altitude</em> <br /> (<em>feet</em>)</th>
+<th><em>Rainfall</em> <br /> (<em>inches</em>)</th>
+<th><em>Approximate <br /> distance from <br /> sequoias</em> (<em>miles</em>)</th>
+     <th>(<em>r</em>)</th>
+     <th>(<em>e</em>)</th>
+     <th>(<em>r/e</em>)</th>
+</tr>
+<tr>
+<td class="tdl">Sacramento,</td>
+<td>1861-1910</td>
+<td>70</td>
+<td>19.40</td>
+<td>200</td>
+<td>+0.398</td>
+<td>&plusmn;0.081</td>
+<td>4.9</td>
+</tr>
+<tr>
+<td class="tdl">Colfax,</td>
+<td>1871-1909</td>
+<td>2400</td>
+<td>48.94</td>
+<td>200</td>
+<td>+0.122</td>
+<td>&plusmn;0.113</td>
+<td>1.1</td>
+</tr>
+<tr><td class="tdl">Summit,</td>
+<td>1871-1909</td>
+<td>7000</td>
+<td>48.07</td>
+<td>200</td>
+<td>+0.148</td>
+<td>&plusmn;0.113</td>
+<td>1.3</td>
+</tr>
+<tr>
+<td class="tdl">Truckee,</td>
+<td>1871-1909</td>
+<td>5800</td>
+<td>27.12</td>
+<td>200</td>
+<td>+0.300</td>
+<td>&plusmn;0.105</td>
+<td>2.9</td>
+</tr>
+<tr>
+<td class="tdl">Boca,</td>
+<td>1871-1909</td>
+<td>5500</td>
+<td>20.34</td>
+<td>200</td>
+<td>+0.604</td>
+<td>&plusmn;0.076</td>
+<td>8.0</td>
+</tr>
+<tr>
+<td class="tdl">Winnemucca,</td>
+<td>1871-1909</td>
+<td>4300</td>
+<td>8.65</td>
+<td>300</td>
+<td>+0.492</td>
+<td>&plusmn;0.089</td>
+<td>5.5</td>
+</tr>
+</table>
+
+<!--File: cc105.png Page 81-->
+<a class="pagenum break" id="Page_81" title="pg 81"></a>
+
+
+
+<p>In the first line of Section A a correlation coefficient
+of only -0.056, which is scarcely six-tenths of the probable
+error, means that there is no appreciable relation
+between the rainfall of a given season and the growth
+during the following spring and summer. The roots of
+the sequoias probably penetrate so deeply that the rain
+and melted snow of the spring months do not sink down
+rapidly enough to influence the trees before the growing
+season comes to an end. The precipitation of two preceding
+seasons, however, has some effect on the trees, as
+appears in the second line of Section A, where the correlation
+coefficient is +0.288, or 3.2 times the probable
+error. When the rainfall of three seasons is taken into
+account the coefficient rises to +0.570, or 8.7 times the
+probable error, while with four years of rainfall the coefficient
+begins to fall off. Thus the growth of these eighteen
+sequoias on relatively dry slopes appears to have depended
+chiefly on the rainfall of the second and third
+preceding rainy seasons. The growth in 1900, for
+example, depended largely on the rainfall in the rainy
+seasons of 1897-1898 and 1898-1899.</p>
+
+<p>Section B of the table shows that with 112 trees, growing
+chiefly in moist depressions where the water supply
+is at a maximum, the correlation between growth and
+rainfall, +0.577 for ten years' rainfall, is even higher
+than with the dry trees. The seepage of the underground
+water is so slow that not until four years' rainfall is
+taken into account is the correlation coefficient more than
+four times the probable error. When only the trees growing
+in moist locations are employed, the coefficient between
+<!--File: cc106.png Page 82-->
+<a class="pagenum break" id="Page_82" title="pg 82"></a>
+tree growth and the rainfall for ten years rises to
+the high figure of +0.605, or 9.8 times the probable error,
+as appears in Section C. These figures, as well as many
+others not here published, make it clear that the curve of
+sequoia growth from 1861 to 1910 affords a fairly close
+indication of the rainfall at Sacramento, provided allowance
+be made for a delay of three to ten years due to the
+fact that the moisture in the soil gradually seeps down
+the mountain-sides and only reaches the sequoias after a
+considerable interval.</p>
+
+<p>If a rainfall record were available for the place where
+the trees actually grow, the relationship would probably
+be still closer.</p>
+
+<p>The record at Fresno, for example, bears out this conclusion
+so far as it goes. But as Fresno lies at a low altitude
+and its rainfall is of essentially the Sacramento
+type, its short record is of less value than that of Sacramento.
+The only rainfall records among the Sierras at
+high levels, where the rainfall and temperature are approximately
+like those of the sequoia region, are found
+along the main line of the Southern Pacific railroad. This
+runs from Oakland northeastward seventy miles across
+the open plain to Sacramento, then another seventy miles,
+as the crow flies, through Colfax and over a high pass
+in the Sierras at Summit, next twenty miles or so down
+through Truckee to Boca, on the edge of the inland basin
+of Nevada, and on northeastward another 160 miles to
+Winnemucca, where it turns east toward Ogden and Salt
+Lake City. Section D of Table 3 shows the correlation
+coefficients between the rainfall along the railroad and
+the growth of the sequoias. At Sacramento, which lies
+fairly open to winds from the Pacific and thus represents
+the general climate of central California, the coefficient
+is nearly five times the probable error, thus indicating a
+<!--File: cc107.png-->
+<a class="pagenum break" id="Page_83" title="pg 83"></a>
+real relation to sequoia growth. Then among the foothills
+of the Sierras at Colfax, the coefficient drops till it is
+scarcely larger than the probable error. It rises rapidly,
+however, as one advances among the mountains, until at
+Boca it attains the high figure of +0.604 or eight times
+the probable error, and continues high in the dry area
+farther east. In other words the growth of the sequoias
+is a good indication of the rainfall where the trees grow
+and in the dry region farther east.</p>
+
+<p>In order to determine the degree to which the sequoia
+record represents the rainfall of other regions, let us
+select Jerusalem for comparison. The reasons for this
+selection are that Jerusalem furnishes the only available
+record that satisfies the following necessary conditions:
+(1) its record is long enough to be important; (2) it is
+located fairly near the latitude of the sequoias, 32&deg;N
+versus 37&deg;N; (3) it is located in a similar type of climate
+with winter rains and a long dry summer; (4) it lies well
+above sea level (2500 feet) and somewhat back from the
+seacoast, thus approximating although by no means
+duplicating the condition of the sequoias; and (5) it lies
+in a region where the evidence of climatic changes during
+historic times is strongest. The ideal place for comparison
+would be the valley in which grow the cedars of Lebanon.
+Those trees resemble the sequoias to an extraordinary
+degree, not only in their location, but in their great age.
+Some day it will be most interesting to compare the
+growth of these two famous groups of old trees.</p>
+
+<p>The correlation coefficients for the sequoia growth
+and the rainfall at Jerusalem are given in Section A,
+Table 4. They are so high and so consistent that they
+scarcely leave room for doubt that where a hundred or
+more sequoias are employed, as in Fig. 5, their curve of
+growth affords a good indication of the fluctuations of
+climate in western Asia. The high coefficient for the
+eleven trees measured by Douglass suggests that where
+the number of trees falls as low as ten, as in the part of
+Fig. 4 from 710 to 840 B. C., the relation between tree
+growth and rainfall is still close even when only one
+year's growth is considered. Where the unit is ten years
+of growth, as in Figs. 4 and 5, the accuracy of the tree
+curve as a measure of rainfall is much greater than when
+a single year is used as in Table 4. When the unit is
+raised to thirty years, as in the smoothed part of Fig. 4
+previous to 240 B. C., even four trees, as from 960 to
+1070, probably give a fair approximation to the general
+changes in rainfall, while a single tree prior to 1110 B. C.
+gives a rough indication.</p>
+<!--File: cc108.png Page 84-->
+<a class="pagenum break" id="Page_84" title="pg 84"></a>
+
+<table id="table_4" class="borders">
+<tr><th colspan="7"><a href="#tables">TABLE 4</a></th></tr>
+<tr><th colspan="7">CORRELATION COEFFICIENTS BETWEEN
+RAINFALL RECORDS<br />IN CALIFORNIA
+AND JERUSALEM</th></tr>
+<tr><td colspan="7">&nbsp;</td></tr>
+<tr><td colspan="7" class="tdi2">
+(<i>r</i>)=<i>Correlation coefficient</i><br />
+(<i>e</i>)=<i>Probable error</i><br />
+(<i>r/e</i>)=<i>Ratio of coefficient to probable error</i>
+</td></tr>
+<tr><td colspan="7">&nbsp;</td></tr>
+<tr><th colspan="7"><span class="smcap">A. Jerusalem Rainfall for 3 Years and Various Groups of
+Sequoias</span><sup><a href="#fn27" id="r27">[27]</a></sup> </th></tr>
+<tr><th colspan="4"></th><th>(<em>r</em>)</th><th>(<em>e</em>)</th><th>(<em>r/e</em>)</th></tr>
+<tr>
+<td colspan="4" class="tdl">11 trees measured by Douglass</td>
+<td>+0.453</td>
+<td>&plusmn;0.078</td>
+<td>5.8</td>
+</tr>
+<tr>
+<td colspan="4" class="tdl">80 trees, moist locations, Groups IA, IIA, IIIA, VA</td><td>+0.500</td>
+<td>&plusmn;0.073</td>
+<td>6.8</td>
+</tr>
+<tr>
+<td colspan="4" class="tdl">101 trees, 69 in moist locations, 32 in dry, I, II, III</td>
+<td> +0.616</td>
+<td>&plusmn;0.061</td>
+<td>10.1</td>
+</tr>
+<tr>
+<td colspan="4" class="tdl">112 trees, 80 in moist locations, 32 in dry, I, II, III, V</td>
+<td>+0.675</td>
+<td>&plusmn;0.053</td>
+<td>12.7</td>
+</tr>
+<tr><td colspan="7">&nbsp;</td></tr>
+ <tr><th colspan="7"><span class="smcap">B. Rainfall at Jerusalem and at Stations in California and Nevada</span></th></tr>
+
+<tr><td colspan="3"></td>
+<td colspan="2">&mdash;&mdash; <em>3 years</em> &mdash;&mdash;</td>
+<td colspan="2">&mdash;&mdash; <em>5 years</em> &mdash;&mdash;</td>
+</tr>
+<tr>
+<th class="tdl">&nbsp;</th>
+<th><em>Altitude</em><br /> (<em>feet</em>)</th>
+<th><em>Years</em></th>
+<th>(<em>r</em>)</th>
+<th>(<em>r/e</em>)</th>
+<th>(<em>r</em>)</th>
+<th>(<em>r/e</em>)</th>
+</tr>
+<tr>
+<td class="tdl">Sacramento,</td>
+<td>70</td>
+<td>1861-1910</td>
+<td>+0.386</td>
+<td>4.7</td>
+<td>+0.352</td>
+<td>4.2</td>
+</tr>
+<tr>
+<td class="tdl">Colfax,</td>
+<td>2400</td>
+<td>1871-1909</td>
+<td>+0.311</td>
+<td>3.1</td>
+<td>+0.308</td>
+<td>3.0</td>
+</tr>
+<tr>
+<td class="tdl">Summit,</td>
+<td>7000</td>
+<td>1871-1909</td>
+<td>+0.099</td>
+<td>0.9</td>
+<td>+0.248</td>
+<td>2.3</td>
+</tr>
+<tr>
+<td class="tdl">Truckee,</td>
+<td>5800</td>
+<td>1871-1909</td>
+<td>+0.229</td>
+<td>2.2</td>
+<td>+0.337</td>
+<td>3.3</td>
+</tr>
+<tr>
+<td class="tdl"><sup><a href="#fn28" id="r28">[28]</a></sup>Boca,</td>
+<td>5500</td>
+<td>1871-1909</td>
+<td>+0.482</td>
+<td>6.4</td>
+<td>+0.617</td>
+<td>8.6</td>
+</tr>
+<tr>
+<td class="tdl">Winnemucca,</td>
+<td>4300</td>
+<td>1871-1909</td>
+<td>+0.235</td>
+<td>2.2</td>
+<td>+0.260</td>
+<td>2.4</td>
+</tr>
+<tr>
+<td class="tdl">San Bernardino,</td>
+<td>1050</td>
+<td>1871-1909</td>
+<td>+0.275</td>
+<td>2.7</td>
+<td>+0.177</td>
+<td>1.8</td>
+</tr>
+
+<tr><td colspan="7">&nbsp;</td></tr>
+<tr><th colspan="7"><span class="smcap">C. Rainfall for 3 Years at California and Nevada Stations, 1871-1909</span></th></tr>
+
+<tr><th colspan="5"></th><th>(<em>r</em>)</th><th>(<em>r/e</em>)</th></tr>
+<tr><td colspan="5" class="tdl">Sacramento and San Bernardino</td><td>+0.663</td> <td>10.7</td></tr>
+<tr><td colspan="5" class="tdl">San Bernardino and Winnemucca</td><td>+0.291</td>  <td>2.8</td></tr>
+</table>
+
+<!--File: cc109.png Page 85-->
+<a class="pagenum break" id="Page_85" title="pg 85"></a>
+
+
+<p>Table 4 shows a peculiar feature in the fact that the
+correlations of Section A between tree growth and the
+rainfall of Jerusalem are decidedly higher than those
+between the rainfall in the two regions. Only at Sacramento
+and Boca are the rainfall coefficients high enough
+to be conclusive. This, however, is not surprising, for
+even between Sacramento and San Bernardino, only 400
+miles apart, the correlation coefficient for the rainfall
+by three-year periods is only 10.7 times the probable
+error, as appears in Section C of Table 4, while between
+San Bernardino and Winnemucca 500 miles away, the
+corresponding figure drops to 2.8. It must be remembered
+that in some respects the growth of the sequoias is
+a much better record of rainfall than are the records kept
+by man. The human record is based on the amount of
+water caught by a little gauge a few inches in diameter.
+Every gust of wind detracts from the accuracy of the
+record; a mile away the rainfall may be double what it
+is at the gauge. Each sequoia, on the other hand, draws
+its moisture from an area thousands of times as large as
+<!--File: cc110.png Page 86-->
+<a class="pagenum break" id="Page_86" title="pg 86"></a>
+a rain gauge. Moreover, the trees on which Figs. 4 and 5
+are based were scattered over an area fifty miles long
+and several hundred square miles in extent. Hence they
+represent the summation of the rainfall over an area
+millions of times as large as that of a rain gauge. This
+fact and the large correlation coefficients between sequoia
+growth and Jerusalem rainfall should be considered in
+connection with the fact that all the coefficients between
+the rainfall of California and Nevada and that of Jerusalem
+are positive. If full records of the complete rainfall
+of California and Nevada on the one hand and of the
+eastern Mediterranean region on the other were available
+for a long period, they would probably agree closely.</p>
+
+<p>Just how widely the sequoias can be used as a measure
+of the climate of the past is not yet certain. In some
+regions, as will shortly be explained, the climatic changes
+seem to have been of an opposite character from those
+of California. In others the Californian or eastern Mediterranean
+type of change seems sometimes to prevail but
+is not always evident. For example, at Malta the rainfall
+today shows a distinct relation to that of Jerusalem and
+to the growth of the sequoias. But the correlation coefficient
+between the rainfall of eight-year periods at Naples,
+a little farther north, and the growth of the sequoias at
+the end of the periods is -0.132, or only 1.4 times the
+probable error and much too small to be significant. This
+is in harmony with the fact that although Naples has
+summer droughts, they are not so pronounced as in California
+and Palestine, and the prevalence of storms is
+much greater. Jerusalem receives only 8 per cent of its
+rain in the seven months from April to October, and
+Sacramento 13, while Malta receives 31 per cent and
+Naples 43. Nevertheless, there is some evidence that in
+the past the climatic fluctuations of southern Italy followed
+<!--File: cc111.png Page 87-->
+<a class="pagenum break" id="Page_87" title="pg 87"></a>
+nearly the same course as those of California and
+Palestine. This apparent discrepancy seems to be explained
+by our previous conclusion that changes of climate
+are due largely to a shifting of storm tracks. When
+sunspots are numerous the storms which now prevail in
+northern Italy seem to be shifted southward and traverse
+the Mediterranean to Palestine just as similar storms
+are shifted southward in the United States. This perhaps
+accounts for the agreement between the sequoia curve
+and the agricultural and social history of Rome from
+about 400 B. C. to 100 A. D., as explained in <em>World Power
+and Evolution</em>. For our present purposes, however, the
+main point is that since rainfall records have been kept
+the fluctuations of climate indicated by the growth of the
+sequoias have agreed closely with fluctuations in the
+rainfall of the eastern Mediterranean region. Presumably
+the same was true in the past. In that case, the sequoia
+curve not only is a good indication of climatic changes or
+pulsations in regions of similar climate, but may serve
+as a guide to coincident but different changes in regions
+of other types.</p>
+
+<p>An enormous body of other evidence points to the same
+conclusion. It indicates that while the average climate
+of the present is drier than that of the past in regions
+having the Mediterranean type of winter rains and
+summer droughts, there have been pronounced pulsations
+during historic times so that at certain times there has
+actually been greater aridity than at present. This conclusion
+is so important that it seems advisable to examine
+the only important arguments that have been raised
+against it, especially against the idea that the general
+rainfall of the eastern Mediterranean was greater in the
+historic past than at present. The first objection is the
+unquestionable fact that droughts and famines have
+<!--File: cc112.png Page 88-->
+<a class="pagenum break" id="Page_88" title="pg 88"></a>
+occurred at periods which seem on other evidence to have
+been moister than the present. This argument has been
+much used, but it seems to have little force. If the rainfall
+of a given region averages thirty inches and varies
+from fifteen to forty-five, a famine will ensue if the rainfall
+drops for a few years to the lower limit and does not
+rise much above twenty for a few years. If the climate of
+the place changes during the course of centuries, so that
+the rainfall averages only twenty inches, and ranges
+from seven to thirty-five, famine will again ensue if the
+rainfall remains near ten inches for a few years. The
+ravages of the first famine might be as bad as those of
+the second. They might even be worse, because when the
+rainfall is larger the population is likely to be greater
+and the distress due to scarcity of food would affect a
+larger number of people. Hence historic records of
+famines and droughts do not indicate that the climate
+was either drier or moister than at present. They merely
+show that at the time in question the climate was drier
+than the normal for that particular period.</p>
+
+<p>The second objection is that deserts existed in the past
+much as at present. This is not a real objection, however,
+for, as we shall see more fully, some parts of the world
+suffer one kind of change and others quite the opposite.
+Moreover, deserts have always existed, and when we talk
+of a change in their climate we merely mean that their
+boundaries have shifted. A concrete example of the mistaken
+use of ancient dryness as proof of climatic uniformity
+is illustrated by the march of Alexander from
+India to Mesopotamia. Hedin gives an excellent presentation
+of the case in the second volume of his <em>Overland
+to India</em>. He shows conclusively that Alexander's army
+suffered terribly from lack of water and provisions. This
+certainly proves that the climate was dry, but it by no
+<!--File: cc113.png Page 89-->
+<a class="pagenum break" id="Page_89" title="pg 89"></a>
+means indicates that there has been no change from the
+past to the present. We do not know whether Alexander's
+march took place during an especially dry or an especially
+wet year. In a desert region like Makran, in
+southern Persia and Beluchistan, where the chief difficulties
+occurred, the rainfall varies greatly from year to
+year. We have no records from Makran, but the conditions
+there are closely similar to those of southern
+Arizona and New Mexico. In 1885 and 1905 the rainfall
+for five stations in that region was as follows:</p>
+
+<table class="borders">
+<tr>
+<th class="tdl"></th>
+<th><em>1885</em></th>
+<th><em>1905</em></th>
+<th><em>Mean rainfall during<br /> period since<br /> observations began</em></th>
+</tr>
+<tr><td class="tdl">Yuma, Arizona,</td>
+<td>2.72</td>
+<td>11.41</td>
+<td>3.13</td>
+</tr>
+<tr>
+<td class="tdl">Phoenix, Arizona,</td>
+<td>3.77</td>
+<td>19.73</td>
+<td>7.27</td>
+</tr>
+<tr>
+<td class="tdl">Tucson, Arizona,</td>
+<td>5.26</td>
+<td>24.17</td>
+<td>11.66</td>
+</tr>
+<tr>
+<td class="tdl">Lordsburg, New Mexico,</td>
+<td>3.99</td>
+<td>19.50</td>
+<td>8.62</td>
+</tr>
+<tr>
+<td class="tdl">El Paso, Texas (on New  Mexico border),</td>
+<td>7.31</td>
+<td>17.80</td>
+<td>9.06</td>
+</tr>
+<tr>
+<th>Average,</th>
+<th class="thl">4.61</th>
+<th class="thl">18.52 </th>
+<th class="thl">7.95</th>
+</tr>
+</table>
+
+<p>These stations are distributed over an area nearly 500
+miles east and west. Manifestly a traveler who spent the
+year 1885 in that region would have had much more difficulty
+in finding water and forage than one who traveled
+in the same places in 1905. During 1885 the rainfall was
+42 per cent less than the average, and during 1905 it was
+134 per cent more than the average. Let us suppose, for
+the sake of argument, that the average rainfall of southeastern
+Persia is six inches today and was ten inches in
+the days of Alexander. If the rainfall from year to year
+varied as much in the past in Persia as it does now in
+New Mexico and Arizona, the rainfall during an ancient
+<!--File: cc114.png Page 90-->
+<a class="pagenum break" id="Page_90" title="pg 90"></a>
+dry year, corresponding in character to 1885, would have
+been about 5.75 inches. On the other hand, if we suppose
+that the rainfall then averaged less than at present,&mdash;let
+us say four inches,&mdash;a wet year corresponding to 1905 in
+the American deserts might have had a rainfall of about
+ten inches. This being the case, it is clear that our estimate
+of what Alexander's march shows as to climate
+must depend largely on whether 325 B. C. was a wet year
+or a dry year. Inasmuch as we know nothing about this,
+we must fall back on the fact that a large army accomplished
+a journey in a place where today even a small
+caravan usually finds great difficulty in procuring forage
+and water. Moreover, elephants were taken 180 miles
+across what is now an almost waterless desert, and yet
+the old historians make no comment on such a feat which
+today would be practically impossible. These things seem
+more in harmony with a change of climate than with
+uniformity. Nevertheless, it is not safe to place much
+reliance on them except when they are taken in conjunction
+with other evidence, such as the numerous ruins,
+which show that Makran was once far more densely
+populated than now seems possible. Taken by itself, such
+incidents as Alexander's march cannot safely be used
+either as an argument for or against changes of climate.</p>
+
+<p>The third and strongest objection to any hypothesis
+of climatic changes during historic times is based on
+vegetation. The whole question is admirably set forth by
+J. W. Gregory,<sup><a href="#fn29" id="r29">[29]</a>
+</sup> who gives not only his own results, but
+those of the ablest scholars who have preceded him. His
+conclusions are important because they represent one of
+the few cases where a definite statistical attempt has been
+made to prove the exact condition of the climate of the
+<!--File: cc115.png Page 91-->
+<a class="pagenum break" id="Page_91" title="pg 91"></a>
+past. After stating various less important reasons for
+believing that the climate of Palestine has not changed,
+he discusses vegetation. The following quotation indicates
+his line of thought. A sentence near the beginning
+is italicized in order to call attention to the importance
+which Gregory and others lay on this particular kind of
+evidence:</p>
+
+<blockquote>
+<p>Some more certain test is necessary than the general conclusions
+which can be based upon the historical and geographical
+evidence of the Bible. In the absence of rain gauge and thermometric
+records, <em>the most precise test of climate is given by the
+vegetation; and fortunately the palm affords a very delicate test
+of the past climate of Palestine and the eastern Mediterranean</em>....
+The date palm has three limits of growth which are determined
+by temperature; thus it does not reach full maturity or
+produce ripe fruit of good quality below the mean annual temperature
+of 69&deg;F. The isothermal of 69&deg; crosses southern Algeria
+near Biskra; it touches the northern coasts of Cyrenaica near
+Derna and passes Egypt near the mouth of the Nile, and then
+bends northward along the coast lands of Palestine.</p>
+
+<p>To the north of this line the date palm grows and produces
+fruit, which only ripens occasionally, and its quality deteriorates
+as the temperature falls below 69&deg;. Between the isotherms of
+68&deg; and 64&deg;, limits which include northern Algeria, most of
+Sicily, Malta, the southern parts of Greece and northern Syria,
+the dates produced are so unripe that they are not edible. In the
+next cooler zone, north of the isotherm of 62&deg;, which enters
+Europe in southwestern Portugal, passes through Sardinia,
+enters Italy near Naples, crosses northern Greece and Asia
+Minor to the east of Smyrna, the date palm is grown only for
+its foliage, since it does not fruit.</p>
+
+<p>Hence at Benghazi, on the north African coast, the date palm
+is fertile, but produces fruit of poor quality. In Sicily and at
+Algiers the fruit ripens occasionally and at Rome and Nice the
+palm is grown only as an ornamental tree.<a class="pagenum break" id="Page_92" title="pg 92"></a></p>
+
+<p>The date palm therefore affords a test of variations in mean
+annual temperature of three grades between 62&deg; and 69&deg;.</p>
+
+<p>This test shows that the mean annual temperature of Palestine
+has not altered since Old Testament times. The palm tree now
+grows dates on the coast of Palestine and in the deep depression
+around the Dead Sea, but it does not produce fruit on the highlands
+of Judea. Its distribution in ancient times, as far as we
+can judge from the Bible, was exactly the same. It grew at
+"Jericho, the city of palm trees" (Deut. xxxiv: 3 and 2 Chron.
+xxviii: 15), and at Engedi, on the western shore of the Dead
+Sea (2 Chron. xx: 2; Sirach xxiv: 14); and though the palm
+does not still live at Jericho&mdash;the last apparently died in 1838&mdash;its
+disappearance must be due to neglect, for the only climatic
+change that would explain it would be an increase in cold or
+moisture. In olden times the date palm certainly grew on the
+highlands of Palestine; but apparently it never produced fruit
+there, for the Bible references to the palm are to its beauty and
+erect growth: "The righteous shall flourish like the palm" (Ps.
+xcii: 12); "They are upright as the palm tree" (Jer. x: 5);
+"Thy stature is like to a palm tree" (Cant. vii: 7). It is used as
+a symbol of victory (Rev. vii: 9), but never praised as a source
+of food.</p>
+
+<p>Dates are not once referred to in the text of the Bible, but
+according to the marginal notes the word translated "honey" in
+2 Chron. xxxi: 5 may mean dates....</p>
+
+<p>It appears, therefore, that the date palm had essentially the
+same distribution in Palestine in Old Testament times as it has
+now; and hence we may infer that the mean temperature was
+then the same as now. If the climate had been moister and cooler,
+the date could not have flourished at Jericho. If it had been
+warmer, the palms would have grown freely at higher levels and
+Jericho would not have held its distinction as <em>the</em> city of palm
+trees.<sup><a href="#fn30" id="r30">[30]</a></sup></p>
+</blockquote>
+
+<p>In the main Gregory's conclusions seem to be well
+grounded, although even according to his data a change
+<!--File: cc117.png Page 93-->
+<a class="pagenum break" id="Page_93" title="pg 93"></a>
+of 2&deg; or 3&deg; in mean temperature would be perfectly
+feasible. It will be noticed, however, that they apply to
+temperature and not to rainfall. They merely prove that
+two thousand years ago the mean temperature of Palestine
+and the neighboring regions was not appreciably different
+from what it is today. This, however, is in no sense
+out of harmony with the hypothesis of climatic pulsations.
+Students of glaciation believe that during the last
+glacial epoch the mean temperature of the earth as a
+whole was only 5&deg; or 6&deg;C. lower than at present. If the
+difference between the climate of today and of the time of
+Christ is a tenth as great as the difference between the
+climate of today and that which prevailed at the culmination
+of the last glacial epoch, the change in two thousand
+years has been of large dimensions. Yet this would require
+a rise of only half a degree Centigrade in the mean
+temperature of Palestine. Manifestly, so slight a change
+would scarcely be detectable in the vegetation.</p>
+
+<p>The slightness of changes in mean temperature as compared
+with changes in rainfall may be judged from a
+comparison of wet and dry years in various regions. For
+example, at Berlin between 1866 and 1905 the ten most
+rainy years had an average precipitation of 670 mm. and
+a mean temperature of 9.15&deg;C. On the other hand, the ten
+years of least rainfall had an average of 483 mm. and a
+mean temperature of 9.35&deg;. In other words, a difference
+of 137 mm., or 39 per cent, in rainfall was accompanied
+by a difference of only 0.2&deg;C. in temperature. Such contrasts
+between the variability of mean rainfall and mean
+temperature are observable not only when individual
+years are selected, but when much longer periods are
+taken. For instance, in the western Gulf region of the
+United States the two inland stations of Vicksburg, Mississippi,
+and Shreveport, Louisiana, and the two maritime
+<!--File: cc118.png Page 94-->
+<a class="pagenum break" id="Page_94" title="pg 94"></a>
+stations of New Orleans, Louisiana, and Galveston,
+Texas, lie at the margins of an area about 400 miles long.
+During the ten years from 1875 to 1884 their rainfall
+averaged 59.4 inches,<sup><a href="#fn31" id="r31">[31]</a></sup> while during the ten years from
+1890 to 1899 it averaged only 42.4 inches. Even in a
+region so well watered as the Gulf States, such a change&mdash;40
+per cent more in the first decade than in the second&mdash;is
+important, and in drier regions it would have a great
+effect on habitability. Yet in spite of the magnitude of
+the change the mean temperature was not appreciably
+different, the average for the four stations being 67.36&deg;F.
+during the more rainy decade and 66.94&deg;F. during the
+less rainy decade&mdash;a difference of only 0.42&deg;F. It is worth
+noticing that in this case the wetter period was also the
+warmer, whereas in Berlin it was the cooler. This is
+probably because a large part of the moisture of the Gulf
+States is brought by winds having a southerly component.
+Similar relationships are apparent in other
+places. We select Jerusalem because we have been discussing
+Palestine. At the time of writing, the data available
+in the <em>Quarterly Journal of the Palestine Exploration
+Fund</em> cover the years from 1882-1899 and 1903-1909.
+Among these twenty-five years the thirteen which had
+most rain had an average of 34.1 inches and a temperature
+of 62.04&deg;F. The twelve with least rain had 24.4 inches
+and a temperature of 62.44&deg;. A difference of 40 per cent
+in rainfall was accompanied by a difference of only
+0.4&deg;F. in temperature.</p>
+
+<p>The facts set forth in the preceding paragraphs seem
+to show that extensive changes in precipitation and
+storminess can take place without appreciable changes of
+mean temperature. If such changed conditions can persist
+<!--File: cc119.png Page 95-->
+<a class="pagenum break" id="Page_95" title="pg 95"></a>
+for ten years, as in one of our examples, there is no
+logical reason why they cannot persist for a hundred or
+a thousand. The evidence of changes in climate during the
+historic period seems to suggest changes in precipitation
+much more than in temperature. Hence the strongest of
+all the arguments against historic changes of climate
+seems to be of relatively little weight, and the pulsatory
+hypothesis seems to be in accord with all the known facts.</p>
+
+<p>Before the true nature of climatic changes, whether
+historic or geologic, can be rightly understood, another
+point needs emphasis. When the pulsatory hypothesis
+was first framed, it fell into the same error as the hypotheses
+of uniformity and of progressive change&mdash;that
+is, the assumption was made that the whole world is
+either growing drier or moister with each pulsation. A
+study of the ruins of Yucatan, in 1912, and of Guatemala,
+in 1913, as is explained in <em>The Climatic Factor</em>, has led to
+the conclusion that the climate of those regions has
+changed in the opposite way from the changes which
+appear to have taken place in the desert regions farther
+south. These Maya ruins in Central America are in many
+cases located in regions of such heavy rainfall, such dense
+forests, and such malignant fevers that habitation is now
+practically impossible. The land cannot be cultivated
+except in especially favorable places. The people are
+terribly weakened by disease and are among the lowest
+in Central America. Only a hundred miles from the unhealthful
+forests we find healthful areas, such as the
+coasts of Yucatan and the plateau of Guatemala. Here
+the vast majority of the population is gathered, the large
+towns are located, and the only progressive people are
+found. Nevertheless, in the past the region of the forests
+was the home of by far the most progressive people who
+are ever known to have lived in America previous to the
+<!--File: cc120.png Page 96-->
+<a class="pagenum break" id="Page_96" title="pg 96"></a>
+days of Columbus. They alone brought to high perfection
+the art of sculpture; they were the only American people
+who invented the art of writing. It seems scarcely credible
+that such a people would have lived in the worst possible
+habitat when far more favored regions were close
+at hand. Therefore it seems as if the climate of eastern
+Guatemala and Yucatan must have been relatively dry
+at some past time. The Maya chronology and traditions
+indicate that this was probably at the same time when
+moister conditions apparently prevailed in the subarid
+or desert portions of the United States and Asia. Fig. 3
+shows that today at times of many sunspots there is
+a similar opposition between a tendency toward storminess
+and rain in subtropical regions and toward aridity
+in low latitudes near the heat equator.</p>
+
+<p>Thus our final conclusion is that during historic times
+there have been pulsatory changes of climate. These
+changes have been of the same type in regions having
+similar kinds of climate, but of different and sometimes
+opposite types in places having diverse climates. As to
+the cause of the pulsations, they cannot have been due to
+the precession of the equinoxes nor apparently to any
+allied astronomical cause, for the time intervals are too
+short and too irregular. They cannot have been due to
+changes in the percentage of carbon dioxide in the atmosphere,
+for not even the strongest believers in the climatic
+efficacy of that gas hold that its amount could fluctuate in
+any such violent way as would be necessary to explain
+the pulsations shown in the California curve of tree
+growth. Volcanic activity seems more probable as at least
+a partial cause, and it would be worth while to investigate
+the matter more fully. Nevertheless, it can apparently
+be only a minor cause. In the first place, the main effect
+of a cloud of dust is to alter the temperature, but
+<!--File: cc121.png Page 97-->
+<a class="pagenum break" id="Page_97" title="pg 97"></a>
+Gregory's summary of the palm and the vine shows that
+variations in temperature are apparently of very slight
+importance during historic times. Again, ruins on the
+bottoms of enclosed salt lakes, old beaches now under the
+water, and signs of irrigation ditches where none are now
+needed indicate a climate drier than the present. Volcanic
+dust, however, cannot account for such a condition,
+for at present the air seems to be practically free
+from such dust for long periods. Thus we now experience
+the greatest extreme which the volcanic hypothesis permits
+in one direction, but there have been greater extremes
+in the same direction. The thermal solar hypothesis
+is likewise unable to explain the observed phenomena,
+for neither it nor the volcanic hypothesis offers any explanation
+of why the climate varies in one way in Mediterranean
+climates and in an opposite way in regions
+near the heat equator.</p>
+
+This leaves the cyclonic hypothesis. It seems to fit the
+facts, for variations in cyclonic storms cause some
+regions to be moister and others drier than usual. At the
+same time the variations in temperature are slight, and
+are apparently different in different regions, some places
+growing warm when others grow cool. In the next chapter
+we shall study this matter more fully, for it can best
+be appreciated by examining the course of events in a
+specific century.
+<!--File: cc122.png Page 98-->
+<a class="pagenum break" id="Page_98" title="pg 98"></a>
+
+
+
+
+<h2 id="Chapt_VI"><a href="#content">CHAPTER VI</a></h2>
+
+<h3>THE CLIMATIC STRESS OF THE FOURTEENTH
+CENTURY</h3>
+
+
+<p>In order to give concreteness to our picture of the
+climatic pulsations of historic times let us take a
+specific period and see how its changes of climate
+were distributed over the globe and how they are related
+to the little changes which now take place in the sunspot
+cycle. We will take the fourteenth century of the Christian
+era, especially the first half. This period is chosen
+because it is the last and hence the best known of the
+times when the climate of the earth seems to have taken
+a considerable swing toward the conditions which now
+prevail when the sun is most active, and which, if intensified,
+would apparently lead to glaciation. It has already
+been discussed in <em>World Power and Evolution</em>, but its
+importance and the fact that new evidence is constantly
+coming to light warrant a fuller discussion.</p>
+
+<p>To begin with Europe; according to the careful account
+of Pettersson<sup><a href="#fn32" id="r32">[32]</a></sup> the fourteenth century shows</p>
+
+<blockquote>
+<p>a record of extreme climatic variations. In the cold winters the
+rivers Rhine, Danube, Thames, and Po were frozen for weeks
+and months. On these cold winters there followed violent floods,
+so that the rivers mentioned inundated their valleys. Such floods
+are recorded in 55 summers in the 14th century. There is, of
+<!--File: cc123.png Page 99-->
+<a class="pagenum break" id="Page_99" title="pg 99"></a>
+course, nothing astonishing in the fact that the inundations of
+the great rivers of Europe were more devastating 600 to 700
+years ago than in our days, when the flow of the rivers has been
+regulated by canals, locks, etc.; but still the inundations in the
+13th and 14th centuries must have surpassed everything of that
+kind which has occurred since then. In 1342 the waters of the
+Rhine rose so high that they inundated the city of Mayence and
+the Cathedral "usque ad cingulum hominis." The walls of
+Cologne were flooded so that they could be passed by boats in
+July. This occurred also in 1374 in the midst of the month of
+February, which is of course an unusual season for disasters of
+the kind. Again in other years the drought was so intense that
+the same rivers, the Danube, Rhine, and others, nearly dried up,
+and the Rhine could be forded at Cologne. This happened at least
+twice in the same century. There is one exceptional summer of
+such evil record that centuries afterwards it was spoken of as
+"the old hot summer of 1357."</p>
+</blockquote>
+
+<p>Pettersson goes on to speak of two oceanic phenomena
+on which the old chronicles lay greater stress than on
+all others:</p>
+
+<blockquote>
+<p>The first [is] the great storm-floods on the coast of the North
+Sea and the Baltic, which occurred so frequently that not less
+than nineteen floods of a destructiveness unparalleled in later
+times are recorded from the 14th century. The coastline of the
+North Sea was completely altered by these floods. Thus on
+January 16, 1300, half of the island Heligoland and many other
+islands were engulfed by the sea. The same fate overtook the
+island of Borkum, torn into several islands by the storm-flood of
+January 16, which remoulded the Frisian Islands into their
+present shape, when also Wendingstadt, on the island of Sylt,
+and Thiryu parishes were engulfed. This flood is known under
+the name of "the great man-drowning." The coasts of the Baltic
+also were exposed to storm-floods of unparalleled violence. On
+November 1, 1304, the island of Ruden was torn asunder from
+Rugen by the force of the waves. Time does not allow me to
+dwell upon individual disasters of this kind, but it will be well
+<!--File: cc124.png Page 100-->
+<a class="pagenum break" id="Page_100" title="pg 100"></a>
+to note that of the nineteen great floods on record eighteen
+occurred in the cold season between the autumnal and vernal
+equinoxes.</p>
+
+<p>The second remarkable phenomenon mentioned by the chronicles
+is the freezing of the entire Baltic, which occurred many
+times during the cold winters of these centuries. On such occasions
+it was possible to travel with carriages over the ice from
+Sweden to Bornholm and from Denmark to the German coast
+(Lubeck), and in some cases even from Gotland to the coast of
+Estland.</p></blockquote>
+
+<p>Norlind<sup><a href="#fn33" id="r33">[33]</a></sup> says that "the only authentic accounts" of
+the complete freezing of the Baltic in the neighborhood
+of the Kattegat are in the years 1296, 1306, 1323, and
+1408. Of these 1296 is "much the most uncertain," while
+1323 was the coldest year ever recorded, as appears from
+the fact that horses and sleighs crossed regularly from
+Sweden to Germany on the ice.</p>
+
+<p>Not only central Europe and the shores of the North
+Sea were marked by climatic stress during the fourteenth
+century, but Scandinavia also suffered. As Pettersson
+puts it:</p>
+
+<blockquote>
+<p>On examining the historic (data) from the last centuries of
+the Middle Ages, Dr. Bull of Christiania has come to the conclusion
+that the decay of the Norwegian kingdom was not so
+much a consequence of the political conditions at that time, as
+of the frequent failures of the harvest so that corn [wheat] for
+bread had to be imported from L&uuml;beck, Rostock, Wismar and so
+forth. The Hansa Union undertook the importation and obtained
+political power by its economic influence. The Norwegian
+land-owners were forced to lower their rents. The population
+decreased and became impoverished. The revenue sank 60 to 70
+per cent. Even the income from Church property decreased.
+<!--File: cc125.png Page 101-->
+<a class="pagenum break" id="Page_101" title="pg 101"></a>
+In 1367 corn was imported from L&uuml;beck to a value of one-half
+million kroner. The trade balance inclined to the disadvantage
+of Norway whose sole article of export at that time was
+dried fish. (The production of fish increased enormously in the
+Baltic regions off south Sweden because of the same changes
+which were influencing the lands, but this did not benefit Norway.)
+Dr. Bull draws a comparison with the conditions described
+in the Sagas when Nordland [at the Arctic Circle] produced
+enough corn to feed the inhabitants of the country. At the time
+of Asbj&ouml;rn Selsbane the chieftains in Trondhen&auml;s [still farther
+north in latitude 69&deg;] grew so much corn that they did not need
+to go southward to buy corn unless three successive years of
+dearth had occurred. The province of Trondheim exported wheat
+to Iceland and so forth. Probably the turbulent political state
+of Scandinavia at the end of the Middle Ages was in a great
+measure due to unfavorable climatic conditions, which lowered
+the standard of life, and not entirely to misgovernment and
+political strife as has hitherto been taken for granted.</p>
+</blockquote>
+
+<p>During this same unfortunate first half of the fourteenth
+century England also suffered from conditions
+which, if sufficiently intensified, might be those of a glacial
+period. According to Thorwald Rogers<sup><a href="#fn34" id="r34">[34]</a></sup> the severest
+famine ever experienced in England was that of 1315-1316,
+and the next worst was in 1321. In fact, from 1308
+to 1322 great scarcity of food prevailed most of the time.
+Other famines of less severity occurred in 1351 and 1369.
+"The same cause was at work in all these cases," says
+Rogers, "incessant rain, and cold, stormy summers. It
+is said that the inclemency of the seasons affected the
+cattle, and that numbers perished from disease and
+want." After the bad harvest of 1315 the price of wheat,
+which was already high, rose rapidly, and in May, 1316,
+was about five times the average. For a year or more
+thereafter it remained at three or four times the ordinary
+<!--File: cc126.png Page 102-->
+<a class="pagenum break" id="Page_102" title="pg 102"></a>
+level. The severity of the famine may be judged from the
+fact that previous to the Great War the most notable
+scarcity of wheat in modern England and the highest
+relative price was in December, 1800. At that time wheat
+cost nearly three times the usual amount, instead of five
+as in 1316. During the famine of the early fourteenth century
+"it is said that people were reduced to subsist upon
+roots, upon horses and dogs, and stories are told of even
+more terrible acts by reason of the extreme famine." The
+number of deaths was so great that the price of labor
+suffered a permanent rise of at least 10 per cent. There
+simply were not people enough left among the peasants
+to do the work demanded by the more prosperous class
+who had not suffered so much.</p>
+
+<p>After the famine came drought. The year 1325 appears
+to have been peculiarly dry, and 1331, 1344, 1362, 1374,
+and 1377 were also dry. In general these conditions do
+little harm in England. They are of interest chiefly as
+showing how excessive rain and drought are apt to
+succeed one another.</p>
+
+<p>These facts regarding northern and central Europe
+during the fourteenth century are particularly significant
+when compared with the conclusions which we have
+drawn in <em>Earth and Sun</em> from the growth of trees in
+Germany and from the distribution of storms. A careful
+study of all the facts shows that we are dealing with two
+distinct types of phenomena. In the first place, the climate
+of central Europe seems to have been peculiarly continental
+during the fourteenth century. The winters were
+so cold that the rivers froze, and the summers were so
+wet that there were floods every other year or oftener.
+This seems to be merely an intensification of the conditions
+which prevail at the present time during periods of
+many sunspots, as indicated by the growth of trees at
+<!--File: cc127.png Page 103-->
+<a class="pagenum break" id="Page_103" title="pg 103"></a>
+Eberswalde in Germany and by the number of storms in
+winter as compared with summer. The prevalence of
+droughts, especially in the spring, is also not inconsistent
+with the existence of floods at other seasons, for one of
+the chief characteristics of a continental climate is that
+the variations from one season to another are more
+marked than in oceanic climates. Even the summer
+droughts are typically continental, for when continental
+conditions prevail, the difference between the same season
+in different years is extreme, as is well illustrated in
+Kansas. It must always be remembered that what causes
+famine is not so much absolute dryness as a temporary
+diminution of the rainfall.</p>
+
+<p>The second type of phenomena is peculiarly oceanic in
+character. It consists of two parts, both of which are
+precisely what would be expected if a highly continental
+climate prevailed over the land. In the first place, at certain
+times the cold area of high pressure, which is the
+predominating characteristic of a continent during the
+winter, apparently spread out over the neighboring
+oceans. Under such conditions an inland sea, such as the
+Baltic, would be frozen, so that horses could cross the ice
+even in the Far West. In the second place, because of the
+unusually high pressure over the continent, the barometric
+gradients apparently became intensified. Hence at
+the margin of the continental high-pressure area the
+winds were unusually strong and the storms of corresponding
+severity. Some of these storms may have
+passed entirely along oceanic tracks, while others invaded
+the borders of the land, and gave rise to the floods
+and to the wearing away of the coast described by
+Pettersson.</p>
+
+<p>Turning now to the east of Europe, Br&uuml;ckner's<sup><a href="#fn35" id="r35">[35]</a></sup> study
+<!--File: cc128.png-->
+<a class="pagenum break" id="Page_104" title="pg 104"></a>
+of the Caspian Sea shows that that region as well as
+western Europe was subject to great climatic vicissitudes
+in the first half of the fourteenth century. In 1306-1307
+the Caspian Sea, after rising rapidly for several years,
+stood thirty-seven feet above the present level and it
+probably rose still higher during the succeeding decades.
+At least it remained at a high level, for Hamdulla, the
+Persian, tells us that in 1325 a place called Aboskun was
+under water.<sup><a href="#fn36" id="r36">[36]</a></sup></p>
+
+<p>Still further east the inland lake of Lop Nor also rose
+at about this time. According to a Chinese account the
+Dragon Town on the shore of Lop Nor was destroyed by
+a flood. From Himley's translation it appears that the
+level of the lake rose so as to overwhelm the city completely.
+This would necessitate the expansion of the lake
+to a point eighty miles east of Lulan, and fully fifty from
+the present eastern end of the Kara Koshun marsh. The
+water would have to rise nearly, or quite, to a strand
+which is now clearly visible at a height of twelve feet
+above the modern lake or marsh.</p>
+
+<p>In India the fourteenth century was characterized by
+what appears to have been the most disastrous drought
+in all history. Apparently the decrease in rainfall here
+was as striking as the increase in other parts of the
+world. No statistics are available but we are told that in
+the great famine which began in 1344 even the Mogul
+emperor was unable to obtain the necessaries of life for
+his household. No rain worth mentioning fell for years.
+In some places the famine lasted three or four years, and
+in some twelve, and entire cities were left without an inhabitant.
+In a later famine, 1769-1770, which occurred in
+Bengal shortly after the foundation of British rule in
+<!--File: cc129.png Page 105-->
+<a class="pagenum break" id="Page_105" title="pg 105"></a>
+India, but while the native officials were still in power,
+a third of the population, or ten out of thirty millions,
+perished. The famine in the first half of the fourteenth
+century seems to have been far worse. These Indian
+famines were apparently due to weak summer monsoons
+caused presumably by the failure of central Asia to warm
+up as much as usual. The heavier snowfall, and the
+greater cloudiness of the summer there, which probably
+accompanied increased storminess, may have been the
+reason.</p>
+
+<p>The New World as well as the Old appears to have
+been in a state of climatic stress during the first half of
+the fourteenth century. According to Pettersson, Greenland
+furnishes an example of this. At first the inhabitants
+of that northland were fairly prosperous and were able
+to approach from Iceland without much hindrance from
+the ice. Today the North Atlantic Ocean northeast of
+Iceland is full of drift ice much of the time. The border
+of the ice varies from season to season, but in general it
+extends westward from Iceland not far from the Arctic
+circle and then follows the coast of Greenland southward
+to Cape Farewell at the southern tip and around to
+the western side for fifty miles or more. Except under
+exceptional circumstances a ship cannot approach the
+coast until well northward on the comparatively ice-free
+west coast. In the old Sagas, however, nothing is said of
+ice in this region. The route from Iceland to Greenland
+is carefully described. In the earliest times it went from
+Iceland a trifle north of west so as to approach the coast
+of Greenland after as short an ocean passage as possible.
+Then it went down the coast in a region where approach
+is now practically impossible because of the ice. At that
+time this coast was icy close to the shore, but there is no
+sign that navigation was rendered difficult as is now the
+<!--File: cc130.png Page 106-->
+<a class="pagenum break" id="Page_106" title="pg 106"></a>
+case. Today no navigator would think of keeping close
+inland. The old route also went <em>north</em> of the island on
+which Cape Farewell is located, although the narrow
+channel between the island and the mainland is now so
+blocked with ice that no modern vessel has ever penetrated
+it. By the thirteenth century, however, there appears
+to have been a change. In the Kungaspegel or
+<em>Kings' Mirror</em>, written at that time, navigators are
+warned not to make the east coast too soon on account
+of ice, but no new route is recommended in the neighborhood
+of Cape Farewell or elsewhere. Finally, however,
+at the end of the fourteenth century, nearly 150 years
+after the Kungaspegel, the old sailing route was abandoned,
+and ships from Iceland sailed directly southwest
+to avoid the ice. As Pettersson says:</p>
+
+<blockquote>
+<p>... At the end of the thirteenth and the beginning of the
+fourteenth century the European civilization in Greenland was
+wiped out by an invasion of the aboriginal population. The colonists
+in the Vesterbygd were driven from their homes and
+probably migrated to America leaving behind their cattle in the
+fields. So they were found by Ivar Bardsson, steward to the
+Bishop of Gardar, in his official journey thither in 1342.</p>
+
+<p>The Eskimo invasion must not be regarded as a common raid.
+It was the transmigration of a people, and like other big movements
+of this kind [was] impelled by altered conditions of
+nature, in this case the alterations of climate caused by [or
+which caused?] the advance of the ice. For their hunting and
+fishing the Eskimos require an at least partially open arctic
+sea. The seal, their principal prey, cannot live where the surface
+of the sea is entirely frozen over. The cause of the favorable
+conditions in the Viking-age was, according to my hypothesis,
+that the ice then melted at a higher latitude in the arctic seas.</p>
+
+<p>The Eskimos then lived further north in Greenland and
+North America. When the climate deteriorated and the sea which
+gave them their living was closed by ice the Eskimos had to find
+<!--File: cc131.png Page 107-->
+<a class="pagenum break" id="Page_107" title="pg 107"></a>
+a more suitable neighborhood. This they found in the land
+colonized by the Norsemen whom they attacked and finally
+annihilated.</p>
+</blockquote>
+
+<p>Finally, far to the south in Yucatan the ancient Maya
+civilization made its last flickering effort at about this
+time. Not much is known of this but in earlier periods
+the history of the Mayas seems to have agreed quite
+closely with the fluctuations in climate.<sup><a href="#fn37" id="r37">[37]</a></sup> Among the
+Mayas, as we have seen, relatively dry periods were the
+times of greatest progress.</p>
+
+<p>Let us turn now to Fig. 3 once more and compare the
+climatic conditions of the fourteenth century with those
+of periods of increasing rainfall. Southern England,
+Ireland, and Scandinavia, where the crops were ruined
+by extensive rain and storms in summer, are places
+where storminess and rainfall now increase when sunspots
+are numerous. Central Europe and the coasts of the
+North Sea, where flood and drought alternated, are regions
+which now have relatively less rain when sunspots
+increase than when they diminish. However, as appears
+from the trees measured by Douglass, the winters become
+more continental and hence cooler, thus corresponding to
+the cold winters of the fourteenth century when people
+walked on the ice from Scandinavia to Denmark. When
+such high pressure prevails in the winter, the total rainfall
+is diminished, but nevertheless the storms are more
+severe than usual, especially in the spring. In southeastern
+Europe, the part of the area whence the Caspian
+derives its water, appears to have less rainfall during
+times of increasing sunspots than when sunspots are few,
+but in an equally large area to the south, where the mountains
+<!--File: cc132.png Page 108-->
+<a class="pagenum break" id="Page_108" title="pg 108"></a>
+are higher and the run-off of the rain is more rapid,
+the reverse is the case. This seems to mean that a slight
+diminution in the water poured in by the Volga would
+be more than compensated by the water derived from
+Persia and from the Oxus and Jaxartes rivers, which in
+the fourteenth century appear to have filled the Sea of
+Aral and overflowed in a large stream to the Caspian.
+Still farther east in central Asia, so far as the records go,
+most of the country receives more rain when sunspots
+are many than when they are few, which would agree
+with what happened when the Dragon Town was inundated.
+In India, on the contrary, there is a large area
+where the rainfall diminishes at times of many sunspots,
+thus agreeing with the terrible famine from which the
+Moguls suffered so severely. In the western hemisphere,
+Greenland, Arizona, and California are all parts of the
+area where the rain increases with many sunspots, while
+Yucatan seems to lie in an area of the opposite type. Thus
+all the evidence seems to show that at times of climatic
+stress, such as the fourteenth century, the conditions
+are essentially the same as those which now prevail at
+times of increasing sunspots.</p>
+
+<p>As to the number of sunspots, there is little evidence
+previous to about 1750. Yet that little is both interesting
+and important. Although sunspots have been observed
+with care in Europe only a little more than three centuries,
+the Chinese have records which go back nearly to
+the beginning of the Christian era. Of course the records
+are far from perfect, for the work was done by individuals
+and not by any great organization which continued
+the same methods from generation to generation.
+The mere fact that a good observer happened to use his
+smoked glass to advantage may cause a particular period
+to appear to have an unusual number of spots. On the
+<!--File: cc133.png Page 109-->
+<a class="pagenum break" id="Page_109" title="pg 109"></a>
+other hand, the fact that such an observer finds spots
+at some times and not at others tends to give a valuable
+check on his results, as does the comparison of one
+observer's work with that of another. Hence, in spite of
+many and obvious defects, most students of the problem
+agree that the Chinese record possesses much value, and
+that for a thousand years or more it gives a fairly true
+idea of the general aspect of the sun. In the Chinese
+records the years with many spots fall in groups, as
+would be expected, and are sometimes separated by long
+intervals. Certain centuries appear to have been marked
+by unusual spottedness. The most conspicuous of these
+is the fourteenth, when the years 1370 to 1385 were particularly
+noteworthy, for spots large enough to be visible
+to the naked eye covered the sun much of the time. Hence
+Wolf,<sup><a href="#fn38" id="r38">[38]</a></sup> who has made an exhaustive study of the matter,
+concludes that there was an absolute maximum of spots
+about 1372. While this date is avowedly open to question,
+the great abundance of sunspots at that time makes it
+probable that it cannot be far wrong. If this is so, it
+seems that the great climatic disturbances of which we
+have seen evidence in the fourteenth century occurred at
+a time when sunspots were increasing, or at least when
+solar activity was under some profoundly disturbing influence.
+Thus the evidence seems to show not merely that
+the climate of historic times has been subject to important
+pulsations, but that those pulsations were magnifications
+of the little climatic changes which now take
+place in sunspot cycles. The past and the present are
+apparently a unit except as to the intensity of the
+changes.</p>
+
+
+<!--File: cc134.png Page 110-->
+<a class="pagenum break" id="Page_110" title="pg 110"></a>
+
+
+
+
+<h2 id="Chapt_VII"><a href="#content">CHAPTER VII</a></h2>
+
+<h3>GLACIATION ACCORDING TO THE SOLAR-CYCLONIC
+HYPOTHESIS<sup><a href="#fn39" id="r39">[39]</a></sup></h3>
+
+
+<p>The remarkable phenomena of glacial periods
+afford perhaps the best available test to which
+any climatic hypothesis can be subjected. In this
+chapter and the two that follow, we shall apply this test.
+Since much more is known about the recent Great Ice
+Age, or Pleistocene glaciation, than about the more
+ancient glaciations, the problems of the Pleistocene will
+receive especial attention. In the present chapter the
+oncoming of glaciation and the subsequent disappearance
+of the ice will be outlined in the light of what would
+be expected according to the solar-cyclonic hypothesis.
+Then in the next chapter several problems of especial
+climatic significance will be considered, such as the localization
+of ice sheets, the succession of severe glacial and
+mild inter-glacial epochs, the sudden commencement of
+glaciation and the peculiar variations in the height of the
+snow line. Other topics to be considered are the occurrence
+of pluvial or rainy climates in non-glaciated regions,
+and glaciation near sea level in subtropical
+latitudes during the Permian and Proterozoic. Then in
+Chapter IX we shall consider the development and distribution
+of the remarkable deposits of wind-blown material
+known as loess.</p>
+
+<p>Facts not considered at the time of framing an hypothesis
+<!--File: cc135.png Page 111-->
+<a class="pagenum break" id="Page_111" title="pg 111"></a>
+are especially significant in testing it. In this particular
+case, the cyclonic hypothesis was framed to explain
+the historic changes of climate revealed by a study of
+ruins, tree rings, and the terraces of streams and lakes,
+without special thought of glaciation or other geologic
+changes. Indeed, the hypothesis had reached nearly its
+present form before much attention was given to geological
+phases of the problem. Nevertheless, it appears
+to meet even this severe test.</p>
+
+<p>According to the solar-cyclonic hypothesis, the Pleistocene
+glacial period was inaugurated at a time when certain
+terrestrial conditions tended to make the earth
+especially favorable for glaciation. How these conditions
+arose will be considered later. Here it is enough to state
+what they were. Chief among them was the fact that the
+continents stood unusually high and were unusually
+large. This, however, was not the primary cause of glaciation,
+for many of the areas which were soon to be
+glaciated were little above sea level. For example, it
+seems clear that New England stood less than a thousand
+feet higher than now. Indeed, Salisbury<sup><a href="#fn40" id="r40">[40]</a></sup> estimates that
+eastern North America in general stood not more than
+a few hundred feet higher than now, and W. B. Wright<sup><a href="#fn41" id="r41">[41]</a></sup>
+reaches the same conclusion in respect to the British
+Isles. Nevertheless, widespread lands, even if they are
+not all high, lead to climatic conditions which favor
+glaciation. For example, enlarged continents cause low
+temperature in high latitudes because they interfere with
+the ocean currents that carry heat polewards. Such continents
+also cause relatively cold winters, for lands cool
+much sooner than does the ocean. Another result is a
+<!--File: cc136.png Page 112-->
+<a class="pagenum break" id="Page_112" title="pg 112"></a>
+diminution of water vapor, not only because cold air
+cannot hold much vapor, but also because the oceanic
+area from which evaporation takes place is reduced by
+the emergence of the continents. Again, when the continents
+are extensive the amount of carbonic acid gas in
+the atmosphere probably decreases, for the augmented
+erosion due to uplift exposes much igneous rock to the
+air, and weathering consumes the atmospheric carbon
+dioxide. When the supply of water vapor and of atmospheric
+carbon dioxide is small, an extreme type of climate
+usually prevails. The combined result of all these conditions
+is that continental emergence causes the climate to
+be somewhat cool and to be marked by relatively great
+contrasts from season to season and from latitude to
+latitude.</p>
+
+<p>When the terrestrial conditions thus permitted glaciation,
+unusual solar activity is supposed to have greatly
+increased the number and severity of storms and to have
+altered their location, just as now happens at times of
+many sunspots. If such a change in storminess had occurred
+when terrestrial conditions were unfavorable for
+glaciation, as, for example, when the lands were low and
+there were widespread epicontinental seas in middle and
+high latitudes, glaciation might not have resulted. In the
+Pleistocene, however, terrestrial conditions permitted
+glaciation, and therefore the supposed increase in storminess
+caused great ice sheets.</p>
+
+<p>The conditions which prevail at times of increased
+storminess have been discussed in detail in <em>Earth and
+Sun</em>. Those which apparently brought on glaciation seem
+to have acted as follows: In the first place the storminess
+lowered the temperature of the earth's surface in several
+ways. The most important of these was the rapid upward
+convection in the centers of cyclonic storms whereby
+<!--File: cc137.png Page 113-->
+<a class="pagenum break" id="Page_113" title="pg 113"></a>
+abundant heat was carried to high levels where most of it
+was radiated away into space. The marked increase in
+the number of tropical cyclones which accompanies increased
+solar activity was probably important in this
+respect. Such cyclones carry vast quantities of heat and
+moisture out of the tropics. The moisture, to be sure,
+liberates heat upon condensing, but as condensation
+occurs above the earth's surface, much of the heat
+escapes into space. Another reason for low temperature
+was that under the influence of the supposedly numerous
+storms of Pleistocene times evaporation over the oceans
+must have increased. This is largely because the velocity
+of the winds is relatively great when storms are strong
+and such winds are powerful agents of evaporation. But
+evaporation requires heat, and hence the strong winds
+lower the temperature.<sup><a href="#fn42" id="r42">[42]</a></sup></p>
+
+<p>The second great condition which enabled increased
+storminess to bring on glaciation was the location of the
+storm tracks. Kullmer's maps, as illustrated in Fig. 2,
+suggest that a great increase in solar activity, such as is
+postulated in the Pleistocene, might shift the main storm
+track poleward even more than it is shifted by the milder
+solar changes during the twelve-year sunspot cycle. If
+this is so, the main track would tend to cross North
+America through the middle of Canada instead of near
+the southern border. Thus there would be an increase in
+precipitation in about the latitude of the Keewatin and
+Labradorean centers of glaciation. From what is known
+of storm tracks in Europe, the main increase in the intensity
+of storms would probably center in Scandinavia.
+Fig. 3 in Chapter V bears this out. That figure, it will be
+recalled, shows what happens to precipitation when solar
+<!--File: cc138.png Page 114-->
+<a class="pagenum break" id="Page_114" title="pg 114"></a>
+activity is increasing. A high rate of precipitation is
+especially marked in the boreal storm track, that is, in
+the northern United States, southern Canada, and northwestern
+Europe.</p>
+
+<p>Another important condition in bringing on glaciation
+would be the fact that when storms are numerous the
+total precipitation appears to increase in spite of the
+slightly lower temperature. This is largely because of the
+greater evaporation. The excessive evaporation arises
+partly from the rapidity of the winds, as already stated,
+and partly from the fact that in areas where the air is
+clear the sun would presumably be able to act more effectively
+than now. It would do so because at times of abundant
+sunspots the sun in our own day has a higher solar
+constant than at times of milder activity. Our whole
+hypothesis is based on the supposition that what now
+happens at times of many sunspots was intensified in
+glacial periods.</p>
+
+<p>A fourth condition which would cause glaciation to
+result from great solar activity would be the fact that
+the portion of the yearly precipitation falling as snow
+would increase, while the proportion of rain would diminish
+in the main storm track. This would arise partly because
+the storms would be located farther north than
+now, and partly because of the diminution in temperature
+due to the increased convection. The snow in itself would
+still further lower the temperature, for snow is an excellent
+reflector of sunlight. The increased cloudiness which
+would accompany the more abundant storms would also
+cause an unusually great reflection of the sunlight and
+still further lower the temperature. Thus at times of
+many sunspots a strong tendency toward the accumulation
+of snow would arise from the rapid convection and
+consequent low temperature, from the northern location
+<!--File: cc139.png Page 115-->
+<a class="pagenum break" id="Page_115" title="pg 115"></a>
+of storms, from the increased evaporation and precipitation,
+from the larger percentage of snowy rather than
+rainy precipitation, and from the great loss of heat due
+to reflection from clouds and snow.</p>
+
+<p>If events at the beginning of the last glacial period
+took place in accordance with the cyclonic hypothesis, as
+outlined above, one of the inevitable results would be the
+production of snowfields. The places where snow would
+accumulate in special quantities would be central Canada,
+the Labrador plateau, and Scandinavia, as well as certain
+mountain regions. As soon as a snowfield became
+somewhat extensive, it would begin to produce striking
+climatic alterations in addition to those to which it owed
+its origin.<sup><a href="#fn43" id="r43">[43]</a></sup> For example, within a snowfield the summers
+remain relatively cold. Hence such a field is likely to be
+an area of high pressure at all seasons. The fact that the
+snowfield is always a place of relatively high pressure
+results in outblowing surface winds except when these
+are temporarily overcome by the passage of strong cyclonic
+storms. The storms, however, tend to be concentrated
+near the margins of the ice throughout the year
+instead of following different paths in each of the four
+seasons. This is partly because cyclonic lows always
+avoid places of high pressure and are thus pushed out
+of the areas where permanent snow has accumulated.
+On the other hand, at times of many sunspots, as Kullmer
+has shown, the main storm track tends to be drawn
+<!--File: cc140.png Page 116-->
+<a class="pagenum break" id="Page_116" title="pg 116"></a>
+poleward, perhaps by electrical conditions. Hence when
+a snowfield is present in the north, the lows, instead of
+migrating much farther north in summer than in winter,
+as they now do, would merely crowd on to the snowfield
+a little farther in summer than in winter. Thus the heavy
+precipitation which is usual in humid climates near the
+centers of lows would take place near the advancing
+margin of the snowfield and cause the field to expand
+still farther southward.</p>
+
+<p>The tendency toward the accumulation of snow on the
+margins of the snowfields would be intensified not only
+by the actual storms themselves, but by other conditions.
+For example, the coldness of the snow would tend to
+cause prompt condensation of the moisture brought by
+the winds that blow toward the storm centers from low
+latitudes. Again, in spite of the general dryness of the
+air over a snowfield, the lower air contains some moisture
+due to evaporation from the snow by day during the
+clear sunny weather of anti-cyclones or highs. Where this
+is sufficient, the cold surface of the snowfields tends to
+produce a frozen fog whenever the snowfield is cooled
+by radiation, as happens at night and during the passage
+of highs. Such a frozen fog is an effective reflector of
+solar radiation. Moreover, because ice has only half the
+specific heat of water, and is much more transparent to
+heat, such a "radiation fog" composed of ice crystals is
+a much less effective retainer of heat than clouds or fog
+made of unfrozen water particles. Shallow fogs of this
+type are described by several polar expeditions. They
+clearly retard the melting of the snow and thus help the
+icefield to grow.</p>
+
+<p>For all these reasons, so long as storminess remained
+great, the Pleistocene snowfields, according to the solar
+hypothesis, must have deepened and expanded. In due
+<!--File: cc141.png Page 117-->
+<a class="pagenum break" id="Page_117" title="pg 117"></a>
+time some of the snow was converted into glacial ice.
+When that occurred, the growth of the snowfield as well
+as of the ice cap must have been accelerated by glacial
+movement. Under such circumstances, as the ice crowded
+southward toward the source of the moisture by which it
+grew, the area of high pressure produced by its low
+temperature would expand. This would force the storm
+track southward in spite of the contrary tendency due to
+the sun. When the ice sheet had become very extensive,
+the track would be crowded relatively near to the northern
+margin of the trade-wind belt. Indeed, the Pleistocene
+ice sheets, at the time of their maximum extension,
+reached almost as far south as the latitude now marking
+the northern limit of the trade-wind belt in summer.
+As the storm track with its frequent low pressure and the
+subtropical belt with its high pressure were forced nearer
+and nearer together, the barometric gradient between
+the two presumably became greater, winds became
+stronger, and the storms more intense.</p>
+
+<p>This zonal crowding would be of special importance in
+summer, at which time it would also be most pronounced.
+In the first place, the storms would be crowded far upon
+the ice cap which would then be protected from the sun
+by a cover of fog and cloud more fully than at any other
+season. Furthermore, the close approach of the trade-wind
+belt to the storm belt would result in a great increase
+in the amount of moisture drawn from the belt of
+evaporation which the trade winds dominate. In the
+trade-wind belt, clear skies and high temperature make
+evaporation especially rapid. Indeed, in spite of the vast
+deserts it is probable that more than three-fourths of the
+total evaporation now taking place on the earth occurs
+in the belt of trades, an area which includes about one-half
+of the earth's surface.</p>
+<!--File: cc142.png Page 118-->
+<a class="pagenum break" id="Page_118" title="pg 118"></a>
+
+<p>The agency which could produce this increased drawing
+northward of moisture from the trade-wind belt
+would be the winds blowing into the lows. According to
+the cyclonic hypothesis, many of these lows would be so
+strong that they would temporarily break down the subtropical
+belt of high pressure which now usually prevails
+between the trades and the zone of westerly winds. This
+belt is even now often broken by tropical cyclones. If the
+storms of more northerly regions temporarily destroyed
+the subtropical high-pressure belt, even though they still
+remained on its northern side, they would divert part of
+the trade winds. Hence the air which now is carried
+obliquely equatorward by those winds would be carried
+spirally northward into the cyclonic lows. Precipitation
+in the storm track on the margin of the relatively cold ice
+sheet would thus be much increased, for most winds from
+low latitudes carry abundant moisture. Such a diversion
+of moisture from low latitudes probably explains the
+deficiency of precipitation along the heat equator at
+times of solar activity, as shown in Fig. 3. Taken as a
+whole, the summer conditions, according to the cyclonic
+hypothesis, would be such that increased evaporation in
+low latitudes would co&ouml;perate with increased storminess,
+cloudiness, and fog in higher latitudes to preserve and
+increase the accumulation of ice upon the borders of the
+ice sheet. The greater the storminess, the more this would
+be true and the more the ice sheet would be able to hold
+its own against melting in summer. Such a combination
+of precipitation and of protection from the sun is especially
+important if an ice sheet is to grow.</p>
+
+<p>The meteorologist needs no geologic evidence that the
+storm track was shoved equatorward by the growth of
+the ice sheet, for he observes a similar shifting whenever
+a winter's snow cap occupies part of the normal storm
+<!--File: cc143.png Page 119-->
+<a class="pagenum break" id="Page_119" title="pg 119"></a>
+tract. The geologist, however, may welcome geologic
+evidence that such an extreme shift of the storm track
+actually occurred during the Pleistocene. Harmer, in
+1901, first pointed out the evidence which was repeated
+with approval by Wright of the Ireland Geological Survey
+in 1914.<sup><a href="#fn44" id="r44">[44]</a></sup> According to these authorities, numerous
+boulders of a distinctive chalk were deposited by Pleistocene
+icebergs along the coast of Ireland. Their distribution
+shows that at the time of maximum glaciation the
+strong winds along the south coast of Ireland were from
+the northeast while today they are from the southwest.
+Such a reversal could apparently be produced only by a
+southward shift of the center of the main storm track
+from its present position in northern Ireland, Scotland,
+and Norway to a position across northern France, central
+Germany, and middle Russia. This would mean that while
+now the centers of the lows commonly move northeastward
+a short distance north of southern Ireland, they
+formerly moved eastward a short distance south of Ireland.
+It will be recalled that in the northern hemisphere
+the winds spiral into a low counter-clockwise and that
+they are strongest near the center. When the centers pass
+not far north of a given point, the strong winds therefore
+blow from the west or southwest, while when the centers
+pass just south of that point, the strong winds come from
+the east or northeast.</p>
+
+<p>In addition to the consequences of the crowding of the
+storm track toward the trade-wind belt, several other
+conditions presumably operated to favor the growth of
+the ice sheet. For example, the lowering of the sea level
+by the removal of water to form the snowfields and
+glaciers interfered with warm currents. It also increased
+the rate of erosion, for it was equivalent to an uplift of
+<!--File: cc144.png Page 120-->
+<a class="pagenum break" id="Page_120" title="pg 120"></a>
+all the land. One consequence of erosion and weathering
+was presumably a diminution of the carbon dioxide in
+the atmosphere, for although the ice covered perhaps a
+tenth of the lands and interfered with carbonation to that
+extent, the removal of large quantities of soil by accelerated
+erosion on the other nine-tenths perhaps more than
+counterbalanced the protective effect of the ice. At the
+same time, the general lowering of the temperature of the
+ocean as well as the lands increased the ocean's capacity
+for carbon dioxide and thus facilitated absorption. At a
+temperature of 50&deg;F. water absorbs 32 per cent more
+carbon dioxide than at 68&deg;. The high waves produced by
+the severe storms must have had a similar effect on a
+small scale. Thus the percentage of carbon dioxide in the
+atmosphere was presumably diminished. Of less significance
+than these changes in the lands and the air, but
+perhaps not negligible, was the increased salinity of the
+ocean which accompanied the removal of water to form
+snow, and the increase of the dissolved mineral load of
+the rejuvenated streams. Increased salinity slows up the
+deep-sea circulation, as we shall see in a later chapter.
+This increases the contrasts from zone to zone.</p>
+
+<p>At times of great solar activity the agencies mentioned
+above would apparently co&ouml;perate to cause an advance of
+ice sheets into lower latitudes. The degree of solar activity
+would have much to do with the final extent of the ice
+sheets. Nevertheless, certain terrestrial conditions would
+tend to set limits beyond which the ice would not greatly
+advance unless the storminess were extraordinarily
+severe. The most obvious of these conditions is the location
+of oceans and of deserts or semi-arid regions. The
+southwestward advance of the European ice sheet and
+the southeastward advance of the Labradorean sheet in
+America were stopped by the Atlantic. The semi-aridity
+<!--File: cc145.png Page 121-->
+<a class="pagenum break" id="Page_121" title="pg 121"></a>
+of the Great Plains, produced by their position in the lee
+of the Rocky Mountains, stopped the advance of the
+Keewatin ice sheet toward the southwest. The advance of
+the European ice sheet southeast seems to have been
+stopped for similar reasons. The cessation of the advance
+would be brought about in such an area not alone by the
+light precipitation and abundant sunshine, but by the
+dryness of the air, and also by the power of dust to absorb
+the sun's heat. Much dust would presumably be
+drawn in from the dry regions by passing cyclonic storms
+and would be scattered over the ice.</p>
+
+<p>The advance of the ice is also slowed up by a rugged
+topography, as among the Appalachians in northern
+Pennsylvania. Such a topography besides opposing a
+physical obstruction to the movement of the ice provides
+bare south-facing slopes which the sun warms effectively.
+Such warm slopes are unfavorable to glacial advance.
+The rugged topography was perhaps quite as effective as
+the altitude of the Appalachians in causing the conspicuous
+northward dent in the glacial margin in Pennsylvania.
+Where glaciers lie in mountain valleys the advance
+beyond a certain point is often interfered with by the
+deployment of the ice at the mouths of gorges. Evaporation
+and melting are more rapid where a glacier is broad
+and thin than where it is narrow and thick, as in a gorge.
+Again, where the topography or the location of oceans or
+dry areas causes the glacial lobes to be long and narrow,
+the elongation of the lobe is apparently checked in several
+ways. Toward the end of the lobe, melting and
+evaporation increase rapidly because the planetary
+westerly winds are more likely to overcome the glacial
+winds and sweep across a long, narrow lobe than across
+a broad one. As they cross the lobe, they accelerate
+evaporation, and probably lessen cloudiness, with a consequent
+<!--File: cc146.png Page 122-->
+<a class="pagenum break" id="Page_122" title="pg 122"></a>
+augmentation of melting. Moreover, although
+lows rarely cross a broad ice sheet, they do cross a
+narrow lobe. For example, Nansen records that strong
+lows occasionally cross the narrow southern part of the
+Greenland ice sheet. The longer the lobe, the more likely
+it is that lows will cross it, instead of following its margin.
+Lows which cross a lobe do not yield so much snow
+to the tip as do those which follow the margin. Hence
+elongation is retarded and finally stopped even without
+a change in the earth's general climate.</p>
+
+<p>Because of these various reasons the advances of the
+ice during the several epochs of a glacial period might
+be approximately equal, even if the durations of the
+periods of storminess and low temperature were different.
+Indeed, they might be sub-equal, even if the periods
+differed in intensity as well as length. Differences in the
+periods would apparently be manifested less in the extent
+of the ice than in the depth of glacial erosion and in
+the thickness of the terminal moraines, outwash plains,
+and other glacial or glacio-fluvial formations.</p>
+
+<p>Having completed the consideration of the conditions
+leading to the advance of the ice, let us now consider the
+condition of North America at the time of maximum
+glaciation.<sup><a href="#fn45" id="r45">[45]</a></sup>
+Over an area of nearly four million square
+miles, occupying practically all the northern half of the
+continent and part of the southern half, as appears in
+Fig. 6, the surface was a monotonous and almost level
+plain of ice covered with snow. When viewed from a
+high altitude, all parts except the margins must have
+presented a uniformly white and sparkling appearance.
+Along the margins, however, except to the north, the
+<!--File: cc145.png Page 123-->
+<a class="pagenum break" id="Page_123" title="pg 123"></a>
+whiteness was irregular, for the view must have included
+not only fresh snow, but moving clouds and dirty snow
+or ice. Along the borders where melting was in progress
+there was presumably more or less spottedness due to
+morainal material or glacial d&eacute;bris brought to the surface
+by ice shearage and wastage. Along the dry southwestern
+border it is also possible that there were numerous
+dark spots due to dust blown onto the ice by the
+wind.</p>
+
+
+<!--File: cc148.png Page 124-->
+<a class="pagenum break" id="Page_124" title="pg 124"></a>
+
+<img src="images/fig6.png" width="800" height="529" alt="Fig. 6"  id="Fig_6" />
+<p class="center"> <em>Fig. 6. Distribution of Pleistocene ice sheets.<br />
+(After Schuchert.</em>)</p>
+
+
+<p>The great white sheet with its ragged border was
+roughly circular in form, with its center in central
+Canada. Yet there were many departures from a perfectly
+circular form. Some were due to the oceans, for,
+except in northern Alaska, the ice extended into the
+ocean all the way from New Jersey around by the north
+to Washington. On the south, topographic conditions
+made the margin depart from a simple arc. From New
+Jersey to Ohio it swung northward. In the Mississippi
+Valley it reached far south; indeed most of the broad
+wedge between the Ohio and the Missouri rivers was
+occupied by ice. From latitude 37&deg; near the junction of
+the Missouri and the Mississippi, however, the ice margin
+extended almost due north along the Missouri to central
+North Dakota. It then stretched westward to the Rockies.
+Farther west lowland glaciation was abundant as far
+south as western Washington. In the Rockies, the Cascades,
+and the Sierra Nevadas glaciation was common as
+far south as Colorado and southern California, respectively,
+and snowfields were doubtless extensive enough to
+make these ranges ribbons of white. Between these lofty
+ranges lay a great unglaciated region, but even in the
+Great Basin itself, in spite of its present aridity, certain
+ranges carried glaciers, while great lakes expanded
+widely.</p>
+<!--File: cc149.png Page 125-->
+<a class="pagenum break" id="Page_125" title="pg 125"></a>
+
+<p>In this vast field of snow the glacial ice slowly crept
+outward, possibly at an average speed of half a foot a
+day, but varying from almost nothing in winter at the
+north, to several feet a day in summer at the south.<sup><a href="#fn46" id="r46">[46]</a></sup> The
+force which caused the movement was the presence of
+the ice piled up not far from the margins. Almost certainly,
+however, there was no great dome from the
+center in Canada outward, as some early writers assumed.
+Such a dome would require that the ice be many
+thousands of feet thick near its center. This is impossible
+because of the fact that ice is more voluminous than
+water (about 9 per cent near the freezing point). Hence
+when subjected to sufficient pressure it changes to the
+liquid form. As friction and internal heat tend to keep
+the bottom of a glacier warm, even in cold regions, the
+probabilities are that only under very special conditions
+was a continental ice sheet much thicker than about 2500
+feet. In Antarctica, where the temperature is much lower
+than was probably attained in the United States, the ice
+sheet is nearly level, several expeditions having traveled
+hundreds of miles with practically no change in altitude.
+In Shackleton's trip almost to the South Pole, he encountered
+a general rise of 3000 feet in 1200 miles. Mountains,
+however, projected through the ice even near the
+pole and the geologists conclude that the ice is not very
+thick even at the world's coldest point, the South Pole.</p>
+
+<p>Along the margin of the ice there were two sorts of
+movement, much more rapid than the slow creep of the
+ice. One was produced by the outward drift of snow
+carried by the outblowing dry winds and the other and
+more important was due to the passage of cyclonic
+storms. Along the border of the ice sheet, except at the
+<!--File: cc150.png Page 126-->
+<a class="pagenum break" id="Page_126" title="pg 126"></a>
+north, storm presumably closely followed storm. Their
+movement, we judge, was relatively slow until near the
+southern end of the Mississippi lobe, but when this point
+was passed they moved much more rapidly, for then they
+could go toward instead of away from the far northern
+path which the sun prescribes when solar activity is
+great. The storms brought much snow to the icefield,
+perhaps sometimes in favored places as much as the hundred
+feet a year which is recorded for some winters in the
+Sierras at present. Even the unglaciated intermontane
+Great Basin presumably received considerable precipitation,
+perhaps twice as much as its present scanty
+supply. The rainfall was enough to support many lakes,
+one of which was ten times as large as Great Salt Lake;
+and grass was doubtless abundant upon many slopes
+which are now dry and barren. The relatively heavy
+precipitation in the Great Basin was probably due primarily
+to the increased number of storms, but may also
+have been much influenced by their slow eastward movement.
+The lows presumably moved slowly in that general
+region not only because they were retarded and turned
+from their normal path by the cold ice to the east, but
+because during the summer the area between the Sierra
+snowfields on the west and the Rocky Mountain and Mississippi
+Valley snowfields on the east was relatively
+warm. Hence it was normally a place of low pressure
+and therefore of inblowing winds. Slow-moving lows are
+much more effective than fast-moving ones in drawing
+moisture northwestward from the Gulf of Mexico, for
+they give the moisture more time to move spirally first
+northeast, under the influence of the normal southwesterly
+winds, then northwest and finally southwest as
+it approaches the storm center. In the case of the present
+lows, before much moisture-laden air can describe such
+<!--File: cc151.png Page 127-->
+<a class="pagenum break" id="Page_127" title="pg 127"></a>
+a circuit, first eastward and then westward, the storm
+center has nearly always moved eastward across the
+Rockies and even across the Great Plains. A result of this
+is the regular decrease in precipitation northward, northwestward,
+and westward from the Gulf of Mexico.</p>
+
+<p>Along the part of the glacial margins where for more
+than 3000 miles the North American ice entered the
+Atlantic and the Pacific oceans, myriads of great blocks
+broke off and floated away as stately icebergs, to scatter
+boulders far over the ocean floor and to melt in warmer
+climes. Where the margin lay upon the lands numerous
+streams issued from beneath the ice, milk-white with
+rock flour, and built up great outwash plains and valley
+trains of gravel and sand. Here and there, just beyond
+the ice, marginal lakes of strange shapes occupied valleys
+which had been dammed by the advancing ice. In many
+of them the water level rose until it reached some low
+point in the divide and then overflowed, forming rapids
+and waterfalls. Indeed, many of the waterfalls of the
+eastern United States and Canada were formed in just
+this way and not a few streams now occupy courses
+through ridges instead of parallel to them, as in pre-glacial
+times.</p>
+
+<p>In the zone to the south of the continental ice sheet,
+the plant and animal life of boreal, cool temperate, and
+warm temperate regions commingled curiously. Heather
+and Arctic willow crowded out elm and oak; musk ox,
+hairy mammoth, and marmot contested with deer, chipmunk,
+and skunk for a chance to live. Near the ice on
+slopes exposed to the cold glacial gales, the immigrant
+boreal species were dominant, but not far away in more
+protected areas the species that had formerly lived there
+held their own. In Europe during the last two advances
+of the great ice sheet the caveman also struggled with
+<!--File: cc152.png Page 128-->
+<a class="pagenum break" id="Page_128" title="pg 128"></a>
+fierce animals and a fiercer climate to maintain life in an
+area whose habitability had long been decreasing.</p>
+
+<p>The next step in our history of glaciation is to outline
+the disappearance of the ice sheets. When a decrease in
+solar activity produced a corresponding decrease in
+storminess, several influences presumably combined to
+cause the disappearance of the ice. Most of their results
+are the reverse of those which brought on glaciation. A
+few special aspects, however, some of which have been
+discussed in <em>Earth and Sun</em>, ought to be brought to mind.
+A diminution in storminess lessens upward convection,
+wind velocity, and evaporation, and these changes, if they
+occurred, must have united to raise the temperature of
+the lower air by reducing the escape of heat. Again a
+decrease in the number and intensity of tropical cyclones
+presumably lessened the amount of moisture carried into
+mid-latitudes, and thus diminished the precipitation. The
+diminution of snowfall on the ice sheets when storminess
+diminished was probably highly important. The amount
+of precipitation on the sheets was presumably lessened
+still further by changes in the storminess of middle
+latitudes. When storminess diminishes, the lows follow a
+less definite path, as Kullmer's maps show, and on the
+average a more southerly path. Thus, instead of all the
+lows contributing snow to the ice sheet, a large fraction
+of the relatively few remaining lows would bring rain to
+areas south of the ice sheet. As storminess decreased, the
+trades and westerlies probably became steadier, and thus
+carried to high latitudes more warm water than when
+often interrupted by storms. Steadier southwesterly
+winds must have produced a greater movement of atmospheric
+as well as oceanic heat to high latitudes. The
+warming due to these two causes was probably the chief
+reason for the disappearance of the European ice sheet
+<!--File: cc153.png Page 129-->
+<a class="pagenum break" id="Page_129" title="pg 129"></a>
+and of those on the Pacific coast of North America. The
+two greater American ice sheets, however, and the
+glaciers elsewhere in the lee of high mountain ranges,
+probably disappeared chiefly because of lessened precipitation.
+If there were no cyclonic storms to draw moisture
+northward from the Gulf of Mexico, most of North
+America east of the Rocky Mountain barrier would be
+arid. Therefore a diminution of storminess would be
+particularly effective in causing the disappearance of ice
+sheets in these regions.</p>
+
+<p>That evaporation was an especially important factor
+in causing the ice from the Keewatin center to disappear,
+is suggested by the relatively small amount of water-sorted
+material in its drift. In South Dakota, for example,
+less than 10 per cent of the drift is stratified.<sup><a href="#fn47" id="r47">[47]</a></sup> On
+the other hand, Salisbury estimates that perhaps a third
+of the Labradorean drift in eastern Wisconsin is crudely
+stratified, about half of that in New Jersey, and more
+than half of the drift in western Europe.</p>
+
+<p>When the sun's activity began to diminish, all these
+conditions, as well as several others, would co&ouml;perate to
+cause the ice sheets to disappear. Step by step with their
+disappearance, the amelioration of the climate would
+progress so long as the period of solar inactivity continued
+and storms were rare. If the inactivity continued
+long enough, it would result in a fairly mild climate in
+high latitudes, though so long as the continents were
+emergent this mildness would not be of the extreme type.
+The inauguration of another cycle of increased disturbance
+of the sun, with a marked increase in storminess,
+would inaugurate another glacial epoch. Thus a succession
+of glacial and inter-glacial epochs might continue so
+long as the sun was repeatedly disturbed.</p>
+<!--File: cc154.png Page 130-->
+<a class="pagenum break" id="Page_130" title="pg 130"></a>
+
+
+
+
+<h2 id="Chapt_VIII"><a href="#content">CHAPTER VIII</a></h2>
+
+<h3>SOME PROBLEMS OF GLACIAL PERIODS</h3>
+
+
+<p>Having outlined in general terms the coming of
+the ice sheets and their disappearance, we are
+now ready to discuss certain problems of compelling
+climatic interest. The discussion will be grouped
+under five heads: (I) the localization of glaciation; (II)
+the sudden coming of glaciation; (III) peculiar variations
+in the height of the snow line and of glaciation;
+(IV) lakes and other evidences of humidity in unglaciated
+regions during the glacial epochs; (V) glaciation
+at sea level and in low latitudes in the Permian and
+Proterozoic eras. The discussion of perhaps the most
+difficult of all climatic problems of glaciation, that of the
+succession of cold glacial and mild inter-glacial epochs,
+has been postponed to the next to the final chapter of this
+book. It cannot be properly considered until we take up
+the history of solar disturbances.</p>
+
+<p>I. The first problem, the localization of the ice sheets,
+arises from the fact that in both the Pleistocene and the
+Permian periods glaciation was remarkably limited. In
+neither period were all parts of high latitudes glaciated;
+yet in both cases glaciation occurred in large regions in
+lower latitudes. Many explanations of this localization
+have been offered, but most are entirely inadequate. Even
+hypotheses with something of proven worth, such as
+those of variations in volcanic dust and in atmospheric
+<!--File: cc155.png-->
+<a class="pagenum break" id="Page_131" title="pg 131"></a>
+carbon dioxide, fail to account for localization. The
+cyclonic form of the solar hypothesis, however, seems to
+afford a satisfactory explanation.</p>
+
+<p>The distribution of the ice in the last glacial period is
+well known, and is shown in Fig. 6. Four-fifths of the
+ice-covered area, which was eight million square miles,
+more or less, was near the borders of the North Atlantic
+in eastern North America and northwestern Europe.
+The ice spread out from two great centers in North
+America, the Labradorean east of Hudson Bay, and the
+Keewatin west of the bay. There were also many glaciers
+in the western mountains, especially in Canada, while
+subordinate centers occurred in Newfoundland, the Adirondacks,
+and the White Mountains. The main ice sheet
+at its maximum extension reached as far south as latitude
+39&deg; in Kansas and Kentucky, and 37&deg; in Illinois.
+Huge boulders were transferred more than one thousand
+miles from their source in Canada. The northward extension
+was somewhat less. Indeed, the northern margin
+of the continent was apparently relatively little glaciated
+and much of Alaska unglaciated. Why should northern
+Kentucky be glaciated when northern Alaska was not?</p>
+
+<p>In Europe the chief center from which the continental
+glacier moved was the Scandinavian highlands. It pushed
+across the depression now occupied by the Baltic to
+southern Russia and across the North Sea depression
+to England and Belgium. The Alps formed a center of
+considerable importance, and there were minor centers
+in Scotland, Ireland, the Pyrenees, Apennines, Caucasus,
+and Urals. In Asia numerous ranges also contained large
+glaciers, but practically all the glaciation was of the
+alpine type and very little of the vast northern lowland
+was covered with ice.</p>
+
+<p>In the southern hemisphere glaciation at low latitudes
+<!--File: cc156.png Page 132-->
+<a class="pagenum break" id="Page_132" title="pg 132"></a>
+was less striking than in the northern hemisphere. Most
+of the increase in the areas of ice was confined to mountains
+which today receive heavy precipitation and still
+contain small glaciers. Indeed, except for relatively slight
+glaciation in the Australian Alps and in Tasmania, most
+of the Pleistocene glaciation in the southern hemisphere
+was merely an extension of existing glaciers, such as
+those of south Chile, New Zealand, and the Andes. Nevertheless,
+fairly extensive glaciation existed much nearer
+the equator than is now the case.</p>
+
+<p>In considering the localization of Pleistocene glaciation,
+three main factors must be taken into account,
+namely, temperature, topography, and precipitation. The
+absence of glaciation in large parts of the Arctic regions
+of North America and of Asia makes it certain that low
+temperature was not the controlling factor. Aside from
+Antarctica, the coldest place in the world is northeastern
+Siberia. There for seven months the average temperature
+is below 0&deg;C., while the mean for the whole year is
+below -10&deg;C. If the temperature during a glacial period
+averaged 6&deg;C. lower than now, as is commonly supposed,
+this part of Siberia would have had a temperature below
+freezing for at least nine months out of the twelve even if
+there were no snowfield to keep the summers cold. Yet
+even under such conditions no glaciation occurred, although
+in other places, such as parts of Canada and
+northwestern Europe, intense glaciation occurred where
+the mean temperature is much higher.</p>
+
+<p>The topography of the lands apparently had much
+more influence upon the localization of glaciation than
+did temperature. Its effect, however, was always to cause
+glaciation exactly where it would be expected and not in
+unexpected places as actually occurred. For example, in
+North America the western side of the Canadian Rockies
+<!--File: cc157.png Page 133-->
+<a class="pagenum break" id="Page_133" title="pg 133"></a>
+suffered intense glaciation, for there precipitation was
+heavy because the westerly winds from the Pacific are
+forced to give up their moisture as they rise. In the same
+way the western side of the Sierra Nevadas was much
+more heavily glaciated than the eastern side. In similar
+fashion the windward slopes of the Alps, the Caucasus,
+the Himalayas, and many other mountain ranges suffered
+extensive glaciation. Low temperature does not
+seem to have been the cause of this glaciation, for in that
+case it is hard to see why both sides of the various ranges
+did not show an equal percentage of increase in the size
+of their icefields.</p>
+
+<p>From what has been said as to temperature and topography,
+it is evident that variations in precipitation have
+had much more to do with glaciation than have variations
+in temperature. In the Arctic lowlands and on the leeward
+side of mountains, the slight development of glaciation
+appears to have been due to scarcity of precipitation.
+On the windward side of mountains, on the other
+hand, a notable increase in precipitation seems to have
+led to abundant glaciation. Such an increase in precipitation
+must be dependent on increased evaporation and
+this could arise either from relatively high temperature
+or strong winds. Since the temperature in the glacial
+period was lower than now, we seem forced to attribute
+the increased precipitation to a strengthening of the
+winds. If the westerly winds from the Pacific should increase
+in strength and waft more moisture to the western
+side of the Canadian Rockies, or if similar winds increased
+the snowfall on the upper slopes of the Alps or
+the Tian-Shan Mountains, the glaciers would extend
+lower than now without any change in temperature.</p>
+
+<p>Although the incompetence of low temperature to cause
+glaciation, and the relative unimportance of the mountains
+<!--File: cc158.png Page 134-->
+<a class="pagenum break" id="Page_134" title="pg 134"></a>
+in northeastern Canada and northwestern Europe
+throw most glacial hypotheses out of court, they are in
+harmony with the cyclonic hypothesis. The answer of
+that hypothesis to the problem of the localization of ice
+sheets seems to be found in certain maps of storminess
+and rainfall in relation to solar activity. In Fig. 2 a
+marked belt of increased storminess at times of many
+sunspots is seen in southern Canada. A comparison of
+this with a series of maps given in <em>Earth and Sun</em> shows
+that the stormy belt tends to migrate northward in harmony
+with an increase in the activity of the sun's atmosphere.
+If the sun were sufficiently active the belt of
+maximum storminess would apparently pass through the
+Keewatin and Labradorean centers of glaciation instead
+of well to the south of them, as at present. It would
+presumably cross another center in Greenland, and then
+would traverse the fourth of the great centers of Pleistocene
+glaciation in Scandinavia. It would not succeed in
+traversing northern Asia, however, any more than it
+does now, because of the great high-pressure area which
+develops there in winter. When the ice sheets expanded
+from the main centers of glaciation, the belt of storms
+would be pushed southward and outward. Thus it might
+give rise to minor centers of glaciers such as the Patrician
+between Hudson Bay and Lake Superior, or the
+centers in Ireland, Cornwall, Wales, and the northern
+Ural Mountains. As the main ice sheets advanced, however,
+the minor centers would be overridden and the
+entire mass of ice would be merged into one vast expanse
+in the Atlantic portion of each of the two continents.</p>
+
+<p>In this connection it may be well to consider briefly the
+most recent hypothesis as to the growth and hence the
+localization of glaciation. In 1911 and more fully in 1915,
+<!--File: cc159.png Page 135-->
+<a class="pagenum break" id="Page_135" title="pg 135"></a>
+Hobbs,<sup><a href="#fn48" id="r48">[48]</a></sup> advanced the anti-cyclonic hypothesis of the
+origin of ice sheets. This hypothesis has the great merit
+of focusing attention upon the fact that ice sheets are
+pronounced anti-cyclonic regions of high pressure. This
+is proved by the strong outblowing winds which prevail
+along their margins. Such winds must, of course, be
+balanced by inward-moving winds at high levels. Abundant
+observations prove that such is the case. For
+example, balloons sent up by Barkow near the margin of
+the Antarctic ice sheet reveal the occurrence of inblowing
+winds, although they rarely occur below a height of
+9000 meters. The abundant data gathered by Guervain
+on the coast of Greenland indicate that outblowing winds
+prevail up to a height of about 4000 meters. At that
+height inblowing winds commence and increase in frequency
+until at an altitude of over 5000 meters they become
+more common than outblowing winds. It should be
+noted, however, that in both Antarctica and Greenland,
+although the winds at an elevation of less than a thousand
+meters generally blow outward, there are frequent and
+decided departures from this rule, so that "variable
+winds" are quite commonly mentioned in the reports of
+expeditions and balloon soundings.</p>
+
+<p>The undoubted anti-cyclonic conditions which Hobbs
+thus calls to the attention of scientists seem to him to
+necessitate a peculiar mechanism in order to produce
+the snow which feeds the glaciers. He assumes that the
+winds which blow toward the centers of the ice sheets
+at high levels carry the necessary moisture by which the
+glaciers grow. When the air descends in the centers of
+the highs, it is supposed to be chilled on reaching the surface
+<!--File: cc160.png Page 136-->
+<a class="pagenum break" id="Page_136" title="pg 136"></a>
+of the ice, and hence to give up its moisture in the
+form of minute crystals. This conclusion is doubtful for
+several reasons. In the first place, Hobbs does not seem
+to appreciate the importance of the variable winds which
+he quotes Arctic and Antarctic explorers as describing
+quite frequently on the edges of the ice sheets. They are
+one of many signs that cyclonic storms are fairly frequent
+on the borders of the ice though not in its interior.
+Thus there is a distinct and sufficient form of precipitation
+actually at work near the margin of the ice, or
+exactly where the thickness of the ice sheet would lead
+us to expect.</p>
+
+<p>Another consideration which throws grave doubt on
+the anti-cyclonic hypothesis of ice sheets is the small
+amount of moisture possible in the highs because of their
+low temperature. Suppose, for the sake of argument,
+that the temperature in the middle of an ice sheet averages
+20&deg;F. This is probably much higher than the actual
+fact and therefore unduly favorable to the anti-cyclonic
+hypothesis. Suppose also that the decrease in temperature
+from the earth's surface upward proceeds at the
+rate of 1&deg;F. for each 300 feet, which is 50 per cent less
+than the actual rate for air with only a slight amount of
+moisture, such as is found in cold regions. Then at a
+height of 10,000 feet, where the inblowing winds begin
+to be felt, the temperature would be -20&deg;F. At that
+temperature the air is able to hold approximately 0.166
+grain of moisture per cubic foot when fully saturated.
+This is an exceedingly small amount of moisture and even
+if it were all precipitated could scarcely build a glacier.
+However, it apparently would not be precipitated because
+when such air descends in the center of the anti-cyclone
+it is warmed adiabatically, that is, by compression. On
+reaching the surface it would have a temperature of 20&deg;
+<!--File: cc161.png Page 137-->
+<a class="pagenum break" id="Page_137" title="pg 137"></a>
+and would be able to hold 0.898 grain of water vapor per
+cubic foot; in other words, it would have a relative humidity
+of about 18 per cent. Under no reasonable assumption
+does the upper air at the center of an ice sheet
+appear to reach the surface with a relative humidity of
+more than 20 or 25 per cent. Such air cannot give up
+moisture. On the contrary, it absorbs it and tends to
+diminish rather than increase the thickness of the sheet
+of ice and snow. But after the surplus heat gained by
+descent has been lost by radiation, conduction, and
+evaporation, the air may become super-saturated with
+the moisture picked up while warm. Hobbs reports that
+explorers in Antarctica and Greenland have frequently
+observed condensation on their clothing. If such moisture
+is not derived directly from the men's own bodies, it is
+apparently picked up from the ice sheet by the descending
+air, and not added to the ice sheet by air from aloft.</p>
+
+<p>The relation of all this to the localization of ice sheets
+is this. If Hobbs' anti-cyclonic hypothesis of glacial
+growth is correct, it would appear that ice sheets should
+grow up where the temperature is lowest and the high-pressure
+areas most persistent; for instance, in northern
+Siberia. It would also appear that so far as the topography
+permitted, the ice sheets ought to move out uniformly
+in all directions; hence the ice sheet ought to be
+as prominent to the north of the Keewatin and Labradorean
+centers as to the south, which is by no means the
+case. Again, in mountainous regions, such as the glacial
+areas of Alaska and Chile, the glaciation ought not to
+be confined to the windward slope of the mountains so
+closely as is actually the fact. In each of these cases the
+glaciated region was large enough so that there was
+probably a true anti-cyclonic area comparable with that
+now prevailing over southern Greenland. In both places
+<!--File: cc162.png Page 138-->
+<a class="pagenum break" id="Page_138" title="pg 138"></a>
+the correlation between glaciation and mountain ranges
+seems much too close to support the anti-cyclonic hypothesis,
+for the inblowing winds which on that hypothesis
+bring the moisture are shown by observation to occur
+at heights far greater than that of all but the loftiest
+ranges.</p>
+
+<p>II. The sudden coming of glaciation is another problem
+which has been a stumbling-block in the way of every
+glacial hypothesis. In his <em>Climates of Geologic Times</em>,
+Schuchert states that the fossils give almost no warning
+of an approaching catastrophe. If glaciation were solely
+due to uplift, or other terrestrial changes aside from vulcanism,
+Schuchert holds that it would have come slowly
+and the stages preceding glaciation would have affected
+life sufficiently to be recorded in the rocks. He considers
+that the suddenness of the coming of glaciation is one
+of the strongest arguments against the carbon dioxide
+hypothesis of glaciation.</p>
+
+<p>According to the cyclonic hypothesis, however, the
+suddenness of the oncoming of glaciation is merely what
+would be expected on the basis of what happens today.
+Changes in the sun occur suddenly. The sunspot cycle is
+only eleven or twelve years long, and even this short
+period of activity is inaugurated more suddenly than it
+declines. Again the climatic record derived from the
+growth of trees, as given in Figs. 4 and 5, also shows that
+marked changes in climate are initiated more rapidly
+than they disappear. In this connection, however, it must
+be remembered that solar activity may arise in various
+ways, as will appear more fully later. Under certain conditions
+storminess may increase and decrease slowly.</p>
+
+<p>III. The height of the snow line and of glaciation furnishes
+another means of testing glacial hypotheses. It is
+well established that in times of glaciation the snow line
+<!--File: cc163.png Page 139-->
+<a class="pagenum break" id="Page_139" title="pg 139"></a>
+was depressed everywhere, but least near the equator.
+For example, according to Penck, permanent snow extended
+4000 feet lower than now in the Alps, whereas
+it stood only 1500 feet below the present level near the
+equator in Venezuela. This unequal depression is not
+readily accounted for by any hypothesis depending solely
+upon the lowering of temperature. By the carbon dioxide
+and the volcanic dust hypotheses, the temperature presumably
+was lowered almost equally in all latitudes, but
+a little more at the equator than elsewhere. If glaciation
+were due to a temporary lessening of the radiation received
+from the sun, such as is demanded by the thermal
+solar hypothesis, and by the longer periods of Croll's
+hypothesis, the lowering would be distinctly greatest at
+the equator. Thus, according to all these hypotheses, the
+snow line should have been depressed most at the equator,
+instead of least.</p>
+
+<p>The cyclonic hypothesis explains the lesser depression
+of the snow line at the equator as due to a diminution of
+precipitation. The effectiveness of precipitation in this
+respect is illustrated by the present great difference in
+the height of the snow line on the humid and dry sides of
+mountains. On the wet eastern side of the Andes near the
+equator, the snow line lies at 16,000 feet; on the dry
+western side, at 18,500 feet. Again, although the humid
+side of the Himalayas lies toward the south, the snow line
+has a level of 15,000 feet, while farther north, on the dry
+side, it is 16,700 feet.<sup><a href="#fn49" id="r49">[49]</a></sup> The fact that the snow line is lower
+near the margin of the Alps than toward the center
+points in the same direction. The bearing of all this on
+the glacial period may be judged by looking again at Fig.
+3 in Chapter V. This shows that at times of sunspot
+activity and hence of augmented storminess, the precipitation
+<!--File: cc164.png Page 140-->
+<a class="pagenum break" id="Page_140" title="pg 140"></a>
+diminishes near the heat equator, that is, where
+the average temperature for the whole year is highest.
+At present the great size of the northern continents and
+their consequent high temperature in summer, cause the
+heat equator to lie north of the "real" equator, except
+where Australia draws it to the southward.<sup><a href="#fn50" id="r50">[50]</a></sup> When large
+parts of the northern continents were covered with ice,
+however, the heat equator and the true equator were
+probably much closer than now, for the continents could
+not become so hot. If so, the diminution in equatorial
+precipitation, which accompanies increased storminess
+throughout the world as a whole, would take place more
+nearly along the true equator than appears in Fig. 3.
+Hence so far as precipitation alone is concerned, we
+should actually expect that the snow line near the equator
+would rise a little during glacial periods. Another factor,
+however, must be considered. K&ouml;ppen's data, it will be
+remembered, show that at times of solar activity the
+earth's temperature falls more at the equator than in
+higher latitudes. If this effect were magnified it would
+lower the snow line. The actual position of the snow line
+at the equator during glacial periods thus appears to be
+the combined effect of diminished precipitation, which
+would raise the line, and of lower temperature, which
+would bring it down.</p>
+
+<p>Before leaving this subject it may be well to recall that
+the relative lessening of precipitation in equatorial latitudes
+during the glacial epochs was probably caused by
+the diversion of moisture from the trade-wind belt. This
+diversion was presumably due to the great number of
+tropical cyclones and to the fact that the cyclonic storms
+of middle latitudes also drew much moisture from the
+trade-wind belt in summer when the northern position of
+<!--File: cc165.png Page 141-->
+<a class="pagenum break" id="Page_141" title="pg 141"></a>
+the sun drew that belt near the storm track which was
+forced to remain south of the ice sheet. Such diversion
+of moisture out of the trade-wind belt must diminish the
+amount of water vapor that is carried by the trades to
+equatorial regions; hence it would lessen precipitation
+in the belt of so-called equatorial calms, which lies along
+the heat equator rather than along the geographical
+equator.</p>
+
+<p>Another phase of the vertical distribution of glaciation
+has been the subject of considerable discussion. In the
+Alps and in many other mountains the glaciation of the
+Pleistocene period appears to have had its upper limit
+no higher than today. This has been variously interpreted.
+It seems, however, to be adequately explained
+as due to decreased precipitation at high altitudes during
+the cold periods. This is in spite of the fact that precipitation
+in general increased with increased storminess.
+The low temperature of glacial times presumably induced
+condensation at lower altitudes than now, and most of
+the precipitation occurred upon the lower slopes of the
+mountains, contributing to the lower glaciers, while little
+of it fell upon the highest glaciers. Above a moderate
+altitude in all lofty mountains the decrease in the amount
+of precipitation is rapid. In most cases the decrease
+begins at a height of less than 3000 feet above the base
+of the main slope, provided the slope is steep. The colder
+the air, the lower the altitude at which this occurs. For
+example, it is much lower in winter than in summer.
+Indeed, the higher altitudes in the Alps are sunny in
+winter even where there are abundant clouds lower down.</p>
+
+<p>IV. The presence of extensive lakes and other evidences
+of a pluvial climate during glacial periods in non-glaciated
+regions which are normally dry is another of the
+facts which most glacial hypotheses fail to explain satisfactorily.
+<!--File: cc166.png Page 142-->
+<a class="pagenum break" id="Page_142" title="pg 142"></a>
+Beyond the ice sheets many regions appear to
+have enjoyed an unusually heavy precipitation during the
+glacial epochs. The evidence of this is abundant, including
+numerous abandoned strand lines of salt lakes and an
+abundance of coarse material in deltas and flood plains.J. D. Whitney,<sup><a href="#fn51" id="r51">[51]</a></sup> in an interesting but neglected volume,
+was one of the first to marshal the evidence of this sort.
+More recently Free<sup><a href="#fn52" id="r52">[52]</a></sup> has amplified this. According to him
+in the Great Basin region of the United States sixty-two
+basins either contain unmistakable evidence of lakes, or
+belong to one of the three great lake groups named below.
+Two of these, the Lake Lahontan and the Lake Bonneville
+groups, comprise twenty-nine present basins, while
+the third, the Owens-Searles chain, contained at least
+five large lakes, the lowest being in Death Valley. In
+western and central Asia a far greater series of salt lakes
+is found and most of these are surrounded by strands at
+high levels. Many of these are described in <em>Explorations
+in Turkestan</em>, <em>The Pulse of Asia</em>, and <em>Palestine and Its
+Transformation</em>. There has been a good deal of debate as
+to whether these lakes actually date from the glacial
+period, as is claimed by C. E. P. Brooks, for example, or
+from some other period. The evidence, however, seems to
+be convincing that the lakes expanded when the ice also
+expanded.</p>
+
+<p>According to the older glacial hypotheses the lower
+temperature which is postulated as the cause of glaciation
+would almost certainly mean less evaporation over
+the oceans and hence less precipitation during glacial
+periods. To counteract this the only way in which the
+<!--File: cc167.png Page 143-->
+<a class="pagenum break" id="Page_143" title="pg 143"></a>
+level of the lakes could be raised would be because the
+lower temperature would cause less evaporation from
+their surfaces. It seems quite impossible, however, that
+the lowering of temperature, which is commonly taken
+to have been not more than 10&deg;C., could counteract the
+lessened precipitation and also cause an enormous expansion
+of most of the lakes. For example, ancient Lake
+Bonneville was more than ten times as large as its
+modern remnant, Great Salt Lake, and its average depth
+more than forty times as great.<sup><a href="#fn53" id="r53">[53]</a></sup> Many small lakes in the
+Old World expanded still more.<sup><a href="#fn54" id="r54">[54]</a></sup> For example, in eastern
+Persia many basins which now contain no lake whatever
+are floored with vast deposits of lacustrine salt and are
+surrounded by old lake bluffs and beaches. In northern
+Africa similar conditions prevail.<sup><a href="#fn55" id="r55">[55]</a></sup> Other, but less obvious,
+evidence of more abundant rainfall in regions that
+are now dry is found in thick strata of gravel, sand, and
+fine silt in the alluvial deposits of flood plains and deltas.<sup><a href="#fn56" id="r56">[56]</a></sup></p>
+
+<p>The cyclonic hypothesis supposes that increased
+storminess accounts for pluvial climates in regions that
+are now dry just as it accounts for glaciation in the
+regions of the ice sheets. Figs. 2 and 3, it will be remembered,
+illustrate what happens when the sun is active.
+Solar activity is accompanied by an increase in storminess
+in the southwestern United States in exactly the region
+where elevated strands of diminished salt lakes are
+most numerous. In Fig. 3, the same condition is seen in
+<!--File: cc168.png Page 144-->
+<a class="pagenum break" id="Page_144" title="pg 144"></a>
+the region of salt lakes in the Old World. Judging by
+these maps, which illustrate what has happened since
+careful meteorological records were kept, an increase in
+solar activity is accompanied by increased rainfall in
+large parts of what are now semi-arid and desert regions.
+Such precipitation would at once cause the level of the
+lakes to rise. Later, when ice sheets had developed in
+Europe and America, the high-pressure areas thus caused
+might force the main storm belt so far south that it would
+lie over these same arid regions. The increase in tropical
+hurricanes at times of abundant sunspots may also have
+a bearing on the climate of regions that are now arid.
+During the glacial period some of the hurricanes probably
+swept far over the lands. The numerous tropical
+cyclones of Australia, for example, are the chief source
+of precipitation for that continent.<sup><a href="#fn57" id="r57">[57]</a></sup> Some of the stronger
+cyclones locally yield more rain in a day or two than
+other sources yield in a year.</p>
+
+<p>V. The occurrence of widespread glaciation near the
+tropics during the Permian, as shown in Fig. 7, has given
+rise to much discussion. The recent discovery of glaciation
+in latitudes as low as 30&deg; in the Proterozoic is correspondingly
+significant. In all cases the occurrence of
+glaciation in low and middle latitudes is probably due to
+the same general causes. Doubtless the position and altitude
+of the mountains had something to do with the
+matter. Yet taken by itself this seems insufficient. Today
+the loftiest range in the world, the Himalayas, is almost
+unglaciated, although its southern slope may seem at first
+thought to be almost ideally located in this respect. Some
+parts rise over 20,000 feet and certain lower slopes receive
+400 inches of rain per year. The small size of the
+Himalayan glaciers in spite of these favorable conditions
+<!--File: cc169.png Page 145-->
+<a class="pagenum break" id="Page_145" title="pg 145"></a>
+
+<!--File: cc170.png Page 146-->
+<a class="pagenum break" id="Page_146" title="pg 146"></a>
+is apparently due largely to the seasonal character of the
+monsoon winds. The strong outblowing monsoons of
+winter cause about half the year to be very dry with clear
+skies and dry winds from the interior of Asia. In all low
+latitudes the sun rides high in the heavens at midday,
+even in winter, and thus melts snow fairly effectively in
+clear weather. This is highly unfavorable to glaciation.
+The inblowing southern monsoons bring all their moisture
+in midsummer at just the time when it is least effective
+in producing snow. Conditions similar to those now
+prevailing in the Himalayas must accompany any great
+uplift of the lands which produces high mountains and
+large continents in subtropical and middle latitudes.
+Hence, uplift alone cannot account for extensive glaciation
+in subtropical latitudes during the Permian and
+Proterozoic.</p>
+
+<img src="images/fig7.png" width="800" height="532" alt="Fig. 7"  id="Fig_7" />
+<p class="center"><em>Fig. 7. Permian geography and glaciation.<br />
+(After Schuchert.)</em></p>
+
+<p>The assumption of a great general lowering of temperature
+is also not adequate to explain glaciation in
+subtropical latitudes. In the first place this would require
+a lowering of many degrees,&mdash;far more than in the Pleistocene
+glacial period. The marine fossils of the Permian,
+however, do not indicate any such condition. In the
+second place, if the lands were widespread as they appear
+to have been in the Permian, a general lowering of
+temperature would diminish rather than increase the
+present slight efficiency of the monsoons in producing
+glaciation. Monsoons depend upon the difference between
+the temperatures of land and water. If the general temperature
+were lowered, the reduction would be much less
+pronounced on the oceans than on the lands, for water
+tends to preserve a uniform temperature, not only because
+of its mobility, but because of the large amount of
+heat given out when freezing takes place, or consumed in
+evaporation. Hence the general lowering of temperature
+<!--File: cc171.png Page 147-->
+<a class="pagenum break" id="Page_147" title="pg 147"></a>
+would make the contrast between continents and oceans
+less than at present in summer, for the land temperature
+would be brought toward that of the ocean. This would
+diminish the strength of the inblowing summer monsoons
+and thus cut off part of the supply of moisture.
+Evidence that this actually happened in the cold fourteenth
+century has already been given in Chapter VI.
+On the other hand, in winter the lands would be much
+colder than now and the oceans only a little colder, so
+that the dry outblowing monsoons of the cold season
+would increase in strength and would also last longer
+than at present. In addition to all this, the mere fact of
+low temperature would mean a general reduction in the
+amount of water vapor in the air. Thus, from almost
+every point of view a mere lowering of temperature
+seems to be ruled out as a cause of Permian glaciation.
+Moreover, if the Permian or Proterozoic glacial periods
+were so cold that the lands above latitude 30&deg; were snow-covered
+most of the time, the normal surface winds in
+subtropical latitudes would be largely equatorward, just
+as the winter monsoons now are. Hence little or no moisture
+would be available to feed the snowfields which give
+rise to the glaciers.</p>
+
+<p>It has been assumed by Marsden Manson and others
+that increased general cloudiness would account for the
+subtropical glaciation of the Permian and Proterozoic.
+Granting for the moment that there could be universal
+persistent cloudiness, this would not prevent or counteract
+the outblowing anti-cyclonic winds so characteristic
+of great snowfields. Therefore, under the hypothesis of
+general cloudiness there would be no supply of moisture
+to cause glaciation in low latitudes. Indeed, persistent
+cloudiness in all higher latitudes would apparently deprive
+the Himalayas of most of their present moisture,
+<!--File: cc172.png Page 148-->
+<a class="pagenum break" id="Page_148" title="pg 148"></a>
+for the interior of Asia would not become hot in summer
+and no inblowing monsoons would develop. In fact, winds
+of all kinds would seemingly be scarce, for they arise
+almost wholly from contrasts of temperature and hence
+of atmospheric pressure. The only way to get winds and
+hence precipitation would be to invoke some other agency,
+such as cyclonic storms, but that would be a departure
+from the supposition that glaciation arose from cloudiness.</p>
+
+<p>Let us now inquire how the cyclonic hypothesis
+accounts for glaciation in low latitudes. We will first
+consider the terrestrial conditions in the early Permian,
+the last period of glaciation in such latitudes. Geologists
+are almost universally agreed that the lands were exceptionally
+extensive and also high, especially in low latitudes.
+One evidence of this is the presence of abundant
+conglomerates composed of great boulders. It is also
+probable that the carbon dioxide in the air during the
+early Permian had been reduced to a minimum by the
+extraordinary amount of coal formed during the preceding
+period. This would tend to produce low temperature
+and thus make the conditions favorable for glaciation as
+soon as an accentuation of solar activity caused unusual
+storminess. If the storminess became extreme when terrestrial
+conditions were thus universally favorable to
+glaciation, it would presumably produce glaciation in low
+latitudes. Numerous and intense tropical cyclones would
+carry a vast amount of moisture out of the tropics, just
+as now happens when the sun is active, but on a far
+larger scale. The moisture would be precipitated on the
+equatorward slopes of the subtropical mountain ranges.
+At high elevations this precipitation would be in the form
+of snow even in summer. Tropical cyclones, however, as
+is shown in <em>Earth and Sun</em>, occur in the autumn and
+<!--File: cc173.png Page 149-->
+<a class="pagenum break" id="Page_149" title="pg 149"></a>
+winter as well as in summer. For example, in the Bay of
+Bengal the number recorded in October is fifty, the
+largest for any month; while in November it is thirty-four,
+and December fourteen as compared with an average
+of forty-two for the months of July to September.
+From January to March, when sunspot numbers averaged
+more than forty, the number of tropical hurricanes
+was 143 per cent greater than when the sunspot numbers
+averaged below forty. During the months from April to
+June, which also would be times of considerable snowy
+precipitation, tropical hurricanes averaged 58 per cent
+more numerous with sunspot numbers above forty than
+with numbers below forty, while from July to September
+the difference amounted to 23 per cent. Even at this
+season some snow falls on the higher slopes, while the
+increased cloudiness due to numerous storms also tends
+to preserve the snow. Thus a great increase in the frequency
+of sunspots is accompanied by increased intensity
+of tropical hurricanes, especially in the cooler autumn and
+spring months, and results not only in a greater accumulation
+of snow but in a decrease in the melting of the
+snow because of more abundant clouds. At such times as
+the Permian, the general low temperature due to rapid
+convection and to the scarcity of carbon dioxide presumably
+joined with the extension of the lands in producing
+great high-pressure areas over the lands in middle
+latitudes during the winters, and thus caused the more
+northern, or mid-latitude type of cyclonic storms to be
+shifted to the equatorward side of the continents at that
+season. This would cause an increase of precipitation in
+winter as well as during the months when tropical hurricanes
+abound. Many other circumstances would co&ouml;perate
+to produce a similar result. For example, the general
+low temperature would cause the sea to be covered with
+<!--File: cc174.png Page 150-->
+<a class="pagenum break" id="Page_150" title="pg 150"></a>
+ice in lower latitudes than now, and would help to create
+high-pressure areas in middle latitudes, thus driving the
+storms far south. If the sea water were fresher than now,
+as it probably was to a notable extent in the Proterozoic
+and perhaps to some slight extent in the Permian, the
+higher freezing point would also further the extension
+of the ice and help to keep the storms away from high
+latitudes. If to this there is added a distribution of land
+and sea such that the volume of the warm ocean currents
+flowing from low to high latitudes was diminished, as
+appears to have been the case, there seems to be no difficulty
+in explaining the subtropical location of the main
+glaciation in both the Permian and the Proterozoic. An
+increase of storminess seems to be the key to the whole
+situation.</p>
+
+<p>One other possibility may be mentioned, although little
+stress should be laid on it. In <em>Earth and Sun</em> it has been
+shown that the main storm track in both the northern
+and southern hemispheres is not concentric with the
+geographical poles. Both tracks are roughly concentric
+with the corresponding magnetic poles, a fact which may
+be important in connection with the hypothesis of an electrical
+effect of the sun upon terrestrial storminess. The
+magnetic poles are known to wander considerably. Such
+wandering gives rise to variations in the direction of
+the magnetic needle from year to year. In 1815 the compass
+in England pointed 24-&frac12;&deg; W. of N. and in 1906
+17&deg; 45' W. Such a variation seems to mean a change of
+many miles in the location of the north magnetic pole.
+Certain changes in the daily march of electromagnetic
+phenomena over the oceans have led Bauer and his associates
+to suggest that the magnetic poles may even be
+subject to a slight daily movement in response to the
+changes in the relative positions of the earth and sun.
+<!--File: cc175.png Page 151-->
+<a class="pagenum break" id="Page_151" title="pg 151"></a>
+Thus there seems to be a possibility that a pronounced
+change in the location of the magnetic pole in Permian
+times, for example, may have had some connection with
+a shifting in the location of the belt of storms. It must be
+clearly understood that there is as yet no evidence of any
+such change, and the matter is introduced merely to call
+attention to a possible line of investigation.</p>
+
+<p>Any hypothesis of Permian and Proterozoic glaciation
+must explain not only the glaciation of low latitudes but
+the lack of glaciation and the accumulation of red desert
+beds in high latitudes. The facts already presented seem
+to explain this. Glaciation could not occur extensively in
+high latitudes partly because during most of the year the
+air was too cold to hold much moisture, but still more
+because the winds for the most part must have blown
+outward from the cold northern areas and the cyclonic
+storm belt was pushed out of high latitudes. Because of
+these conditions precipitation was apparently limited to
+a relatively small number of storms during the summer.
+Hence great desert areas must have prevailed at high
+latitudes. Great aridity now prevails north of the Himalayas
+and related ranges, and red beds are accumulating
+in the centers of the great deserts, such as those of the
+Tarim Basin and the Transcaspian. The redness is not
+due to the original character of the rock, but to intense
+oxidation, as appears from the fact that along the edges
+of the desert and wherever occasional floods carry sediment
+far out into the midst of the sand, the material has
+the ordinary brownish shades. As soon as one goes out
+into the places where the sand has been exposed to the air
+for a long time, however, it becomes pink, and then red.
+Such conditions may have given rise to the high degree
+of oxidation in the famous Permian red beds. If the air
+of the early Permian contained an unusual percentage of
+<!--File: cc176.png Page 152-->
+<a class="pagenum break" id="Page_152" title="pg 152"></a>
+oxygen because of the release of that gas by the great
+plant beds which formed coal in the preceding era, as
+Chamberlin has thought probable, the tendency to produce
+red beds would be still further increased.</p>
+
+<p>It must not be supposed, however, that these conditions
+would absolutely limit glaciation to subtropical
+latitudes. The presence of early Permian glaciation in
+North America at Boston and in Alaska and in the Falkland
+Islands of the South Atlantic Ocean proves that at
+least locally there was sufficient moisture to form glaciers
+near the coast in relatively high latitudes. The possibility
+of this would depend entirely upon the form of the lands
+and the consequent course of ocean currents. Even in
+those high latitudes cyclonic storms would occur unless
+they were kept out by conditions of pressure such as have
+been described above.</p>
+
+<p>The marine faunas of Permian age in high latitudes
+have been interpreted as indicating mild oceanic temperatures.
+This is a point which requires further investigation.
+Warm oceans during times of slight solar activity
+are a necessary consequence of the cyclonic hypothesis,
+as will appear later. The present cold oceans seem to be
+the expectable result of the Pleistocene glaciation and of
+the present relatively disturbed condition of the sun. If
+a sudden disturbance threw the solar atmosphere into
+violent commotion within a few thousand years during
+Permian times, glaciation might occur as described above,
+while the oceans were still warm. In fact their warmth
+would increase evaporation while the violent cyclonic
+storms and high winds would cause heavy rain and keep
+the air cool by constantly raising it to high levels where
+it would rapidly radiate its heat into space.</p>
+
+<p>Nevertheless it is not yet possible to determine how
+warm the oceans were at the actual time of the Permian
+<!--File: cc177.png Page 153-->
+<a class="pagenum break" id="Page_153" title="pg 153"></a>
+glaciation. Some faunas formerly reported as Permian
+are now known to be considerably older. Moreover, others
+of undoubted Permian age are probably not strictly contemporaneous
+with the glaciation. So far back in the
+geological record it is very doubtful whether we can date
+fossils within the limits of say 100,000 years. Yet a difference
+of 100,000 years would be more than enough to
+allow the fossils to have lived either before or after the
+glaciation, or in an inter-glacial epoch. One such epoch
+is known to have occurred and nine others are suggested
+by the inter-stratification of glacial till and marine sediments
+in eastern Australia. The warm currents which
+would flow poleward in inter-glacial epochs must have
+favored a prompt reintroduction of marine faunas driven
+out during times of glaciation. Taken all and all, the
+Permian glaciation seems to be accounted for by the
+cyclonic hypothesis quite as well as does the Pleistocene.
+In both these cases, as well as in the various pulsations
+of historic times, it seems to be necessary merely to magnify
+what is happening today in order to reproduce the
+conditions which prevailed in the past. If the conditions
+which now prevail at times of sunspot minima were magnified,
+they would give the mild conditions of inter-glacial
+epochs and similar periods. If the conditions which now
+prevail at times of sunspot maxima are magnified a little
+they seem to produce periods of climatic stress such as
+those of the fourteenth century. If they are magnified
+still more the result is apparently glacial epochs like
+those of the Pleistocene, and if they are magnified to a
+still greater extent, the result is Permian or Proterozoic
+glaciation. Other factors must indeed be favorable, for
+climatic changes are highly complex and are unquestionably
+due to a combination of circumstances. The point
+which is chiefly emphasized in this book is that among
+<!--File: cc178.png Page 154-->
+<a class="pagenum break" id="Page_154" title="pg 154"></a>
+those several circumstances, changes in cyclonic storms
+due apparently to activity of the sun's atmosphere must
+always be reckoned.</p>
+<!--File: cc179.png Page 155-->
+<a class="pagenum break" id="Page_155" title="pg 155"></a>
+
+
+
+<h2 id="Chapt_IX"><a href="#content">CHAPTER IX</a></h2>
+
+<h3>THE ORIGIN OF LOESS</h3>
+
+
+<p>One of the most remarkable formations associated
+with glacial deposits consists of vast sheets
+of the fine-grained, yellowish, wind-blown material
+called loess. Somewhat peculiar climatic conditions
+evidently prevailed when it was formed. At present
+similar deposits are being laid down only near the leeward
+margin of great deserts. The famous loess deposits
+of China in the lee of the Desert of Gobi are examples.
+During the Pleistocene period, however, loess accumulated
+in a broad zone along the margin of the ice sheet
+at its maximum extent. In the Old World it extended
+from France across Germany and through the Black
+Earth region of Russia into Siberia. In the New World
+a still larger area is loess-covered. In the Mississippi
+Valley, tens of thousands of square miles are mantled by
+a layer exceeding twenty feet in thickness and in many
+places approaching a hundred feet. Neither the North
+American nor the European deposits are associated with
+a desert. Indeed, loess is lacking in the western and
+drier parts of the great plains and is best developed in
+the well-watered states of Iowa, Illinois, and Missouri.
+Part of the loess overlies the non-glacial materials of the
+great central plain, but the northern portions overlie the
+drift deposits of the first three glaciations. A few traces
+of loess are associated with the Kansan and Illinoian,
+the second and third glaciations, but most of the American
+<!--File: cc180.png Page 156-->
+<a class="pagenum break" id="Page_156" title="pg 156"></a>
+loess appears to have been formed at approximately
+the time of the Iowan or fourth glaciation, while only a
+little overlies the drift sheets of the Wisconsin age. The
+loess is thickest near the margin of the Iowan till sheet
+and thins progressively both north and south. The
+thinning southward is abrupt along the stream divides,
+but very gradual along the larger valleys. Indeed, loess is
+abundant along the bluffs of the Mississippi, especially
+the east bluff, almost to the Gulf of Mexico.<sup><a href="#fn58" id="r58">[58]</a></sup></p>
+
+<p>It is now generally agreed that all typical loess is wind
+blown. There is still much question, however, as to its
+time of origin, and thus indirectly as to its climatic implications.
+Several American and European students
+have thought that the loess dates from inter-glacial times.
+On the other hand, Penck has concluded that the loess
+was formed shortly before the commencement of the
+glacial epochs; while many American geologists hold that
+the loess accumulated while the ice sheets were at approximately
+their maximum size. W. J. McGee, Chamberlin
+and Salisbury, Keyes, and others lean toward this
+view. In this chapter the hypothesis is advanced that it
+was formed at the one other possible time, namely, immediately
+following the retreat of the ice.</p>
+
+<p>These four hypotheses as to the time of origin of loess
+imply the following differences in its climatic relations.
+If loess was formed during typical inter-glacial epochs,
+or toward the close of such epochs, profound general
+aridity must seemingly have prevailed in order to kill
+off the vegetation and thus enable the wind to pick up
+sufficient dust. If the loess was formed during times of
+extreme glaciation when the glaciers were supplying
+large quantities of fine material to outflowing streams,
+less aridity would be required, but there must have been
+<!--File: cc181.png Page 157-->
+<a class="pagenum break" id="Page_157" title="pg 157"></a>
+sharp contrasts between wet seasons in summer when
+the snow was melting and dry seasons in winter when
+the storms were forced far south by the glacial high pressure.
+Alternate floods and droughts would thus affect
+broad areas along the streams. Hence arises the hypothesis
+that the wind obtained the loess from the flood plains
+of streams at times of maximum glaciation. If the loess
+was formed during the rapid retreat of the ice, alternate
+summer floods and winter droughts would still prevail,
+but much material could also be obtained by the winds
+not only from flood plains, but also from the deposits
+exposed by the melting of the ice and not yet covered by
+vegetation.</p>
+
+<p>The evidence for and against the several hypotheses
+may be stated briefly. In support of the hypothesis of the
+inter-glacial origin of loess, Shimek and others state that
+the glacial drift which lies beneath the loess commonly
+gives evidence that some time elapsed between the disappearance
+of the ice and the deposition of the loess. For
+example, abundant shells of land snails in the loess are
+not of the sort now found in colder regions, but resemble
+those found in the drier regions. It is probable that if
+they represented a glacial epoch they would be depauperated
+by the cold as are the snails of far northern regions.
+The gravel pavement discussed below seems to be strong
+evidence of erosion between the retreat of the ice and
+the deposition of the loess.</p>
+
+<p>Turning to the second hypothesis, namely, that the
+loess accumulated near the close of the inter-glacial epoch
+rather than in the midst of it, we may follow Penck. The
+mammalian fossils seem to him to prove that the loess
+was formed while boreal animals occupied the region, for
+they include remains of the hairy mammoth, woolly rhinoceros,
+and reindeer. On the other hand, the typical
+<!--File: cc182.png Page 158-->
+<a class="pagenum break" id="Page_158" title="pg 158"></a>
+inter-glacial beds not far away yield remains of species
+characteristic of milder climates, such as the elephant,
+the smaller rhinoceros, and the deer. In connection with
+these facts it should be noted that occasional remains of
+tundra vegetation and of trees are found beneath the
+loess, while in the loess itself certain steppe animals,
+such as the common gopher or spermaphyl, are found.
+Penck interprets this as indicating a progressive desiccation
+culminating just before the oncoming of the next ice
+sheet.</p>
+
+<p>The evidence advanced in favor of the hypothesis that
+the loess was formed when glaciation was near its maximum
+includes the fact that if the loess does not represent
+the outwash from the Iowan ice, there is little else that
+does, and presumably there must have been outwash.
+Also the distribution of loess along the margins of
+streams suggests that much of the material came from
+the flood plains of overloaded streams flowing from the
+melting ice.</p>
+
+<p>Although there are some points in favor of the hypothesis
+that the loess originated (1) in strictly inter-glacial
+times, (2) at the end of inter-glacial epochs, and
+(3) at times of full glaciation, each hypothesis is much
+weakened by evidence that supports the others. The evidence
+of boreal animals seems to disprove the hypothesis
+that the loess was formed in the middle of a mild inter-glacial
+epoch. On the other hand, Penck's hypothesis as
+to loess at the end of inter-glacial times fails to account
+for certain characteristics of the lowest part of the loess
+deposits and of the underlying topography. Instead of
+normal valleys and consequent prompt drainage such as
+ought to have developed before the end of a long inter-glacial
+epoch, the surface on which the loess lies shows
+many undrained depressions. Some of these can be seen
+<!--File: cc183.png Page 159-->
+<a class="pagenum break" id="Page_159" title="pg 159"></a>
+in exposed banks, while many more are inferred from the
+presence of shells of pond snails here and there in the
+overlying loess. The pond snails presumably lived in
+shallow pools occupying depressions in the uneven surface
+left by the ice. Another reason for questioning
+whether the loess was formed at the end of an inter-glacial
+epoch is that this hypothesis does not provide a
+reasonable origin for the material which composes the
+loess. Near the Alps where the loess deposits are small
+and where glaciers probably persisted in the inter-glacial
+epochs and thus supplied flood plain material in large
+quantities, this does not appear important. In the broad
+upper Mississippi Basin, however, and also in the Black
+Earth region of Russia there seems to be no way to get
+the large body of material composing the loess except by
+assuming the existence of great deserts to windward.
+But there seems to be little or no evidence of such deserts
+where they could be effective. The mineralogical character
+of the loess of Iowan age proves that the material
+came from granitic rocks, such as formed a large part of
+the drift. The nearest extensive outcrops of granite are
+in the southwestern part of the United States, nearly a
+thousand miles from Iowa and Illinois. But the loess is
+thickest near the ice margin and thins toward the southwest
+and in other directions, whereas if its source were
+the southwestern desert, its maximum thickness would
+probably be near the margin of the desert.</p>
+
+<p>The evidence cited above seems inconsistent not only
+with the hypothesis that the loess was formed at the end
+of an inter-glacial epoch, but also with the idea that it
+originated at times of maximum glaciation either from
+river-borne sediments or from any other source. A
+further and more convincing reason for this last conclusion
+is the probability and almost the certainty that
+<!--File: cc184.png Page 160-->
+<a class="pagenum break" id="Page_160" title="pg 160"></a>
+when the ice advanced, its front lay close to areas where
+the vegetation was not much thinner than that which
+today prevails under similar climatic conditions. If the
+average temperature of glacial maxima was only 6&deg;C.
+lower than that of today, the conditions just beyond the
+ice front when it was in the loess region from southern
+Illinois to Minnesota would have been like those now prevailing
+in Canada from New Brunswick to Winnipeg.
+The vegetation there is quite different from the grassy,
+semi-arid vegetation of which evidence is found in the
+loess. The roots and stalks of such grassy vegetation are
+generally agreed to have helped produce the columnar
+structure which enables the loess to stand with almost
+vertical surfaces.</p>
+
+<p>We are now ready to consider the probability that loess
+accumulated mainly during the retreat of the ice. Such a
+retreat exposed a zone of drift to the outflowing glacial
+winds. Most glacial hypotheses, such as that of uplift,
+or depleted carbon dioxide, call for a gradual retreat
+of the ice scarcely faster than the vegetation could advance
+into the abandoned area. Under the solar-cyclonic
+hypothesis, on the other hand, the climatic changes may
+have been sudden and hence the retreat of the ice may
+have been much more rapid than the advance of vegetation.
+Now wind-blown materials are derived from places
+where vegetation is scanty. Scanty vegetation on good
+soil, it is true, is usually due to aridity, but may also
+result because the time since the soil was exposed to the
+air has not been long enough for the soil to be sufficiently
+weathered to support vegetation. Even when weathering
+has had full opportunity, as when sand bars, mud flats,
+and flood plains are exposed, vegetation takes root only
+slowly. Moreover, storms and violent winds may prevent
+the spread of vegetation, as is seen on sandy beaches even
+<!--File: cc185.png Page 161-->
+<a class="pagenum break" id="Page_161" title="pg 161"></a>
+in distinctly humid regions like New Jersey and Denmark.
+Thus it appears that unless the retreat of the ice
+were as slow as the advance of vegetation, a barren area
+of more or less width must have bordered the retreating
+ice and formed an ideal source of loess.</p>
+
+<p>Several other lines of evidence seemingly support the
+conclusion that the loess was formed during the retreat
+of the ice. For example, Shimek, who has made almost
+a lifelong study of the Iowan loess, emphasizes the fact
+that there is often an accumulation of stones and pebbles
+at its base. This suggests that the underlying till was
+eroded before the loess was deposited upon it. The first
+reaction of most students is to assume that of course
+this was due to running water. That is possible in many
+cases, but by no means in all. So widespread a sheet of
+gravel could not be deposited by streams without destroying
+the irregular basins and hollows of which we have
+seen evidence where the loess lies on glacial deposits. On
+the other hand, the wind is competent to produce a similar
+gravel pavement without disturbing the old topography.
+"Desert pavements" are a notable feature in most
+deserts. On the edges of an ice sheet, as Hobbs has made
+us realize, the commonest winds are outward. They often
+attain a velocity of eighty miles an hour in Antarctica
+and Greenland. Such winds, however, usually decline
+rapidly in velocity only a few score miles from the ice.
+Thus their effect would be to produce rapid erosion
+of the freshly bared surface near the retreating ice.
+The pebbles would be left behind as a pavement, while
+sand and then loess would be deposited farther from the
+ice where the winds were weaker and where vegetation
+was beginning to take root. Such a decrease in wind
+velocity may explain the occasional vertical gradation
+from gravel through sand to coarse loess and then to
+<!--File: cc186.png Page 162-->
+<a class="pagenum break" id="Page_162" title="pg 162"></a>
+normal fine loess. As the ice sheet retreated the wind in
+any given place would gradually become less violent.
+As the ice continued to retreat the area where loess was
+deposited would follow at a distance, and thus each part
+of the gravel pavement would in turn be covered with the
+loess.</p>
+
+<p>The hypothesis that loess is deposited while the ice is
+retreating is in accord with many other lines of evidence.
+For example, it accords with the boreal character of the
+mammal remains as described above. Again, the advance
+of vegetation into the barren zone along the front of the
+ice would be delayed by the strong outblowing winds.
+The common pioneer plants depend largely on the wind
+for the distribution of their seeds, but the glacial winds
+would carry them away from the ice rather than toward
+it. The glacial winds discourage the advance of vegetation
+in another way, for they are drying winds, as are
+almost all winds blowing from a colder to a warmer
+region. The fact that remains of trees sometimes occur
+at the bottom of the loess probably means that the deposition
+of loess extended into the forests which almost
+certainly persisted not far from the ice. This seems more
+likely than that a period of severe aridity before the advance
+of the ice killed the trees and made a steppe or
+desert. Penck's chief argument in favor of the formation
+of loess before the advance of the ice rather than after,
+is that since loess is lacking upon the youngest drift sheet
+in Europe it must have been formed before rather than
+after the last or W&uuml;rm advance of the ice. This breaks
+down on two counts. First, on the corresponding (Wisconsin)
+drift sheet in America, loess is present,&mdash;in small
+quantities to be sure, but unmistakably present. Second,
+there is no reason to assume that conditions were identical
+at each advance and retreat of the ice. Indeed, the
+<!--File: cc187.png Page 163-->
+<a class="pagenum break" id="Page_163" title="pg 163"></a>
+fact that in Europe, as in the United States, nearly all
+the loess was formed at one time, and only a little is associated
+with the other ice advances, points clearly against
+Penck's fundamental assumption that the accumulation
+of loess was due to the approach of a cold climate.</p>
+
+<p>Having seen that the loess was probably formed during
+the retreat of the ice, we are now ready to inquire what
+conditions the cyclonic hypothesis would postulate in the
+loess areas during the various stages of a glacial cycle.
+Fig. 2, in Chapter IV, gives the best idea of what would
+apparently happen in North America, and events in Europe
+would presumably be similar. During the nine maximum
+years on which Fig. 2 is based the sunspot numbers averaged
+seventy, while during the nine minimum years they
+averaged less than five. It seems fair to suppose that the
+maximum years represent the average conditions which
+prevailed in the past at times when the sun was in a
+median stage between the full activity which led to glaciation
+and the mild activity of the minimum years which
+appear to represent inter-glacial conditions. This would
+mean that when a glacial period was approaching, but
+before an ice sheet had accumulated to any great extent,
+a crescent-shaped strip from Montana through Illinois to
+Maine would suffer a diminution in storminess ranging
+up to 60 per cent as compared with inter-glacial conditions.
+This is in strong contrast with an increase in
+storminess amounting to 75 or even 100 per cent both in
+the boreal storm belt in Canada and in the subtropical
+belt in the Southwest. Such a decrease in storminess in
+the central United States would apparently be most
+noticeable in summer, as is shown in <em>Earth and Sun</em>.
+Hence it would have a maximum effect in producing
+aridity. This would favor the formation of loess, but it is
+In discussions of climate, as of most subjects, a
+peculiar psychological phenomenon is observable.
+Everyone sees the necessity of explaining conditions
+different from those that now exist, but few realize that
+present conditions may be abnormal, and that they need
+explanation just as much as do others. Because of this
+tendency glaciation has been discussed with the greatest
+fullness, while there has been much neglect not only of
+the periods when the climate of the earth resembled that
+of the present, but also of the vastly longer periods when
+it was even milder than now.
+doubtful whether the aridity would become extreme
+<!--File: cc188.png Page 164-->
+<a class="pagenum break" id="Page_164" title="pg 164"></a>
+enough to explain such vast deposits as are found
+throughout large parts of the Mississippi Basin. That
+would demand that hundreds of thousands of square miles
+should become almost absolute desert, and it is not probable
+that any such thing occurred. Nevertheless, according
+to the cyclonic hypothesis the period immediately
+before the advent of the ice would be relatively dry in
+the central United States, and to that extent favorable to
+the work of the wind.</p>
+
+<p>As the climatic conditions became more severe and the
+ice sheet expanded, the dryness and lack of storms would
+apparently diminish. The reason, as has been explained,
+would be the gradual pushing of the storms southward
+by the high-pressure area which would develop over the
+ice sheet. Thus at the height of a glacial epoch there
+would apparently be great storminess in the area where
+the loess is found, especially in summer. Hence the
+cyclonic hypothesis does not accord with the idea of great
+deposition of loess at the time of maximum glaciation.</p>
+
+<p>Finally we come to the time when the ice was retreating.
+We have already seen that not only the river flood
+plains, but also vast areas of fresh glacial deposits would
+be exposed to the winds, and would remain without vegetation
+for a long time. At that very time the retreat of
+the ice sheet would tend to permit the storms to follow
+paths determined by the degree of solar activity, in place
+of the far southerly paths to which the high atmospheric
+pressure over the expanded ice sheet had previously
+forced them. In other words, the conditions shown in
+Fig. 2 would tend to reappear when the sun's activity
+was diminishing and the ice sheet was retreating, just as
+they had appeared when the sun was becoming more
+active and the ice sheet was advancing. This time, however,
+the semi-arid conditions arising from the scarcity
+<!--File: cc189.png Page 165-->
+<a class="pagenum break" id="Page_165" title="pg 165"></a>
+of storms would prevail in a region of glacial deposits
+and widely spreading river deposits, few or none of
+which would be covered with vegetation. The conditions
+would be almost ideal for eolian erosion and for the
+transportation of loess by the wind to areas a little more
+remote from the ice where grassy vegetation had made a
+start.</p>
+
+<p>The cyclonic hypothesis also seems to offer a satisfactory
+explanation of variations in the amount of loess
+associated with the several glacial epochs. It attributes
+these to differences in the rate of disappearance of the
+ice, which in turn varied with the rate of decline of solar
+activity and storminess. This is supposed to be the reason
+why the Iowan loess deposits are much more extensive
+than those of the other epochs, for the Iowan ice sheet
+presumably accomplished part of its retreat much more
+suddenly than the other ice sheets.<sup><a href="#fn59" id="r59">[59]</a></sup> The more sudden the
+retreat, the greater the barren area where the winds
+could gather fine bits of dust. Temporary readvances may
+also have been so distributed and of such intensity that
+they frequently accentuated the condition shown in Fig.
+2, thus making the central United States dry soon after
+the exposure of great amounts of glacial d&eacute;bris. The
+closeness with which the cyclonic hypothesis accords with
+the facts as to the loess is one of the pleasant surprises of
+the hypothesis. The first draft of Fig. 2 and the first outlines
+of the hypothesis were framed without thought of
+the loess. Yet so far as can now be seen, both agree
+closely with the conditions of loess formation.</p>
+
+
+<!--File: cc190.png Page 166-->
+<a class="pagenum break" id="Page_166" title="pg 166"></a>
+
+
+
+
+<h2 id="Chapt_X"><a href="#content">CHAPTER X</a></h2>
+
+<h2>CAUSES OF MILD GEOLOGICAL CLIMATES</h2>
+
+
+<p>In discussions of climate, as of most subjects, a
+peculiar psychological phenomenon is observable.
+Everyone sees the necessity of explaining conditions
+different from those that now exist, but few realize that
+present conditions may be abnormal, and that they need
+explanation just as much as do others. Because of this
+tendency glaciation has been discussed with the greatest
+fullness, while there has been much neglect not only of
+the periods when the climate of the earth resembled that
+of the present, but also of the vastly longer periods when
+it was even milder than now.</p>
+
+<p>How important the periods of mild climate have been
+in geological times may be judged from the relative
+length of glacial compared with inter-glacial epochs, and
+still more from the far greater relative length of the mild
+parts of periods and eras when compared with the severe
+parts. Recent estimates by R. T. Chamberlin<sup><a href="#fn60" id="r60">[60]</a></sup> indicate
+that according to the consensus of opinion among geologists
+the average inter-glacial epoch during the Pleistocene
+was about five times as long as the average glacial
+epoch, while the whole of a given glacial epoch averaged
+five times as long as the period when the ice was at a
+maximum. Climatic periods far milder, longer, and more
+monotonous than any inter-glacial epoch appear repeatedly
+<!--File: cc191.png Page 167-->
+<a class="pagenum break" id="Page_167" title="pg 167"></a>
+during the course of geological history. Our task in
+this chapter is to explain them.</p>
+
+<p>Knowlton<sup><a href="#fn61" id="r61">[61]</a></sup> has done geology a great service by collecting
+the evidence as to the mild type of climate which
+has again and again prevailed in the past. He lays special
+stress on botanical evidence since that pertains to the
+variable atmosphere of the lands, and hence furnishes a
+better guide than does the evidence of animals that lived
+in the relatively unchanging water of the oceans. The
+nature of the evidence has already been indicated in
+various parts of this book. It includes palms, tree ferns,
+and a host of other plants which once grew in regions
+which are now much too cold to support them. With this
+must be placed the abundant reef-building corals and
+other warmth-loving marine creatures in latitudes now
+much too cold for them. Of a piece with this are the conditions
+of inter-glacial epochs in Europe, for example,
+when elephants and hippopotamuses, as well as many
+species of plants from low latitudes, were abundant.
+These conditions indicate not only that the climate was
+warmer than now, but that the contrast from season to
+season was much less. Indeed, Knowlton goes so far as
+to say that "relative uniformity, mildness, and comparative
+equability of climate, accompanied by high humidity,
+have prevailed over the greater part of the earth, extending
+to, or into, polar circles, during the greater part of
+geologic time&mdash;since, at least, the Middle Paleozoic. This
+is the regular, the ordinary, the normal condition." ... "By
+many it is thought that one of the strongest arguments
+against a gradually cooling globe and a humid,
+non-zonally disposed climate in the ages before the Pleistocene
+is the discovery of evidences of glacial action
+<!--File: cc192.png Page 168-->
+<a class="pagenum break" id="Page_168" title="pg 168"></a>
+practically throughout the entire geologic column.
+Hardly less than a dozen of these are now known, ranging
+in age from Huronian to Eocene. It seems to be a very
+general assumption by those who hold this view that
+these evidences of glacial activities are to be classed as
+ice ages, largely comparable in effect and extent to the
+Pleistocene refrigeration, but as a matter of fact only
+three are apparently of a magnitude to warrant such
+designation. These are the Huronian glaciation, that of
+the 'Permo-Carboniferous,' and that of the Pleistocene.
+The others, so far as available data go, appear to be
+explainable as more or less local manifestations that had
+no widespread effect on, for instance, ocean temperatures,
+distribution of life, et cetera. They might well have
+been of the type of ordinary mountain glaciers, due entirely
+to local elevation and precipitation." ... "If the
+sun had been the principal source of heat in pre-Pleistocene
+time, terrestrial temperatures would of necessity
+have been disposed in zones, whereas the whole trend of
+this paper has been the presentation of proof that these
+temperatures were distinctly non-zonal. Therefore it
+seems to follow that the sun&mdash;at least the present small-angle
+sun&mdash;could not have been the sole or even the principal
+source of heat that warmed the early oceans."</p>
+
+<p>Knowlton is so strongly impressed by the widespread
+fossil floras that usually occur in the middle parts of the
+geological periods, that as Schuchert<sup><a href="#fn62">[62]</a></sup> puts it, he neglects
+the evidence of other kinds. In the middle of the periods
+and eras the expansion of the warm oceans over the continents
+was greatest, while the lands were small and
+hence had more or less insular climates of the oceanic
+type. At such times, the marine fauna agrees with the
+<!--File: cc193.png Page 169-->
+<a class="pagenum break" id="Page_169" title="pg 169"></a>
+flora in indicating a mild climate. Large colony-forming
+foraminifera, stony corals, shelled cephalopods, gastropods
+and thick-shelled bivalves, generally the cemented
+forms, were common in the Far North and even in the
+Arctic. This occurred in the Silurian, Devonian, Pennsylvanian,
+and Jurassic periods, yet at other times, such
+as the Cretaceous and Eocene, such forms were very
+greatly reduced in variety in the northern regions or else
+wholly absent. These things, as Schuchert<sup><a href="#fn62" id="r62">[62]</a></sup> says, can only
+mean that Knowlton is right when he states that "climatic
+zoning such as we have had since the beginning of
+the Pleistocene did not obtain in the geologic ages prior
+to the Pleistocene." It does not mean, however, that
+there was a "non-zonal arrangement" and that the temperature
+of the oceans was everywhere the same and
+"without widespread effect on the distribution of life."</p>
+
+<p>Students of paleontology hold that as far back as we
+can go in the study of plants, there are evidences of seasons
+and of relatively cool climates in high latitudes. The
+cycads, for instance, are one of the types most often used
+as evidence of a warm climate. Yet Wieland,<sup><a href="#fn63" id="r63">[63]</a></sup> who has
+made a lifelong study of these plants, says that many of
+them "might well grow in temperate to cool climates.
+Until far more is learned about them they should at least
+be held as valueless as indices of tropic climates." The
+inference is "that either they or their close relatives had
+the capacity to live in every clime. There is also a suspicion
+that study of the associated ferns may compel revision
+of the long-accepted view of the universality of
+tropic climates throughout the Mesozoic." Nathorst is
+quoted by Wieland as saying, "I think ... that during
+the time when the Gingkophytes and Cycadophytes dominated,
+<!--File: cc194.png Page 170-->
+<a class="pagenum break" id="Page_170" title="pg 170"></a>
+many of them must have adapted themselves for
+living in cold climates also. Of this I have not the least
+doubt."</p>
+
+<p>Another important line of evidence which Knowlton
+and others have cited as a proof of the non-zonal arrangement
+of climate in the past, is the vast red beds which are
+found in the Proterozoic, late Silurian, Devonian, Permian,
+and Triassic, and in some Tertiary formations.
+These are believed to resemble laterite, a red and highly
+oxidized soil which is found in great abundance in equatorial
+regions. Knowlton does not attempt to show that
+the red beds present equatorial characteristics in other
+respects, but bases his conclusion on the statement that
+"red beds are not being formed at the present time in
+any desert region." This is certainly an error. As has
+already been said, in both the Transcaspian and Takla
+Makan deserts, the color of the sand regularly changes
+from brown on the borders to pale red far out in the
+desert. Kuzzil Kum, or Red Sand, is the native name.
+The sands in the center of the desert apparently were
+originally washed down from the same mountains as
+those on the borders, and time has turned them red.
+Since the same condition is reported from the Arabian
+Desert, it seems that redness is characteristic of some of
+the world's greatest deserts. Moreover, beds of salt and
+gypsum are regularly found in red beds, and they can
+scarcely originate except in deserts, or in shallow almost
+landlocked bays on the coasts of deserts, as appears to
+have happened in the Silurian where marine fossils are
+found interbedded with gypsum.</p>
+
+<p>Again, Knowlton says that red beds cannot indicate
+deserts because the plants found in them are not
+"pinched or depauperate, nor do they indicate xerophytic
+adaptations. Moreover, very considerable deposits
+<!--File: cc195.png Page 171-->
+<a class="pagenum break" id="Page_171" title="pg 171"></a>
+of coal are found in red beds in many parts of the world,
+which implies the presence of swamps but little above
+sea-level."</p>
+
+<p>Students of desert botany are likely to doubt the force
+of these considerations. As MacDougal<sup><a href="#fn64" id="r64">[64]</a></sup> has shown, the
+variety of plants in deserts is greater than in moist
+regions. Not only do xerophytic desert species prevail,
+but halophytes are present in the salty areas, and hygrophytes
+in the wet swampy areas, while ordinary mesophytes
+prevail along the water courses and are washed
+down from the mountains. The ordinary plants, not the
+xerophytes, are the ones that are chiefly preserved since
+they occur in most abundance near streams where deposition
+is taking place. So far as swamps are concerned, few
+are of larger size than those of Seistan in Persia, Lop
+Nor in Chinese Turkestan, and certain others in the midst
+of the Asiatic deserts. Streams flowing from the mountains
+into deserts are almost sure to form large swamps,
+such as those along the Tarim River in central Asia.
+Lake Chad in Africa is another example. In it, too, reeds
+are very numerous.</p>
+
+<p>Putting together the evidence on both sides in this disputed
+question, it appears that throughout most of geological
+time there is some evidence of a zonal arrangement
+of climate. The evidence takes the form of traces of
+cool climates, of seasons, and of deserts. Nevertheless,
+there is also strong evidence that these conditions were
+in general less intense than at present and that times of
+relatively warm, moist climate without great seasonal
+extremes have prevailed very widely during periods
+much longer than those when a zonal arrangement as
+<!--File: cc196.png Page 172-->
+<a class="pagenum break" id="Page_172" title="pg 172"></a>
+marked as that of today prevailed. As Schuchert<sup><a href="#fn65" id="r65">[65]</a></sup> puts it:
+"Today the variation on land between the tropics and the
+poles is roughly between 110&deg; and -60&deg;F., in the oceans
+between 85&deg; and 31&deg;F. In the geologic past the temperature
+of the oceans for the greater parts of the periods
+probably was most often between 85&deg; and 55&deg;F., while on
+land it may have varied between 90&deg; and 0&deg;F. At rare
+intervals the extremes were undoubtedly as great as they
+are today. The conclusion is therefore that at all times
+the earth had temperature zones, varying between the
+present-day intensity and times which were almost without
+such belts, and at these latter times the greater part
+of the earth had an almost uniformly mild climate, without
+winters."</p>
+
+<p>It is these mild climates which we must now attempt
+to explain. This leads us to inquire what would happen to
+the climate of the earth as a whole if the conditions which
+now prevail at times of few sunspots were to become
+intensified. That they could become greatly intensified
+seems highly probable, for there is good reason to think
+that aside from the sunspot cycle the sun's atmosphere
+is in a disturbed condition. The prominences which
+sometimes shoot out hundreds of thousands of miles
+seem to be good evidence of this. Suppose that the sun's
+atmosphere should become very quiet. This would apparently
+mean that cyclonic storms would be much less
+numerous and less severe than during the present times
+of sunspot minima. The storms would also apparently
+follow paths in middle latitudes somewhat as they do
+now when sunspots are fewest. The first effect of such a
+condition, if we can judge from what happens at present,
+would be a rise in the general temperature of the earth,
+because less heat would be carried aloft by storms.</p>
+<!--File: cc197.png Page 173-->
+<a class="pagenum break" id="Page_173" title="pg 173"></a>
+<p>Today, as is shown in <em>Earth and Sun</em>, a difference of
+perhaps 10 per cent in the average storminess during
+periods of sunspot maxima and minima is correlated with
+a difference of 3&deg;C. in the temperature at the earth's
+surface. This includes not only an actual lowering of
+0.6&deg;C. at times of sunspot maxima, but the overcoming
+of the effect of increased insolation at such times, an
+effect which Abbot calculates as about 2.5&deg;C. If the
+storminess were to be reduced to one-half or one-quarter
+its present amount at sunspot minima, not only would the
+loss of heat by upward convection in storms be diminished,
+but the area covered by clouds would diminish so
+that the sun would have more chance to warm the lower
+air. Hence the average rise of temperature might amount
+to as much at 5&deg; or 10&deg;C.</p>
+
+<p>Another effect of the decrease in storminess would be
+to make the so-called westerly winds, which are chiefly
+southwesterly in the northern hemisphere and northwesterly
+in the southern hemisphere, more strong and
+steady than at present. They would not continually suffer
+interruption by cyclonic winds from other directions, as
+is now the case, and would have a regularity like that
+of the trades. This conclusion is strongly re&euml;nforced in
+a paper by Clayton<sup><a href="#fn66" id="r66">[66]</a></sup> which came to hand after this chapter
+had been completed. From his studies of the solar
+constant and the temperature of the earth which are
+described in <em>Earth and Sun</em>, he reaches the following
+conclusion: "The results of these researches have led
+me to believe: 1. That if there were no variation in solar
+radiation the atmospheric motions would establish a
+stable system with exchanges of air between equator and
+pole and between ocean and land, in which the only variations
+<!--File: cc198.png Page 174-->
+<a class="pagenum break" id="Page_174" title="pg 174"></a>
+would be daily and annual changes set in operation
+by the relative motions of the earth and sun. 2. The existing
+abnormal changes, which we call weather, have their
+origins chiefly, if not entirely, in the variations of solar
+radiation."</p>
+
+<p>If cyclonic storms and "weather" were largely eliminated
+and if the planetary system of winds with its
+steady trades and southwesterlies became everywhere
+dominant, the regularity and volume of the poleward-flowing
+currents, such as the Gulf Stream and the
+Atlantic Drift in one ocean, and the Japanese Current in
+another, would be greatly increased. How important this
+is may be judged from the work of Helland-Hansen and
+Nansen.<sup><a href="#fn67" id="r67">[67]</a></sup> These authors find that with the passage of each
+cyclonic storm there is a change in the temperature of
+the surface water of the Atlantic Ocean. Winds at right
+angles to the course of the Drift drive the water first in
+one direction and then in the other but do not advance it
+in its course. Winds with an easterly component, on the
+other hand, not only check the Drift but reverse it, driving
+the warm water back toward the southwest and
+allowing cold water to well up in its stead. The driving
+force in the Atlantic Drift is merely the excess of the
+winds with a westerly component over those with an
+easterly component.</p>
+
+<p>Suppose that the numbers in Fig. 8 represent the
+strength of the winds in a certain part of the North
+Atlantic or North Pacific, that is, the total number of
+miles moved by the air per year. In quadrant A of the
+left-hand part all the winds move from a more or less
+southwesterly direction and produce a total movement
+<!--File: cc199.png Page 175-->
+<a class="pagenum break" id="Page_175" title="pg 175"></a>
+of the air amounting to thirty units per year. Those
+coming from points between north and west move twenty-five
+units; those between north and east, twenty units;
+and those between east and south, twenty-five units.
+Since the movement of the winds in quadrants B and
+D is the same, these winds have no effect in producing
+currents. They merely move the water back and forth,
+and thus give it time to lose whatever heat it has brought
+from more southerly latitudes. On the other hand, since
+the easterly winds in quadrant C do not wholly check the
+currents caused by the westerly winds of quadrant A,
+the effective force of the westerly winds amounts to ten,
+or the difference between a force of thirty in quadrant A
+and of twenty in quadrant C. Hence the water is moved
+forward toward the northeast, as shown by the thick
+part of arrow A.</p>
+<img src="images/fig8.png" width="798" height="433" alt="Fig. 8"  id="Fig_8" />
+<p class="center"><em>Fig. 8. Effect of diminution of storms on
+movement of water.</em></p>
+
+<p>Now suppose that cyclonic storms should be greatly
+reduced in number so that in the zone of prevailing
+westerlies they were scarcely more numerous than tropical
+<!--File: cc200.png Page 176-->
+<a class="pagenum break" id="Page_176" title="pg 176"></a>
+hurricanes now are in the trade-wind belt. Then the
+more or less southwesterly winds in quadrant A&acute; in the
+right-hand part of Fig. 8 would not only become more
+frequent but would be stronger than at present. The
+total movement from that quarter might rise to sixty
+units, as indicated in the figure. In quadrants B&acute; and D&acute;
+the movement would fall to fifteen and in quadrant C&acute; to
+ten. B&acute; and D&acute; would balance one another as before. The
+movement in A&acute;, however, would exceed that in C&acute; by fifty
+instead of ten. In other words, the current-making force
+would become five times as great as now. The actual
+effect would be increased still more, for the winds from
+the southwest would be stronger as well as steadier if
+there were no storms. A strong wind which causes whitecaps
+has much more power to drive the water forward
+than a weaker wind which does not cause whitecaps. In a
+wave without a whitecap the water returns to practically
+the original point after completing a circle beneath the
+surface. In a wave with a whitecap, however, the cap
+moves forward. Any increase in velocity beyond the rate
+at which whitecaps are formed has a great influence upon
+the amount of water which is blown forward. Several
+times as much water is drifted forward by a persistent
+wind of twenty miles an hour as by a ten-mile wind.<sup><a href="#fn68" id="r68">[68]</a></sup></p>
+
+<p>In this connection a suggestion which is elaborated in
+Chapter XIII may be mentioned. At present the salinity
+of the oceans checks the general deep-sea circulation and
+thereby increases the contrasts from zone to zone. In the
+past, however, the ocean must have been fresher than
+now. Hence the circulation was presumably less impeded,
+and the transfer of heat from low latitudes to high was
+facilitated.</p>
+
+<!--File: cc201.png Page 177-->
+<a class="pagenum break" id="Page_177" title="pg 177"></a>
+
+
+<p>Consider now the magnitude of the probable effect of
+a diminution in storms. Today off the coast of Norway
+in latitude 65&deg;N. and longitude 10&deg;E., the mean temperature
+in January is 2&deg;C. and in July 12&deg;C. This represents
+a plus anomaly of about 22&deg; in January and 2&deg; in July;
+that is, the Norwegian coast is warmer than the normal
+for its latitude by these amounts. Suppose that in some
+past time the present distribution of lands and seas prevailed,
+but Norway was a lowland where extensive deposits
+could accumulate in great flood plains. Suppose,
+also, that the sun's atmosphere was so inactive that few
+cyclonic storms occurred, steady winds from the west-southwest
+prevailed, and strong, uninterrupted ocean
+currents brought from the Caribbean Sea and Gulf of
+Mexico much greater supplies of warm water than at
+present. The Norwegian winters would then be warmer
+than now not only because of the general increase in temperature
+which the earth regularly experiences at sunspot
+minima, but because the currents would accentuate
+this condition. In summer similar conditions would prevail
+except that the warming effect of the winds and
+currents would presumably be less than in winter, but
+this might be more than balanced by the increased heat
+of the sun during the long summer days, for storms and
+clouds would be rare.</p>
+
+<p>If such conditions raised the winter temperature only
+8&deg;C. and the summer temperature 4&deg;C., the climate would
+be as warm as that of the northern island of New Zealand
+(latitude 35&deg;-43&deg;S.). The flora of that part of New Zealand
+is subtropical and includes not only pines and
+beeches, but palms and tree ferns. A climate scarcely
+warmer than that of New Zealand would foster a flora
+like that which existed in far northern latitudes during
+some of the milder geological periods. If, however, the
+<!--File: cc202.png Page 178-->
+<a class="pagenum break" id="Page_178" title="pg 178"></a>
+general temperature of the earth's surface were raised
+5&deg; because of the scarcity of storms, if the currents were
+strong enough so that they increased the present anomaly
+by 50 per cent, and if more persistent sunshine in summer
+raised the temperature at that season about 4&deg;C., the
+January temperature would be 18&deg;C. and the July temperature
+22&deg;C. These figures perhaps make summer and
+winter more nearly alike than was ever really the case in
+such latitudes. Nevertheless, they show that a diminution
+of storms and a consequent strengthening and steadying
+of the southwesterlies might easily raise the temperature
+of the Norwegian coast so high that corals could flourish
+within the Arctic Circle.</p>
+
+<p>Another factor would co&ouml;perate in producing mild
+temperatures in high latitudes during the winter, namely,
+the fogs which would presumably accumulate. It is well
+known that when saturated air from a warm ocean is
+blown over the lands in winter, as happens so often in the
+British Islands and around the North Sea, fog is formed.
+The effect of such a fog is indeed to shut out the sun's
+radiation, but in high latitudes during the winter when
+the sun is low, this is of little importance. Another effect
+is to retain the heat of the earth itself. When a constant
+supply of warm water is being brought from low latitudes
+this blanketing of the heat by the fog becomes of
+great importance. In the past, whenever cyclonic storms
+were weak and westerly winds were correspondingly
+strong, winter fogs in high latitudes must have been much
+more widespread and persistent than now.</p>
+
+<p>The bearing of fogs on vegetation is another interesting
+point. If a region in high latitudes is constantly protected
+by fog in winter, it can support types of vegetation
+characteristic of fairly low latitudes, for plants are
+oftener killed by dry cold than by moist cold. Indeed,
+<!--File: cc203.png Page 179-->
+<a class="pagenum break" id="Page_179" title="pg 179"></a>
+excessive evaporation from the plant induced by dry
+cold when the evaporated water cannot be rapidly replaced
+by the movement of sap is a chief reason why
+large plants are winterkilled. The growing of transplanted
+palms on the coast of southwestern Ireland, in
+spite of its location in latitude 50&deg;N., is possible only because
+of the great fogginess in winter due to the marine
+climate. The fogs prevent the escape of heat and ward off
+killing frosts. The tree ferns in latitude 46&deg;S. in New
+Zealand, already referred to, are often similarly protected
+in winter. Therefore, the relative frequency of fogs
+in high latitudes when storms were at a minimum would
+apparently tend not merely to produce mild winters but
+to promote tropical vegetation.</p>
+
+<p>The strong steady trades and southwesterlies which
+would prevail at times of slight solar activity, according
+to our hypothesis, would have a pronounced effect on the
+water of the deep seas as well as upon that of the surface.
+In the first place, the deep-sea circulation would be hastened.
+For convenience let us speak of the northern hemisphere.
+In the past, whenever the southwesterly winds
+were steadier than now, as was probably the case when
+cyclonic storms were relatively rare, more surface water
+than at present was presumably driven from low latitudes
+and carried to high latitudes. This, of course, means that
+a greater volume of water had to flow back toward the
+equator in the lower parts of the ocean, or else as a cool
+surface current. The steady southwesterly winds, however,
+would interfere with south-flowing surface currents,
+thus compelling the polar waters to find their way
+equatorward beneath the surface. In low latitudes the
+polar waters would rise and their tendency would be to
+lower the temperature. Hence steadier westerlies would
+make for lessened latitudinal contrasts in climate not
+<!--File: cc204.png Page 180-->
+<a class="pagenum break" id="Page_180" title="pg 180"></a>
+only by driving more warm water poleward but by causing
+more polar water to reach low latitudes.</p>
+
+<p>At this point a second important consideration must be
+faced. Not only would the deep-sea circulation be hastened,
+but the ocean depths might be warmed. The deep
+parts of the ocean are today cold because they receive
+their water from high latitudes where it sinks because of
+low temperature. Suppose, however, that a diminution in
+storminess combined with other conditions should permit
+corals to grow in latitude 70&deg;N. The ocean temperature
+would then have to average scarcely lower than 20&deg;C.
+and even in the coldest month the water could scarcely
+fall below about 15&deg;C. Under such conditions, if the polar
+ocean were freely connected with the rest of the oceans,
+no part of it would probably have a temperature much
+below 10&deg;C., for there would be no such thing as ice caps
+and snowfields to reflect the scanty sunlight and radiate
+into space what little heat there was. On the contrary,
+during the winter an almost constant state of dense fogginess
+would prevail. So great would be the blanketing
+effect of this that a minimum monthly temperature of
+10&deg;C. for the coldest part of the ocean may perhaps be
+too low for a time when corals thrived in latitude 70&deg;.</p>
+
+<p>The temperature of the ocean depths cannot permanently
+remain lower than that of the coldest parts of the
+surface. Temporarily this might indeed happen when a
+solar change first reduced the storminess and strengthened
+the westerlies and the surface currents. Gradually,
+however, the persistent deep-sea circulation would bring
+up the colder water in low latitudes and carry downward
+the water of medium temperature at the coldest part of
+the surface. Thus in time the whole body of the ocean
+would become warm. The heat which at present is carried
+away from the earth's surface in storms would slowly
+<!--File: cc205.png Page 181-->
+<a class="pagenum break" id="Page_181" title="pg 181"></a>
+accumulate in the oceans. As the process went on, all
+parts of the ocean's surface would become warmer, for
+equatorial latitudes would be less and less cooled by cold
+water from below, while the water blown from low latitudes
+to high would be correspondingly warmer. The
+warming of the ocean would come to an end only with
+the attainment of a state of equilibrium in which the loss
+of heat by radiation and evaporation from the ocean's
+surface equaled the loss which under other circumstances
+would arise from the rise of warm air in cyclonic storms.
+When once the oceans were warmed, they would form an
+extremely strong conservative force tending to preserve
+an equable climate in all latitudes and at all seasons.
+According to the solar cyclonic hypothesis such conditions
+ought to have prevailed throughout most of geological
+time. Only after a strong and prolonged solar
+disturbance with its consequent storminess would conditions
+like those of today be expected.</p>
+
+<p>In this connection another possibility may be mentioned.
+It is commonly assumed that the earth's axis is
+held steadily in one direction by the fact that the rotating
+earth is a great gyroscope. Having been tilted to a certain
+position, perhaps by some extraneous force, the axis
+is supposed to maintain that position until some other
+force intervenes. Cordeiro,<sup><a href="#fn69" id="r69">[69]</a></sup> however, maintains that this
+is true only of an absolutely rigid gyroscope. He believes
+that it is mathematically demonstrable that if an elastic
+gyroscope be gradually tilted by some extraneous force,
+and if that force then ceases to act, the gyroscope as a
+whole will oscillate back and forth. The earth appears to
+be slightly elastic. Cordeiro therefore applies his formul&aelig;
+to it, on the following assumptions: (1) That the
+original position of the axis was nearly vertical to the
+<!--File: cc206.png Page 182-->
+<a class="pagenum break" id="Page_182" title="pg 182"></a>
+plane of the ecliptic in which the earth revolves around
+the sun; (2) that at certain times the inclination has been
+even greater than now; and (3) that the position of the
+axis with reference to the earth has not changed to any
+great extent, that is, the earth's poles have remained
+essentially stationary with reference to the earth, although
+the whole earth has been gyroscopically tilted
+back and forth repeatedly.</p>
+
+<p>With a vertical axis the daylight and darkness in all
+parts of the earth would be of equal duration, being
+always twelve hours. There would be no seasons, and the
+climate would approach the average condition now experienced
+at the two equinoxes. On the whole the climate
+of high latitudes would give the impression of being
+milder than now, for there would be less opportunity for
+the accumulation of snow and ice with their strong cooling
+effect. On the other hand, if the axis were tilted more
+than now, the winter nights would be longer and the
+winters more severe than at present, and there would be
+a tendency toward glaciation. Thus Cordeiro accounts
+for alternating mild and glacial epochs. The entire swing
+from the vertical position to the maximum inclination
+and back to the vertical may last millions of years depending
+on the earth's degree of elasticity. The swing
+beyond the vertical position in the other direction would
+be equally prolonged. Since the axis is now supposed to
+be much nearer its maximum than its minimum degree of
+tilting, the duration of epochs having a climate more
+severe than that of the present would be relatively short,
+while the mild epochs would be long.</p>
+
+<p>Cordeiro's hypothesis has been almost completely
+ignored. One reason is that his treatment of geological
+facts, and especially his method of riding rough-shod
+over widely accepted conclusions, has not commended his
+<!--File: cc207.png Page 183-->
+<a class="pagenum break" id="Page_183" title="pg 183"></a>
+work to geologists. Therefore they have not deemed it
+worth while to urge mathematicians to test the assumptions
+and methods by which he reached his results. It is
+perhaps unfair to test Cordeiro by geology, for he lays
+no claim to being a geologist. In mathematics he labors
+under the disadvantage of having worked outside the
+usual professional channels, so that his work does not
+seem to have been subjected to sufficiently critical
+analysis.</p>
+
+<p>Without expressing any opinion as to the value of
+Cordeiro's results we feel that the subject of the earth's
+gyroscopic motion and of a possible secular change in
+the direction of the axis deserves investigation for two
+chief reasons. In the first place, evidences of seasonal
+changes and of seasonal uniformity seem to occur more
+or less alternately in the geological record. Second, the
+remarkable discoveries of Garner and Allard<sup><a href="#fn70" id="r70">[70]</a></sup> show that
+the duration of daylight has a pronounced effect upon
+the reproduction of plants. We have referred repeatedly
+to the tree ferns, corals, and other forms of life which
+now live in relatively low latitudes and which cannot
+endure strong seasonal contrasts, but which once lived
+far to the north. On the other hand, Sayles,<sup><a href="#fn71" id="r71">[71]</a></sup> for example,
+finds that microscopical examination of the banding of
+ancient shales and slates indicates distinct seasonal banding
+like that of recent Pleistocene clays or of the Squantum
+slate formed during or near the Permian glacial
+period. Such seasonal banding is found in rocks of various
+ages: (a) Huronian, in cobalt shales previously
+reported by Coleman; (b) late Proterozoic or early Cambrian,
+<!--File: cc208.png Page 184-->
+<a class="pagenum break" id="Page_184" title="pg 184"></a>
+in Hiwassee slate; (c) lower Cambrian, in Georgian
+slates of Vermont; (d) lower Ordovician, in Georgia
+(Rockmart slate), Tennessee (Athens shale), Vermont
+(slates), and Quebec (Beekmantown formation); and (e)
+Permian in Massachusetts (Squantum slate). How far
+the periods during which such evidence of seasons was
+recorded really alternated with mild periods, when tropical
+species lived in high latitudes and the contrast of
+seasons was almost or wholly lacking, we have as yet no
+means of knowing. If periods characterized by marked
+seasonal changes should be found to have alternated with
+those when the seasons were of little importance, the fact
+would be of great geological significance.</p>
+
+<p>The discoveries of Garner and Allard as to the effect
+of light on reproduction began with a peculiar tobacco
+plant which appeared in some experiments at Washington.
+The plant grew to unusual size, and seemed to
+promise a valuable new variety. It formed no seeds, however,
+before the approach of cold weather. It was therefore
+removed to a greenhouse where it flowered and
+produced seed. In succeeding years the flowering was
+likewise delayed till early winter, but finally it was discovered
+that if small plants were started in the greenhouse
+in the early fall they flowered at the same time as
+the large ones. Experiments soon demonstrated that the
+time of flowering depends largely upon the length of the
+daily period when the plants are exposed to light. The
+same is true of many other plants, and there is great
+variety in the conditions which lead to flowering. Some
+plants, such as witch hazel, appear to be stimulated to
+bloom by very short days, while others, such as evening
+primrose, appear to require relatively long days. So
+sensitive are plants in this respect that Garner and
+Allard, by changing the length of the period of light, have
+<!--File: cc209.png Page 185-->
+<a class="pagenum break" id="Page_185" title="pg 185"></a>
+caused a flowerbud in its early stages not only to stop
+developing but to return once more to a vegetative shoot.</p>
+
+<blockquote>
+<p>Common iris, which flowers in May and June, will not blossom
+under ordinary conditions when grown in the greenhouse in
+winter, even under the same temperature conditions that prevail
+in early summer. Again, one variety of soy beans will regularly
+begin to flower in June of each year, a second variety in July,
+and a third in August, when all are planted on the same date.
+There are no temperature differences during the summer months
+which could explain these differences in time of flowering; and,
+since "internal causes" alone cannot be accepted as furnishing
+a satisfactory explanation, some external factor other than temperature
+must be responsible.</p>
+
+<p>The ordinary varieties of cosmos regularly flower in the fall
+in northern latitudes if they are planted in the spring or summer.
+If grown in a warm greenhouse during the winter months the
+plants also flower readily, so that the cooler weather of fall is
+not a necessary condition. If successive plantings of cosmos are
+made in the greenhouse during the late winter and early spring
+months, maintaining a uniform temperature throughout, the
+plantings made after a certain date will fail to blossom promptly,
+but, on the contrary, will continue to grow till the following fall,
+thus flowering at the usual season for this species. This curious
+reversal of behavior with advance of the season cannot be attributed
+to change in temperature. Some other factor is responsible
+for the failure of cosmos to blossom during the summer months.
+In this respect the behavior of cosmos is just the opposite of that
+observed in iris.</p>
+
+<p>Certain varieties of soy beans change their behavior in a
+peculiar manner with advance of the summer season. The variety
+known as Biloxi, for example, when planted early in the spring
+in the latitude of Washington, D. C., continues to grow throughout
+the summer, flowering in September. The plants maintain
+growth without flowering for fifteen to eighteen weeks, attaining
+a height of five feet or more. As the dates of successive plantings
+are moved forward through the months of June and July, however,
+<!--File: cc210.png Page 186-->
+<a class="pagenum break" id="Page_186" title="pg 186"></a>
+there is a marked tendency for the plants to cut short the
+period of growth which precedes flowering. This means, of course,
+that there is a tendency to flower at approximately the same time
+of year regardless of the date of planting. As a necessary consequence,
+the size of the plants at the time of flowering is reduced
+in proportion to the delay in planting.</p>
+</blockquote>
+
+<p>The bearing of this on geological problems lies in a
+query which it raises as to the ability of a genus or family
+of plants to adapt itself to days of very different length
+from those to which it is wonted. Could tree ferns, ginkgos,
+cycads, and other plants whose usual range of location
+never subjects them to daylight for more than
+perhaps fourteen hours or less than ten, thrive and reproduce
+themselves if subjected to periods of daylight
+ranging all the way from nothing up to about twenty-four
+hours? No answer to this is yet possible, but the
+question raises most interesting opportunities of investigation.
+If Cordeiro is right as to the earth's elastic
+gyroscopic motion, there may have been certain periods
+when a vertical or almost vertical axis permitted the
+days to be of almost equal length at all seasons in all
+latitudes. If such an absence of seasons occurred when
+the lands were low, when the oceans were extensive and
+widely open toward the poles, and when storms were
+relatively inactive, the result might be great mildness of
+climate such as appears sometimes to have prevailed in
+the middle of geological eras. Suppose on the other hand
+that the axis should be tilted more than now, and that
+the lands should be widely emergent and the storm belt
+highly active in low latitudes, perhaps because of the
+activity of the sun. The conditions might be favorable for
+glaciation at latitudes as low as those where the Permo-Carboniferous
+ice sheets appear to have centered. The
+possibilities thus suggested by Cordeiro's hypothesis are
+<!--File: cc211.png Page 187-->
+<a class="pagenum break" id="Page_187" title="pg 187"></a>
+so interesting that the gyroscopic motion of the earth
+ought to be investigated more thoroughly. Even if no
+such gyroscopic motion takes place, however, the other
+causes of mild climate discussed in this chapter may
+be enough to explain all the observed phenomena.</p>
+
+<p>Many important biological consequences might be
+drawn from this study of mild geological climates, but
+this book is not the place for them. In the first chapter
+we saw that one of the most remarkable features of the
+climate of the earth is its wonderful uniformity through
+hundreds of millions of years. As we come down through
+the vista of years the mild geological periods appear to
+represent a return as nearly as possible to this standard
+condition of uniformity. Certain changes of the earth
+itself, as we shall see in the next chapter, may in the long
+run tend slightly to change the exact conditions of this
+climatic standard, as we might perhaps call it. Yet they
+act so slowly that their effect during hundreds of millions
+of years is still open to question. At most they seem
+merely to have produced a slight increase in diversity
+from season to season and from zone to zone. The normal
+climate appears still to be of a milder type than that
+which happens to prevail at present. Some solar condition,
+whose possible nature will be discussed later, seems
+even now to cause the number of cyclonic storms to be
+greater than normal. Hence the earth's climate still
+shows something of the great diversity of seasons and
+of zones which is so marked a characteristic of glacial
+epochs.</p>
+<!--File: cc212.png Page 188-->
+<a class="pagenum break" id="Page_188" title="pg 188"></a>
+
+
+
+
+<h2 id="Chapt_XI"><a href="#content">CHAPTER XI</a></h2>
+
+<h3>TERRESTRIAL CAUSES OF CLIMATIC CHANGES</h3>
+
+
+<p>The major portion of this book has been concerned
+with the explanation of the more abrupt and extreme
+changes of climate. This chapter and the
+next consider two other sorts of climatic changes, the
+slight secular progression during the hundreds of millions
+of years of recorded earth history, and especially
+the long slow geologic oscillations of millions or tens of
+millions of years. It is generally agreed among geologists
+that the progressive change has tended toward greater
+extremes of climate; that is, greater seasonal contrasts,
+and greater contrasts from place to place and from zone
+to zone.<sup><a href="#fn72" id="r72">[72]</a></sup> The slow cyclic changes have been those that
+favored widespread glaciation at one extreme near the
+ends of geologic periods and eras, and mild temperatures
+even in subpolar regions at the other extreme during the
+medial portions of the periods.</p>
+
+<p>As has been pointed out in an earlier chapter, it has
+often been assumed that all climatic changes are due to
+terrestrial causes. We have seen, however, that there is
+strong evidence that solar variations play a large part in
+modifying the earth's climate. We have also seen that no
+known terrestrial agency appears to be able to produce
+the abrupt changes noted in recent years, the longer
+<!--File: cc213.png Page 189-->
+<a class="pagenum break" id="Page_189" title="pg 189"></a>
+cycles of historical times, or geological changes of the
+shorter type, such as glaciation. Nevertheless, terrestrial
+changes doubtless have assisted in producing both the
+progressive change and the slow cyclic changes recorded
+in the rocks, and it is the purpose of this chapter and the
+two that follow to consider what terrestrial changes have
+taken place and the probable effect of such changes.</p>
+
+<p>The terrestrial changes that have a climatic significance
+are numerous. Some, such as variations in the
+amount of volcanic dust in the higher air, have been considered
+in an earlier chapter. Others are too imperfectly
+known to warrant discussion, and in addition there are
+presumably others which are entirely unknown. Doubtless
+some of these little known or unknown changes have
+been of importance in modifying climate. For example,
+the climatic influence of vegetation, animals, and man
+may be appreciable. Here, however, we shall confine ourselves
+to purely physical causes, which will be treated in
+the following order: First, those concerned with the solid
+parts of the earth, namely: (I) amount of land; (II) distribution
+of land; (III) height of land; (IV) lava flows;
+and (V) internal heat. Second, those which arise from
+the salinity of oceans, and third, those depending on the
+composition and amount of atmosphere.</p>
+
+<p>The terrestrial change which appears indirectly to
+have caused the greatest change in climate is the contraction
+of the earth. The problem of contraction is
+highly complex and is as yet only imperfectly understood.
+Since only its results and not its processes influence climate,
+the following section as far as page 196 is not
+necessary to the general reader. It is inserted in order to
+explain why we assume that there have been oscillations
+between certain types of distribution of the lands.</p>
+
+<p>The extent of the earth's contraction may be judged
+<!--File: cc214.png Page 190-->
+<a class="pagenum break" id="Page_190" title="pg 190"></a>
+from the shrinkage indicated by the shortening of the
+rock formations in folded mountains such as the Alps,
+Juras, Appalachians, and Caucasus. Geologists are continually
+discovering new evidence of thrust faults of
+great magnitude where masses of rock are thrust bodily
+over other rocks, sometimes for many miles. Therefore,
+the estimates of the amount of shrinkage based on the
+measurements of folds and faults need constant revision
+upward. Nevertheless, they have already reached a considerable
+figure. For example, in 1919, Professor A. Heim
+estimated the shortening of the meridian passing through
+the modern Alps and the ancient Hercynian and Caledonian
+mountains as fully a thousand miles in Europe,
+and over five hundred miles for the rest of this meridian.<sup><a href="#fn73" id="r73">[73]</a></sup>
+This is a radial shortening of about 250 miles. Possibly
+the shrinkage has been even greater than this. Chamberlin<sup><a href="#fn74" id="r74">[74]</a></sup>
+has compared the density of the earth, moon, Mars,
+and Venus with one another, and found it probable that
+the radial shrinkage of the earth may be as much as
+570 miles. This result is not so different from Heim's as
+appears at first sight, for Heim made no allowance for
+unrecognized thrust faults and for the contraction incident
+to metamorphism. Moreover, Heim did not include
+shrinkage during the first half of geological time before
+the above-mentioned mountain systems were upheaved.</p>
+
+<p>According to a well-established law of physics, contraction
+of a rotating body results in more rapid rotation
+and greater centrifugal force. These conditions must increase
+the earth's equatorial bulge and thereby cause
+changes in the distribution of land and water. Opposed
+to the rearrangement of the land due to increased rotation
+<!--File: cc215.png Page 191-->
+<a class="pagenum break" id="Page_191" title="pg 191"></a>
+caused by contraction, there has presumably been
+another rearrangement due to tidal retardation of the
+earth's rotation and a consequent lessening of the equatorial
+bulge. G. H. Darwin long ago deduced a relatively
+large retardation due to lunar tides. A few years ago
+W. D. MacMillan, on other assumptions, deduced only a
+negligible retardation. Still more recently Taylor<sup><a href="#fn75" id="r75">[75]</a></sup> has
+studied the tides of the Irish Sea, and his work has led
+Jeffreys<sup><a href="#fn76" id="r76">[76]</a></sup> and
+Brown<sup><a href="#fn77" id="r77">[77]</a></sup> to conclude that there has been considerable
+retardation, perhaps enough, according to
+Brown, to equal the acceleration due to the earth's contraction.
+From a prolonged and exhaustive study of the
+motions of the moon Brown concludes that tidal friction
+or some other cause is now lengthening the day at the rate
+of one second per thousand years, or an hour in almost
+four million years if the present rate continues. He makes
+it clear that the retardation due to tides would not correspond
+in point of time with the acceleration due to contraction.
+The retardation would occur slowly, and would
+take place chiefly during the long quiet periods of geologic
+history, while the acceleration would occur rapidly
+at times of diastrophic deformation. As a consequence,
+the equatorial bulge would alternately be reduced at a
+slow rate, and then somewhat suddenly augmented.</p>
+
+<p>The less rigid any part of the earth is, the more quickly
+it responds to the forces which lead to bulging or which
+tend to lessen the bulge. Since water is more fluid than
+land, the contraction of the earth and the tidal retardation
+presumably tend alternately to increase and decrease
+the amount of water near the equator more than the
+<!--File: cc216.png Pag2 192-->
+<a class="pagenum break" id="Page_192" title="pg 192"></a>
+amount of land. Thus, throughout geological history we
+should look for cyclic changes in the relative area of the
+lands within the tropics and similar changes of opposite
+phase in higher latitudes. The extent of the change would
+depend upon (a) the amount of alteration in the speed
+of rotation, and (b) the extent of low land in low latitudes
+and of shallow sea in high latitudes. According to
+Slichter's tables, if the earth should rotate in twenty-three
+hours instead of twenty-four, the great Amazon
+lowland would be submerged by the inflow of oceanic
+water, while wide areas in Hudson Bay, the North Sea,
+and other northern regions, would become land because
+the ocean water would flow away from them.<sup><a href="#fn78" id="r78">[78]</a></sup></p>
+
+<p>Following the prompt equatorward movement of water
+which would occur as the speed of rotation increased,
+there must also be a gradual movement or creepage of the
+solid rocks toward the equator, that is, a bulging of the
+ocean floor and of the lands in low latitudes, with a consequent
+emergence of the lands there and a relative
+rise of sea level in higher latitudes. Tidal retardation
+would have a similar effect. Suess<sup><a href="#fn79" id="r79">[79]</a></sup> has described widespread
+elevated strand lines in the tropics which he interprets
+as indicating a relatively sudden change in sea
+level, though he does not suggest a cause of the change.
+However, in speaking of recent geological times, Suess
+reports that a movement more recent than the old
+strands "was an accumulation of water toward the
+equator, a diminution toward the poles, and (it appears)
+as though this last movement were only one of the many
+oscillations which succeed each other with the same tendency,
+i.e., with a positive excess at the equator, a negative
+<!--File: cc217.png Page 193-->
+<a class="pagenum break" id="Page_193" title="pg 193"></a>
+excess at the poles." (Vol. II, p. 551.) This creepage
+of the rocks equatorward seemingly might favor the
+growth of mountains in tropical and subtropical regions,
+because it is highly improbable that the increase in the
+bulge would go on in all longitudes with perfect uniformity.
+Where it went on most rapidly mountains would
+arise. That such irregularity of movement has actually
+occurred is suggested not only by the fact that many
+Cenozoic and older mountain ranges extend east and
+west, but by the further fact that these include some of
+our greatest ranges, many of which are in fairly low latitudes.
+The Himalayas, the Javanese ranges, and the half-submerged
+Caribbean chains are examples. Such mountains
+suggest a thrust in a north and south direction
+which is just what would happen if the solid mass of the
+earth were creeping first equatorward and then poleward.</p>
+
+<p>A fact which is in accord with the idea of a periodic
+increase in the oceans in low latitudes because of renewed
+bulging at the equator is the exposure in moderately
+high latitudes of the greatest extent of ancient rocks.
+This seems to mean that in low latitudes the frequent
+deepening of the oceans has caused the old rocks to be
+largely covered by sediments, while the old lands in
+higher latitudes have been left more fully exposed to
+erosion.</p>
+
+<p>Another suggestion of such periodic equatorward movements
+of the ocean water is found in the reported contrast
+between the relative stability with which the northern part
+of North America has remained slightly above sea level
+except at times of widespread submergence, while the
+southern parts have suffered repeated submergence alternating
+with great emergence.<sup><a href="#fn80" id="r80">[80]</a></sup> Furthermore, although
+<!--File: cc218.png Page 194-->
+<a class="pagenum break" id="Page_194" title="pg 194"></a>
+the northern part of North America has been generally
+exposed to erosion since the Proterozoic, it has supplied
+much less sediment than have the more southern land
+areas.<sup><a href="#fn81" id="r81">[81]</a></sup> This apparently means that much of Canada has
+stood relatively low, while repeated and profound uplift
+alternating with depression has occurred in subtropical
+latitudes, apparently in adjustment to changes in the
+earth's speed of rotation. The uplifts generally followed
+the times of submergence due to equatorward movement
+of the water, though the buckling of the crust which accompanies
+shrinkage doubtless caused some of the submergence.
+The evidence that northern North America
+stood relatively low throughout much of geological time
+depends not only on the fact that little sediment came to
+the south from the north, but also on the fact that at
+times of especially widespread epicontinental seas, the
+submergence was initiated at the north.<sup><a href="#fn82" id="r82">[82]</a></sup> This is especially
+true for Ordovician, Silurian, Devonian, and Jurassic
+times in North America. General submergence of this
+kind is supposed to be due chiefly to the overflowing of
+the ocean when its level is slowly raised by the deposition
+of sediment derived from the erosion of what once were
+continental highlands but later are peneplains. The fact
+that such submergence began in high latitudes, however,
+seems to need a further explanation. The bulging of the
+rock sphere at the equator and the consequent displacement
+of some of the water in low latitudes would furnish
+such an explanation, as would also a decrease in the speed
+of rotation induced by tidal retardation, if that retardation
+were great enough and rapid enough to be geologically
+effective.</p>
+<!--File: cc219.png Page 195-->
+<a class="pagenum break" id="Page_195" title="pg 95"></a>
+
+<p>The climatic effects of the earth's contraction, which
+we shall shortly discuss, are greatly complicated by the
+fact that contraction has taken place irregularly. Such
+irregularity has occurred in spite of the fact that the
+processes which cause contraction have probably gone
+on quite steadily throughout geological history. These
+processes include the chemical reorganization of the minerals
+of the crust, a process which is illustrated by the
+metamorphism of sedimentary rocks into crystalline
+forms. The escape of gases through volcanic action or
+otherwise has been another important process.</p>
+
+<p>Although the processes which cause contraction probably
+go on steadily, their effect, as Chamberlin<sup><a href="#fn83" id="r83">[83]</a></sup> and
+others have pointed out, is probably delayed by inertia.
+Thus the settling of the crust or its movement on a large
+scale is delayed. Perhaps the delay continues until the
+stresses become so great that of themselves they overcome
+the inertia, or possibly some outside agency, whose
+nature we shall consider later, re&euml;nforces the stresses and
+gives the slight impulse which is enough to release them
+and allow the earth's crust to settle into a new state of
+equilibrium. When contraction proceeds actively, the
+ocean segments, being largest and heaviest, are likely to
+settle most, resulting in a deepening of the oceans and an
+emergence of the lands. Following each considerable contraction
+there would be an increase in the speed of rotation.
+The repeated contractions with consequent growth
+of the equatorial bulge would alternate with long quiet
+periods during which tidal retardation would again decrease
+the speed of rotation and hence lessen the bulge.
+The result would be repeated changes of distribution of
+land and water, with consequent changes in climate.</p>
+<!--File: cc220.png0 Page 196-->
+<a class="pagenum break" id="Page_196" title="pg 196"></a>
+
+<p>I. We shall now consider the climatic effect of the
+repeated changes in the relative amounts of land and
+water which appear to have resulted from the earth's
+contraction and from changes in its speed of rotation.
+During many geologic epochs a larger portion of the
+earth was covered with water than at present. For example,
+during at least twelve out of about twenty epochs,
+North America has suffered extensive inundations,<sup><a href="#fn84" id="r84">[84]</a></sup> and
+in general the extensive submergence of Europe, the
+other area well known geologically, has coincided with
+that of North America. At other times, the ocean has
+been less extensive than now, as for example during the
+recent glacial period, and probably during several of
+the glacial periods of earlier date. Each of the numerous
+changes in the relative extent of the lands must have
+resulted in a modification of climate.<sup><a href="#fn85" id="r85">[85]</a></sup> This modification
+would occur chiefly because water becomes warm far
+more slowly than land, and cools off far more slowly.</p>
+
+<p>An increase in the lands would cause changes in several
+climatic conditions. (a) The range of temperature between
+day and night and between summer and winter
+would increase, for lands become warmer by day and in
+summer than do oceans, and cooler at night and in
+winter. The higher summer temperature when the lands
+are widespread is due chiefly to the fact that the land, if
+not snow-covered, absorbs more of the sun's radiant
+energy than does the ocean, for its reflecting power is
+low. The lower winter temperature when lands are widespread
+occurs not only because they cool off rapidly but
+<!--File: cc221.png Page 197-->
+<a class="pagenum break" id="Page_197" title="pg 197"></a>
+because the reduced oceans cannot give them so much
+heat. Moreover, the larger the land, the more generally
+do the winds blow outward from it in winter and thus
+prevent the ocean heat from being carried inland. So
+long as the ocean is not frozen in high latitudes, it is
+generally the chief source of heat in winter, for the nights
+are several months long near the poles, and even when
+the sun does shine its angle is so low that reflection from
+the snow is very great. Furthermore, although on the
+average there is more reflection from water than from
+land, the opposite is true in high latitudes in winter
+when the land is snow-covered while the ocean is relatively
+dark and is roughened by the waves. Another
+factor in causing large lands to have extremely low temperature
+in winter is the fact that in proportion to their
+size they are less protected by fog and cloud than are
+smaller areas. The belt of cloud and fog which is usually
+formed when the wind blows from the ocean to the relatively
+cold land is restricted to the coastal zone. Thus the
+larger the land, the smaller the fraction in which loss of
+heat by radiation is reduced by clouds and fogs. Hence
+an increase in the land area is accompanied by an increase
+in the contrasts in temperature between land and
+water.</p>
+
+<p>(b) The contrasts in temperature thus produced must
+cause similar contrasts in atmospheric pressure, and
+hence stronger barometric gradients. (c) The strong
+gradients would mean strong winds, flowing from land
+to sea or from sea to land. (d) Local convection would
+also be strengthened in harmony with the expansion of
+the lands, for the more rapid heating of land than of
+water favors active convection.</p>
+
+<p>(e) As the extent of the ocean diminished, there would
+normally be a decrease in the amount of water vapor for
+<!--File: cc222.png Page 198-->
+<a class="pagenum break" id="Page_198" title="pg 198"></a>
+three reasons: (1) Evaporation from the ocean is the
+great source of water vapor. Other conditions being
+equal, the smaller the ocean becomes, the less the evaporation.
+(2) The amount of water vapor in the air diminishes
+as convection increases, since upward convection
+is a chief method by which condensation and precipitation
+are produced, and water vapor removed from the
+atmosphere. (3) Nocturnal cooling sufficient to produce
+dew and frost is very much more common upon land than
+upon the ocean. The formation of dew and frost diminishes
+the amount of water vapor at least temporarily.
+(f) Any diminution in water vapor produced in these
+ways, or otherwise, is significant because water vapor is
+the most essential part of the atmosphere so far as regulation
+of temperature is concerned. It tends to keep the
+days from becoming hot or the nights cold. Therefore
+any decrease in water vapor would increase the diurnal
+and seasonal range of temperature, making the climate
+more extreme and severe. Thus a periodic increase in the
+area of the continents would clearly make for periodic
+increased climatic contrasts, with great extremes, a type
+of climatic change which has recurred again and again.
+Indeed, each great glaciation accompanied or followed
+extensive emergence of the lands.<sup><a href="#fn86" id="r86">[86]</a></sup></p>
+
+<p>Whether or not there has been a <em>progressive</em> increase
+from era to era in the area of the lands is uncertain.
+Good authorities disagree widely. There is no doubt,
+however, that at present the lands are more extensive
+than at most times in the past, though smaller, perhaps,
+than at certain periods. The wide expanse of lands helps
+explain the prominence of seasons at present as compared
+with the past.</p>
+
+<!--File: cc223.png Page 199-->
+<a class="pagenum break" id="Page_199" title="pg 199"></a>
+
+<p>II. The contraction of the earth, as we have seen, has
+produced great changes in the distribution as well as in
+the extent of land and water. Large parts of the present
+continents have been covered repeatedly by the sea, and
+extensive areas now covered with water have been land.
+In recent geological times, that is, during the Pliocene and
+Pleistocene, much of the present continental shelf, the
+zone less than 600 feet below sea level, was land. If the
+whole shelf had been exposed, the lands would have been
+greater than at present by an area larger than North
+America. When the lands were most elevated, or a little
+earlier, North America was probably connected with
+Asia and almost with Europe. Asia in turn was apparently
+connected with the larger East Indian islands. In
+much earlier times land occupied regions where now the
+ocean is fairly deep. Groups of islands, such as the East
+Indies and Malaysia and perhaps the West Indies, were
+united into widespreading land masses. Figs. 7 and 9,
+illustrating the paleography of the Permian and the
+Cretaceous periods, respectively, indicate a land distribution
+radically different from that of today.</p>
+
+<p>So far as appears from the scattered facts of geological
+history, the changes in the distribution of land seem
+to have been marked by the following characteristics: (1)
+Accompanying the differentiation of continental and
+oceanic segments of the earth's crust, the oceans have
+become somewhat deeper, and their basins perhaps
+larger, while the continents, on the average, have been
+more elevated and less subject to submergence. Hence
+there have been less radical departures from the present
+distribution during the relatively recent Cenozoic era
+than in the ancient Paleozoic because the submergence of
+continental areas has become less general and less frequent.
+For example, the last extensive epeiric or interior
+<!--File: cc224.png Page 200-->
+<a class="pagenum break" id="Page_200" title="pg 200"></a>
+sea in North America was in the Cretaceous, at least ten
+million years ago, and according to Barrell perhaps fifty
+million, while in Europe, according to de Lapparent,<sup><a href="#fn87" id="r87">[87]</a></sup> a
+smaller share of the present continent has been submerged
+since the Cretaceous than before. Indeed, as in
+North America, the submergence has decreased on the
+average since the Paleozoic era. (2) The changes in distribution
+of land which have taken place during earth
+history have been cyclic. Repeatedly, at the close of each
+of the score or so of geologic periods, the continents
+emerged more or less, while at the close of the groups of
+periods known as eras, the lands were especially large
+and emergent. After each emergence, a gradual encroachment
+of the sea took place, and toward the close of several
+of the earlier periods, the sea appears to have
+covered a large fraction of the present land areas. (3) On
+the whole, the amount of land in the middle and high latitudes
+of the northern hemisphere appears to have increased
+during geologic time. Such an increase does not
+require a growth of the continents, however, in the
+broader sense of the term, but merely that a smaller
+fraction of the continent and its shelf should be submerged.
+(4) In tropical latitudes, on the other hand, the
+extent of the lands seems to have decreased, apparently
+by the growth of the ocean basins. South America and
+Africa are thought by many students to have been connected,
+and Africa was united with India via Madagascar,
+as is suggested in Fig. 9. The most radical cyclic
+as well as the most radical progressive changes in land
+distribution also seem to have taken place in tropical
+regions.<sup><a href="#fn88" id="r88">[88]</a></sup>
+<!--File: cc225.png Page 201-->
+<a class="pagenum break" id="Page_201" title="pg 201"></a></p>
+
+
+<img src="images/fig9.png" width="800" height="531" alt="Fig. 9"  id="Fig_9" />
+<p class="center"><em>Fig. 9. Cretaceous Paleogeography</em>.<br />
+(<em>After Schuchert.</em>)</p>
+
+
+<p>Although there is much evidence of periodic increase
+<!--File: cc226.png Page 202-->
+<a class="pagenum break" id="Page_202" title="pg 202"></a>
+of the sea in equatorial latitudes and of land in high latitudes,
+it has remained for the zo&ouml;logist Metcalf to present
+a very pretty bit of evidence that at certain times
+submergence along the equator coincided with emergence
+in high latitudes, and vice versa. Certain fresh water
+frogs which carry the same internal parasite are confined
+to two widely separated areas in tropical and south temperate
+America and in Australia. The extreme improbability
+that both the frogs and the parasites could have
+originated independently in two unconnected areas and
+could have developed by convergent evolution so that
+they are almost identical in the two continents makes it
+almost certain that there must have been a land connection
+between South America and Australia, presumably
+by way of Antarctica. The facts as to the parasites
+seem also to prove that while the land connection existed
+there was a sea across South America in equatorial latitudes.
+The parasite infests not only the frogs but the
+American toads known as Bufo. Now Bufo originated
+north of the equator in America and differs from the
+frogs which originated in southern South America in
+not being found in Australia. This raises the question of
+how the frogs could go to Australia via Antarctica carrying
+the parasite with them, while the toads could not go.
+Metcalf's answer is that the toads were cut off from the
+southern part of South America by an equatorial sea
+until after the Antarctic connection between the Old
+World and the New was severed.</p>
+
+<blockquote><p>
+As Patagonia let go of Antarctica by subsidence of the intervening
+land area, there was a probable concomitant rise of land
+through what is now middle South America and the northern
+and southern portions of this continent came together.<sup><a href="#fn89" id="r89">[89]</a></sup></p>
+</blockquote>
+
+
+<!--File: cc227.png Page 203-->
+<a class="pagenum break" id="Page_203" title="pg 203"></a>
+
+<p>These various changes in the earth's crust have given
+rise to certain specific types of distribution of the lands,
+which will now be considered. We shall inquire what climatic
+conditions would arise from changes in (a) the
+continuity of the lands from north to south, (b) the
+amount of land in tropical latitudes, and (c) the amount
+of land in middle and high latitudes.</p>
+
+<p>(a) At present the westward drift of warm waters, set
+in motion by the trade winds, is interrupted by land
+masses and turned poleward, producing the important
+Gulf Stream Drift and Japan Current in the northern
+hemisphere, and corresponding, though less important,
+currents in the southern hemisphere. During the past,
+quite different sets of ocean currents doubtless have
+existed in response to a different distribution of land.
+Repeatedly, in the mid-Cretaceous (Fig. 9) and several
+other periods, the present American barrier to the westward
+moving tropical current was broken in Central
+America. Even if the supposed continent of "Gondwana
+Land" extended from Africa to South America in equatorial
+latitudes, strong currents must still have flowed
+westward along its northern shore under the impulse of
+the peculiarly strong trade winds which the equatorial
+land would create. Nevertheless at such times relatively
+little warm tropical water presumably entered the North
+Atlantic, for it escaped into the Pacific. At several other
+times, such as the late Ordovician and mid-Devonian,
+when the isthmian barrier existed, it probably turned an
+important current northward into what is now the Mississippi
+Basin instead of into the Atlantic. There it
+traversed an epeiric, or mid-continental sea open to both
+north and south. Hence its effectiveness in warming
+Arctic regions must have been quite different from that
+of the present Gulf Stream.</p>
+<!--File: cc228.png Page 204-->
+<a class="pagenum break" id="Page_204" title="pg 204"></a>
+
+<p>(b) We will next consider the influences of changes in
+the amount of equatorial and tropical land. As such lands
+are much hotter than the corresponding seas, the intensity
+and width of the equatorial belt of low pressure must
+be great when they are extensive. Hence the trade winds
+must have been stronger than now whenever tropical
+lands were more extensive than at present. This is because
+the trades are produced by the convection due to
+excessive heat along the heat equator. There the air
+expands upward and flows poleward at high altitudes.
+The trade wind consists of air moving toward the heat
+equator to take the place of the air which there rises.
+When the lands in low latitudes were wide the trade
+winds must also have dominated a wide belt. The greater
+width of the trade-wind belt today over Africa than over
+the Atlantic illustrates the matter. The belt must have
+been still wider when Gondwana Land was large, as it is
+believed to have been during the Paleozoic era and the
+early Mesozoic.</p>
+
+<p>An increase in the width of the equatorial belt of
+low pressure under the influence of broad tropical lands
+would be accompanied not only by stronger and more
+widespread trade winds, but by a corresponding strengthening
+of the subtropical belts of high pressure. The chief
+reason would be the greater expansion of the air in the
+equatorial low pressure belt and the consequent more
+abundant outflow of air at high altitudes in the form of
+anti-trades or winds returning poleward above the trades.
+Such winds would pile up the air in the region of the high-pressure
+belt. Moreover, since the meridians converge as
+one proceeds away from the equator, the air of the poleward-moving
+anti-trades tends to be crowded as it
+reaches higher latitudes, thus increasing the pressure.
+Unless there were a corresponding increase in tropical
+<!--File: cc229.png Page 205-->
+<a class="pagenum break" id="Page_205" title="pg 205"></a>
+cyclones, one of the most prominent results of the
+strengthened trades and the intensified subtropical high-pressure
+belt at times of broad lands in low latitudes
+would be great deserts. It will be recalled that the trade-wind
+lowlands and the extra-tropical belt of highs are the
+great desert belts at present. The trade-wind lowlands
+are desert because air moving into warmer latitudes
+takes up water except where it is cooled by rising on
+mountain-sides. The belt of highs is arid because there,
+too, air is being warmed, but in this case by descending
+from aloft.</p>
+
+<p>Again, if the atmospheric pressure in the subtropical
+belt should be intensified, the winds flowing poleward
+from this belt would necessarily become stronger. These
+would begin as southwesterlies in the northern hemisphere
+and northwesterlies in the southern. In the preceding
+chapter we have seen that such winds, especially
+when cyclonic storms are few and mild, are a powerful
+agent in transferring subtropical heat poleward. If the
+strength of the westerlies were increased because of
+broad lands in low latitudes, their efficacy in transferring
+heat would be correspondingly augmented. It is thus
+evident that any change in the extent of tropical lands
+during the geologic past must have had important climatic
+consequences in changing the velocity of the
+atmospheric circulation and in altering the transfer of
+heat from low latitudes to high. When the equatorial and
+tropical lands were broad the winds and currents must
+have been strong, much heat must have been carried
+away from low latitudes, and the contrast between low
+and high latitudes must have been relatively slight. As
+we have already remarked, leading paleogeographers
+believe that changes in the extent of the lands have been
+especially marked in low latitudes, and that on the average
+<!--File: cc230.png Page 206-->
+<a class="pagenum break" id="Page_206" title="pg 206"></a>
+there has been a decrease in the extent of land within
+the tropics. Gondwana Land is the greatest illustration
+of this. In the same way, on the numerous paleogeographic
+maps of North America, most paleogeographers
+have shown fairly extensive lands south of the latitude of
+the United States during most of the geologic epochs.<sup><a href="#fn90" id="r90">[90]</a></sup></p>
+
+<p>(c) There is evidence that during geologic history the
+area of the lands in middle and high latitudes, as well as
+in low latitudes, has changed radically. An increase in
+such lands would cause the winters to grow colder. This
+would be partly because of the loss of heat by radiation
+into the cold dry air over the continents in winter, and
+partly because of increased reflection from snow and
+frost, which gather much more widely upon the land than
+upon the ocean. Furthermore, in winter when the continents
+are relatively cold, there is a strong tendency for
+winds to blow out from the continent toward the ocean.
+The larger the land the stronger this tendency. In Asia
+it gives rise to strong winter monsoons. The effect of
+such winds is illustrated by the way in which the westerlies
+prevent the Gulf Stream from warming the eastern
+United States in winter. The Gulf Stream warms northwestern
+Europe much more than the United States because,
+in Europe, the prevailing winds are onshore.</p>
+
+<p>Another effect of an increase in the area of the lands in
+middle and high latitudes would be to interpose barriers
+to oceanic circulation and thus lower the temperature
+of polar regions. This would not mean glaciation in
+high latitudes, however, even when the lands were widespread
+as in the Mesozoic and early Tertiary. Students
+of glaciology are more and more thoroughly convinced
+
+
+<!--File: cc231.png Page 207-->
+<a class="pagenum break" id="Page_207" title="pg 207"></a>
+that glaciation depends on the availability of moisture
+even more than upon low temperature.</p>
+
+<p>In conclusion it may be noted that each of the several
+climatic influences of increased land area in the high
+latitudes would tend to increase the contrasts between
+land and sea, between winter and summer, and between
+low latitudes and high. In other words, so far as the
+effect upon high latitudes themselves is concerned, an
+expansion of the lands there would tend in the same
+direction as a diminution in low latitudes. In so far as
+the general trend of geological evolution has been toward
+more land in high latitudes and less in low, it would help
+to produce a progressive increase in climatic diversity
+such as is faintly indicated in the rock strata. On the
+other hand, the oscillations in the distribution of the
+lands, of which geology affords so much evidence, must
+certainly have played an important part in producing the
+periodic changes of climate which the earth has undergone.</p>
+
+<p>III. Throughout geological history there is abundant
+evidence that the process of contraction has led to
+marked differences not only in the distribution and area
+of the lands, but in their height. On the whole the lands
+have presumably increased in height since the Proterozoic,
+somewhat in proportion to the increased differentiation
+of continents and oceans.<sup><a href="#fn91" id="r91">[91]</a></sup> If there has been such an
+increase, the contrast between the climate of ocean and
+land must have been accentuated, for highlands have a
+greater diurnal and seasonal range of temperature than
+do lowlands. The ocean has very little range of either
+sort. The large range at high altitudes is due chiefly to
+the small quantity of water vapor, for this declines
+<!--File: cc232.png Page 208-->
+<a class="pagenum break" id="Page_208" title="pg 208"></a>
+steadily with increased altitude. A diminution in the
+density of the other constituents of the air also decreases
+the blanketing effect of the atmosphere. In conformity
+with the great seasonal range in temperature at times
+when the lands stand high, the direction of the wind
+would be altered. When the lands are notably warmer
+than the oceans, the winds commonly flow from land to
+sea, and when the continents are much colder than the
+oceans, the direction is reversed. The monsoons of Asia
+are examples. Strong seasonal winds disturb the normal
+planetary circulation of the trade winds in low latitudes
+and of the westerlies in middle latitudes. They also interfere
+with the ocean currents set in motion by the planetary
+winds. The net result is to hinder the transfer of
+heat from low latitudes to high, and thus to increase the
+contrasts between the zones. Local as well as zonal contrasts
+are also intensified. The higher the land, the
+greater, relatively speaking, are the cloudiness and precipitation
+on seaward slopes, and the drier the interior.
+Indeed, most highlands are arid. Henry's<sup><a href="#fn92" id="r92">[92]</a></sup> recent study
+of the vertical distribution of rainfall on mountain-sides
+indicates that a decrease sets in at about 3500 feet in the
+tropics and only a little higher in mid-latitudes.</p>
+
+<p>In addition to the main effects upon atmospheric circulation
+and precipitation, each of the many upheavals
+of the lands must have been accompanied by many minor
+conditions which tended toward diversity. For example,
+the streams were rejuvenated, and instead of meandering
+perhaps over vast flood plains they intrenched their
+channels and in many cases dug deep gorges. The water
+table was lowered, soil was removed from considerable
+areas, the bare rock was exposed, and the type of dominant
+<!--File: cc233.png Page 209-->
+<a class="pagenum break" id="Page_209" title="pg 209"></a>
+vegetation altered in many places. An almost barren
+ridge may represent all that remains of what was once a
+vast forested flood plain. Thus, increased elevation of the
+land produces contrasted conditions of slope, vegetation,
+availability of ground water, exposure to wind and so
+forth, and these unite in diversifying climate. Where
+mountains are formed, strong contrasts are sure to
+occur. The windward slopes may be very rainy, while
+neighboring leeward slopes are parched by a dry foehn
+wind. At the same time the tops may be snow-covered.
+Increased local contrasts in climatic conditions are
+known to influence the intensity of cyclonic storms,<sup><a href="#fn93" id="r93">[93]</a></sup> and
+these affect the climatic conditions of all middle and high
+latitudes, if not of the entire earth. The paths followed
+by cyclonic storms are also altered by increased contrast
+between land and water. When the continents are notably
+colder than the neighboring oceans, high atmospheric
+pressure develops on the lands and interferes with the
+passage of lows, which are therefore either deflected
+around the continent or forced to move slowly.</p>
+
+<p>The distribution of lofty mountains has an even more
+striking climatic effect than the general uplift of a region.
+In Proterozoic times there was a great range in the Lake
+Superior region; in the late Devonian the Acadian mountains
+of New England and the Maritime Provinces of
+Canada possibly attained a height equal to the present
+Rockies. Subsequently, in the late Paleozoic a significant
+range stood where the Ouachitas now are. Accompanying
+the uplift of each of these ranges, and all others, the
+climate of the surrounding area, especially to leeward,
+must have been altered greatly. Many extensive salt deposits
+<!--File: cc234.png Page 210-->
+<a class="pagenum break" id="Page_210" title="pg 210"></a>
+found now in fairly humid regions, for example,
+the Pennsylvanian and Permian deposits of Kansas and
+Oklahoma, were probably laid down in times of local
+aridity due to the cutting off of moisture-bearing winds
+by the mountains of Llanoria in Louisiana and Texas.
+Hence such deposits do not necessarily indicate periods
+of widespread and profound aridity.</p>
+
+<p>When the causes of ancient glaciation were first considered
+by geologists, about the middle of the nineteenth
+century, it was usually assumed that the glaciated areas
+had been elevated to great heights, and thus rendered
+cold enough to permit the accumulation of glaciers. The
+many glaciers occurring in the Alps of central Europe
+where glaciology arose doubtless suggested this explanation.
+However, it is now known that most of the ancient
+glaciation was not of the alpine type, and there is adequate
+proof that the glacial periods cannot be explained
+as due directly and solely to uplift. Nevertheless, upheavals
+of the lands are among the most important factors
+in controlling climate, and variations in the height
+of the lands have doubtless assisted in producing climate
+oscillations, especially those of long duration. Moreover,
+the progressive increase in the height of the lands has
+presumably played a part in fostering local and zonal
+diversity in contrast with the relative uniformity of
+earlier geological times.</p>
+
+<p>IV. The contraction of the earth has been accompanied
+by volcanic activity as well as by changes in the extent,
+distribution, and altitude of the lands. The probable part
+played by volcanic dust as a contributory factor in producing
+short sudden climatic variations has already been
+discussed. There is, however, another though probably
+less important respect in which volcanic activity may
+have had at least a slight climatic significance. The oldest
+<!--File: cc235.png Page 211-->
+<a class="pagenum break" id="Page_211" title="pg 211"></a>
+known rocks, those of the Archean era, contain so much
+igneous matter that many students have assumed that
+they show that the entire earth was once liquid. It is now
+considered that they merely indicate igneous activity of
+great magnitude. In the later part of Proterozoic time,
+during the second quarter of the earth's history according
+to Schuchert's estimate, there were again vast outflowings
+of lava. In the Lake Superior district, for example,
+a thickness of more than a mile accumulated over
+a large area, and lavas are common in many areas where
+rocks of this age are known. The next quarter of the
+earth's history elapsed without any correspondingly
+great outflows so far as is known, though several lesser
+ones occurred. Toward the end of the last quarter, and
+hence quite recently from the geological standpoint,
+another period of outflows, perhaps as noteworthy as
+that of the Proterozoic, occurred in the Cretaceous and
+Tertiary.</p>
+
+<p>The climatic effects of such extensive lava flows would
+be essentially as follows: In the first place so long as the
+lavas were hot they would set up a local system of convection
+with inflowing winds. This would interfere at
+least a little with the general winds of the area. Again,
+where the lava flowed out into water, or where rain fell
+upon hot lava, there would be rapid evaporation which
+would increase the rainfall. Then after the lava had
+cooled, it would still influence climate a trifle in so far as
+its color was notably darker or lighter than that of the
+average surface. Dark surfaces absorb solar heat and
+become relatively warm when the sun shines upon them.
+Dark objects likewise radiate heat more rapidly than
+light-colored objects. Hence they cool more rapidly at
+night, and in the winter. As most lavas are relatively
+dark they increase the average diurnal range of temperature.
+<!--File: cc236.png Page 212-->
+<a class="pagenum break" id="Page_212" title="pg 212"></a>
+Hence even after they are cool they increase the
+climatic diversity of the land.</p>
+
+<p>The amount of heat given to the atmosphere by an
+extensive lava flow, though large according to human
+standards, is small compared with the amount received
+from the sun by a like area, except during the first few
+weeks or months before the lava has formed a thick
+crust. Furthermore, probably only a small fraction of
+any large series of flows occurred in a given century or
+millennium. Moreover, even the largest lava flows covered
+an area of only a few hundredths of one per cent of the
+earth's surface. Nevertheless, the conditions which modify
+climate are so complicated that it would be rash to
+state that this amount of additional heat has been of
+no climatic significance. Like the proverbial "straw that
+broke the camel's back," the changes it would surely
+produce in local convection, atmospheric pressure, and
+the direction of the wind may have helped to shift the
+paths of storms and to produce other complications which
+were of appreciable climatic significance.</p>
+
+<p>V. The last point which we shall consider in connection
+with the effect of the earth's interior upon climate is
+internal heat. The heat given off by lavas is merely a
+small part of that which is emitted by the earth as a
+whole. In the earliest part of geological history enough
+heat may have escaped from the interior of the earth to
+exert a profound influence on the climate. Knowlton,<sup><a href="#fn94" id="r94">[94]</a></sup>
+as we have seen, has recently built up an elaborate theory
+on this assumption. At present, however, accurate measurements
+show that the escape of heat is so slight that
+it has no appreciable influence except in a few volcanic
+<!--File: cc237.png Page 213-->
+<a class="pagenum break" id="Page_213" title="pg 213"></a>
+areas. It is estimated to raise the average temperature
+of the earth's surface less than 0.1&deg;C.<sup><a href="#fn95" id="r95">[95]</a></sup></p>
+
+<p>In order to contribute enough heat to raise the surface
+temperature 1&deg;C., the temperature gradient from the
+interior of the earth to the surface would need to be ten
+times as great as now, for the rate of conduction varies
+directly with the gradient. If the gradient were ten times
+as great as now, the rocks at a depth of two and one-half
+miles would be so hot as to be almost liquid according to
+Barrell's<sup><a href="#fn96" id="r96">[96]</a></sup> estimates. The thick strata of unmetamorphosed
+Paleozoic rocks indicate that such high temperatures
+have not prevailed at such slight depths since the
+Proterozoic. Furthermore, the fact that the climate was
+cold enough to permit glaciation early in the Proterozoic
+era and at from one to three other times before the opening
+of the Paleozoic suggests that the rate of escape of
+heat was not rapid even in the first half of the earth's
+recorded history. Yet even if the general escape of heat
+has never been large since the beginning of the better-known
+part of geological history, it was presumably
+greater in early times than at present.</p>
+
+<p>If there actually has been an appreciable decrease in
+the amount of heat given out by the earth's interior, its
+effects would agree with the observed conditions of the
+geological record. It would help to explain the relative
+mildness of zonal, seasonal, and local contrasts of climate
+in early geological times, but it would not help to explain
+the long oscillations from era to era which appear to have
+been of much greater importance. Those oscillations, so
+far as we can yet judge, may have been due in part to
+solar changes, but in large measure they seem to be
+<!--File: cc238.png Page 214-->
+<a class="pagenum break" id="Page_214" title="pg 214"></a>
+explained by variations in the extent, distribution, and
+altitude of the lands. Such variations appear to be the
+inevitable result of the earth's contraction.</p>
+<!--File: cc239.png Page 215-->
+<a class="pagenum break" id="Page_215" title="pg 215"></a>
+
+
+
+
+<h2 id="Chapt_XII"><a href="#content">CHAPTER XII</a></h2>
+
+<h3>POST-GLACIAL CRUSTAL MOVEMENTS AND
+CLIMATIC CHANGES</h3>
+
+
+<p>An interesting practical application of some of the
+preceding generalizations is found in an attempt
+by C. E. P. Brooks<sup><a href="#fn97" id="r97">[97]</a></sup> to interpret post-glacial
+climatic changes almost entirely in terms of crustal movement.
+We believe that he carries the matter much too far,
+but his discussion is worthy of rather full recapitulation,
+not only for its theoretical value but because it gives a
+good summary of post-glacial changes. His climatic table
+for northwest Europe as reprinted from the annual report
+of the Smithsonian Institution for 1917, p. 366, is
+as follows:</p>
+
+
+<table class="borders">
+<tr>
+<th><em>Phase</em></th>
+<th><em>Climate</em> </th>
+<th> <em>Date</em></th>
+</tr>
+<tr>
+<td class="tdl">1. The Last Great Glaciation.</td>
+<td class="tdl">Arctic climate.</td>
+<td>30,000-18,000 B. C.</td>
+</tr>
+<tr><td class="tdl">2. The Retreat of the Glaciers.</td>
+<td class="tdl">Severe continental climate.</td>
+<td>18,000-6000 B. C.</td>
+</tr>
+<tr>
+<td class="tdl">3. The Continental Phase.</td>
+<td class="tdl">Continental climate.</td>
+<td>6000-4000 B. C.</td></tr>
+<tr><td class="tdl">4. The Maritime Phase.</td>
+<td class="tdl">Warm and moist.</td>
+<td>4000-3000 B. C.</td>
+</tr>
+<tr>
+<td class="tdl">5. The Later Forest Phase.</td>
+<td class="tdl">Warm and dry.</td>
+<td>3000-1800 B. C.</td></tr>
+<tr><td class="tdl">6. The Peat-Bog Phase.</td>
+<td class="tdl">Cooler and moister.</td>
+<td>1800 B. C.-300 A. D.</td></tr>
+<tr>
+<td class="tdl">7. The Recent Phase.</td>
+<td class="tdl">Becoming drier.</td>
+<td>300 A. D.-</td></tr>
+</table>
+
+
+<p>Brooks bases his chronology largely on De Geer's
+measurements of the annual layers of clay in lake
+<!--File: cc240.png Page 216-->
+<a class="pagenum break" id="Page_216" title="pg 216"></a>
+bottoms but makes much use of other evidence. According
+to Brooks the last glacial epoch lasted roughly from
+30,000 to 18,000 B. C., but this includes a slight amelioration
+of climate followed by a readvance of the ice, known
+as the Buhl stage. During the time of maximum glaciation
+the British Isles stood twenty or thirty feet higher
+than now and Scandinavia was "considerably" more
+elevated. The author believes that this caused a fall of
+1&deg;C. in the temperature of the British Isles and of 2&deg;C.
+in Scandinavia. By an ingenious though not wholly convincing
+method of calculation he concludes that this
+lowering of temperature, aided by an increase in the area
+of the lands, sufficed to start an ice sheet in Scandinavia.
+The relatively small area of ice cooled the air and gave
+rise to an area of high barometric pressure. This in turn
+is supposed to have caused further expansion of the ice
+and to have led to full-fledged glaciation.</p>
+
+<p>About 18,000 B. C. the retreat of the ice began in good
+earnest. Even though no evidence has yet been found,
+Brooks believes there must have been a change in the distribution
+of land and sea to account for the diminution of
+the ice. The ensuing millenniums formed the Magdalenian
+period in human history, the last stage of the Paleolithic,
+when man lived in caves and reindeer were abundant
+in central Europe.<sup><a href="#fn98" id="r98">[98]</a></sup> At first the ice retreated very
+slowly and there were periods when for scores of years
+the ice edge remained stationary or even readvanced.
+About 10,000 B. C. the edge of the ice lay along the
+southern coast of Sweden. During the next 2000 years it
+withdrew more rapidly to about 59&deg;N. Then came the
+Fennoscandian pause, or Gschnitz stage, when for about
+<!--File: cc241.png Page 217-->
+<a class="pagenum break" id="Page_217" title="pg 217"></a>
+200 years the ice edge remained in one position, forming
+a great moraine. Brooks suggests that this pause about
+8000 B. C. was due to the closing of the connection between
+the Atlantic Ocean and the Baltic Sea and the
+synchronous opening of a connection between the Baltic
+and the White Seas, whereby cold Arctic waters replaced
+the warmer Atlantic waters. He notes, however, that
+about 7500 B. C. the obliquity of the ecliptic was probably
+nearly 1&deg; greater than at present. This he calculates to
+have caused the climate of Germany and Sweden to be
+1&deg;F. colder than at present in winter and 1&deg;F. warmer in
+summer.</p>
+
+<p>The next climatic stage was marked by a rise of temperature
+till about 6000 B. C. During this period the ice
+at first retreated, presumably because the climate was
+ameliorating, although no cause of such amelioration is
+assigned. At length the ice lay far enough north to allow
+a connection between the Baltic and the Atlantic by way
+of Lakes Wener and Wetter in southern Sweden. This is
+supposed to have warmed the Baltic Sea and to have
+caused the climate to become distinctly milder. Next the
+land rose once more so that the Baltic was separated
+from the Atlantic and was converted into the Ancylus
+lake of fresh water. The southwest Baltic region then
+stood 400 feet higher than now. The result was the Daun
+stage, about 5000 B. C., when the ice halted or perhaps
+readvanced a little, its front being then near Ragunda
+in about latitude 63&deg;. Why such an elevation did not
+cause renewed glaciation instead of merely the slight
+Daun pause, Brooks does not explain, although his calculations
+as to the effect of a slight elevation of the land
+during the main period of glaciation from 30,000 to
+18,000 B. C. would seem to demand a marked readvance.</p>
+<!--File: cc242.png Page 218-->
+<a class="pagenum break" id="Page_218" title="pg 218"></a>
+
+<p>After 5000 B. C. there ensued a period when the climate,
+although still distinctly continental, was relatively
+mild. The winters, to be sure, were still cold but the
+summers were increasingly warm. In Sweden, for example,
+the types of vegetation indicate that the summer
+temperature was 7&deg;F. higher than now. Storms, Brooks
+assumes, were comparatively rare except on the outer
+fringe of Great Britain. There they were sufficiently
+abundant so that in the Northwest they gave rise to the
+first Peat-Bog period, during which swamps replaced
+forests of birch and pine. Southern and eastern England,
+however, probably had a dry continental climate. Even
+in northwest Norway storms were rare as is indicated by
+remains of forests on islands now barren because of the
+strong winds and fierce storms. Farther east most parts
+of central and northern Europe were relatively dry. This
+was the early Neolithic period when man advanced from
+the use of unpolished to polished stone implements.</p>
+
+<p>Not far from 4000 B. C. the period of continental climate
+was replaced by a comparatively moist maritime
+climate. Brooks believes that this was because submergence
+opened the mouth of the Baltic and caused the
+fresh Ancylus lake to give place to the so-called Litorina
+sea. The temperature in Sweden averaged about 3&deg;F.
+higher than at present and in southwestern Norway 2&deg;.
+More important than this was the small annual range of
+temperature due to the fact that the summers were cool
+while the winters were mild. Because of the presence of
+a large expanse of water in the Baltic region, storms, as
+our author states, then crossed Great Britain and followed
+the Baltic depression, carrying the moisture far
+inland. In spite of the additional moisture thus available
+the snow line in southern Norway was higher than now.</p>
+
+<p>At this point Brooks turns to other parts of the world.
+<!--File: cc243.png Page 219-->
+<a class="pagenum break" id="Page_219" title="pg 219"></a>
+He states that not far from 4000 B. C., a submergence
+of the lands, rarely amounting to more than twenty-five
+feet, took place not only in the Baltic region but in Ireland,
+Iceland, Spitzbergen, and other parts of the Arctic
+Ocean, as well as in the White Sea, Greenland, and the
+eastern part of North America. Evidences of a mild climate
+are found in all those places. Similar evidence of a
+mild warm climate is found in East Africa, East Australia,
+Tierra del Fuego, and Antarctica. The dates are
+not established with certainty but they at least fall in the
+period immediately preceding the present epoch. In explanation
+of these conditions Brooks assumes a universal
+change of sea level. He suggests with some hesitation
+that this may have been due to one of Pettersson's
+periods of maximum "tide-generating force." According
+to Pettersson the varying positions of the moon, earth,
+and sun cause the tides to vary in cycles of about 9, 90,
+and 1800 years, though the length of the periods is not
+constant. When tides are high there is great movement
+of ocean waters and hence a great mixture of the water
+at different latitudes. This is supposed to cause an
+amelioration of climate. The periods of maximum and
+minimum tide-generating force are as follows:</p>
+
+<blockquote>
+<p>Maxima &nbsp; &nbsp; &nbsp;  3500 B. C. &mdash;&mdash;&mdash;&mdash; 2100 B. C. &mdash;&mdash;&mdash;&mdash; 350 B. C. &mdash;&mdash;&mdash;&mdash; A. D. 1434<br />
+Minima &nbsp; &nbsp;  &mdash;&mdash;&mdash;&mdash;&mdash; 2800 B. C. &mdash;&mdash;&mdash;&mdash; 1200 B. C. &mdash;&mdash;&mdash;&mdash; A. D. 530 &mdash;&mdash;&mdash;&mdash;</p>
+</blockquote>
+
+<p>Brooks thinks that the big trees in California and the
+Norse sagas and Germanic myths indicate a rough agreement
+of climatic phenomena with Pettersson's last three
+dates, while the mild climate of 4000 B. C. may really
+belong to 3500 B. C. He gives no evidence confirming
+Pettersson's view at the other three dates.</p>
+
+<p>To return to Brooks' sketch of the relation of climatic
+pulsations to the altitude of the lands, by 3000 B. C., that
+<!--File: cc244.png Page 220-->
+<a class="pagenum break" id="Page_220" title="pg 220"></a>
+is, toward the close of the Neolithic period, further elevation
+is supposed to have taken place over the central
+latitudes of western Europe. Southern Britain, which had
+remained constantly above its present level ever since
+30,000 B. C., was perhaps ninety feet higher than now.
+Ireland was somewhat enlarged by elevation, the Straits
+of Dover were almost closed, and parts of the present
+North Sea were land. To these conditions Brooks ascribes
+the prevalence of a dry continental climate. The storms
+shifted northward once more, the winds were mild, as
+seems to be proved by remains of trees in exposed places;
+and forests replaced fields of peat and heath in Britain
+and Germany. The summers were perhaps warmer than
+now but the winters were severe. The relatively dry climate
+prevailed as far west as Ireland. For example, in
+Drumkelin Bog in Donegal County a corded oak road and
+a two-story log cabin appear to belong to this time. Fourteen
+feet of bog lie below the floor and twenty-six above.
+This period, perhaps 3000-2000 B. C., was the legendary
+heroic age of Ireland when "the vigour of the Irish
+reached a level not since attained." This, as Brooks
+points out, may have been a result of the relatively dry
+climate, for today the extreme moisture of Ireland seems
+to be a distinct handicap. In Scandinavia, civilization, or
+at least the stage of relative progress, was also high at
+this time.</p>
+
+<p>By 1600 B. C. the land had assumed nearly its present
+level in the British Isles and the southern Baltic
+region, while northern Scandinavia still stood lower than
+now. The climate of Britain and Germany was so humid
+that there was an extensive formation of peat even on
+high ground not before covered. This moist stage seems
+to have lasted almost to the time of Christ, and may have
+been the reason why the Romans described Britain as
+<!--File: cc245.png Page 221-->
+<a class="pagenum break" id="Page_221" title="pg 221"></a>
+peculiarly wet and damp. At this point Brooks again departs
+from northwest Europe to a wider field:</p>
+
+<blockquote>
+<p>It is possible that we have to attribute this damp period in
+Northwest Europe to some more general cause, for Ellsworth
+Huntington's curves of tree-growth in California and climate
+in Western Asia both show moister conditions from about
+1000 B. C. to A. D. 200, and the same author believes that the
+Mediterranean lands had a heavier rainfall about 500 B. C. to
+A. D. 200. It seems that the phase was marked by a general increase
+of the storminess of the temperate regions of the northern
+hemisphere at least, with a maximum between Ireland and North
+Germany, indicating probably that the Baltic again became the
+favourite track of depressions from the Atlantic.</p>
+</blockquote>
+
+<p>Brooks ends his paper with a brief r&eacute;sum&eacute; of glacial
+changes in North America, but as the means of dating
+events are unreliable the degree of synchronism with
+Europe is not clear. He sums up his conclusions as
+follows:</p>
+
+<blockquote>
+<p>On the whole it appears that though there is a general similarity
+in the climatic history of the two sides of the North
+Atlantic, the changes are not really contemporaneous, and such
+relationship as appears is due mainly to the natural similarity
+in the geographical history of two regions both recovering from
+an Ice Age, and only very partially to world-wide pulsations
+of climate. Additional evidence on this head will be available
+when Baron de Geer publishes the results of his recent investigations
+of the seasonal glacial clays of North America, especially
+if, as he hopes, he is able to correlate the banding of these clays
+with the growth-rings of the big trees.</p>
+
+<p>When we turn to the northwest of North America, this is
+brought out very markedly. For in Yukon and Alaska the Ice
+Age was a very mild affair compared with its severity in eastern
+America and Scandinavia. As the land had not a heavy ice-load
+to recover from, there were no complicated geographical
+<!--File: cc246.png Page 222-->
+<a class="pagenum break" id="Page_222" title="pg 222"></a>
+changes. Also, there were no fluctuations of climate, but simply
+a gradual passage to present conditions. The latter circumstance
+especially seems to show that the emphasis laid on geographical
+rather than astronomical factors of <em>great</em> climatic changes is
+not misplaced.</p>
+</blockquote>
+
+<p>Brooks' painstaking discussion of post-glacial climatic
+changes is of great value because of the large body of
+material which he has so carefully wrought together. His
+strong belief in the importance of changes in the level of
+the lands deserves serious consideration. It is difficult,
+however, to accept his final conclusion that such changes
+are the main factors in recent climatic changes. It is almost
+impossible, for example, to believe that movements
+of the land could produce almost the same series of
+climatic changes in Europe, Central Asia, the western
+and eastern parts of North America, and the southern
+hemisphere. Yet such changes appear to have occurred
+during and since the glacial period. Again there is no
+evidence whatever that movements of the land have anything
+to do with the historic cycles of climate or with the
+cycles of weather in our own day, which seem to be the
+same as glacial cycles on a small scale. Also, as Dr.
+Simpson points out in discussing Brooks' paper, there
+appears "no solution along these lines of the problem
+connected with rich vegetation in both polar circles and
+the ice-age which produced the ice-sheet at sea-level in
+Northern India." Nevertheless, we may well believe that
+Brooks is right in holding that changes in the relative
+level and relative area of land and sea have had important
+local effects. While they are only one of the
+factors involved in climatic changes, they are certainly
+one that must constantly be kept in mind.</p>
+<!--File: cc247.png Page 223-->
+<a class="pagenum break" id="Page_223" title="pg 223"></a>
+
+
+
+
+<h2 id="Chapt_XIII"><a href="#content">CHAPTER XIII</a></h2>
+
+<h3>THE CHANGING COMPOSITION OF OCEANS AND
+ATMOSPHERE</h3>
+
+
+<p>Having discussed the climatic effect of movements
+of the earth's crust during the course of
+geological time, we are now ready to consider
+the corresponding effects due to changes in the movable
+envelopes&mdash;the oceans and the atmosphere. Variations in
+the composition of sea water and of air and in the
+amount of air must almost certainly have occurred, and
+must have produced at least slight climatic consequences.
+It should be pointed out at once that such variations
+appear to be far less important climatically than do
+movements of the earth's crust and changes in the activity
+of the sun. Moreover, in most cases, they are not
+reversible as are the crustal and solar phenomena. Hence,
+while most of them appear to have been unimportant so
+far as climatic oscillations and fluctuations are concerned,
+they seemingly have aided in producing the slight secular
+progression to which we have so often referred.</p>
+
+<p>There is general agreement among geologists that the
+ocean has become increasingly saline throughout the
+ages. Indeed, calculations of the rate of accumulation of
+salt have been a favorite method of arriving at estimates
+of the age of the ocean, and hence of the earliest marine
+sediments. So far as known, however, no geologist or
+climatologist has discussed the probable climatic effects
+<!--File: cc248.png Page 224-->
+<a class="pagenum break" id="Page_224" title="pg 224"></a>
+of increased salinity. Yet it seems clear that an increase
+in salinity must have a slight effect upon climate.</p>
+
+<p>Salinity affects climate in four ways: (1) It appreciably
+influences the rate of evaporation; (2) it alters
+the freezing point; (3) it produces certain indirect
+effects through changes in the absorption of carbon
+dioxide; and (4) it has an effect on oceanic circulation.</p>
+
+<p>(1) According to the experiments of Mazelle and
+Okada, as reported by Kr&uuml;mmel,<sup><a href="#fn99" id="r99">[99]</a></sup> evaporation from ordinary
+sea water is from 9 to 30 per cent less rapid than
+from fresh water under similar conditions. The variation
+from 9 to 30 per cent found in the experiments depends,
+perhaps, upon the wind velocity. When salt water
+is stagnant, rapid evaporation tends to result in the
+development of a film of salt on the top of the water,
+especially where it is sheltered from the wind. Such a film
+necessarily reduces evaporation. Hence the relatively
+low salinity of the oceans in the past probably had a
+tendency to increase the amount of water vapor in the
+air. Even a little water vapor augments slightly the
+blanketing effect of the air and to that extent diminishes
+the diurnal and seasonal range of temperature and the
+contrast from zone to zone.</p>
+
+<p>(2) Increased salinity means a lower freezing temperature
+of the oceans and hence would have an effect
+during cold periods such as the present and the Pleistocene
+ice age. It would not, however, be of importance
+during the long warm periods which form most of
+geologic time. A salinity of about 3.5 per cent at present
+lowers the freezing point of the ocean roughly 2&deg;C. below
+that of fresh water. If the ocean were fresh and our
+winters as cold as now, all the harbors of New England
+and the Middle Atlantic States would be icebound. The
+<!--File: cc249.png Page 225-->
+<a class="pagenum break" id="Page_225" title="pg 225"></a>
+Baltic Sea would also be frozen each winter, and even
+the eastern harbors of the British Isles would be frequently
+locked in ice. At high latitudes the area of permanently
+frozen oceans would be much enlarged. The
+effect of such a condition upon marine life in high latitudes
+would be like that of a change to a warmer climate.
+It would protect the life on the continental shelf from the
+severe battering of winter storms. It would also lessen
+the severity of the winter temperature in the water for
+when water freezes it gives up much latent heat,&mdash;eighty
+calories per cubic centimeter. Part of this raises the
+temperature of the underlying water.</p>
+
+<p>The expansion of the ice near northern shores would
+influence the life of the lands quite differently from that
+of the oceans. It would act like an addition of land to the
+continents and would, therefore, increase the atmospheric
+contrasts from zone to zone and from continental interior
+to ocean. In summer the ice upon the sea would tend to
+keep the coastal lands cool, very much as happens now
+near the Arctic Ocean, where the ice floes have a great
+effect through their reflection of light and their absorption
+of heat in melting. In winter the virtual enlargement
+of the continents by the addition of an ice fringe would
+decrease the snowfall upon the lands. Still more important
+would be the effect in intensifying the anti-cyclonic
+conditions which normally prevail in winter not
+only over continents but over ice-covered oceans. Hence
+the outblowing cold winds would he strengthened.<sup><a href="#fn100" id="r100">[100]</a></sup> The
+net effect of all these conditions would apparently be a
+diminution of snowfall in high latitudes upon the lands
+even though the summer snowfall upon the ocean and the
+<!--File: cc250.png Page 226-->
+<a class="pagenum break" id="Page_226" title="pg 226"></a>
+coasts may have increased. This condition may have been
+one reason why widespread glaciation does not appear to
+have prevailed in high latitudes during the Proterozoic
+and Permian glaciations, even though it occurred farther
+south. If the ocean during those early glacial epochs
+were ice-covered down to middle latitudes, a lack of extensive
+glaciation in high latitudes would be no more
+surprising than is the lack of Pleistocene glaciation in
+the northern parts of Alaska and Asia. Great ice sheets
+are impossible without a large supply of moisture.</p>
+
+<p>(3) Among the indirect effects of salinity one of the
+chief appears to be that the low salinity of the water in
+the past and the greater ease with which it froze presumably
+allowed the temperature of the entire ocean to be
+slightly higher than now. This is because ice serves as a
+blanket and hinders the radiation of heat from the underlying
+water. The temperature of the ocean has a climatic
+significance not only directly, but indirectly through its
+influence on the amount of carbon dioxide held by the
+oceans. A change of even 1&deg;C. from the present mean
+temperature of 2&deg;C. would alter the ability of the entire
+ocean to absorb carbon dioxide by about 4 per cent. This,
+according to F. W. Clarke,<sup><a href="#fn101" id="r101">[101]</a></sup> is because the oceans contain
+from eighteen to twenty-seven times as much carbon
+dioxide as the air when only the free carbon dioxide is
+considered, and about seventy times as much according
+to Johnson and Williamson<sup><a href="#fn102" id="r102">[102]</a></sup> when the partially combined
+carbon dioxide is also considered. Moreover, the capacity
+of water for carbon dioxide varies sharply with the temperature.<sup><a href="#fn103" id="r103">[103]</a></sup>
+Hence a rise in temperature of only 1&deg;C.
+would theoretically cause the oceans to give up from 30
+<!--File: cc251.png Page 227-->
+<a class="pagenum break" id="Page_227" title="pg 227"></a>
+to 280 times as much carbon dioxide as the air now holds.
+This, however, is on the unfounded assumption that the
+oceans are completely saturated. The important point is
+merely that a slight change in ocean temperature would
+cause a disproportionately large change in the amount
+of carbon dioxide in the air with all that this implies in
+respect to blanketing the earth, and thus altering temperature.</p>
+
+<p>(4) Another and perhaps the most important effect of
+salinity upon climate depends upon the rapidity of the
+deep-sea circulation. The circulation is induced by differences
+of temperature, but its speed is affected at least
+slightly by salinity. The vertical circulation is now dominated
+by cold water from subpolar latitudes. Except in
+closed seas like the Mediterranean the lower portions
+of the ocean are near the freezing point. This is because
+cold water sinks in high latitudes by reason of its superior
+density, and then "creeps" to low latitudes. There
+it finally rises and replaces either the water driven poleward
+by the winds, or that which has evaporated from
+the Surface.<sup><a href="#fn104" id="r104">[104]</a></sup></p>
+
+<p>During past ages, when the sea water was less salty,
+the circulation was presumably more rapid than now.
+This was because, in tropical regions, the rise of cold
+<!--File: cc252.png Page 228-->
+<a class="pagenum break" id="Page_228" title="pg 228"></a>
+water is hindered by the sinking of warm surface water
+which is relatively dense because evaporation has removed
+part of the water and caused an accumulation of
+salt. According to Kr&uuml;mmel and Mill,<sup><a href="#fn105" id="r105">[105]</a></sup> the surface salinity
+of the subtropical belt of the North Atlantic commonly
+exceeds 3.7 per cent and sometimes reaches 3.77 per cent,
+whereas the underlying waters have a salinity of less
+than 3.5 per cent and locally as little as 3.44 per cent.
+The other oceans are slightly less saline than the North
+Atlantic at all depths, but the vertical salinity gradients
+along the tropics are similar. According to the Smithsonian
+Physical Tables, the difference in salinity between
+the surface water and that lying below is equivalent to
+a difference of .003 in density, where the density of fresh
+water is taken as 1.000. Since the decrease in density produced
+by warming water from the temperature of its
+greatest density (4&deg;C.) to the highest temperatures
+which ever prevail in the ocean (30&deg;C. or 86&deg;F.) is only
+.004, the more saline surface waters of the dry tropics
+are at most times almost as dense as the less saline but
+colder waters beneath the surface, which have come from
+higher latitudes. During days of especially great evaporation,
+however, the most saline portions of the surface
+waters in the dry tropics are denser than the underlying
+waters and therefore sink, and produce a temporary local
+stagnation in the general circulation. Such a sinking of
+the warm surface waters is reported by Kr&uuml;mmel, who
+detected it by means of the rise in temperature which it
+produces at considerable depths. If such a hindrance to
+the circulation did not exist, the velocity of the deep-sea
+movements would be greater.</p>
+
+<p>If in earlier times a more rapid circulation occurred,
+low latitudes must have been cooled more than now by
+<!--File: cc253.png Page 229-->
+<a class="pagenum break" id="Page_229" title="pg 229"></a>
+the rise of cold waters. At the same time higher latitudes
+were presumably warmed by a greater flow of warm
+water from tropical regions because less of the surface
+heat sank in low latitudes. Such conditions would tend to
+lessen the climatic contrast between the different latitudes.
+Hence, in so far as the rate of deep-sea circulation
+depends upon salinity, the slowly increasing amount of
+salt in the oceans must have tended to increase the contrasts
+between low and high latitudes. Thus for several
+reasons, the increase of salinity during geologic history
+seems to deserve a place among the minor agencies which
+help to explain the apparent tendency toward a secular
+progression of climate in the direction of greater contrasts
+between tropical and subpolar latitudes.</p>
+
+<p>Changes in the composition and amount of the atmosphere
+have presumably had a climatic importance greater
+than that of changes in the salinity of the oceans. The
+atmospheric changes may have been either progressive
+or cyclic, or both. In early times, according to the nebular
+hypothesis, the atmosphere was much more dense than
+now and contained a larger percentage of certain constituents,
+notably carbon dioxide and water. The planetesimal
+hypothesis, on the other hand, postulates an increase
+in the density of the atmosphere, for according to
+this hypothesis the density of the atmosphere depends
+upon the power of the earth to hold gases, and this power
+increases as the earth grows bigger with the infall of
+material from without.<sup><a href="#fn106" id="r106">[106]</a></sup></p>
+
+<p>Whichever hypothesis may be correct, it seems probable
+that when life first appeared on the land the atmosphere
+resembled that of today in certain fundamental
+respects. It contained the elements essential to life, and
+<!--File: cc254.png Page 230-->
+<a class="pagenum break" id="Page_230" title="pg 230"></a>
+its blanketing effect was such as to maintain temperatures
+not greatly different from those of the present. The
+evidence of this depends largely upon the narrow limits
+of temperature within which the activities of modern
+life are possible, and upon the cumulative evidence that
+ancient life was essentially similar to the types now
+living. The resemblance between some of the oldest
+forms and those of today is striking. For example,
+according to Professor Schuchert:<sup><a href="#fn107" id="r107">[107]</a></sup> "Many of the living
+genera of forest trees had their origin in the Cretaceous,
+and the giant sequoias of California go back to the Triassic,
+while Ginkgo is known in the Permian. Some of the
+fresh-water molluscs certainly were living in the early
+periods of the Mesozoic, and the lung-fish of today
+(Ceratodus) is known as far back as the Triassic and is
+not very unlike other lung-fishes of the Devonian. The
+higher vertebrates and insects, on the other hand, are
+very sensitive to their environment, and therefore do not
+extend back generically beyond the Cenozoic, and only in
+a few instances even as far as the Oligocene. Of marine
+invertebrates the story is very different, for it is well
+known that the horseshoe crab (Limulus) lived in the
+Upper Jurassic, and Nautilus in the Triassic, with forms
+in the Devonian not far removed from this genus. Still
+longer-ranging genera occur among the brachiopods, for
+living Lingula and Crania have specific representatives
+as far back as the early Ordovician. Among living foraminifers,
+Lagena, Globigerina, and Nodosaria are known
+in the later Cambrian or early Ordovician. In the Middle
+Cambrian near Field, British Columbia, Walcott has
+found a most varied array of invertebrates among which
+are crustaceans not far removed from living forms.
+Zo&ouml;logists who see these wonderful fossils are at once
+<!--File: cc255.png Page 231-->
+<a class="pagenum break" id="Page_231" title="pg 231"></a>
+struck with their modernity and the little change that has
+taken place in certain stocks since that far remote time.
+Back of the Paleozoic, little can be said of life from the
+generic standpoint, since so few fossils have been recovered,
+but what is at hand suggests that the marine
+environment was similar to that of today."</p>
+
+<p>At present, as we have repeatedly seen, little growth
+takes place either among animals or plants at temperatures
+below 0&deg;C. or above 40&deg;C., and for most species
+the limiting temperatures are about 10&deg; and 30&deg;. The
+maintenance of so narrow a scale of temperature is a
+function of the atmosphere, as well as of the sun. Without
+an atmosphere, the temperature by day would mount
+fatally wherever the sun rides high in the sky. By night
+it would fall everywhere to a temperature approaching
+absolute zero, that is -273&deg;C. Some such temperature
+prevails a few miles above the earth's surface,
+beyond the effective atmosphere. Indeed, even if the
+atmosphere were almost as it is now, but only lacked one
+of the minor constituents, a constituent which is often
+actually ignored in statements of the composition of the
+air, life would be impossible. Tyndall concludes that if
+water vapor were entirely removed from the atmosphere
+for a single day and night, all life&mdash;except that which is
+dormant in the form of seeds, eggs, or spores&mdash;would be
+exterminated. Part would be killed by the high temperature
+developed by day when the sun was high, and part,
+by the cold night.</p>
+
+<p>The testimony of ancient glaciation as to the slight
+difference in the climate and therefore in the atmosphere
+of early and late geological times is almost as clear
+as that of life. Just as life proves that the earth can never
+have been extremely cold during hundreds of millions of
+years, so glaciation in moderately low latitudes near
+<!--File: cc256.png Page 232-->
+<a class="pagenum break" id="Page_232" title="pg 232"></a>
+the dawn of earth history and at several later times,
+proves that the earth was not particularly hot even in
+those early days. The gentle progressive change of climate
+which is recorded in the rocks appears to have been only
+in slight measure a change in the mean temperature of
+the earth as a whole, and almost entirely a change in the
+distribution of temperature from place to place and
+season to season. Hence it seems probable that neither
+the earth's own emission of heat, nor the supply of solar
+heat, nor the power of the atmosphere to retain heat can
+have been much greater a few hundred million years ago
+than now. It is indeed possible that these three factors
+may have varied in such a way that any variation in one
+has been offset by variations of the others in the opposite
+direction. This, however, is so highly improbable that it
+seems advisable to assume that all three have remained
+relatively constant. This conclusion together with a
+realization of the climatic significance of carbon dioxide
+has forced most of the adherents of the nebular hypothesis
+to abandon their assumption that carbon dioxide, the
+heaviest gas in the air, was very abundant until taken
+out by coal-forming plants or combined with the calcium
+oxide of igneous rocks to form the limestone secreted
+by animals. In the same way the presence of sun cracks
+in sedimentary rocks of all ages suggests that the air
+cannot have contained vast quantities of water vapor
+such as have been assumed by Knowlton and others in
+order to account for the former lack of sharp climatic
+contrast between the zones. Such a large amount of water
+vapor would almost certainly be accompanied by well-nigh
+universal and continual cloudiness so that there
+would be little chance for the pools on the earth's water-soaked
+surface to dry up. Furthermore, there is only one
+way in which such cloudiness could be maintained and
+<!--File: cc257.png Page 233-->
+<a class="pagenum break" id="Page_233" title="pg 233"></a>
+that is by keeping the air at an almost constant temperature
+night and day. This would require that the chief
+source of warmth be the interior of the earth, a condition
+which the Proterozoic, Permian, and other widespread
+glaciations seem to disprove.</p>
+
+<p>Thus there appears to be strong evidence against the
+radical changes in the atmosphere which are sometimes
+postulated. Yet some changes must have taken place, and
+even minor changes would be accompanied by some sort
+of climatic effect. The changes would take the form of
+either an increase or a decrease in the atmosphere as a
+whole, or in its constituent elements. The chief means by
+which the atmosphere has increased appear to be as
+follows: (a) By contributions from the interior of the
+earth via volcanoes and springs and by the weathering of
+igneous rocks with the consequent release of their enclosed
+gases;<sup><a href="#fn108" id="r108">[108]</a></sup> (b) by the escape of some of the abundant
+gases which the ocean holds in solution; (c) by the arrival
+on the earth of gases from space, either enclosed in
+meteors or as free-flying molecules; (d) by the release of
+gases from organic compounds by oxidation, or by exhalation
+from animals and plants. On the other hand, one
+or another of the constituents of the atmosphere has presumably
+decreased (a) by being locked up in newly
+formed rocks or organic compounds; (b) by being dissolved
+in the ocean; (c) by the escape of molecules into
+space; and (d) by the condensation of water vapor.</p>
+
+<p>The combined effect of the various means of increase
+and decrease depends partly on the amount of each constituent
+received from the earth's interior or from space,
+and partly on the fact that the agencies which tend to
+deplete the atmosphere are highly selective in their
+<!--File: cc258.png Page 234-->
+<a class="pagenum break" id="Page_234" title="pg 234"></a>
+action. Our knowledge of how large a quantity of new
+gases the air has received is very scanty, but judging by
+present conditions the general tendency is toward a slow
+increase chiefly because of meteorites, volcanic action,
+and the work of deep-seated springs. As to decrease, the
+case is clearer. This is because the chemically active
+gases, oxygen, CO<sub>2</sub>, and water vapor, tend to be locked
+up in the rocks, while the chemically inert gases, nitrogen
+and argon, show almost no such tendency. Though oxygen
+is by far the most abundant element in the earth's
+crust, making up more than 50 per cent of the total, it
+forms only about one-fifth of the air. Nitrogen, on the
+other hand, is very rare in the rocks, but makes up nearly
+four-fifths of the air. It would, therefore, seem probable
+that throughout the earth's history, there has been a
+progressive increase in the amount of atmospheric nitrogen,
+and presumably a somewhat corresponding increase
+in the mass of the air. On the other hand, it is not clear
+what changes have occurred in the amount of atmospheric
+oxygen. It may have increased somewhat or
+perhaps even notably. Nevertheless, because of the
+greater increase in nitrogen, it may form no greater percentage
+of the air now than in the distant past.</p>
+
+<p>As to the absolute amounts of oxygen, Barrell<sup><a href="#fn109" id="r109">[109]</a></sup>
+thought that atmospheric oxygen began to be present
+only after plants had appeared. It will be recalled that
+plants absorb carbon dioxide and separate the carbon
+from the oxygen, using the carbon in their tissues and
+setting free the oxygen. As evidence of a paucity of
+oxygen in the air in early Proterozoic times, Barrell
+cites the fact that the sedimentary rocks of that remote
+<!--File: cc259.png Page 235-->
+<a class="pagenum break" id="Page_235" title="pg 235"></a>
+time commonly are somewhat greyish or greenish-grey
+wackes, or other types, indicating incomplete oxidation.
+He admits, however, that the stupendous thicknesses of
+red sandstones, quartzite, and hematitic iron ores of the
+later Proterozoic prove that by that date there was an
+abundance of atmospheric oxygen. If so, the change from
+paucity to abundance must have occurred before fossils
+were numerous enough to give much clue to climate.
+However, Barrell's evidence as to a former paucity of
+atmospheric oxygen is not altogether convincing. In the
+first place, it does not seem justifiable to assume that
+there could be no oxygen until plants appeared to break
+down the carbon dioxide, for some oxygen is contributed
+by volcanoes,<sup><a href="#fn110" id="r110">[110]</a></sup> and lightning decomposes water into its
+elements. Part of the hydrogen thus set free escapes into
+space, for the earth's gravitative force does not appear
+great enough to hold this lightest of gases, but the oxygen
+remains. Thus electrolysis of water results in the
+accumulation of oxygen. In the second place, there is no
+proof that the ancient greywackes are not deoxidized
+sediments. Light colored rock formations do not necessarily
+indicate a paucity of atmospheric oxygen, for such
+rocks are abundant even in recent times. For example,
+the Tertiary formations are characteristically light
+colored, a result, however, of deoxidation. Finally, the
+fact that sedimentary rocks, irrespective of their age,
+contain an average of about 1.5 per cent more oxygen
+than do igneous rocks,<sup><a href="#fn111" id="r111">[111]</a></sup> suggests that oxygen was present
+in the air in quantity even when the earliest shales
+and sandstones were formed, for atmospheric oxygen
+seems to be the probable source of the extra oxygen they
+<!--File: cc260.png Page 236-->
+<a class="pagenum break" id="Page_236" title="pg 236"></a>
+contain. The formation of these particular sedimentary
+rocks by weathering of igneous rocks involves only a
+little carbon dioxide and water. Although it seems probable
+that oxygen was present in the atmosphere even at
+the beginning of the geological record, it may have been
+far less abundant then than now. It may have been removed
+from the atmosphere by animals or by the oxidation
+of the rocks almost as rapidly as it was added by
+volcanoes, plants, and other agencies.</p>
+
+<p>After this chapter was in type, St. John<sup><a href="#fn112" id="r112">[112]</a></sup> announced
+his interesting discovery that oxygen is apparently lacking
+in the atmosphere of Venus. He considers that this
+proves that Venus has no life. Furthermore he concludes
+that so active an element as oxygen cannot be abundant
+in the atmosphere of a planet unless plants continually
+supply large quantities by breaking down carbon dioxide.</p>
+
+<p>But even if the earth has experienced a notable increase
+in atmospheric oxygen since the appearance of
+life, this does not necessarily involve important climatic
+changes except those due to increased atmospheric density.
+This is because oxygen has very little effect upon
+the passage of light or heat, being transparent to all but
+a few wave lengths. Those absorbed are chiefly in the
+ultra violet.</p>
+
+<p>The distinct possibility that oxygen has increased in
+amount, makes it the more likely that there has been an
+increase in the total atmosphere, for the oxygen would
+supplement the increase in the relatively inert nitrogen
+and argon, which has presumably taken place. The climatic
+effects of an increase in the atmosphere include, in
+the first place, an increased scattering of light as it
+approaches the earth. Nitrogen, argon, and oxygen all
+<!--File: cc261.png Page 237-->
+<a class="pagenum break" id="Page_237" title="pg 237"></a>
+scatter the short waves of light and thus interfere with
+their reaching the earth. Abbot and Fowle,<sup><a href="#fn113" id="r113">[113]</a></sup> who have
+carefully studied the matter, believe that at present the
+scattering is quantitatively important in lessening insolation.
+Hence our supposed general increase in the volume
+of the air during part of geological times would tend to
+reduce the amount of solar energy reaching the earth's
+surface. On the other hand, nitrogen and argon do not
+appear to absorb the long wave lengths known as heat,
+and oxygen absorbs so little as to be almost a non-absorber.
+Therefore the reduced penetration of the air
+by solar radiation due to the scattering of light would
+apparently not be neutralized by any direct increase in
+the blanketing effect of the atmosphere, and the temperature
+near the earth's surface would be slightly lowered
+by a thicker atmosphere. This would diminish the amount
+of water vapor which would be held in the air, and
+thereby lower the temperature a trifle more.</p>
+
+<p>In the second place, the higher atmospheric pressure
+which would result from the addition of gases to the
+air would cause a lessening of the rate of evaporation,
+for that rate declines as pressure increases. Decreased
+evaporation would presumably still further diminish the
+vapor content of the atmosphere. This would mean a
+greater daily and seasonal range of temperature, as is
+very obvious when we compare clear weather with cloudy.
+Cloudy nights are relatively warm while clear nights are
+cool, because water vapor is an almost perfect absorber
+of radiant heat, and there is enough of it in the air on
+moist nights to interfere greatly with the escape of the
+heat accumulated during the day. Therefore, if atmospheric
+<!--File: cc262.png Page 238-->
+<a class="pagenum break" id="Page_238" title="pg 238"></a>
+moisture were formerly much more abundant
+than now, the temperature must have been much more
+uniform. The tendency toward climatic severity as time
+went on would be still further increased by the cooling
+which would result from the increased wind velocity discussed
+below; for cooling by convection increases with
+the velocity of the wind, as does cooling by conduction.</p>
+
+<p>Any persistent lowering of the general temperature of
+the air would affect not only its ability to hold water
+vapor, but would produce a lessening in the amount of
+atmospheric carbon dioxide, for the colder the ocean
+becomes the more carbon dioxide it can hold in solution.
+When the oceanic temperature falls, part of the atmospheric
+carbon dioxide is dissolved in the ocean. This
+minor constituent of the air is important because
+although it forms only 0.003 per cent of the earth's atmosphere,
+Abbot and Fowle's<sup><a href="#fn114" id="r114">[114]</a></sup> calculations indicate that
+it absorbs over 10 per cent of the heat radiated outward
+from the earth. Hence variations in the amount of carbon
+dioxide may have caused an appreciable variation in
+temperature and thus in other climatic conditions.
+Humphreys, as we have seen, has calculated that a
+doubling of the carbon dioxide in the air would directly
+raise the earth's temperature to the extent of 1.3&deg;C., and
+a halving would lower it a like amount. The indirect
+results of such an increase or decrease might be greater
+than the direct results, for the change in temperature
+due to variations in carbon dioxide would alter the
+capacity of the air to hold moisture.</p>
+
+<p>Two conditions would especially help in this respect;
+first, changes in nocturnal cooling, and second, changes
+in local convection. The presence of carbon dioxide diminishes
+nocturnal cooling because it absorbs the heat radiated
+<!--File: cc263.png Page 239-->
+<a class="pagenum break" id="Page_239" title="pg 239"></a>
+by the earth, and re-radiates part of it back again.
+Hence with increased carbon dioxide and with the
+consequent warmer nights there would be less nocturnal
+condensation of water vapor to form dew and frost.
+Local convection is influenced by carbon dioxide because
+this gas lessens the temperature gradient. In general, the
+less the gradient, that is, the less the contrast between
+the temperature at the surface and higher up, the less
+convection takes place. This is illustrated by the seasonal
+variation in convection. In summer, when the gradient is
+steepest, convection reaches its maximum. It will be recalled
+that when air rises it is cooled by expansion, and
+if it ascends far the moisture is soon condensed and
+precipitated. Indeed, local convection is considered by
+C. P. Day to be the chief agency which keeps the lower
+air from being continually saturated with moisture. The
+presence of carbon dioxide lessens convection because it
+increases the absorption of heat in the zone above the
+level in which water vapor is abundant, thus warming
+these higher layers. The lower air may not be warmed
+correspondingly by an increase in carbon dioxide if
+Abbot and Fowle are right in stating that near the earth's
+surface there is enough water vapor to absorb practically
+all the wave lengths which carbon dioxide is capable of
+absorbing. Hence carbon dioxide is chiefly effective at
+heights to which the low temperature prevents water
+vapor from ascending. Carbon dioxide is also effective
+in cold winters and in high latitudes when even the lower
+air is too cold to contain much water vapor. Moreover,
+carbon dioxide, by altering the amount of atmospheric
+water vapor, exerts an indirect as well as a direct effect
+upon temperature.</p>
+
+<p>Other effects of the increase in air pressure which we
+are here assuming during at least the early part of geological
+<!--File: cc264.png Page 240-->
+<a class="pagenum break" id="Page_240" title="pg 240"></a>
+times are corresponding changes in barometric
+contrasts, in the strength of winds, and in the mass of air
+carried by the winds along the earth's surface. The increase
+in the mass of the air would re&euml;nforce the greater
+velocity of the winds in their action as eroding and transporting
+agencies. Because of the greater weight of the
+air, the winds would be capable of picking up more dust
+and of carrying it farther and higher; while the increased
+atmospheric friction would keep it aloft a longer time.
+The significance of dust at high levels and its relation to
+solar radiation have already been discussed in connection
+with volcanoes. It will be recalled that on the average it
+lowers the surface temperature. At lower levels, since
+dust absorbs heat quickly and gives it out quickly, its
+presence raises the temperature of the air by day and
+lowers it by night. Hence an increase in dustiness tends
+toward greater extremes.</p>
+
+<p>From all these considerations it appears that if the
+atmosphere has actually evolved according to the supposition
+which is here tentatively entertained, the general
+tendency of the resultant climatic changes must have
+been partly toward long geological oscillations and partly
+toward a general though very slight increase in climatic
+severity and in the contrasts between the zones. This
+seems to agree with the geological record, although the
+fact that we are living in an age of relative climatic
+severity may lead us astray.</p>
+
+<p>The significant fact about the whole matter is that the
+three great types of terrestrial agencies, namely, those
+of the earth's interior, those of the oceans, and those of
+the air, all seem to have suffered changes which lead to
+slow variations of climate. Many reversals have doubtless
+taken place, and the geologic oscillations thus induced
+are presumably of much greater importance than
+<!--File: cc265.png Page 241-->
+<a class="pagenum break" id="Page_241" title="pg 241"></a>
+the progressive change, yet so far as we can tell the
+purely terrestrial changes throughout the hundreds of
+millions of years of geological time have tended toward
+complexity and toward increased contrasts from continent
+to ocean, from latitude to latitude, from season to
+season, and from day to night.</p>
+
+<p>Throughout geological history the slow and almost
+imperceptible differentiation of the earth's surface has
+been one of the most noteworthy of all changes. It has
+been opposed by the extraordinary conservatism of the
+universe which causes the average temperature today to
+be so like that of hundreds of millions of years ago that
+many types of life are almost identical. Nevertheless, the
+differentiation has gone on. Often, to be sure, it has presumably
+been completely masked by the disturbances of
+the solar atmosphere which appear to have been the
+cause of the sharper, shorter climatic pulsations. But
+regardless of cosmic conservatism and of solar impulses
+toward change, the slow differentiation of the earth's
+surface has apparently given to the world of today much
+of the geographical complexity which is so stimulating
+a factor in organic evolution. Such complexity&mdash;such
+diversity from place to place&mdash;appears to be largely
+accounted for by purely terrestrial causes. It may be
+regarded as the great terrestrial contribution to the
+climatic environment which guides the development of
+life.</p>
+<!--File: cc266.png Page 242-->
+<a class="pagenum break" id="Page_242" title="pg 242"></a>
+
+
+
+
+<h2 id="Chapt_XIV"><a href="#content">CHAPTER XIV</a></h2>
+
+<h3>THE EFFECT OF OTHER BODIES ON THE SUN</h3>
+
+
+<p>If solar activity is really an important factor in
+causing climatic changes, it behooves us to subject
+the sun to the same kind of inquiry to which we
+have subjected the earth. We have inquired into the nature
+of the changes through which the earth's crust, the
+oceans, and the atmosphere have influenced the climate
+of geological times. It has not been necessary, however,
+to study the origin of the earth, nor to trace its earlier
+stages. Our study of the geological record begins only
+when the earth had attained practically its present mass,
+essentially its present shape, and a climate so similar to
+that of today that life as we know it was possible. In
+other words, the earth had passed the stages of infancy,
+childhood, youth, and early maturity, and had reached
+full maturity. As it still seems to be indefinitely far from
+old age, we infer that during geological times its relative
+changes have been no greater than those which a man
+experiences between the ages of perhaps twenty-five and
+forty.</p>
+
+<p>Similar reasoning applies with equal or greater force
+to the sun. Because of its vast size it presumably passes
+through its stages of development much more slowly than
+the earth. In the first chapter of this book we saw that
+the earth's relative uniformity of climate for hundreds of
+millions of years seems to imply a similar uniformity in
+solar activity. This accords with a recent tendency among
+<!--File: cc267.png-->
+<a class="pagenum break" id="Page_243" title="pg 243"></a>
+astronomers who are more and more recognizing that the
+stars and the solar system possess an extraordinary degree
+of conservatism. Changes that once were supposed
+to take place in thousands of years are now thought to
+have required millions. Hence in this chapter we shall
+assume that throughout geological times the condition
+of the sun has been almost as at present. It may have
+been somewhat larger, or different in other ways, but it
+was essentially a hot, gaseous body such as we see today
+and it gave out essentially the same amount of energy.
+This assumption will affect the general validity of what
+follows only if it departs widely from the truth. With this
+assumption, then, let us inquire into the degree to which
+the sun's atmosphere has probably been disturbed
+throughout geological times.</p>
+
+<p>In <em>Earth and Sun</em>, as already explained, a detailed
+study has led to the conclusion that cyclonic storms are
+influenced by the electrical action of the sun. Such action
+appears to be most intense in sunspots, but apparently
+pertains also to other disturbed areas in the sun's
+atmosphere. A study of sunspots suggests that their
+true periodicity is almost if not exactly identical with
+that of the orbital revolution of Jupiter, 11.8 years. Other
+investigations show numerous remarkable coincidences
+between sunspots and the orbital revolution of the other
+planets, including especially Saturn and Mercury. This
+seems to indicate that there is some truth in the hypothesis
+that sunspots and other related disturbances of the
+solar atmosphere owe their periodicity to the varying
+effects of the planets as they approach and recede from
+the sun in their eccentric orbits and as they combine or
+oppose their effects according to their relative positions.
+This does not mean that the energy of the solar disturbances
+is supposed to come from the planets, but merely
+<!--File: cc268.png-->
+<a class="pagenum break" id="Page_244" title="pg 244"></a>
+that their variations act like the turning of a switch to
+determine when and how violently the internal forces of
+the sun shall throw the solar atmosphere into commotion.
+This hypothesis is by no means new, for in one form or
+another it has been advocated by Wolfer, Birkeland,
+E. W. Brown, Schuster, Arctowski, and others.</p>
+
+<p>The agency through which the planets influence the
+solar atmosphere is not yet clear. The suggested agencies
+are the direct pull of gravitation, the tidal effect of the
+planets, and an electro-magnetic effect. In <em>Earth and
+Sun</em> the conclusion is reached that the first two are out
+of the question, a conclusion in which E. W. Brown
+acquiesces. Unless some unknown cause is appealed to,
+this leaves an electro-magnetic hypothesis as the only one
+which has a reasonable foundation. Schuster inclines to
+this view. The conclusions set forth in <em>Earth and Sun</em> as
+to the electrical nature of the sun's influence on the earth
+point somewhat in the same direction. Hence in this
+chapter we shall inquire what would happen to the sun,
+and hence to the earth, on their journey through space,
+if the solar atmosphere is actually subject to disturbance
+by the electrical or other effects of other heavenly bodies.
+It need hardly be pointed out that we are here venturing
+into highly speculative ground, and that the verity or
+falsity of the conclusions reached in this chapter has
+nothing to do with the validity of the reasoning in previous
+chapters. Those chapters are based on the assumption
+that terrestrial causes of climatic changes are supplemented
+by solar disturbances which produce their
+effect partly through variations in temperature but also
+through variations in the intensity and paths of cyclonic
+storms. The present chapter seeks to shed some light on
+the possible causes and sequence of solar disturbances.</p>
+
+<p>Let us begin by scanning the available evidence as to
+<!--File: cc269.png-->
+<a class="pagenum break" id="Page_245" title="pg 245"></a>
+solar disturbances previous to the time when accurate
+sunspot records are available. Two rather slender bits of
+evidence point to cycles of solar activity lasting hundreds
+of years. One of these has already been discussed in
+Chapter VI, where the climatic stress of the fourteenth
+century was described. At that time sunspots are known
+to have been unusually numerous, and there were great
+climatic extremes. Lakes overflowed in Central Asia;
+storms, droughts, floods, and cold winters were unusually
+severe in Europe; the Caspian Sea rose with great
+rapidity; the trees of California grew with a vigor unknown
+for centuries; the most terrible of recorded famines
+occurred in England and India; the Eskimos were
+probably driven south by increasing snowiness in Greenland;
+and the Mayas of Yucatan appear to have made
+their last weak attempt at a revival of civilization under
+the stimulus of greater storminess and less constant
+rainfall.</p>
+
+<p>The second bit of evidence is found in recent exhaustive
+studies of periodicities by Turner<sup><a href="#fn115" id="r115">[115]</a></sup> and other
+astronomers. They have sought every possible natural
+occurrence for which a numerical record is available for
+a long period. The most valuable records appear to be
+those of tree growth, Nile floods, Chinese earthquakes,
+and sunspots. Turner reaches the conclusion that all four
+types of phenomena show the same periodicity, namely,
+cycles with an average length of about 260 to 280 years.
+He suggests that if this is true, the cycles in tree growth
+and in floods, both of which are climatic, are probably
+due to a non-terrestrial cause. The fact that the sunspots
+<!--File: cc270.png-->
+<a class="pagenum break" id="Page_246" title="pg 246"></a>
+show similar cycles suggests that the sun's variations
+are the cause.</p>
+
+<p>These two bits of evidence are far too slight to form
+the foundation of any theory as to changes in solar
+activity in the geological past. Nevertheless it may be
+helpful to set forth certain possibilities as a stimulus to
+further research. For example, it has been suggested that
+meteoric bodies may have fallen into the sun and caused
+it suddenly to flare up, as it were. This is not impossible,
+although it does not appear to have taken place since
+men became advanced enough to make careful observations.
+Moreover, the meteorites which now fall on the
+earth are extremely small, the average size being computed
+as no larger than a grain of wheat. The largest
+ever found on the earth's surface, at Bacubirito in
+Mexico, weighs only about fifty tons, while within the
+rocks the evidences of meteorites are extremely scanty
+and insignificant. If meteorites had fallen into the sun
+often enough and of sufficient size to cause glacial fluctuations
+and historic pulsations of climate, it seems highly
+probable that the earth would show much more evidence
+of having been similarly disturbed. And even if the sun
+should be bombarded by large meteors the result would
+probably not be sudden cold periods, which are the most
+notable phenomena of the earth's climatic history, but
+sudden warm periods followed by slow cooling. Nevertheless,
+the disturbance of the sun by collision with meteoric
+matter can by no means be excluded as a possible cause
+of climatic variations.</p>
+
+<p>Allied to the preceding hypothesis is Shapley's<sup><a href="#fn116" id="r116">[116]</a></sup> nebular
+hypothesis. At frequent intervals, averaging about
+<!--File: cc271.png-->
+<a class="pagenum break" id="Page_247" title="pg 247"></a>
+once a year during the last thirty years, astronomers have
+discovered what are known as nov&aelig;. These are stars
+which were previously faint or even invisible, but which
+flash suddenly into brilliancy. Often their light-giving
+power rises seven or eight magnitudes&mdash;a thousand-fold.
+In addition to the spectacular nov&aelig; there are numerous
+irregular variables whose brilliancy changes in every
+ratio from a few per cent up to several magnitudes. Most
+of them are located in the vicinity of nebul&aelig;, as is also
+the case with nov&aelig;. This, as well as other facts, makes it
+probable that all these stars are "friction variables," as
+Shapley calls them. Apparently as they pass through the
+nebul&aelig; they come in contact with its highly diffuse
+matter and thereby become bright much as the earth
+would become bright if its atmosphere were filled with
+millions of almost infinitesimally small meteorites. A star
+may also lose brilliancy if nebulous matter intervenes
+between it and the observer. If our sun has been subjected
+to any of these changes some sort of climatic effect
+must have been produced.</p>
+
+<p>In a personal communication Shapley amplifies the
+nebular climatic hypothesis as follows:</p>
+
+<blockquote>
+<p>Within 700 light years of the sun in many directions (Taurus,
+Cygnus, Ophiuchus, Scorpio) are great diffuse clouds of nebulosity,
+some bright, most of them dark. The probability that stars
+moving in the general region of such clouds will encounter this
+material is very high, for the clouds fill enormous volumes of
+space,&mdash;e.g., probably more than a hundred thousand cubic light
+years in the Orion region, and are presumably composed of rarefied
+gases or of dust particles. Probably throughout all our
+part of space such nebulosity exists (it is all around us, we are
+sure), but only in certain regions is it dense enough to affect
+conspicuously the stars involved in it. If a star moving at high
+velocity should collide with a dense part of such a nebulous
+<!--File: cc272.png-->
+<a class="pagenum break" id="Page_248" title="pg 248"></a>
+cloud, we should probably have a typical nova. If the relative
+velocity of nebulous material and star were low or moderate, or
+if the material were rare, we should not expect a conspicuous
+effect on the star's light.</p>
+
+<p>In the nebulous region of Orion, which is probably of unusually
+high density, there are about 100 known stars, varying
+between 20% and 80% of their total light&mdash;all of them irregularly&mdash;some
+slowly, some suddenly. Apparently they are
+"friction variables." Some of the variables suddenly lose 40%
+of their light as if blanketed by nebulous matter. In the Trifid
+Nebula there are variables like those of Orion, in Messier 8 also,
+and probably many of the 100 or so around the Rho Ophiuchi
+region belong to this kind.</p>
+
+<p>I believe that our sun could not have been a typical nova, at
+least not since the Archeozoic, that is for perhaps a billion years.
+I believe we have in geological climates final proof of this, because
+an increase in the amount of solar radiation by 1000 times
+as in the typical nova, would certainly punctuate emphatically
+the life cycle on the earth, even if the cause of the nova would
+not at the same time eliminate the smaller planets. But the sun
+may have been one of these miniature nov&aelig; or friction variables;
+and I believe it very probable that its wanderings through
+this part of space could not long leave its mean temperature
+unaffected to the amount of a few per cent.</p>
+
+<p>One reason we have not had this proposal insisted upon
+before is that the data back of it are mostly new&mdash;the Orion
+variables have been only recently discovered and studied, the
+distribution and content of the dark nebul&aelig; are hardly as yet
+generally known.</p>
+</blockquote>
+
+<p>This interesting hypothesis cannot be hastily dismissed.
+If the sun should pass through a nebula it seems
+inevitable that there would be at least slight climatic
+effects and perhaps catastrophic effects through the
+action of the gaseous matter not only on the sun but on
+the earth's own atmosphere. As an explanation of the
+<!--File: cc273.png-->
+<a class="pagenum break" id="Page_249" title="pg 249"></a>
+general climatic conditions of the past, however, Shapley
+points out that the hypothesis has the objection of being
+vague, and that nebulosity should not be regarded as
+more than "a possible factor." One of the chief difficulties
+seems to be the enormously wide distribution of as
+yet undiscovered nebulous matter which must be assumed
+if any large share of the earth's repeated climatic
+changes is to be ascribed to such matter. If such matter
+is actually abundant in space, it is hard to see how any
+but the nearest stars would be visible. Another objection
+is that there is no known nebulosity near at hand with
+which to connect the climatic vicissitudes of the last
+glacial period. Moreover, the known nebul&aelig; are so much
+less numerous than stars that the chances that the sun
+will encounter one of them are extremely slight. This,
+however, is not an objection, for Shapley points out that
+during geological times the sun can never have varied
+as much as do the nov&aelig;, or even as most of the friction
+variables. Thus the hypothesis stands as one that is worth
+investigating, but that cannot be finally rejected or accepted
+until it is made more definite and until more information
+is available.</p>
+
+<p>Another suggested cause of solar variations is the relatively
+sudden contraction of the sun such as that which
+sometimes occurs on the earth when continents are uplifted
+and mountains upheaved. It seems improbable that
+this could have occurred in a gaseous body like the sun.
+Lacking, as it does, any solid crust which resists a change
+of form, the sun probably shrinks steadily. Hence any
+climatic effects thus produced must be extremely gradual
+and must tend steadily in one direction for millions of
+years.</p>
+
+<p>Still another suggestion is that the tidal action of the
+stars and other bodies which may chance to approach
+<!--File: cc274.png-->
+<a class="pagenum break" id="Page_250" title="pg 250"></a>
+the sun's path may cause disturbances of the solar
+atmosphere. The vast kaleidoscope of space is never
+quiet. The sun, the stars, and all the other heavenly
+bodies are moving, often with enormous speed. Hence the
+effect of gravitation upon the sun must vary constantly
+and irregularly, as befits the geological requirements. In
+the case of the planets, however, the tidal effect does not
+seem competent to produce the movements of the solar
+atmosphere which appear to be concerned in the inception
+of sunspots. Moreover, there is only the most remote
+probability that a star and the sun will approach near
+enough to one another to produce a pronounced gravitational
+disturbance in the solar atmosphere. For instance,
+if it be assumed that changes in Jupiter's tidal effect on
+the sun are the main factor in regulating the present difference
+between sunspot maxima and sunspot minima,
+the chances that a star or some non-luminous body of
+similar mass will approach near enough to stimulate
+solar activity and thereby bring on glaciation are only
+one in twelve billion years, as will be explained below.
+This seems to make a gravitational hypothesis impossible.</p>
+
+<p>Another possible cause of solar disturbances is that
+the stars in their flight through space may exert an
+electrical influence which upsets the equilibrium of the
+solar atmosphere. At first thought this seems even more
+impossible than a gravitational effect. Electrostatic
+effects, however, differ greatly from those of tides. They
+vary as the diameter of a body instead of as its mass;
+their differentials also vary inversely as the square of
+the distance instead of as the cube. Electrostatic effects
+also increase as the fourth power of the temperature or
+at least would do so if they followed the law of black
+bodies; they are stimulated by the approach of one body
+<!--File: cc275.png-->
+<a class="pagenum break" id="Page_251" title="pg 251"></a>
+to another; and they are cumulative, for if ions arrive
+from space they must accumulate until the body to which
+they have come begins to discharge them. Hence, on the
+basis of assumptions such as those used in the preceding
+paragraph, the chances of an electrical disturbance of
+the solar atmosphere sufficient to cause glaciation on the
+earth may be as high as one in twenty or thirty million
+years. This seems to put an electrical hypothesis within
+the bounds of possibility. Further than that we cannot
+now go. There may be other hypotheses which fit the facts
+much better, but none seems yet to have been suggested.</p>
+
+<p>In the rest of this chapter the tidal and electrical
+hypotheses of stellar action on the sun will be taken up
+in detail. The tidal hypothesis is considered because in
+discussions of the effect of the planets it has hitherto
+held almost the entire field. The electrical hypothesis will
+be considered because it appears to be the best yet suggested,
+although it still seems doubtful whether electrical
+effects can be of appreciable importance over such vast
+distances as are inevitably involved. The discussion of
+both hypotheses will necessarily be somewhat technical,
+and will appeal to the astronomer more than to the layman.
+It does not form a necessary part of this book, for
+it has no bearing on our main thesis of the effect of the
+sun on the earth. It is given here because ultimately the
+question of changes in solar activity during geological
+times must be faced.</p>
+
+<p>In the astronomical portion of the following discussion
+we shall follow Jeans<sup><a href="#fn117" id="r117">[117]</a></sup> in his admirable attempt at a
+mathematical analysis of the motions of the universe.
+Jeans divides the heavenly bodies into five main types.
+(1) Spiral nebul&aelig;, which are thought by some astronomers
+<!--File: cc276.png-->
+<a class="pagenum break" id="Page_252" title="pg 252"></a>
+to be systems like our own in the making, and by
+others to be independent universes lying at vast distances
+beyond the limits of our Galactic universe, as it is called
+from the Galaxy or Milky Way. (2) Nebul&aelig; of a smaller
+type, called planetary. These lie within the Galactic portion
+of the universe and seem to be early stages of what
+may some day be stars or solar systems. (3) Binary or
+multiple stars, which are extraordinarily numerous. In
+some parts of the heavens they form 50 or even 60 per
+cent of the stars and in the galaxy as a whole they seem to
+form "fully one third." (4) Star clusters. These consist
+of about a hundred groups of stars in each of which the
+stars move together in the same direction with approximately
+the same velocity. These, like the spiral nebul&aelig;,
+are thought by some astronomers to lie outside the limits
+of the galaxy, but this is far from certain. (5) The solar
+system. According to Jeans this seems to be unique. It
+does not fit into the general mathematical theory by
+which he explains spiral nebul&aelig;, planetary nebul&aelig;, binary
+stars, and star clusters. It seems to demand a special
+explanation, such as is furnished by tidal disruption due
+to the passage of the sun close to another star.</p>
+
+<p>The part of Jeans' work which specially concerns us is
+his study of the probability that some other star will
+approach the sun closely enough to have an appreciable
+gravitative or electrical effect, and thus cause disturbances
+in the solar atmosphere. Of course both the star
+and the sun are moving, but to avoid circumlocution we
+shall speak of such mutual approaches simply as approaches
+of the sun. For our present purpose the most
+fundamental fact may be summed up in a quotation from
+Jeans in which he says that most stars "show evidence
+of having experienced considerable disturbance by other
+systems; there is no reason why our solar system should
+<!--File: cc277.png-->
+<a class="pagenum break" id="Page_253" title="pg 253"></a>
+be expected to have escaped the common fate." Jeans
+gives a careful calculation from which it is possible to
+derive some idea of the probability of any given degree of
+approach of the sun and some other star. Of course all
+such calculations must be based on certain assumptions.
+The assumptions made by Jeans are such as to make the
+probability of close approaches as great as possible. For
+example, he allows only 560 million years for the entire
+evolution of the sun, whereas some astronomers and
+geologists would put the figure ten or more times as
+high. Nevertheless, Jeans' assumptions at least show
+the order of magnitude which we may expect on the basis
+of reasonable astronomical conclusions.</p>
+
+<p>According to the planetary hypothesis of sunspots, the
+difference in the effect of Jupiter when it is nearest and
+farthest from the sun is the main factor in starting the
+sunspot cycle and hence the corresponding terrestrial
+cycle. The climatic difference between sunspot maxima
+and minima, as measured by temperature, apparently
+amounts to at least a twentieth and perhaps a tenth of
+the difference between the climate of the last glacial
+epoch and the present. We may suppose, then, that a body
+which introduced a gravitative or electrical factor twenty
+times as great as the difference in Jupiter's effect at its
+maximum and minimum distances from the sun would
+cause a glacial epoch if the effect lasted long enough. Of
+course the other planets combine their effects with that
+of Jupiter, but for the sake of simplicity we will leave
+the others out of account. The difference between Jupiter's
+maximum and minimum tidal effect on the sun
+amounts to 29 per cent of the planet's average effect.
+The corresponding difference, according to the electrical
+hypothesis, is about 19 per cent, for electrostatic action
+varies as the square of the distance instead of as the cube.
+<!--File: cc278.png-->
+<a class="pagenum break" id="Page_254" title="pg 254"></a>
+Let us assume that a body exerting four times Jupiter's
+present tidal effect and placed at the average distance of
+Jupiter from the sun would disturb the sun's atmosphere
+twenty times as much as the present difference between
+sunspot maxima and minima, and thus, perhaps, cause a
+glacial period on the earth.</p>
+
+<p>On the basis of this assumption our first problem is to
+estimate the frequency with which a star, visible or
+dark, is likely to approach near enough to the sun to
+produce a <em>tidal</em> effect four times that of Jupiter. The
+number of visible stars is known or at least well estimated.
+As to dark stars, which have grown cool, Arrhenius
+believed that they are a hundred times as numerous
+as bright stars; few astronomers believe that there are
+less than three or four times as many. Dr. Shapley of
+the Harvard Observatory states that a new investigation
+of the matter suggests that eight or ten is probably a
+maximum figure. Let us assume that nine is correct.
+The average visible star, so far as measured, has a mass
+about twice that of the sun, or about 2100 times that of
+Jupiter. The distances of the stars have been measured
+in hundreds of cases and thus we can estimate how many
+stars, both visible and invisible, are on an average contained
+in a given volume of space. On this basis Jeans
+estimates that there is only one chance in thirty billion
+years that a visible star will approach within 2.8 times
+the distance of Neptune from the sun, that is, within about
+eight billion miles. If we include the invisible stars the
+chances become one in three billion years. In order to
+produce four times the tidal effect of Jupiter, however,
+the average star would have to approach within about
+four billion miles of the sun, and the chances of that
+are only one in twelve billion years. The disturbing star
+<!--File: cc279.png-->
+<a class="pagenum break" id="Page_255" title="pg 255"></a>
+would be only 40 per cent farther from the sun than
+Neptune, and would almost pass within the solar system.</p>
+
+<p>Even though Jeans holds that the frequency of the
+mutual approach of the sun and a star was probably
+much greater in the distant past than at present, the
+figures just given lend little support to the tidal hypothesis.
+In fact, they apparently throw it out of court. It will
+be remembered that Jeans has made assumptions which
+give as high a frequency of stellar encounters as is consistent
+with the astronomical facts. We have assumed
+nine dark stars for every bright one, which may be a
+liberal estimate. Also, although we have assumed that a
+disturbance of the sun's atmosphere sufficient to cause
+a glacial period would arise from a tidal effect only
+twenty times as great as the difference in Jupiter's effect
+when nearest the sun and farthest away, in our computations
+this has actually been reduced to thirteen. With all
+these favorable assumptions the chances of a stellar approach
+of the sort here described are now only one in
+twelve billion years. Yet within a hundred million years,
+according to many estimates of geological time, and
+almost certainly within a billion, there have been at least
+half a dozen glaciations.</p>
+
+<p>Our use of Jeans' data interposes another and equally
+insuperable difficulty to any tidal hypothesis. Four billion
+miles is a very short distance in the eyes of an
+astronomer. At that distance a star twice the size of the
+sun would attract the outer planets more strongly than
+the sun itself, and might capture them. If a star should
+come within four billion miles of the sun, its effect in
+distorting the orbits of all the planets would be great.
+If this had happened often enough to cause all the glaciations
+known to geologists, the planetary orbits would
+be strongly elliptical instead of almost circular. The considerations
+<!--File: cc280.png-->
+<a class="pagenum break" id="Page_256" title="pg 256"></a>
+here advanced militate so strongly against
+the tidal hypothesis of solar disturbances that it seems
+scarcely worth while to consider it further.</p>
+
+<p>Let us turn now to the electrical hypothesis. Here the
+conditions are fundamentally different from those of the
+tidal hypothesis. In the first place the electrostatic effect
+of a body has nothing to do with its mass, but depends on
+the area of its surface; that is, it varies as the square of
+the radius. Second, the emission of electrons varies exponentially.
+If hot glowing stars follow the same law as
+black bodies at lower temperatures, the emission of
+electrons, like the emission of other kinds of energy,
+varies as the fourth power of the absolute temperature.
+In other words, suppose there are two black bodies, otherwise
+alike, but one with a temperature of 27&deg; C. or 300&deg;
+on the absolute scale, and the other with 600&deg; on the
+absolute scale. The temperature of one is twice as high
+as that of the other, but the electrostatic effect will be
+sixteen times as great.<sup><a href="#fn118" id="r118">[118]</a></sup> Third, the number of electrons
+<!--File: cc281.png-->
+<a class="pagenum break" id="Page_257" title="pg 257"></a>
+that reach a given body varies inversely as the square of
+the distance, instead of as the cube which is the case
+with tide-making forces.</p>
+
+<p>In order to use these three principles in calculating
+the effect of the stars we must know the diameters, distances,
+temperature, and number of the stars. The distances
+and number may safely be taken as given by Jeans
+in the calculations already cited. As to the diameters, the
+measurements of the stars thus far made indicate that
+the average mass is about twice that of the sun. The
+average density, as deduced by Shapley<sup><a href="#fn119" id="r119">[119]</a></sup> from the movements
+of double stars, is about one-eighth the solar
+density. This would give an average diameter about two
+and a half times that of the sun. For the dark stars, we
+shall assume for convenience that they are ten times as
+numerous as the bright ones. We shall also assume that
+their diameter is half that of the sun, for being cool they
+must be relatively dense, and that their temperature is
+the same as that which we shall assume for Jupiter.</p>
+
+<p>As to Jupiter we shall continue our former assumption
+that a body with four times the effectiveness of that
+planet, which here means with twice as great a radius,
+would disturb the sun enough to cause glaciation. It
+would produce about twenty times the electrostatic effect
+<!--File: cc282.png-->
+<a class="pagenum break" id="Page_258" title="pg 258"></a>
+which now appears to be associated with the difference in
+Jupiter's effect at maximum and minimum. The temperature
+of Jupiter must also be taken into account. The
+planet is supposed to be hot because its density is low,
+being only about 1.25 that of water. Nevertheless, it is
+probably not luminous, for as Moulton<sup><a href="#fn120" id="r120">[120]</a></sup> puts it, shadows
+upon it are black and its moons show no sign of illumination
+except from the sun. Hence a temperature of about
+600&deg;C., or approximately 900&deg; on the absolute scale,
+seems to be the highest that can reasonably be assigned
+to the cold outer layer whence electrons are emitted. As
+to the temperature of the sun, we shall adopt the common
+estimate of about 6300&deg;C. on the absolute scale. The
+other stars will be taken as averaging the same, although
+of course they vary greatly.</p>
+
+<p>When Jeans' method of calculating the probability of
+a mutual approach of the sun and a star is applied to the
+assumptions given above, the results are as shown in
+Table 5. On that basis the dark stars seem to be of
+negligible importance so far as the electrical hypothesis
+is concerned. Even though they may be ten times as
+numerous as the bright ones there appears to be only
+one chance in 130 billion years that one of them will approach
+the sun closely enough to cause the assumed disturbance
+of the solar atmosphere. On the other hand, if
+all the visible stars were the size of the sun, and as hot
+as that body, their electrical effect would be fourfold
+that of our assumed dark star because of their size, and
+2401 times as great because of their temperature, or approximately
+10,000 times as great. Under such conditions
+the theoretical chance of an approach that would cause
+glaciation is one in 130 million years. If the average
+visible star is somewhat cooler than the sun and has a
+<!--File: cc283.png-->
+<a class="pagenum break" id="Page_259" title="pg 259"></a>
+radius about two and one-half times as great, as appears
+to be the fact, the chances rise to one in thirty-eight million
+years. A slight and wholly reasonable change in our
+assumptions would reduce this last figure to only five or
+ten million. For instance, the earth's mean temperature
+during the glacial period has been assumed as 10&deg;C.
+lower than now, but the difference may have been only 6&deg;.
+Again, the temperature of the outer atmosphere of Jupiter
+where the electrons are shot out may be only 500&deg; or
+700&deg; absolute, instead of 900&deg;. Or the diameter of the
+average star may be five or ten times that of the sun,
+instead of only two and one-half times as great. All this,
+however, may for the present be disregarded. The essential
+point is that even when the assumptions err on the
+side of conservatism, the results are of an order of magnitude
+which puts the electrical hypothesis within the
+bounds of possibility, whereas similar assumptions put
+the tidal hypothesis, with its single approach in twelve
+billion years, far beyond those limits.</p>
+
+<p>The figures for Betelgeuse in Table 5 are interesting.
+At a meeting of the American Association for the Advancement
+of Science in December, 1920, Michelson
+reported that by measurements of the interference of
+light coming from the two sides of that bright star in
+Orion, the observers at Mount Wilson had confirmed the
+recent estimates of three other authorities that the star's
+diameter is about 218 million miles, or 250 times that of
+the sun. If other stars so much surpass the estimates of
+only a decade or two ago, the average diameter of all the
+visible stars must be many times that of the sun. The low
+figure for Betelgeuse in section D of the table means that
+if all the stars were as large as Betelgeuse, several might
+often be near enough to cause profound disturbances of
+the solar atmosphere. Nevertheless, because of the low
+<!--File: cc284.png-->
+<a class="pagenum break" id="Page_260" title="pg 260"></a>
+temperature of the giant red stars of the Betelgeuse type,
+the distance at which one of them would produce a given
+electrical effect is only about five times the distance at
+which our assumed average star would produce the same
+effect. This, to be sure, is on the assumption that the
+<!--File: cc285.png-->
+<a class="pagenum break" id="Page_261" title="pg 261"></a>
+radiation of energy from incandescent bodies varies
+according to temperature in the same ratio as the radiation
+from black bodies. Even if this assumption departs
+somewhat from the truth, it still seems almost certain
+that the lower temperature of the red compared with the
+high temperature of the white stars must to a considerable
+degree reduce the difference in electrical effect which
+would otherwise arise from their size.</p>
+
+<table id="table_5" class="borders">
+<tr><th colspan="5"><a href="#tables">TABLE 5</a></th></tr>
+<tr><th colspan="5">THEORETICAL PROBABILITY OF STELLAR APPROACHES</th>
+</tr>
+<tr>
+<td></td>
+<th><em>1<br />Dark Stars</em></th>
+<th><em>2<br />Sun</em></th>
+<th><em>3<br />Average Star</em></th>
+<th><em>4<br />Betelgeuse</em></th>
+</tr>
+<tr>
+<td>A. Approximate radius in miles.</td>
+<td>430,000</td>
+<td>860,000</td>
+<td>2,150,000</td>
+<td>218,000,000</td>
+</tr>
+<tr>
+<td>B. Assumed temperature above absolute zero.</td>
+<td>900&deg; C.</td>
+<td>6300&deg; C.</td>
+<td>5400&deg; C.</td>
+<td>3150&deg; C.</td>
+</tr>
+<tr>
+<td>C. Approximate theoretical distance
+at which star would cause solar disturbance great enough
+to cause glaciation (billions<sup><a href="#fn121" id="r121">[121]</a></sup> of miles).</td>
+<td>1.2</td>
+<td>120</td>
+<td>220</td>
+<td>3200 </td>
+</tr>
+<tr>
+<td>D. Average interval between approaches close enough to cause glaciation if all stars were of given type. Years.</td>
+<td>130,000,000,000<sup><a href="#fn122" id="r122">[122]</a></sup></td>
+<td>130,000,000</td>
+<td>38,000,000</td>
+<td>700,000</td>
+</tr>
+</table>
+
+
+<p>Thus far in our attempt to estimate the distance at
+which a star might disturb the sun enough to cause glaciation
+on the earth, we have considered only the star's
+size and temperature. No account has been taken of the
+degree to which its atmosphere is disturbed. Yet in the
+case of the sun this seems to be one of the most important
+factors. The magnetic field of sunspots is sometimes 50
+or 100 times as strong as that of the sun in general. The
+strength of the magnetic field appears to depend on the
+strength of the electrical currents in the solar atmosphere.
+But the intensity of the sunspots and, by inference,
+of the electrical currents, may depend on the electrical
+action of Jupiter and the other planets. If we apply a
+similar line of reasoning to the stars, we are at once led
+to question whether the electrical activity of double stars
+may not be enormously greater than that of isolated
+stars like the sun.</p>
+
+<p>If this line of reasoning is correct, the atmosphere of
+every double star must be in a state of commotion vastly
+greater than that of the sun's atmosphere even when it
+is most disturbed. For example, suppose the sun were
+accompanied by a companion of equal size at a distance
+of one million miles, which would make it much like many
+known double stars. Suppose also that in accordance with
+the general laws of physics the electrical effect of the
+two suns upon one another is proportional to the fourth
+<!--File: cc286.png-->
+<a class="pagenum break" id="Page_262" title="pg 262"></a>
+power of the temperature, the square of the radius, and
+the inverse square of the distance. Then the effect of each
+sun upon the other would be sixty billion (6 &times; 10<sup>10</sup>) times
+as great as the present electrical effect of Jupiter upon
+the sun. Just what this would mean as to the net effect
+of a pair of such suns upon the electrical potential of
+other bodies at a distance we can only conjecture. The
+outstanding fact is that the electrical conditions of a
+double star must be radically different and vastly more
+intense than those of a single star like the sun.</p>
+
+<p>This conclusion carries weighty consequences. At present
+twenty or more stars are known to be located within
+about 100 trillion miles of the sun (five parsecs, as the
+astronomers say), or 16.5 light years. According to the
+assumptions employed in Table 5 an average single star
+would influence the sun enough to cause glaciation if it
+came within approximately 200 billion miles. If the star
+were double, however, it might have an electrical capacity
+enormously greater than that of the sun. Then it would
+be able to cause glaciation at a correspondingly great
+distance. Today Alpha Centauri, the nearest known star
+about twenty-five trillion miles, or 4.3 light years from
+the sun, and Sirius, the brightest star in the heavens, is
+about fifty trillion miles away, or 8.5 light years. If these
+stars were single and had a diameter three times that of
+the sun, and if they were of the same temperature as has
+been assumed for Betelgeuse, which is about fifty times as
+far away as Alpha Centauri, the relative effects of the
+three stars upon the sun would be, approximately, Betelgeuse
+700, Alpha Centauri 250, Sirius 1. But Alpha Centauri
+is triple and Sirius double, and both are much hotter
+than Betelgeuse. Hence Alpha Centauri and even Sirius
+may be far more effective than Betelgeuse.</p>
+
+<p>The two main components of Alpha Centauri are separated
+<!--File: cc287.png-->
+<a class="pagenum break" id="Page_263" title="pg 263"></a>
+by an average distance of about 2,200,000,000 miles,
+or somewhat less than that of Neptune from the sun. A
+third and far fainter star, one of the faintest yet measured,
+revolves around them at a great distance. In mass
+and brightness the two main components are about like
+the sun, and we will assume that the same is true of their
+radius. Then, according to the assumptions made above,
+their effect in disturbing one another electrically would
+be about 10,000 times the total effect of Jupiter upon the
+sun, or 2500 times the effect that we have assumed to be
+necessary to produce a glacial period. We have already
+seen in Table 5 that, according to our assumptions, a
+single star like the sun would have to approach within
+120 billion miles of the solar system, or within 2 per cent
+of a light year, in order to cause glaciation. By a similar
+process of reasoning it appears that if the mutual electrical
+excitation of the two main parts of Alpha Centauri,
+regardless of the third part, is proportional to the apparent
+excitation of the sun by Jupiter, Alpha Centauri
+would be 5000 times as effective as the sun. In other
+words, if it came within 8,500,000,000,000 miles of the sun,
+or 1.4 light years, it would so change the electrical conditions
+as to produce a glacial epoch. In that case Alpha
+Centauri is now so near that it introduces a disturbing
+effect equal to about one-sixth of the effect needed to
+cause glaciation on the earth. Sirius and perhaps others
+of the nearer and brighter or larger stars may also create
+appreciable disturbances in the electrical condition of the
+sun's atmosphere, and may have done so to a much
+greater degree in the past, or be destined to do so in the
+future. Thus an electrical hypothesis of solar disturbances
+seems to indicate that the position of the sun in
+respect to other stars may be a factor of great importance
+in determining the earth's climate.</p>
+<!--File: cc288.png-->
+<a class="pagenum break" id="Page_264" title="pg 264"></a>
+
+
+
+
+<h2 id="Chapt_XV"><a href="#content">CHAPTER XV</a></h2>
+
+<h3>THE SUN'S JOURNEY THROUGH SPACE</h3>
+
+
+<p>Having gained some idea of the nature of the
+electrical hypothesis of solar disturbances and
+of the possible effect of other bodies upon the
+sun's atmosphere, let us now compare the astronomical
+data with those of geology. Let us take up five chief
+points for which the geologist demands an explanation,
+and which any hypothesis must meet if it is to be permanently
+accepted. These are (1) the irregular intervals
+at which glacial periods occur; (2) the division of glacial
+periods into epochs separated sometimes by hundreds
+of thousands of years; (3) the length of glacial periods
+and epochs; (4) the occurrence of glacial stages and historic
+pulsations in the form of small climatic waves
+superposed upon the larger waves of glacial epochs; (5)
+the occurrence of climatic conditions much milder than
+those of today, not only in the middle portion of the great
+geological eras, but even in some of the recent inter-glacial
+epochs.</p>
+
+<p>1. The irregular duration of the interval from one
+glacial epoch to another corresponds with the irregular
+distribution of the stars. If glaciation is indirectly due
+to stellar influences, the epochs might fall close together,
+or might be far apart. If the average interval were ten
+million years, one interval might be thirty million or
+more and the next only one or two hundred thousand.
+<!--File: cc289.png-->
+<a class="pagenum break" id="Page_265" title="pg 265"></a>
+According to Schuchert, the known periods of glacial or
+semi-glacial climate have been approximately as follows:</p>
+
+<h4>LIST OF GLACIAL PERIODS</h4>
+<ol>
+<li>Archeozoic.<br />
+  (&frac14; of geological time or perhaps much more)
+  <ul class="none">
+     <li>No known glacial periods.</li>
+  </ul>
+</li>
+
+<li>Proterozoic.<br />
+  (&frac14; of geological time)
+  <ul class="none">
+   <li>a. Oldest known glacial period near base of Proterozoic in
+       Canada. Evidence widely distributed.</li>
+   <li>b. Indian glacial period; time unknown.</li>
+   <li>c. African glacial period; time unknown.</li>
+   <li>d. Glaciation near end of Proterozoic in Australia, Norway,
+       and China.</li>
+    </ul>
+</li>
+
+<li>Paleozoic.<br />
+  (&frac14; of geological time)
+  <ul class="none">
+    <li>a. Late Ordovician(?). Local in Arctic Norway.</li>
+    <li>b. Silurian. Local in Alaska.</li>
+    <li>c. Early Devonian. Local in South Africa.</li>
+    <li>d. Early Permian. World-wide and very severe.</li>
+  </ul>
+</li>
+
+<li>Mesozoic and Cenozoic.<br />
+  (&frac14; of geological time)<br />
+  <ul class="none">
+     <li>a-b. None definitely determined during Mesozoic, although there appears to have been periods of cooling
+     (a) in the late Triassic, and (b) in the late Cretacic, with at least local glaciation in early Eocene.</li>
+     <li>c. Severe glacial period during Pleistocene.</li>
+  </ul>
+</li>
+</ol>
+
+<p>This table suggests an interesting inquiry. During the
+last few decades there has been great interest in ancient
+glaciation and geologists have carefully examined rocks
+of all ages for signs of glacial deposits. In spite of the
+large parts of the earth which are covered with deposits
+belonging to the Mesozoic and Cenozoic, which form the
+<!--File: cc290.png-->
+<a class="pagenum break" id="Page_266" title="pg 266"></a>
+last quarter of geological time, the only signs of actual
+glaciation are those of the great Pleistocene period and a
+few local occurrences at the end of the Mesozoic or beginning
+of the Cenozoic. Late in the Triassic and early
+in the Jurassic, the climate appears to have been rigorous,
+although no tillites have been found to demonstrate
+glaciation. In the preceding quarter, that is, the Paleozoic,
+the Permian glaciation was more severe than that
+of the Pleistocene, and the Devonian than that of the
+Eocene, while the Ordovician evidences of low temperature
+are stronger than those at the end of the Triassic.
+In view of the fact that rocks of Paleozoic age cover
+much smaller areas than do those of later age, the three
+Paleozoic glaciations seem to indicate a relative frequency
+of glaciation. Going back to the Proterozoic, it
+is astonishing to find that evidence of two highly developed
+glacial periods, and possibly four, has been discovered.
+Since the Indian and the African glaciations of
+Proterozoic times are as yet undated, we cannot be sure
+that they are not of the same date as the others. Nevertheless,
+even two is a surprising number, for not only
+are most Proterozoic rocks so metamorphosed that possible
+evidences of glacial origin are destroyed, but rocks
+of that age occupy far smaller areas than either those
+of Paleozoic or, still more, Mesozoic and Cenozoic age.
+Thus the record of the last three-quarters of geological
+time suggests that if rocks of all ages were as abundant
+and as easily studied as those of the later periods, the
+frequency of glacial periods would be found to increase
+as one goes backward toward the beginnings of the
+earth's history. This is interesting, for Jeans holds that
+the chances that the stars would approach one another
+were probably greater in the past than at present. This
+conclusion is based on the assumption that our universe
+<!--File: cc291.png-->
+<a class="pagenum break" id="Page_267" title="pg 267"></a>
+is like the spiral nebul&aelig; in which the orbits of the various
+members are nearly circular during the younger stages.
+Jeans considers it certain that in such cases the orbits
+will gradually become larger and more elliptical because
+of the attraction of one body for another. Thus as time
+goes on the stars will be more widely distributed and
+the chances of approach will diminish. If this is correct,
+the agreement between astronomical theory and geological
+conclusions suggests that the two are at least not in
+opposition.</p>
+
+<p>The first quarter of geological time as well as the last
+three must be considered in this connection. During the
+Archeozoic, no evidence of glaciation has yet been discovered.
+This suggests that the geological facts disprove
+the astronomical theory. But our knowledge of early
+geological times is extremely limited, so limited that
+lack of evidence of glaciation in the Archeozoic may have
+no significance. Archeozoic rocks have been studied
+minutely over a very small percentage of the earth's land
+surface. Moreover, they are highly metamorphosed so
+that, even if glacial tills existed, it would be hard to
+recognize them. Third, according to both the nebular and
+the planetesimal hypotheses, it seems possible that
+during the earliest stages of geological history the
+earth's interior was somewhat warmer than now, and the
+surface may have been warmed more than at present by
+conduction, by lava flows, and by the fall of meteorites.
+If the earth during the Archeozoic period emitted enough
+heat to raise its surface temperature a few degrees, the
+heat would not prevent the development of low forms of
+life but might effectively prevent all glaciation. This
+does not mean that it would prevent changes of climate,
+but merely changes so extreme that their record would
+be preserved by means of ice. It will be most interesting
+<!--File: cc292.png-->
+<a class="pagenum break" id="Page_268" title="pg 268"></a>
+to see whether future investigations in geology and
+astronomy indicate either a semi-uniform distribution of
+glacial periods throughout the past, or a more or less
+regular decrease in frequency from early times down to
+the present.</p>
+
+<p>2. The Pleistocene glacial period was divided into at
+least four epochs, while in the Permian at least one
+inter-glacial epoch seems certain, and in some places the
+alternation between glacial and non-glacial beds suggests
+no less than nine. In the other glaciations the evidence is
+not yet clear. The question of periodicity is so important
+that it overthrows most glacial hypotheses. Indeed, had
+their authors known the facts as established in recent
+years, most of the hypotheses would never have been
+advanced. The carbon dioxide hypothesis is the only one
+which was framed with geologically rapid climatic alternations
+in mind. It certainly explains the facts of periodicity
+better than does any of its predecessors, but even
+so it does not account for the intimate way in which
+variations of all degrees from those of the weather up to
+glacial epochs seem to grade into one another.</p>
+
+<p>According to our stellar hypothesis, occasional groups
+of glacial epochs would be expected to occur close together
+and to form long glacial periods. This is because
+many of the stars belong to groups or clusters in which
+the stars move in parallel paths. A good example is the
+cluster in the Hyades, where Boss has studied thirty-nine
+stars with special care.<sup><a href="#fn123" id="r123">[123]</a></sup> The stars are grouped about a
+center about 130 light years from the sun. The stars
+themselves are scattered over an area about thirty
+light years in diameter. They average about the same
+distance apart as do those near the sun, but toward the
+<!--File: cc293.png-->
+<a class="pagenum break" id="Page_269" title="pg 269"></a>
+center of the group they are somewhat closer together.
+The whole thirty-nine sweep forward in essentially
+parallel paths. Boss estimates that 800,000 years ago
+the cluster was only half as far from the sun as at present,
+but probably that was as near as it has been during
+recent geological times. All of the thirty-nine stars of this
+cluster, as Moulton<sup><a href="#fn124" id="r124">[124]</a></sup> puts it, "are much greater in light-giving
+power than the sun. The luminosities of even the
+five smallest are from five to ten times that of the sun,
+while the largest are one hundred times greater in light-giving
+power than our own luminary. Their masses are
+probably much greater than that of the sun." If the sun
+were to pass through such a cluster, first one star and
+then another might come so near as to cause a profound
+disturbance in the sun's atmosphere.</p>
+
+<p>3. Another important point upon which a glacial hypothesis
+may come to grief is the length of the periods
+or rather of the epochs which compose the periods.
+During the last or Pleistocene glacial period the evidence
+in America and Europe indicates that the inter-glacial
+epochs varied in length and that the later ones were
+shorter than the earlier. Chamberlin and Salisbury, from
+a comparison of various authorities, estimate that the
+intervals from one glacial epoch to another form a declining
+series, which may be roughly expressed as follows:
+16-8-4-2-1, where unity is the interval from the
+climax of the late Wisconsin, or last glacial epoch, to the
+present. Most authorities estimate the culmination of the
+late Wisconsin glaciation as twenty or thirty thousand
+years ago. Penck estimates the length of the last inter-glacial
+period as 60,000 years and the preceding one as
+240,000.<sup><a href="#fn125" id="r125">[125]</a></sup> R. T. Chamberlin, as already stated, finds that
+<!--File: cc294.png-->
+<a class="pagenum break" id="Page_270" title="pg 270"></a>
+the consensus of opinion is that inter-glacial epochs have
+averaged five times as long as glacial epochs. The actual
+duration of the various glaciations probably did not vary
+in so great a ratio as did the intervals from one glaciation
+to another. The main point, however, is the irregularity
+of the various periods.</p>
+
+<p>The relation of the stellar electrical hypothesis to the
+length of glacial epochs may be estimated from column
+C, in Table 5. There we see that the distances at which
+a star might possibly disturb the sun enough to cause
+glaciation range all the way from 120 billion miles in
+the case of a small star like the sun, to 3200 billion in
+the case of Betelgeuse, while for double stars the figure
+may rise a hundred times higher. From this we can calculate
+how long it would take a star to pass from a point
+where its influence would first amount to a quarter of the
+assumed maximum to a similar point on the other side of
+the sun. In making these calculations we will assume that
+the relative rate at which the star and the sun approach
+each other is about twenty-two miles per second, or 700
+million miles per year, which is the average rate of
+motion of all the known stars. According to the distances
+in Table 5 this gives a range from about 500 years up to
+about 10,000, which might rise to a million in the case of
+double stars. Of course the time might be relatively short
+if the sun and a rapidly moving star were approaching
+one another almost directly, or extremely long if the sun
+and the star were moving in almost the same direction
+and at somewhat similar rates,&mdash;a condition more
+common than the other. Here, as in so many other cases,
+the essential point is that the figures which we thus obtain
+seem to be of the right order of magnitude.</p>
+
+<p>4. Post-glacial climatic stages are so well known that
+in Europe they have definite names. Their sequence has
+<!--File: cc295.png-->
+<a class="pagenum break" id="Page_271" title="pg 271"></a>
+already been discussed in Chapter XII. Fossils found in
+the peat bogs of Denmark and Scandinavia, for example,
+prove that since the final disappearance of the continental
+ice cap at the close of the Wisconsin there has
+been at least one period when the climate of Europe was
+distinctly milder than now. Directly overlying the sheets
+of glacial drift laid down by the ice there is a flora corresponding
+to that of the present tundras. Next come remains
+of a forest vegetation dominated by birches and
+poplars, showing that the climate was growing a little
+warmer. Third, there follow evidences of a still more
+favorable climate in the form of a forest dominated by
+pines; fourth, one where oak predominates; and fifth, a
+flora similar to that of the Black Forest of Germany,
+indicating that in Scandinavia the temperature was then
+decidedly higher than today. This fifth flora has retreated
+southward once more, having been driven back to its
+present latitude by a slight recurrence of a cool stormy
+climate.<sup><a href="#fn126" id="r126">[126]</a></sup> In central Asia evidence of post-glacial stages
+is found not only in five distinct moraines but in a corresponding
+series of elevated strands surrounding salt
+lakes and of river terraces in non-glaciated arid regions.<sup><a href="#fn127" id="r127">[127]</a></sup></p>
+
+<p>In historic as well as prehistoric times, as we have
+already seen, there have been climatic fluctuations. For
+instance, the twelfth or thirteenth century B. C. appears
+to have been almost as mild as now, as does the seventh
+century B. C. On the other hand about 1000 B. C., at the
+time of Christ, and in the fourteenth century there were
+times of relative severity. Thus it appears that both on
+<!--File: cc296.png-->
+<a class="pagenum break" id="Page_272" title="pg 272"></a>
+a large and on a small scale pulsations of climate are the
+rule. Any hypothesis of climatic changes must satisfy
+the periods of these pulsations. These conditions furnish
+a problem which makes difficulty for almost all hypotheses
+of climatic change. According to the present hypothesis,
+earth movements such as are discussed in Chapter
+XII may co&ouml;perate with two astronomical factors. One is
+the constant change in the positions of the stars, a change
+which we have already called kaleidoscopic, and the other
+is the fact that a large proportion of the stars are double
+or multiple. When one star in a group approaches the
+sun closely enough to cause a great solar disturbance,
+numerous others may approach or recede and have a
+minor effect. Thus, whenever the sun is near groups of
+stars we should expect that the earth would show many
+minor climatic pulsations and stages which might or
+might not be connected with glaciation. The historic
+pulsations shown in the curve of tree growth in California,
+Fig. 4, are the sort of changes that would be
+expected if movements of the stars have an effect on the
+solar atmosphere.</p>
+
+<p>Not only are fully a third of all the visible stars double,
+as we have already seen, but at least a tenth of these are
+known to be triple or multiple. In many of the double
+stars the two bodies are close together and revolve so
+rapidly that whatever periodicity they might create in
+the sun's atmosphere would be very short. In the triplets,
+however, the third star is ordinarily at least ten times
+as far from the other two as they are from each other,
+and its period of rotation sometimes runs into hundreds
+or thousands of years. An actual multiple star in the
+constellation Polaris will serve as an example. The main
+star is believed by Jeans to consist of two parts which
+are almost in contact and whirl around each other with
+<!--File: cc297.png-->
+<a class="pagenum break" id="Page_273" title="pg 273"></a>
+extraordinary speed in four days. If this is true they
+must keep each other's atmospheres in a state of intense
+commotion. Much farther away a third star revolves
+around this pair in twelve years. At a much greater distance
+a fourth star revolves around the common center
+of gravity of itself and the other three in a period which
+may be 20,000 years. Still more complicated cases probably
+exist. Suppose such a system were to traverse a
+path where it would exert a perceptible influence on the
+sun for thirty or forty thousand years. The varying
+movements of its members would produce an intricate
+series of cycles which might show all sorts of major and
+minor variations in length and intensity. Thus the varied
+and irregular stages of glaciation and the pulsations of
+historic times might be accounted for on the hypothesis
+of the proximity of the sun to a multiple star, as well as
+on that of the less pronounced approach and recession
+of a number of stars. In addition to all this, an almost
+infinitely complex series of climatic changes of long and
+short duration might arise if the sun passed through a
+nebula.</p>
+
+<p>5. We have seen in Chapter VIII that the contrast
+between the somewhat severe climate of the present and
+the generally mild climate of the past is one of the great
+geological problems. The glacial period is not a thing
+of the distant past. Geologists generally recognize that
+it is still with us. Greenland and Antarctica are both
+shrouded in ice sheets in latitudes where fossil floras
+prove that at other periods the climate was as mild as
+in England or even New Zealand. The present glaciated
+regions, be it noted, are on the polar borders of the
+world's two most stormy oceanic areas, just where ice
+would be expected to last longest according to the solar
+cyclonic hypothesis. In contrast with the semi-glacial
+<!--File: cc298.png-->
+<a class="pagenum break" id="Page_274" title="pg 274"></a>
+conditions of the present, the last inter-glacial epoch was
+so mild that not only men but elephants and hippopotamuses
+flourished in central Europe, while at earlier times
+in the middle of long eras, such as the Paleozoic and
+Mesozoic, corals, cycads, and tree ferns flourished within
+the Arctic circle.</p>
+
+<p>If the electro-stellar hypothesis of solar disturbances
+proves well founded, it may explain these peculiarities.
+Periods of mild climate would represent a return of the
+sun and the earth to their normal conditions of quiet. At
+such times the atmosphere of the sun is assumed to be
+little disturbed by sunspots, facul&aelig;, prominences, and
+other allied evidences of movements; and the rice-grain
+structure is perhaps the most prominent of the solar
+markings. The earth at such times is supposed to be
+correspondingly free from cyclonic storms. Its winds are
+then largely of the purely planetary type, such as trade
+winds and westerlies. Its rainfall also is largely planetary
+rather than cyclonic. It falls in places such as the
+heat equator where the air rises under the influence of
+heat, or on the windward slopes of mountains, or in regions
+where warm winds blow from the ocean over cold
+lands.</p>
+
+<p>According to the electro-stellar hypothesis, the conditions
+which prevailed during hundreds of millions of
+years of mild climate mean merely that the solar system
+was then in parts of the heavens where stars&mdash;especially
+double stars&mdash;were rare or small, and electrical disturbances
+correspondingly weak. Today, on the other hand,
+the sun is fairly near a number of stars, many of which
+are large doubles. Hence it is supposed to be disturbed,
+although not so much as at the height of the last glacial
+epoch.</p>
+
+<p>After the preceding parts of this book had been
+<!--File: cc299.png-->
+<a class="pagenum break" id="Page_275" title="pg 275"></a>
+written, the assistance of Dr. Schlesinger made it possible
+to test the electro-stellar hypothesis by comparing
+actual astronomical dates with the dates of climatic or
+solar phenomena. In order to make this possible, Dr.
+Schlesinger and his assistants have prepared Table 6,
+giving the position, magnitude, and motions of the thirty-eight
+nearest stars, and especially the date at which each
+was nearest the sun. In column 10 where the dates are
+given, a minus sign indicates the past and a plus sign the
+future. Dr. Shapley has kindly added column 12, giving
+the absolute magnitudes of the stars, that of the sun
+being 4.8, and column 13, showing their luminosity or
+absolute radiation, that of the sun being unity. Finally,
+column 14 shows the effective radiation received by the
+sun from each star when the star is at a minimum distance.
+Unity in this case is the effect of a star like the
+sun at a distance of one light year.</p>
+
+<p>It is well known that radiation of all kinds, including
+light, heat, and electrical emissions, varies in direct proportion
+to the exposed surface, that is, as the square
+of the radius of a sphere, and inversely as the square of
+the distance. From black bodies, as we have seen, the
+total radiation varies as the fourth power of the absolute
+temperature. It is not certain that either light or
+electrical emissions from incandescent bodies vary in
+quite this same proportion, nor is it yet certain whether
+luminous and electrical emissions vary exactly together.
+Nevertheless they are closely related. Since the light
+coming from each star is accurately measured, while no
+information is available as to electrical emissions, we
+have followed Dr. Shapley's suggestion and used the
+luminosity of the stars as the best available measure of
+total radiation. This is presumably an approximate
+measure of electrical activity, provided some allowance
+be made for disturbances by outside bodies such as companion
+stars. Hence the inclusion of column 14.</p>
+<!--File: cc300.png-->
+<a class="pagenum break" id="Page_276" title="pg 276"></a>
+
+
+<table id="table_6" class="borders1">
+<tr><th colspan="15"><a href="#tables">TABLE 6</a></th></tr>
+
+<tr><th colspan="15">THIRTY-EIGHT STARS HAVING LARGEST KNOWN PARALLAXES</th></tr>
+<tr>
+<th></th>
+<th>(<em>1</em>) <br /><em>Right Ascension &alpha; 1900</em></th>
+<th>(<em>2</em>) <br /><em>Declination &delta; 1900</em></th>
+<th>(<em>3</em>) <br /><em>Visual Mag. m</em></th>
+<th>(<em>4</em>) <br /><em>Spectrum</em></th>
+<th>(<em>5</em>) <br /><em>Proper Motion</em></th>
+<th>(<em>6</em>) <br /><em>Radial Velocity<br />km. per sec.</em></th>
+<th>(<em>7</em>) <br /><em>Present Parallax &pi;</em></th>
+<th>(<em>8</em>) <br /><em>Maximum Parallax</em></th>
+<th>(<em>9</em>) <br /><em>Minimum Distance Light Yrs.</em></th>
+<th>(<em>10</em>) <br /><em>Time of<br />Minimum<br />Distance</em></th>
+<th>(<em>11</em>) <br /><em>Magnitude<br />at Min. Dist.</em></th>
+<th>(<em>12</em>) <br /><em>Absolute<br /> Magnitude</em></th>
+<th>(<em>13</em>) <br /><em>Luminosity</em></th>
+<th>(<em>14</em>) <br /><em>Effective<br /> radiation <br />at minimum<br />distance<br />from sun</em></th>
+</tr>
+<tr>
+<td>Groombr. 34</td>
+<td>0<sup>h</sup>12<sup>m</sup>.7</td>
+<td>+43&deg;27'</td>
+<td>8.1</td>
+<td>Ma</td>
+<td>2".89</td>
+<td>+  3</td>
+<td>".28</td>
+<td>".28</td>
+<td>11.6</td>
+<td>-  4000</td>
+<td>8.1</td>
+<td>10.3</td>
+<td>0.0063</td>
+<td>0.000051</td>
+</tr>
+<tr>
+<td><sup><a href="#fn128" id="r128">[128]</a></sup>&eta; Cassiop.</td>
+<td>43 .0</td>
+<td>+57 17</td>
+<td>3.6</td>
+<td>F8</td>
+<td>1 .24</td>
+<td>+ 10</td>
+<td>.18</td>
+<td>.19</td>
+<td>17.1</td>
+<td>- 47000</td>
+<td>3.5 </td>
+<td>4.9</td>
+<td>0.91</td>
+<td>0.003110</td>
+</tr>
+<tr>
+<td></td>
+<td>43 .9</td>
+<td>+4 55</td>
+<td>12.3</td>
+<td>F0</td>
+<td>3 .01</td>
+<td>.....</td>
+<td>.24</td>
+<td>....</td>
+<td>....</td>
+<td>.......</td>
+<td>....</td>
+<td>14.2</td>
+<td>0.00017</td>
+<td>........</td>
+</tr>
+<tr>
+<td><sup><a href="#fn128">[128]</a></sup>&kappa; Tucan&aelig;</td>
+<td>1 12 .4</td>
+<td>-69 24</td>
+<td>5.0</td>
+<td>F8</td>
+<td>.39</td>
+<td>+ 12</td>
+<td>.16</td>
+<td>.23</td>
+<td>14.2</td>
+<td>-264000</td>
+<td>4.2</td>
+<td>6.0</td>
+<td>0.33</td>
+<td>0.001610</td>
+</tr>
+<tr>
+<td>&tau; Ceti</td>
+<td>39 .4</td>
+<td>-16 28</td>
+<td>3.6</td>
+<td>K0</td>
+<td>1 .92</td>
+<td>- 16</td>
+<td>.32</td>
+<td>.37</td>
+<td>8.8</td>
+<td>+ 46000</td>
+<td>3.3</td>
+<td>6.1</td>
+<td>0.30</td>
+<td>0.003840</td>
+</tr>
+<tr>
+<td>&delta;<sub>2</sub> Eridani</td>
+<td>3 15 .9</td>
+<td>-43 27</td>
+<td>4.3</td>
+<td>G5</td>
+<td>3 .16</td>
+<td>+ 87</td>
+<td>.16</td>
+<td>.22</td>
+<td>14.8</td>
+<td>- 33000</td>
+<td>3.6</td>
+<td>5.3</td>
+<td>0.63</td>
+<td>0.002960</td>
+</tr>
+<tr>
+<td><sup><a href="#fn128">[128]</a></sup>&epsilon; Eridani</td>
+<td>28 .2</td>
+<td>- 9 48</td>
+<td>3.8</td>
+<td>K0</td>
+<td>.97</td>
+<td>+ 16</td>
+<td>.31</td>
+<td>.46</td>
+<td>7.1</td>
+<td>-106000</td>
+<td>3.0</td>
+<td>6.3</td>
+<td>0.25</td>
+<td>0.004970</td>
+</tr>
+<tr>
+<td><sup><a href="#fn128">[128]</a></sup>40(0)<sup>2</sup> Eridani</td>
+<td>4 10 .7</td>
+<td>- 7 49</td>
+<td>4.5</td>
+<td>G5</td>
+<td>4 .08</td>
+<td>- 42</td>
+<td>.21</td>
+<td>.23</td>
+<td>14.2</td>
+<td>+ 19000</td>
+<td>4.3</td>
+<td>6.1</td>
+<td>0.30</td>
+<td>0.001470</td>
+</tr>
+<tr>
+<td>Cordoba Z. 243</td>
+<td>5  7 .7</td>
+<td>-44 59</td>
+<td>9.2</td>
+<td>K2</td>
+<td>8 .75</td>
+<td>+242</td>
+<td>.32</td>
+<td>.68</td>
+<td>4.8</td>
+<td>- 10000</td>
+<td>7.6</td>
+<td>11.7</td>
+<td>0.0017</td>
+<td>0.000074</td>
+</tr>
+<tr>
+<td>Weisse 592</td>
+<td>26 .4</td>
+<td>- 3 42</td>
+<td>8.8</td>
+<td>K2</td>
+<td>2 .22</td>
+<td>.....</td>
+<td>.17</td>
+<td>....</td>
+<td>....</td>
+<td>.......</td>
+<td>....</td>
+<td>9.9</td>
+<td>0.009</td>
+<td>........</td>
+</tr>
+<tr>
+<td><sup><a href="#fn128">[128]</a></sup>&alpha; Can. Maj. (Sirius)</td>
+<td>6 40 .7</td>
+<td>-16 35</td>
+<td>-1.6</td>
+<td>A0</td>
+<td>1 .32</td>
+<td>-  8</td>
+<td>.37</td>
+<td>.41</td>
+<td>8.0</td>
+<td>+ 65000</td>
+<td>-1.8</td>
+<td>1.2</td>
+<td>27.50</td>
+<td>0.429000</td>
+</tr>
+<tr>
+<td><sup><a href="#fn128">[128]</a></sup>&alpha; Can. Min. (Procyon)</td>
+<td>7 34 .1</td>
+<td>+ 5 29</td>
+<td>0.5</td>
+<td>F5</td>
+<td>1 .24</td>
+<td>-  4</td>
+<td>.31</td>
+<td>.32</td>
+<td>10.2</td>
+<td>+ 34000</td>
+<td>0.5</td>
+<td>3.0</td>
+<td>5.25</td>
+<td>0.051300</td>
+</tr>
+<tr>
+<td><sup><a href="#fn128">[128]</a></sup>Fedorenko 1457-8</td>
+<td>9  7 .6</td>
+<td>+53  7</td>
+<td>7.9</td>
+<td>Ma</td>
+<td>1 .68</td>
+<td>+ 10</td>
+<td>.16</td>
+<td>.16</td>
+<td>20.4</td>
+<td>- 24000</td>
+<td>7.9</td>
+<td>8.9</td>
+<td>0.023</td>
+<td>0.000055</td>
+</tr>
+<tr>
+<td>Groombr. 1618</td>
+<td>10  5 .3 </td>
+<td>+49 58</td>
+<td>6.8</td>
+<td>K5p</td>
+<td>1 .45</td>
+<td>- 30</td>
+<td>.18</td>
+<td>.23</td>
+<td>14.2</td>
+<td>+ 69000</td>
+<td>6.3</td>
+<td>8.1</td>
+<td>0.048</td>
+<td>0.000238</td>
+</tr>
+<tr>
+<td>Weisse 234</td>
+<td>14 .2</td>
+<td>+20 22</td>
+<td>9.0</td>
+<td>...</td>
+<td>.49</td>
+<td>.....</td>
+<td>.19</td>
+<td>....</td>
+<td>....</td>
+<td>.......</td>
+<td>....</td>
+<td>10.4</td>
+<td>0.0057</td>
+<td>........</td>
+</tr>
+<tr>
+<td>Lalande 21185</td>
+<td>57 .9</td>
+<td>+36 38</td>
+<td>7.6</td>
+<td>Mb</td>
+<td>4 .78</td>
+<td>- 87</td>
+<td>.41</td>
+<td>.76</td>
+<td>4.3</td>
+<td>+ 20000</td>
+<td>6.2</td>
+<td>10.7</td>
+<td>0.0044</td>
+<td>0.000238</td>
+</tr>
+<tr>
+<td>Lalande 21258</td>
+<td>11  0 .5</td>
+<td>+44  2</td>
+<td>8.5</td>
+<td>K5</td>
+<td>4 .52</td>
+<td>+ 65</td>
+<td>.19</td>
+<td>.22</td>
+<td>14.8</td>
+<td>- 20000</td>
+<td>8.2</td>
+<td>9.9</td>
+<td>0.009</td>
+<td>0.000041</td>
+</tr>
+<tr>
+<td></td>
+<td>12 .0</td>
+<td>-57  2</td>
+<td>12.0</td>
+<td>...</td>
+<td>2 .69</td>
+<td>.....</td>
+<td>.34</td>
+<td>....</td>
+<td>....</td>
+<td>........</td>
+<td>...</td>
+<td>14.7</td>
+<td>0.00011</td>
+<td>........</td>
+</tr>
+<tr>
+<td>Lalande 25372</td>
+<td>13 40 .7</td>
+<td>+15 26</td>
+<td>8.5</td>
+<td>K5</td>
+<td>2 .30</td>
+<td>.....</td>
+<td>.19</td>
+<td>....</td>
+<td>....</td>
+<td>.......</td>
+<td>....</td>
+<td>9.9</td>
+<td>0.009</td>
+<td>........</td>
+</tr>
+<tr>
+<td><sup><a href="#fn128">[128]</a></sup>&alpha; Centauri</td>
+<td>14 32 .8</td>
+<td>-60 25</td>
+<td>0.2</td>
+<td>G</td>
+<td>3 .68</td>
+<td>+ 22</td>
+<td>.76</td>
+<td>1.03</td>
+<td>3.2</td>
+<td>- 28000</td>
+<td>-0.5</td>
+<td>4.6</td>
+<td>1.20</td>
+<td>0.117500</td>
+</tr>
+<tr>
+<td><sup><a href="#fn128">[128]</a></sup>&xi; Bootes</td>
+<td>14 46 .8</td>
+<td>+19 31</td>
+<td>4.6</td>
+<td>K5p</td>
+<td>.17</td>
+<td>+  4</td>
+<td>.17 </td>
+<td>.22</td>
+<td>14.8</td>
+<td>-598000</td>
+<td>4.0</td>
+<td>5.8</td>
+<td>0.40</td>
+<td>0.001815</td>
+</tr>
+<tr>
+<td><sup><a href="#fn128">[128]</a></sup>Lalande 27173</td>
+<td>51 .6</td>
+<td>-20 58</td>
+<td>5.8</td>
+<td>Kp</td>
+<td>1 .96</td>
+<td>+ 20</td>
+<td>.18</td>
+<td>.19</td>
+<td>17.1</td>
+<td>- 36000</td>
+<td>5.6</td>
+<td>7.1</td>
+<td>0.12</td>
+<td>0.000412</td>
+</tr>
+<tr>
+<td>Weisse 1259</td>
+<td>16 41 .4</td>
+<td>+33 41</td>
+<td>8.4</td>
+<td>...</td>
+<td>.37</td>
+<td>.....</td>
+<td>.18</td>
+<td>....</td>
+<td>....</td>
+<td>.......</td>
+<td>....</td>
+<td>9.7</td>
+<td>0.011</td>
+<td>........</td>
+</tr>
+<tr>
+<td>Lacaille 7194</td>
+<td>17 11 .5</td>
+<td>-46 32</td>
+<td>5.7</td>
+<td>K</td>
+<td>.97</td>
+<td>.....</td>
+<td>.19</td>
+<td>....</td>
+<td>....</td>
+<td>.......</td>
+<td>....</td>
+<td>7.1</td>
+<td>0.12</td>
+<td>........</td>
+</tr>
+<tr>
+<td><sup><a href="#fn128">[128]</a></sup>&beta; 416</td>
+<td>12 .1</td>
+<td>-34 53</td>
+<td>5.9</td>
+<td>K5</td>
+<td>1 .19</td>
+<td>-  4</td>
+<td>.17</td>
+<td>.17</td>
+<td>19.2</td>
+<td>+ 21000</td>
+<td>5.7</td>
+<td>7.1</td>
+<td>0.12</td>
+<td>0.000329</td>
+</tr>
+<tr>
+<td>Argel <br />-0.17415-6</td>
+<td>37 .0</td>
+<td>+68 26</td>
+<td>9.1</td>
+<td> K </td>
+<td>1 .33</td>
+<td>.....</td>
+<td>.22</td>
+<td>....</td>
+<td>....</td>
+<td>.......</td>
+<td>....</td>
+<td>10.8</td>
+<td>0.004</td>
+<td>........</td>
+</tr>
+<tr>
+<td>Barnard's star</td>
+<td>52 .9</td>
+<td>+ 4 25</td>
+<td>9.7</td>
+<td>Mb</td>
+<td>10 .30</td>
+<td>- 80</td>
+<td>.53</td>
+<td>.70</td>
+<td>4.7</td>
+<td>+ 10000</td>
+<td>9.1</td>
+<td>13.3</td>
+<td>0.0025</td>
+<td>0.000114</td>
+</tr>
+<tr>
+<td><sup><a href="#fn128">[128]</a></sup>70p Ophiuchi</td>
+<td>18  0 .4</td>
+<td>+ 2 31</td>
+<td>4.3</td>
+<td>K</td>
+<td>1 .13</td>
+<td>.....</td>
+<td>.19</td>
+<td>....</td>
+<td>....</td>
+<td>.......</td>
+<td>....</td>
+<td>5.7</td>
+<td>0.44</td>
+<td>........</td>
+</tr>
+<tr>
+<td><sup><a href="#fn128">[128]</a></sup>&Sigma; 2398</td>
+<td>41 .7</td>
+<td>+59 29</td>
+<td>8.8</td>
+<td>K</td>
+<td>2 .31</td>
+<td>.....</td>
+<td>.29</td>
+<td>....</td>
+<td>....</td>
+<td>.......</td>
+<td>....</td>
+<td>11.1</td>
+<td>0.0030</td>
+<td>........</td>
+</tr>
+<tr>
+<td>&sigma; Draconis</td>
+<td>19 32 .5</td>
+<td>+69 29</td>
+<td>4.8</td>
+<td>G5</td>
+<td>1 .84</td>
+<td>+ 26</td>
+<td>.20</td>
+<td>.23</td>
+<td>14.2</td>
+<td>- 49000</td>
+<td>4.5</td>
+<td>6.3</td>
+<td>0.25</td>
+<td>0.001238</td>
+</tr>
+<tr>
+<td><sup><a href="#fn128">[128]</a></sup>&alpha; Aquil&aelig; (Altair)</td>
+<td>45 .9</td>
+<td>+ 8 36</td>
+<td>1.2</td>
+<td>A5</td>
+<td>.66</td>
+<td>- 33</td>
+<td>.21</td>
+<td>.51</td>
+<td>6.4</td>
+<td>+117000</td>
+<td>-0.7</td>
+<td>2.8</td>
+<td>6.30</td>
+<td>0.153600</td>
+</tr>
+<tr>
+<td><sup><a href="#fn128">[128]</a></sup>61 Cygni</td>
+<td>21  2 .4</td>
+<td>+38 15</td>
+<td>5.6</td>
+<td>K5</td>
+<td>5 .20</td>
+<td>- 64</td>
+<td>.30</td>
+<td>.38</td>
+<td>8.6</td>
+<td>+ 19000</td>
+<td>5.1</td>
+<td>8.0</td>
+<td>0.053</td>
+<td>0.000715</td>
+</tr>
+<tr>
+<td>Lacaille 8760</td>
+<td>11 .4</td>
+<td>-39 15</td>
+<td>6.6</td>
+<td>G</td>
+<td>3 .53</td>
+<td>+ 13</td>
+<td>.25</td>
+<td>.26</td>
+<td>12.6</td>
+<td>- 11000</td>
+<td>6.6</td>
+<td>8.6</td>
+<td>0.030</td>
+<td>0.000189</td>
+</tr>
+<tr>
+<td>&epsilon; Indi</td>
+<td>55 .7</td>
+<td>-57 12</td>
+<td>4.8</td>
+<td>K5</td>
+<td>4 .70</td>
+<td>- 39</td>
+<td>.28</td>
+<td>.31</td>
+<td>10.5</td>
+<td>+ 17000</td>
+<td>4.6</td>
+<td>7.0</td>
+<td>0.13</td>
+<td>0.001230</td>
+</tr>
+<tr>
+<td><sup><a href="#fn128">[128]</a></sup>Kr&uuml;ger 60</td>
+<td>22 24 .4</td>
+<td>+57 12</td>
+<td>9.2</td>
+<td>...</td>
+<td>.87</td>
+<td>.....</td>
+<td>.26</td>
+<td>....</td>
+<td>....</td>
+<td>.......</td>
+<td>....</td>
+<td>11.3</td>
+<td>0.0025</td>
+<td>.......</td>
+</tr>
+<tr>
+<td>Lacaille 9352</td>
+<td>59 .4</td>
+<td>-36 26</td>
+<td>7.1</td>
+<td>K</td>
+<td>6 .90</td>
+<td>+ 12</td>
+<td>.29</td>
+<td>.29</td>
+<td>11.2</td>
+<td>-  3000</td>
+<td>7.1</td>
+<td>9.4</td>
+<td>0.014</td>
+<td>0.000111</td>
+</tr>
+<tr>
+<td>Lalande 46650</td>
+<td>23 44 .0</td>
+<td>+ 1 52</td>
+<td>8.7</td>
+<td>Ma</td>
+<td>1 .39</td>
+<td>.....</td>
+<td>.17</td>
+<td>....</td>
+<td>....</td>
+<td>.......</td>
+<td>....</td>
+<td>9.9</td>
+<td>0.009</td>
+<td>.......</td>
+</tr>
+<tr>
+<td>C. G. A. 32416</td>
+<td>59 .5</td>
+<td>-37 51</td>
+<td>8.2</td>
+<td>G</td>
+<td>6 .05</td>
+<td>+ 26</td>
+<td>.22</td>
+<td>.22</td>
+<td>14.8</td>
+<td>-  7000</td>
+<td>8.2</td>
+<td>9.9</td>
+<td>0.009</td>
+<td>0.000041</td>
+</tr>
+</table>
+
+<!--File: cc301.png-->
+<a class="pagenum break" id="Page_277" title="pg 277"></a>
+
+<!--File: cc302.png-->
+<a class="pagenum break" id="Page_278" title="pg 278"></a>
+
+
+<p>On the basis of column 14 and of the movements and
+distances of the stars as given in the other columns Fig.
+10 has been prepared. This gives an estimate of the
+approximate electrical energy received by the sun from
+the nearest stars for 70,000 years before and after the
+present. It is based on the twenty-six stars for which
+complete data are available in Table 6. The inclusion of
+the other twelve would not alter the form of the curve,
+for even the largest of them would not change any part
+by more than about half of 1 per cent, if as much.
+Nor would the curve be visibly altered by the omission
+of all except four of the twenty-six stars actually used.
+The four that are important, and their relative luminosity
+when nearest the sun, are Sirius 429,000, Altair
+153,000, Alpha Centauri 117,500, and Procyon 51,300.
+The figure for the next star is only 4970, while for this
+star combined with the other twenty-one that are unimportant
+it is only 24,850.</p>
+
+<p>Figure 10 is not carried more than 70,000 years into
+the past or into the future because the stars near the
+sun at more remote times are not included among the
+thirty-eight having the largest known parallaxes. That
+is, they have either moved away or are not yet near
+enough to be included. Indeed, as Dr. Schlesinger
+strongly emphasizes, there may be swiftly moving, bright
+or gigantic stars which are now quite far away, but whose
+inclusion would alter Fig. 10 even within the limits of
+the 140,000 years there shown. It is almost certain, however,
+that the most that these would do would be to raise,
+but not obliterate, the minima on either side of the main
+maximum.</p>
+
+<p>In preparing Fig. 10 it has been necessary to make
+<!--File: cc303.png-->
+<a class="pagenum break" id="Page_279" title="pg 279"></a>
+
+
+<!--File: cc304.png-->
+<a class="pagenum break" id="Page_280" title="pg 280"></a>
+allowance for double stars. Passing by the twenty-two
+unimportant stars, it appears that the companion of
+Sirius is eight or ten magnitudes smaller than that star,
+while the companions of Procyon and Altair are five or
+more magnitudes smaller than their bright comrades.
+This means that the luminosity of the faint components
+is at most only 1 per cent of that of their bright companions
+and in the case of Sirius not a hundredth of 1
+per cent. Hence their inclusion would have no visible
+effect on Fig. 10. In Alpha Centauri, on the other hand,
+the two components are of almost the same magnitude.
+For this reason the effective radiation of that star as
+given in column 14 is doubled in Fig. 10, while for
+another reason it is raised still more. The other reason
+is that if our inferences as to the electrical effect of the
+sun on the earth and of the planets on the sun are correct,
+double stars, as we have seen, must be much more
+effective electrically than single stars. By the same
+reasoning two bright stars close together must excite
+one another much more than a bright star and a very
+faint one, even if the distances in both cases are the same.
+So, too, other things being equal, a triple star must be
+more excited electrically than a double star. Hence in
+preparing Fig. 10 all double stars receive double weight
+and each part of Alpha Centauri receives an additional
+50 per cent because both parts are bright and because
+they have a third companion to help in exciting them.</p>
+
+<img src="images/fig10.png" alt="Fig. 10"  id="Fig_10" />
+<p class="center"><em>Fig. 10. Climatic changes of 140,000 years as inferred from the stars.</em></p>
+
+<p>According to the electro-stellar hypothesis, Alpha Centauri
+is more important climatically than any other star
+in the heavens not only because it is triple and bright, but
+because it is the nearest of all stars, and moves fairly
+rapidly. Sirius and Procyon move slowly in respect to
+the sun, only about eleven and eight kilometers per
+second respectively, and their distances at minimum are
+<!--File: cc305.png-->
+<a class="pagenum break" id="Page_281" title="pg 281"></a>
+fairly large, that is, 8 and 10.2 light years. Hence their
+effect on the sun changes slowly. Altair moves faster,
+about twenty-six kilometers per second, and its minimum
+distance is 6.4 light years, so that its effect changes fairly
+rapidly. Alpha Centauri moves about twenty-four kilometers
+per second, and its minimum distance is only 3.2
+light years. Hence its effect changes very rapidly, the
+change in its apparent luminosity as seen from the sun
+amounting at maximum to about 30 per cent in 10,000
+years against 14 per cent for Altair, 4 for Sirius, and 2
+for Procyon. The vast majority of the stars change so
+much more slowly than even Procyon that their effect is
+almost uniform. All the stars at a distance of more than
+perhaps twenty or thirty light years may be regarded as
+sending to the sun a practically unchanging amount of
+radiation. It is the bright stars within this limit which
+are important, and their importance increases with their
+proximity, their speed of motion, and the brightness and
+number of their companions. Hence Alpha Centauri
+causes the main maximum in Fig. 10, while Sirius, Altair,
+and Procyon combine to cause a general rise of the curve
+from the past to the future.</p>
+
+<p>Let us now interpret Fig. 10 geologically. The low position
+of the curve fifty to seventy thousand years ago
+suggests a mild inter-glacial climate distinctly less severe
+than that of the present. Geologists say that such was the
+case. The curve suggests a glacial epoch culminating
+about 28,000 years ago. The best authorities put the climax
+of the last glacial epoch between twenty-five and
+thirty thousand years ago. The curve shows an amelioration
+of climate since that time, although it suggests that
+there is still considerable severity. The retreat of the ice
+from North America and Europe, and its persistence in
+Greenland and Antarctica agree with this. And the curve
+<!--File: cc306.png-->
+<a class="pagenum break" id="Page_282" title="pg 282"></a>
+indicates that the change of climate is still persisting, a
+conclusion in harmony with the evidence as to historic
+changes.</p>
+
+<p>If Alpha Centauri is really so important, the effect of
+its variations, provided it has any, ought perhaps to be
+evident in the sun. The activity of the star's atmosphere
+presumably varies, for the orbits of the two components
+have an eccentricity of 0.51. Hence during their period
+of revolution, 81.2 years, the distance between them
+ranges from 1,100,000,000 to 3,300,000,000 miles. They
+were at a minimum distance in 1388, 1459, 1550, 1631,
+1713, 1794, 1875, and will be again in 1956. In Fig.
+11, showing sunspot variations, it is noticeable that the
+years 1794 and 1875 come just at the ends of periods of
+unusual solar activity, as indicated by the heavy horizontal
+line. A similar period of great activity seems to
+have begun about 1914. If its duration equals the average
+of its two predecessors, it will end about 1950. Back in
+the fourteenth century a period of excessive solar activity,
+which has already been described, culminated from
+1370 to 1385, or just before the two parts of Alpha Centauri
+were at a minimum distance. Thus in three and
+perhaps four cases the sun has been unusually active
+during a time when the two parts of the star were most
+rapidly approaching each other and when their atmospheres
+were presumably most disturbed and their electrical
+emanations strongest.</p>
+
+<p>The fact that Alpha Centauri, the star which would be
+expected most strongly to influence the sun, and hence
+the earth, was nearest the sun at the climax of the last
+glacial epoch, and that today the solar atmosphere is
+most active when the star is presumably most disturbed
+may be of no significance. It is given for what it is worth.
+Its importance lies not in the fact that it proves anything,
+
+but that no contradiction is found when we test
+the electro-stellar hypothesis by facts which were not
+thought of when the hypothesis was framed. A vast
+amount of astronomical work is still needed before the
+matter can be brought to any definite conclusion. In case
+the hypothesis stands firm, it may be possible to use the
+stars as a help in determining the exact chronology of the
+later part of geological times. If the hypothesis is disproved,
+it will merely leave the question of solar variations
+where it is today. It will not influence the main
+conclusions of this book as to the causes and nature of
+climatic changes. Its value lies in the fact that it calls
+attention to new lines of research.</p>
+<!--File: cc307.png-->
+<a class="pagenum break" id="Page_283" title="pg 283"></a>
+
+<img src="images/fig11.png" width="800" height="193" alt="Fig. 11"  id="Fig_11" />
+<p class="center"><em>Fig. 11. Sunspot curve showing cycles, 1750 to 1920.</em></p>
+
+<p class="blockquot"><em>Note.</em> The asterisks indicate two absolute minima of sunspots in 1810 and 1913, and the middle years (1780 and
+1854) of two periods when the sunspot maxima never fell below 95. If Alpha Centauri has an effect on the sun's
+atmosphere, the end of another such period would be expected not far from 1957.</p>
+<!--File: cc308.png-->
+<a class="pagenum break" id="Page_284" title="pg 284"></a>
+<!--File: cc309.png-->
+<a class="pagenum break" id="Page_285" title="pg 285"></a>
+
+
+
+
+<h2 id="Chapt_XVI"><a href="#content">CHAPTER XVI</a></h2>
+
+<h3>THE EARTH'S CRUST AND THE SUN</h3>
+
+
+<p>Although the problems of this book may lead far
+afield, they ultimately bring us back to the earth
+and to the present. Several times in the preceding
+pages there has been mention of the fact that periods of
+extreme climatic fluctuations are closely associated with
+great movements of the earth's crust whereby mountains
+are uplifted and continents upheaved. In attempting to
+explain this association the general tendency has been
+to look largely at the past instead of the present. Hence
+it has been almost impossible to choose among three
+possibilities, all beset with difficulties. First, the movements
+of the crust may have caused the climatic fluctuations;
+second, climatic changes may cause crustal movements;
+and third, variations in solar activity or in some
+other outside agency may give rise to both types of terrestrial
+phenomena.</p>
+
+<p>The idea that movements of the earth's crust are the
+main cause of geological changes of climate is becoming
+increasingly untenable as the complexity and rapidity of
+climatic changes become more clear, especially during
+post-glacial times. It implies that the earth's surface
+moves up and down with a speed and facility which
+appear to be out of the question. If volcanic activity be
+invoked the problem becomes no clearer. Even if volcanic
+dust should fill the air frequently and completely, neither
+its presence nor absence would produce such peculiar features
+<!--File: cc310.png-->
+<a class="pagenum break" id="Page_286" title="pg 286"></a>
+as the localization of glaciers, the distribution of
+loess, and the mild climate of most parts of geological
+time. Nevertheless, because of the great difficulties presented
+by the other two possibilities many geologists
+still hold that directly or indirectly the greater climatic
+changes have been mainly due to movements of the
+earth's crust and to the reaction of the crustal movements
+on the atmosphere.</p>
+
+<p>The possibility that climatic changes are in themselves
+a cause of movements of the earth's crust seems so improbable
+that no one appears to have investigated it with
+any seriousness. Nevertheless, it is worth while to raise
+the question whether climatic extremes may co&ouml;perate
+with other agencies in setting the time when the earth's
+crust shall be deformed.</p>
+
+<p>As to the third possibility, it is perfectly logical to
+ascribe both climatic changes and crustal deformation to
+some outside agency, solar or otherwise, but hitherto
+there has been so little evidence on this point that such
+an ascription has merely begged the question. If heavenly
+bodies should approach the earth closely enough so that
+their gravitational stresses caused crustal deformation,
+all life would presumably be destroyed. As to the sun,
+there has hitherto been no conclusive evidence that it is
+related to crustal movements, although various writers
+have made suggestions along this line. In this chapter
+we shall carry these suggestions further and shall see
+that they are at least worthy of study.</p>
+
+<p>As a preliminary to this study it may be well to note
+that the coincidence between movements of the earth's
+crust and climatic changes is not so absolute as is sometimes
+supposed. For example, the profound crustal
+changes at the end of the Mesozoic were not accompanied
+by widespread glaciation so far as is yet known, although
+<!--File: cc311.png-->
+<a class="pagenum break" id="Page_287" title="pg 287"></a>
+the temperature appears to have been lowered. Nor was
+the violent volcanic and diastrophic activity in the Miocene
+associated with extreme climates. Indeed, there
+appears to have been little contrast from zone to zone,
+for figs, bread fruit trees, tree ferns, and other plants of
+low latitudes grew in Greenland. Nevertheless, both at
+the end of the Mesozoic and in the Miocene the climate
+may possibly have been severe for a time, although the
+record is lost. On the other hand, Kirk's recent discovery
+of glacial till in Alaska between beds carrying an undoubted
+Middle Silurian fauna indicates glaciation at a
+time when there was little movement of the crust so far
+as yet appears.<sup><a href="#fn129" id="r129">[129]</a></sup> Thus we conclude that while climatic
+changes and crustal movements usually occur together,
+they may occur separately.</p>
+
+<p>According to the solar-cyclonic hypothesis such a condition
+is to be expected. If the sun were especially active
+when the terrestrial conditions prohibited glaciation,
+changes of climate would still occur, but they would be
+milder than under other circumstances, and would leave
+little record in the rocks. Or there might be glaciation in
+high latitudes, such as that of southern Alaska in the
+Middle Silurian, and none elsewhere. On the other hand,
+when the sun was so inactive that no great storminess
+occurred, the upheaval of continents and the building of
+mountains might go on without the formation of ice
+sheets, as apparently happened at the end of the Mesozoic.
+The lack of absolute coincidence between glaciation
+and periods of widespread emergence of the lands is
+evident even today, for there is no reason to suppose
+that the lands are notably lower or less extensive now
+than they were during the Pleistocene glaciation. In
+fact, there is much evidence that many areas have risen
+<!--File: cc312.png-->
+<a class="pagenum break" id="Page_288" title="pg 288"></a>
+since that time. Yet glaciation is now far less extensive
+than in the Pleistocene. Any attempt to explain this difference
+on the basis of terrestrial changes is extremely
+difficult, for the shape and altitude of continents and
+mountains have not changed much in twenty or thirty
+thousand years. Yet the present moderately mild epoch,
+like the puzzling inter-glacial epochs of earlier times, is
+easily explicable on the assumption that the sun's atmosphere
+may sometimes vary in harmony with crustal
+activity, but does not necessarily do so at all times.</p>
+
+<p>Turning now to the main problem of how climatic
+changes may be connected with movements of the earth's
+crust, let us follow our usual method and examine what
+is happening today. Let us first inquire whether earthquakes,
+which are one of the chief evidences that crustal
+movements are actually taking place in our own times,
+show any connection with sunspots. In order to test this,
+we have compared <em>Milne's Catalogue of Destructive
+Earthquakes</em> from 1800 to 1899, with Wolf's sunspot
+numbers for the same period month by month. The earthquake
+catalogue, as its compiler describes it, "is an
+attempt to give a list of earthquakes which have announced
+changes of geological importance in the earth's
+crust; movements which have probably resulted in the
+creation or the extension of a line of fault, the vibrations
+accompanying which could, with proper instruments,
+have been recorded over a continent or the whole surface
+of our world. Small earthquakes have been excluded,
+while the number of large earthquakes both for ancient
+and modern times has been extended. As an illustration
+of exclusion, I may mention that between 1800 and 1808,
+which are years taken at random, I find in Mallet's catalogue
+407 entries. Only thirty-seven of these, which were
+accompanied by structural damage, have been retained.
+<!--File: cc313.png-->
+<a class="pagenum break" id="Page_289" title="pg 289"></a>
+Other catalogues such as those of Perry and Fuchs have
+been treated similarly."<sup><a href="#fn130" id="r130">[130]</a></sup></p>
+
+<p>If the earthquakes in such a carefully selected list bear
+a distinct relation to sunspots, it is at least possible and
+perhaps probable that a similar relation may exist between
+solar activity and geological changes in the earth's
+crust. The result of the comparison of earthquakes and
+sunspots is shown in Table 7. The first column gives the
+sunspot numbers; the second, the number of months that
+had the respective spot numbers during the century from
+1800 to 1899. Column C shows the total number of earthquakes
+during the months having any particular degree
+of spottedness; while D, which is the significant column,
+gives the average number of destructive earthquakes per
+month under each of the six conditions of solar spottedness.</p>
+
+<table id="table_7" class="borders2">
+<tr><th colspan="6"><a href="#tables">TABLE 7</a></th></tr>
+<tr><td colspan='6'>&nbsp;</td></tr>
+<tr><th colspan="6">DESTRUCTIVE EARTHQUAKES FROM 1800 TO
+1899 COMPARED WITH SUNSPOTS</th></tr>
+<tr><td colspan='6'>&nbsp;</td></tr>
+<tr>
+<th><em>A</em></th>
+<th><em>B</em></th>
+<th><em>C</em></th>
+<th><em>D</em></th>
+<th><em>E</em></th>
+<th><em>F</em></th>
+</tr>
+<tr>
+<th><em>Sunspot numbers</em></th>
+<th><em>Number of months per Wolf's Table</em></th>
+<th><em>Number of earthquakes</em></th>
+<th><em>Average number of earthquakes per month</em></th>
+<th><em>Number of earthquakes in succeeding month</em></th>
+<th><em>Average number of earthquakes in succeeding month</em></th>
+</tr>
+<tr>
+<td> 0-15</td>
+<td>344</td>
+<td>522</td>
+<td>1.52</td>
+<td>512</td>
+<td>1.49</td>
+</tr>
+<tr>
+<td>15-30</td>
+<td>194</td>
+<td>306</td>
+<td>1.58</td>
+<td>310</td>
+<td>1.60</td>
+</tr>
+<tr>
+<td>30-50</td>
+<td>237</td>
+<td>433</td>
+<td>1.83</td>
+<td>439</td>
+<td>1.85</td>
+</tr>
+<tr>
+<td>50-70</td>
+<td>195</td>
+<td>402</td>
+<td>2.06</td>
+<td>390</td>
+<td>2.00</td>
+</tr>
+<tr>
+<td>70-100</td>
+<td>135</td>
+<td>286</td>
+<td>2.12</td>
+<td>310</td>
+<td>2.30</td>
+</tr>
+<tr>
+<td>over 100</td>
+<td>95</td>
+<td>218</td>
+<td>2.30</td>
+<td>175</td>
+<td>1.84</td>
+</tr>
+</table>
+
+<!--File: cc314.png Page 290-->
+<a class="pagenum break" id="Page_290" title="pg 290"></a>
+<p>The regularity of column D is so great as to make
+it almost certain that we are here dealing with a real
+relationship. Column F, which shows the average number
+of earthquakes in the month succeeding any given condition
+of the sun, is still more regular except for the last
+entry.</p>
+
+<p>The chance that six numbers taken at random will
+arrange themselves in any given order is one in 720. In
+other words, there is one chance in 720 that the regularity
+of column D is accidental. But column F is as regular as
+column D except for the last entry. If columns D and E
+were independent there would be one chance in about
+500,000 that the six numbers in both columns would
+fall in the same order, and one chance in 14,400 that
+five numbers in each would fall in the same order.
+But the two columns are somewhat related, for although
+the after-shocks of a great earthquake are never included
+in Milne's table, a world-shaking earthquake in one
+region during a given month probably creates conditions
+that favor similar earthquakes elsewhere during the next
+month. Hence the probability that we are dealing with a
+purely accidental arrangement in Table 7 is less than one
+in 14,400 and greater than one in 500,000. It may be one
+in 20,000 or 100,000. In any event it is so slight that there
+is high probability that directly or indirectly sunspots
+and earthquakes are somehow connected.</p>
+
+<p>In ascertaining the relation between sunspots and
+earthquakes it would be well if we could employ the strict
+method of correlation coefficients. This, however, is impossible
+for the entire century, for the record is by no
+means homogeneous. The earlier decades are represented
+by only about one-fourth as many earthquakes as the
+later ones, a condition which is presumably due to lack of
+information. This makes no difference with the method
+<!--File: cc315.png-->
+<a class="pagenum break" id="Page_291" title="pg 291"></a>
+employed in Table 7, since years with many and few sunspots
+are distributed almost equally throughout the
+entire nineteenth century, but it renders the method of
+correlation coefficients inapplicable. During the period
+from 1850 onward the record is much more nearly homogeneous,
+though not completely so. Even in these later
+decades, however, allowance must be made for the fact
+that there are more earthquakes in winter than in
+summer, the average number per month for the fifty
+years being as follows:</p>
+
+<table>
+<tr><td class="tdl">Jan. 2.8</td>              <td class="tdl">May  2.4  </td>            <td class="tdl">Sept. 2.5</td></tr>
+<tr><td class="tdl">Feb. 2.4</td>              <td class="tdl">June 2.3</td>              <td class="tdl">Oct.  2.6</td></tr>
+<tr><td class="tdl">Mar. 2.5</td>              <td class="tdl">July 2.4</td>              <td class="tdl">Nov.  2.7</td></tr>
+<tr><td class="tdl">Apr. 2.4 </td>             <td class="tdl">Aug. 2.4</td>              <td class="tdl">Dec.  2.8</td></tr>
+</table>
+
+<p>The correlation coefficient between the departures from
+these monthly averages and the corresponding departures
+from the monthly averages of the sunspots for the
+same period, 1850-1899, are as follows:</p>
+
+<blockquote>
+<p>Sunspots and earthquakes of same month: +0.042, or 1.5
+times the probable error.</p>
+
+<p>Sunspots of a given month and earthquakes of that month
+and the next: +0.084, or 3.1 times the probable error.</p>
+
+<p>Sunspots of three consecutive months and earthquakes of
+three consecutive months allowing a lag of one month, i.e., sunspots
+of January, February, and March compared with earthquakes
+of February, March, and April; sunspots of February,
+March, and April with earthquakes of March, April, and May,
+etc.; +0.112, or 4.1 times the probable error.</p>
+</blockquote>
+
+<p>These coefficients are all small, but the number of individual
+cases, 600 months, is so large that the probable
+error is greatly reduced, being only &plusmn;0.027 or &plusmn;0.028.
+Moreover, the nature of our data is such that even if
+<!--File: cc316.png-->
+<a class="pagenum break" id="Page_292" title="pg 292"></a>
+there is a strong connection between solar changes and
+earth movements, we should not expect a large correlation
+coefficient. In the first place, as already mentioned,
+the earthquake data are not strictly homogeneous.
+Second, an average of about two and one-half strong
+earthquakes per month is at best only a most imperfect
+indication of the actual movement of the earth's crust.
+Third, the sunspots are only a partial and imperfect
+measure of the activity of the sun's atmosphere. Fourth,
+the relation between solar activity and earthquakes is
+almost certainly indirect. In view of all these conditions,
+the regularity of Table 7 and the fact that the most important
+correlation coefficient rises to more than four
+times the probable error makes it almost certain that the
+solar and terrestrial phenomena are really connected.</p>
+
+<p>We are now confronted by the perplexing question of
+how this connection can take place. Thus far only three
+possibilities present themselves, and each is open to
+objections. The chief agencies concerned in these three
+possibilities are heat, electricity, and atmospheric pressure.
+Heat may be dismissed very briefly. We have seen
+that the earth's surface becomes relatively cool when
+the sun is active. Theoretically even the slightest change
+in the temperature of the earth's surface must influence
+the thermal gradient far into the interior and hence cause
+a change of volume which might cause movements of the
+crust. Practically the heat of the surface ceases to be of
+appreciable importance at a depth of perhaps twenty
+feet, and even at that depth it does not act quickly enough
+to cause the relatively prompt response which seems to be
+characteristic of earthquakes in respect to the sun.</p>
+
+<p>The second possibility is based on the relationship
+between solar and terrestrial electricity. When the sun
+is active the earth's atmospheric electrical potential is
+<!--File: cc317.png-->
+<a class="pagenum break" id="Page_293" title="pg 293"></a>
+subject to slight variations. It is well known that when
+two opposing points of an ionized solution are oppositely
+charged electrically, a current passes through the liquid
+and sets up electrolysis whereby there is a segregation
+of materials, and a consequent change in the volume of
+the parts near the respective electrical poles. The same
+process takes place, although less freely, in a hot mass
+such as forms the interior of the earth. The question
+arises whether internal electrical currents may not pass
+between the two oppositely charged poles of the earth,
+or even between the great continental masses and the
+regions of heavier rock which underlie the oceans. Could
+this lead to electrolysis, hence to differentiation in volume,
+and thus to movements of the earth's crust? Could
+the results vary in harmony with the sun? Bowie<sup><a href="#fn131" id="r131">[131]</a></sup> has
+shown that numerous measurements of the strength and
+direction of the earth's gravitative pull are explicable
+only on the assumption that the upheaval of a continent
+or a mountain range is due in part not merely to pressure,
+or even to flowage of the rocks beneath the crust,
+but also to an actual change in volume whereby the rocks
+beneath the continent attain relatively great volume and
+those under the oceans a small volume in proportion to
+their weight. The query arises whether this change of
+volume may be related to electrical currents at some
+depth below the earth's surface.</p>
+
+<p>The objections to this hypothesis are numerous. First,
+there is little evidence of electrolytic differentiation in
+the rocks. Second, the outer part of the earth's crust is a
+very poor conductor so that it is doubtful whether even
+a high degree of electrification of the surface would have
+much effect on the interior. Third, electrolysis due to any
+<!--File: cc318.png-->
+<a class="pagenum break" id="Page_294" title="pg 294"></a>
+such mild causes as we have here postulated must be an
+extremely slow process, too slow, presumably, to have
+any appreciable result within a month or two. Other
+objections join with these three in making it seem improbable
+that the sun's electrical activity has any direct
+effect upon movements of the earth's crust.</p>
+
+<p>The third, or meteorological hypothesis, which makes
+barometric pressure the main intermediary between solar
+activity and earthquakes, seems at first sight almost as
+improbable as the thermal and electrical hypotheses.
+Nevertheless, it has a certain degree of observational
+support of a kind which is wholly lacking in the other two
+cases. Among the extensive writings on the periodicity of
+earthquakes one main fact stands out with great distinctness:
+earthquakes vary in number according to the
+season. This fact has already been shown incidentally in
+the table of earthquake frequency by months. If allowance
+is made for the fact that February is a short month,
+there is a regular decrease in the frequency of severe
+earthquakes from December and January to June. Since
+most of Milne's earthquakes occurred in the northern
+hemisphere, this means that severe earthquakes occur in
+winter about 20 per cent oftener than in summer.</p>
+
+<!--File: cc319.png-->
+<a class="pagenum break" id="Page_295" title="pg 295"></a>
+<!--File: cc320.png-->
+<table id="table_8" class="borders">
+<tr><th colspan="7"><a href="#tables">TABLE 8</a></th></tr>
+<tr><td colspan='7'>&nbsp;</td></tr>
+<tr><th colspan="7">SEASONAL MARCH OF EARTHQUAKES</th></tr>
+
+<tr><th colspan="7"><small>AFTER DAVISSON AND KNOTT</small></th></tr>
+<tr><td colspan='7'>&nbsp;</td></tr>
+<tr>
+<th><em>A</em></th>
+<th><em>B</em></th>
+<th><em>C</em></th>
+<th><em>D</em></th>
+<th><em>E</em></th>
+<th><em>F</em></th>
+<th><em>G</em></th>
+</tr>
+<tr>
+<th><em>Region</em></th>
+<th><em>Limiting Dates</em></th>
+<th><em>Number of Shocks</em></th>
+<th><em>Maximum Month</em></th>
+<th><em>Amplitude</em></th>
+<th><em>Expected Amplitude</em></th>
+<th><em>Ratio of<br />  Actual to<br />  Expected Amplitude</em></th>
+</tr>
+
+<tr>
+<td class="tdl">Northern Hemisphere</td>
+<td>223-1850</td>
+<td>5879</td>
+<td>Dec.</td>
+<td>0.110</td>
+<td>0.023</td>
+<td>4.8</td>
+</tr>
+
+<tr>
+<td class="tdl">Northern Hemisphere</td>
+<td>1865-1884</td>
+<td>8133</td>
+<td>Dec.</td>
+<td>0.290</td>
+<td>0.020</td>
+<td>14.5</td>
+</tr>
+<tr>
+<td class="tdl">Europe </td>
+<td>1865-1884</td>
+<td>5499</td>
+<td>Dec.</td>
+<td>0.350</td>
+<td>0.024</td>
+<td>14.6</td>
+</tr>
+<tr>
+<td class="tdl">Europe</td>
+<td>306-1843</td>
+<td>1961</td>
+<td>Dec.</td>
+<td>0.220</td>
+<td>0.040</td>
+<td>5.5</td>
+</tr>
+<tr>
+<td class="tdl">Southeast Europe</td>
+<td>1859-1887</td>
+<td>3470</td>
+<td>Dec.</td>
+<td>0.210</td>
+<td>0.030</td>
+<td>7.0</td>
+</tr>
+<tr>
+<td class="tdl">Vesuvius District</td>
+<td>1865-1883</td>
+<td>513</td>
+<td>Dec.</td>
+<td>0.250</td>
+<td>0.078 </td>
+<td>3.2</td>
+</tr>
+<tr>
+<td class="tdl" colspan="7">Italy:</td>
+</tr>
+<tr>
+  <td class="tdi2">Old Tromometre</td>
+  <td>1872-1887</td>
+  <td>61732</td>
+  <td>Dec.</td>
+  <td>0.490</td>
+  <td>0.007</td>
+  <td>70.0</td>
+  </tr>
+<tr>
+  <td class="tdi2">Old Tromometre</td>
+  <td>1876-1887</td>
+  <td>38546</td>
+  <td>Dec.</td>
+  <td>0.460</td>
+  <td>0.009</td>
+  <td>49.5</td>
+</tr>
+<tr>
+  <td class="tdi2">Normal Tromometre</td>
+  <td>1876-1887</td>
+  <td>38546</td>
+  <td>Dec.</td>
+  <td>0.490</td>
+  <td>0.009</td>
+  <td>52.8</td>
+</tr>
+<tr>
+<td class="tdl">Balkan, etc.</td>
+<td>1865-1884</td>
+<td>624</td>
+<td>Dec.</td>
+<td>0.270</td>
+<td>0.071</td>
+<td>3.8</td>
+</tr>
+<tr>
+<td class="tdl">Hungary, etc.</td>
+<td>1865-1884</td>
+<td>384</td>
+<td>Dec.</td>
+<td>0.310</td>
+<td>0.090</td>
+<td>3.4</td>
+</tr>
+<tr>
+<td class="tdl">Italy</td>
+<td>1865-1883</td>
+<td>2350</td>
+<td>Dec.(Sept.)</td>
+<td>0.140</td>
+<td>0.037</td>
+<td>3.8</td>
+</tr>
+<tr>
+<td class="tdl">Grecian Archip.</td>
+<td>1859-1881</td>
+<td>3578</td>
+<td>Dec.-Jan.</td>
+<td>0.164</td>
+<td>0.030</td>
+<td>5.5</td>
+</tr>
+<tr>
+<td class="tdl">Austria</td>
+<td>1865-1884</td>
+<td>461</td>
+<td>Jan.</td>
+<td>0.370</td>
+<td>0.083</td>
+<td>4.4</td>
+</tr>
+<tr>
+<td class="tdl">Switzerland, etc.</td>
+<td>1865-1883</td>
+<td>524</td>
+<td>Jan.</td>
+<td>0.560</td>
+<td>0.077</td>
+<td>7.3</td>
+</tr>
+<tr>
+<td class="tdl">Asia</td>
+<td>1865-1884</td>
+<td>458</td>
+<td>Feb.</td>
+<td>0.330</td>
+<td>0.083</td>
+<td>4.0</td>
+</tr>
+<tr>
+<td class="tdl">North America</td>
+<td>1865-1884</td>
+<td>552</td>
+<td>Nov.</td>
+<td>0.350</td>
+<td>0.075</td>
+<td>4.7</td>
+</tr>
+<tr>
+<td class="tdl">California</td>
+<td>1850-1886</td>
+<td>949</td>
+<td>Oct.</td>
+<td>0.300</td>
+<td>0.058</td>
+<td>5.2</td>
+</tr>
+<tr>
+<td class="tdl">Japan</td>
+<td>1878-1881</td>
+<td>246</td>
+<td>Dec.</td>
+<td>0.460</td>
+<td>0.113</td>
+<td>4.1</td>
+</tr>
+<tr>
+<td class="tdl">Japan</td>
+<td>1872-1880</td>
+<td>367</td>
+<td>Dec.-Jan.</td>
+<td>0.256</td>
+<td>0.093</td>
+<td>2.8</td>
+</tr>
+<tr>
+<td class="tdl">Japan</td>
+<td>1876-1891</td>
+<td>1104</td>
+<td>Feb.</td>
+<td>0.190</td>
+<td>0.053</td>
+<td>3.6</td>
+</tr>
+<tr>
+<td class="tdl">Japan</td>
+<td>1885-1889</td>
+<td>2997</td>
+<td>Oct.</td>
+<td>0.080</td>
+<td>0.032</td>
+<td>2.5</td>
+</tr>
+<tr>
+<td class="tdl">Zante</td>
+<td>1825-1863</td>
+<td>1326</td>
+<td>Aug.</td>
+<td>0.100</td>
+<td>0.049</td>
+<td>2.0</td>
+</tr>
+<tr>
+<td class="tdl">Italy, North of Naples</td>
+<td>1865-1883</td>
+<td>1513</td>
+<td>Sept.(Nov.)</td>
+<td>0.210</td>
+<td>0.046</td>
+<td>4.6</td>
+</tr>
+<tr>
+<td class="tdl">East Indies </td>
+<td>1873-1881</td>
+<td>515</td>
+<td>Aug., Oct., or Dec.?</td>
+<td>0.071?</td>
+<td>0.078 </td>
+<td>0.9</td>
+</tr>
+ <tr>
+<td class="tdl">Malay Archip.</td>
+<td>1865-1884</td>
+<td>598</td>
+<td>May</td>
+<td>0.190</td>
+<td>0.072</td>
+<td>2.6</td>
+</tr>
+<tr>
+<td class="tdl">New Zealand</td>
+<td>1869-1879</td>
+<td>585</td>
+<td>Aug.-Sept.</td>
+<td>0.203</td>
+<td>0.073</td>
+<td>2.8</td>
+</tr>
+<tr>
+<td class="tdl">Chile</td>
+<td>1873-1881</td>
+<td>212</td>
+<td>July</td>
+<td>0.480</td>
+<td>0.122</td>
+<td>3.9</td>
+</tr>
+<tr>
+<td class="tdl">Southern Hemisphere</td>
+<td>1865-1884</td>
+<td>751</td>
+<td>July</td>
+<td>0.370</td>
+<td>0.065</td>
+<td>5.7</td>
+</tr>
+<tr>
+<td class="tdl">New Zealand</td>
+<td>1868-1890</td>
+<td>641</td>
+<td>March, May</td>
+<td>0.050</td>
+<td>0.070</td>
+<td>0.7</td>
+</tr>
+<tr>
+<td class="tdl">Chile</td>
+<td>1865-1883?</td>
+<td>316</td>
+<td>July, Dec.</td>
+<td>0.270</td>
+<td>0.100</td>
+<td>2.7</td>
+</tr>
+<tr>
+<td class="tdl">Peru, Bolivia</td>
+<td>1865-1884</td>
+<td>350</td>
+<td>July</td>
+<td>0.480</td>
+<td>0.095</td>
+<td>5.1</td>
+</tr>
+</table>
+<p>The most thorough investigation of this subject seems
+to have been that of Davisson.<sup><a href="#fn132" id="r132">[132]</a></sup> His results have been
+worked over and amplified by Knott,<sup><a href="#fn133" id="r133">[133]</a></sup> who has tested
+them by Schuster's exact mathematical methods. His results
+are given in Table 8.<sup><a href="#fn134" id="r134">[134]</a></sup> Here the northern hemisphere
+<a class="pagenum break" id="Page_296" title="pg 296"></a>
+is placed first; then come the East Indies and the Malay
+Archipelago lying close to the equator; and finally the
+southern hemisphere. In the northern hemisphere practically
+all the maxima come in the winter, for the month
+of December appears in fifteen cases out of the twenty-five
+in column D, while January, February, or November
+appears in six others. It is also noticeable that in sixteen
+cases out of twenty-five the ratio of the actual to the expected
+amplitude in column G is four or more, so that a
+real relationship is indicated, while the ratio falls below
+three only in Japan and Zante. The equatorial data,
+unlike those of the northern hemisphere, are indefinite,
+for in the East Indies no month shows a marked maximum
+and the expected amplitude exceeds the actual amplitude.
+Even in the Malay Archipelago, which shows a
+maximum in May, the ratio of actual to expected amplitude
+is only 2.6. Turning to the southern hemisphere, the
+winter months of that hemisphere are as strongly marked
+by a maximum as are the winter months of the northern
+<!--File: cc321.png-->
+<a class="pagenum break" id="Page_297" title="pg 297"></a>
+hemisphere. July or August appears in five out of six
+cases. Here the ratio between the actual and expected
+amplitudes is not so great as in the northern hemisphere.
+Nevertheless, it is practically four in Chile, and exceeds
+five in Peru and Bolivia, and in the data for the entire
+southern hemisphere.</p>
+
+
+
+<p>The whole relationship between earthquakes and the
+seasons in the northern and southern hemispheres is
+summed up in Fig. 12 taken from Knott. The northern
+hemisphere shows a regular diminution in earthquake
+frequency from December until June, and an increase
+the rest of the year. In the southern hemisphere the
+course of events is the same so far as summer and winter
+are concerned, for August with its maximum comes in
+winter, while February with its minimum comes in
+summer. In the southern hemisphere the winter month
+of greatest seismic activity has over 100 per cent more
+earthquakes than the summer month of least activity. In
+the northern hemisphere this difference is about 80 per
+cent, but this smaller figure occurs partly because the
+northern data include certain interesting and significant
+regions like Japan and China where the usual conditions
+are reversed.<sup><a href="#fn135" id="r135">[135]</a></sup> If equatorial regions were included
+in Fig. 12, they would give an almost straight line.</p>
+
+<p>The connection between earthquakes and the seasons is
+so strong that almost no students of seismology question
+it, although they do not agree as to its cause. A meteorological
+hypothesis seems to be the only logical explanation.<sup><a href="#fn136" id="r136">[136]</a></sup>
+Wherever sufficient data are available, earthquakes
+<!--File: cc322.png-->
+<a class="pagenum break" id="Page_298" title="pg 298"></a>
+appear to be most numerous when climatic conditions
+cause the earth's surface to be most heavily loaded or to
+change its load most rapidly. The main factor in the
+loading is apparently atmospheric pressure. This acts in
+two ways. First, when the continents become cold in
+winter the pressure increases. On an average the air
+at sea level presses upon the earth's surface at the rate
+of 14.7 pounds per square inch, or over a ton per square
+foot, and only a little short of thirty million tons per
+square mile. An average difference of one inch between
+the atmospheric pressure of summer and winter over ten
+million square miles of the continent of Asia, for example,
+means that the continent's load in winter is about
+ten million million tons heavier than in summer. Second,
+the changes in atmospheric pressure due to the passage
+of storms are relatively sharp and sudden. Hence they
+are probably more effective than the variations in the
+load from season to season. This is suggested by the
+rapidity with which the terrestrial response seems to
+follow the supposed solar cause of earthquakes. It is also
+suggested by the fact that violent storms are frequently
+followed by violent earthquakes. "Earthquake weather,"
+as Dr. Schlesinger suggests, is a common phrase in the
+typhoon region of Japan, China, and the East Indies.
+During tropical hurricanes a change of pressure amounting
+to half an inch in two hours is common. On September
+<!--File: cc323.png-->
+<a class="pagenum break" id="Page_299" title="pg 299"></a>
+22, 1885, at False Point Lighthouse on the Bay of
+Bengal, the barometer fell about an inch in six hours,
+then nearly an inch and a half in not much over two
+hours, and finally rose fully two inches inside of two
+hours. A drop of two inches in barometric pressure
+means that a load of about two million tons is removed
+
+<!--File: cc324.png-->
+<a class="pagenum break" id="Page_300" title="pg 300"></a>
+from each square mile of land; the corresponding rise of
+pressure means the addition of a similar load. Such a
+storm, and to a less degree every other storm, strikes a
+blow upon the earth's surface, first by removing millions
+of tons of pressure and then by putting them on again.<sup><a href="#fn137" id="r137">[137]</a></sup>
+Such storms, as we have seen, are much more frequent
+and severe when sunspots are numerous than at other
+times. Moreover, as Veeder<sup><a href="#fn138" id="r138">[138]</a></sup> long ago showed, one of the
+most noteworthy evidences of a connection between sunspots
+and the weather is a sudden increase of pressure in
+certain widely separated high pressure areas. In most
+parts of the world winter is not only the season of
+highest pressure and of most frequent changes of
+Veeder's type, but also of severest storms. Hence a
+meteorological hypothesis would lead to the expectation
+that earthquakes would occur more frequently in winter
+than in summer. On the Chinese coast, however, and also
+on the oceanic side of Japan, as well as in some more
+tropical regions, the chief storms come in summer in the
+form of typhoons. These are the places where earthquakes
+also are most abundant in summer. Thus, wherever
+we turn, storms and the related barometric changes
+seem to be most frequent and severe at the very times
+when earthquakes are also most frequent.</p>
+
+<img src="images/fig12.png" width="715" height="867" alt="Fig. 12"  id="Fig_12" />
+<p class="center"><em>Fig. 12. Seasonal distribution of earthquakes.
+(After Davisson and Knott.)</em><br />
+
+&mdash;&mdash;&nbsp; Northern Hemisphere.<br />
+- - - - Southern Hemisphere.</p>
+
+<p>Other meteorological factors, such as rain, snow,
+winds, and currents, probably have some effect on earthquakes
+<!--File: cc325.png-->
+<a class="pagenum break" id="Page_301" title="pg 301"></a>
+through their ability to load the earth's crust.
+The coming of vegetation may also help. These agencies,
+however, appear to be of small importance compared
+with the storms. In high latitudes and in regions of
+abundant storminess most of these factors generally
+combine with barometric pressure to produce frequent
+changes in the load of the earth's crust, especially in
+winter. In low latitudes, on the other hand, there are few
+severe storms, and relatively little contrast in pressure
+and vegetation from season to season; there is no snow;
+and the amount of ground water changes little. With this
+goes the twofold fact that there is no marked seasonal
+distribution of earthquakes, and that except in certain
+local volcanic areas, earthquakes appear to be rare. In
+proportion to the areas concerned, for example, there is
+little evidence of earthquakes in equatorial Africa and
+South America.</p>
+
+<p>The question of the reality of the connection between
+meteorological conditions and crustal movements is so
+important that every possible test should be applied. At
+the suggestion of Professor Schlesinger we have looked
+up a very ingenious line of inquiry. During the last
+decades of the nineteenth century, a long series of extremely
+accurate observations of latitude disclosed a fact
+which had previously been suspected but not demonstrated,
+namely, that the earth wabbles a little about its
+axis. The axis itself always points in the same direction,
+and since the earth slides irregularly around it the latitude
+of all parts of the earth keeps changing. Chandler
+has shown that the wabbling thus induced consists of
+two parts. The first is a movement in a circle with a
+radius of about fifteen feet which is described in approximately
+430 days. This so-called Eulerian movement is a
+normal gyroscopic motion like the slow gyration of a
+<!--File: cc326.png-->
+<a class="pagenum break" id="Page_302" title="pg 302"></a>
+spinning top. This depends on purely astronomical
+causes, and no terrestrial cause can stop it or eliminate
+it. The period appears to be constant, but there are certain
+puzzling irregularities. The usual amplitude of this
+movement, as Schlesinger<sup><a href="#fn139" id="r139">[139]</a></sup> puts it, "is about 0".27, but
+twice in recent years it has jumped to 0".40. Such a
+change could be accounted for by supposing that the
+earth had received a severe blow or a series of milder
+blows tending in the same direction." These blows, which
+were originally suggested by Helmert are most interesting
+in view of our suggestion as to the blows struck by
+storms.</p>
+
+<p>The second movement of the pole has a period of a
+year, and is roughly an ellipse whose longest radius is
+fourteen feet and the shortest, four feet; or, to put it
+technically, there is an annual term with a maximum
+amplitude of about 0".20. This, however, varies irregularly.
+The result is that the pole seems to wander over
+the earth's surface in the spiral fashion illustrated in
+Fig. 13. It was early suggested that this peculiar wandering
+of the pole in an annual period must be due to
+meteorological causes. Jeffreys<sup><a href="#fn140" id="r140">[140]</a></sup> has investigated the
+matter exhaustively. He assumes certain reasonable
+values for the weight of air added or subtracted from
+different parts of the earth's surface according to the
+seasons. He also considers the effect of precipitation,
+vegetation, and polar ice, and of variations of temperature
+and atmospheric pressure in their relation to movements
+of the ocean. Then he proceeds to compare all
+<!--File: cc327.png-->
+<a class="pagenum break" id="Page_303" title="pg 303"></a>
+
+these with the actual wandering of the pole from 1907
+to 1913. While it is as yet too early to say that any
+special movement of the pole was due to the specific
+meteorological conditions of any particular year,
+Jeffreys' work makes it clear that meteorological causes,
+especially atmospheric pressure, are sufficient to cause
+the observed irregular wanderings. Slight wanderings
+may arise from various other sources such as movements
+of the rocks when geological faults occur or the rush of
+a great wave due to a submarine earthquake. So far as
+known, however, all these other agencies cause insignificant
+displacements compared with those arising from
+movements of the air. This fact coupled with the mathematical
+certainty that meteorological phenomena must
+produce some wandering of the pole, has caused most
+astronomers to accept Jeffreys' conclusion. If we follow
+their example we are led to conclude that changes in
+atmospheric pressure and in the other meteorological
+conditions strike blows which sometimes shift the earth
+<!--File: cc328.png-->
+<a class="pagenum break" id="Page_304" title="pg 304"></a>
+several feet from its normal position in respect to the
+axis.</p>
+
+<img src="images/fig13.png" width="443" height="402" alt="Fig. 13"  id="Fig_13" />
+<p class="center"><em>Fig. 13. Wandering of the
+pole from 1890 to 1898.</em>
+(<em>After Moulton.</em>)</p>
+
+<p>If the foregoing reasoning is correct, the great and
+especially the sudden departures from the smooth
+gyroscopic circle described by the pole in the Eulerian
+motion would be expected to occur at about the same time
+as unusual earthquake activity. This brings us to an
+interesting inquiry carried out by Milne<sup><a href="#fn141" id="r141">[141]</a></sup> and amplified
+by Knott.<sup><a href="#fn142" id="r142">[142]</a></sup> Taking Albrecht's representation of the
+irregular spiral-like motion of the pole, as given in Fig.
+13, they show that there is a preponderance of severe
+earthquakes at times when the direction of motion of the
+earth in reference to its axis departs from the smooth
+Eulerian curve. A summary of their results is given in
+Table 9. The table indicates that during the period from
+1892 to 1905 there were nine different times when the
+curve of Fig. 13 changed its direction or was deflected by
+less than 10&deg; during a tenth of a year. In other words,
+during those periods it did not curve as much as it ought
+according to the Eulerian movement. At such times there
+were 179 world-shaking earthquakes, or an average of
+about 19.9 per tenth of a year. According to the other
+lines of Table 9, in thirty-two cases the deflection during
+a tenth of a year was between 10&deg; and 25&deg;, while in fifty-six
+cases it was from 25&deg; to 40&deg;. During these periods
+the curve remained close to the Eulerian path and the
+world-shaking earthquakes averaged only 8.2 and 12.9.
+Then, when the deflection was high, that is, when meteorological
+conditions threw the earth far out of its Eulerian
+course, the earthquakes were again numerous, the
+number rising to 23.4 when the deflection amounted to
+more than 55&deg;.</p>
+
+<!--File: cc329.png-->
+<a class="pagenum break" id="Page_305" title="pg 305"></a>
+
+<table id="table_9" class="borders2">
+<tr><th colspan="4"><a href="#tables">TABLE 9</a></th></tr>
+<tr><td colspan='4'>&nbsp;</td></tr>
+<tr><th colspan="4">DEFLECTION OF PATH OF POLE COMPARED WITH EARTHQUAKES</th></tr>
+<tr><td colspan='4'>&nbsp;</td></tr>
+<tr>
+<th><em>Deflection</em></th>
+<th><em>No. of Deflections</em></th>
+<th><em>No. of Earthquakes</em></th>
+<th><em>Average No. of Earthquakes</em></th>
+</tr>
+<tr>
+<td>0-10&deg;</td>
+<td>9</td>
+<td>179</td>
+<td>19.9</td>
+</tr>
+<tr>
+<td>10-25&deg; </td>
+<td>32</td>
+<td>263 </td>
+<td>8.2</td>
+</tr>
+<tr>
+<td>25-40&deg;</td>
+<td>56</td>
+<td>722</td>
+<td>12.9</td>
+</tr>
+<tr>
+<td>40-55&deg;</td>
+<td>19</td>
+<td>366</td>
+<td>19.3</td>
+</tr>
+<tr>
+<td>over 55&deg;</td>
+<td>7</td>
+<td>164</td>
+<td>23.4</td>
+</tr>
+</table>
+
+<p>In order to test this conclusion in another way we have
+followed a suggestion of Professor Schlesinger. Under
+his advice the Eulerian motion has been eliminated and
+a new series of earthquake records has been compared
+with the remaining motions of the poles which presumably
+arise largely from meteorological causes. For this
+purpose use has been made of the very full records of
+earthquakes published under the auspices of the International
+Seismological Commission for the years 1903
+to 1908, the only years for which they are available.
+These include every known shock of every description
+which was either recorded by seismographs or by direct
+observation in any part of the world. Each shock is given
+the same weight, no matter what its violence or how
+closely it follows another. The angle of deflection has
+been measured as Milne measured it, but since the Eulerian
+motion is eliminated, our zero is approximately
+the normal condition which would prevail if there were
+no meteorological complications. Dividing the deflections
+into six equal groups according to the size of the angle,
+we get the result shown in Table 10.</p>
+<!--File: cc330.png Page 306-->
+<a class="pagenum break" id="Page_306" title="pg 306"></a>
+
+<table id="table_10" class="borders2">
+<tr><th colspan="2"><a href="#tables">TABLE 10</a></th></tr>
+<tr><td colspan='2'>&nbsp;</td></tr>
+<tr><th colspan="2">EARTHQUAKES IN 1903-1908 COMPARED
+WITH DEPARTURES OF THE PROJECTED
+CURVE OF THE EARTH'S
+AXIS FROM THE EULERIAN
+POSITION</th>
+</tr>
+<tr><td colspan='2'>&nbsp;</td></tr>
+<tr>
+<th><em>Average angle of deflection<br />(10 periods of 1/10 year each)</em> </th>
+<th><em>Average daily number<br />of earthquakes</em></th>
+</tr>
+<tr>
+<td>-10.5&deg;</td>
+<td>8.31</td>
+</tr>
+<tr>
+<td> 11.5&deg;</td>
+<td> 8.35</td>
+</tr>
+<tr>
+<td>25.8&deg;</td>
+<td>8.23</td>
+</tr>
+<tr>
+<td>40.2&deg;</td>
+<td> 8.14</td>
+</tr>
+<tr>
+<td> 54.7&deg;  </td>
+<td>8.86</td>
+</tr>
+<tr>
+<td>90.3&deg;</td>
+<td>11.81</td>
+</tr>
+</table>
+
+<p>Here where some twenty thousand earthquakes are
+employed the result agrees closely with that of Milne for
+a different series of years and for a much smaller number
+of earthquakes. So long as the path of the pole departs
+less than about 45&deg; from the smooth gyroscopic Eulerian
+path, the number of earthquakes is almost constant, about
+eight and a quarter per day. When the angle becomes
+large, however, the number increases by nearly 50 per
+cent. Thus the work of Milne, Knott, and Jeffreys is confirmed
+by a new investigation. Apparently earthquakes
+and crustal movements are somehow related to sudden
+changes in the load imposed on the earth's crust by
+meteorological conditions.</p>
+
+<p>This conclusion is quite as surprising to the authors
+as to the reader&mdash;perhaps more so. At the beginning of
+this investigation we had no faith whatever in any important
+<!--File: cc331.png Page 307-->
+<a class="pagenum break" id="Page_307" title="pg 307"></a>
+relation between climate and earthquakes. At its
+end we are inclined to believe that the relation is close
+and important.</p>
+
+<p>It must not be supposed, however, that meteorological
+conditions are the <em>cause</em> of earthquakes and of movements
+of the earth's crust. Even though the load that the
+climatic agencies can impose upon the earth's crust runs
+into millions of tons per square mile, it is a trifle compared
+with what the crust is able to support. There is,
+however, a great difference between the cause and the
+occasion of a phenomenon. Suppose that a thick sheet of
+glass is placed under an increasing strain. If the strain
+is applied slowly enough, even so rigid a material as glass
+will ultimately bend rather than break. But suppose that
+while the tension is high the glass is tapped. A gentle
+tap may be followed by a tiny crack. A series of little
+taps may be the signal for small cracks to spread in
+every direction. A few slightly harder taps may cause
+the whole sheet to break suddenly into many pieces. Yet
+even the hardest tap may be the merest trifle compared
+with the strong force which is keeping the glass in a state
+of strain and which would ultimately bend it if given
+time.</p>
+
+<p>The earth as a whole appears to stand between steel
+and glass in rigidity. It is a matter of common observation
+that rocks stand high in this respect and in the
+consequent difficulty with which they can be bent without
+breaking. Because of the earth's contraction the crust
+endures a constant strain, which must gradually become
+enormous. This strain is increased by the fact that sediment
+is transferred from the lands to the borders of the
+sea and there forms areas of thick accumulation. From
+this has arisen the doctrine of isostasy, or of the equalization
+of crustal pressure. An important illustration of
+<!--File: cc332.png-->
+<a class="pagenum break" id="Page_308" title="pg 308"></a>
+this is the oceanward and equatorial creep which has
+been described in Chapter XI. There we saw that when
+the lands have once been raised to high levels or when a
+shortening of the earth's axis by contraction has increased
+the oceanic bulge at the equator, or when the
+reverse has happened because of tidal retardation, the
+outer part of the earth appears to creep slowly back
+toward a position of perfect isostatic adjustment. If the
+sun had no influence upon the earth, either direct or
+indirect, isostasy and other terrestrial processes might
+flex the earth's crust so gradually that changes in the
+form and height of the lands would always take place
+slowly, even from the geological point of view. Thus
+erosion would usually be able to remove the rocks as
+rapidly as they were domed above the general level. If
+this happened, mountains would be rare or unknown, and
+hence climatic contrasts would be far less marked than is
+actually the case on our earth where crustal movements
+have repeatedly been rapid enough to produce mountains.</p>
+
+<p>Nature's methods rarely allow so gradual an adjustment
+to the forces of isostasy. While the crust is under a
+strain, not only because of contraction, but because of
+changes in its load through the transference of sediments
+and the slow increase or decrease in the bulge at the
+equator, the atmosphere more or less persistently carries
+on the tapping process. The violence of that process
+varies greatly, and the variations depend largely on the
+severity of the climatic contrasts. If the main outlines of
+the cyclonic hypothesis are reliable, one of the first effects
+of a disturbance of the sun's atmosphere is increased
+storminess upon the earth. This is accompanied by increased
+intensity in almost every meteorological process.
+The most important effect, however, so far as the earth's
+crust is concerned would apparently be the rapid and
+<!--File: cc333.png Page 309-->
+<a class="pagenum break" id="Page_309" title="pg 309"></a>
+intense changes of atmospheric pressure which would
+arise from the swift passage of one severe storm after
+another. Each storm would be a little tap on the tensely
+strained crust. Any single tap might be of little consequence,
+even though it involved a change of a billion
+tons in the pressure on an area no larger than the state
+of Rhode Island. Yet a rapid and irregular succession of
+such taps might possibly cause the crust to crack, and
+finally to collapse in response to stresses arising from
+the shrinkage of the earth.</p>
+
+<p>Another and perhaps more important effect of variations
+in storminess and especially in the location of the
+stormy areas would be an acceleration of erosion in some
+places and a retardation elsewhere. A great increase in
+rainfall may almost denude the slopes of soil, while a
+diminution to the point where much of the vegetation
+dies off has a similar effect. If such changes should take
+place rapidly, great thicknesses of sediment might be
+concentrated in certain areas in a short time, thus disturbing
+the isostatic adjustment of the earth's crust. This
+might set up a state of strain which would ultimately
+have to be relieved, thus perhaps initiating profound
+crustal movements. Changes in the load of the earth's
+crust due to erosion and the deposition of sediment, no
+matter how rapid they may be from the geological standpoint,
+are slow compared with those due to changes in
+barometric pressure. A drop of an inch in barometric
+pressure is equivalent to the removal of about five inches
+of solid rock. Even under the most favorable circumstances,
+the removal of an average depth of five inches
+of rock or its equivalent in soil over millions of square
+miles would probably take several hundred years, while
+the removal of a similar load of air might occur in half
+a day or even a few hours. Thus the erosion and deposition
+<!--File: cc334.png Page 310-->
+<a class="pagenum break" id="Page_310" title="pg 310"></a>
+due to climatic variations presumably play their
+part in crustal deformation chiefly by producing crustal
+stresses, while the storms, as it were, strike sharp, sudden
+blows.</p>
+
+<p>Suppose now that a prolonged period of world-wide
+mild climate, such as is described in Chapter X, should
+permit an enormous accumulation of stresses due to contraction
+and tidal retardation. Suppose that then a
+sudden change of climate should produce a rapid shifting
+of the deep soil that had accumulated on the lands, with a
+corresponding localization and increase in strains. Suppose
+also that frequent and severe storms play their part,
+whether great or small, by producing an intensive tapping
+of the crust. In such a case the ultimate collapse would
+be correspondingly great, as would be evident in the
+succeeding geological epoch. The sea floor might sink
+lower, the continents might be elevated, and mountain
+ranges might be shoved up along lines of special weakness.
+This is the story of the geological period as known
+to historical geology. The force that causes such movements
+would be the pull of gravity upon the crust surrounding
+the earth's shrinking interior. Nevertheless
+climatic changes might occasionally set the date when the
+gravitative pull would finally overcome inertia, and thus
+usher in the crustal movements that close old geologic
+periods and inaugurate new ones. This, however, could
+occur only if the crust were under sufficient strain. As
+Lawson<sup><a href="#fn143" id="r143">[143]</a></sup> says in his discussion of the "elastic rebound
+theory," the sudden shifts of the crust which seem to be
+the underlying cause of earthquakes "can occur only
+after the accumulation of strain to a limit and ... this
+accumulation involves a slow creep of the region affected.
+<!--File: cc335.png Page 311-->
+<a class="pagenum break" id="Page_311" title="pg 311"></a>
+In the long periods between great earthquakes the energy
+necessary for such shocks is being stored up in the rocks
+as elastic compression."</p>
+
+<p>If a period of intense storminess should occur when
+the earth as a whole was in such a state of strain, the
+sudden release of the strains might lead to terrestrial
+changes which would alter the climate still further, making
+it more extreme, and perhaps permitting the storminess
+due to the solar disturbances to bring about glaciation.
+At the same time if volcanic activity should
+increase it would add its quota to the tendency toward
+glaciation. Nevertheless, it might easily happen that a
+very considerable amount of crustal movement would
+take place without causing a continental ice sheet or even
+a marked alpine ice sheet. Or again, if the strains in the
+earth's crust had already been largely released through
+other agencies before the stormy period began, the climate
+might become severe enough to cause glaciation
+in high latitudes without leading to any very marked
+movements of the earth's crust, as apparently happened
+in the Mid-Silurian period.</p>
+
+
+<h2 id="conclusion"><a href="#content">CONCLUSION</a></h2>
+
+<p>Here we must bring this study of the earth's evolution
+to a close. Its fundamental principle has been that the
+present, if rightly understood, affords a full key to the
+past. With this as a guide we have touched on many
+hypotheses, some essential and some unessential to the
+general line of thought. The first main hypothesis is that
+the earth's present climatic variations are correlated
+with changes in the solar atmosphere. This is the keynote
+of the whole book. It is so well established, however,
+<!--File: cc336.pngPage 312-->
+<a class="pagenum break" id="Page_312" title="pg 312"></a>
+that it ranks as a theory rather than as an hypothesis.
+Next comes the hypothesis that variations in the solar
+atmosphere influence the earth's climate chiefly by causing
+variations not only in temperature but also in
+atmospheric pressure and thus in storminess, wind, and
+rainfall. This, too, is one of the essential foundations on
+which the rest of the book is built, but though this
+cyclonic hypothesis is still a matter of discussion, it
+seems to be based on strong evidence. These two hypotheses
+might lead us astray were they not balanced by
+another. This other is that many climatic conditions are
+due to purely terrestrial causes, such as the form and
+altitude of the lands, the degree to which the continents
+are united, the movement of ocean currents, the activity
+of volcanoes, and the composition of the atmosphere and
+the ocean. Only by combining the solar and the terrestrial
+can the truth be perceived. Finally, the last main hypothesis
+of this book holds that if the climatic conditions which
+now prevail at times of solar activity were magnified
+sufficiently and if they occurred in conjunction with certain
+important terrestrial conditions of which there is
+good evidence, they would produce most of the notable
+phenomena of glacial periods. For example, they would
+explain such puzzling conditions as the localization and
+periodicity of glaciation, the formation of loess, and the
+occurrence of glaciation in low latitudes during Permian
+and Proterozoic times. The converse of this is that if
+the conditions which now prevail at times when the sun
+is relatively inactive should be intensified, that is, if
+the sun's atmosphere should become calmer than now,
+and if the proper terrestrial conditions of topographic
+form and atmospheric composition should prevail, there
+would arise the mild climatic conditions which appear
+to have prevailed during the greater part of geological
+<!--File: cc337.png Page 313-->
+<a class="pagenum break" id="Page_313" title="pg 313"></a>
+time. In short, there seems thus far to be no phase of
+the climate of the past which is not in harmony with an
+hypothesis which combines into a single unit the three
+main hypotheses of this book, solar, cyclonic, and terrestrial.</p>
+
+<p>Outside the main line of thought lie several other
+hypotheses. Several of these, as well as some of the main
+hypotheses, are discussed chiefly in <em>Earth and Sun</em>, but
+as they are given a practical application in this book
+they deserve a place in this final summary. Each of these
+secondary hypotheses is in its way important. Yet any or
+all may prove untrue without altering our main conclusions.
+This point cannot be too strongly emphasized, for
+there is always danger that differences of opinion as to
+minor hypotheses and even as to details may divert attention
+from the main point. Among the non-essential
+hypotheses is the idea that the sun's atmosphere influences
+that of the earth electrically as well as thermally.
+This idea is still so new that it has only just entered the
+stage of active discussion, and naturally the weight of
+opinion is against it. Although not necessary to the main
+purpose of this book, it plays a minor r&ocirc;le in the chapter
+dealing with the relation of the sun to other astronomical
+bodies. It also has a vital bearing on the further advance
+of the science of meteorology and the art of weather
+forecasting. Another secondary hypothesis holds that
+sunspots are set in motion by the planets. Whether the
+effect is gravitational or more probably electrical, or
+perhaps of some other sort, does not concern us at present,
+although the weight of evidence seems to point
+toward electronic emissions. This question, like that of
+the relative parts played by heat and electricity in terrestrial
+climatic changes, can be set aside for the moment.
+What does concern us is a third hypothesis, namely, that
+<!--File: cc338.png-->
+<a class="pagenum break" id="Page_314" title="pg 314"></a>
+if the planets really determine the periodicity of sunspots,
+even though not supplying the energy, the sun in
+its flight through space must have been repeatedly and
+more strongly influenced in the same way by many other
+heavenly bodies. In that case, climatic changes like those
+of the present, but sometimes greatly magnified, have presumably
+arisen because of the constantly changing position
+of the solar system in respect to other parts of the
+universe. Finally, the fourth of our secondary hypotheses
+postulates that at present the date of movements of the
+earth's crust is often determined by the fact that storms
+and other meteorological conditions keep changing the
+load upon first one part of the earth's surface and then
+upon another. Thus stresses that have accumulated in the
+earth's isostatic shell during the preceding months are
+released. In somewhat the same way epochs of extreme
+storminess and rapid erosion in the past may possibly
+have set the date for great movements of the earth's
+crust. This hypothesis, like the other three in our secondary
+or non-essential group, is still so new that only the
+first steps have been taken in testing it. Yet it seems to
+deserve careful study.</p>
+
+<p>In testing all the hypotheses here discussed, primary
+and secondary alike, the first necessity is a far greater
+amount of quantitative work. In this book there has been
+a constant attempt to subject every hypothesis to the test
+of statistical facts of observation. Nevertheless, we have
+been breaking so much new ground that in many cases
+exact facts are not yet available, while in others they
+can be properly investigated only by specialists in
+physics, astronomy, or mathematics. In most cases the
+next great step is to ascertain whether the forces here
+called upon are actually great enough to produce the
+observed results. Even though they act only as a means
+<!--File: cc339.png Page 315-->
+<a class="pagenum break" id="Page_315" title="pg 315"></a>
+of releasing the far greater forces due to the contraction
+of the earth and the sun, they need to be rigidly tested
+as to their ability to play even this minor r&ocirc;le. Still
+another line of study that cries aloud for research is a
+fuller comparison between earthquakes on the one hand
+and meteorological conditions and the wandering of the
+poles on the other. Finally, an extremely interesting and
+hopeful quest is the determination of the positions and
+movements of additional stars and other celestial bodies,
+the faint and invisible as well as the bright, in order to
+ascertain the probable magnitude of their influence upon
+the sun and thus upon the earth at various times in the
+past and in the future. Perhaps we are even now approaching
+some star that will some day give rise to a
+period of climatic stress like that of the fourteenth century,
+or possibly to a glacial epoch. Or perhaps the variations
+in others of the nearer stars as well as Alpha
+Centauri may show a close relation to changes in the sun.</p>
+
+<p>Throughout this volume we have endeavored to discover
+new truth concerning the physical environment
+that has molded the evolution of all life. We have seen
+how delicate is the balance among the forces of nature,
+even though they be of the most stupendous magnitude.
+We have seen that a disturbance of this balance in one
+of the heavenly bodies may lead to profound changes in
+another far away. Yet during the billion years, more or
+less, of which we have knowledge, there appears never
+to have been a complete cataclysm involving the destruction
+of all life. One star after another, if our hypothesis
+is correct, has approached the solar system closely
+enough to set the atmosphere of the sun in such commotion
+that great changes of climate have occurred upon
+the earth. Yet never has the solar system passed so close
+to any other body or changed in any other way sufficiently
+<!--File: cc340.png Page 316-->
+<a class="pagenum break" id="Page_316" title="pg 316"></a>
+to blot out all living things. The effect of climatic
+changes has always been to alter the environment and
+therefore to destroy part of the life of a given time, but
+with this there has invariably gone a stimulus to other
+organic types. New adaptations have occurred, new lines
+of evolutionary progress have been initiated, and the net
+result has been greater organic diversity and richness.
+Temporarily a great change of climate may seem to
+retard evolution, but only for a moment as the geologist
+counts time. Then it becomes evident that the march of
+progress has actually been more rapid than usual. Thus
+the main periods of climatic stress are the most conspicuous
+milestones upon the upward path toward more varied
+adaptation. The end of each such period of stress has
+found the life of the world nearer to the high mentality
+which reaches out to the utmost limits of space, of time,
+and of thought in the search for some explanation of the
+meaning of the universe. Each approach of the sun to
+other bodies, if such be the cause of the major climatic
+changes, has brought the organic world one step nearer
+to the solution of the greatest of all problems,&mdash;the problem
+of whether there is a psychic goal beyond the mental
+goal toward which we are moving with ever accelerating
+speed. Throughout the vast eons of geological time the
+adjustment of force to force, of one body of matter to
+another, and of the physical environment to the organic
+response has been so delicate, and has tended so steadily
+toward the one main line of mental progress that there
+seems to be a purpose in it all. If the cosmic uniformity
+of climate continues to prevail and if the uniformity is
+varied by changes as stimulating as those of the past, the
+imagination can scarcely picture the wonders of the
+future. In the course of millions or even billions of years
+the development of mind, and perhaps of soul, many excel
+<!--File: cc341.png Page 317-->
+<a class="pagenum break" id="Page_317" title="pg 317"></a>
+that of today as far as the highest known type of mentality
+excels the primitive plasma from which all life
+appears to have arisen.</p>
+<!--File: cc342.png Page 318-->
+<a class="pagenum break" id="Page_318" title="pg 318"></a>
+
+<!--[Blank Page]-->
+<!--File: cc343.png Page 319-->
+<a class="pagenum break" id="Page_319" title="pg 319"></a>
+
+
+
+<h2 id="index"><a href="#content">INDEX</a></h2>
+
+<h3>* Indicates illustrations.</h3>
+
+<ul class="none">
+<li>Abbot, C. G., cited, <a href="#Page_45">45</a>, <a href="#Page_52">52</a>, <a href="#Page_237">237</a>, <a href="#Page_238">238</a>, <a href="#Page_239">239.</a></li>
+
+<li>Aboskun, <a href="#Page_104">104.</a></li>
+
+<li>Africa, earthquakes, <a href="#Page_301">301</a>;
+  <ul class="none"><li>East, <em>see</em> East Africa;</li>
+  <li>lakes, <a href="#Page_143">143;</a></li>
+ <li> North, <em>see</em> North Africa.</li>
+ </ul>
+ </li>
+
+<li>African glaciation, <a href="#Page_266">266</a>.</li>
+
+<li>Air, <em>see</em> Atmosphere.</li>
+
+<li>Alaska, glacial till in, <a href="#Page_287">287</a>;
+ <ul class="none">
+ <li>Ice Age in, <a href="#Page_221">221.</a></li>
+ </ul>
+ </li>
+
+<li>Albrecht, cited, <a href="#Page_304">304</a>.</li>
+
+<li>Alexander, march of, <a href="#Page_88">88</a> f.</li>
+
+<li>Allard, H. A., cited, <a href="#Page_183">183</a>, <a href="#Page_184">184</a>.</li>
+
+<li>Alpha Centauri, companion of, <a href="#Page_280">280</a>;
+<ul class="none">
+  <li>distance from sun, <a href="#Page_262">262</a>;</li>
+  <li>luminosity, <a href="#Page_278">278</a>;</li>
+  <li>speed of, <a href="#Page_281">281</a>;</li>
+  <li>variations, <a href="#Page_282">282</a>.</li>
+</ul>
+  </li>
+
+<li>Alps, loess in, <a href="#Page_159">159</a>;
+<ul class="none">
+  <li>precipitation in, <a href="#Page_141">141</a>;</li>
+  <li>snow level in, <a href="#Page_139">139</a>.</li>
+  </ul>
+</li>
+<li>Altair, companion of, <a href="#Page_280">280</a>;
+<ul class="none">
+  <li>luminosity, <a href="#Page_278">278</a>;</li>
+  <li>speed of, <a href="#Page_281">281</a>.</li>
+  </ul>
+</li>
+
+<li>Amazon forest, temperature, <a href="#Page_17">17</a>.</li>
+
+<li>Ancylus lake, <a href="#Page_217">217</a>.</li>
+
+<li>Andes, snow line, <a href="#Page_139">139</a>.</li>
+
+<li>Animals, climate and, <a href="#Page_1">1</a>.</li>
+
+<li>Antarctica, mild climate, <a href="#Page_219">219</a>;
+<ul class="none">
+  <li>thickness of ice in, <a href="#Page_125">125</a>;</li>
+  <li>winds, <a href="#Page_135">135</a>, <a href="#Page_161">161</a>.</li>
+  </ul>
+  </li>
+
+<li>Anti-cyclonic hypothesis, <a href="#Page_135">135</a> ff.</li>
+
+<li>Appalachians, effect on ice sheet, <a href="#Page_121">121</a>.</li>
+
+<li>Arabia, civilization in, <a href="#Page_67">67</a>.</li>
+
+<li>Aral, Sea of, <a href="#Page_108">108</a>.</li>
+
+<li>Archean rocks, <a href="#Page_211">211</a>.</li>
+
+<li>Archeozoic, <a href="#Page_3">3</a> f.;
+<ul class="none">
+  <li>climate of, <a href="#Page_267">267</a>.</li>
+  </ul>
+  </li>
+
+<li>Arctic Ocean, submergence, <a href="#Page_219">219</a>.</li>
+
+<li>Arctowski, H., cited, <a href="#Page_29">29</a>, <a href="#Page_46">46</a>, <a href="#Page_244">244</a>.</li>
+
+<li>Argon, increase of, <a href="#Page_236">236</a>.</li>
+
+<li>Arizona, rainfall, <a href="#Page_89">89</a>, <a href="#Page_108">108</a>;
+<ul class="none">
+  <li>trees measured in, <a href="#Page_73">73</a>.</li>
+</ul>
+</li>
+<li>Arrhenius, S., cited, <a href="#Page_36">36</a>, <a href="#Page_254">254</a>.</li>
+
+<li>Arsis, of pulsation, <a href="#Page_24">24</a>.</li>
+
+<li>Asbj&ouml;rn Selsbane, corn of, <a href="#Page_101">101</a>.</li>
+
+<li>Asia, atmospheric pressure, <a href="#Page_298">298</a>;
+<ul class="none">
+  <li>central,changes of climate, * <a href="#Page_75">75</a>;</li>
+  <li>central, post-glacial climate, <a href="#Page_271">271</a>;</li>
+  <li>climate, <a href="#Page_66">66</a>;</li>
+  <li>glaciation in, <a href="#Page_131">131</a>;</li>
+  <li>storminess in, <a href="#Page_60">60</a>;</li>
+  <li>western, climate in, <a href="#Page_84">84</a> f.</li>
+  </ul>
+</li>
+<li>Atlantic Ocean, storminess, <a href="#Page_57">57</a>.</li>
+
+<li>Atmosphere, changes, <a href="#Page_19">19</a> f., <a href="#Page_229">229</a>;
+<ul class="none">
+  <li>composition of, <a href="#Page_223">223-241</a>;</li>
+  <li>effect on temperature, <a href="#Page_231">231</a>.</li>
+  </ul>
+</li>
+<li>Atmospheric circulation, glaciation and, <a href="#Page_42">42</a>.</li>
+
+<li>Atmospheric electricity, solar relations of, <a href="#Page_56">56</a>.</li>
+
+<li>Atmospheric pressure, earthquakes and, <a href="#Page_298">298</a>;
+<ul class="none">
+  <li>evaporation and, <a href="#Page_237">237</a>;</li>
+  <li>increase in, <a href="#Page_239">239</a>;</li>
+  <li>redistribution of, <a href="#Page_49">49</a>;</li>
+  <li>variation, <a href="#Page_53">53</a>.</li>
+  </ul>
+</li>
+<li>Australia, East, mild climate, <a href="#Page_219">219</a>;
+<ul class="none">
+  <li>precipitation, <a href="#Page_144">144</a>.</li>
+  </ul>
+</li>
+<li>Axis, earth's, <a href="#Page_48">48</a>;
+<ul class="none">
+  <li>wabbling of, <a href="#Page_301">301</a>.</li>
+</ul>
+</li>
+
+<li>&nbsp;</li>
+
+<li>Bacon, Sir Francis, cited,  <a href="#Page_27">27</a>.</li>
+
+<li>Bacubirito, meteor at,  <a href="#Page_246">246</a>.</li>
+
+<li>Baltic Sea, as lake,  <a href="#Page_217">217</a>;
+<ul class="none">
+  <li>freezing of,  <a href="#Page_100">100</a>;</li>
+  <li>ice,  <a href="#Page_26">26</a>;</li>
+  <li>storm-floods,  <a href="#Page_99">99</a>;</li>
+  <li>submergence,  <a href="#Page_219">219</a>.</li>
+</ul>
+</li>
+
+<li>Bardsson, Ivar,  <a href="#Page_106">106</a>.</li>
+
+<li>Barkow, cited,  <a href="#Page_135">135</a>.</li>
+
+<li>Barometric pressure, solar relations of,  <a href="#Page_56">56</a>.</li>
+
+<li>Barrell, J., cited,  <a href="#Page_3">3</a>,  <a href="#Page_200">200</a>,  <a href="#Page_213">213</a>,  <a href="#Page_234">234</a>.</li>
+
+<li>Bartoli, A. G., cited,  <a href="#Page_257">257</a>.</li>
+
+<!--File: cc344.png-->
+<!--<a class="pagenum break" id="Page_320" title="pg 320"></a>-->
+
+<li>Bauer, L. A., cited,  <a href="#Page_150">150</a>.</li>
+
+<li>Beaches, under water,  <a href="#Page_97">97</a>.</li>
+
+<li>Beadnell, H. J. L., cited,  <a href="#Page_143">143</a>.</li>
+
+<li>Beluchistan, rainfall,  <a href="#Page_89">89</a>.</li>
+
+<li>Bengal, Bay of, cyclones in,  <a href="#Page_149">149</a>.</li>
+
+<li>Bengal, famine in,  <a href="#Page_104">104</a> f.</li>
+
+<li>Berlin, rainfall and temperature,  <a href="#Page_93">93</a>.</li>
+
+<li>Betelgeuse,  <a href="#Page_259">259</a> f.;
+<ul class="none">
+  <li>distance from sun,  <a href="#Page_262">262</a>.</li>
+  </ul>
+
+  </li>
+
+<li>Bible, climatic evidence in,  <a href="#Page_91">91</a> f.;
+<ul class="none">
+  <li>palms in,  <a href="#Page_92">92</a>.</li>
+  </ul>
+  </li>
+
+<li>Binary stars,  <a href="#Page_252">252</a>.</li>
+
+<li>Birkeland, K., cited,  <a href="#Page_244">244</a>.</li>
+
+<li>Black Earth region, loess in,  <a href="#Page_159">159</a>.</li>
+
+<li>Boca, Cal., correlation coefficients,  <a href="#Page_83">83</a>,  <a href="#Page_85">85</a>.</li>
+
+<li>Boltzmann, L., cited,  <a href="#Page_257">257</a>.</li>
+
+<li>Bonneville, Lake,  <a href="#Page_142">142</a>,  <a href="#Page_143">143</a>.</li>
+
+<li>Borkum, storm-flood in,  <a href="#Page_99">99</a>.</li>
+
+<li>Boss, L. cited,  <a href="#Page_268">268</a>,  <a href="#Page_269">269</a>.</li>
+
+<li>Botanical evidence of mild climates,  <a href="#Page_167">167</a> ff.</li>
+
+<li>Boulders, on Irish coast,  <a href="#Page_119">119</a>.</li>
+
+<li>Bowie, W., cited,  <a href="#Page_293">293</a>.</li>
+
+<li>Bowman, I., cited,  <a href="#Page_213">213</a>.</li>
+
+<li>Britain, forests,  <a href="#Page_220">220</a>;
+<ul class="none">
+  <li>level of land,  <a href="#Page_220">220</a>.</li>
+  </ul>
+  </li>
+
+<li>British Isles, height of land,  <a href="#Page_111">111</a>;
+<ul class="none">
+  <li>temperature,  <a href="#Page_216">216</a>.</li>
+  </ul>
+  </li>
+
+<li>Brooks, C. E. P., cited,  <a href="#Page_115">115</a>,  <a href="#Page_143">143</a>,  <a href="#Page_196">196</a>,  <a href="#Page_215">215</a>,  <a href="#Page_225">225</a>.</li>
+
+<li>Brooks, C. F., cited,  <a href="#Page_209">209</a>.</li>
+
+<li>Brown, E. W., cited,  <a href="#Page_191">191</a>,  <a href="#Page_244">244</a>.</li>
+
+<li>Br&uuml;ckner, E., cited,  <a href="#Page_27">27</a>.</li>
+
+<li>Br&uuml;ckner periods,  <a href="#Page_27">27</a> f.</li>
+
+<li>Bufo, habitat of,  <a href="#Page_202">202</a>.</li>
+
+<li>Buhl stage,  <a href="#Page_216">216</a>.</li>
+
+<li>Bull, Dr., cited,  <a href="#Page_100">100</a>,  <a href="#Page_101">101</a>.</li>
+
+<li>Butler, H. C., cited,  <a href="#Page_66">66</a>,  <a href="#Page_67">67</a> ff.,  <a href="#Page_70">70</a>,  <a href="#Page_76">76</a>.</li>
+
+<li>&nbsp;</li>
+
+<li>California, changes of climate, *  <a href="#Page_75">75</a>;
+<ul class="none">
+  <li>correlations of rainfall,  <a href="#Page_86">86</a>;</li>
+  <li>measurements of sequoias in,  <a href="#Page_73">73</a>,  <a href="#Page_74">74</a> ff.;</li>
+  <li>rainfall,  <a href="#Page_108">108</a>.</li>
+  </ul>
+  </li>
+
+<li>Cambrian period,  <a href="#Page_4">4</a> f.</li>
+
+<li>Canada, storminess,  <a href="#Page_53">53</a> f.,  <a href="#Page_57">57</a>;
+<ul class="none">
+  <li>storm tracks in,  <a href="#Page_113">113</a>.</li>
+  </ul>
+  </li>
+
+<li>Cape Farewell, shore ice at,  <a href="#Page_105">105</a>.</li>
+
+<li>Carbon dioxide, erosion and,  <a href="#Page_119">119</a> f.;
+<ul class="none">
+  <li>from volcanoes,  <a href="#Page_23">23</a>;</li>
+  <li>hypothesis,  <a href="#Page_139">139</a>;</li>
+  <li>importance of,  <a href="#Page_9">9</a>,  <a href="#Page_11">11</a> f.;</li>
+  <li>in Permian,  <a href="#Page_148">148</a>;.</li>
+  <li>in atmosphere,  <a href="#Page_20">20</a>,  <a href="#Page_96">96</a>,  <a href="#Page_238">238</a>;</li>
+  <li>in ocean,  <a href="#Page_226">226</a>;</li>
+  <li>nebular hypothesis and,  <a href="#Page_232">232</a>;</li>
+  <li>theory of glaciation,  <a href="#Page_36">36</a> ff.</li>
+  </ul>
+</li>
+<li>Caribbean mountains, origin of,  <a href="#Page_193">193</a>.</li>
+
+<li>Carnegie Institution of Washington,  <a href="#Page_74">74</a>.</li>
+
+<li>Caspian Sea, climatic stress,  <a href="#Page_104">104</a>;
+<ul class="none">
+  <li>rainfall,  <a href="#Page_107">107</a> f.;</li>
+  <li>rise and fall,  <a href="#Page_27">27</a>;</li>
+  <li>ruins in,  <a href="#Page_71">71</a>.</li>
+  </ul>
+  </li>
+
+<li>Cenozoic, climate,  <a href="#Page_266">266</a>;
+<ul class="none">
+  <li>fossils,  <a href="#Page_21">21</a>.</li>
+  </ul>
+  </li>
+
+<li>Central America, Maya ruins,  <a href="#Page_95">95</a>.</li>
+
+<li>Chad, Lake, swamps of,  <a href="#Page_171">171</a>.</li>
+
+<li>Chamberlin, R. T., cited,  <a href="#Page_166">166</a>,  <a href="#Page_233">233</a>,  <a href="#Page_269">269</a>.</li>
+
+<li>Chamberlin, T. C., cited,  <a href="#Page_19">19</a>,  <a href="#Page_36">36</a>,  <a href="#Page_38">38</a>,  <a href="#Page_39">39</a>,  <a href="#Page_42">42</a> f.,  <a href="#Page_42">48</a>,  <a href="#Page_122">122</a>,  <a href="#Page_125">125</a>,  <a href="#Page_152">152</a>,  <a href="#Page_156">156</a>,  <a href="#Page_190">190</a>,  <a href="#Page_195">195</a>,  <a href="#Page_227">227</a>,  <a href="#Page_269">269</a>.</li>
+
+<li>Chandler, S. C., cited,  <a href="#Page_301">301</a>.</li>
+
+<li>Chinese earthquakes, periodicity of,  <a href="#Page_245">245</a>.</li>
+
+<li>Chinese, sunspot observations,  <a href="#Page_108">108</a> f.</li>
+
+<li>Chinese Turkestan, desiccation in,  <a href="#Page_66">66</a>.</li>
+
+<li>Chronology, glacial,  <a href="#Page_215">215</a>.</li>
+
+<li>Clarke, F. W., cited,  <a href="#Page_226">226</a>,  <a href="#Page_235">235</a>.</li>
+
+<li>Clayton, H. H., cited,  <a href="#Page_173">173</a> f.</li>
+
+<li>Climate, effect of contraction,  <a href="#Page_189">189</a> ff.;
+<ul class="none">
+  <li>affect of salinity,  <a href="#Page_224">224</a>;</li>
+  <li>in history,  <a href="#Page_64">64-97</a>;</li>
+  <li>uniformity,  <a href="#Page_1">1-15</a>;</li>
+  <li>variability,  <a href="#Page_16">16-32</a>.</li>
+  </ul>
+  </li>
+
+<li>Climates, mild, causes of,  <a href="#Page_166">166-187</a>;
+<ul class="none">
+  <li>mild, periods of,  <a href="#Page_274">274</a>.</li>
+  </ul>
+  </li>
+
+<li>Climatic changes, and crustal movements,  <a href="#Page_285">285</a> ff.;
+<ul class="none">
+  <li>hypotheses of,  <a href="#Page_33">33-50</a>;</li>
+  <li>mountain-building and, *  <a href="#Page_25">25</a>;</li>
+  <li>post-glacial crustal movements and,  <a href="#Page_215">215-222</a>;</li>
+  <li>terrestrial causes of,  <a href="#Page_188">188-214</a>.</li>
+  </ul>
+  </li>
+
+<li>Climatic sequence,  <a href="#Page_16">16</a> f.</li>
+
+<li>Climatic stages, post-glacial,  <a href="#Page_270">270</a>.</li>
+
+<li>Climatic stress, in fourteenth century,  <a href="#Page_98">98-109</a>.</li>
+
+<li>Climatic uniformity, hypothesis of,  <a href="#Page_65">65</a>,  <a href="#Page_71">71</a> f.</li>
+
+<li>Climatic zoning,  <a href="#Page_169">169</a>.</li>
+
+<!--File: cc345.png-->
+<!--<a class="pagenum break" id="Page_321" title="pg 321"></a>-->
+
+<li>Cloudiness, glaciation and,  <a href="#Page_114">114</a>,  <a href="#Page_147">147</a>.</li>
+
+<li>Clouds, as protection,  <a href="#Page_197">197</a>.</li>
+
+<li>Colfax, Cal., correlation coefficients,  <a href="#Page_83">83</a>.</li>
+
+<li>Cologne, flood at,  <a href="#Page_99">99</a>.</li>
+
+<li>Compass, variations,  <a href="#Page_150">150</a>.</li>
+
+<li>Continental climate, variations,  <a href="#Page_103">103</a>.</li>
+
+<li>Continents, effect on climate,  <a href="#Page_111">111</a> f.</li>
+
+<li>Contraction, effect on climate,  <a href="#Page_189">189</a> ff.,  <a href="#Page_199">199</a>,  <a href="#Page_207">207</a>;
+<ul class="none">
+  <li>effect on lands,  <a href="#Page_207">207</a>;</li>
+  <li>heat of sun and,  <a href="#Page_13">13</a> f.;</li>
+  <li>irregular,  <a href="#Page_195">195</a>;</li>
+  <li>of the earth,  <a href="#Page_18">18</a>;</li>
+  <li>of the sun,  <a href="#Page_249">249</a>;</li>
+  <li>stresses caused by,  <a href="#Page_310">310</a>.</li>
+  </ul>
+  </li>
+
+<li>Convection, carbon dioxide and,  <a href="#Page_239">239</a>.</li>
+
+<li>Corals, in high latitudes,  <a href="#Page_21">21</a>,  <a href="#Page_39">39</a>,  <a href="#Page_167">167</a>,  <a href="#Page_178">178</a>.</li>
+
+<li>Cordeiro, F. J. B., cited,  <a href="#Page_181">181</a>,  <a href="#Page_183">183</a>,  <a href="#Page_186">186</a>.</li>
+
+<li>Correlation coefficients, earthquakes and sunspots,  <a href="#Page_291">291</a>;
+<ul class="none">
+  <li>Jerusalem rainfall and sequoia growth,  <a href="#Page_83">83</a> ff.;</li>
+  <li>rainfall and tree growth,  <a href="#Page_79">79</a> ff.</li>
+  </ul>
+  </li>
+
+<li>Cosmos, effect of light,  <a href="#Page_185">185</a>.</li>
+
+<li>Cressey, G. B., cited,  <a href="#Page_80">80</a>.</li>
+
+<li>Cretaceous, lava,  <a href="#Page_211">211</a>;
+<ul class="none">
+  <li>mountain ranges,  <a href="#Page_44">44</a>;</li>
+  <li>paleogeography, *  <a href="#Page_201">201</a>;</li>
+  <li>submergence of North America,  <a href="#Page_200">200</a>.</li>
+  </ul>
+  </li>
+
+<li>Croll, J., cited, <a href="#Page_34">34</a> ff.,  <a href="#Page_176">176</a>.</li>
+
+<li>Croll's hypothesis, snow line,  <a href="#Page_139">139</a>.</li>
+
+<li>Crust, climate and movements of,  <a href="#Page_63">63</a>,  <a href="#Page_287">287</a>,  <a href="#Page_310">310</a>;
+<ul class="none">
+  <li>movements of,  <a href="#Page_43">43</a>;</li>
+  <li>strains in,  <a href="#Page_22">22</a>.</li>
+  </ul>
+  </li>
+
+<li>Currents and planetary winds,  <a href="#Page_174">174</a>.</li>
+
+<li>Cycads,  <a href="#Page_169">169</a>.</li>
+
+<li>Cyclonic hypothesis,  <a href="#Page_97">97</a>;
+<ul class="none">
+  <li>loess and,  <a href="#Page_163">163</a>;</li>
+  <li>Permian glaciation and,  <a href="#Page_148">148</a>;</li>
+  <li>snow line,  <a href="#Page_139">139</a>.</li>
+  </ul>
+  </li>
+
+<li>Cyclonic storms, in glacial epochs,  <a href="#Page_140">140</a> f.;
+<ul class="none">
+  <li>solar electricity and,  <a href="#Page_243">243</a> (<em>see</em> Storms, Storminess).</li>
+  </ul>
+  </li>
+
+<li>Cyclonic vacillations,  <a href="#Page_30">30</a> f.;
+<ul class="none">
+  <li>nature of,  <a href="#Page_57">57</a> ff.</li>
+  </ul>
+  </li>
+
+<li>&nbsp;</li>
+
+<li>Daily vibrations,  <a href="#Page_28">28</a> f.</li>
+
+<li>Danube, frozen,  <a href="#Page_98">98</a>.</li>
+
+<li>Darwin, G. H., cited, <a href="#Page_191">191</a>.</li>
+
+<li>Daun stage, <a href="#Page_217">217</a>.</li>
+
+<li>Davis, W. M., cited, <a href="#Page_271">271</a>.</li>
+
+<li>Davisson, C., cited, <a href="#Page_294">294</a>, <a href="#Page_295">295</a>, <a href="#Page_299">299</a>.</li>
+
+<li>Day, C. P., cited, <a href="#Page_239">239</a>.</li>
+
+<li>Day, length of, <a href="#Page_18">18</a>, <a href="#Page_191">191</a>.</li>
+
+<li>Dead Sea, palms near, <a href="#Page_92">92</a>.</li>
+
+<li>Death Valley, <a href="#Page_142">142</a>.</li>
+
+<li>De Ballore, M., cited, <a href="#Page_297">297</a>, <a href="#Page_298">298</a>.</li>
+
+<li>Deep-sea circulation, rapidity, <a href="#Page_227">227</a>;
+<ul class="none">
+  <li>salinity and, <a href="#Page_176">176</a>;</li>
+  <li>solar activity and, <a href="#Page_179">179</a>.</li>
+  </ul>
+  </li>
+
+<li>De Geer, S., cited, <a href="#Page_215">215</a>, <a href="#Page_221">221</a>.</li>
+
+<li>De Lapparent, A., cited, <a href="#Page_200">200</a>.</li>
+
+<li>Denmark, fossils, <a href="#Page_271">271</a>.</li>
+
+<li>"Desert pavements," <a href="#Page_161">161</a>.</li>
+
+<li>Deserts, abundant flora of, <a href="#Page_171">171</a>;
+<ul class="none">
+  <li>and pulsations theory, <a href="#Page_88">88</a> ff.;</li>
+  <li>red beds of, <a href="#Page_170">170</a>.</li>
+  </ul>
+  </li>
+
+<li>Devonian, climate, <a href="#Page_266">266</a>;
+<ul class="none">
+  <li>mountains, <a href="#Page_209">209</a>.</li>
+  </ul>
+  </li>
+
+<li>Dog, climate and, <a href="#Page_1">1</a>.</li>
+
+<li>Donegal County, Ireland, <a href="#Page_220">220</a>.</li>
+
+<li>Double stars, <a href="#Page_272">272</a>, <a href="#Page_280">280</a>;
+<ul class="none">
+  <li>electrical effect of, <a href="#Page_261">261</a>.</li>
+  </ul>
+  </li>
+
+<li>Douglass, A. E., cited, <a href="#Page_28">28</a>, <a href="#Page_73">73</a>, <a href="#Page_74">74</a> f., <a href="#Page_84">84</a>, <a href="#Page_85">85</a>, <a href="#Page_107">107</a>.</li>
+
+<li>Dragon Town, destruction of, <a href="#Page_104">104</a>, <a href="#Page_108">7108</a>.</li>
+
+<li>Drake, N. F., cited, <a href="#Page_297">297</a>, <a href="#Page_298">298</a>.</li>
+
+<li>Droughts, and pulsations theory, <a href="#Page_87">87</a> f.;
+<ul class="none">
+  <li>in England, <a href="#Page_102">102</a>;</li>
+  <li>in India, <a href="#Page_104">104</a> f.</li>
+  </ul>
+  </li>
+
+<li>Drumkelin Bog, Ireland, log cabin in, <a href="#Page_220">220</a>.</li>
+
+<li>Dust, at high levels, <a href="#Page_240">240</a>.</li>
+
+<li>&nbsp;</li>
+
+<li>Earth, crust of and the sun,  <a href="#Page_285">285-317</a>;
+<ul class="none">
+  <li>internal heat, <a href="#Page_212">212</a>;</li>
+  <li>nature of mild climate, <a href="#Page_274">274</a>;</li>
+  <li>position of axis, <a href="#Page_181">181</a>;</li>
+  <li>rigidity of, <a href="#Page_307">307</a>;</li>
+  <li>temperature gradient, <a href="#Page_213">213</a>;</li>
+  <li>temperature of surface, <a href="#Page_8">8</a>.</li>
+  </ul>
+  </li>
+
+<li>Earthquakes, and seasons, <a href="#Page_294">294</a>, <a href="#Page_297">297</a>;
+<ul class="none">
+  <li>and sunspots, <a href="#Page_288">288</a> f.;</li>
+  <li>and tropical hurricanes, <a href="#Page_300">300</a>;</li>
+  <li>and wandering of pole, <a href="#Page_304">304</a> f.;</li>
+  <li>cause of, <a href="#Page_307">307</a>;</li>
+  <li>compared with departures from Eulerian position, <a href="#Page_306">306</a>;</li>
+  <li>seasonal distribution of, <a href="#Page_299">299</a>;</li>
+  <li>seasonal march, <a href="#Page_295">295</a>.</li>
+  </ul>
+  </li>
+
+<li>"Earthquake weather," <a href="#Page_298">298</a>.</li>
+
+<!--File: cc346.png-->
+<!--<a class="pagenum break" id="Page_322" title="pg 322"></a>-->
+
+<li>East Africa, mild climate, <a href="#Page_219">219</a>.</li>
+
+<li>East Indies, earthquakes of, <a href="#Page_296">296</a>.</li>
+
+<li>Eberswalde, tree growth at, <a href="#Page_102">102</a> f.</li>
+
+<li>Ecliptic, obliquity of, <a href="#Page_217">217</a>.</li>
+
+<li>Electrical currents, in solar atmosphere, <a href="#Page_261">261</a>.</li>
+
+<li>Electrical emissions, variation of, <a href="#Page_275">275</a>.</li>
+
+<li>Electrical hypothesis, <a href="#Page_150">150</a>, <a href="#Page_250">250</a> f., <a href="#Page_256">256</a> ff.</li>
+
+<li>Electrical phenomena, storminess and, <a href="#Page_56">56</a>.</li>
+
+<li>Electricity, and earthquakes, <a href="#Page_292">292</a>;
+<ul class="none">
+  <li>solar, <a href="#Page_243">243</a>.</li>
+  </ul>
+  </li>
+
+<li>Electro-magnetic hypothesis, <a href="#Page_244">244</a>.</li>
+
+<li>Electrons, solar, <a href="#Page_56">56</a>;
+<ul class="none">
+  <li>variation of, <a href="#Page_256">256</a>.</li>
+  </ul>
+  </li>
+
+<li>Electro-stellar hypothesis, <a href="#Page_274">274</a>.</li>
+
+<li>Elevation, climatic changes and, <a href="#Page_39">39</a>.</li>
+
+<li>Engedi, palms in, <a href="#Page_92">92</a>.</li>
+
+<li>England, climatic stress, <a href="#Page_101">101</a> f.;
+<ul class="none">
+  <li>storminess and rainfall, <a href="#Page_107">107</a>.</li>
+  </ul>
+  </li>
+
+<li>Eocene, climate, <a href="#Page_266">266</a>.</li>
+
+<li>Equinoxes, precession of, <a href="#Page_96">96</a>.</li>
+
+<li>Erosion, storminess and, <a href="#Page_309">309</a>.</li>
+
+<li>Eskimo, in Greenland, <a href="#Page_106">106</a>.</li>
+
+<li>Eulerian movement, <a href="#Page_301">301</a>, <a href="#Page_304">304</a>.</li>
+
+<li>Euphrates, <a href="#Page_67">67</a>.</li>
+
+<li>Europe, climatic stress, <a href="#Page_98">98</a> ff., <a href="#Page_102">102</a> f.;
+<ul class="none">
+  <li>climatic table, <a href="#Page_215">215</a>;</li>
+  <li>glaciation in,  <a href="#Page_131">131</a>,</li>
+  <li>ice sheet, <a href="#Page_121">121</a>;</li>
+  <li>inundations of rivers, <a href="#Page_99">99</a>;</li>
+  <li>post-glacial climate, <a href="#Page_271">271</a>;</li>
+  <li>rainfall, <a href="#Page_107">107</a>;</li>
+  <li>submergence, <a href="#Page_196">196</a>, <a href="#Page_200">200</a>.</li>
+  </ul>
+  </li>
+
+<li>Evaporation, and glaciation, <a href="#Page_112">112</a>, <a href="#Page_114">114</a>;
+<ul class="none">
+  <li>atmospheric pressure and, <a href="#Page_237">237</a>;</li>
+  <li>from plants, <a href="#Page_179">179</a>;</li>
+  <li>importance, <a href="#Page_129">129</a>;</li>
+  <li>in trade-wind belt, <a href="#Page_117">117</a>;</li>
+  <li>rapidity of, <a href="#Page_224">224</a>.</li>
+  </ul>
+  </li>
+
+<li>Evening primrose, effect of light, <a href="#Page_184">184</a>.</li>
+
+<li>Evolution, climate and, <a href="#Page_20">20</a>;
+<ul class="none">
+  <li>geographical complexity and, <a href="#Page_241">241</a>;</li>
+  <li>glaciation and, <a href="#Page_33">33</a>;</li>
+  <li>of the earth, <a href="#Page_311">311</a>.</li>
+  </ul>
+  </li>
+
+<li>&nbsp;</li>
+
+<li>Facul&aelig;, cause of, <a href="#Page_61">61</a>.</li>
+
+<li>False Point Lighthouse, barometric pressure at, <a href="#Page_299">299</a>.</li>
+
+<li>Famine, cause of, <a href="#Page_103">103</a>;
+<ul class="none">
+  <li>in England, <a href="#Page_101">101</a> f.;</li>
+  <li>in India, <a href="#Page_104">104</a> f.;</li>
+  <li>pulsations theory and, <a href="#Page_87">87</a> f.</li>
+  </ul>
+  </li>
+
+<li>Faunas, and mild climates, <a href="#Page_168">168</a> f.;
+<ul class="none">
+  <li>in Permian, <a href="#Page_152">152</a> f.</li>
+  </ul>
+  </li>
+
+<li>Fennoscandian pause, <a href="#Page_216">216</a>.</li>
+
+<li>Flowering, light and, <a href="#Page_184">184</a>.</li>
+
+<li>Fog, and glaciation, <a href="#Page_116">116</a>;
+<ul class="none">
+  <li>as protection, <a href="#Page_197">197</a>;</li>
+  <li>temperature and, <a href="#Page_178">178</a>.</li>
+  </ul>
+  </li>
+
+<li>Forests, climate and, <a href="#Page_66">66</a>.</li>
+
+<li>Form of the land, <a href="#Page_43">43</a> ff.</li>
+
+<li>Fossil floras, and mild climates, <a href="#Page_168">168</a>;
+<ul class="none">
+  <li>in Antarctica, <a href="#Page_273">273</a>;</li>
+  <li>in Greenland, <a href="#Page_273">273</a>.</li>
+  </ul>
+  </li>
+
+<li>Fossils, <a href="#Page_169">169</a>, <a href="#Page_230">230</a>;
+<ul class="none">
+  <li>and loess, <a href="#Page_158">158</a>;</li>
+  <li>Archeozoic, <a href="#Page_3">3</a> f.;</li>
+  <li>Cenozoic, <a href="#Page_21">21</a>;</li>
+  <li>dating of, <a href="#Page_153">153</a>;</li>
+  <li>glaciation and, <a href="#Page_138">138</a>;</li>
+  <li>in peat bogs, <a href="#Page_271">271</a>;</li>
+  <li>mild climate, <a href="#Page_167">167</a>;</li>
+  <li>Proterozoic, <a href="#Page_4">4</a>, <a href="#Page_6">6</a> f.</li>
+  </ul>
+  </li>
+
+<li>Fourteenth century, climatic stress in, <a href="#Page_98">98</a>-109.</li>
+
+<li>Fowle, F. E., cited, <a href="#Page_45">45</a>, <a href="#Page_237">237</a>,  <a href="#Page_238">238</a>, <a href="#Page_239">239</a>.</li>
+
+<li>Frech, F., cited, <a href="#Page_36">36</a>.</li>
+
+<li>Free, E. E., cited, <a href="#Page_142">142</a>.</li>
+
+<li>Freezing, salinity and, <a href="#Page_224">224</a>.</li>
+
+<li>Fresno, rainfall record, <a href="#Page_82">82</a>.</li>
+
+<li>"Friction variables," <a href="#Page_247">247</a>.</li>
+
+<li>Frisian Islands, storm-flood, <a href="#Page_99">99</a>.</li>
+
+<li>Fritz, H., cited, <a href="#Page_109">109</a>.</li>
+
+<li>Frogs, distribution of, <a href="#Page_202">202</a>.</li>
+
+<li>Fuchs, cited, <a href="#Page_289">289</a>.</li>
+
+<li>&nbsp;</li>
+
+<li>Galaxy,  <a href="#Page_252">252</a>.</li>
+
+<li>Galveston, Tex., rainfall and temperature, <a href="#Page_94">94</a>.</li>
+
+<li>Garner, W. W., cited, <a href="#Page_183">183</a>, <a href="#Page_184">184</a>.</li>
+
+<li>Gasses, in air, <a href="#Page_233">233</a>.</li>
+
+<li>Geographers, and climatic changes, <a href="#Page_65">65</a> ff.</li>
+
+<li>Geological time table, * <a href="#Page_5">5</a>.</li>
+
+<li>Geologic oscillations, <a href="#Page_18">18</a> f., <a href="#Page_21">21</a> ff., <a href="#Page_188">188</a>, <a href="#Page_240">240</a>.</li>
+
+<li>Geologists, changes in ideas of, <a href="#Page_64">64</a> f.</li>
+
+<li>Germanic myths, <a href="#Page_219">219</a>.</li>
+
+<li>Germany, forests, <a href="#Page_220">220</a>;
+<ul class="none">
+  <li>growth of trees in, <a href="#Page_102">102</a>;</li>
+  <li>storms in, <a href="#Page_102">102</a>.</li>
+  </ul>
+  </li>
+
+<li>Gilbert, G. K., cited, <a href="#Page_143">143</a>.</li>
+
+<!--File: cc347.png-->
+<!--<a class="pagenum break" id="Page_323" title="pg 323"></a>-->
+
+<li>Glacial epochs, causes of, <a href="#Page_268">268</a>;
+<ul class="none">
+  <li>dates of, <a href="#Page_216">216</a>;</li>
+  <li>intervals between, <a href="#Page_264">264</a> f.;</li>
+  <li>length of, <a href="#Page_166">166</a> f.</li>
+  </ul>
+  </li>
+
+<li>Glacial fluctuations, <a href="#Page_24">24</a> ff.;
+<ul class="none">
+  <li>nature of, <a href="#Page_57">57</a> ff.</li>
+  </ul>
+  </li>
+
+<li>Glacial period, at present, <a href="#Page_272">272</a>;
+<ul class="none">
+  <li>ice in, <a href="#Page_57">57</a> f.;</li>
+  <li>length of, <a href="#Page_269">269</a>;</li>
+  <li>list, <a href="#Page_265">265</a>;</li>
+  <li>temperature, <a href="#Page_38">38</a>.</li>
+  </ul>
+  </li>
+
+<li>Glaciation, and loess,  <a href="#Page_155">155</a> f.;
+<ul class="none">
+  <li>and movement of crust, <a href="#Page_287">287</a>;</li>
+  <li>conditions favorable for,  <a href="#Page_111">111</a>,</li>
+  <li>extent of, <a href="#Page_124">124</a>;</li>
+  <li>hypotheses of, <a href="#Page_33">33</a> ff.;</li>
+  <li>in southern Canada, <a href="#Page_18">18</a>;</li>
+  <li>localization of, <a href="#Page_130">130</a> ff.;</li>
+  <li>Permian, * <a href="#Page_145">145</a>;</li>
+  <li>solar-cyclonic hypothesis of, <a href="#Page_110">110-129</a>;</li>
+  <li>suddenness of, <a href="#Page_138">138</a>;</li>
+  <li>upper limit of, <a href="#Page_141">141</a>.</li>
+  </ul>
+  </li>
+
+<li>Goldthwait, J. W., cited, <a href="#Page_271">271</a>.</li>
+
+<li>Gondwana land, <a href="#Page_21">21</a>, <a href="#Page_204">204</a>.</li>
+
+<li>Gravitation, effect on sun, <a href="#Page_250">250</a>;
+<ul class="none">
+  <li>pull of, <a href="#Page_244">244</a>.</li>
+  </ul>
+  </li>
+
+<li>Great Basin, in glacial period, <a href="#Page_126">126</a>;
+<ul class="none">
+  <li>salt lakes in, <a href="#Page_142">142</a>.</li>
+  </ul>
+  </li>
+
+<li>Great Ice Age, see Pleistocene.</li>
+
+<li>Great Plains, effect on ice sheet, <a href="#Page_120">120</a>.</li>
+
+<li>Greenland, climatic stress, <a href="#Page_105">105</a> ff.;
+<ul class="none">
+  <li>ice, <a href="#Page_26">26</a>;</li>
+  <li>rainfall, <a href="#Page_108">7108</a>;</li>
+  <li>storminess, <a href="#Page_57">57</a>;</li>
+  <li>submergence, <a href="#Page_219">219</a>;</li>
+  <li>vegetation, <a href="#Page_21">21</a>, <a href="#Page_37">37</a>, <a href="#Page_287">287</a>;</li>
+  <li>winds, <a href="#Page_135">135</a>, <a href="#Page_161">161</a>.</li>
+  </ul>
+  </li>
+
+<li>Gregory, J. W., cited, <a href="#Page_90">90</a> ff., <a href="#Page_97">97</a>.</li>
+
+<li>Gschnitz stage, <a href="#Page_216">216</a>.</li>
+
+<li>Guatemala, ruins in, <a href="#Page_95">95</a>.</li>
+
+<li>Guervain, cited, <a href="#Page_135">135</a>.</li>
+
+<li>Gyroscope, earth as, <a href="#Page_181">181</a>.</li>
+
+<li>&nbsp;</li>
+
+<li>Hale, G. E., cited, <a href="#Page_56">56</a>, <a href="#Page_62">62</a>.</li>
+
+<li>Hamdulla, cited, <a href="#Page_104">104</a>.</li>
+
+<li>Hann, J., cited, <a href="#Page_66">66</a>.</li>
+
+<li>Hansa Union, operations of, <a href="#Page_100">100</a>.</li>
+
+<li>Harmer, F. W., cited, <a href="#Page_115">115</a>, <a href="#Page_119">119</a>.</li>
+
+<li>Heat, and earthquakes, <a href="#Page_292">292</a>;
+<ul class="none">
+  <li>earth's internal, <a href="#Page_18">18</a>.</li>
+  </ul>
+  </li>
+
+<li>Hedin, S., cited, <a href="#Page_88">88</a>.</li>
+
+<li>Heim, A., cited, <a href="#Page_190">190</a>.</li>
+
+<li>Heligoland, flood in, <a href="#Page_99">99</a>.</li>
+
+<li>Helland-Hansen, B., cited, <a href="#Page_174">174</a>.</li>
+
+<li>Helmert, F. R., cited, <a href="#Page_302">302</a>.</li>
+
+<li>Henderson, L. J., cited, <a href="#Page_9">9</a>, <a href="#Page_10">10</a>, <a href="#Page_11">11</a>, <a href="#Page_12">12</a>.</li>
+
+<li>Henry, A. J., cited, <a href="#Page_94">94</a>, <a href="#Page_208">208</a>.</li>
+
+<li>Hercynian Mountains, <a href="#Page_45">45</a>.</li>
+
+<li>High pressure and glaciation, <a href="#Page_115">115</a>, <a href="#Page_135">135</a>.</li>
+
+<li>Himalayas, glaciation, <a href="#Page_144">144</a>;
+<ul class="none">
+  <li>origin of,  <a href="#Page_193">193</a>,</li>
+  <li>snow line, <a href="#Page_139">139</a>.</li>
+  </ul>
+  </li>
+
+<li>Himley, cited, <a href="#Page_104">104</a>.</li>
+
+<li>Historic pulsations, <a href="#Page_24">24</a> f.;
+<ul class="none">
+  <li>nature of, <a href="#Page_57">57</a> ff.</li>
+  </ul>
+  </li>
+
+<li>History, climate of, <a href="#Page_64">64</a>-97;
+<ul class="none">
+  <li>climatic pulsations and, <a href="#Page_26">26</a>.</li>
+  </ul>
+  </li>
+
+<li>Hobbs, W. H., cited, <a href="#Page_115">115</a>, <a href="#Page_125">125</a>, <a href="#Page_135">135</a>, <a href="#Page_161">161</a>.</li>
+
+<li>Hot springs, temperature of, <a href="#Page_6">6</a>.</li>
+
+<li>Humphreys, W. J., cited, <a href="#Page_2">2</a>, <a href="#Page_37">37</a> f., <a href="#Page_45">45</a>, <a href="#Page_46">46</a>, <a href="#Page_50">50</a>, <a href="#Page_56">56</a>,  <a href="#Page_238">238</a>.</li>
+
+<li>Hurricanes, in arid regions, <a href="#Page_144">144</a>;
+<ul class="none">
+  <li>sunspots and, <a href="#Page_53">53</a>.</li>
+  </ul>
+  </li>
+
+<li>Hyades, cluster in, <a href="#Page_268">268</a>.</li>
+
+<li>&nbsp;</li>
+
+<li>Ice, accumulations, <a href="#Page_57">57</a> f.;
+<ul class="none">
+  <li>advances of, <a href="#Page_122">122</a>;</li>
+  <li>distribution of,  <a href="#Page_131">131</a>,</li>
+  <li>drift, <a href="#Page_105">105</a>.</li>
+  </ul>
+  </li>
+
+<li>Ice sheets, disappearance, <a href="#Page_128">128</a>;
+<ul class="none">
+  <li>limits, <a href="#Page_120">120</a>;</li>
+  <li>localization, <a href="#Page_130">130</a> ff.;</li>
+  <li>rate of retreat, <a href="#Page_165">165</a>;</li>
+  <li>thickness, <a href="#Page_125">125</a>.</li>
+  </ul>
+  </li>
+
+<li>Iceland, submergence, <a href="#Page_219">219</a>.</li>
+
+<li>Iowan ice sheet, rapid retreat, <a href="#Page_165">165</a>.</li>
+
+<li>Iowan loess, <a href="#Page_158">158</a>.</li>
+
+<li>India, drought, <a href="#Page_104">104</a> f.;
+<ul class="none">
+  <li>famine, <a href="#Page_104">104</a> f.;</li>
+  <li>rainfall, <a href="#Page_108">7108</a>.</li>
+  </ul>
+  </li>
+
+<li>Indian glaciation, <a href="#Page_266">266</a>.</li>
+
+<li>Inter-glacial epoch, Permian, <a href="#Page_153">153</a>.</li>
+
+<li>Internal heat of earth, <a href="#Page_212">212</a>.</li>
+
+<li>Ireland, Drumkelin Bog, <a href="#Page_220">220</a>;
+<ul class="none">
+  <li>in glacial period, <a href="#Page_119">119</a>;</li>
+  <li>level of land, <a href="#Page_220">220</a>;</li>
+  <li>storminess and rainfall, <a href="#Page_107">107</a>;</li>
+  <li>submergence, <a href="#Page_219">219</a>.</li>
+  </ul>
+  </li>
+
+<li>Irish Sea, tides, <a href="#Page_191">191</a>.</li>
+
+<li>Irrigation ditches, abandoned, <a href="#Page_97">97</a>.</li>
+
+<li>Isostasy, <a href="#Page_307">307</a> ff.</li>
+
+<li>Italy, southern, climate of, <a href="#Page_86">86</a> f.</li>
+
+<li>&nbsp;</li>
+
+<li>Japan, earthquakes of, <a href="#Page_296">296</a>.</li>
+
+<li>Javanese mountains, origin of, <a href="#Page_193">193</a>.</li>
+
+<li>Jaxartes, <a href="#Page_108">7108</a>.</li>
+
+<li>Jeans, J. H., cited, <a href="#Page_251">251</a>,  <a href="#Page_252">252</a>, <a href="#Page_253">253</a>, <a href="#Page_266">266</a>, <a href="#Page_272">272</a>.</li>
+
+<!--File: cc348.png-->
+<!--<a class="pagenum break" id="Page_324" title="pg 324"></a>-->
+
+<li>Jeffreys, H., cited, <a href="#Page_302">302</a>, <a href="#Page_303">303</a>, <a href="#Page_306">306</a>.</li>
+
+<li>Jeffreys, J., cited, <a href="#Page_191">191</a>.</li>
+
+<li>Jericho, palms in, <a href="#Page_92">92</a>.</li>
+
+<li>Jerusalem, rainfall, <a href="#Page_86">86</a>;
+<ul class="none">
+  <li>rainfall and temperature, <a href="#Page_94">94</a>;</li>
+  <li>rainfall in, and sequoia growth, <a href="#Page_83">83</a> ff.</li>
+  </ul>
+  </li>
+
+<li>Johnson, cited, <a href="#Page_226">226</a>.</li>
+
+<li>Judea, palms in, <a href="#Page_92">92</a>.</li>
+
+<li>Jupiter, and sunspots, <a href="#Page_243">243</a>;
+<ul class="none">
+  <li>effect of, <a href="#Page_253">253</a>;</li>
+  <li>periodicity of, <a href="#Page_61">61</a> f.;</li>
+  <li>temperature of, <a href="#Page_258">258</a>;</li>
+  <li>tidal effect of, <a href="#Page_250">250</a>.</li>
+  </ul>
+  </li>
+
+<li>Jurassic, climate, <a href="#Page_266">266</a>;
+<ul class="none">
+  <li>mountain ranges, <a href="#Page_44">44</a>.</li>
+  </ul>
+  </li>
+
+<li>&nbsp;</li>
+
+<li>Kansas, variations of seasons, <a href="#Page_103">103</a>.</li>
+
+<li>Kara Koshun marsh, Lop Nor, <a href="#Page_104">104</a>.</li>
+
+<li>Keewatin center, <a href="#Page_113">113</a>;
+<ul class="none">
+  <li>evaporation in, <a href="#Page_129">129</a>.</li>
+  </ul>
+  </li>
+
+<li>Keewatin ice sheet, <a href="#Page_121">121</a>.</li>
+
+<li>Kelvin, Lord, cited, <a href="#Page_13">13</a> f.</li>
+
+<li>Keyes, C. R., cited,  <a href="#Page_156">156</a>.</li>
+
+<li>Kirk, E., cited, <a href="#Page_287">287</a>.</li>
+
+<li>Knott, C. G., cited, <a href="#Page_294">294</a>, <a href="#Page_295">295</a>, <a href="#Page_297">297</a>, <a href="#Page_299">299</a>, <a href="#Page_304">304</a>, <a href="#Page_306">306</a>.</li>
+
+<li>Knowlton, F. H., cited <a href="#Page_167">167</a>, <a href="#Page_169">169</a>, <a href="#Page_170">170</a>, <a href="#Page_212">212</a>, <a href="#Page_232">232</a>.</li>
+
+<li>K&ouml;ppen, W.,, <a href="#Page_52">52</a>, <a href="#Page_140">140</a>.</li>
+
+<li>Krakatoa, glaciation and, <a href="#Page_48">48</a>;
+<ul class="none">
+  <li>volcanic hypothesis and, <a href="#Page_45">45</a>.</li>
+  </ul>
+  </li>
+
+<li>Kr&uuml;mmel, O., cited, <a href="#Page_224">224</a>, <a href="#Page_228">228</a>.</li>
+
+<li>Kullmer, C. J., cited, <a href="#Page_113">113</a>, <a href="#Page_115">115</a>, <a href="#Page_128">128</a>;
+<ul class="none">
+  <li>map of storminess, * <a href="#Page_54">54</a>.</li>
+  </ul>
+  </li>
+
+<li><em>Kungaspegel</em>, sea routes described, <a href="#Page_106">106</a>.</li>
+
+<li>&nbsp;</li>
+
+<li>Labor, price in England, <a href="#Page_102">102</a>.</li>
+
+<li>Labradorean center of glaciation, <a href="#Page_113">113</a>.</li>
+
+<li>Lahontan, Lake, <a href="#Page_142">142</a>.</li>
+
+<li>Lake strands, <em>see</em> Strands.</li>
+
+<li>Lake Superior, lava, <a href="#Page_211">211</a>.</li>
+
+<li>Lakes, during glacial periods, <a href="#Page_141">141</a> f.;
+<ul class="none">
+  <li>in semi-arid regions, <a href="#Page_60">60</a>;</li>
+  <li>of Great Basin, <a href="#Page_126">126</a>;</li>
+  <li>ruins in, <a href="#Page_97">97</a>.</li>
+  </ul>
+  </li>
+
+<li>Land, and water, climatic effect of, <a href="#Page_196">196</a> ff.;
+<ul class="none">
+  <li>distribution of, <a href="#Page_200">200</a>, form of, <a href="#Page_43">43</a> ff.;</li>
+  <li>range of temperature and, <a href="#Page_196">196</a>.</li>
+  </ul>
+  </li>
+
+<li>Lavas, climatic effect of, <a href="#Page_211">211</a>.</li>
+
+<li>Lawson, A. C., cited, <a href="#Page_310">310</a>.</li>
+
+<li>Lebanon, cedars of, <a href="#Page_83">83</a>.</li>
+
+<li>Leiter, H., cited, <a href="#Page_71">71</a>.</li>
+
+<li>Leverett, F., cited, <a href="#Page_271">271</a>.</li>
+
+<li>Life, atmosphere and, <a href="#Page_229">229</a> f.;
+<ul class="none">
+  <li>chemical characteristic of, <a href="#Page_12">12</a>;</li>
+  <li>effect of salinity, <a href="#Page_225">225</a>;</li>
+  <li>of glacial period, <a href="#Page_127">127</a>;</li>
+  <li>persistence of forms, <a href="#Page_230">230</a>.</li>
+  </ul>
+  </li>
+
+<li>Light, effect of atmosphere on, <a href="#Page_236">236</a>;
+<ul class="none">
+  <li>effect on plants, <a href="#Page_184">184</a> ff.;</li>
+  <li>ultra-violet, storminess and, <a href="#Page_56">56</a>;</li>
+  <li>variation of, <a href="#Page_275">275</a>.</li>
+  </ul>
+  </li>
+
+<li>Litorina sea, <a href="#Page_218">218</a>.</li>
+
+<li>Loess, date of, <a href="#Page_156">156</a> ff.;
+<ul class="none">
+  <li>origin of,  <a href="#Page_155">155</a>, <a href="#Page_165">165</a>.</li>
+  </ul>
+  </li>
+
+<li>Lop Nor, rise of, <a href="#Page_104">104</a>;
+<ul class="none">
+  <li>swamps, <a href="#Page_171">171</a>.</li>
+  </ul>
+  </li>
+
+<li>Lows, and glacial lobes, <a href="#Page_122">122</a>;
+<ul class="none">
+  <li>movements of, <a href="#Page_126">126</a>;</li>
+  <li>see Storms and Cyclones.</li>
+  </ul>
+  </li>
+
+<li>Lulan, <a href="#Page_104">104</a>.</li>
+
+<li>Lull, R. S., cited, <a href="#Page_5">5</a>, <a href="#Page_188">188</a>.</li>
+
+<li>&nbsp;</li>
+
+<li>MacDougal, D. T., cited, <a href="#Page_171">171</a>.</li>
+
+<li>McGee, W. J., cited,  <a href="#Page_156">156</a>.</li>
+
+<li>Macmillan, W. D., cited, <a href="#Page_191">191</a>.</li>
+
+<li>Magdalenian period, <a href="#Page_216">216</a>.</li>
+
+<li>Magnetic fields of sunspots, <a href="#Page_56">56</a>.</li>
+
+<li>Magnetic poles, relation to storm tracks, <a href="#Page_150">150</a>.</li>
+
+<li>Makran, climate, <a href="#Page_89">89</a>;
+<ul class="none">
+  <li>rainfall, <a href="#Page_89">89</a>.</li>
+  </ul>
+  </li>
+
+<li>Malay Archipelago, earthquakes of, <a href="#Page_296">296</a>.</li>
+
+<li>Mallet, R., cited, <a href="#Page_288">288</a>.</li>
+
+<li>Malta, rainfall, <a href="#Page_86">86</a>.</li>
+
+<li>Manson, M., cited, <a href="#Page_147">147</a>.</li>
+
+<li>Mayas, civilization, <a href="#Page_26">26</a>;
+<ul class="none">
+  <li>ruins, <a href="#Page_95">95</a>.</li>
+  </ul>
+  </li>
+
+<li>Mayence, flood at, <a href="#Page_99">99</a>.</li>
+
+<li>Mazelle, E., cited, <a href="#Page_224">224</a>.</li>
+
+<li>Mediterranean, climate of, <a href="#Page_72">72</a>;
+<ul class="none">
+  <li>rainfall records, <a href="#Page_86">86</a>;</li>
+  <li>storminess in, <a href="#Page_60">60</a>.</li>
+  </ul>
+  </li>
+
+<li>Mercury, and sunspots, <a href="#Page_243">243</a>.</li>
+
+<li>Mesozoic, climate, <a href="#Page_266">266</a>;
+<ul class="none">
+  <li>crustal changes, <a href="#Page_286">286</a>;</li>
+  <li>emergence of lands, <a href="#Page_287">287</a>.</li>
+  </ul>
+  </li>
+
+<li>Messier, <a href="#Page_8">8</a>;
+<ul class="none">
+  <li>variables, <a href="#Page_248">248</a>.</li>
+  </ul>
+  </li>
+
+<li>Metcalf, M. M., cited, <a href="#Page_202">202</a>.</li>
+
+<li>Meteorological factors and earthquakes, <a href="#Page_300">300</a> f.</li>
+
+<!--File: cc349.png-->
+<!--<a class="pagenum break" id="Page_325" title="pg 325"></a>-->
+
+<li>Meteorological hypothesis of crustal movements, <a href="#Page_294">294</a>.</li>
+
+<li>Meteors, and sun's heat, <a href="#Page_13">13</a>, <a href="#Page_246">246</a>.</li>
+
+<li>Michelson, A. A., cited, <a href="#Page_259">259</a>.</li>
+
+<li>Middle Silurian, fauna in Alaska, <a href="#Page_287">287</a>.</li>
+
+<li>Mild climates, <em>see</em> Climates, mild.</li>
+
+<li>Milky Way,  <a href="#Page_252">252</a>.</li>
+
+<li>Mill, H. R., cited, <a href="#Page_228">228</a>.</li>
+
+<li>Milne, J., cited, <a href="#Page_288">288</a>, <a href="#Page_290">290</a>, <a href="#Page_294">294</a>, <a href="#Page_304">304</a>, <a href="#Page_306">306</a>.</li>
+
+<li>Miocene, crustal changes, <a href="#Page_287">287</a>.</li>
+
+<li>Mississippi Basin, loess in, <a href="#Page_159">159</a>.</li>
+
+<li>Mogul emperor, and famine, <a href="#Page_104">104</a>.</li>
+
+<li>Monsoons, character of, <a href="#Page_146">146</a>;
+<ul class="none">
+  <li>direction of, <a href="#Page_208">208</a>;</li>
+  <li>Indian famines and, <a href="#Page_105">105</a>.</li>
+  </ul>
+  </li>
+
+<li>Moulton, F. R., cited, <a href="#Page_13">13</a>, <a href="#Page_258">258</a>, <a href="#Page_269">269</a>.</li>
+
+<li>Mountain building, climatic changes and, * <a href="#Page_25">25</a>.</li>
+
+<li>Mountains, folding of, <a href="#Page_190">190</a>;
+<ul class="none">
+  <li>rainfall, on, <a href="#Page_208">208</a>.</li>
+  </ul>
+  </li>
+
+<li>Multiple stars,  <a href="#Page_252">252</a>.</li>
+
+<li>&nbsp;</li>
+
+<li>Nansen, F., cited, <a href="#Page_122">122</a>, <a href="#Page_174">174</a>.</li>
+
+<li>Naples, rainfall, <a href="#Page_86">86</a>.</li>
+
+<li>Nathorst, cited, <a href="#Page_169">169</a>.</li>
+
+<li>Nebul&aelig;, <a href="#Page_247">247</a>.</li>
+
+<li>Nebular hypothesis, <a href="#Page_232">232</a>, <a href="#Page_267">267</a>.</li>
+
+<li>Neolithic period, <a href="#Page_218">218</a>.</li>
+
+<li>Nevada, correlations of rainfall, <a href="#Page_86">86</a>.</li>
+
+<li>New England, height of land, <a href="#Page_111">111</a>.</li>
+
+<li>New Mexico, rainfall, <a href="#Page_89">89</a>.</li>
+
+<li>New Orleans, La., rainfall and temperature, <a href="#Page_94">94</a>.</li>
+
+<li>New Zealand, climate, <a href="#Page_177">177</a>;
+<ul class="none">
+  <li>tree ferns, <a href="#Page_179">179</a>.</li>
+  </ul>
+  </li>
+
+<li>Newcomb, S., cited, <a href="#Page_52">52</a>.</li>
+
+<li>Nile floods, periodicity in, <a href="#Page_245">245</a>.</li>
+
+<li>Nitrogen, in atmosphere, <a href="#Page_19">19</a>.</li>
+
+<li>Niya, Chinese Turkestan, desiccation at, <a href="#Page_66">66</a>.</li>
+
+<li>Nocturnal cooling, changes in,  <a href="#Page_238">238</a> f.</li>
+
+<li>Norlind, A., cited, <a href="#Page_100">100</a>.</li>
+
+<li>Norsemen, route to Greenland, <a href="#Page_26">26</a>.</li>
+
+<li>Norse sagas, <a href="#Page_219">219</a>.</li>
+
+<li>North Africa, climate of, <a href="#Page_71">71</a>;
+<ul class="none">
+  <li>Roman aqueducts in, <a href="#Page_71">71</a>.</li>
+  </ul>
+  </li>
+
+<li>North America, at maximum glaciation, <a href="#Page_122">122</a> ff.;
+<ul class="none">
+  <li>emergence of lands,  <a href="#Page_193">193</a>,</li>
+  <li>glaciation in,  <a href="#Page_131">131</a>,</li>
+  <li>height of land,  <a href="#Page_111">111</a>,</li>
+  <li>interior sea in,  <a href="#Page_200">200</a>,</li>
+  <li>inundations,  <a href="#Page_196">196</a>,</li>
+  <li>loess in,  <a href="#Page_155">155</a>,</li>
+  <li>submergence of lands, <a href="#Page_19">19</a>, <a href="#Page_21">21</a>.</li>
+  </ul>
+  </li>
+
+<li>North Atlantic Ocean, salinity, <a href="#Page_228">228</a>.</li>
+
+<li>North Sea, climatic stress, <a href="#Page_98">98</a> ff.;
+<ul class="none">
+  <li>floods around, <a href="#Page_26">26</a>, <a href="#Page_99">99</a>;</li>
+  <li>rainfall, <a href="#Page_107">107</a>;</li>
+  <li>storminess, <a href="#Page_57">57</a>.</li>
+  </ul>
+  </li>
+
+<li>Northern hemisphere, earthquakes of, <a href="#Page_294">294</a>.</li>
+
+<li>Norway, decay, <a href="#Page_100">100</a>;
+<ul class="none">
+  <li>temperature, <a href="#Page_177">177</a>.</li>
+  </ul>
+  </li>
+
+<li>Nov&aelig;, <a href="#Page_247">247</a>.</li>
+
+<li>&nbsp;</li>
+
+<li>Oceanic circulation, carbon dioxide and, <a href="#Page_39">39</a> ff.</li>
+
+<li>Oceanic climate, characteristics, <a href="#Page_103">103</a>.</li>
+
+<li>Oceanic currents, diversion, <a href="#Page_44">44</a>;
+<ul class="none">
+  <li>influence of land distribution, <a href="#Page_203">203</a>.</li>
+  </ul>
+  </li>
+
+<li>Oceans, age of, <a href="#Page_223">223</a>;
+<ul class="none">
+  <li>composition of, <a href="#Page_223">223</a>-241;</li>
+  <li>deepening of, <a href="#Page_199">199</a>;</li>
+  <li>salinity, <a href="#Page_19">19</a>, <a href="#Page_223">223</a>;</li>
+  <li>temperature, <a href="#Page_6">6</a>, <a href="#Page_152">152</a>, <a href="#Page_180">180</a>, <a href="#Page_226">226</a>.</li>
+  </ul>
+  </li>
+
+<li>Okada, T., cited, <a href="#Page_224">224</a>.</li>
+
+<li>Old Testament, temperature, <a href="#Page_92">92</a>.</li>
+
+<li>Orbital precessions, <a href="#Page_27">27</a>.</li>
+
+<li>Ordovician, climate, <a href="#Page_266">266</a>.</li>
+
+<li>Organic evolution, glacial fluctuations and, <a href="#Page_26">26</a>.</li>
+
+<li>Orion, nebulosity near, <a href="#Page_247">247</a>;
+<ul class="none">
+  <li>stars near, <a href="#Page_248">248</a>.</li>
+  </ul>
+  </li>
+
+<li>Orontes, <a href="#Page_67">67</a>.</li>
+
+<li>Osborn, H. F., cited, <a href="#Page_216">216</a>.</li>
+
+<li>Owens-Searles, lakes, <a href="#Page_142">142</a>.</li>
+
+<li>Oxus, <a href="#Page_108">7108</a>.</li>
+
+<li>Oxygen, in atmosphere, <a href="#Page_20">20</a>, <a href="#Page_234">234</a>;
+<ul class="none">
+  <li>in Permian, <a href="#Page_152">152</a>.</li>
+  </ul>
+  </li>
+
+<li>Ozone, cause of, <a href="#Page_56">56</a>.</li>
+
+<li>&nbsp;</li>
+
+<li>Paleolithic, <a href="#Page_216">216</a>.</li>
+
+<li>Paleozoic, climate, <a href="#Page_266">266</a>;
+<ul class="none">
+  <li>mountains in, <a href="#Page_209">209</a>.</li>
+  </ul>
+  </li>
+
+<li>Palestine, change of climate, <a href="#Page_91">91</a> f.</li>
+
+<li>Palms, climatic change and, <a href="#Page_91">91</a> f.;
+<ul class="none">
+  <li>in Ireland, <a href="#Page_179">179</a>.</li>
+  </ul>
+  </li>
+
+<li>Palmyra, ruins of, <a href="#Page_66">66</a>.</li>
+
+<li>Parallaxes of stars, <a href="#Page_276">276</a> f.</li>
+
+<li>Patrician center, <a href="#Page_134">134</a>.</li>
+
+<!--File: cc350.png-->
+<!--<a class="pagenum break" id="Page_326" title="pg 326"></a>-->
+
+<li>Peat-bog period, first, <a href="#Page_218">218</a>.</li>
+
+<li>Penck, A., cited, <a href="#Page_139">139</a>,  <a href="#Page_156">156</a>, <a href="#Page_157">157</a>, <a href="#Page_158">158</a>, <a href="#Page_269">269</a>.</li>
+
+<li>Pennsylvanian, life of, <a href="#Page_26">26</a>.</li>
+
+<li>Periodicities, <a href="#Page_245">245</a> f.</li>
+
+<li>Periodicity, of climatic phenomena, <a href="#Page_60">60</a> f.;
+<ul class="none">
+  <li>of glaciation, <a href="#Page_268">268</a>;</li>
+  <li>of sunspots, <a href="#Page_243">243</a>.</li>
+  </ul>
+  </li>
+
+<li>Permian, climate, <a href="#Page_266">266</a>;
+<ul class="none">
+  <li>distribution of glaciation, <a href="#Page_152">152</a>;</li>
+  <li>glaciation, <a href="#Page_60">60</a>, <a href="#Page_144">144</a>, * <a href="#Page_145">145</a>, <a href="#Page_226">226</a>;</li>
+  <li>glaciation and mountains, <a href="#Page_45">45</a>;</li>
+  <li>life of, <a href="#Page_26">26</a>;</li>
+  <li>red beds, <a href="#Page_151">151</a>;</li>
+  <li>temperature, <a href="#Page_146">146</a> f.</li>
+  </ul>
+  </li>
+
+<li>Perry, cited, <a href="#Page_289">289</a>.</li>
+
+<li>Persia, lakes, <a href="#Page_143">143</a>;
+<ul class="none">
+  <li>rainfall, <a href="#Page_89">89</a>.</li>
+  </ul>
+  </li>
+
+<li>Pettersson, O., cited, <a href="#Page_98">98</a> ff., <a href="#Page_100">100</a> f., <a href="#Page_103">103</a>, <a href="#Page_106">106</a>, <a href="#Page_219">219</a>.</li>
+
+<li>Pirsson, L. V., cited, <a href="#Page_3">3</a>, <a href="#Page_196">196</a>.</li>
+
+<li>Planetary hypothesis, <a href="#Page_253">253</a>, <a href="#Page_267">267</a>.</li>
+
+<li>Planetary nebul&aelig;,  <a href="#Page_252">252</a>.</li>
+
+<li>Planets, and sunspots, <a href="#Page_243">243</a>;
+<ul class="none">
+  <li>effect of star on, <a href="#Page_255">255</a>;</li>
+  <li>sunspot cycle and, <a href="#Page_62">62</a>;</li>
+  <li>temperatures, <a href="#Page_8">8</a> f.</li>
+  </ul>
+  </li>
+
+<li>Plants, climate and, <a href="#Page_1">1</a> f.;
+<ul class="none">
+  <li>effect of light, <a href="#Page_184">184</a> ff.</li>
+  </ul>
+  </li>
+
+<li>Pleion, defined, <a href="#Page_29">29</a>.</li>
+
+<li>Pleionian migrations, <a href="#Page_29">29</a> f.</li>
+
+<li>Pleistocene, climate, <a href="#Page_266">266</a>;
+<ul class="none">
+  <li>duration of, <a href="#Page_48">48</a>;</li>
+  <li>glaciation, <a href="#Page_110">110</a> ff.;</li>
+  <li>ice sheets, * <a href="#Page_123">123</a>.</li>
+  </ul>
+  </li>
+
+<li>Pluvial climate, causes of, <a href="#Page_143">143</a>;
+<ul class="none">
+  <li>during glacial periods, <a href="#Page_141">141</a>.</li>
+  </ul>
+  </li>
+
+<li>Po, frozen, <a href="#Page_98">98</a>.</li>
+
+<li>Polaris, <a href="#Page_272">272</a>.</li>
+
+<li>Polar wandering, hypothesis of, <a href="#Page_48">48</a> f.</li>
+
+<li>Pole and earthquakes, <a href="#Page_305">305</a>.</li>
+
+<li>Post-glacial crustal movements and climatic changes, <a href="#Page_215">215-222</a>.</li>
+
+<li>Poynting, J. H., cited, <a href="#Page_8">8</a>.</li>
+
+<li>Precessional hypothesis, <a href="#Page_34">34</a> f.</li>
+
+<li>Precipitation, and glaciation, <a href="#Page_114">114</a>, <a href="#Page_133">133</a>;
+<ul class="none">
+  <li>during glacial period, <a href="#Page_118">118</a>;</li>
+  <li>snow line and, <a href="#Page_139">139</a>;</li>
+  <li>temperature and, <a href="#Page_94">94</a>.</li>
+  </ul>
+  </li>
+
+<li>Procyon, companion of, <a href="#Page_280">280</a>;
+<ul class="none">
+  <li>luminosity, <a href="#Page_278">278</a>;</li>
+  <li>speed of, <a href="#Page_281">281</a>.</li>
+  </ul>
+  </li>
+
+<li>Progressive change, <a href="#Page_241">241</a>.</li>
+
+<li>Progressive desiccation, hypothesis of, <a href="#Page_65">65</a> ff.</li>
+
+<li>Proterozoic, <a href="#Page_4">4</a> f.;
+<ul class="none">
+  <li>fossils, <a href="#Page_6">6</a> f.;</li>
+  <li>glaciation, <a href="#Page_18">18</a>, <a href="#Page_144">144</a>, <a href="#Page_226">226</a>, <a href="#Page_266">266</a>;</li>
+  <li>lava, <a href="#Page_211">211</a>;</li>
+  <li>mountains in, <a href="#Page_209">209</a>;</li>
+  <li>oceanic salinity, <a href="#Page_42">42</a> f.;</li>
+  <li>oxygen in air, <a href="#Page_234">234</a>;</li>
+  <li>red beds, <a href="#Page_151">151</a>;</li>
+  <li>temperature, <a href="#Page_146">146</a> f.</li>
+  </ul>
+  </li>
+
+<li>Pulsations, hypothesis of, <a href="#Page_65">65</a>, <a href="#Page_72">72</a> ff.</li>
+
+<li>Pulsatory climatic changes, <a href="#Page_72">72</a> ff.</li>
+
+<li>Pulsatory hypothesis, <a href="#Page_272">272</a>.</li>
+
+<li>Pumpelly, R., cited, <a href="#Page_271">271</a>.</li>
+
+<li>&nbsp;</li>
+
+<li>Radiation, variation of, <a href="#Page_275">275</a>.</li>
+
+<li>Radioactivity, heat of sun and, <a href="#Page_14">14</a> f.</li>
+
+<li>Rainfall, changes in, <a href="#Page_93">93</a> f.;
+<ul class="none">
+  <li>glaciation and, <a href="#Page_50">50</a>;</li>
+  <li>sunspots and, <a href="#Page_53">53</a>, * <a href="#Page_58">58</a>, <a href="#Page_59">59</a>;</li>
+  <li>tree growth and, <a href="#Page_79">79</a>.</li>
+  </ul>
+  </li>
+
+<li>Red beds, <a href="#Page_151">151</a>, <a href="#Page_170">170</a>.</li>
+
+<li>Rhine, flood, <a href="#Page_99">99</a>;
+<ul class="none">
+  <li>frozen, <a href="#Page_98">98</a>.</li>
+  </ul>
+  </li>
+
+<li>Rho Ophiuchi, variables, <a href="#Page_248">248</a>.</li>
+
+<li>"Rice grains," <a href="#Page_61">61</a>.</li>
+
+<li>Richardson, O. W., cited, <a href="#Page_256">256</a>.</li>
+
+<li>Rigidity, of earth, <a href="#Page_307">307</a>.</li>
+
+<li>Roads, climate and, <a href="#Page_66">66</a>.</li>
+
+<li>Rogers, Thorwald, cited, <a href="#Page_101">101</a>.</li>
+
+<li>Romans, aqueduct of, <a href="#Page_71">71</a>.</li>
+
+<li>Rome, history of, <a href="#Page_87">87</a>.</li>
+
+<li>Rotation, of earth, <a href="#Page_18">18</a> f.</li>
+
+<li>Ruden, storm-flood, <a href="#Page_99">99</a>.</li>
+
+<li>Rugen, storm-flood, <a href="#Page_99">99</a>.</li>
+
+<li>Ruins, as climatic evidence, <a href="#Page_66">66</a>;
+<ul class="none">
+  <li>rainfall and, <a href="#Page_60">60</a>.</li>
+  </ul>
+  </li>
+
+<li>&nbsp;</li>
+
+<li>Sacramento, correlation coefficients, <a href="#Page_82">82</a> f., <a href="#Page_85">85</a>;
+<ul class="none">
+  <li>rainfall, <a href="#Page_86">86</a>;</li>
+  <li>rainfall record, <a href="#Page_79">79</a>.</li>
+  </ul>
+  </li>
+
+<li>Sagas, cited, <a href="#Page_105">105</a> f.</li>
+
+<li>St. John, C. E., cited, <a href="#Page_236">236</a>.</li>
+
+<li>Salinity, deep-sea circulation and, <a href="#Page_176">176</a>;
+<ul class="none">
+  <li>effect on climate, <a href="#Page_224">224</a>;</li>
+  <li>in North Atlantic, <a href="#Page_228">228</a>;</li>
+  <li>ocean temperature and, <a href="#Page_226">226</a>;</li>
+  <li>of ocean, <a href="#Page_19">19</a>, <a href="#Page_120">120</a>.</li>
+  </ul>
+  </li>
+
+<li>Salisbury, R. D., cited, <a href="#Page_111">111</a>, <a href="#Page_125">125</a>, <a href="#Page_129">129</a>, <a href="#Page_139">139</a>,  <a href="#Page_156">156</a>, <a href="#Page_206">206</a>, <a href="#Page_269">269</a>, <a href="#Page_271">271</a>.</li>
+
+<li>Salt, in ocean, <a href="#Page_223">223</a>.</li>
+
+<li>San Bernardino, correlation of rainfall, <a href="#Page_85">85</a>.</li>
+
+<li>Saturn, and sunspots, <a href="#Page_243">243</a>;
+<ul class="none">
+  <li>sunspot cycle and, <a href="#Page_62">62</a>.</li>
+  </ul>
+  </li>
+
+<!--File: cc351.png-->
+<!--<a class="pagenum break" id="Page_327" title="pg 327"></a>-->
+
+<li>Sayles, R. W., cited, <a href="#Page_183">183</a>.</li>
+
+<li>Scandinavia, climatic stress, <a href="#Page_100">100</a> f.;
+<ul class="none">
+  <li>fossils, <a href="#Page_271">271</a>;</li>
+  <li>post-glacial climate, <a href="#Page_271">271</a>;</li>
+  <li>rainfall, <a href="#Page_107">107</a>;</li>
+  <li>storminess, <a href="#Page_57">57</a>, <a href="#Page_107">107</a>;</li>
+  <li>temperature, <a href="#Page_216">216</a>.</li>
+  </ul>
+  </li>
+
+<li>Scandinavian center of glaciation, <a href="#Page_113">113</a>.</li>
+
+<li>Schlesinger, F., cited, <a href="#Page_275">275</a>, <a href="#Page_278">278</a>, <a href="#Page_298">298</a>, <a href="#Page_301">301</a>, <a href="#Page_305">305</a>.</li>
+
+<li>Schuchert, C., cited, <a href="#Page_3">3</a>, <a href="#Page_5">5</a>, <a href="#Page_23">23</a>, * <a href="#Page_25">25</a>, * <a href="#Page_123">123</a>, <a href="#Page_138">138</a>, * <a href="#Page_145">145</a>, <a href="#Page_168">168</a>, <a href="#Page_169">169</a>, <a href="#Page_172">172</a>, <a href="#Page_188">188</a>, <a href="#Page_193">193</a>, <a href="#Page_196">196</a>, <a href="#Page_198">198</a>, <a href="#Page_200">200</a>, * <a href="#Page_201">201</a>, <a href="#Page_206">206</a>, <a href="#Page_211">211</a>, <a href="#Page_230">230</a>, <a href="#Page_265">265</a>.</li>
+
+<li>Schuster, A., cited, <a href="#Page_61">61</a>, <a href="#Page_244">244</a>, <a href="#Page_294">294</a>, <a href="#Page_296">296</a>.</li>
+
+<li>Sculpture, Maya, <a href="#Page_96">96</a>.</li>
+
+<li>Sea level and glaciation, <a href="#Page_119">119</a>.</li>
+
+<li>Seasonal alternations, <a href="#Page_28">28</a> f.</li>
+
+<li>Seasonal banding, <a href="#Page_183">183</a> f.</li>
+
+<li>Seasonal changes, geological, <a href="#Page_183">183</a>.</li>
+
+<li>Seasons, and earthquakes, <a href="#Page_294">294</a>, <a href="#Page_295">295</a>, <a href="#Page_297">297</a>, <a href="#Page_299">299</a>;
+<ul class="none">
+  <li>evidences of, <a href="#Page_169">169</a>.</li>
+  </ul>
+  </li>
+
+<li>Secular progression, <a href="#Page_17">17</a> ff., <a href="#Page_188">188</a>.</li>
+
+<li>Seistan, swamps, <a href="#Page_171">171</a>.</li>
+
+<li>Sequoias, measurements of, <a href="#Page_74">74</a> ff.;
+<ul class="none">
+  <li>rainfall record, <a href="#Page_79">79</a>.</li>
+  </ul>
+  </li>
+
+<li>Setchell, W. A., cited, <a href="#Page_1">1</a>.</li>
+
+<li>Shackleton, E., cited, <a href="#Page_125">125</a>.</li>
+
+<li>Shapley, H., cited, <a href="#Page_246">246</a>, <a href="#Page_247">247</a>, <a href="#Page_254">254</a>, <a href="#Page_256">256</a>, <a href="#Page_275">275</a>.</li>
+
+<li>Shimek, E., cited, <a href="#Page_157">157</a>, <a href="#Page_161">161</a>.</li>
+
+<li>Shreveport, La., rainfall and temperature, <a href="#Page_93">93</a> f.</li>
+
+<li>Shrinkage of the earth, <a href="#Page_190">190</a>.</li>
+
+<li>Siberia, and glaciation, <a href="#Page_132">132</a>.</li>
+
+<li>Sierras, rainfall records, <a href="#Page_82">82</a>.</li>
+
+<li>Simpson, G. C., cited, <a href="#Page_222">222</a>.</li>
+
+<li>Sirius, companion of, <a href="#Page_280">280</a>;
+<ul class="none">
+  <li>distance from sun, <a href="#Page_262">262</a>;</li>
+  <li>luminosity, <a href="#Page_278">278</a>;</li>
+  <li>speed of, <a href="#Page_281">281</a>.</li>
+  </ul>
+  </li>
+
+<li>Slichter, C. S., cited, <a href="#Page_192">192</a>.</li>
+
+<li>Smith, J. W., cited, <a href="#Page_73">73</a>.</li>
+
+<li>Snowfall, glaciation and, <a href="#Page_50">50</a>, <a href="#Page_114">114</a>.</li>
+
+<li>Snowfield, climatic effects of, <a href="#Page_115">115</a>.</li>
+
+<li>Snow line, height of, <a href="#Page_138">138</a>;
+<ul class="none">
+  <li>in Andes, <a href="#Page_139">139</a>;</li>
+  <li>in Himalayas, <a href="#Page_139">139</a>.</li>
+  </ul>
+  </li>
+
+<li>Solar activity, cycles of, <a href="#Page_245">245</a>;
+<ul class="none">
+  <li>deep-sea circulation and, <a href="#Page_179">179</a>;</li>
+  <li>ice and, <a href="#Page_134">134</a>.</li>
+  </ul>
+  </li>
+
+<li>Solar constant, <a href="#Page_114">114</a>.</li>
+
+<li>Solar-cyclonic hypothesis,  <a href="#Page_51">51-63</a>, <a href="#Page_287">287</a>;
+<ul class="none">
+  <li>glaciation and,  <a href="#Page_110">110-129</a>.</li>
+  </ul>
+  </li>
+
+<li>Solar prominences, cause of, <a href="#Page_61">61</a>.</li>
+
+<li>Solar system,  <a href="#Page_252">252</a>;
+<ul class="none">
+  <li>conservation of, <a href="#Page_243">243</a>;</li>
+  <li>proximity to stars, <a href="#Page_63">63</a>.</li>
+  </ul>
+  </li>
+
+<li>Solar variations, storms and, <a href="#Page_31">31</a>.</li>
+
+<li>South America, earthquakes, <a href="#Page_301">301</a>.</li>
+
+<li>South Pole, thickness of ice at, <a href="#Page_125">125</a>.</li>
+
+<li>Southern hemisphere, earthquakes, <a href="#Page_296">296</a>;
+<ul class="none">
+  <li>glaciation in, <a href="#Page_131">131</a> f.</li>
+  </ul>
+  </li>
+
+<li>Southern Pacific railroad, rainfall records along, <a href="#Page_82">82</a>.</li>
+
+<li>Soy beans, effect of light, <a href="#Page_185">185</a> f.</li>
+
+<li>Space, sun's journey through, <a href="#Page_264">264-284</a>.</li>
+
+<li>Spiral nebul&aelig;, <a href="#Page_251">251</a> f.;
+<ul class="none">
+  <li>universe of, <a href="#Page_267">267</a>.</li>
+  </ul>
+  </li>
+
+<li>Spitzbergen, submergence, <a href="#Page_219">219</a>.</li>
+
+<li>Springs, climate and, <a href="#Page_66">66</a>.</li>
+
+<li>Stars, approach to sun, <a href="#Page_253">253</a>;
+<ul class="none">
+  <li>binary,  <a href="#Page_252">252</a>;</li>
+  <li>clusters,  <a href="#Page_252">252</a>, <a href="#Page_268">268</a>;</li>
+  <li>effect on solar atmosphere, <a href="#Page_63">63</a>;</li>
+  <li>dark, <a href="#Page_254">254</a>;</li>
+  <li>parallaxes of, <a href="#Page_276">276</a> f.;</li>
+  <li>tidal action of, <a href="#Page_249">249</a>.</li>
+  </ul>
+  </li>
+
+<li>Stefan's Law, <a href="#Page_257">257</a>.</li>
+
+<li>Stein, M. A., cited, <a href="#Page_78">78</a>.</li>
+
+<li>Stellar approaches, probability of, <a href="#table_5">260</a>.</li>
+
+<li>Storm belt in arid regions, <a href="#Page_144">144</a>.</li>
+
+<li>Storm-floods, in fourteenth century, <a href="#Page_99">99</a>.</li>
+
+<li>Storminess, and erosion, <a href="#Page_309">309</a>;
+<ul class="none">
+  <li>and ice, <a href="#Page_134">134</a>;</li>
+  <li>effect on glaciation, <a href="#Page_112">112</a>;</li>
+  <li>sunspots and, <a href="#Page_163">163</a>;</li>
+  <li>temperature and, <a href="#Page_94">94</a>, <a href="#Page_173">173</a>.</li>
+  </ul>
+  </li>
+
+<li>Storms, blows of, <a href="#Page_300">300</a>, <a href="#Page_302">302</a>;
+<ul class="none">
+  <li>increase, <a href="#Page_60">60</a>;</li>
+  <li>movement of, <a href="#Page_125">125</a> f.;</li>
+  <li>movement of water and, * <a href="#Page_175">175</a>;</li>
+  <li>origin of, <a href="#Page_30">30</a> f.;</li>
+  <li>sunspots and, <a href="#Page_28">28</a>, <a href="#Page_53">53</a>;</li>
+  <li><em>see</em> Cyclones and Lows.</li>
+  </ul>
+  </li>
+
+<li>Storm tracks, during glacial period, <a href="#Page_117">117</a>;
+<ul class="none">
+  <li>location, <a href="#Page_113">113</a>;</li>
+  <li>relation to magnetic poles, <a href="#Page_150">150</a>;</li>
+  <li>shifting of, <a href="#Page_119">119</a>.</li>
+  </ul>
+  </li>
+
+<li>Strands, climate and, <a href="#Page_66">66</a>;
+<ul class="none">
+  <li>in semi-arid regions, <a href="#Page_60">60</a>;</li>
+  <li>of salt lakes, <a href="#Page_142">142</a>.</li>
+  </ul>
+  </li>
+
+<li>Suess, E., cited, <a href="#Page_192">192</a>.</li>
+
+<li>Sun, and the earth's crust, <a href="#Page_285">285-317</a>;
+<ul class="none">
+  <li>approach to star, <a href="#Page_253">253</a>;</li>
+  <li>atmosphere of, <a href="#Page_61">61</a>, <a href="#Page_274">274</a>;</li>
+
+<!--File: cc352.png-->
+<!--<a class="pagenum break" id="Page_328" title="pg 328"></a>-->
+
+  <li>atmosphere of, and weather,  <a href="#Page_52">52;</a></li>
+  <li>cooling of,  <a href="#Page_49">49;</a></li>
+  <li>contraction of,  <a href="#Page_249">249;</a></li>
+  <li>disturbances of, <a href="#Page_172">172</a>;</li>
+  <li>effect of other bodies on,  <a href="#Page_242">242-263</a>;</li>
+  <li>heat, <a href="#Page_13">13</a>;</li>
+  <li>journey through space,  <a href="#Page_264">264-284</a>;</li>
+  <li>Knowlton's hypothesis of,  <a href="#Page_168">168</a>.</li>
+  </ul>
+  </li>
+
+<li>Suncracks,  <a href="#Page_232">232.</a></li>
+
+<li>Sunspot cycles, <a href="#Page_27">27</a> f.</li>
+
+<li>Sunspots, and earthquakes, <a href="#Page_289">289</a>;
+<ul class="none">
+  <li>causes of, <a href="#Page_61">61</a>;</li>
+  <li>magnetic field of, <a href="#Page_261">261</a>;</li>
+  <li>maximum of, <a href="#Page_109">109</a>;</li>
+  <li>mild climates and, <a href="#Page_172">172</a>;</li>
+  <li>number, <a href="#Page_108">7108</a> f.;</li>
+  <li>periodicity, <a href="#Page_243">243</a>;</li>
+  <li>planetary hypothesis of, <a href="#Page_253">253</a>;</li>
+  <li>records, <a href="#Page_245">245</a>;</li>
+  <li>storminess and, <a href="#Page_163">163</a>;</li>
+  <li>storms and, <a href="#Page_300">300</a>;</li>
+  <li>temperature of earth and, <a href="#Page_52">52</a>, <a href="#Page_173">173</a>.</li>
+  </ul>
+  </li>
+
+<li>Sunspot variations, <a href="#Page_282">282</a>.</li>
+
+<li>Swamps, as desert phenomena, <a href="#Page_171">171</a>.</li>
+
+<li>Sylt, storm-flood, <a href="#Page_99">99</a>.</li>
+
+<li>Syria, civilization in, <a href="#Page_67">67</a>;
+<ul class="none">
+  <li>inscriptions in, <a href="#Page_76">76</a>;</li>
+  <li>Roman aqueducts in, <a href="#Page_71">71</a>.</li>
+  </ul>
+  </li>
+
+<li>Syrian Desert, ruins in, <a href="#Page_66">66</a>.</li>
+
+<li>&nbsp;</li>
+
+<li>Talbert, cited, <a href="#Page_213">213</a>.</li>
+
+<li>Tarim Basin, red beds, <a href="#Page_151">151</a>.</li>
+
+<li>Tarim Desert, desiccation, <a href="#Page_66">66</a>.</li>
+
+<li>Tarim River, swamps, <a href="#Page_171">171</a>.</li>
+
+<li>Taylor, G., cited, <a href="#Page_140">140</a>, <a href="#Page_144">144</a>, <a href="#Page_191">191</a>, <a href="#Page_271">271</a>.</li>
+
+<li>Temperature, change of in Atlantic, <a href="#Page_174">174</a>;
+<ul class="none">
+  <li>changes in, <a href="#Page_93">93</a>;</li>
+  <li>climatic change and, <a href="#Page_49">49</a>;</li>
+  <li>critical, <a href="#Page_9">9</a>;</li>
+  <li>geological time and,  <a href="#Page_3">3</a>;</li>
+  <li>glacial period,  <a href="#Page_38">38</a>;</li>
+  <li>glaciation and,  <a href="#Page_42">42</a>,  <a href="#Page_132">132</a>,  <a href="#Page_139">139</a>;</li>
+  <li>gradient of earth,  <a href="#Page_213">213</a>;</li>
+  <li>of ocean,  <a href="#Page_180">180</a>;</li>
+  <li>in Norway,  <a href="#Page_177">177</a>;</li>
+  <li>in Permian,  <a href="#Page_146">146</a> f.;</li>
+  <li>in Proterozoic,  <a href="#Page_146">146</a> f.;</li>
+  <li>limits,  <a href="#Page_6">6</a> ff.;</li>
+  <li>precipitation and,  <a href="#Page_94">94</a>;</li>
+  <li>range of,  <a href="#Page_3">3</a>,  <a href="#Page_8">8</a>;</li>
+  <li>solar activity and,  <a href="#Page_140">140</a>;</li>
+  <li>storminess and,  <a href="#Page_94">94</a>,  <a href="#Page_112">112</a>,  <a href="#Page_173">173</a>;</li>
+  <li>sunspots and,  <a href="#Page_28">28</a>,  <a href="#Page_173">173</a>;</li>
+  <li>volcanic eruptions and,  <a href="#Page_46">46</a>;</li>
+  <li>zones,  <a href="#Page_172">172</a>.</li>
+  </ul>
+  </li>
+
+<li>Terrestial causes of climatic changes,  <a href="#Page_188">188-214</a>.</li>
+
+<li>Tertiary, lava,  <a href="#Page_211">211</a>.</li>
+
+<li>Thames, frozen,  <a href="#Page_98">98</a>.</li>
+
+<li>Thermal solar hypothesis,  <a href="#Page_49">49</a> f.,  <a href="#Page_97">97</a>.</li>
+
+<li>Thermo-pleion, movements of,  <a href="#Page_30">30</a>.</li>
+
+<li>Thesis, of pulsations,  <a href="#Page_24">24</a>.</li>
+
+<li>Thiryu, storm-flood,  <a href="#Page_99">99</a>.</li>
+
+<li>Tian-Shan Mountains, irrigation in,  <a href="#Page_71">71</a>.</li>
+
+<li>Tidal action of stars,  <a href="#Page_249">249</a>.</li>
+
+<li>Tidal effect, of Jupiter,  <a href="#Page_253">253</a>;
+<ul class="none">
+  <li>of planets,  <a href="#Page_244">244</a>.</li>
+  </ul>
+  </li>
+
+<li>Tidal hypothesis,  <a href="#Page_251">251</a>.</li>
+
+<li>Tidal retardation, effect on land and sea,  <a href="#Page_191">191</a>;
+<ul class="none">
+  <li>rotation of earth and,  <a href="#Page_18">18</a> f.;</li>
+  <li>stress caused by,  <a href="#Page_310">310</a>.</li>
+  </ul>
+  </li>
+
+<li>Tides, cycles of,  <a href="#Page_219">219</a>.</li>
+
+<li>Time, geological, <em>see</em> Geological time.</li>
+
+<li>Toads, distribution of,  <a href="#Page_202">202</a>.</li>
+
+<li>Tobacco plant, effect of light,  <a href="#Page_184">184</a>.</li>
+
+<li>Topography, and glaciation,  <a href="#Page_132">132</a>.</li>
+
+<li>Transcaspian Basin, red beds,  <a href="#Page_151">151</a>.</li>
+
+<li>Tree ferns, in New Zealand,  <a href="#Page_179">179</a>.</li>
+
+<li>Tree growth, periodicity in,  <a href="#Page_245">245</a>;
+<ul class="none">
+  <li>rainfall and,  <a href="#Page_79">79</a>.</li>
+  </ul>
+  </li>
+
+<li>Tress, in California,  <a href="#Page_219">219</a>;
+<ul class="none">
+  <li>measurement of,  <a href="#Page_73">73</a> ff.</li>
+  </ul>
+  </li>
+
+<li>Triassic, climate,  <a href="#Page_266">266</a>.</li>
+
+<li>Trifid Nebula, variables,  <a href="#Page_248">248</a>.</li>
+
+<li>Trondheim, wheat in,  <a href="#Page_101">101</a>.</li>
+
+<li>Trondhen&auml;s, corn in,  <a href="#Page_101">101</a>.</li>
+
+<li>Tropical cyclones, in glacial epochs,  <a href="#Page_140">140</a> f.;
+<ul class="none">
+  <li>occurrence,  <a href="#Page_148">148</a>;</li>
+  <li>solar activity and,  <a href="#Page_113">113</a>.</li>
+  </ul>
+  </li>
+
+<li>Tropical hurricanes, earthquakes and,  <a href="#Page_300">300</a>;
+<ul class="none">
+  <li>sunspots and,  <a href="#Page_149">149</a>.</li>
+  </ul>
+  </li>
+
+<li>Turfan, temperature,  <a href="#Page_17">17</a>.</li>
+
+<li>Turner, H. H., cited,  <a href="#Page_245">245</a>.</li>
+
+<li>Tyler, J. M., cited,  <a href="#Page_216">216</a>.</li>
+
+<li>Tyndall, J., cited,  <a href="#Page_36">36</a>, <a href="#Page_37">37</a>.</li>
+
+<li>Typhoon region, "earthquake weather,"  <a href="#Page_298">298</a>.</li>
+
+<li>Typhoons, occurrence,  <a href="#Page_300">300</a>.</li>
+
+<li>&nbsp;</li>
+
+<li>United States, rainfall and temperature in Gulf region,  <a href="#Page_93">93</a> f.;
+<ul class="none">
+  <li>salt lakes in,  <a href="#Page_142">142</a>;</li>
+  <li>southwestern, climate,  <a href="#Page_66">66</a>;</li>
+  <li>storminess,  <a href="#Page_53">53</a> f.,  <a href="#Page_60">60</a>.</li>
+  </ul>
+  </li>
+
+<li>&nbsp;</li>
+
+<li>Variables,  <a href="#Page_247">247</a>.</li>
+
+<li>Veeder, M. A., cited,  <a href="#Page_300">300</a>.</li>
+
+<li>Vegetation, theory of pulsations and,  <a href="#Page_90">90</a>.</li>
+
+<!--File: cc353.png-->
+<!--<a class="pagenum break" id="Page_329" title="pg 329"></a>-->
+
+<li>Venus, atmosphere of,  <a href="#Page_236">236</a>.</li>
+
+<li>Vesterbygd, invasion of,  <a href="#Page_106">106</a>.</li>
+
+<li>Vicksburg, Miss., rainfall and temperature,  <a href="#Page_93">93</a> f.</li>
+
+<li>Volcanic activity, climate and,  <a href="#Page_210">210</a>;
+<ul class="none">
+  <li>movement of the earth's crust and,  <a href="#Page_285">285</a>;</li>
+  <li>times of uplifting lands and,  <a href="#Page_23">23</a>.</li>
+  </ul>
+  </li>
+
+<li>Volcanic dust, climatic changes and,  <a href="#Page_97">97</a>.</li>
+
+<li>Volcanic hypothesis, climatic change and,  <a href="#Page_45">45</a> ff.;
+<ul class="none">
+  <li>snow line,  <a href="#Page_139">139</a>.</li>
+  </ul>
+  </li>
+
+<li>Volcanoes, activity of,  <a href="#Page_96">96</a>.</li>
+
+<li>Volga,  <a href="#Page_108">108</a>.</li>
+
+<li>&nbsp;</li>
+
+<li>Walcott, C. D., cited,  <a href="#Page_4">4</a>,  <a href="#Page_230">230</a>.</li>
+
+<li>Wandering of the pole,  <a href="#Page_302">302</a>.</li>
+
+<li>Water, importance,  <a href="#Page_9">9</a>.</li>
+
+<li>Water vapor, condensation of,  <a href="#Page_56">56</a>;
+<ul class="none">
+  <li>effect on life,  <a href="#Page_231">231</a>;</li>
+  <li>in atmosphere,  <a href="#Page_19">19</a>.</li>
+  </ul>
+  </li>
+
+<li>Wave, effect on movement of water,  <a href="#Page_176">176</a>.</li>
+
+<li>Weather, changes of,  <a href="#Page_31">31</a> f.;
+<ul class="none">
+  <li>origin of,  <a href="#Page_174">174</a>;</li>
+  <li>variations,  <a href="#Page_52">52</a>.</li>
+  </ul>
+  </li>
+
+<li>Wells, H. G., cited,  <a href="#Page_35">35</a>.</li>
+
+<li>Wendingstadt, storm-flood,  <a href="#Page_99">99</a>.</li>
+
+<li>Westerlies,  <a href="#Page_21">21</a> f.</li>
+
+<li>Wheat, price in England,  <a href="#Page_102">102</a>.</li>
+
+<li>White Sea, submergence,  <a href="#Page_219">219</a>.</li>
+
+<li>Whitney, J. D., cited,  <a href="#Page_142">142</a>.</li>
+
+<li>Wieland, G. R., cited,  <a href="#Page_169">169</a>.</li>
+
+<li>Williamson, E. D., cited,  <a href="#Page_226">226</a>.</li>
+
+<li>Willis, B., cited,  <a href="#Page_206">206</a>.</li>
+
+<li>Winds, at ice front,  <a href="#Page_162">162</a>;
+<ul class="none">
+  <li>effect on currents,  <a href="#Page_174">174</a>;</li>
+  <li>glaciation and,  <a href="#Page_133">133</a>;</li>
+  <li>in Antarctica,  <a href="#Page_161">161</a>;</li>
+  <li>in glacial period,  <a href="#Page_119">119</a>;</li>
+  <li>in Greenland,  <a href="#Page_161">161</a>;</li>
+  <li>planetary system of,  <a href="#Page_174">174</a>;</li>
+  <li>velocity,  <a href="#Page_240">240</a>.</li>
+  </ul>
+  </li>
+
+<li>Witch hazel, effect of light,  <a href="#Page_184">184</a>.</li>
+
+<li>Wolf, J. R., cited,  <a href="#Page_61">61</a>,  <a href="#Page_109">109</a>,  <a href="#Page_288">288</a>.</li>
+
+<li>Wolfer, cited,  <a href="#Page_244">244</a>.</li>
+
+<li>Wright, W. B., cited,  <a href="#Page_35">35</a>,  <a href="#Page_111">111</a>,  <a href="#Page_119">119</a>.</li>
+
+<li>Writing, among Mayas,  <a href="#Page_96">96</a>.</li>
+
+<li>&nbsp;</li>
+
+<li>Yucatan, Maya civilization,  <a href="#Page_26">26</a>,  <a href="#Page_107">107</a>;
+<ul class="none">
+  <li>rainfall,  <a href="#Page_108">108</a>;</li>
+  <li>ruins,  <a href="#Page_95">95</a>.</li>
+  </ul>
+  </li>
+
+<li>Yukon, Ice Age in,  <a href="#Page_221">221</a>.</li>
+
+<li>&nbsp;</li>
+
+<li>Zante, earthquakes of,  <a href="#Page_296">296</a>.</li>
+
+<li>Zonal crowding,  <a href="#Page_117">117</a>.</li>
+
+<!--File: cc354.png-->
+<!--<a class="pagenum break" id="Page_330" title="pg 330"></a>-->
+
+</ul>
+
+
+<h4>PRINTED IN THE UNITED STATES OF AMERICA</h4>
+<p>&nbsp;</p>
+<p>&nbsp;</p>
+
+<!--Footnotes:-->
+<div class="footnotes">
+<h3>FOOTNOTES:</h3>
+<div id="fn1"><p><a href="#r1">[1]</a> &nbsp; W. A. Setchell:  The Temperature Interval in the Geographical Distribution
+of Marine Alg&aelig;; Science, Vol. 52, 1920, p. 187.</p></div>
+
+<div id="fn2"><p><a href="#r2">[2]</a> &nbsp; J. Barrell: Rhythms and the Measurements of Geologic Time; Bull.
+Geol. Soc. Am., Vol. 28, Dec., 1917, pp. 745-904.</p></div>
+
+<div id="fn3"><p><a href="#r3">[3]</a> &nbsp; Pirsson and Schuchert: Textbook of Geology, 1915, pp. 538-550.</p></div>
+
+<div id="fn4"><p><a href="#r4">[4]</a> &nbsp; From Charles Schuchert in The Evolution of the Earth and Its Inhabitants:
+Edited by R. S. Lull, New Haven, 1918, but with revisions by Professor Schuchert.</p></div>
+
+<div id="fn5"><p><a href="#r5">[5]</a> &nbsp; J. H. Poynting: Radiation in the Solar System; Phil. Trans. A, 1903,
+202, p. 525.</p></div>
+
+<div id="fn6"><p><a href="#r6">[6]</a> &nbsp; L. J. Henderson: The Fitness of the Environment, 1913.</p></div>
+
+<div id="fn7"><p><a href="#r7">[7]</a> &nbsp; Henderson: <em>loc. cit.</em>, p. 138.</p></div>
+
+<div id="fn8"><p><a href="#r8">[8]</a> &nbsp; F. R. Moulton: Introduction to Astronomy, 1916.</p></div>
+
+<div id="fn9"><p><a href="#r9">[9]</a> &nbsp; Moulton: <em>loc. cit.</em></p></div>
+
+<div id="fn10"><p><a href="#r10">[10]</a> &nbsp; James Croll: Climate and Time, 1876.</p></div>
+
+<div id="fn11"><p><a href="#r11">[11]</a> &nbsp; T. C. Chamberlin:  An attempt to frame a working hypothesis of the
+cause of glacial periods on an atmospheric basis; Jour. Geol., Vol. VII,
+1899, pp. 545-584, 667-685, 757-787.</p>
+
+<p>T. C. Chamberlin and R. D. Salisbury: Geology, Vol. II, 1906, pp. 93-106,
+655-677, and Vol. III, pp. 432-446.</p>
+
+<p>S. Arrhenius (Kosmische Physik, Vol. II, 1903, p. 503) carried out some
+investigations on carbon dioxide which have had a pronounced effect on
+later conclusions.</p>
+
+<p>F. Frech adopted Arrhenius' idea and developed it in a paper entitled
+Ueber die Klima-Aenderungen der Geologischen Vergangenheit. Compte
+Rendu, Tenth (Mexico) Congr. Geol. Intern., 1907 (=1908), pp. 299-325.</p>
+
+<p>The exact origin of the carbon dioxide theory has been stated so variously
+that it seems worth while to give the exact facts. Prompted by the suggestion,
+of Tyndall that glaciation might be due to depletion of atmospheric
+carbon dioxide, Chamberlin worked up the essentials of his early views
+before he saw any publication from Arrhenius, to whom the idea has often
+been attributed. In 1895 or earlier Chamberlin began to give the carbon
+dioxide hypothesis to his students and to discuss it before local scientific
+bodies. In 1897 he prepared a paper on "A Group of Hypotheses Bearing
+on Climatic Changes," Jour. Geol., Vol. V (1897), to be read at the meeting
+of the British Association at Toronto, basing his conclusions on Tyndall's
+determination of the competency of carbon dioxide as an absorber of heat
+radiated from the earth. He had essentially completed this when a paper by
+Arrhenius, "On the influence of carbonic acid in the air upon the temperature
+of the ground," Phil. Mag., 1896, pp. 237-276, first came to his
+attention. Chamberlin then changed his conservative, tentative statement of
+the functions of carbon dioxide to a more sweeping one based on Arrhenius'
+very definite quantitative deductions from Langley's experiments. Both
+Langley and Arrhenius were then in the ascendancy of their reputations
+and seemingly higher authorities could scarcely have been chosen, nor a
+finer combination than experiment and physico-mathematical development.
+Arrhenius' deductions were later proved to have been overstrained, while
+Langley's interpretation and even his observations were challenged. Chamberlin's
+latest views are more like his earlier and more conservative statement.</p></div>
+
+<div id="fn12"><p><a href="#r12">[12]</a> &nbsp; C. G. Abbot and F. E. Fowle: Volcanoes and Climate; Smiths. Misc.
+Coll., Vol. 60, 1913, 24 pp.</p>
+
+<p>W. J. Humphreys: Volcanic dust and other factors in the production of
+climatic and their possible relation to ice ages; Bull. Mount Weather
+Observatory, Vol. 6, Part 1, 1913, 26 pp. Also, Physics of the Air, 1920.</p>
+</div>
+
+<div id="fn13"><p><a href="#r13">[13]</a> &nbsp; H. Arctowski: The Pleonian Cycle of Climatic Fluctuations; Am.
+Jour. Sci., Vol. 42, 1916, pp. 27-33. See also Annals of the New York
+Academy of Sciences, Vol. 24, 1914.</p></div>
+
+<div id="fn14"><p><a href="#r14">[14]</a> &nbsp; W. K&ouml;ppen: &Uuml;ber mehrj&auml;hrige Perioden der Witterung ins besondere
+&uuml;zer die II-j&auml;hrige Periode der Temperatur. Also, Lufttemperaturen
+Sonnenflecke und Vulcanausbr&uuml;che; Meteorologische Zeitschrift, Vol. 7,
+1914, pp. 305-328.</p></div>
+
+<div id="fn15"><p><a href="#r15">[15]</a> &nbsp; The so-called sunspot numbers to which reference is made again and
+again in this book are based on a system devised by Wolf and revised by
+A. Wolfer. The number and size of the spots are both taken into account.
+The numbers from 1749 to 1900 may be found in the Monthly Weather
+Review for April, 1902, and from 1901 to 1918 in the same journal for
+1920.</p></div>
+
+<div id="fn16"><p><a href="#r16">[16]</a> &nbsp; Much of this chapter is taken from The Solar Hypothesis of Climatic
+Changes; Bull. Geol. Soc. Am., Vol. 25, 1914.</p></div>
+
+<div id="fn17"><p><a href="#r17">[17]</a> &nbsp; Ellsworth Huntington: Explorations in Turkestan, 1905; The Pulse of
+Asia, 1907; Palestine and Its Transformation, 1911; The Climatic Factor,
+1915; World Power and Evolution, 1919.</p></div>
+
+<div id="fn18"><p><a href="#r18">[18]</a> &nbsp; J. Hann: Klimatologie, Vol. 1, 1908, p. 352.</p></div>
+
+<div id="fn19"><p><a href="#r19">[19]</a> &nbsp; H. C. Butler: Desert Syria, the Land of a Lost Civilization; Geographical
+Review, Feb., 1920, pp. 77-108.
+</p></div>
+
+<div id="fn20"><p><a href="#r20">[20]</a> &nbsp; This is due to the fact that where these forests occur, in Gilead for
+example, the mountains to the west break down, so that the west winds with
+water from the Mediterranean are able to reach the inner range without
+having lost all their water. It is one of the misfortunes of Syria that its
+mountains generally rise so close to the sea that they shut off rainfall from
+the interior and cause the rain to fall on slopes too steep for easy cultivation.</p></div>
+
+<div id="fn21"><p><a href="#r21">[21]</a> &nbsp; H. Leiter: Die Frage der Klimaanderung waherend geschichtlicher Zeit
+in Nordafrika. Abhandl. K. K. Geographischen Gesellschaft, Wien, 1909, p. 143.</p></div>
+
+<div id="fn22"><p><a href="#r22">[22]</a> &nbsp; A most careful and convincing study of this problem is embodied in
+an article by J. W. Smith: The Effects of Weather upon the Yield of Corn;
+Monthly Weather Review, Vol. 42, 1914, pp. 78-92. On the basis of the
+yield of corn in Ohio for 60 years and in other states for shorter periods,
+he shows that the rainfall of July has almost as much influence on the crop
+as has the rainfall of all other months combined. See his Agricultural
+Meteorology, New York, 1920.</p></div>
+
+<div id="fn23"><p><a href="#r23">[23]</a> &nbsp; See chapter by A. E. Douglass in The Climatic Factor; and his book on
+Climatic Cycles and Tree-Growth; Carnegie Inst., 1919. Also article by
+M. N. Stewart: The Relation of Precipitation to Tree Growth, in the
+Monthly Weather Review, Vol. 41, 1913.</p></div>
+
+<div id="fn24"><p><a href="#r24">[24]</a> &nbsp; The dotted line is taken from Palestine and Its Transformation, pp.
+327 and 403.</p></div>
+
+<div id="fn25"><p><a href="#r25">[25]</a> &nbsp; M. A. Stein: Ruins of Desert Cathay, London, 1912.</p></div>
+
+<div id="fn26"><p><a href="#r26">[26]</a> &nbsp; In the preparation and interpretation of this table the help of Mr.
+G. B. Cressey is gratefully acknowledged.</p></div>
+
+
+<div id="fn27"><p><a href="#r27">[27]</a> &nbsp; For the tree data used in these comparisons, see The Climatic Factor
+P. 328, and A. E. Douglass: Climatic Cycles and Tree Growth, p. 123.</p></div>
+
+<div id="fn28"><p><a href="#r28">[28]</a> &nbsp; One year interpolated.</p></div>
+
+<div id="fn29"><p><a href="#r29">[29]</a> &nbsp; J. W. Gregory: Is the Earth Drying Up?  Geog. Jour., Vol. 43, 1914,
+pp. 148-172 and 293-318.</p></div>
+
+<div id="fn30"><p><a href="#r30">[30]</a> &nbsp; Geog. Jour., Vol. 43, pp. 159-161.</p></div>
+
+<div id="fn31"><p><a href="#r31">[31]</a> &nbsp; See A. J. Henry: Secular Variation of Precipitation in the United
+States; Bull. Am. Geog. Soc., Vol. 46, 1914, pp. 192-201.</p></div>
+
+<div id="fn32"><p><a href="#r32">[32]</a> &nbsp; O. Pettersson: The connection between hydrographical and meteorological
+phenomena; Quarterly Journal of the Royal Meteorological Society, Vol.
+38, pp. 174-175.</p></div>
+
+<div id="fn33"><p><a href="#r33">[33]</a> &nbsp; A. Norlind: Einige Bemerkungen &uuml;ber das Klima der historischen Zeit
+nebst einem Verzeichnis mittelaltlicher Witterungs erscheinungen; Lunds
+Univ. Arsskrift, N. F., Vol. 10, 1914, 53 pp.</p></div>
+
+<div id="fn34"><p><a href="#r34">[34]</a> &nbsp; Thorwald Rogers: A History of Agriculture and Prices in England.</p></div>
+
+<div id="fn35"><p><a href="#r35">[35]</a> &nbsp; E. Br&uuml;ckner: Klimaschwankungen seit 1700, Vienna, 1891.</p></div>
+
+<div id="fn36"><p><a href="#r36">[36]</a> &nbsp; For a full discussion of the changes in the Caspian Sea see The Pulse
+of Asia, pp. 329-358.</p></div>
+
+<div id="fn37"><p><a href="#r37">[37]</a> &nbsp; S. Q. Morley: The Inscriptions at Cop&aacute;n; Carnegie Inst. of Wash., No.
+219, 1920.</p>
+
+<p>Ellsworth Huntington: The Red Man's Continent, 1919.</p></div>
+
+<div id="fn38"><p><a href="#r38">[38]</a> &nbsp; See summary of Wolf's work with additional information by H. Fritz;
+Z&uuml;rich Vierteljahrschrift, Vol. 38, 1893, pp. 77-107.</p></div>
+
+<div id="fn39"><p><a href="#r39">[39]</a> &nbsp; This chapter is an amplification and revision of the sketch of the glacial
+period contained in The Solar Hypothesis of Climatic Changes; Bull. Geol.
+Soc. Am., Vol. 25, 1914.</p></div>
+
+<div id="fn40"><p><a href="#r40">[40]</a> &nbsp; R. D. Salisbury: Physical Geography of the Pleistocene, in Outlines of
+Geologic History, by Willis, Salisbury, and others, 1910, p. 265.</p></div>
+
+<div id="fn41"><p><a href="#r41">[41]</a> &nbsp; The Quaternary Ice Age, 1914, p. 364.</p></div>
+
+<div id="fn42"><p><a href="#r42">[42]</a> &nbsp; For fuller discussion of climatic controls see S. S. Visher: Seventy
+Laws of Climate, Annals Assoc. Am. Geographers, 1922.</p></div>
+
+<div id="fn43"><p><a href="#r43">[43]</a> &nbsp; Many of these alterations are implied or discussed in the following
+papers:</p>
+<ol>
+<li>F. W. Harmer: Influence of Winds upon the Climate of the Pleistocene;
+Quart. Jour. Geol. Soc., Vol. 57, 1901, p. 405.</li>
+
+<li>C. E. P. Brooks: Meteorological Conditions of an Ice Sheet; Quart.
+Jour. Royal Meteorol. Soc., Vol. 40, 1914, pp. 53-70, and The Evolution of
+Climate in Northwest Europe; <em>op. cit.</em>, Vol. 47, 1921, pp. 173-194.</li>
+
+<li>W. H. Hobbs: The R&ocirc;le of the Glacial Anticyclone in the Air Circulation
+of the Globe; Proc. Am. Phil. Soc., Vol. 54, 1915, pp. 185-225.</li>
+</ol></div>
+
+<div id="fn44"><p><a href="#r44">[44]</a> &nbsp; W. B. Wright: The Quaternary Ice Age, 1914, p. 100.</p></div>
+
+
+<div id="fn45"><p><a href="#r45">[45]</a> &nbsp; The description of the distribution of the ice sheet is based on T. C.
+Chamberlin's wall map of North America at the maximum of glaciation, 1913.</p></div>
+
+<div id="fn46"><p><a href="#r46">[46]</a> &nbsp; Chamberlin and Salisbury: Geology, 1906, Vol. 3, and W. H. Hobbs:
+Characteristics of Existing Glaciers, 1911.</p></div>
+
+<div id="fn47"><p><a href="#r47">[47]</a> &nbsp; S. S. Visher: The Geography of South Dakota; S. D. Geol. Surv., 1918.</p></div>
+
+<div id="fn48"><p><a href="#r48">[48]</a> &nbsp; W. H. Hobbs: Characteristics of Existing Glaciers, 1911. The R&ocirc;le of
+the Glacial Anticyclones in the Air Circulation of the Globe; Proc. Am.
+Phil. Soc., Vol. 54, 1915, pp. 185-225.</p></div>
+
+<div id="fn49"><p><a href="#r49">[49]</a> &nbsp; R. D. Salisbury: Physiography, 1919.</p></div>
+
+<div id="fn50"><p><a href="#r50">[50]</a> &nbsp; Griffith Taylor: Australian Meteorology, 1920, p. 283.</p></div>
+
+<div id="fn51"><p><a href="#r51">[51]</a> &nbsp; J. D. Whitney: Climatic Changes of the Later Geological Times, 1882.</p></div>
+
+<div id="fn52"><p><a href="#r52">[52]</a> &nbsp; E. E. Free: U. S. Dept. of Agriculture, Bull. 54, 1914. Mr. Free has
+prepared a summary of this Bulletin which appears in The Solar Hypothesis,
+Bull. Geol. Sec. of Am., Vol. 25, pp. 559-562.</p></div>
+
+<div id="fn53"><p><a href="#r53">[53]</a> &nbsp; G. K. Gilbert: Lake Bonneville; Monograph 1, U. S. Geol. Surv.</p></div>
+
+<div id="fn54"><p><a href="#r54">[54]</a> &nbsp; C. E. P. Brooks: Quart. Jour. Royal Meteorol. Soc., 1914, pp. 63-66.</p></div>
+
+<div id="fn55"><p><a href="#r55">[55]</a> &nbsp; H. J. L. Beadnell: A. Egyptian Oasis, London, 1909.</p>
+
+<p>Ellsworth Huntington: The Libyan Oasis of Kharga; Bull. Am. Geog.
+Soc., Vol. 42, Sept., 1910, pp. 641-661.</p></div>
+
+<div id="fn56"><p><a href="#r56">[56]</a> &nbsp; S. S. Visher: The Bajada of the Tucson Bolson of Southern Arizona;
+Science, N. S., Mar. 23, 1913.</p>
+
+<p>Ellsworth Huntington: The Basins of Eastern Persia and Seistan, in
+Explorations in Turkestan.</p></div>
+
+<div id="fn57"><p><a href="#r57">[57]</a> &nbsp; Griffith Taylor: Australian Meteorology, 1920, p. 189.</p></div>
+
+<div id="fn58"><p><a href="#r58">[58]</a> &nbsp; Chamberlin and Salisbury: Geology, 1906, Vol. III, pp. 405-412.</p></div>
+
+<div id="fn59"><p><a href="#r59">[59]</a> &nbsp; It may have retreated soon after reaching its maximum. If so, the
+general lack of thick terminal moraines would be explained. See page 122.</p></div>
+
+<div id="fn60"><p><a href="#r60">[60]</a> &nbsp; Rollin T. Chamberlin: Personal Communication.</p></div>
+
+<div id="fn61"><p><a href="#r61">[61]</a> &nbsp; F. H. Knowlton: Evolution of Geologic Climates; Bull. Geol. Soc. Am.,
+Vol. 30, 1919, pp. 499-566.</p></div>
+
+<div id="fn62"><p><a href="#r62">[62]</a> &nbsp; Chas. Schuchert: Review of Knowlton's Evolution of Geological Climates,
+in Am. Jour. Sci., 1921.</p></div>
+
+<div id="fn63"><p><a href="#r63">[63]</a> &nbsp; G. R. Wieland: Distribution and Relationships of the Cycadeoids; Am.
+Jour. Bot., Vol. 7, 1920, pp. 125-145.</p></div>
+
+<div id="fn64"><p><a href="#r64">[64]</a> &nbsp; D. T. MacDougal: Botanical Features of North American Deserts;
+Carnegie Instit. of Wash., No. 99, 1908.</p></div>
+
+<div id="fn65"><p><a href="#r65">[65]</a> &nbsp; <em>Loc. cit.</em></p></div>
+
+<div id="fn66"><p><a href="#r66">[66]</a> &nbsp; H. H. Clayton: Variation in Solar Radiation and the Weather; Smiths.
+Misc. Coll., Vol. 71, No. 3, Washington, 1920.</p></div>
+
+<div id="fn67"><p><a href="#r67">[67]</a> &nbsp; B. Helland Hansen and F. Nansen: Temperature Variations in the
+North Atlantic Ocean and in the Atmosphere; Misc. Coll., Smiths. Inst., Vol. 70, No. 4, Washington, 1920.</p></div>
+
+<div id="fn68"><p><a href="#r68">[68]</a> &nbsp; The climatic significance of ocean currents is well discussed in Croll's
+Climate and Time, 1875, and his Climate and Cosmogony, 1889.</p></div>
+
+<div id="fn69"><p><a href="#r69">[69]</a> &nbsp; F. J. B. Cordeiro: The Gyroscope, 1913.</p></div>
+
+<div id="fn70"><p><a href="#r70">[70]</a> &nbsp; W. W. Garner and H. A. Allard: Flowering and Fruition of Plants
+as Controlled by Length of Day; Yearbook Dept. Agri., 1920, pp. 377-400.</p></div>
+
+<div id="fn71"><p><a href="#r71">[71]</a> &nbsp; Report of Committee on Sedimentation, National Research Council,
+April, 1922.</p></div>
+
+<div id="fn72"><p><a href="#r72">[72]</a> &nbsp; Chas. Schuchert: The Earth's Changing Surface and Climate during
+Geologic Time; in Lull: The Evolution of the Earth and Its Inhabitants,
+1918, p. 55.</p></div>
+
+<div id="fn73"><p><a href="#r73">[73]</a> &nbsp; Quoted by J. Cornet: Cours de G&eacute;ologie, 1920, p. 330.</p></div>
+
+<div id="fn74"><p><a href="#r74">[74]</a> &nbsp; T. C. Chamberlin: The Order of Magnitude of the Shrinkage of the
+Earth; Jour. Geol., Vol. 28, 1920, pp. 1-17, 126-157.</p></div>
+
+<div id="fn75"><p><a href="#r75">[75]</a> &nbsp; G. I. Taylor: Philosophical Transactions, A. 220, 1919, pp. 1-33;
+Monthly Notices Royal Astron. Soc., Jan., 1920, Vol. 80, p. 308.</p></div>
+
+<div id="fn76"><p><a href="#r76">[76]</a> &nbsp; J. Jeffreys: Monthly Notices Royal Astron. Soc., Jan., 1920, Vol. 80,
+p. 309.</p></div>
+
+<div id="fn77"><p><a href="#r77">[77]</a> &nbsp; E. W. Brown: personal communication.</p></div>
+
+<div id="fn78"><p><a href="#r78">[78]</a> &nbsp; C. S. Slichter: The Rotational Period of a Heterogeneous Spheroid; in
+Contributions to the Fundamental Problems of Geology, by T. C. Chamberlin,
+<em>et al.</em>, Carnegie Inst. of Wash., No. 107, 1909.</p></div>
+
+<div id="fn79"><p><a href="#r79">[79]</a> &nbsp; E. Suess: The Face of the Earth, Vol. II, p. 553, 1901.</p></div>
+
+<div id="fn80"><p><a href="#r80">[80]</a> &nbsp; Chas. Schuchert: The Earth's Changing Surface and Climate; in Lull:
+The Evolution of the Earth and Its Inhabitants, 1918, p. 78.</p></div>
+
+<div id="fn81"><p><a href="#r81">[81]</a> &nbsp; J. Barren: Rhythms and the Measurement of Geologic Time; Bull.
+Geol. Soc. Am., Vol. 28, 1917, p. 838.</p></div>
+
+<div id="fn82"><p><a href="#r82">[82]</a> &nbsp; Chas. Schuchert: <i>loc. cit.</i>, p. 78.</p></div>
+
+<div id="fn83"><p><a href="#r83">[83]</a> &nbsp; T. C. Chamberlin: Diastrophism, the Ultimate Basis of Correlation;
+Jour. Geol., Vol. 16, 1909; Chas. Schuchert: <em>loc. cit.</em></p></div>
+
+<div id="fn84"><p><a href="#r84">[84]</a> &nbsp; Pirsson-Schuchert: Textbook of Geology, 1915, Vol. II, p. 982; Chas.
+Schuchert: Paleogeography of North America; Bull. Geol. Soc. Am., Vol.
+20, pp. 427-606; reference on p. 499.</p></div>
+
+<div id="fn85"><p><a href="#r85">[85]</a> &nbsp; The general subject of the climatic significance of continentality is
+discussed by C. E. P. Brooks: continentality and Temperature; Quart.
+Jour. Royal Meteorol. Soc., April, 1917, and Oct., 1918.</p></div>
+
+<div id="fn86"><p><a href="#r86">[86]</a> &nbsp; Chas. Schuchert: Climates of Geologic Time; in The Climatic Factor;
+Carnegie Institution, 1914, p. 286.</p></div>
+
+<div id="fn87"><p><a href="#r87">[87]</a> &nbsp; A. de Lapparent: Trait&eacute; de G&eacute;ologie, 1906.</p></div>
+
+<div id="fn88"><p><a href="#r88">[88]</a> &nbsp; Chas. Schuchert: Historical Geology, 1915, p. 464.</p></div>
+
+<div id="fn89"><p><a href="#r89">[89]</a> &nbsp; M. M. Metcalf: Upon an important method of studying problems of
+relationship and of geographical distribution; Proceedings National Academy
+of Sciences, Vol. 6, July, 1920, pp. 432-433.</p></div>
+
+<div id="fn90"><p><a href="#r90">[90]</a> &nbsp; Chas. Schuchert: Paleogeography of North America; Bull. Geol. Soc.
+Am., Vol. 20, 1910; and Willis, Salisbury, and others: Outlines of Geologic
+History, 1910.</p></div>
+
+<div id="fn91"><p><a href="#r91">[91]</a> &nbsp; Chas. Schuchert: The Earth's Changing Surface and Climate; in Lull:
+The Evolution of the Earth and Its Inhabitants, 1918, p. 50.</p></div>
+
+<div id="fn92"><p><a href="#r92">[92]</a> &nbsp; A. J. Henry: The Decrease of Precipitation with Altitude; Monthly
+Weather Review, Vol. 47, 1919, pp. 33-41.</p></div>
+
+<div id="fn93"><p><a href="#r93">[93]</a> &nbsp; Chas. F. Brooks: Monthly Weather Review, Vol. 46, 1918, p. 511; and
+also A. J. Henry and others: Weather Forecasting in the United States,
+1913.</p></div>
+
+<div id="fn94"><p><a href="#r94">[94]</a> &nbsp; F. H. Knowlton: Evolution of Geologic Climates; Bull. Geol. Soc. Am.,
+Vol. 30, Dec., 1919, pp. 499-566.</p></div>
+
+<div id="fn95"><p><a href="#r95">[95]</a> &nbsp; Talbert, quoted by I. Bowman: Forest Physiography, 1911, p. 63.</p></div>
+
+<div id="fn96"><p><a href="#r96">[96]</a> &nbsp; J. Barrell: Rhythms and the Measurement of Geologic Time; Bull.
+Geol. Soc. Am., Vol. 28, 1917, pp. 745-904.</p></div>
+
+<div id="fn97"><p><a href="#r97">[97]</a> &nbsp; C. E. P. Brooks: The Evolution of Climate in Northwest Europe. Quart.
+Jour. Royal Meteorol. Soc., Vol. 47, 1921, pp. 173-194.</p></div>
+
+<div id="fn98"><p><a href="#r98">[98]</a> &nbsp; H. F. Osborn: Men of the Old Stone Age, N. Y., 1915; J. M. Tyler:
+The New Stone Age in Northwestern Europe, N. Y., 1920.</p></div>
+
+<div id="fn99"><p><a href="#r99">[99]</a> &nbsp; Encyclop&aelig;dia Britannica, 11th edition: article "Ocean."</p></div>
+
+<div id="fn100"><p><a href="#r100">[100]</a> &nbsp; C. E. P. Brooks: The Meteorological Conditions of an Ice sheet and
+Their Bearing on the Desiccation of the Globe; Quart. Jour. Royal Meteorol.
+Soc., Vol. 40, 1914, pp. 53-70.</p></div>
+
+<div id="fn101"><p><a href="#r101">[101]</a> &nbsp; Data of Geochemistry, Fourth Ed., 1920; Bull. No. 695, U. S. Geol.
+Survey.</p></div>
+
+<div id="fn102"><p><a href="#r102">[102]</a> &nbsp; Quoted by Schuchert in The Evolution of the Earth.</p></div>
+
+<div id="fn103"><p><a href="#r103">[103]</a> &nbsp; Smithsonian Physical Tables, Sixth Revision, 1914, p. 142.</p></div>
+
+<div id="fn104"><p><a href="#r104">[104]</a> &nbsp; Chamberlin, in a very suggestive article "On a possible reversal of
+oceanic circulation" (Jour. of Geol., Vol. 14, pp. 363-373, 1906), discusses
+the probable climatic consequences of a reversal in the direction of deep-sea
+circulation. It is not wholly beyond the bounds of possibility that,
+in the course of ages the increasing drainage of salt from the lands not
+only by nature but by man's activities in agriculture and drainage, may
+ultimately cause such a reversal by increasing the ocean's salinity until the
+more saline tropical portion is heavier than the cooler but fresher subpolar
+waters. If that should happen, Greenland, Antarctica, and the northern
+shores of America and Asia would be warmed by the tropical heat which
+had been transferred poleward beneath the surface of the ocean, without
+loss <em>en route</em>. Subpolar regions, under such a condition of reversed deep-sea
+circulation, might have a mild climate. Indeed, they might be among the
+world's most favorable regions climatically.</p></div>
+
+<div id="fn105"><p><a href="#r105">[105]</a> &nbsp; Encyclop&aelig;dia Britannica: article "Ocean."</p></div>
+
+<div id="fn106"><p><a href="#r106">[106]</a> &nbsp; Chamberlin and Salisbury: Geology, Vol. II, pp. 1-132, 1906; and T. C.
+Chamberlin: The Origin of the Earth, 1916.</p></div>
+
+<div id="fn107"><p><a href="#r107">[107]</a> &nbsp; Personal communication.</p></div>
+
+<div id="fn108"><p><a href="#r108">[108]</a> &nbsp; R. T. Chamberlin: Gases in Rocks, Carnegie Inst. of Wash., No. 106,
+1908.</p></div>
+
+<div id="fn109"><p><a href="#r109">[109]</a> &nbsp; J. Barrell: The Origin of the Earth, in Evolution of the Earth and
+Its Inhabitants, 1918, p. 44, and more fully in an unpublished manuscript.</p></div>
+
+<div id="fn110"><p><a href="#r110">[110]</a> &nbsp; F. W. Clarke: Data of Geochemistry, Fourth Ed., 1920, Bull. No. 695,
+U. S. Geol. Survey, p. 256.</p></div>
+
+<div id="fn111"><p><a href="#r111">[111]</a> &nbsp; F. W. Clarke: <em>loc. cit.</em>, pp. 27-34 et al.</p></div>
+
+<div id="fn112"><p><a href="#r112">[112]</a> &nbsp; Chas. E. St. John: Science Service Press Reports from the Mt. Wilson
+Observatory, May, 1922.</p></div>
+
+<div id="fn113"><p><a href="#r113">[113]</a> &nbsp; Abbot and Fowle: Annals Astrophysical Observatory; Smiths. Inst.,
+Vol. II, 1908, p. 163.</p>
+
+<p>F. E. Fowle: Atmospheric Scattering of Light; Misc. Coll. Smiths. Inst.,
+Vol. 69, 1918.</p></div>
+
+<div id="fn114"><p><a href="#r114">[114]</a> &nbsp; Abbot and Fowle: <em>loc. cit.</em>, p. 172.</p></div>
+
+<div id="fn115"><p><a href="#r115">[115]</a> &nbsp; H. H. Turner: On a Long Period in Chinese Earthquake Records; Mon.
+Not. Royal Astron. Soc., Vol. 79, 1919, pp. 531-539; Vol. 80, 1920, pp. 617-619;
+Long Period Terms in the Growth of Trees; <em>idem</em>, pp. 793-808.</p></div>
+
+<div id="fn116"><p><a href="#r116">[116]</a> &nbsp; Harlow Shapley: Note on a Possible Factor in Geologic Climates;
+Jour. Geol., Vol. 29, No. 4, May, 1921; Nov&aelig; and Variable Stars, Pub.
+Astron. Soc. Pac., No. 194, Aug., 1921.</p></div>
+
+<div id="fn117"><p><a href="#r117">[117]</a> &nbsp; J. H. Jeans: Problems of Cosmogony and Stellar Dynamics, Cambridge,
+1919.</p></div>
+
+<div id="fn118"><p><a href="#r118">[118]</a> &nbsp; This fact is so important and at the same time so surprising to the
+layman, that a quotation from The Electron Theory of Matter by O. W.
+Richardson, 1914, pp. 326 and 334 is here added.</p>
+<blockquote>
+<p>"It is a very familiar fact that when material bodies are heated they
+emit electromagnetic radiations, in the form of thermal, luminous, and
+actinic rays, in appreciable quantities. Such an effect is a natural consequence
+of the electron and kinetic theories of matter. On the kinetic theory, temperature
+is a measure of the violence of the motion of the ultimate particles;
+and we have seen that on the electron theory, electromagnetic
+radiation is a consequence of their acceleration. The calculation of this
+emission from the standpoint of the electron theory alone is a very complex
+problem which takes us deeply into the structure of matter and which has
+probably not yet been satisfactorily resolved. Fortunately, we can find out
+a great deal about these phenomena by the application of general principles
+like the conservation of energy and the second law of thermodynamics
+without considering special assumptions about the ultimate constitution
+of matter. It is to be borne in mind that the emission under
+consideration occurs at all temperatures although it is more marked the
+higher the temperature.... The energy per unit volume, <em>in vacuo</em>, of the
+radiation in equilibrium in an enclosure at the absolute temperature, T, is
+equal to a universal constant, A, multiplied by the fourth power of the
+absolute temperature. Since the intensity of the radiation is equal to the
+energy per unit volume multiplied by the velocity of light, it follows that
+the former must also be proportional to the fourth power of the absolute
+temperature. Moreover, if E is the total emission from unit area of a
+perfectly black body, we see from p. 330 that E=A&acute;T<sup>4</sup>, where A&acute; is a new
+universal constant. This result is usually known as Stefan's Law. It was
+suggested by Stefan in the inaccurate form that the total radiant energy
+of emission from bodies varies as the fourth power of the absolute temperature,
+as a generalization from the results of experiments. The credit for
+showing that it is a consequence of the existence of radiation pressure
+combined with the principles of thermodynamics is due to Bartoli and
+Boltzmann."</p></blockquote></div>
+
+<div id="fn119"><p><a href="#r119">[119]</a> &nbsp; Quoted by Moulton in his Introduction to Astronomy.</p></div>
+
+<div id="fn120"><p><a href="#r120">[120]</a> &nbsp; Introduction to Astronomy.</p></div>
+
+<div id="fn121"><p><a href="#r121">[121]</a> &nbsp; The term billions, here and elsewhere, is used in the American sense, 10<sup>9</sup>.</p></div>
+
+<div id="fn122"><p><a href="#r122">[122]</a> &nbsp; The assumed number of stars here is ten times as great as in the other
+parts of this line.</p></div>
+
+<div id="fn123"><p><a href="#r123">[123]</a> &nbsp; Lewis Boss: Convergent of a Moving Cluster in Taurus; Astronom.
+Jour., Vol. 26, No. 4, 1908, pp. 31-36.</p></div>
+
+<div id="fn124"><p><a href="#r124">[124]</a> &nbsp; F. R. Moulton: in Introduction to Astronomy, 1916.</p></div>
+
+<div id="fn125"><p><a href="#r125">[125]</a> &nbsp; A. Penck: Die Alpen im Eiszeitalter, Leipzig, 1909.</p></div>
+
+<div id="fn126"><p><a href="#r126">[126]</a> &nbsp; R. D. Salisbury: Physical Geography of the Pleistocene, in Outlines of
+Geologic History, by Willis and Salisbury, 1910, pp. 273-274.</p></div>
+
+<div id="fn127"><p><a href="#r127">[127]</a> &nbsp; Davis, Pumpelly, and Huntington: Explorations in Turkestan, Carnegie
+Inst. of Wash., No. 26, 1905.</p>
+
+<p>In North America the stages have been the subject of intensive studies
+on the part of Taylor, Leverett, Goldthwait, and many others.</p></div>
+
+<div id="fn128"><p><a href="#r128">[128]</a> &nbsp; Double star.</p></div>
+
+<div id="fn129"><p><a href="#r129">[129]</a> &nbsp; E. Kirk: Paleozoic Glaciation in Alaska; Am. Jour. Sci., 1918, p. 511.</p></div>
+
+<div id="fn130"><p><a href="#r130">[130]</a> &nbsp; J. Milne: Catalogue of Destructive Earthquakes; Rep. Brit. Asso. Adv.
+Sci., 1911.</p></div>
+
+<div id="fn131"><p><a href="#r131">[131]</a> &nbsp; Wm. Bowie: Lecture before the Geological Club of Yale University.
+See Am. Jour. Sci., 1921.</p></div>
+
+<div id="fn132"><p><a href="#r132">[132]</a> &nbsp; Chas. Davisson: On the Annual and Semi-annual Seismic Periods;
+Roy. Soc. of London, Philosophical Transactions, Vol. 184, 1893, 1107 <em>ff.</em></p></div>
+
+<div id="fn133"><p><a href="#r133">[133]</a> &nbsp; C. G. Knott: The Physics of Earthquake Phenomena, Oxford, 1908.</p></div>
+
+<div id="fn134"><p><a href="#r134">[134]</a> &nbsp; In Table 8 the first column indicates the region; the second, the dates;
+and the third, the number of shocks. The fourth column gives the month in
+which the annual maximum occurs when the crude figures are smoothed by
+the use of overlapping six-monthly means. In other words, the average for
+each successive six months has been placed in the middle of the period.
+Thus the average of January to June, inclusive, is placed between March
+and April, that for February to July between April and May, and so on.
+This method eliminates the minor fluctuations and also all periodicities
+having a duration of less than a year. If there were no annual periodicity
+the smoothing would result in practically the same figure for each month.
+The column marked "Amplitude" gives the range from the highest month
+to the lowest divided by the number of earthquakes and then corrected
+according to Schuster's method which is well known to mathematicians,
+but which is so confusing to the layman that it will not be described. Next,
+in the column marked "Expected Amplitude," we have the amplitude that
+would be expected if a series of numbers corresponding to the earthquake
+numbers and having a similar range were arranged in accidental order
+throughout the year. This also is calculated by Schuster's method in which
+the expected amplitude is equal to the square root of "pi" divided by the
+number of shocks. When the actual amplitude is four or more times the
+expected amplitude, the probability that there is a real periodicity in the
+observed phenomena becomes so great that we may regard it as practically
+certain. If there is no periodicity the two are equal. The last column gives
+the number of times by which the actual exceeds the expected amplitude,
+and thus is a measure of the probability that earthquakes vary systematically
+in a period of a year.</p></div>
+
+<div id="fn135"><p><a href="#r135">[135]</a> &nbsp; N. F. Drake: Destructive Earthquakes in China; Bull. Seism. Soc. Am.,
+Vol. 2, 1912, pp. 40-91, 124-133.</p></div>
+
+<div id="fn136"><p><a href="#r136">[136]</a> &nbsp; The only other explanation that seems to have any standing is the
+psychological hypothesis of Montessus de Ballore as given in Les Tremblements
+de Terre. He attributes the apparent seasonal variation in earthquakes
+to the fact that in winter people are within doors, and hence notice movements of the earth much more than in summer when they are out of
+doors. There is a similar difference between people's habits in high latitudes
+and low. Undoubtedly this does have a marked effect upon the degree
+to which minor earthquake shocks are noticed. Nevertheless, de Ballore's
+contention, as well as any other psychological explanation, is completely
+upset by two facts: First, instrumental records show the same seasonal distribution
+as do records based on direct observation, and instruments certainly
+are not influenced by the seasons. Second, in some places, notably
+China, as Drake has shown, the summer rather than the winter is very
+decidedly the time when earthquakes are most frequent.</p></div>
+
+<div id="fn137"><p><a href="#r137">[137]</a> &nbsp; A comparison of tropical hurricanes with earthquakes is interesting.
+Taking all the hurricanes recorded in August, September, and October, from
+1880 to 1899, and the corresponding earthquakes in Milne's catalogue, the
+correlation coefficient between hurricanes and earthquakes is +0.236, with a
+probable error of &plusmn;0.082, the month being used as the unit. This is not a
+large correlation, yet when it is remembered that the hurricanes represent
+only a small part of the atmospheric disturbances in any given month, it
+suggests that with fuller data the correlation might be large.</p></div>
+
+<div id="fn138"><p><a href="#r138">[138]</a> &nbsp; Ellsworth Huntington: The Geographic Work of Dr. M. A. Veeder;
+Geog. Rev., Vol. 3, March and April, 1917, Nos. 3 and 4.</p></div>
+
+<div id="fn139"><p><a href="#r139">[139]</a> &nbsp; Frank Schlesinger: Variations of Latitude; Their Bearing upon Our
+Knowledge of the Interior of the Earth; Proc. Am. Phil. Soc., Vol. 54,
+1915, pp. 351-358. Also Smithsonian Report for 1916, pp. 248-254.</p></div>
+
+<div id="fn140"><p><a href="#r140">[140]</a> &nbsp; Harold Jeffreys: Causes Contributory to the Annual Variations of
+Latitude; Monthly Notices, Royal Astronomical Soc., Vol. 76, 1916, pp.
+499-525.</p></div>
+
+<div id="fn141"><p><a href="#r141">[141]</a> &nbsp; John Milne: British Association Reports for 1903 and 1906.</p></div>
+
+<div id="fn142"><p><a href="#r142">[142]</a> &nbsp; C. G. Knott: The Physics of Earthquake Phenomena, Oxford, 1908.</p></div>
+
+<div id="fn143"><p><a href="#r143">[143]</a> &nbsp; A. C. Lawson: The Mobility of the Coast Ranges of California; Univ.
+of Calif. Pub., Geology, Vol. 12, No. 7, pp. 431-473.</p></div>
+</div>
+<!--End Footnotes-->
+</div><!-- End of Book -->
+
+
+
+
+
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+<pre>
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@@ -0,0 +1,11720 @@
+The Project Gutenberg EBook of Climatic Changes, by
+Ellsworth Huntington and Stephen Sargent Visher
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: Climatic Changes
+       Their Nature and Causes
+
+Author: Ellsworth Huntington
+        Stephen Sargent Visher
+
+Release Date: October 26, 2011 [EBook #37855]
+
+Language: English
+
+Character set encoding: ASCII
+
+*** START OF THIS PROJECT GUTENBERG EBOOK CLIMATIC CHANGES ***
+
+
+
+
+Produced by Robin Monks, Wayne Hammond and the Online
+Distributed Proofreading Team at http://www.pgdp.net (This
+file was produced from images generously made available
+by The Internet Archive/Canadian Libraries)
+
+
+
+
+
+
+
+  [TRANSCRIBER'S NOTE: Spelling maintained as closely as possible to the
+  original document, while obvious typos have been corrected. Emdashes
+  in original text for negative temperatures changed to minus signs to
+  standardize temperatures.]
+
+
+
+
+  CLIMATIC CHANGES
+
+  THEIR NATURE AND CAUSES
+
+
+  PUBLISHED ON THE FOUNDATION
+  ESTABLISHED IN MEMORY OF
+  THEODORE L. GLASGOW
+
+
+
+
+  OTHER BOOKS BY THE SAME AUTHORS
+
+
+  ELLSWORTH HUNTINGTON
+
+  A. _Four books showing the development of knowledge as to Historical
+     Pulsations of Climate._
+
+      The Pulse of Asia. Boston, 1907. Explorations in Turkestan.
+      Expedition of 1903. Washington, 1905.
+      Palestine and Its Transformation. Boston, 1911.
+      The Climatic Factor, as Illustrated in Arid America. Washington,
+        1914.
+
+  B. _Two books illustrating the effect of climate on man._
+
+      Civilization and Climate. New Haven, 1915.
+      World Power and Evolution. New Haven, 1919.
+
+  C. _Four books illustrating the general principles of Geography._
+
+      Asia: A Geography Reader. Chicago, 1912.
+      The Red Man's Continent. New Haven, 1919.
+      Principles of Human Geography (with S. W. Cushing). New York,
+        1920.
+      Business Geography (with F. E. Williams). New York, 1922.
+
+  D. _A companion to the present volume._
+
+      Earth and Sun: An Hypothesis of Weather and Sunspots. New Haven.
+        In press.
+
+
+  STEPHEN SARGENT VISHER
+
+  Geography, Geology and Biology of Southern Dakota. Vermilion, 1912.
+  The Biology of Northwestern South Dakota. Vermilion, 1914.
+  The Geography of South Dakota. Vermilion, 1918.
+  Handbook of the Geology of Indiana (with others). Indianapolis, 1922.
+  Hurricanes of Australia and the South Pacific. Melbourne, 1922.
+
+
+
+
+  CLIMATIC CHANGES
+
+  THEIR NATURE AND CAUSES
+
+  BY
+
+  ELLSWORTH HUNTINGTON
+  Research Associate in Geography in Yale University
+
+  AND
+
+  STEPHEN SARGENT VISHER
+  Associate Professor of Geology in Indiana University
+
+  [Illustration]
+
+  NEW HAVEN
+  YALE UNIVERSITY PRESS
+  LONDON: HUMPHREY MILFORD: OXFORD UNIVERSITY PRESS
+  MDCCCCXXII
+
+
+
+  COPYRIGHT 1922 BY
+  YALE UNIVERSITY PRESS
+  Published 1922.
+
+
+
+  THE THEODORE L. GLASGOW MEMORIAL
+  PUBLICATION FUND
+
+
+  The present volume is the fifth work published by the Yale
+  University Press on the Theodore L. Glasgow Memorial Publication
+  Fund. This foundation was established September 17, 1918, by an
+  anonymous gift to Yale University in memory of Flight Sub-Lieutenant
+  Theodore L. Glasgow, R.N. He was born in Montreal, Canada, and was
+  educated at the University of Toronto Schools and at the Royal
+  Military College, Kingston. In August, 1916, he entered the Royal
+  Naval Air Service and in July, 1917, went to France with the Tenth
+  Squadron attached to the Twenty-second Wing of the Royal Flying
+  Corps. A month later, August 19, 1917, he was killed in action on
+  the Ypres front.
+
+
+
+  TO
+
+  THOMAS CHROWDER CHAMBERLIN
+
+  OF THE UNIVERSITY OF CHICAGO
+
+  WHOSE CLEAR AND MASTERLY DISCUSSION OF THE GREAT PROBLEMS
+  OF TERRESTRIAL EVOLUTION HAS BEEN ONE OF THE MOST INSPIRING
+  FACTORS IN THE WRITING OF THIS BOOK
+
+
+
+  _There is a toy, which I have heard, and I would not have it given
+  over, but waited upon a little. They say it is observed in the Low
+  Countries (I know not in what part), that every five and thirty
+  years the same kind and suit of years and weathers comes about
+  again; as great frosts, great wet, great droughts, warm winters,
+  summers with little heat, and the like, and they call it the prime;
+  it is a thing I do the rather mention, because, computing backwards,
+  I have found some concurrence._
+
+  FRANCIS BACON
+
+
+
+
+PREFACE
+
+
+Unity is perhaps the keynote of modern science. This means unity in
+time, for the present is but the outgrowth of the past, and the future
+of the present. It means unity of process, for there seems to be no
+sharp dividing line between organic and inorganic, physical and mental,
+mental and spiritual. And the unity of modern science means also a
+growing tendency toward cooeperation, so that by working together
+scientists discover much that would else have remained hid.
+
+This book illustrates the modern trend toward unity in all of these
+ways. First, it is a companion volume to _Earth and Sun_. That volume is
+a discussion of the causes of weather, but a consideration of the
+weather of the present almost inevitably leads to a study of the climate
+of the past. Hence the two books were written originally as one, and
+were only separated from considerations of convenience. Second, the
+unity of nature is so great that when a subject such as climatic changes
+is considered, it is almost impossible to avoid other subjects, such as
+the movements of the earth's crust. Hence this book not only discusses
+climatic changes, but considers the causes of earthquakes and attempts
+to show how climatic changes may be related to great geological
+revolutions in the form, location, and altitude of the lands. Thus the
+book has a direct bearing on all the main physical factors which have
+molded the evolution of organic life, including man.
+
+In the third place, this volume illustrates the unity of modern science
+because it is preeminently a cooeperative product. Not only have the two
+authors shared in its production, but several of the Yale Faculty have
+also cooeperated. From the geological standpoint, Professor Charles
+Schuchert has read the entire manuscript in its final form as well as
+parts at various stages. He has helped not only by criticisms,
+suggestions, and facts, but by paragraphs ready for the printer. In the
+same way in the domain of physics, Professor Leigh Page has repeatedly
+taken time to assist, and either in writing or by word of mouth has
+contributed many pages. In astronomy, the same cordial cooeperation has
+come with equal readiness from Professor Frank Schlesinger. Professors
+Schuchert, Schlesinger, and Page have contributed so materially that
+they are almost co-authors of the volume. In mathematics, Professor
+Ernest W. Brown has been similarly helpful, having read and criticised
+the entire book. In certain chemical problems, Professor Harry W. Foote
+has been our main reliance. The advice and suggestions of these men have
+frequently prevented errors, and have again and again started new and
+profitable lines of thought. If we have made mistakes, it has been
+because we have not profited sufficiently by their cooeperation. If the
+main hypothesis of this book proves sound, it is largely because it has
+been built up in constant consultation with men who look at the problem
+from different points of vision. Our appreciation of their generous and
+unstinted cooeperation is much deeper than would appear from this brief
+paragraph.
+
+Outside the Yale Faculty we have received equally cordial assistance.
+Professor T. C. Chamberlin of the University of Chicago, to whom, with
+his permission, we take great pleasure in dedicating this volume, has
+read the entire proof and has made many helpful suggestions. We cannot
+speak too warmly of our appreciation not only of this, but of the way
+his work has served for years as an inspiration in the preliminary work
+of gathering data for this volume. Professor Harlow Shapley of Harvard
+University has contributed materially to the chapter on the sun and its
+journey through space; Professor Andrew E. Douglass of the University of
+Arizona has put at our disposal some of his unpublished results;
+Professors S. B. Woodworth and Reginald A. Daly, and Mr. Robert W.
+Sayles of Harvard, and Professor Henry F. Reid of Johns Hopkins have
+suggested new facts and sources of information; Professor E. R. Cumings
+of Indiana University has critically read the entire proof;
+conversations with Professor John P. Buwalda of the University of
+California while he was teaching at Yale make him another real
+contributor; and Mr. Wayland Williams has contributed the interesting
+quotation from Bacon on page x of this book. Miss Edith S. Russell has
+taken great pains in preparing the manuscript and in suggesting many
+changes that make for clearness. Many others have also helped, but it is
+impossible to make due acknowledgment because such contributions have
+become so thoroughly a part of the mental background of the book that
+their source is no longer distinct in the minds of the authors.
+
+The division of labor between the two authors has not followed any set
+rules. Both have had a hand in all parts of the book. The main draft of
+Chapters VII, VIII, IX, XI, and XIII was written by the junior author;
+his contributions are also especially numerous in Chapters X and XV; the
+rest of the book was written originally by the senior author.
+
+
+
+
+CONTENTS
+
+
+                                                                PAGE
+
+     I. The Uniformity of Climate                                  1
+
+    II. The Variability of Climate                                16
+
+   III. Hypotheses of Climatic Change                             33
+
+    IV. The Solar Cyclonic Hypothesis                             51
+
+     V. The Climate of History                                    64
+
+    VI. The Climatic Stress of the Fourteenth Century             98
+
+   VII. Glaciation According to the Solar Cyclonic Hypothesis    110
+
+  VIII. Some Problems of Glacial Periods                         130
+
+    IX. The Origin of Loess                                      155
+
+     X. Causes of Mild Geological Climates                       166
+
+    XI. Terrestrial Causes of Climatic Changes                   188
+
+   XII. Post-Glacial Crustal Movements and Climatic Changes      215
+
+  XIII. The Changing Composition of Oceans and Atmosphere        223
+
+   XIV. The Effect of Other Bodies on the Sun                    242
+
+    XV. The Sun's Journey through Space                          264
+
+   XVI. The Earth's Crust and the Sun                            285
+
+
+
+
+LIST OF ILLUSTRATIONS
+
+
+                                                                PAGE
+
+  Fig. 1. Climatic changes and mountain building                  25
+
+  Fig. 2. Storminess at sunspot maxima vs. minima                 54
+
+  Fig. 3. Relative rainfall at times of increasing and
+          decreasing sunspots                                 58, 59
+
+  Fig. 4. Changes of climate in California and in western
+          and central Asia                                        75
+
+  Fig. 5. Changes in California climate for 2000 years, as
+           measured by growth of Sequoia trees                    77
+
+  Fig. 6. Distribution of Pleistocene ice sheets                 123
+
+  Fig. 7. Permian geography and glaciation                       145
+
+  Fig. 8. Effect of diminution of storms on movement of water    175
+
+  Fig. 9. Cretaceous Paleogeography                              201
+
+  Fig. 10. Climatic changes of 140,000 years as inferred from
+           the stars                                             279
+
+  Fig. 11. Sunspot curve showing cycles, 1750 to 1920            283
+
+  Fig. 12. Seasonal distribution of earthquakes                  299
+
+  Fig. 13. Wandering of the pole from 1890 to 1898               303
+
+
+
+
+TABLES
+
+
+                                                                PAGE
+
+  1. The Geological Time Table                                     5
+
+  2. Types of Climatic Sequence                                   16
+
+  3. Correlation Coefficients between Rainfall and Growth of
+     Sequoias in California                                       80
+
+  4. Correlation Coefficients between Rainfall Records in
+     California and Jerusalem                                     84
+
+  5. Theoretical Probability of Stellar Approaches               260
+
+  6. Thirty-Eight Stars Having Largest Known Parallaxes     276, 277
+
+  7. Destructive Earthquakes from 1800 to 1899 Compared with
+     Sunspots                                                    289
+
+  8. Seasonal March of Earthquakes                               295
+
+  9. Deflection of Path of Pole Compared with Earthquakes        305
+
+  10. Earthquakes in 1903 to 1908 Compared with Departures of
+      the Projected Curve of the Earth's Axis from the
+      Eulerian Position                                          306
+
+
+
+
+CHAPTER I
+
+THE UNIFORMITY OF CLIMATE
+
+
+The role of climate in the life of today suggests its importance in the
+past and in the future. No human being can escape from the fact that his
+food, clothing, shelter, recreation, occupation, health, and energy are
+all profoundly influenced by his climatic surroundings. A change of
+season brings in its train some alteration in practically every phase of
+human activity. Animals are influenced by climate even more than man,
+for they have not developed artificial means of protecting themselves.
+Even so hardy a creature as the dog becomes notably different with a
+change of climate. The thick-haired "husky" of the Eskimos has outwardly
+little in common with the small and almost hairless canines that grovel
+under foot in Mexico. Plants are even more sensitive than animals and
+men. Scarcely a single species can flourish permanently in regions which
+differ more than 20 deg.C. in average yearly temperature, and for most the
+limit of successful growth is 10 deg..[1] So far as we yet know every living
+species of plant and animal, including man, thrives best under definite
+and limited conditions of temperature, humidity, and sunshine, and of
+the composition and movement of the atmosphere or water in which it
+lives. Any departure beyond the limits means lessened efficiency, and in
+the long run a lower rate of reproduction and a tendency toward changes
+in specific characteristics. Any great departure means suffering or
+death for the individual and destruction for the species.
+
+Since climate has so profound an influence on life today, it has
+presumably been equally potent at other times. Therefore few scientific
+questions are more important than how and why the earth's climate has
+varied in the past, and what changes it is likely to undergo in the
+future. This book sets forth what appear to be the chief reasons for
+climatic variations during historic and geologic times. It assumes that
+causes which can now be observed in operation, as explained in a
+companion volume entitled _Earth and Sun_, and in such books as
+Humphreys' _Physics of the Air_, should be carefully studied before less
+obvious causes are appealed to. It also assumes that these same causes
+will continue to operate, and are the basis of all valid predictions as
+to the weather or climate of the future.
+
+In our analysis of climatic variations, we may well begin by inquiring
+how the earth's climate has varied during geological history. Such an
+inquiry discloses three great tendencies, which to the superficial view
+seem contradictory. All, however, have a similar effect in providing
+conditions under which organic evolution is able to make progress. The
+first tendency is toward uniformity, a uniformity so pronounced and of
+such vast duration as to stagger the imagination. Superposed upon this
+there seems to be a tendency toward complexity. During the greater part
+of geological history the earth's climate appears to have been
+relatively monotonous, both from place to place and from season to
+season; but since the Miocene the rule has been diversity and
+complexity, a condition highly favorable to organic evolution. Finally,
+the uniformity of the vast eons of the past and the tendency toward
+complexity are broken by pulsatory changes, first in one direction and
+then in another. To our limited human vision some of the changes, such
+as glacial periods, seem to be waves of enormous proportions, but
+compared with the possibilities of the universe they are merely as the
+ripples made by a summer zephyr.
+
+The uniformity of the earth's climate throughout the vast stretches of
+geological time can best be realized by comparing the range of
+temperature on the earth during that period with the possible range as
+shown in the entire solar system. As may be seen in Table 1, the
+geological record opens with the Archeozoic era, or "Age of Unicellular
+Life," as it is sometimes called, for the preceding cosmic time has left
+no record that can yet be read. Practically no geologists now believe
+that the beginning of the Archeozoic was less than one hundred million
+years ago; and since the discovery of the peculiar properties of radium
+many of the best students do not hesitate to say a billion or a billion
+and a half.[2] Even in the Archeozoic the rocks testify to a climate
+seemingly not greatly different from that of the average of geologic
+time. The earth's surface was then apparently cool enough so that it was
+covered with oceans and warm enough so that the water teemed with
+microscopic life. The air must have been charged with water vapor and
+with carbon dioxide, for otherwise there seems to be no possible way of
+explaining the formation of mudstones and sandstones, limestones of vast
+thickness, carbonaceous shales, graphites, and iron ores.[3] Although
+the Archeozoic has yielded no generally admitted fossils, yet what seem
+to be massive algae and sponges have been found in Canada. On the other
+hand, abundant life is believed to have been present in the oceans, for
+by no other known means would it be possible to take from the air the
+vast quantities of carbon that now form carbonaceous shales and
+graphite.
+
+In the next geologic era, the Proterozoic, the researches of Walcott
+have shown that besides the marine algae there must have been many other
+kinds of life. The Proterozoic fossils thus far discovered include not
+only microscopic radiolarians such as still form the red ooze of the
+deepest ocean floors, but the much more significant tubes of annelids or
+worms. The presence of the annelids, which are relatively high in the
+scale of organization, is generally taken to mean that more lowly forms
+of animals such as coelenterates and probably even the mollusca and
+primitive arthropods must already have been evolved. That there were
+many kinds of marine invertebrates living in the later Proterozoic is
+indicated by the highly varied life and more especially the trilobites
+found in the oldest Cambrian strata of the next succeeding period. In
+fact the Cambrian has sponges, primitive corals, a great variety of
+brachiopods, the beginnings of gastropods, a wonderful array of
+trilobites, and other lowly forms of arthropods. Since, under the
+postulate of evolution, the life of that time forms an unbroken sequence
+with that of the present, and since many of the early forms differ only
+in minor details from those of today, we infer that the climate then was
+not very different from that of today. The same line of reasoning leads
+to the conclusion that even in the middle of the Proterozoic, when
+multicellular marine animals must already have been common, the climate
+of the earth had already for an enormous period been such that all the
+lower types of oceanic invertebrates had already evolved.
+
+TABLE 1
+
+THE GEOLOGICAL TIME TABLE[4]
+
+  COSMIC TIME
+
+    FORMATIVE ERA. Birth and growth of the earth. Beginnings of the
+    atmosphere, hydrosphere, continental platforms, oceanic basins,
+    and possibly of life. No known geological record.
+
+  GEOLOGIC TIME
+
+    ARCHEOZOIC ERA. Origin of simplest life.
+
+    PROTEROZOIC ERA. Age of invertebrate origins. An early and a late
+    ice age, with one or more additional ones indicated.
+
+    PALEOZOIC ERA. Age of primitive vertebrate dominance.
+
+      _Cambrian Period._ First abundance of marine animals and dominance
+      of trilobites.
+
+      _Ordovician Period._ First known fresh-water fishes.
+
+      _Silurian Period._ First known land plants.
+
+      _Devonian Period._ First known amphibians. "Table Mountain" ice
+      age.
+
+      _Mississippian Period._ Rise of marine fishes (sharks).
+
+      _Pennsylvanian Period._ Rise of insects and first period of marked
+      coal accumulation.
+
+      _Permian Period._ Rise of reptiles. Another great ice age.
+
+    MESOZOIC ERA. Age of reptile dominance.
+
+      _Triassic Period._ Rise of dinosaurs. The period closes with a
+      cool climate.
+
+      _Jurassic Period._ Rise of birds and flying reptiles.
+
+      _Comanchean Period._ Rise of flowering plants and higher insects.
+
+      _Cretaceous Period._ Rise of archaic or primitive mammalia.
+
+    CENOZOIC ERA. Age of mammal dominance.
+
+      _Early Cenozoic or Eocene and Oligocene time._ Rise of higher
+      mammals. Glaciers in early Eocene of the Laramide Mountains.
+
+      _Late Cenozoic or Miocene and Pliocene time._ Transformation of
+      ape like animals into man.
+
+      _Glacial or Pleistocene time._ Last great ice age.
+
+  PRESENT TIME
+
+    PSYCHOZOIC ERA. Age of man or age of reason. Includes the present or
+    "Recent time," estimated to be probably less than 30,000 years.
+
+Moreover, they could live in most latitudes, for the indirect evidences
+of life in the Archeozoic and Proterozoic rocks are widely distributed.
+Thus it appears that at an almost incredibly early period, perhaps many
+hundred million years ago, the earth's climate differed only a little
+from that of the present.
+
+The extreme limits of temperature beyond which the climate of geological
+times cannot have departed can be approximately determined. Today the
+warmest parts of the ocean have an average temperature of about 30 deg.C. on
+the surface. Only a few forms of life live where the average temperature
+is much higher than this. In deserts, to be sure, some highly organized
+plants and animals can for a short time endure a temperature as high as
+75 deg.C. (167 deg.F.). In certain hot springs, some of the lowest unicellular
+plant forms exist in water which is only a little below the boiling
+point. More complex forms, however, such as sponges, worms, and all the
+higher plants and animals, seem to be unable to live either in water or
+air where the temperature averages above 45 deg.C. (113 deg.F.) for any great
+length of time and it is doubtful whether they can thrive permanently
+even at that temperature. The obvious unity of life for hundreds of
+millions of years and its presence at all times in middle latitudes so
+far as we can tell seem to indicate that since the beginning of marine
+life the temperature of the oceans cannot have averaged much above 50 deg.C.
+even in the warmest portions. This is putting the limit too high rather
+than too low, but even so the warmest parts of the earth can scarcely
+have averaged much more than 20 deg. warmer than at present.
+
+Turning to the other extreme, we may inquire how much colder than now
+the earth's surface may have been since life first appeared. Proterozoic
+fossils have been found in places where the present average temperature
+approaches 0 deg.C. If those places should be colder than now by 30 deg.C., or
+more, the drop in temperature at the equator would almost certainly be
+still greater, and the seas everywhere would be permanently frozen. Thus
+life would be impossible. Since the contrasts between summer and winter,
+and between the poles and the equator seem generally to have been less
+in the past than at present, the range through which the mean
+temperature of the earth as a whole could vary without utterly
+destroying life was apparently less than would now be the case.
+
+These considerations make it fairly certain that for at least several
+hundred million years the average temperature of the earth's surface has
+never varied more than perhaps 30 deg.C. above or below the present level.
+Even this range of 60 deg.C. (108 deg.F.) may be double or triple the range that
+has actually occurred. That the temperature has not passed beyond
+certain narrow limits, whatever their exact degree, is clear from the
+fact that if it had done so, all the higher forms of life would have
+been destroyed. Certain of the lowest unicellular forms might indeed
+have persisted, for when dormant they can stand great extremes of dry
+heat and of cold for a long time. Even so, evolution would have had to
+begin almost anew. The supposition that such a thing has happened is
+untenable, for there is no hint of any complete break in the record of
+life during geological times,--no sudden disappearance of the higher
+organisms followed by a long period with no signs of life other than
+indirect evidence such as occurs in the Archeozoic.
+
+A change of 60 deg.C. or even of 20 deg. in the average temperature of the
+earth's surface may seem large when viewed from the limited standpoint
+of terrestrial experience. Viewed, however, from the standpoint of
+cosmic evolution, or even of the solar system, it seems a mere trifle.
+Consider the possibilities. The temperature of empty space is the
+absolute zero, or -273 deg.C. To this temperature all matter must fall,
+provided it exists long enough and is not appreciably heated by
+collisions or by radiation. At the other extreme lies the temperature of
+the stars. As stars go, our sun is only moderately hot, but the
+temperature of its surface is calculated to be nearly 7000 deg.C., while
+thousands of miles in the interior it may rise to 20,000 deg. or 100,000 deg. or
+some other equally unknowable and incomprehensible figure. Between the
+limits of the absolute zero on the one hand, and the interior of a sun
+or star on the other, there is almost every conceivable possibility of
+temperature. Today the earth's surface averages not far from 14 deg.C., or
+287 deg. above the absolute zero. Toward the interior, the temperature in
+mines and deep wells rises about 1 deg.C. for every 100 meters. At this rate
+it would be over 500 deg.C. at a depth of ten miles, and over 5000 deg. at 100
+miles.
+
+Let us confine ourselves to surface temperatures, which are all that
+concern us in discussing climate. It has been calculated by Poynting[5]
+that if a small sphere absorbed and re-radiated all the heat that fell
+upon it, its temperature at the distance of Mercury from the sun would
+average about 210 deg.C.; at the distance of Venus, 85 deg.; the earth 27 deg.; Mars
+-30 deg.; Neptune -219 deg.. A planet much nearer the sun than is Mercury might
+be heated to a temperature of a thousand, or even several thousand,
+degrees, while one beyond Neptune would remain almost at absolute zero.
+It is well within the range of possibility that the temperature of a
+planet's surface should be anywhere from near -273 deg.C. up to perhaps
+5000 deg.C. or more, although the probability of low temperature is much
+greater than of high. Thus throughout the whole vast range of
+possibilities extending to perhaps 10,000 deg., the earth claims only 60 deg. at
+most, or less than 1 per cent. This may be remarkable, but what is far
+more remarkable is that the earth's range of 60 deg. includes what seem to
+be the two most critical of all possible temperatures, namely, the
+freezing point of water, 0 deg.C., and the temperature where water can
+dissolve an amount of carbon dioxide equal to its own volume. The most
+remarkable fact of all is that the earth has preserved its temperature
+within these narrow limits for a hundred million years, or perchance a
+thousand million.
+
+To appreciate the extraordinary significance of this last fact, it is
+necessary to realize how extremely critical are the temperatures from
+about 0 deg. to 40 deg.C., and how difficult it is to find any good reason for a
+relatively uniform temperature through hundreds of millions of years.
+Since the dawn of geological time the earth's temperature has apparently
+always included the range from about the freezing point of water up to
+about the point where protoplasm begins to disintegrate. Henderson, in
+_The Fitness of the Environment_, rightly says that water is "the most
+familiar and the most important of all things." In many respects water
+and carbon dioxide form the most unique pair of substances in the whole
+realm of chemistry. Water has a greater tendency than any other known
+substance to remain within certain narrowly defined limits of
+temperature. Not only does it have a high specific heat, so that much
+heat is needed to raise its temperature, but on freezing it gives up
+more heat than any substance except ammonia, while none of the common
+liquids approach it in the amount of additional heat required for
+conversion into vapor after the temperature of vaporization has been
+reached. Again, water substance, as the physicists call all forms of
+H_{2}O, is unique in that it not only contracts on melting, but
+continues to contract until a temperature several degrees above its
+melting point is reached. That fact has a vast importance in helping to
+keep the earth's surface at a uniform temperature. If water were like
+most liquids, the bottoms of all the oceans and even the entire body of
+water in most cases would be permanently frozen.
+
+Again, as a solvent there is literally nothing to compare with water. As
+Henderson[6] puts it: "Nearly the whole science of chemistry has been
+built up around water and aqueous solution." One of the most significant
+evidences of this is the variety of elements whose presence can be
+detected in sea water. According to Henderson they include hydrogen,
+oxygen, nitrogen, carbon, chlorine, sodium, magnesium, sulphur,
+phosphorus, which are easily detected; and also arsenic, caesium, gold,
+lithium, rubidium, barium, lead, boron, fluorine, iron, iodine, bromine,
+potassium, cobalt, copper, manganese, nickel, silver, silicon, zinc,
+aluminium, calcium, and strontium. Yet in spite of its marvelous power
+of solution, water is chemically rather inert and relatively stable. It
+dissolves all these elements and thousands of their compounds, but still
+remains water and can easily be separated and purified. Another unique
+property of water is its power of ionizing dissolved substances, a
+property which makes it possible to produce electric currents in
+batteries. This leads to an almost infinite array of electro-chemical
+reactions which play an almost dominant role in the processes of life.
+Finally, no common liquid except mercury equals water in its power of
+capillarity. This fact is of enormous moment in biology, most obviously
+in respect to the soil.
+
+Although carbon dioxide is far less familiar than water, it is almost as
+important. "These two simple substances," says Henderson, "are the
+common source of every one of the complicated substances which are
+produced by living beings, and they are the common end products of the
+wearing away of all the constituents of protoplasm, and of the
+destruction of those materials which yield energy to the body." One of
+the remarkable physical properties of carbon dioxide is its degree of
+solubility in water. This quality varies enormously in different
+substances. For example, at ordinary pressures and temperatures, water
+can absorb only about 5 per cent of its own volume of oxygen, while it
+can take up about 1300 times its own volume of ammonia. Now for carbon
+dioxide, unlike most gases, the volume that can be absorbed by water is
+nearly the same as the volume of the water. The volumes vary, however,
+according to temperature, being absolutely the same at a temperature of
+about 15 deg.C. or 59 deg.F., which is close to the ideal temperature for man's
+physical health and practically the same as the mean temperature of the
+earth's surface when all seasons are averaged together. "Hence, when
+water is in contact with air, and equilibrium has been established, the
+amount of free carbonic acid in a given volume of water is almost
+exactly equal to the amount in the adjacent air. Unlike oxygen,
+hydrogen, and nitrogen, carbonic acid enters water freely; unlike
+sulphurous oxide and ammonia, it escapes freely from water. Thus the
+waters can never wash carbonic acid completely out of the air, nor can
+the air keep it from the waters. It is the one substance which thus, in
+considerable quantities relative to its total amount, everywhere
+accompanies water. In earth, air, fire, and water alike these two
+substances are always associated.
+
+"Accordingly, if water be the first primary constituent of the
+environment, carbonic acid is inevitably the second,--because of its
+solubility possessing an equal mobility with water, because of the
+reservoir of the atmosphere never to be depleted by chemical action in
+the oceans, lakes, and streams. In truth, so close is the association
+between these two substances that it is scarcely correct logically to
+separate them at all; together they make up the real environment and
+they never part company."[7]
+
+The complementary qualities of carbon dioxide and water are of supreme
+importance because these two are the only known substances which are
+able to form a vast series of complex compounds with highly varying
+chemical formulae. No other known compounds can give off or take on atoms
+without being resolved back into their elements. No others can thus
+change their form freely without losing their identity. This power of
+change without destruction is the fundamental chemical characteristic of
+life, for life demands complexity, change, and growth.
+
+In order that water and carbon dioxide may combine to form the compounds
+on which life is based, the water must be in the liquid form, it must be
+able to dissolve carbon dioxide freely, and the temperature must not be
+high enough to break up the highly complex and delicate compounds as
+soon as they are formed. In other words, the temperature must be above
+freezing, while it must not rise higher than some rather indefinite
+point between 50 deg.C. and the boiling point, where all water finally turns
+into vapor. In the whole range of temperature, so far as we know, there
+is no other interval where any such complex reactions take place. The
+temperature of the earth for hundreds of millions of years has remained
+firmly fixed within these limits.
+
+The astonishing quality of the earth's uniformity of temperature becomes
+still more apparent when we consider the origin of the sun's heat. What
+that origin is still remains a question of dispute. The old ideas of a
+burning sun, or of one that is simply losing an original supply of heat
+derived from some accident, such as collision with another body, were
+long ago abandoned. The impact of a constant supply of meteors affords
+an almost equally unsatisfactory explanation. Moulton[8] states that if
+the sun were struck by enough meteorites to keep up its heat, the earth
+would almost certainly be struck by enough so that it would receive
+about half of 1 per cent as much heat from them as from the sun. This is
+millions of times more heat than is now received from meteors. If the
+sun owes its heat to the impact of larger bodies at longer intervals,
+the geological record should show a series of interruptions far more
+drastic than is actually the case.
+
+It has also been supposed that the sun owes its heat to contraction. If
+a gaseous body contracts it becomes warmer. Finally, however, it must
+become so dense that its rate of contraction diminishes and the process
+ceases. Under the sun's present condition of size and density a radial
+contraction of 120 feet per year would be enough to supply all the
+energy now radiated by that body. This seems like a hopeful source of
+energy, but Kelvin calculated that twenty million years ago it was
+ineffective and ten million years hence it will be equally so. Moreover,
+if this is the source of heat, the amount of radiation from the sun
+would have to vary enormously. Twenty million years ago the sun would
+have extended nearly to the earth's orbit and would have been so tenuous
+that it would have emitted no more heat than some of the nebulae in
+space. Some millions of years later, when the sun's radius was twice as
+great as at present, that body would have emitted only one-fourth as
+much heat as now, which would mean that on the earth's surface the
+theoretical temperature would have been 200 deg. below the present level.
+This is utterly out of accord with the uniformity of climate shown by
+the geological record. In the future, if the sun's contraction is the
+only source of heat, the sun can supply the present amount for only ten
+million years, which would mean a change utterly unlike anything of
+which the geological record holds even the faintest hint.[9]
+
+Altogether the problem of how the sun can have remained so uniform and
+how the earth's atmosphere and other conditions can also have remained
+so uniform throughout hundreds of millions of years is one of the most
+puzzling in the whole realm of nature. If appeal is taken to
+radioactivity and the breaking up of uranium into radium and helium,
+conditions can be postulated which will give the required amount of
+energy. Such is also the case if it be supposed that there is some
+unknown process which may induce an atomic change like radioactivity in
+bodies which are now supposed to be stable elements. In either case,
+however, there is as yet no satisfactory explanation of the _uniformity_
+of the earth's climate. A hundred million or a thousand million years
+ago the temperature of the earth's surface was very much the same as
+now. The earth had then presumably ceased to emit any great amount of
+heat, if we may judge from the fact that its surface was cool enough so
+that great ice sheets could accumulate on low lands within 40 deg. of the
+equator. The atmosphere was apparently almost like that of today, and
+was almost certainly not different enough to make up for any great
+divergence of the sun from its present condition. We cannot escape the
+stupendous fact that in those remote times the sun must have been
+essentially the same as now, or else that some utterly unknown factor is
+at work.
+
+FOOTNOTES:
+
+[Footnote 1: W. A. Setchell: The Temperature Interval in the
+Geographical Distribution of Marine Algae; Science, Vol. 52, 1920,
+p. 187.]
+
+[Footnote 2: J. Barrell: Rhythms and the Measurements of Geologic Time;
+Bull. Geol. Soc. Am., Vol. 28, Dec., 1917, pp. 745-904.]
+
+[Footnote 3: Pirsson and Schuchert: Textbook of Geology, 1915, pp.
+538-550.]
+
+[Footnote 4: From Charles Schuchert in The Evolution of the Earth and
+Its Inhabitants: Edited by R. S. Lull, New Haven, 1918, but with
+revisions by Professor Schuchert.]
+
+[Footnote 5: J. H. Poynting: Radiation in the Solar System; Phil. Trans.
+A, 1903, 202, p. 525.]
+
+[Footnote 6: L. J. Henderson: The Fitness of the Environment, 1913.]
+
+[Footnote 7: Henderson: _loc. cit._, p. 138.]
+
+[Footnote 8: F. R. Moulton: Introduction to Astronomy, 1916.]
+
+[Footnote 9: Moulton: _loc. cit._]
+
+
+
+
+CHAPTER II
+
+THE VARIABILITY OF CLIMATE
+
+
+The variability of the earth's climate is almost as extraordinary as its
+uniformity. This variability is made up partly of a long, slow tendency
+in one direction and partly of innumerable cycles of every conceivable
+duration from days, or even hours, up to millions of years. Perhaps the
+easiest way to grasp the full complexity of the matter is to put the
+chief types of climatic sequence in the form of a table.
+
+   +-----------------------------------------------------------+
+   |                                                           |
+   |                            TABLE 2                        |
+   |                                                           |
+   |                   TYPES OF CLIMATIC SEQUENCE              |
+   |                                                           |
+   |  1. Cosmic uniformity.         7. Brueckner periods.       |
+   |  2. Secular progression.       8. Sunspot cycles.         |
+   |  3. Geologic oscillations.     9. Seasonal alternations.  |
+   |  4. Glacial fluctuations.     10. Pleionian migrations.   |
+   |  5. Orbital precessions.      11. Cyclonic vacillations.  |
+   |  6. Historical pulsations.    12. Daily vibrations.       |
+   |                                                           |
+   +-----------------------------------------------------------+
+
+In assigning names to the various types an attempt has been made to
+indicate something of the nature of the sequence so far as duration,
+periodicity, and general tendencies are concerned. Not even the rich
+English language of the twentieth century, however, furnishes words with
+enough shades of meaning to express all that is desired. Moreover,
+except in degree, there is no sharp distinction between some of the
+related types, such as glacial fluctuations and historic pulsations.
+Yet, taken as a whole, the table brings out the great contrast between
+two absolutely diverse extremes. At the one end lies well-nigh eternal
+uniformity, or an extremely slow progress in one direction throughout
+countless ages; at the other, rapid and regular vibrations from day to
+day, or else irregular and seemingly unsystematic vacillations due to
+cyclonic storms, both of which types are repeated millions of times
+during even a single glacial fluctuation.
+
+The meaning of cosmic uniformity has been explained in the preceding
+chapter. Its relation to the other types of climatic sequences seems to
+be that it sets sharply defined limits beyond which no changes of any
+kind have ever gone since life, as we know it, first began. Secular
+progression, on the other hand, means that in spite of all manner of
+variations, now this way and then the other, the normal climate of the
+earth, if there is such a thing, has on the whole probably changed a
+little, perhaps becoming more complex. After each period of continental
+uplift and glaciation--for such are preeminently the times of
+complexity--it is doubtful whether the earth has ever returned to quite
+its former degree of monotony. Today the earth has swung away from the
+great diversity of the glacial period. Yet we still have contrasts of
+what seem to us great magnitude. In low depressions, such as Turfan in
+the central deserts of Eurasia, the thermometer sometimes ranges from
+0 deg.F. in the morning to 60 deg. in the shade at noon. On a cloudy day in the
+Amazon forest close to the seashore, on the contrary, the temperature
+for months may rise to 85 deg. by day and sink no lower than 75 deg. at night.
+
+The reasons for the secular progression of the earth's climate appear to
+be intimately connected with those which have caused the next, and, in
+many respects, more important type of climatic sequence, which consists
+of geological oscillations. Both the progression and the oscillations
+seem to depend largely on three purely terrestrial factors: first, the
+condition of the earth's interior, including both internal heat and
+contraction; second, the salinity and movement of the ocean; and third,
+the composition and amount of the atmosphere. To begin with the earth's
+interior--its loss of heat appears to be an almost negligible factor in
+explaining either secular progression or geologic oscillation. According
+to both the nebular and the planetesimal hypotheses, the earth's crust
+appears to be colder now than it was hundreds or thousands of millions
+of years ago. The emission of internal heat, however, had probably
+ceased to be of much climatic significance near the beginning of the
+geological record, for in southern Canada glaciation occurred very early
+in the Proterozoic era. On the other hand, the contraction of the earth
+has produced remarkable effects throughout the whole of geological time.
+It has lessened the earth's circumference by a thousand miles or more,
+as appears from the way in which the rocks have been folded and thrust
+bodily over one another. According to the laws of dynamics this must
+have increased the speed of the earth's rotation, thus shortening the
+day, and also having the more important effect of increasing the bulge
+at the equator. On the other hand, recent investigations indicate that
+tidal retardation has probably diminished the earth's rate of rotation
+more than seemed probable a few years ago, thus lengthening the day and
+diminishing the bulge at the equator. Thus two opposing forces have been
+at work, one causing acceleration and one retardation. Their combined
+effect may have been a factor in causing secular progression of climate.
+It almost certainly was of much importance in causing pronounced
+oscillations first one way and then the other. This matter, together
+with most of those touched in these first chapters, will be expanded in
+later parts of the book. On the whole the tendency appears to have been
+to create climatic diversity in place of uniformity.
+
+The increasing salinity of the oceans may have been another factor in
+producing secular progression, although of slight importance in respect
+to oscillations. While the oceans were still growing in volume, it is
+generally assumed that they must have been almost fresh for a vast
+period, although Chamberlin thinks that the change in salinity has been
+much less than is usually supposed. So far as the early oceans were
+fresher than those of today, their deep-sea circulation must have been
+less hampered than now by the heavy saline water which is produced by
+evaporation in warm regions. Although this saline water is warm, its
+weight causes it to descend, instead of moving poleward in a surface
+current; this descent slows up the rise of the cold water which has
+moved along in the depths of the ocean from high latitudes, and thus
+checks the general oceanic circulation. If the ancient oceans were
+fresher and hence had a freer circulation than now, a more rapid
+interchange of polar and equatorial water presumably tended to equalize
+the climate of all latitudes.
+
+Again, although the earth's atmosphere has probably changed far less
+during geological times than was formerly supposed, its composition has
+doubtless varied. The total volume of nitrogen has probably increased,
+for that gas is so inert that when it once becomes a part of the air it
+is almost sure to stay there. On the other hand, the proportions of
+oxygen, carbon dioxide, and water vapor must have fluctuated. Oxygen is
+taken out constantly by animals and by all the processes of rock
+weathering, but on the other hand the supply is increased when plants
+break up new carbon dioxide derived from volcanoes. As for the carbon
+dioxide, it appears probable that in spite of the increased supply
+furnished by volcanoes the great amounts of carbon which have gradually
+been locked up in coal and limestone have appreciably depleted the
+atmosphere. Water vapor also may be less abundant now than in the past,
+for the presence of carbon dioxide raises the temperature a little and
+thereby enables the air to hold more moisture. When the area of the
+oceans has diminished, and this has recurred very often, this likewise
+would tend to reduce the water vapor. Moreover, even a very slight
+diminution in the amount of heat given off by the earth, or a decrease
+in evaporation because of higher salinity in the oceans, would tend in
+the same direction. Now carbon dioxide and water vapor both have a
+strong blanketing effect whereby heat is prevented from leaving the
+earth. Therefore, the probable reduction in the carbon dioxide and water
+vapor of the earth's atmosphere has apparently tended to reduce the
+climatic monotony and create diversity and complexity. Hence, in spite
+of many reversals, the general tendency of changes, not only in the
+earth's interior and in the oceans, but also in the atmosphere, appears
+to be a secular progression from a relatively monotonous climate in
+which the evolution of higher organic forms would scarcely be rapid to
+an extremely diverse and complex climate highly favorable to progressive
+evolution. The importance of these purely terrestrial agencies must not
+be lost sight of when we come to discuss other agencies outside the
+earth.
+
+In Table 2 the next type of climatic sequence is geologic oscillation.
+This means slow swings that last millions of years. At one extreme of
+such an oscillation the climate all over the world is relatively
+monotonous; it returns, as it were, toward the primeval conditions at
+the beginning of the secular progression. At such times magnolias,
+sequoias, figs, tree ferns, and many other types of subtropical plants
+grew far north in places like Greenland, as is well known from their
+fossil remains of middle Cenozoic time, for example. At these same
+times, and also at many others before such high types of plants had
+evolved, reef-making corals throve in great abundance in seas which
+covered what is now Wisconsin, Michigan, Ontario, and other equally cool
+regions. Today these regions have an average temperature of only about
+70 deg.F. in the warmest month, and average well below freezing in winter.
+No reef-making corals can now live where the temperature averages below
+68 deg.F. The resemblance of the ancient corals to those of today makes it
+highly probable that they were equally sensitive to low temperature.
+Thus, in the mild portions of a geologic oscillation the climate seems
+to have been so equable and uniform that many plants and animals could
+live 1500 and at other times even 4000 miles farther from the equator
+than now.
+
+At such times the lands in middle and high latitudes were low and small,
+and the oceans extended widely over the continental platforms. Thus
+unhampered ocean currents had an opportunity to carry the heat of low
+latitudes far toward the poles. Under such conditions, especially if the
+conception of the great subequatorial continent of Gondwana land is
+correct, the trade winds and the westerlies must have been stronger and
+steadier than now. This would not only enable the westerlies, which are
+really southwesterlies, to carry more heat than now to high latitudes,
+but would still further strengthen the ocean currents. At the same time,
+the air presumably contained an abundance of water vapor derived from
+the broad oceans, and an abundance of atmospheric carbon dioxide
+inherited from a preceding time when volcanoes contributed much carbon
+dioxide to the air. These two constituents of the atmosphere may have
+exercised a pronounced blanketing effect whereby the heat of the earth
+with its long wave lengths was kept in, although the energy of the sun
+with its shorter wave lengths was not markedly kept out. Thus everything
+may have combined to produce mild conditions in high latitudes, and to
+diminish the contrast between equator and pole, and between summer and
+winter.
+
+Such conditions perhaps carry in themselves the seeds of decay. At any
+rate while the lands lie quiet during a period of mild climate great
+strains must accumulate in the crust because of the earth's contraction
+and tidal retardation. At the same time the great abundance of plants
+upon the lowlying plains with their mild climates, and the marine
+creatures upon the broad continental platforms, deplete the atmospheric
+carbon dioxide. Part of this is locked up as coal and part as limestone
+derived from marine plants as well as animals. Then something happens so
+that the strains and stresses of the crust are released. The sea floors
+sink; the continents become relatively high and large; mountain ranges
+are formed; and the former plains and emergent portions of the
+continental platforms are eroded into hills and valleys. The large size
+of the continents tends to create deserts and other types of climatic
+diversity; the presence of mountain ranges checks the free flow of winds
+and also creates diversity; the ocean currents are likewise checked,
+altered, and diverted so that the flow of heat from low to high
+latitudes is diminished. At the same time evaporation from the ocean
+diminishes so that a decrease in water vapor combines with the previous
+depletion of carbon dioxide to reduce the blanketing effect of the
+atmosphere. Thus upon periods of mild monotony there supervene periods
+of complexity, diversity, and severity. Turn to Table 1 and see how a
+glacial climate again and again succeeds a time when relative mildness
+prevailed almost everywhere. Or examine Fig. 1 and notice how the lines
+representing temperatures go up and down. In the figure Schuchert makes
+it clear that when the lands have been large and mountain-making has
+been important, as shown by the high parts of the lower shaded area, the
+climate has been severe, as shown by the descent of the snow line, the
+upper shaded area. In the diagram the climatic oscillations appear
+short, but this is merely because they have been crowded together,
+especially in the left hand or early part. There an inch in length may
+represent a hundred million years. Even at the right-hand end an inch is
+equivalent to several million years.
+
+The severe part of a climatic oscillation, as well as the mild part,
+will be shown in later chapters to bear in itself certain probable seeds
+of decay. While the lands are being uplifted, volcanic activity is
+likely to be vigorous and to add carbon dioxide to the air. Later, as
+the mountains are worn down by the many agencies of water, wind, ice,
+and chemical decay, although much carbon dioxide is locked up by the
+carbonation of the rocks, the carbon locked up in the coal is set free
+and increases the carbon dioxide of the air. At the same time the
+continents settle slowly downward, for the earth's crust though rigid as
+steel is nevertheless slightly viscous and will flow if subjected to
+sufficiently great and enduring pressure. The area from which
+evaporation can take place is thereby increased because of the spread of
+the oceans over the continents, and water vapor joins with the carbon
+dioxide to blanket the earth and thus tends to keep it uniformly warm.
+Moreover, the diminution of the lands frees the ocean currents from
+restraint and permits them to flow more freely from low latitudes to
+high. Thus in the course of millions of years there is a return toward
+monotony. Ultimately, however, new stresses accumulate in the earth's
+crust, and the way is prepared for another great oscillation. Perhaps
+the setting free of the stresses takes place simply because the strain
+at last becomes irresistible. It is also possible, as we shall see, that
+an external agency sometimes adds to the strain and thereby determines
+the time at which a new oscillation shall begin.
+
+In Table 2 the types of climatic sequences which follow "geologic
+oscillations" are "glacial fluctuations," "orbital precessions" and
+"historical pulsations." Glacial fluctuations and historical pulsations
+appear to be of the same type, except as to severity and duration, and
+hence may be considered together. They will be treated briefly here
+because the theories as to their causes are outlined in the next two
+chapters. Oddly enough, although the historic pulsations lie much closer
+to us than do the glacial fluctuations, they were not discovered until
+two or three generations later, and are still much less known. The most
+important feature of both sequences is the swing from a glacial to an
+inter-glacial epoch or from the arsis or accentuated part of an
+historical pulsation to the thesis or unaccented part. In a glacial
+epoch or in the arsis of an historic pulsation, storms are usually
+abundant and severe, the mean temperature is lower than usual, snow
+accumulates in high latitudes or upon lofty mountains. For example, in
+the last such period during the fourteenth century, great floods and
+droughts occurred alternately around the North Sea; it was several times
+possible to cross the Baltic Sea from Germany to Sweden on the ice, and
+the ice of Greenland advanced so much that shore ice caused the Norsemen
+to change their sailing route between Iceland and the Norse colonies in
+southern Greenland. At the same time in low latitudes and in parts of
+the continental interior there is a tendency toward diminished rainfall
+and even toward aridity and the formation of deserts. In Yucatan, for
+example, a diminution in tropical rainfall in the fourteenth century
+seems to have given the Mayas a last opportunity for a revival of their
+decaying civilization.
+
+[Illustration: _Fig. 1. Climatic changes and mountain building._
+
+(_After Schuchert, in The Evolution of the Earth and Its Inhabitants,
+edited by R. S. Lull._) Diagram showing the times and probable extent of
+the more or less marked climate changes in the geologic history of North
+America, and of its elevation into chains of mountains.]
+
+Among the climatic sequences, glacial fluctuations are perhaps of the
+most vital import from the standpoint of organic evolution; from the
+standpoint of human history the same is true of climatic pulsations.
+Glacial epochs have repeatedly wiped out thousands upon thousands of
+species and played a part in the origin of entirely new types of plants
+and animals. This is best seen when the life of the Pennsylvanian is
+contrasted with that of the Permian. An historic pulsation may wipe out
+an entire civilization and permit a new one to grow up with a radically
+different character. Hence it is not strange that the causes of such
+climatic phenomena have been discussed with extraordinary vigor. In few
+realms of science has there been a more imposing or more interesting
+array of theories. In this book we shall consider the more important of
+these theories. A new solar or cyclonic hypothesis and the hypothesis of
+changes in the form and altitude of the land will receive the most
+attention, but the other chief hypotheses are outlined in the next
+chapter, and are frequently referred to throughout the volume.
+
+Between glacial fluctuations and historical pulsations in duration, but
+probably less severe than either, come orbital precessions. These stand
+in a group by themselves and are more akin to seasonal alternations than
+to any other type of climatic sequence. They must have occurred with
+absolute regularity ever since the earth began to revolve around the sun
+in its present elliptical orbit. Since the orbit is elliptical and since
+the sun is in one of the two foci of the ellipse, the earth's distance
+from the sun varies. At present the earth is nearest the sun in the
+northern winter. Hence the rigor of winter in the northern hemisphere is
+mitigated, while that of the southern hemisphere is increased. In about
+ten thousand years this condition will be reversed, and in another ten
+thousand the present conditions will return once more. Such climatic
+precessions, as we may here call them, must have occurred unnumbered
+times in the past, but they do not appear to have been large enough to
+leave in the fossils of the rocks any traces that can be distinguished
+from those of other climatic sequences.
+
+We come now to Brueckner periods and sunspot cycles. The Brueckner periods
+have a length of about thirty-three years. Their existence was suggested
+at least as long ago as the days of Sir Francis Bacon, whose statement
+about them is quoted on the flyleaf of this book. They have since been
+detected by a careful study of the records of the time of harvest,
+vintage, the opening of rivers to navigation, and the rise or fall of
+lakes like the Caspian Sea. In his book on _Klimaschwankungen seit
+1700_, Brueckner has collected an uncommonly interesting assortment of
+facts as to the climate of Europe for more than two centuries. More
+recently, by a study of the rate of growth of trees, Douglass, in his
+book on _Climatic Cycles and Tree Growth_, has carried the subject still
+further. In general the nature of the 33-year periods seems to be
+identical with that of the 11- or 12- year sunspot cycle, on the one
+hand, and of historic pulsations on the other. For a century observers
+have noted that the variations in the weather which everyone notices
+from year to year seem to have some relation to sunspots. For
+generations, however, the relationship was discussed without leading to
+any definite conclusion. The trouble was that the same change was
+supposed to take place in all parts of the world. Hence, when every sort
+of change was found somewhere at any given sunspot stage, it seemed as
+though there could not be a relationship. Of late years, however, the
+matter has become fairly clear. The chief conclusions are, first, that
+when sunspots are numerous the average temperature of the earth's
+surface is lower than normal. This does not mean that all parts are
+cooler, for while certain large areas grow cool, others of less extent
+become warm at times of many sunspots. Second, at times of many sunspots
+storms are more abundant than usual, but are also confined somewhat
+closely to certain limited tracks so that elsewhere a diminution of
+storminess may be noted. This whole question is discussed so fully in
+_Earth and Sun_ that it need not detain us further in this preliminary
+view of the whole problem of climate. Suffice it to say that a study of
+the sunspot cycle leads to the conclusion that it furnishes a clue to
+many of the unsolved problems of the climate of the past, as well as a
+key to prediction of the future.
+
+Passing by the seasonal alternations which are fully explained as the
+result of the revolution of the earth around the sun, we may merely
+point out that, like the daily vibrations which bring Table 2 to a
+close, they emphasize the outstanding fact that the main control of
+terrestrial climate is the amount of energy received from the sun. This
+same principle is illustrated by pleionian migrations. The term "pleion"
+comes from a Greek word meaning "more." It was taken by Arctowski to
+designate areas or periods where there is an excess of some climatic
+element, such as atmospheric pressure, rainfall, or temperature. Even if
+the effect of the seasons is eliminated, it appears that the course of
+these various elements does not run smoothly. As everyone knows, a
+period like the autumn of 1920 in the eastern United States may be
+unusually warm, while a succeeding period may be unseasonably cool.
+These departures from the normal show a certain rough periodicity. For
+example, there is evidence of a period of about twenty-seven days,
+corresponding to the sun's rotation and formerly supposed to be due to
+the moon's revolution which occupies almost the same length of time.
+Still other periods appear to have an average duration of about three
+months and of between two and three years. Two remarkable discoveries
+have recently been made in respect to such pleions. One is that a given
+type of change usually occurs simultaneously in a number of well-defined
+but widely separated centers, while a change of an opposite character
+arises in another equally well-defined, but quite different, set of
+centers. In general, areas of high pressure have one type of change and
+areas of low pressure the other type. So systematic are these
+relationships and so completely do they harmonize in widely separated
+parts of the earth, that it seems certain that they must be due to some
+outside cause, which in all probability can be only the sun. The second
+discovery is that pleions, when once formed, travel irregularly along
+the earth's surface. Their paths have not yet been worked out in detail,
+but a general migration seems well established. Because of this, it is
+probable that if unusually warm weather prevails in one part of a
+continent at a given time, the "thermo-pleion," or excess of heat, will
+not vanish but will gradually move away in some particular direction. If
+we knew the path that it would follow we might predict the general
+temperature along its course for some months in advance. The paths are
+often irregular, and the pleions frequently show a tendency to break up
+or suddenly revive. Probably this tendency is due to variations in the
+sun. When the sun is highly variable, the pleions are numerous and
+strong, and extremes of weather are frequent. Taken as a whole the
+pleions offer one of the most interesting and hopeful fields not only
+for the student of the causes of climatic variations, but for the man
+who is interested in the practical question of long-range weather
+forecasts. Like many other climatic phenomena they seem to represent the
+combined effect of conditions in the sun and upon the earth itself.
+
+The last of the climatic sequences which require explanation is the
+cyclonic vacillations. These are familiar to everyone, for they are the
+changes of weather which occur at intervals of a few days, or a week or
+two, at all seasons, in large parts of the United States, Europe, Japan,
+and some of the other progressive parts of the earth. They do not,
+however, occur with great frequency in equatorial regions, deserts, and
+many other regions. Up to the end of the last century, it was generally
+supposed that cyclonic storms were purely terrestrial in origin. Without
+any adequate investigation it was assumed that all irregularities in the
+planetary circulation of the winds arise from an irregular distribution
+of heat due to conditions within or upon the earth itself. These
+irregularities were supposed to produce cyclonic storms in certain
+limited belts, but not in most parts of the world. Today this view is
+being rapidly modified. Undoubtedly, the irregularities due to purely
+terrestrial conditions are one of the chief contributory causes of
+storms, but it begins to appear that solar variations also play a part.
+It has been found, for example, that not only the mean temperature of
+the earth's surface varies in harmony with the sunspot cycle, but that
+the frequency and severity of storms vary in the same way. Moreover, it
+has been demonstrated that the sun's radiation is not constant, but is
+subject to innumerable variations. This does not mean that the sun's
+general temperature varies, but merely that at some times heated gases
+are ejected rapidly to high levels so that a sudden wave of energy
+strikes the earth. Thus, the present tendency is to believe that the
+cyclonic variations, the changes of weather which come and go in such a
+haphazard, irresponsible way, are partly due to causes pertaining to the
+earth itself and partly to the sun.
+
+From this rapid survey of the types of climatic sequences, it is evident
+that they may be divided into four great groups. First comes cosmic
+uniformity, one of the most marvelous and incomprehensible of all known
+facts. We simply have no explanation which is in any respect adequate.
+Next come secular progression and geologic oscillations, two types of
+change which seem to be due mainly to purely terrestrial causes, that
+is, to changes in the lands, the oceans, and the air. The general
+tendency of these changes is toward complexity and diversity, thus
+producing progression, but they are subject to frequent reversals which
+give rise to oscillations lasting millions of years. The processes by
+which the oscillations take place are fully discussed in this book.
+Nevertheless, because they are fairly well understood, they are deferred
+until after the third group of sequences has been discussed. This group
+includes glacial fluctuations, historic pulsations, Brueckner periods,
+sunspot cycles, pleionian migrations, and cyclonic vacillations. The
+outstanding fact in regard to all of these is that while they are
+greatly modified by purely terrestrial conditions, they seem to owe
+their origin to variations in the sun. They form the chief subject of
+_Earth and Sun_ and in their larger phases are the most important topic
+of this book also. The last group of sequences includes orbital
+precessions, seasonal alternations, and daily variations. These may be
+regarded as purely solar in origin. Yet their influence, like that of
+each of the other groups, is much modified by the earth's own
+conditions. Our main problem is to separate and explain the two great
+elements in climatic changes,--the effects of the sun, on the one hand,
+and of the earth on the other.
+
+
+
+
+CHAPTER III
+
+HYPOTHESES OF CLIMATIC CHANGE
+
+
+The next step in our study of climate is to review the main hypotheses
+as to the causes of glaciation. These hypotheses apply also to other
+types of climatic changes. We shall concentrate on glacial periods,
+however, not only because they are the most dramatic and well-known
+types of change, but because they have been more discussed than any
+other and have also had great influence on evolution. Moreover, they
+stand near the middle of the types of climatic sequences, and an
+understanding of them does much to explain the others. In reviewing the
+various theories we shall not attempt to cover all the ground, but shall
+merely state the main ideas of the few theories which have had an
+important influence upon scientific thought.
+
+The conditions which any satisfactory climatic hypothesis must satisfy
+are briefly as follows:
+
+(1) Due weight must be given to the fact that changes of climate are
+almost certainly due to the combined effect of a variety of causes, both
+terrestrial and solar or cosmic.
+
+(2) Attention must also be paid to both sides in the long controversy as
+to whether glaciation is due primarily to a diminution in the earth's
+supply of heat or to a _redistribution_ of the heat through changes in
+atmospheric and oceanic circulation. At present the great majority of
+authorities are on the side of a diminution of heat, but the other view
+also deserves study.
+
+(3) A satisfactory hypothesis must explain the frequent synchronism
+between two great types of phenomena; first, movements of the earth's
+crust whereby continents are uplifted and mountains upheaved; and,
+second, great changes of climate which are usually marked by relatively
+rapid oscillations from one extreme to another.
+
+(4) No hypothesis can find acceptance unless it satisfies the somewhat
+exacting requirements of the geological record, with its frequent but
+irregular repetition of long, mild periods, relatively cool or
+intermediate periods like the present, and glacial periods of more or
+less severity and perhaps accompanying the more or less widespread
+uplifting of continents. At least during the later glacial periods the
+hypothesis must explain numerous climatic epochs and stages superposed
+upon a single general period of continental upheaval. Moreover, although
+historical geology demands cycles of varied duration and magnitude, it
+does not furnish evidence of any rigid periodicity causing the cycles to
+be uniform in length or intensity.
+
+(5) Most important of all, a satisfactory explanation of climatic
+changes and crustal deformation must take account of all the agencies
+which are now causing similar phenomena. Whether any other agencies
+should be considered is open to question, although the relative
+importance of existing agencies may have varied.
+
+I. _Croll's Eccentricity Theory._ One of the most ingenious and most
+carefully elaborated scientific hypotheses is Croll's[10] precessional
+hypothesis as to the effect of the earth's own motions. So well was this
+worked out that it was widely accepted for a time and still finds a
+place in popular but unscientific books, such as Wells' _Outline of
+History_, and even in scientific works like Wright's _Quaternary Ice
+Age_. The gist of the hypothesis has already been given in connection
+with the type of climatic sequence known as orbital precessions. The
+earth is 93 million miles away from the sun in January and 97 million in
+July. The earth's axis "precesses," however, just as does that of a
+spinning top. Hence arises what is known as the precession of the
+equinoxes, that is, a steady change in the season at which the earth is
+in perihelion, or nearest to the sun. In the course of 21,000 years the
+time of perihelion varies from early in January through the entire
+twelve months and back to January. Moreover, the earth's orbit is
+slightly more elliptical at certain periods than at others, for the
+planets sometimes become bunched so that they all pull the earth in one
+direction. Hence, once in about one hundred thousand years the effect of
+the elliptical shape of the earth's orbit is at a maximum.
+
+Croll argued that these astronomical changes must alter the earth's
+climate, especially by their effect on winds and ocean currents. His
+elaborate argument contains a vast amount of valuable material. Later
+investigation, however, seems to have proven the inadequacy of his
+hypothesis. In the first place, the supposed cause does not seem nearly
+sufficient to produce the observed results. Second, Croll's hypothesis
+demands that glaciation in the northern and southern hemisphere take
+place alternately. A constantly growing collection of facts, however,
+indicates that glaciation does not occur in the two hemispheres
+alternately, but at the same time. Third, the hypothesis calls for the
+constant and frequent repetition of glaciation at absolutely regular
+intervals. The geological record shows no such regularity, for sometimes
+several glacial epochs follow in relatively close succession at
+irregular intervals of perhaps fifty to two hundred thousand years, and
+thus form a glacial period; and then for millions of years there are
+none. Fourth, the eccentricity hypothesis provides no adequate
+explanation for the glacial stages or subepochs, the historic
+pulsations, and the other smaller climatic variations which are
+superposed upon glacial epochs and upon one another in bewildering
+confusion. In spite of these objections, there can be little question
+that the eccentricity of the earth's orbit and the precession of the
+equinoxes with the resulting change in the season of perihelion must
+have some climatic effect. Hence Croll's theory deserves a permanent
+though minor place in any full discussion of the causes of climatic
+changes.
+
+II. _The Carbon Dioxide Theory._ At about the time that the eccentricity
+theory was being relegated to a minor niche, a new theory was being
+developed which soon exerted a profound influence upon geological
+thought. Chamberlin,[11] adopting an idea suggested by Tyndall, fired
+the imagination of geologists by his skillful exposition of the part
+played by carbon dioxide in causing climatic changes. Today this theory
+is probably more widely accepted than any other. We have already seen
+that the amount of carbon dioxide gas in the atmosphere has a decided
+climatic importance. Moreover, there can be little doubt that the amount
+of that gas in the atmosphere varies from age to age in response to the
+extent to which it is set free by volcanoes, consumed by plants,
+combined with rocks in the process of weathering, dissolved in the ocean
+or locked up in the form of coal and limestone. The main question is
+whether such variations can produce changes so rapid as glacial epochs
+and historical pulsations.
+
+Abundant evidence seems to show that the degree to which the air can be
+warmed by carbon dioxide is sharply limited. Humphreys, in his excellent
+book on the _Physics of the Air_, calculates that a layer of carbon
+dioxide forty centimeters thick has practically as much blanketing
+effect as a layer indefinitely thicker. In other words, forty
+centimeters of carbon dioxide, while having no appreciable effect on
+sunlight coming toward the earth, would filter out and thus retain in
+the atmosphere all the outgoing terrestrial heat that carbon dioxide is
+capable of absorbing. Adding more would be like adding another filter
+when the one in operation has already done all that that particular kind
+of filter is capable of doing. According to Humphreys' calculations, a
+doubling of the carbon dioxide in the air would in itself raise the
+average temperature about 1.3 deg.C. and further carbon dioxide would have
+practically no effect. Reducing the present supply by half would reduce
+the temperature by essentially the same amount.
+
+The effect must be greater, however, than would appear from the figures
+given above, for any change in temperature has an effect on the amount
+of water vapor, which in turn causes further changes of temperature.
+Moreover, as Chamberlin points out, it is not clear whether Humphreys
+allows for the fact that when the 40 centimeters of CO_{2} nearest the
+earth has been heated by terrestrial radiation, it in turn radiates half
+its heat outward and half inward. The outward half is all absorbed in
+the next layer of carbon dioxide, and so on. The process is much more
+complex than this, but the end result is that even the last increment of
+CO_{2}, that is, the outermost portions in the upper atmosphere, must
+apparently absorb an infinitesimally small amount of heat. This fact,
+plus the effect of water vapor, would seem to indicate that a doubling
+or halving of the amount of CO_{2}, would have an effect of more than
+1.3 deg.C. A change of even 2 deg.C. above or below the present level of the
+earth's mean temperature would be of very appreciable climatic
+significance, for it is commonly believed that during the height of the
+glacial period the mean temperature was only 5 deg. to 8 deg.C. lower than now.
+
+Nevertheless, variations in atmospheric carbon dioxide do not
+necessarily seem competent to produce the relatively rapid climatic
+fluctuations of glacial epochs and historic pulsations as distinguished
+from the longer swings of glacial periods and geological eras. In
+Chamberlin's view, as in ours, the elevation of the land, the
+modification of the currents of the air and of the ocean, and all that
+goes with elevation as a topographic agency constitute a primary cause
+of climatic changes. A special effect of this is the removal of carbon
+dioxide from the air by the enhanced processes of weathering. This, as
+he carefully states, is a very slow process, and cannot of itself lead
+to anything so sudden as the oncoming of glaciation. But here comes
+Chamberlin's most distinctive contribution to the subject, namely, the
+hypothesis that changes in atmospheric temperature arising from
+variations in atmospheric carbon dioxide are able to cause a reversal of
+the deep-sea oceanic circulation.
+
+According to Chamberlin's view, the ordinary oceanic circulation of the
+greater part of geological time was the reverse of the present
+circulation. Warm water descended to the ocean depths in low latitudes,
+kept its heat while creeping slowly poleward, and rose in high latitudes
+producing the warm climate which enabled corals, for example, to grow in
+high latitudes. Chamberlin holds this opinion largely because there
+seems to him to be no other reasonable way to account for the enormously
+long warm periods when heat-loving forms of life lived in what are now
+polar regions of ice and snow. He explains this reversed circulation by
+supposing that an abundance of atmospheric carbon dioxide, together with
+a broad distribution of the oceans, made the atmosphere so warm that the
+evaporation in low latitudes was far more rapid than now. Hence the
+surface water of the ocean became a relatively concentrated brine. Such
+a brine is heavy and tends to sink, thereby setting up an oceanic
+circulation the reverse of that which now prevails. At present the polar
+waters sink because they are cold and hence contract. Moreover, when
+they freeze a certain amount of salt leaves the ice and thereby
+increases the salinity of the surrounding water. Thus the polar water
+sinks to the depths of the ocean, its place is taken by warmer and
+lighter water from low latitudes which moves poleward along the surface,
+and at the same time the cold water of the ocean depths is forced
+equatorward below the surface. But if the equatorial waters were so
+concentrated that a steady supply of highly saline water kept descending
+to low levels, the direction of the circulation would have to be
+reversed. The time when this would occur would depend upon the delicate
+balance between the downward tendencies of the cold polar water and of
+the warm saline equatorial water.
+
+Suppose that while such a reversed circulation prevailed, the
+atmospheric CO_{2} should be depleted, and the air cooled so much that
+the concentration of the equatorial waters by evaporation was no longer
+sufficient to cause them to sink. A reversal would take place, the
+present type of circulation would be inaugurated, and the whole earth
+would suffer a chill because the surface of the ocean would become cool.
+The cool surface-water would absorb carbon dioxide faster than the
+previous warm water had done, for heat drives off gases from water. This
+would hasten the cooling of the atmosphere still more, not only directly
+but by diminishing the supply of atmospheric moisture. The result would
+be glaciation. But ultimately the cold waters of the higher latitudes
+would absorb all the carbon dioxide they could hold, the slow
+equatorward creep would at length permit the cold water to rise to the
+surface in low latitudes. There the warmth of the equatorial sun and the
+depleted supply of carbon dioxide in the air would combine to cause the
+water to give up its carbon dioxide once more. If the atmosphere had
+been sufficiently depleted by that time, the rising waters in low
+latitudes might give up more carbon dioxide than the cold polar waters
+absorbed. Thus the atmospheric supply would increase, the air would
+again grow warm, and a tendency toward deglaciation, or toward an
+inter-glacial condition would arise. At such times the oceanic
+circulation is not supposed to have been reversed, but merely to have
+been checked and made slower by the increasing warmth. Thus
+inter-glacial conditions like those of today, or even considerably
+warmer, are supposed to have been produced with the present type of
+circulation.
+
+The emission of carbon dioxide in low latitudes could not permanently
+exceed the absorption in high latitudes. After the present type of
+circulation was finally established, which might take tens of thousands
+of years, the two would gradually become equal. Then the conditions
+which originally caused the oceanic circulation to be reversed would
+again destroy the balance; the atmospheric carbon dioxide would be
+depleted; the air would grow cooler; and the cycle of glaciation would
+be repeated. Each cycle would be shorter than the last, for not only
+would the swings diminish like those of a pendulum, but the agencies
+that were causing the main depletion of the atmospheric carbon dioxide
+would diminish in intensity. Finally as the lands became lower through
+erosion and submergence, and as the processes of weathering became
+correspondingly slow, the air would gradually be able to accumulate
+carbon dioxide; the temperature would increase; and at length the
+oceanic circulation would be reversed again. When the warm saline waters
+of low latitudes finally began to sink and to set up a flow of warm
+water poleward in the depths of the ocean, a glacial period would
+definitely come to an end.
+
+This hypothesis has been so skillfully elaborated, and contains so many
+important elements that one can scarcely study it without profound
+admiration. We believe that it is of the utmost value as a step toward
+the truth, and especially because it emphasizes the great function of
+oceanic circulation. Nevertheless, we are unable to accept it in full
+for several reasons, which may here be stated very briefly. Most of them
+will be discussed fully in later pages.
+
+(1) While a reversal of the deep-sea circulation would undoubtedly be of
+great climatic importance and would produce a warm climate in high
+latitudes, we see no direct evidence of such a reversal. It is equally
+true that there is no conclusive evidence against it, and the
+possibility of a reversal must not be overlooked. There seem, however,
+to be other modifications of atmospheric and oceanic circulation which
+are able to produce the observed results.
+
+(2) There is much, and we believe conclusive, evidence that a mere
+lowering of temperature would not produce glaciation. What seems to be
+needed is changes in atmospheric circulation and in precipitation. The
+carbon dioxide hypothesis has not been nearly so fully developed on the
+meteorological side as in other respects.
+
+(3) The carbon dioxide hypothesis seems to demand that the oceans should
+have been almost as saline as now in the Proterozoic era at the time of
+the first known glaciation. Chamberlin holds that such was the case, but
+the constant supply of saline material brought to the ocean by rivers
+and the relatively small deposition of such material on the sea floor
+seem to indicate that the early oceans must have been much fresher than
+those of today.
+
+(4) The carbon dioxide hypothesis does not attempt to explain minor
+climatic fluctuations such as post-glacial stages and historic
+pulsations, but these appear to be of the same nature as glacial epochs,
+differing only in degree.
+
+(5) Another reason for hesitation in accepting the carbon dioxide
+hypothesis as a full explanation of glacial fluctuations is the highly
+complex and non-observational character of the explanation of the
+alternation of glacial and inter-glacial epochs and of their constantly
+decreasing length.
+
+(6) Most important of all, a study of the variations of weather and of
+climate as they are disclosed by present records and by the historic
+past suggests that there are now in action certain other causes which
+are competent to explain glaciation without recourse to a process whose
+action is beyond the realm of observation.
+
+These considerations lead to the conclusion that the carbon dioxide
+hypothesis and the reversal of the oceanic circulation should be
+regarded as a tentative rather than a final explanation of glaciation.
+Nevertheless, the action of carbon dioxide seems to be an important
+factor in producing the longer oscillations of climate from one
+geological era to another. It probably plays a considerable part in
+preparing the way for glacial periods and in making it possible for
+other factors to produce the more rapid changes which have so deeply
+influenced organic evolution.
+
+III. _The Form of the Land._ Another great cause of climatic change
+consists of a group of connected phenomena dependent upon movements of
+the earth's crust. As to the climatic potency of changes in the lands
+there is practical agreement among students of climatology and
+glaciation. That the height and extent of the continents, the location,
+size, and orientation of mountain ranges, and the opening and closing of
+oceanic gateways at places like Panama, and the consequent diversion of
+oceanic currents, exert a profound effect upon climate can scarcely be
+questioned. Such changes may be introduced rapidly, but their
+disappearance is usually slow compared with the rapid pulsations to
+which climate has been subject during historic times and during stages
+of glacial retreat and advance, or even in comparison with the epochs
+into which the Pleistocene, Permian, and perhaps earlier glacial periods
+have been divided. Hence, while crustal movements appear to be more
+important than the eccentricity of the earth's orbit or the amount of
+carbon dioxide in the air, they do not satisfactorily explain glacial
+fluctuations, historic pulsations, and especially the present little
+cycles of climatic change. All these changes involve a relatively rapid
+swing from one extreme to another, while an upheaval of a continent,
+which is at best a slow geologic process, apparently cannot be undone
+for a long, long time. Hence such an upheaval, if acting alone, would
+lead to a relatively long-lived climate of a somewhat extreme type. It
+would help to explain the long swings, or geologic oscillations between
+a mild and uniform climate at one extreme, and a complex and varied
+climate at the other, but it would not explain the rapid climatic
+pulsations which are closely associated with great movements of the
+earth's crust. It might prepare the way for them, but could not cause
+them. That this conclusion is true is borne out by the fact that vast
+mountain ranges, like those at the close of the Jurassic and Cretaceous,
+are upheaved without bringing on glacial climates. Moreover, the marked
+Permian ice age follows long after the birth of the Hercynian Mountains
+and before the rise of others of later Permian origin.
+
+IV. _The Volcanic Hypothesis._ In the search for some cause of climatic
+change which is highly efficient and yet able to vary rapidly and
+independently, Abbot, Fowle, Humphreys, and others,[12] have concluded
+that volcanic eruptions are the missing agency. In _Physics of the Air_,
+Humphreys gives a careful study of the effect of volcanic dust upon
+terrestrial temperature. He begins with a mathematical investigation of
+the size of dust particles, and their quantity after certain eruptions.
+He demonstrates that the power of such particles to deflect light of
+short wave-lengths coming from the sun is perhaps thirty times more than
+their power to retain the heat radiated in long waves from the earth.
+Hence it is estimated that if a Krakatoa were to belch forth dust every
+year or two, the dust veil might cause a reduction of about 6 deg.C. in the
+earth's surface temperature. As in every such complicated problem, some
+of the author's assumptions are open to question, but this touches their
+quantitative and not their qualitative value. It seems certain that if
+volcanic explosions were frequent enough and violent enough, the
+temperature of the earth's surface would be considerably lowered.
+
+Actual observation supports this theoretical conclusion. Humphreys
+gathers together and amplifies all that he and Abbot and Fowle have
+previously said as to observations of the sun's thermal radiation by
+means of the pyrheliometer. This summing up of the relations between the
+heat received from the sun, and the occurrence of explosive volcanic
+eruptions leaves little room for doubt that at frequent intervals during
+the last century and a half a slight lowering of terrestrial temperature
+has actually occurred after great eruptions. Nevertheless, it does not
+justify Humphreys' final conclusion that "phenomena within the earth
+itself suffice to modify its own climate, ... that these and these alone
+have actually caused great changes time and again in the geologic past."
+Humphreys sees so clearly the importance of the purely terrestrial point
+of view that he unconsciously slights the cosmic standpoint and ignores
+the important solar facts which he himself adduces elsewhere at
+considerable length.
+
+In addition to this the _degree_ to which the temperature of the earth
+as a whole is influenced by volcanic eruptions is by no means so clear
+as is the fact that there is some influence. Arctowski,[13] for example,
+has prepared numerous curves showing the march of temperature month
+after month for many years. During the period from 1909 to 1913, which
+includes the great eruption of Katmai in Alaska, low temperature is
+found to have prevailed at the time of the eruption, but, as Arctowski
+puts it, on the basis of the curves for 150 stations in all parts of the
+world: "The supposition that these abnormally low temperatures were due
+to the veil of volcanic dust produced by the Katmai eruption of June 6,
+1912, is completely out of the question. If that had been the case,
+temperature would have decreased from that date on, whereas it was
+decreasing for more than a year before that date."
+
+Koeppen,[14] in his comprehensive study of temperature for a hundred
+years, also presents a strong argument against the idea that volcanic
+eruptions have an important place in determining the present temperature
+of the earth. A volcanic eruption is a sudden occurrence. Whatever
+effect is produced by dust thrown into the air must occur within a few
+months, or as soon as the dust has had an opportunity to be wafted to
+the region in question. When the dust arrives, there will be a rapid
+drop through the few degrees of temperature which the dust is supposed
+to be able to account for, and thereafter a slow rise of temperature. If
+volcanic eruptions actually caused a frequent lowering of terrestrial
+temperature in the hundred years studied by Koeppen, there should be more
+cases where the annual temperature is decidedly below the normal than
+where it shows a large departure in the opposite direction. The contrary
+is actually the case.
+
+A still more important argument is the fact that the earth is now in an
+intermediate condition of climate. Throughout most of geologic time, as
+we shall see again and again, the climate of the earth has been milder
+than now. Regions like Greenland have not been the seat of glaciers, but
+have been the home of types of plants which now thrive in relatively low
+latitudes. In other words, the earth is today only part way from a
+glacial epoch to what may be called the normal, mild climate of the
+earth--a climate in which the contrast from zone to zone was much less
+than now, and the lower air averaged warmer. Hence it seems impossible
+to avoid the conclusion that the cause of glaciation is still operating
+with considerable although diminished efficiency. But volcanic dust is
+obviously not operating to any appreciable extent at present, for the
+upper air is almost free from dust a large part of the time.
+
+Again, as Chamberlin suggests, let it be supposed that a Krakatoan
+eruption every two years would produce a glacial period. Unless the most
+experienced field workers on the glacial formations are quite in error,
+the various glacial epochs of the Pleistocene glacial period had a joint
+duration of at least 150,000 years and perhaps twice as much. That would
+require 75,000 Krakatoan eruptions. But where are the pits and cones of
+such eruptions? There has not been time to erode them away since the
+Pleistocene glaciation. Their beds of volcanic ash would presumably be
+as voluminous as the glacial beds, but there do not seem to be
+accumulations of any such size. Even though the same volcano suffered
+repeated explosions, it seems impossible to find sufficient fresh
+volcanic debris. Moreover, the volcanic hypothesis has not yet offered
+any mechanism for systematic glacial variations. Hence, while the
+hypothesis is important, we must search further for the full explanation
+of glacial fluctuations, historic pulsations, and the earth's present
+quasi-glacial climate.
+
+V. _The Hypothesis of Polar Wandering._ Another hypothesis, which has
+some adherents, especially among geologists, holds that the position of
+the earth's axis has shifted repeatedly during geological times, thus
+causing glaciation in regions which are not now polar. Astrophysicists,
+however, are quite sure that no agency could radically change the
+relation between the earth and its axis without likewise altering the
+orbits of the planets to a degree that would be easily recognized.
+Moreover, the distribution of the centers of glaciation both in the
+Permian and Pleistocene periods does not seem to conform to this
+hypothesis.
+
+VI. _The Thermal Solar Hypothesis._ The only other explanations of the
+climatic changes of glacial and historic times which now seem to have
+much standing are two distinct and almost antagonistic solar hypotheses.
+One is the idea that changes in the earth's climate are due to
+variations in the heat emitted by the sun and hence in the temperature
+of the earth. The other is the entirely different idea that climatic
+changes arise from solar conditions which cause a _redistribution of the
+earth's atmospheric pressure_ and hence produce changes in winds, ocean
+currents, and especially storms. This second, or "cyclonic," hypothesis
+is the subject of a book entitled _Earth and Sun_, which is to be
+published as a companion to the present volume. It will be outlined in
+the next chapter. The other, or thermal, hypothesis may be dismissed
+briefly. Unquestionably a permanent change in the amount of heat emitted
+by the sun would permanently alter the earth's climate. There is
+absolutely no evidence, however, of any such change during geologic
+time. The evidence as to the earth's cosmic uniformity and as to secular
+progression is all against it. Suppose that for thirty or forty thousand
+years the sun cooled off enough so that the earth was as cool as during
+a glacial epoch. As glaciation is soon succeeded by a mild climate, some
+agency would then be needed to raise the sun's temperature. The impact
+of a shower of meteorites might accomplish this, but that would mean a
+very sudden heating, such as there is no evidence of in geological
+history. In fact, there is far more evidence of sudden cooling than of
+sudden heating. Moreover, it is far beyond the bounds of probability
+that such an impact should be repeated again and again with just such
+force as to bring the climate back almost to where it started and yet to
+allow for the slight changes which cause secular progression. Another
+and equally cogent objection to the thermal form of solar hypothesis is
+stated by Humphreys as follows: "A change of the solar constant
+obviously alters all surface temperatures by a roughly constant
+percentage. Hence a decrease of the heat from the sun would in general
+cause a decrease of the interzonal temperature gradients; and this in
+turn a less vigorous atmospheric circulation, and a less copious rain or
+snowfall--exactly the reverse of the condition, namely, abundant
+precipitation, most favorable to extensive glaciation."
+
+This brings us to the end of the main hypotheses as to climatic changes,
+aside from the solar cyclonic hypothesis which will be discussed in the
+next chapter. It appears that variations in the position of the earth at
+perihelion have a real though slight influence in causing cycles with a
+length of about 21,000 years. Changes in the carbon dioxide of the air
+probably have a more important but extremely slow influence upon
+geologic oscillations. Variations in the size, shape, and height of the
+continents are constantly causing all manner of climatic complications,
+but do not cause rapid fluctuations and pulsations. The eruption of
+volcanic dust appears occasionally to lower the temperature, but its
+potency to explain the complex climatic changes recorded in the rocks
+has probably been exaggerated. Finally, although minor changes in the
+amount of heat given out by the sun occur constantly and have been
+demonstrated to have a climatic effect, there is no evidence that such
+changes are the main cause of the climatic phenomena which we are trying
+to explain. Nevertheless, in connection with other solar changes they
+may be of high importance.
+
+FOOTNOTES:
+
+[Footnote 10: James Croll: Climate and Time, 1876.]
+
+[Footnote 11: T. C. Chamberlin: An attempt to frame a working hypothesis
+of the cause of glacial periods on an atmospheric basis; Jour. Geol.,
+Vol. VII, 1899, pp. 545-584, 667-685, 757-787.
+
+T. C. Chamberlin and R. D. Salisbury: Geology, Vol. II, 1906, pp.
+93-106, 655-677, and Vol. III, pp. 432-446.
+
+S. Arrhenius (Kosmische Physik, Vol. II, 1903, p. 503) carried out some
+investigations on carbon dioxide which have had a pronounced effect on
+later conclusions.
+
+F. Frech adopted Arrhenius' idea and developed it in a paper entitled
+Ueber die Klima-Aenderungen der Geologischen Vergangenheit. Compte
+Rendu, Tenth (Mexico) Congr. Geol. Intern., 1907 (=1908), pp. 299-325.
+
+The exact origin of the carbon dioxide theory has been stated so
+variously that it seems worth while to give the exact facts. Prompted by
+the suggestion, of Tyndall that glaciation might be due to depletion of
+atmospheric carbon dioxide, Chamberlin worked up the essentials of his
+early views before he saw any publication from Arrhenius, to whom the
+idea has often been attributed. In 1895 or earlier Chamberlin began to
+give the carbon dioxide hypothesis to his students and to discuss it
+before local scientific bodies. In 1897 he prepared a paper on "A Group
+of Hypotheses Bearing on Climatic Changes," Jour. Geol., Vol. V (1897),
+to be read at the meeting of the British Association at Toronto, basing
+his conclusions on Tyndall's determination of the competency of carbon
+dioxide as an absorber of heat radiated from the earth. He had
+essentially completed this when a paper by Arrhenius, "On the influence
+of carbonic acid in the air upon the temperature of the ground," Phil.
+Mag., 1896, pp. 237-276, first came to his attention. Chamberlin then
+changed his conservative, tentative statement of the functions of carbon
+dioxide to a more sweeping one based on Arrhenius' very definite
+quantitative deductions from Langley's experiments. Both Langley and
+Arrhenius were then in the ascendancy of their reputations and seemingly
+higher authorities could scarcely have been chosen, nor a finer
+combination than experiment and physico-mathematical development.
+Arrhenius' deductions were later proved to have been overstrained, while
+Langley's interpretation and even his observations were challenged.
+Chamberlin's latest views are more like his earlier and more
+conservative statement.]
+
+[Footnote 12: C. G. Abbot and F. E. Fowle: Volcanoes and Climate;
+Smiths. Misc. Coll., Vol. 60, 1913, 24 pp.
+
+W. J. Humphreys: Volcanic dust and other factors in the production of
+climatic and their possible relation to ice ages; Bull. Mount Weather
+Observatory, Vol. 6, Part 1, 1913, 26 pp. Also, Physics of the Air,
+1920.]
+
+[Footnote 13: H. Arctowski: The Pleonian Cycle of Climatic Fluctuations;
+Am. Jour. Sci., Vol. 42, 1916, pp. 27-33. See also Annals of the New
+York Academy of Sciences, Vol. 24, 1914.]
+
+[Footnote 14: W. Koeppen: Ueber mehrjaehrige Perioden der Witterung
+ins besondere uezer die II-jaehrige Periode der Temperatur. Also,
+Lufttemperaturen Sonnenflecke und Vulcanausbrueche; Meteorologische
+Zeitschrift, Vol. 7, 1914, pp. 305-328.]
+
+
+
+
+CHAPTER IV
+
+THE SOLAR CYCLONIC HYPOTHESIS
+
+
+The progress of science is made up of a vast succession of hypotheses.
+The majority die in early infancy. A few live and are for a time widely
+accepted. Then some new hypothesis either destroys them completely or
+shows that, while they contain elements of truth, they are not the whole
+truth. In the previous chapter we have discussed a group of hypotheses
+of this kind, and have tried to point out fairly their degree of truth
+so far as it can yet be determined. In this chapter we shall outline
+still another hypothesis, the relation of which to present climatic
+conditions has been fully developed in _Earth and Sun_; while its
+relation to the past will be explained in the present volume. This
+hypothesis is not supposed to supersede the others, for so far as they
+are true they cannot be superseded. It merely seems to explain some of
+the many conditions which the other hypotheses apparently fail to
+explain. To suppose that it will suffer a fate more glorious than its
+predecessors would be presumptuous. The best that can be hoped is that
+after it has been pruned, enriched, and modified, it may take its place
+among the steps which finally lead to the goal of truth.
+
+In this chapter the new hypothesis will be sketched in broad outline in
+order that in the rest of this book the reader may appreciate the
+bearing of all that is said. Details of proof and methods of work will
+be omitted, since they are given in _Earth and Sun_. For the sake of
+brevity and clearness the main conclusions will be stated without the
+qualifications and exceptions which are fully explained in that volume.
+Here it will be necessary to pass quickly over points which depart
+radically from accepted ideas, and which therefore must arouse serious
+question in the minds of thoughtful readers. That, however, is a
+necessary consequence of the attempt which this book makes to put the
+problem of climate in such form that the argument can be followed by
+thoughtful students in any branch of knowledge and not merely by
+specialists. Therefore, the specialist can merely be asked to withhold
+judgment until he has read all the evidence as given in _Earth and Sun_,
+and then to condemn only those parts that are wrong and not the whole
+argument.
+
+Without further explanation let us turn to our main problem. In the
+realm of climatology the most important discovery of the last generation
+is that variations in the weather depend on variations in the activity
+of the sun's atmosphere. The work of the great astronomer, Newcomb, and
+that of the great climatologist, Koeppen, have shown beyond question that
+the temperature of the earth's surface varies in harmony with variations
+in the number and area of sunspots.[15] The work of Abbot has shown that
+the amount of heat radiated from the sun also varies, and that in
+general the variations correspond with those of the sunspots, although
+there are exceptions, especially when the spots are fewest. Here,
+however, there at once arises a puzzling paradox. The earth certainly
+owes its warmth to the sun. Yet when the sun emits the most energy, that
+is, when sunspots are most numerous, the earth's surface is coolest.
+Doubtless the earth receives more heat than usual at such times, and the
+upper air may be warmer than usual. Here we refer only to the air at the
+earth's surface.
+
+Another large group of investigators have shown that atmospheric
+pressure also varies in harmony with the number of sunspots. Some parts
+of the earth's surface have one kind of variation at times of many
+sunspots and other parts the reverse. These differences are systematic
+and depend largely on whether the region in question happens to have
+high atmospheric pressure or low. The net result is that when sunspots
+are numerous the earth's storminess increases, and the atmosphere is
+thrown into commotion. This interferes with the stable planetary winds,
+such as the trades of low latitudes and the prevailing westerlies of
+higher latitudes. Instead of these regular winds and the fair weather
+which they bring, there is a tendency toward frequent tropical
+hurricanes in the lower latitudes and toward more frequent and severe
+storms of the ordinary type in the latitudes where the world's most
+progressive nations now live. With the change in storminess there
+naturally goes a change in rainfall. Not all parts of the world,
+however, have increased storminess and more abundant rainfall when
+sunspots are numerous. Some parts change in the opposite way. Thus when
+the sun's atmosphere is particularly disturbed, the contrasts between
+different parts of the earth's surface are increased. For example, the
+northern United States and southern Canada become more stormy and rainy,
+as appears in Fig. 2, and the same is true of the Southwest and along
+the south Atlantic coast. In a crescent-shaped central area, however,
+extending from Wyoming through Missouri to Nova Scotia, the number of
+storms and the amount of rainfall decrease.
+
+[Illustration: _Fig. 2. Storminess at sunspot maxima vs. minima._
+(_After Kullmer._)
+
+Based on nine years' nearest sunspot minima and nine years' nearest
+sunspot maxima in the three sunspot cycles from 1888 to 1918. Heavy
+shading indicates excess of storminess when sunspots are numerous.
+Figures indicate average yearly number of storms by which years of
+maximum sunspots exceed those of minimum sunspots.]
+
+The two controlling factors of any climate are the temperature and the
+atmospheric pressure, for they determine the winds, the storms, and thus
+the rainfall. A study of the temperature seems to show that the peculiar
+paradox of a hot sun and a cool earth is due largely to the increased
+storminess during times of many sunspots. The earth's surface is heated
+by the rays of the sun, but most of the rays do not in themselves heat
+the air as they pass through it. The air gets its heat largely from the
+heat absorbed by the water vapor which is intimately mingled with its
+lower portions, or from the long heat waves sent out by the earth after
+it has been warmed by the sun. The faster the air moves along the
+earth's surface the less it becomes heated, and the more heat it takes
+away. This sounds like a contradiction, but not to anyone who has tried
+to heat a stove in the open air. If the air is still, the stove rapidly
+becomes warm and so does the air around it. If the wind is blowing, the
+cool air delays the heating of the stove and prevents the surface from
+ever becoming as hot as it would otherwise. That seems to be what
+happens on a large scale when sunspots are numerous. The sun actually
+sends to the earth more energy than usual, but the air moves with such
+unusual rapidity that it actually cools the earth's surface a trifle by
+carrying the extra heat to high levels where it is lost into space.
+
+There has been much discussion as to why storms are numerous when the
+sun's atmosphere is disturbed. Many investigators have supposed it was
+due entirely and directly to the heating of the earth's surface by the
+sun. This, however, needs modification for several reasons. In the first
+place, recent investigations show that in a great many cases changes in
+barometric pressure precede changes in temperature and apparently cause
+them by altering the winds and producing storms. This is the opposite of
+what would happen if the effect of solar heat upon the earth's surface
+were the only agency. In the second place, if storms were due
+exclusively to variations in the ordinary solar radiation which comes to
+the earth as light and is converted into heat, the solar effect ought to
+be most pronounced when the center of the sun's visible disk is most
+disturbed. As a matter of fact the storminess is notably greatest when
+the edges of the solar disk are most disturbed. These facts and others
+lead to the conclusion that some agency other than heat must also play
+some part in producing storminess.
+
+The search for this auxiliary agency raises many difficult questions
+which cannot yet be answered. On the whole the weight of evidence
+suggests that electrical phenomena of some kind are involved, although
+variations in the amount of ultra-violet light may also be important.
+Many investigators have shown that the sun emits electrons. Hale has
+proved that the sun, like the earth, is magnetized. Sunspots also have
+magnetic fields the strength of which is often fifty times as great as
+that of the sun as a whole. If electrons are sent to the earth, they
+must move in curved paths, for they are deflected by the sun's magnetic
+field and again by the earth's magnetic field. The solar deflection may
+cause their effects to be greatest when the spots are near the sun's
+margin; the terrestrial deflection may cause concentration in bands
+roughly concentric with the magnetic poles of the earth. These
+conditions correspond with the known facts.
+
+Farther than this we cannot yet go. The calculations of Humphreys seem
+to indicate that the direct electrical effect of the sun's electrons
+upon atmospheric pressure is too small to be of appreciable significance
+in intensifying storms. On the other hand the peculiar way in which
+activity upon the margins of the sun appears to be correlated not only
+with atmospheric electricity, but with barometric pressure, seems to be
+equally strong evidence in the other direction. Possibly the sun's
+electrons and its electrical waves produce indirect effects by being
+converted into heat, or by causing the formation of ozone and the
+condensation of water vapor in the upper air. Any one of these processes
+would raise the temperature of the upper air, for the ozone and the
+water vapor would be formed there and would tend to act as a blanket to
+hold in the earth's heat. But any such change in the temperature of the
+upper air would influence the lower air through changes in barometric
+pressure. These considerations are given here because the thoughtful
+reader is likely to inquire how solar activity can influence storminess.
+Moreover, at the end of this book we shall take up certain speculative
+questions in which an electrical hypothesis will be employed. For the
+main portions of this book it makes no difference how the sun's
+variations influence the earth's atmosphere. The only essential point is
+that when the solar atmosphere is active the storminess of the earth
+increases, and that is a matter of direct observation.
+
+Let us now inquire into the relation between the small cyclonic
+vacillations of the weather and the types of climatic changes known as
+historic pulsations and glacial fluctuations. One of the most
+interesting results of recent investigations is the evidence that
+sunspot cycles on a small scale present almost the same phenomena as do
+historic pulsations and glacial fluctuations. For instance, when
+sunspots are numerous, storminess increases markedly in a belt near the
+northern border of the area of greatest storminess, that is, in southern
+Canada and thence across the Atlantic to the North Sea and Scandinavia.
+(See Figs. 2 and 3.) Corresponding with this is the fact that the
+evidence as to climatic pulsations in historic times indicates that
+regions along this path, for instance Greenland, the North Sea region,
+and southern Scandinavia, were visited by especially frequent and severe
+storms at the climax of each pulsation. Moreover, the greatest
+accumulations of ice in the glacial period were on the poleward border
+of the general regions where now the storms appear to increase most at
+times of solar activity.
+
+[Illustration: _Fig. 3A. Relative rainfall at times of increasing and
+decreasing sunspots._
+
+Heavy shading, more rain with increasing spots. Light shading, more rain
+with decreasing spots. No data for unshaded areas.
+
+Figures indicate percentages of the average rainfall by which the
+rainfall during periods of increasing spots exceeds or falls short of
+rainfall during periods of decreasing spots. The excess or deficiency is
+stated in percentages of the average. Rainfall data from Walker:
+Sunspots and Rainfall.]
+
+[Illustration: _Fig. 3B. Relative rainfall at times of increasing and
+decreasing sunspots._
+
+Heavy shading, more rain with increasing spots. Light shading, more rain
+with decreasing spots. No data for unshaded areas. Figures indicate
+percentages of the average rainfall by which the rainfall during periods
+of increasing spots exceeds or falls short of rainfall during periods of
+decreasing spots. The excess or deficiency is stated in percentages of
+the average. Rainfall data from Walker: Sunspots and Rainfall.]
+
+Even more clear is the evidence from other regions where storms increase
+at times of many sunspots. One such region includes the southwestern
+United States, while another is the Mediterranean region and the
+semi-arid or desert parts of Asia farther east. In these regions
+innumerable ruins and other lines of evidence show that at the climax of
+each climatic pulsation there was more storminess and rainfall than at
+present, just as there now is when the sun is most active. In still
+earlier times, while ice was accumulating farther north, the basins of
+these semi-arid regions were filled with lakes whose strands still
+remain to tell the tale of much-increased rainfall and presumable
+storminess. If we go back still further in geological times to the
+Permian glaciation, the areas where ice accumulated most abundantly
+appear to be the regions where tropical hurricanes produce the greatest
+rainfall and the greatest lowering of temperature at times of many
+sunspots. From these and many other lines of evidence it seems probable
+that historic pulsations and glacial fluctuations are nothing more than
+sunspot cycles on a large scale. It is one of the fundamental rules of
+science to reason from the known to the unknown, from the near to the
+far, from the present to the past. Hence it seems advisable to
+investigate whether any of the climatic phenomena of the past may have
+arisen from an intensification of the solar conditions which now appear
+to give rise to similar phenomena on a small scale.
+
+The rest of this chapter will be devoted to a _resume_ of certain
+tentative conclusions which have no bearing on the main part of this
+book, but which apply to the closing chapters. There we shall inquire
+into the periodicity of the climatic phenomena of geological times, and
+shall ask whether there is any reason to suppose that the sun's activity
+has exhibited similar periodicity. This leads to an investigation of the
+possible causes of disturbances in the sun's atmosphere. It is generally
+assumed that sunspots, solar prominences, the bright clouds known as
+faculae, and other phenomena denoting a perturbed state of the solar
+atmosphere, are due to some cause within the sun. Yet the limitation of
+these phenomena, especially the sunspots, to restricted latitudes, as
+has been shown in _Earth and Sun_, does not seem to be in harmony with
+an internal solar origin, even though a banded arrangement may be normal
+for a rotating globe. The fairly regular periodicity of the sunspots
+seems equally out of harmony with an internal origin. Again, the solar
+atmosphere has two kinds of circulation, one the so-called "rice
+grains," and the other the spots and their attendant phenomena. Now the
+rice grains present the appearance that would be expected in an
+atmospheric circulation arising from the loss of heat by the outer part
+of a gaseous body like the sun. For these reasons and others numerous
+good thinkers from Wolf to Schuster have held that sunspots owe their
+periodicity to causes outside the sun. The only possible cause seems to
+be the planets, acting either through gravitation, through forces of an
+electrical origin, or through some other agency. Various new
+investigations which are described in _Earth and Sun_ support this
+conclusion. The chief difficulty in accepting it hitherto has been that
+although Jupiter, because of its size, would be expected to dominate the
+sunspot cycle, its period of 11.86 years has not been detected. The
+sunspot cycle has appeared to average 11.2 years in length, and has been
+called the 11-year cycle. Nevertheless, a new analysis of the sunspot
+data shows that when attention is concentrated upon the major maxima,
+which are least subject to retardation or acceleration by other causes,
+a periodicity closely approaching that of Jupiter is evident. Moreover,
+when the effects of Jupiter, Saturn, and the other planets are combined,
+they produce a highly variable curve which has an extraordinary
+resemblance to the sunspot curve. The method by which the planets
+influence the sun's atmosphere is still open to question. It may be
+through tides, through the direct effect of gravitation, through
+electro-magnetic forces, or in some other way. Whichever it may be, the
+result may perhaps be slight differences of atmospheric pressure upon
+the sun. Such differences may set in motion slight whirling movements
+analogous to terrestrial storms, and these presumably gather momentum
+from the sun's own energy. Since the planetary influences vary in
+strength because of the continuous change in the relative distances and
+positions of the planets, the sun's atmosphere appears to be swayed by
+cyclonic disturbances of varying degrees of severity. The cyclonic
+disturbances known as sunspots have been proved by Hale to become more
+highly electrified as they increase in intensity. At the same time hot
+gases presumably well up from the lower parts of the solar atmosphere
+and thereby cause the sun to emit more heat. Thus by one means or
+another, the earth's atmosphere appears to be set in commotion and
+cycles of climate are inaugurated.
+
+If the preceding reasoning is correct, any disturbance of the solar
+atmosphere must have an effect upon the earth's climate. If the
+disturbance were great enough and of the right nature it might produce a
+glacial epoch. The planets are by no means the only bodies which act
+upon the sun, for that body sustains a constantly changing relation to
+millions of other celestial bodies of all sizes up to vast universes,
+and at all sorts of distances. If the sun and another star should
+approach near enough to one another, it is certain that the solar
+atmosphere would be disturbed much more than at present.
+
+Here we must leave the cyclonic hypothesis of climate and must refer the
+reader once more to _Earth and Sun_ for fuller details. In the rest of
+this book we shall discuss the nature of the climatic changes of past
+times and shall inquire into their relation to the various climatic
+hypotheses mentioned in the last two chapters. Then we shall inquire
+into the possibility that the solar system has ever been near enough to
+any of the stars to cause appreciable disturbances of the solar
+atmosphere. We shall complete our study by investigating the vexed
+question of why movements of the earth's crust, such as the uplifting of
+continents and mountain chains, have generally occurred at the same time
+as great climatic fluctuations. This would not be so surprising were it
+not that the climatic phenomena appear to have consisted of highly
+complex cycles while the uplift has been a relatively steady movement in
+one direction. We shall find some evidence that the solar disturbances
+which seem to cause climatic changes also have a relation to movements
+of the crust.
+
+FOOTNOTES:
+
+[Footnote 15: The so-called sunspot numbers to which reference is made
+again and again in this book are based on a system devised by Wolf and
+revised by A. Wolfer. The number and size of the spots are both taken
+into account. The numbers from 1749 to 1900 may be found in the Monthly
+Weather Review for April, 1902, and from 1901 to 1918 in the same
+journal for 1920.]
+
+
+
+
+CHAPTER V
+
+THE CLIMATE OF HISTORY[16]
+
+We are now prepared to consider the climate of the past. The first
+period to claim attention is the few thousand years covered by written
+history. Strangely enough, the conditions during this time are known
+with less accuracy than are those of geological periods hundreds of
+times more remote. Yet if pronounced changes have occurred since the
+days of the ancient Babylonians and since the last of the post-glacial
+stages, they are of great importance not only because of their possible
+historic effects, but because they bridge the gap between the little
+variations of climate which are observable during a single lifetime and
+the great changes known as glacial epochs. Only by bridging the gap can
+we determine whether there is any genetic relation between the great
+changes and the small. A full discussion of the climate of historic
+times is not here advisable, for it has been considered in detail in
+numerous other publications.[17] Our most profitable course would seem
+to be to consider first the general trend of opinion and then to take up
+the chief objections to each of the main hypotheses.
+
+In the hot debate over this problem during recent decades the ideas of
+geographers seem to have gone through much the same metamorphosis as
+have those of geologists in regard to the climate of far earlier times.
+
+As every geologist well knows, at the dawn of geology people believed in
+climatic uniformity--that is, it was supposed that since the completion
+of an original creative act there had been no important changes. This
+view quickly disappeared and was superseded by the hypothesis of
+progressive cooling and drying, an hypothesis which had much to do with
+the development of the nebular hypothesis, and which has in turn been
+greatly strengthened by that hypothesis. The discovery of evidence of
+widespread continental glaciation, however, necessitated a modification
+of this view, and succeeding years have brought to light a constantly
+increasing number of glacial, or at least cool, periods distributed
+throughout almost the whole of geological time. Moreover, each year,
+almost, brings new evidence of the great complexity of glacial periods,
+epochs, and stages. Thus, for many decades, geologists have more and
+more been led to believe that in spite of surprising uniformity, when
+viewed in comparison with the cosmic possibilities, the climate of the
+past has been highly unstable from the viewpoint of organic evolution,
+and its changes have been of all degrees of intensity.
+
+Geographers have lately been debating the reality of historic changes of
+climate in the same way in which geologists debated the reality of
+glacial epochs and stages. Several hypotheses present themselves but
+these may all be grouped under three headings; namely, the hypotheses of
+(1) progressive desiccation, (2) climatic uniformity, and (3)
+pulsations. The hypothesis of progressive desiccation has been widely
+advocated. In many of the drier portions of the world, especially
+between 30 deg. and 40 deg. from the equator, and preeminently in western and
+central Asia and in the southwestern United States, almost innumerable
+facts seem to indicate that two or three thousand years ago the climate
+was distinctly moister than at present. The evidence includes old lake
+strands, the traces of desiccated springs, roads in places now too dry
+for caravans, other roads which make detours around dry lake beds where
+no lakes now exist, and fragments of dead forests extending over
+hundreds of square miles where trees cannot now grow for lack of water.
+Still stronger evidence is furnished by ancient ruins, hundreds of which
+are located in places which are now so dry that only the merest fraction
+of the former inhabitants could find water. The ruins of Palmyra, in the
+Syrian Desert, show that it must once have been a city like modern
+Damascus, with one or two hundred thousand inhabitants, but its water
+supply now suffices for only one or two thousand. All attempts to
+increase the water supply have had only a slight effect and the water is
+notoriously sulphurous, whereas in the former days, when it was
+abundant, it was renowned for its excellence. Hundreds of pages might be
+devoted to describing similar ruins. Some of them are even more
+remarkable for their dryness than is Niya, a site in the Tarim Desert of
+Chinese Turkestan. Yet there the evidence of desiccation within 2000
+years is so strong that even so careful and conservative a man as
+Hann,[18] pronounces it "ueberzeugend."
+
+A single quotation from scores that might be used will illustrate the
+conclusions of some of the most careful archaeologists.[19]
+
+     Among the regions which were once populous and highly civilized,
+     but which are now desert and deserted, there are few which were
+     more closely connected with the beginnings of our own civilization
+     than the desert parts of Syria and northern Arabia. It is only of
+     recent years that the vast extent and great importance of this lost
+     civilization has been fully recognized and that attempts have been
+     made to reduce the extent of the unexplored area and to discover
+     how much of the territory which has long been known as desert was
+     formerly habitable and inhabited. The results of the explorations
+     of the last twenty years have been most astonishing in this regard.
+     It has been found that practically all of the wide area lying
+     between the coast range of the eastern Mediterranean and the
+     Euphrates, appearing upon the maps as the Syrian Desert, an area
+     embracing somewhat more than 20,000 square miles, was more thickly
+     populated than any area of similar dimensions in England or in the
+     United States is today if one excludes the immediate vicinity of
+     the large modern cities. It has also been discovered that an
+     enormous desert tract lying to the east of Palestine, stretching
+     eastward and southward into the country which we know as Arabia,
+     was also a densely populated country. How far these settled regions
+     extended in antiquity is still unknown, but the most distant
+     explorations in these directions have failed to reach the end of
+     ruins and other signs of former occupation.
+
+     The traveler who has crossed the settled, and more or less
+     populous, coast range of northern Syria and descended into the
+     narrow fertile valley of the Orontes, encounters in any farther
+     journey toward the east an irregular range of limestone hills lying
+     north and south and stretching to the northeast almost halfway to
+     the Euphrates. These hills are about 2,500 feet high, rising in
+     occasional peaks from 3,000 to 3,500 feet above sea level. They are
+     gray and unrelieved by any visible vegetation. On ascending into
+     the hills the traveler is astonished to find at every turn remnants
+     of the work of men's hands, paved roads, walls which divided
+     fields, terrace walls of massive structure. Presently he comes upon
+     a small deserted and partly ruined town composed of buildings large
+     and small constructed of beautifully wrought blocks of limestone,
+     all rising out of the barren rock which forms the ribs of the
+     hills. If he mounts an eminence in the vicinity, he will be still
+     further astonished to behold similar ruins lying in all directions.
+     He may count ten or fifteen or twenty, according to the commanding
+     position of his lookout. From a distance it is often difficult to
+     believe that these are not inhabited places; but closer inspection
+     reveals that the gentle hand of time or the rude touch of
+     earthquake has been laid upon every building. Some of the towns are
+     better preserved than others; some buildings are quite perfect but
+     for their wooden roofs which time has removed, others stand in
+     picturesque ruins, while others still are level with the ground. On
+     a far-off hilltop stands the ruin of a pagan temple, and crowning
+     some lofty ridge lie the ruins of a great Christian monastery. Mile
+     after mile of this barren gray country may be traversed without
+     encountering a single human being. Day after day may be spent in
+     traveling from one ruined town to another without seeing any green
+     thing save a terebinth tree or two standing among the ruins, which
+     have sent their roots down into earth still preserved in the
+     foundations of some ancient building. No soil is visible anywhere
+     except in a few pockets in the rock from which it could not be
+     washed by the torrential rains of the wet season; yet every ruin is
+     surrounded with the remains of presses for the making of oil and
+     wine. Only one oasis has been discovered in these high plateaus.
+
+     Passing eastward from this range of hills, one descends into a
+     gently rolling country that stretches miles away toward the
+     Euphrates. At the eastern foot of the hills one finds oneself in a
+     totally different country, at first quite fertile and dotted with
+     frequent villages of flat-roofed houses. Here practically all the
+     remains of ancient times have been destroyed through ages of
+     building and rebuilding. Beyond this narrow fertile strip the soil
+     grows drier and more barren, until presently another kind of desert
+     is reached, an undulating waste of dead soil. Few walls or towers
+     or arches rise to break the monotony of the unbroken landscape; but
+     the careful explorer will find on closer examination that this
+     region was more thickly populated in antiquity even than the hill
+     country to the west. Every unevenness of the surface marks the site
+     of a town, some of them cities of considerable extent.
+
+     We may draw certain very definite conclusions as to the former
+     conditions of the country itself. There was soil upon the northern
+     hills where none now exists, for the buildings now show unfinished
+     foundation courses which were not intended to be seen; the soil in
+     depressions without outlets is deeper than it formerly was; there
+     are hundreds of olive and wine presses in localities where no tree
+     or vine could now find footing; and there are hillsides with ruined
+     terrace walls rising one above the other with no sign of earth near
+     them. There was also a large natural water supply. In the north as
+     well as in the south we find the dry beds of rivers, streams, and
+     brooks with sand and pebbles and well-worn rocks but no water in
+     them from one year's end to the other. We find bridges over these
+     dry streams and crudely made washing boards along their banks
+     directly below deserted towns. Many of the bridges span the beds of
+     streams that seldom or never have water in them and give clear
+     evidence of the great climatic changes that have taken place. There
+     are well heads and well houses, and inscriptions referring to
+     springs; but neither wells nor springs exist today except in the
+     rarest instances. Many of the houses had their rock-hewn cisterns,
+     never large enough to have supplied water for more than a brief
+     period, and corresponding to the cisterns which most of our recent
+     forefathers had which were for convenience rather than for
+     dependence. Some of the towns in southern Syria were provided with
+     large public reservoirs, but these are not large enough to have
+     supplied water to their original populations. The high plateaus
+     were of course without irrigation; but there are no signs, even in
+     the lower flatter country, that irrigation was ever practiced; and
+     canals for this purpose could not have completely disappeared.
+     There were forests in the immediate vicinity, forests producing
+     timbers of great length and thickness; for in the north and
+     northeast practically all the buildings had wooden roofs, wooden
+     intermediate floors, and other features of wood. Costly buildings,
+     such as temples and churches, employed large wooden beams; but wood
+     was used in much larger quantities in private dwellings, shops,
+     stables, and barns. If wood had not been plentiful and cheap--which
+     means grown near by--the builders would have adopted the building
+     methods of their neighbors in the south, who used very little wood
+     and developed the most perfect type of lithic architecture the
+     world has ever seen. And here there exists a strange anomaly:
+     Northern Syria, where so much wood was employed in antiquity, is
+     absolutely treeless now; while in the mountains of southern Syria,
+     where wood must have been scarce in antiquity to have forced upon
+     the inhabitants an almost exclusive use of stone, there are still
+     groves of scrub oak and pine, and travelers of half a century ago
+     reported large forests of chestnut trees.[20] It is perfectly
+     apparent that large parts of Syria once had soil and forests and
+     springs and rivers, while it has none of these now, and that it had
+     a much larger and better distributed rainfall in ancient times than
+     it has now.
+
+Professor Butler's careful work is especially interesting because of its
+contrast to the loose statements of those who believe in climatic
+uniformity. So far as I am aware, no opponent of the hypothesis of
+climatic changes has ever even attempted to show by careful statistical
+analysis that the ancient water supply of such ruins was no greater than
+that of the present. The most that has been done is to suggest that
+there may have been sources of water which are now unknown. Of course,
+this might be true in a single instance, but it could scarcely be the
+case in many hundreds or thousands of ruins.
+
+Although the arguments in favor of a change of climate during the last
+two thousand years seem too strong to be ignored, their very strength
+seems to have been a source of error. A large number of people have
+jumped to the conclusion that the change which appears to have occurred
+in certain regions occurred everywhere, and that it consisted of a
+gradual desiccation.
+
+Many observers, quite as careful as those who believe in progressive
+desiccation, point to evidences of aridity in past times in the very
+regions where the others find proof of moisture. Lakes such as the
+Caspian Sea fell to such a low level that parts of their present floors
+were exposed and were used as sites for buildings whose ruins are still
+extant. Elsewhere, for instance in the Tian-Shan Mountains, irrigation
+ditches are found in places where irrigation never seems to be necessary
+at present. In Syria and North Africa during the early centuries of the
+Christian era the Romans showed unparalleled activity in building great
+aqueducts and in watering land which then apparently needed water almost
+as much as it does today. Evidence of this sort is abundant and is as
+convincing as is the evidence of moister conditions in the past. It is
+admirably set forth, for example, in the comprehensive and ably written
+monograph of Leiter on the climate of North Africa.[21] The evidence
+cited there and elsewhere has led many authors strongly to advocate the
+hypothesis of climatic uniformity. They have done exactly as have the
+advocates of progressive change, and have extended their conclusions
+over the whole world and over the whole of historic times.
+
+The hypotheses of climatic uniformity and of progressive change both
+seem to be based on reliable evidence. They may seem to be diametrically
+opposed to one another, but this is only when there is a failure to
+group the various lines of evidence according to their dates, and
+according to the types of climate in which they happen to be located.
+When the facts are properly grouped in both time and space, it appears
+that evidence of moist conditions in the historic Mediterranean lands is
+found during certain periods; for instance, four or five hundred years
+before Christ, at the time of Christ, and 1000 A. D. The other kind of
+evidence, on the contrary, culminates at other epochs, such as about
+1200 B. C. and in the seventh and thirteenth centuries after Christ. It
+is also found during the interval from the culmination of a moist epoch
+to the culmination of a dry one, for at such times the climate was
+growing drier and the people were under stress. This was seemingly the
+case during the period from the second to the fourth centuries of our
+era. North Africa and Syria must then have been distinctly better
+watered than at present, as appears from Butler's vivid description; but
+they were gradually becoming drier, and the natural effect on a
+vigorous, competent people like the Romans was to cause them to
+construct numerous engineering works to provide the necessary water.
+
+The considerations which have just been set forth have led to a third
+hypothesis, that of pulsatory climatic changes. According to this, the
+earth's climate is not stable, nor does it change uniformly in one
+direction. It appears to fluctuate back and forth not only in the little
+waves which we see from year to year or decade to decade, but in much
+larger waves, which take hundreds of years or even a thousand. These in
+turn seem to merge into and be imposed on the greater waves which form
+glacial stages, glacial epochs, and glacial periods. At the present time
+there seems to be no way of determining whether the general tendency is
+toward aridity or toward glaciation. The seventh century of our era was
+apparently the driest time during the historic period--distinctly drier
+than the present--but the thirteenth century was almost equally dry, and
+the twelfth or thirteenth before Christ may have been very dry.
+
+The best test of an hypothesis is actual measurements. In the case of
+the pulsatory hypothesis we are fortunately able to apply this test by
+means of trees. The growth of vegetation depends on many factors--soil,
+exposure, wind, sun, temperature, rain, and so forth. In a dry region
+the most critical factor in determining how a tree's growth shall vary
+from year to year is the supply of moisture during the few months of
+most rapid growth.[22] The work of Douglass[23] and others has shown
+that in Arizona and California the thickness of the annual rings affords
+a reliable indication of the amount of moisture available during the
+period of growth. This is especially true when the growth of several
+years is taken as the unit and is compared with the growth of a similar
+number of years before or after. Where a long series of years is used,
+it is necessary to make corrections to eliminate the effects of age, but
+this can be done by mathematical methods of considerable accuracy. It is
+difficult to determine whether the climate at the beginning and end of a
+tree's life was the same, but it is easily possible to determine whether
+there have been pulsations while the tree was making its growth. If a
+large number of trees from various parts of a given district all formed
+thick rings at a certain period and then formed thin ones for a hundred
+years, after which the rings again become thick, we seem to be safe in
+concluding that the trees have lived through a long, dry period. The
+full reasons for this belief and details as to the methods of estimating
+climate from tree growth are given in _The Climatic Factor_.
+
+The results set forth in that volume may be summarized as follows:
+During the years 1911 and 1912, under the auspices of the Carnegie
+Institution of Washington, measurements were made of the thickness of
+the rings of growth on the stumps of about 450 sequoia trees in
+California. These trees varied in age from 250 to nearly 3250 years. The
+great majority were over 1000 years of age, seventy-nine were over 2000
+years, and three over 3000. Even where only a few trees are available
+the record is surprisingly reliable, except where occasional accidents
+occur. Where the number approximates 100, accidental variations are
+largely eliminated and we may accept the record with considerable
+confidence. Accordingly, we may say that in California we have a fairly
+accurate record of the climate for 2000 years and an approximate record
+for 1000 years more. The final results of the measurements of the
+California trees are shown in Fig. 4, where the climatic variations for
+3000 years in California are indicated by the solid line. The high parts
+of the line indicate rainy conditions, the low parts, dry. An
+examination of this curve shows that during 3000 years there have
+apparently been climatic variations more important than any which have
+taken place during the past century. In order to bring out the details
+more clearly, the more reliable part of the California curve, from 100
+B. C. to the present time, has been reproduced in Fig. 5. This is
+identical with the corresponding part of Fig. 4, except that the
+vertical scale is three times as great.
+
+[Illustration: _Fig. 4. Changes of climate in California (solid line)
+and in western and central Asia (dotted line)._
+
+Note. The curves of Figs. 4 and 5 are reproduced as published in _The
+Solar Hypothesis_ in 1914. Later work, however, has indicated that in
+the Asiatic curve the dash lines, which were tentatively inserted in
+1914, are probably more nearly correct than the dotted lines. Still
+further evidence indicates that the Asiatic curve is nearly like that of
+California in its main features.]
+
+The curve of tree growth in California seems to be a true representation
+of the general features of climatic pulsations in the Mediterranean
+region. This conclusion was originally based on the resemblance between
+the solid line of Fig. 4, representing tree growth, and the dotted line
+representing changes of climate in the eastern Mediterranean region as
+inferred from the study of ruins and of history before any work on this
+subject had been done in America.[24] The dotted line is here reproduced
+for its historical significance as a stage in the study of climatic
+changes. If it were to be redrawn today on the basis of the knowledge
+acquired in the last twelve years, it would be much more like the tree
+curve. For example, the period of aridity suggested by the dip of the
+dotted line about 300 A. D. was based largely on Professor Butler's data
+as to the paucity of inscriptions and ruins dating from that period in
+Syria. In the recent article, from which a long quotation has been
+given, he shows that later work proves that there is no such paucity. On
+the other hand, it has accentuated the marked and sudden decay in
+civilization and population which occurred shortly after 600 A. D. He
+reached the same conclusion to which the present authors had come on
+wholly different grounds, namely, that the dip in the dotted line about
+300 A. D. is not warranted, whereas the dip about 630 A. D. is extremely
+important. In similar fashion the work of Stein[25] in central Asia
+makes it clear that the contrast between the water supply about 200 B.C.
+and in the preceding and following centuries was greater than was
+supposed on the basis of the scanty evidence available when the dotted
+line of Fig. 4 was drawn in 1910.
+
+[Illustration: _Fig. 5. Changes in California climate for 2000 years,
+as measured by growth of Sequoia trees._
+
+Fig. 5 is the same as the later portion of Fig. 4, except that the
+vertical scale has been magnified threefold. It seems probable that the
+dotted line at the right is more nearly correct than the solid line.
+During the thirty years since the end of the curve the general tendency
+appears in general to have been somewhat upward.]
+
+Since the curve of the California trees is the only continuous and
+detailed record yet available for the climate of the last three thousand
+years, it deserves most careful study. It is especially necessary to
+determine the degree of accuracy with which the growth of the trees
+represents (1) the local rainfall and (2) the rainfall of remote regions
+such as Palestine. Perhaps the best way to determine these matters is
+the standard mathematical method of correlation coefficients. If two
+phenomena vary in perfect unison, as in the case of the turning of the
+wheels and the progress of an automobile when the brakes are not
+applied, the correlation coefficient is 1.00, being positive when the
+automobile goes forward and negative when it goes backward. If there is
+no relation between two phenomena, as in the case of the number of miles
+run by a given automobile each year and the number of chickens hatched
+in the same period, the coefficient is zero. A partial relationship
+where other factors enter into the matter is represented by a
+coefficient between zero and one, as in the case of the movement of the
+automobile and the consumption of gasoline. In this case the relation is
+very obvious, but is modified by other factors, including the roughness
+and grade of the road, the amount of traffic, the number of stops, the
+skill of the driver, the condition and load of the automobile, and the
+state of the weather. Such partial relationships are the kind for which
+correlation coefficients are most useful, for the size of the
+coefficients shows the relative importance of the various factors. A
+correlation coefficient four times the probable error, which can always
+be determined by a formula well known to mathematicians, is generally
+considered to afford evidence of some kind of relation between two
+phenomena. When the ratio between coefficient and error rises to six,
+the relationship is regarded as strong.
+
+Few people would question that there is a connection between tree growth
+and rainfall, especially in a climate with a long summer dry season like
+that of California. But the growth of the trees also depends on their
+position, the amount of shading, the temperature, insect pests, blights,
+the wind with its tendency to break the branches, and a number of other
+factors. Moreover, while rain commonly favors growth, great extremes are
+relatively less helpful than more moderate amounts. Again, the roots of
+a tree may tap such deep sources of water that neither drought nor
+excessive rain produces much effect for several years. Hence in
+comparing the growth of the huge sequoias with the rainfall we should
+expect a correlation coefficient high enough to be convincing, but
+decidedly below 1.00. Unfortunately there is no record of the rainfall
+where the sequoias grow, the nearest long record being that of
+Sacramento, nearly 200 miles to the northwest and close to sea level
+instead of at an altitude of about 6000 feet.
+
+Applying the method of correlation coefficients to the annual rainfall
+of Sacramento and the growth of the sequoias from 1863 to 1910, we
+obtain the results shown in Table 3. The trees of Section A of the table
+grew in moderately dry locations although the soil was fairly deep, a
+condition which seems to be essential to sequoias. In this case, as in
+all the others, the rainfall is reckoned from July to June, which
+practically means from October to May, since there is almost no summer
+rain. Thus the tree growth in 1861 is compared with the rainfall of the
+preceding rainy season, 1860-1861, or of several preceding rainy seasons
+as the table indicates.
+
+   +-------------------------------------------------------------------+
+   |                            TABLE 3                                |
+   |                                                                   |
+   |         CORRELATION COEFFICIENTS BETWEEN RAINFALL AND             |
+   |              GROWTH OF SEQUOIAS IN CALIFORNIA[26]                 |
+   |                                                                   |
+   |     (_r_) = _Correlation coefficient_                             |
+   |     (_e_) = _Probable error_                                      |
+   | (_r_/_e_) = _Ratio of coefficient to probable error_              |
+   |                                                                   |
+   | A. SACRAMENTO RAINFALL AND GROWTH OF 18 SEQUOIAS IN DRY           |
+   |                       LOCATIONS, 1861-1910                        |
+   |                                                                   |
+   |                                     (_r_)     (_e_)  (_r_/_e_)    |
+   |                                     ------    ------    -----     |
+   |   1 year of rainfall                -0.059    +-0.096     0.6      |
+   |   2 years of rainfall               +0.288    +-0.090     3.2      |
+   |   3 years of rainfall               +0.570    +-0.066     8.7      |
+   |   4 years of rainfall               +0.470    +-0.076     6.2      |
+   |                                                                   |
+   | B. SACRAMENTO RAINFALL AND GROWTH OF 112 SEQUOIAS MOSTLY IN       |
+   |                  MOIST LOCATIONS, 1861-1910                       |
+   |                                                                   |
+   |   3 years of rainfall               +0.340    +-0.087     3.9      |
+   |   4 years of rainfall               +0.371    +-0.084     4.5      |
+   |   5 years of rainfall               +0.398    +-0.082     4.9      |
+   |   6 years of rainfall               +0.418    +-0.079     5.3      |
+   |   7 years of rainfall               +0.471    +-0.076     6.2      |
+   |   8 years of rainfall              (+0.520)   +-0.071     7.3      |
+   |   9 years of rainfall               +0.575    +-0.065     8.8      |
+   |  10 years of rainfall               +0.577    +-0.065     8.8      |
+   |                                                                   |
+   | C. SACRAMENTO RAINFALL AND GROWTH OF 80 SEQUOIAS IN MOIST         |
+   |                  LOCATIONS, 1861-1910                             |
+   |                                                                   |
+   |  10 years of rainfall               +0.605    +-0.062     9.8      |
+   |                                                                   |
+   | D. ANNUAL SEQUOIA GROWTH AND RAINFALL OF PRECEDING 5 YEARS        |
+   |          AT STATIONS ON SOUTHERN PACIFIC RAILROAD                 |
+   |                                                                   |
+   |        1 = _Years_                                                |
+   |        2 = _Altitude_ (_feet_)                                    |
+   |        3 = _Rainfall_ (_inches_)                                  |
+   |        4 = _Approximate distance from sequoias_ (_miles_)         |
+   |                                                                   |
+   |                 1        2     3     4   (_r_)   (_e_)  (_r_/_e_) |
+   |             ---------  ----  -----  ---  ------  ------ --------- |
+   | Sacramento, 1861-1910    70  19.40  200  +0.398  +-0.081     4.9   |
+   | Colfax,     1871-1909  2400  48.94  200  +0.122  +-0.113     1.1   |
+   | Summit,     1871-1909  7000  48.07  200  +0.148  +-0.113     1.3   |
+   | Truckee,    1871-1909  5800  27.12  200  +0.300  +-0.105     2.9   |
+   | Boca,       1871-1909  5500  20.34  200  +0.604  +-0.076     8.0   |
+   | Winnemucca, 1871-1909  4300   8.65  300  +0.492  +-0.089     5.5   |
+   |                                                                   |
+   +-------------------------------------------------------------------+
+
+In the first line of Section A a correlation coefficient of only -0.056,
+which is scarcely six-tenths of the probable error, means that there is
+no appreciable relation between the rainfall of a given season and the
+growth during the following spring and summer. The roots of the sequoias
+probably penetrate so deeply that the rain and melted snow of the spring
+months do not sink down rapidly enough to influence the trees before the
+growing season comes to an end. The precipitation of two preceding
+seasons, however, has some effect on the trees, as appears in the second
+line of Section A, where the correlation coefficient is +0.288, or 3.2
+times the probable error. When the rainfall of three seasons is taken
+into account the coefficient rises to +0.570, or 8.7 times the probable
+error, while with four years of rainfall the coefficient begins to fall
+off. Thus the growth of these eighteen sequoias on relatively dry slopes
+appears to have depended chiefly on the rainfall of the second and third
+preceding rainy seasons. The growth in 1900, for example, depended
+largely on the rainfall in the rainy seasons of 1897-1898 and 1898-1899.
+
+Section B of the table shows that with 112 trees, growing chiefly in
+moist depressions where the water supply is at a maximum, the
+correlation between growth and rainfall, +0.577 for ten years' rainfall,
+is even higher than with the dry trees. The seepage of the underground
+water is so slow that not until four years' rainfall is taken into
+account is the correlation coefficient more than four times the probable
+error. When only the trees growing in moist locations are employed, the
+coefficient between tree growth and the rainfall for ten years rises to
+the high figure of +0.605, or 9.8 times the probable error, as appears
+in Section C. These figures, as well as many others not here published,
+make it clear that the curve of sequoia growth from 1861 to 1910 affords
+a fairly close indication of the rainfall at Sacramento, provided
+allowance be made for a delay of three to ten years due to the fact that
+the moisture in the soil gradually seeps down the mountain-sides and
+only reaches the sequoias after a considerable interval.
+
+If a rainfall record were available for the place where the trees
+actually grow, the relationship would probably be still closer.
+
+The record at Fresno, for example, bears out this conclusion so far as
+it goes. But as Fresno lies at a low altitude and its rainfall is of
+essentially the Sacramento type, its short record is of less value than
+that of Sacramento. The only rainfall records among the Sierras at high
+levels, where the rainfall and temperature are approximately like those
+of the sequoia region, are found along the main line of the Southern
+Pacific railroad. This runs from Oakland northeastward seventy miles
+across the open plain to Sacramento, then another seventy miles, as the
+crow flies, through Colfax and over a high pass in the Sierras at
+Summit, next twenty miles or so down through Truckee to Boca, on the
+edge of the inland basin of Nevada, and on northeastward another 160
+miles to Winnemucca, where it turns east toward Ogden and Salt Lake
+City. Section D of Table 3 shows the correlation coefficients between
+the rainfall along the railroad and the growth of the sequoias. At
+Sacramento, which lies fairly open to winds from the Pacific and thus
+represents the general climate of central California, the coefficient is
+nearly five times the probable error, thus indicating a real relation to
+sequoia growth. Then among the foothills of the Sierras at Colfax, the
+coefficient drops till it is scarcely larger than the probable error. It
+rises rapidly, however, as one advances among the mountains, until at
+Boca it attains the high figure of +0.604 or eight times the probable
+error, and continues high in the dry area farther east. In other words
+the growth of the sequoias is a good indication of the rainfall where
+the trees grow and in the dry region farther east.
+
+In order to determine the degree to which the sequoia record represents
+the rainfall of other regions, let us select Jerusalem for comparison.
+The reasons for this selection are that Jerusalem furnishes the only
+available record that satisfies the following necessary conditions: (1)
+its record is long enough to be important; (2) it is located fairly near
+the latitude of the sequoias, 32 deg.N versus 37 deg.N; (3) it is located in a
+similar type of climate with winter rains and a long dry summer; (4) it
+lies well above sea level (2500 feet) and somewhat back from the
+seacoast, thus approximating although by no means duplicating the
+condition of the sequoias; and (5) it lies in a region where the
+evidence of climatic changes during historic times is strongest. The
+ideal place for comparison would be the valley in which grow the cedars
+of Lebanon. Those trees resemble the sequoias to an extraordinary
+degree, not only in their location, but in their great age. Some day it
+will be most interesting to compare the growth of these two famous
+groups of old trees.
+
+   +-------------------------------------------------------------------+
+   |                            TABLE 4                                |
+   |                                                                   |
+   |                CORRELATION COEFFICIENTS BETWEEN                   |
+   |                 RAINFALL RECORDS IN CALIFORNIA                    |
+   |                        AND JERUSALEM                              |
+   |                                                                   |
+   |     (_r_) = _Correlation coefficient_                             |
+   |     (_e_) = _Probable error_                                      |
+   | (_r_/_e_) = _Ratio of coefficient to probable error_              |
+   |                                                                   |
+   | A. JERUSALEM RAINFALL FOR 3 YEARS AND VARIOUS GROUPS OF           |
+   |                           SEQUOIAS[27]                            |
+   |                                                                   |
+   |                                          (_r_)    (_e_)  (_r_/_e_)|
+   |                                          ------   -----  ---------|
+   |  11 trees measured by Douglass           +0.453  +-0.078    5.8    |
+   |  80 trees, moist locations, Groups IA,                            |
+   |       IIA, IIIA, VA                      +0.500  +-0.073    6.8    |
+   | 101 trees, 69 in moist locations, 32 in                           |
+   |       dry, I, II, III                    +0.616  +-0.061   10.1    |
+   | 112 trees, 80 in moist locations, 32 in                           |
+   |       dry, I, II, III, V                 +0.675  +-0.053   12.7    |
+   |                                                                   |
+   | B. RAINFALL AT JERUSALEM AND AT STATIONS IN CALIFORNIA AND NEVADA |
+   |                                                                   |
+   |                   1 = _Altitude_ (_feet_)                         |
+   |                   2 = _Years_                                     |
+   |                                                                   |
+   |                                    -- 3 years --   -- 5 years --  |
+   |                    1       2      (_r_) (_r_/_e_) (_r_) (_r_/_e_) |
+   |                  ----  ---------  ------ -------  ------ -------  |
+   |  Sacramento,       70  1861-1910  +0.386   4.7    +0.352   4.2    |
+   |  Colfax,         2400  1871-1909  +0.311   3.1    +0.308   3.0    |
+   |  Summit,         7000  1871-1909  +0.099   0.9    +0.248   2.3    |
+   |  Truckee,        5800  1871-1909  +0.229   2.2    +0.337   3.3    |
+   |[A]Boca,          5500  1871-1909  +0.482   6.4    +0.617   8.6    |
+   |  Winnemucca,     4300  1871-1909  +0.235   2.2    +0.260   2.4    |
+   |  San Bernardino, 1050  1871-1909  +0.275   2.7    +0.177   1.8    |
+   |                                                                   |
+   | C. RAINFALL FOR 3 YEARS AT CALIFORNIA AND NEVADA STATIONS,        |
+   |                               1871-1909                           |
+   |                                                                   |
+   |                                                (_r_)  (_r_/_e_)   |
+   |                                                ------  -------    |
+   |  Sacramento and San Bernardino                 +0.663    10.7     |
+   |  San Bernardino and Winnemucca                 +0.291     2.8     |
+   |                                                                   |
+   +-------------------------------------------------------------------+
+
+The correlation coefficients for the sequoia growth and the rainfall at
+Jerusalem are given in Section A, Table 4. They are so high and so
+consistent that they scarcely leave room for doubt that where a hundred
+or more sequoias are employed, as in Fig. 5, their curve of growth
+affords a good indication of the fluctuations of climate in western
+Asia. The high coefficient for the eleven trees measured by Douglass
+suggests that where the number of trees falls as low as ten, as in the
+part of Fig. 4 from 710 to 840 B. C., the relation between tree growth
+and rainfall is still close even when only one year's growth is
+considered. Where the unit is ten years of growth, as in Figs. 4 and 5,
+the accuracy of the tree curve as a measure of rainfall is much greater
+than when a single year is used as in Table 4. When the unit is raised
+to thirty years, as in the smoothed part of Fig. 4 previous to
+240 B. C., even four trees, as from 960 to 1070, probably give a fair
+approximation to the general changes in rainfall, while a single tree
+prior to 1110 B. C. gives a rough indication.
+
+Table 4 shows a peculiar feature in the fact that the correlations of
+Section A between tree growth and the rainfall of Jerusalem are
+decidedly higher than those between the rainfall in the two regions.
+Only at Sacramento and Boca are the rainfall coefficients high enough to
+be conclusive. This, however, is not surprising, for even between
+Sacramento and San Bernardino, only 400 miles apart, the correlation
+coefficient for the rainfall by three-year periods is only 10.7 times
+the probable error, as appears in Section C of Table 4, while between
+San Bernardino and Winnemucca 500 miles away, the corresponding figure
+drops to 2.8. It must be remembered that in some respects the growth of
+the sequoias is a much better record of rainfall than are the records
+kept by man. The human record is based on the amount of water caught by
+a little gauge a few inches in diameter. Every gust of wind detracts
+from the accuracy of the record; a mile away the rainfall may be double
+what it is at the gauge. Each sequoia, on the other hand, draws its
+moisture from an area thousands of times as large as a rain gauge.
+Moreover, the trees on which Figs. 4 and 5 are based were scattered over
+an area fifty miles long and several hundred square miles in extent.
+Hence they represent the summation of the rainfall over an area millions
+of times as large as that of a rain gauge. This fact and the large
+correlation coefficients between sequoia growth and Jerusalem rainfall
+should be considered in connection with the fact that all the
+coefficients between the rainfall of California and Nevada and that of
+Jerusalem are positive. If full records of the complete rainfall of
+California and Nevada on the one hand and of the eastern Mediterranean
+region on the other were available for a long period, they would
+probably agree closely.
+
+Just how widely the sequoias can be used as a measure of the climate of
+the past is not yet certain. In some regions, as will shortly be
+explained, the climatic changes seem to have been of an opposite
+character from those of California. In others the Californian or eastern
+Mediterranean type of change seems sometimes to prevail but is not
+always evident. For example, at Malta the rainfall today shows a
+distinct relation to that of Jerusalem and to the growth of the
+sequoias. But the correlation coefficient between the rainfall of
+eight-year periods at Naples, a little farther north, and the growth of
+the sequoias at the end of the periods is -0.132, or only 1.4 times the
+probable error and much too small to be significant. This is in harmony
+with the fact that although Naples has summer droughts, they are not so
+pronounced as in California and Palestine, and the prevalence of storms
+is much greater. Jerusalem receives only 8 per cent of its rain in the
+seven months from April to October, and Sacramento 13, while Malta
+receives 31 per cent and Naples 43. Nevertheless, there is some evidence
+that in the past the climatic fluctuations of southern Italy followed
+nearly the same course as those of California and Palestine. This
+apparent discrepancy seems to be explained by our previous conclusion
+that changes of climate are due largely to a shifting of storm tracks.
+When sunspots are numerous the storms which now prevail in northern
+Italy seem to be shifted southward and traverse the Mediterranean to
+Palestine just as similar storms are shifted southward in the United
+States. This perhaps accounts for the agreement between the sequoia
+curve and the agricultural and social history of Rome from about
+400 B. C. to 100 A. D., as explained in _World Power and Evolution_. For
+our present purposes, however, the main point is that since rainfall
+records have been kept the fluctuations of climate indicated by the
+growth of the sequoias have agreed closely with fluctuations in the
+rainfall of the eastern Mediterranean region. Presumably the same was
+true in the past. In that case, the sequoia curve not only is a good
+indication of climatic changes or pulsations in regions of similar
+climate, but may serve as a guide to coincident but different changes in
+regions of other types.
+
+An enormous body of other evidence points to the same conclusion. It
+indicates that while the average climate of the present is drier than
+that of the past in regions having the Mediterranean type of winter
+rains and summer droughts, there have been pronounced pulsations during
+historic times so that at certain times there has actually been greater
+aridity than at present. This conclusion is so important that it seems
+advisable to examine the only important arguments that have been raised
+against it, especially against the idea that the general rainfall of the
+eastern Mediterranean was greater in the historic past than at present.
+The first objection is the unquestionable fact that droughts and famines
+have occurred at periods which seem on other evidence to have been
+moister than the present. This argument has been much used, but it seems
+to have little force. If the rainfall of a given region averages thirty
+inches and varies from fifteen to forty-five, a famine will ensue if the
+rainfall drops for a few years to the lower limit and does not rise much
+above twenty for a few years. If the climate of the place changes during
+the course of centuries, so that the rainfall averages only twenty
+inches, and ranges from seven to thirty-five, famine will again ensue if
+the rainfall remains near ten inches for a few years. The ravages of the
+first famine might be as bad as those of the second. They might even be
+worse, because when the rainfall is larger the population is likely to
+be greater and the distress due to scarcity of food would affect a
+larger number of people. Hence historic records of famines and droughts
+do not indicate that the climate was either drier or moister than at
+present. They merely show that at the time in question the climate was
+drier than the normal for that particular period.
+
+The second objection is that deserts existed in the past much as at
+present. This is not a real objection, however, for, as we shall see
+more fully, some parts of the world suffer one kind of change and others
+quite the opposite. Moreover, deserts have always existed, and when we
+talk of a change in their climate we merely mean that their boundaries
+have shifted. A concrete example of the mistaken use of ancient dryness
+as proof of climatic uniformity is illustrated by the march of Alexander
+from India to Mesopotamia. Hedin gives an excellent presentation of the
+case in the second volume of his _Overland to India_. He shows
+conclusively that Alexander's army suffered terribly from lack of water
+and provisions. This certainly proves that the climate was dry, but it
+by no means indicates that there has been no change from the past to the
+present. We do not know whether Alexander's march took place during an
+especially dry or an especially wet year. In a desert region like
+Makran, in southern Persia and Beluchistan, where the chief difficulties
+occurred, the rainfall varies greatly from year to year. We have no
+records from Makran, but the conditions there are closely similar to
+those of southern Arizona and New Mexico. In 1885 and 1905 the rainfall
+for five stations in that region was as follows:
+
+   +------------------------------------------------------------+
+   |                                                            |
+   |                                            _Mean rainfall  |
+   |                                            during period   |
+   |                        _1885_    _1905_    since           |
+   |                                            observations    |
+   |                                            began_          |
+   | Yuma, Arizona,          2.72     11.41        3.13         |
+   | Phoenix, Arizona,       3.77     19.73        7.27         |
+   | Tucson, Arizona,        5.26     24.17       11.66         |
+   | Lordsburg, New Mexico,  3.99     19.50        8.62         |
+   | El Paso, Texas (on New                                     |
+   |   Mexico border),       7.31     17.80        9.06         |
+   |                         ----     -----       -----         |
+   | Average,                4.61     18.52        7.95         |
+   |                                                            |
+   +------------------------------------------------------------+
+
+These stations are distributed over an area nearly 500 miles east and
+west. Manifestly a traveler who spent the year 1885 in that region would
+have had much more difficulty in finding water and forage than one who
+traveled in the same places in 1905. During 1885 the rainfall was 42 per
+cent less than the average, and during 1905 it was 134 per cent more
+than the average. Let us suppose, for the sake of argument, that the
+average rainfall of southeastern Persia is six inches today and was ten
+inches in the days of Alexander. If the rainfall from year to year
+varied as much in the past in Persia as it does now in New Mexico and
+Arizona, the rainfall during an ancient dry year, corresponding in
+character to 1885, would have been about 5.75 inches. On the other hand,
+if we suppose that the rainfall then averaged less than at present,--let
+us say four inches,--a wet year corresponding to 1905 in the American
+deserts might have had a rainfall of about ten inches. This being the
+case, it is clear that our estimate of what Alexander's march shows as
+to climate must depend largely on whether 325 B. C. was a wet year or a
+dry year. Inasmuch as we know nothing about this, we must fall back on
+the fact that a large army accomplished a journey in a place where today
+even a small caravan usually finds great difficulty in procuring forage
+and water. Moreover, elephants were taken 180 miles across what is now
+an almost waterless desert, and yet the old historians make no comment
+on such a feat which today would be practically impossible. These things
+seem more in harmony with a change of climate than with uniformity.
+Nevertheless, it is not safe to place much reliance on them except when
+they are taken in conjunction with other evidence, such as the numerous
+ruins, which show that Makran was once far more densely populated than
+now seems possible. Taken by itself, such incidents as Alexander's march
+cannot safely be used either as an argument for or against changes of
+climate.
+
+The third and strongest objection to any hypothesis of climatic changes
+during historic times is based on vegetation. The whole question is
+admirably set forth by J. W. Gregory,[28] who gives not only his own
+results, but those of the ablest scholars who have preceded him. His
+conclusions are important because they represent one of the few cases
+where a definite statistical attempt has been made to prove the exact
+condition of the climate of the past. After stating various less
+important reasons for believing that the climate of Palestine has not
+changed, he discusses vegetation. The following quotation indicates his
+line of thought. A sentence near the beginning is italicized in order to
+call attention to the importance which Gregory and others lay on this
+particular kind of evidence:
+
+    Some more certain test is necessary than the general conclusions
+    which can be based upon the historical and geographical evidence of
+    the Bible. In the absence of rain gauge and thermometric records,
+    _the most precise test of climate is given by the vegetation; and
+    fortunately the palm affords a very delicate test of the past
+    climate of Palestine and the eastern Mediterranean_.... The date
+    palm has three limits of growth which are determined by temperature;
+    thus it does not reach full maturity or produce ripe fruit of good
+    quality below the mean annual temperature of 69 deg.F. The isothermal of
+    69 deg. crosses southern Algeria near Biskra; it touches the northern
+    coasts of Cyrenaica near Derna and passes Egypt near the mouth of
+    the Nile, and then bends northward along the coast lands of
+    Palestine.
+
+    To the north of this line the date palm grows and produces fruit,
+    which only ripens occasionally, and its quality deteriorates as the
+    temperature falls below 69 deg.. Between the isotherms of 68 deg. and 64 deg.,
+    limits which include northern Algeria, most of Sicily, Malta, the
+    southern parts of Greece and northern Syria, the dates produced are
+    so unripe that they are not edible. In the next cooler zone, north
+    of the isotherm of 62 deg., which enters Europe in southwestern
+    Portugal, passes through Sardinia, enters Italy near Naples, crosses
+    northern Greece and Asia Minor to the east of Smyrna, the date palm
+    is grown only for its foliage, since it does not fruit.
+
+    Hence at Benghazi, on the north African coast, the date palm is
+    fertile, but produces fruit of poor quality. In Sicily and at
+    Algiers the fruit ripens occasionally and at Rome and Nice the palm
+    is grown only as an ornamental tree.
+
+    The date palm therefore affords a test of variations in mean annual
+    temperature of three grades between 62 deg. and 69 deg..
+
+    This test shows that the mean annual temperature of Palestine has
+    not altered since Old Testament times. The palm tree now grows dates
+    on the coast of Palestine and in the deep depression around the Dead
+    Sea, but it does not produce fruit on the highlands of Judea. Its
+    distribution in ancient times, as far as we can judge from the
+    Bible, was exactly the same. It grew at "Jericho, the city of palm
+    trees" (Deut. xxxiv: 3 and 2 Chron. xxviii: 15), and at Engedi, on
+    the western shore of the Dead Sea (2 Chron. xx: 2; Sirach xxiv: 14);
+    and though the palm does not still live at Jericho--the last
+    apparently died in 1838--its disappearance must be due to neglect,
+    for the only climatic change that would explain it would be an
+    increase in cold or moisture. In olden times the date palm certainly
+    grew on the highlands of Palestine; but apparently it never produced
+    fruit there, for the Bible references to the palm are to its beauty
+    and erect growth: "The righteous shall flourish like the palm" (Ps.
+    xcii: 12); "They are upright as the palm tree" (Jer. x: 5); "Thy
+    stature is like to a palm tree" (Cant. vii: 7). It is used as a
+    symbol of victory (Rev. vii: 9), but never praised as a source of
+    food.
+
+    Dates are not once referred to in the text of the Bible, but
+    according to the marginal notes the word translated "honey" in
+    2 Chron. xxxi: 5 may mean dates....
+
+    It appears, therefore, that the date palm had essentially the same
+    distribution in Palestine in Old Testament times as it has now; and
+    hence we may infer that the mean temperature was then the same as
+    now. If the climate had been moister and cooler, the date could not
+    have flourished at Jericho. If it had been warmer, the palms would
+    have grown freely at higher levels and Jericho would not have held
+    its distinction as _the_ city of palm trees.[29]
+
+In the main Gregory's conclusions seem to be well grounded, although
+even according to his data a change of 2 deg. or 3 deg. in mean temperature
+would be perfectly feasible. It will be noticed, however, that they
+apply to temperature and not to rainfall. They merely prove that two
+thousand years ago the mean temperature of Palestine and the neighboring
+regions was not appreciably different from what it is today. This,
+however, is in no sense out of harmony with the hypothesis of climatic
+pulsations. Students of glaciation believe that during the last glacial
+epoch the mean temperature of the earth as a whole was only 5 deg. or 6 deg.C.
+lower than at present. If the difference between the climate of today
+and of the time of Christ is a tenth as great as the difference between
+the climate of today and that which prevailed at the culmination of the
+last glacial epoch, the change in two thousand years has been of large
+dimensions. Yet this would require a rise of only half a degree
+Centigrade in the mean temperature of Palestine. Manifestly, so slight a
+change would scarcely be detectable in the vegetation.
+
+The slightness of changes in mean temperature as compared with changes
+in rainfall may be judged from a comparison of wet and dry years in
+various regions. For example, at Berlin between 1866 and 1905 the ten
+most rainy years had an average precipitation of 670 mm. and a mean
+temperature of 9.15 deg.C. On the other hand, the ten years of least
+rainfall had an average of 483 mm. and a mean temperature of 9.35 deg.. In
+other words, a difference of 137 mm., or 39 per cent, in rainfall was
+accompanied by a difference of only 0.2 deg.C. in temperature. Such
+contrasts between the variability of mean rainfall and mean temperature
+are observable not only when individual years are selected, but when
+much longer periods are taken. For instance, in the western Gulf region
+of the United States the two inland stations of Vicksburg, Mississippi,
+and Shreveport, Louisiana, and the two maritime stations of New Orleans,
+Louisiana, and Galveston, Texas, lie at the margins of an area about 400
+miles long. During the ten years from 1875 to 1884 their rainfall
+averaged 59.4 inches,[30] while during the ten years from 1890 to 1899
+it averaged only 42.4 inches. Even in a region so well watered as the
+Gulf States, such a change--40 per cent more in the first decade than in
+the second--is important, and in drier regions it would have a great
+effect on habitability. Yet in spite of the magnitude of the change the
+mean temperature was not appreciably different, the average for the four
+stations being 67.36 deg.F. during the more rainy decade and 66.94 deg.F. during
+the less rainy decade--a difference of only 0.42 deg.F. It is worth noticing
+that in this case the wetter period was also the warmer, whereas in
+Berlin it was the cooler. This is probably because a large part of the
+moisture of the Gulf States is brought by winds having a southerly
+component. Similar relationships are apparent in other places. We select
+Jerusalem because we have been discussing Palestine. At the time of
+writing, the data available in the _Quarterly Journal of the Palestine
+Exploration Fund_ cover the years from 1882-1899 and 1903-1909. Among
+these twenty-five years the thirteen which had most rain had an average
+of 34.1 inches and a temperature of 62.04 deg.F. The twelve with least rain
+had 24.4 inches and a temperature of 62.44 deg.. A difference of 40 per cent
+in rainfall was accompanied by a difference of only 0.4 deg.F. in
+temperature.
+
+The facts set forth in the preceding paragraphs seem to show that
+extensive changes in precipitation and storminess can take place without
+appreciable changes of mean temperature. If such changed conditions can
+persist for ten years, as in one of our examples, there is no logical
+reason why they cannot persist for a hundred or a thousand. The evidence
+of changes in climate during the historic period seems to suggest
+changes in precipitation much more than in temperature. Hence the
+strongest of all the arguments against historic changes of climate seems
+to be of relatively little weight, and the pulsatory hypothesis seems to
+be in accord with all the known facts.
+
+Before the true nature of climatic changes, whether historic or
+geologic, can be rightly understood, another point needs emphasis. When
+the pulsatory hypothesis was first framed, it fell into the same error
+as the hypotheses of uniformity and of progressive change--that is, the
+assumption was made that the whole world is either growing drier or
+moister with each pulsation. A study of the ruins of Yucatan, in 1912,
+and of Guatemala, in 1913, as is explained in _The Climatic Factor_, has
+led to the conclusion that the climate of those regions has changed in
+the opposite way from the changes which appear to have taken place in
+the desert regions farther south. These Maya ruins in Central America
+are in many cases located in regions of such heavy rainfall, such dense
+forests, and such malignant fevers that habitation is now practically
+impossible. The land cannot be cultivated except in especially favorable
+places. The people are terribly weakened by disease and are among the
+lowest in Central America. Only a hundred miles from the unhealthful
+forests we find healthful areas, such as the coasts of Yucatan and the
+plateau of Guatemala. Here the vast majority of the population is
+gathered, the large towns are located, and the only progressive people
+are found. Nevertheless, in the past the region of the forests was the
+home of by far the most progressive people who are ever known to have
+lived in America previous to the days of Columbus. They alone brought to
+high perfection the art of sculpture; they were the only American people
+who invented the art of writing. It seems scarcely credible that such a
+people would have lived in the worst possible habitat when far more
+favored regions were close at hand. Therefore it seems as if the climate
+of eastern Guatemala and Yucatan must have been relatively dry at some
+past time. The Maya chronology and traditions indicate that this was
+probably at the same time when moister conditions apparently prevailed
+in the subarid or desert portions of the United States and Asia. Fig. 3
+shows that today at times of many sunspots there is a similar opposition
+between a tendency toward storminess and rain in subtropical regions and
+toward aridity in low latitudes near the heat equator.
+
+Thus our final conclusion is that during historic times there have been
+pulsatory changes of climate. These changes have been of the same type
+in regions having similar kinds of climate, but of different and
+sometimes opposite types in places having diverse climates. As to the
+cause of the pulsations, they cannot have been due to the precession of
+the equinoxes nor apparently to any allied astronomical cause, for the
+time intervals are too short and too irregular. They cannot have been
+due to changes in the percentage of carbon dioxide in the atmosphere,
+for not even the strongest believers in the climatic efficacy of that
+gas hold that its amount could fluctuate in any such violent way as
+would be necessary to explain the pulsations shown in the California
+curve of tree growth. Volcanic activity seems more probable as at least
+a partial cause, and it would be worth while to investigate the matter
+more fully. Nevertheless, it can apparently be only a minor cause. In
+the first place, the main effect of a cloud of dust is to alter the
+temperature, but Gregory's summary of the palm and the vine shows that
+variations in temperature are apparently of very slight importance
+during historic times. Again, ruins on the bottoms of enclosed salt
+lakes, old beaches now under the water, and signs of irrigation ditches
+where none are now needed indicate a climate drier than the present.
+Volcanic dust, however, cannot account for such a condition, for at
+present the air seems to be practically free from such dust for long
+periods. Thus we now experience the greatest extreme which the volcanic
+hypothesis permits in one direction, but there have been greater
+extremes in the same direction. The thermal solar hypothesis is likewise
+unable to explain the observed phenomena, for neither it nor the
+volcanic hypothesis offers any explanation of why the climate varies in
+one way in Mediterranean climates and in an opposite way in regions near
+the heat equator.
+
+This leaves the cyclonic hypothesis. It seems to fit the facts, for
+variations in cyclonic storms cause some regions to be moister and
+others drier than usual. At the same time the variations in temperature
+are slight, and are apparently different in different regions, some
+places growing warm when others grow cool. In the next chapter we shall
+study this matter more fully, for it can best be appreciated by
+examining the course of events in a specific century.
+
+FOOTNOTES:
+
+[Footnote 16: Much of this chapter is taken from The Solar Hypothesis of
+Climatic Changes; Bull. Geol. Soc. Am., Vol. 25, 1914.]
+
+[Footnote 17: Ellsworth Huntington: Explorations in Turkestan, 1905; The
+Pulse of Asia, 1907; Palestine and Its Transformation, 1911; The
+Climatic Factor, 1915; World Power and Evolution, 1919.]
+
+[Footnote 18: J. Hann: Klimatologie, Vol. 1, 1908, p. 352.]
+
+[Footnote 19: H. C. Butler: Desert Syria, the Land of a Lost
+Civilization; Geographical Review, Feb., 1920, pp. 77-108.]
+
+[Footnote 20: This is due to the fact that where these forests occur, in
+Gilead for example, the mountains to the west break down, so that the
+west winds with water from the Mediterranean are able to reach the inner
+range without having lost all their water. It is one of the misfortunes
+of Syria that its mountains generally rise so close to the sea that they
+shut off rainfall from the interior and cause the rain to fall on slopes
+too steep for easy cultivation.]
+
+[Footnote 21: H. Leiter: Die Frage der Klimaanderung waherend
+geschichtlicher Zeit in Nordafrika. Abhandl. K. K. Geographischen
+Gesellschaft, Wien, 1909, p. 143.]
+
+[Footnote 22: A most careful and convincing study of this problem is
+embodied in an article by J. W. Smith: The Effects of Weather upon the
+Yield of Corn; Monthly Weather Review, Vol. 42, 1914, pp. 78-92. On the
+basis of the yield of corn in Ohio for 60 years and in other states for
+shorter periods, he shows that the rainfall of July has almost as much
+influence on the crop as has the rainfall of all other months combined.
+See his Agricultural Meteorology, New York, 1920.]
+
+[Footnote 23: See chapter by A. E. Douglass in The Climatic Factor; and
+his book on Climatic Cycles and Tree-Growth; Carnegie Inst., 1919. Also
+article by M. N. Stewart: The Relation of Precipitation to Tree Growth,
+in the Monthly Weather Review, Vol. 41, 1913.]
+
+[Footnote 24: The dotted line is taken from Palestine and Its
+Transformation, pp. 327 and 403.]
+
+[Footnote 25: M. A. Stein: Ruins of Desert Cathay, London, 1912.]
+
+[Footnote 26: In the preparation and interpretation of this table the
+help of Mr. G. B. Cressey is gratefully acknowledged.]
+
+[Footnote 27: For the tree data used in these comparisons, see The
+Climatic Factor P. 328, and A. E. Douglass: Climatic Cycles and Tree
+Growth, p. 123.]
+
+[Footnote A: One year interpolated.]
+
+[Footnote 28: J. W. Gregory: Is the Earth Drying Up? Geog. Jour., Vol.
+43, 1914, pp. 148-172 and 293-318.]
+
+[Footnote 29: Geog. Jour., Vol. 43, pp. 159-161.]
+
+[Footnote 30: See A. J. Henry: Secular Variation of Precipitation in the
+United States; Bull. Am. Geog. Soc., Vol. 46, 1914, pp. 192-201.]
+
+
+
+
+CHAPTER VI
+
+THE CLIMATIC STRESS OF THE FOURTEENTH CENTURY
+
+
+In order to give concreteness to our picture of the climatic pulsations
+of historic times let us take a specific period and see how its changes
+of climate were distributed over the globe and how they are related to
+the little changes which now take place in the sunspot cycle. We will
+take the fourteenth century of the Christian era, especially the first
+half. This period is chosen because it is the last and hence the best
+known of the times when the climate of the earth seems to have taken a
+considerable swing toward the conditions which now prevail when the sun
+is most active, and which, if intensified, would apparently lead to
+glaciation. It has already been discussed in _World Power and
+Evolution_, but its importance and the fact that new evidence is
+constantly coming to light warrant a fuller discussion.
+
+To begin with Europe; according to the careful account of Pettersson[31]
+the fourteenth century shows
+
+    a record of extreme climatic variations. In the cold winters the
+    rivers Rhine, Danube, Thames, and Po were frozen for weeks and
+    months. On these cold winters there followed violent floods, so that
+    the rivers mentioned inundated their valleys. Such floods are
+    recorded in 55 summers in the 14th century. There is, of course,
+    nothing astonishing in the fact that the inundations of the great
+    rivers of Europe were more devastating 600 to 700 years ago than in
+    our days, when the flow of the rivers has been regulated by canals,
+    locks, etc.; but still the inundations in the 13th and 14th
+    centuries must have surpassed everything of that kind which has
+    occurred since then. In 1342 the waters of the Rhine rose so high
+    that they inundated the city of Mayence and the Cathedral "usque ad
+    cingulum hominis." The walls of Cologne were flooded so that they
+    could be passed by boats in July. This occurred also in 1374 in the
+    midst of the month of February, which is of course an unusual season
+    for disasters of the kind. Again in other years the drought was so
+    intense that the same rivers, the Danube, Rhine, and others, nearly
+    dried up, and the Rhine could be forded at Cologne. This happened at
+    least twice in the same century. There is one exceptional summer of
+    such evil record that centuries afterwards it was spoken of as "the
+    old hot summer of 1357."
+
+Pettersson goes on to speak of two oceanic phenomena on which the old
+chronicles lay greater stress than on all others:
+
+    The first [is] the great storm-floods on the coast of the North Sea
+    and the Baltic, which occurred so frequently that not less than
+    nineteen floods of a destructiveness unparalleled in later times are
+    recorded from the 14th century. The coastline of the North Sea was
+    completely altered by these floods. Thus on January 16, 1300, half
+    of the island Heligoland and many other islands were engulfed by the
+    sea. The same fate overtook the island of Borkum, torn into several
+    islands by the storm-flood of January 16, which remoulded the
+    Frisian Islands into their present shape, when also Wendingstadt, on
+    the island of Sylt, and Thiryu parishes were engulfed. This flood is
+    known under the name of "the great man-drowning." The coasts of the
+    Baltic also were exposed to storm-floods of unparalleled violence.
+    On November 1, 1304, the island of Ruden was torn asunder from Rugen
+    by the force of the waves. Time does not allow me to dwell upon
+    individual disasters of this kind, but it will be well to note that
+    of the nineteen great floods on record eighteen occurred in the cold
+    season between the autumnal and vernal equinoxes.
+
+    The second remarkable phenomenon mentioned by the chronicles is the
+    freezing of the entire Baltic, which occurred many times during the
+    cold winters of these centuries. On such occasions it was possible
+    to travel with carriages over the ice from Sweden to Bornholm and
+    from Denmark to the German coast (Lubeck), and in some cases even
+    from Gotland to the coast of Estland.
+
+Norlind[32] says that "the only authentic accounts" of the complete
+freezing of the Baltic in the neighborhood of the Kattegat are in the
+years 1296, 1306, 1323, and 1408. Of these 1296 is "much the most
+uncertain," while 1323 was the coldest year ever recorded, as appears
+from the fact that horses and sleighs crossed regularly from Sweden to
+Germany on the ice.
+
+Not only central Europe and the shores of the North Sea were marked by
+climatic stress during the fourteenth century, but Scandinavia also
+suffered. As Pettersson puts it:
+
+    On examining the historic (data) from the last centuries of the
+    Middle Ages, Dr. Bull of Christiania has come to the conclusion that
+    the decay of the Norwegian kingdom was not so much a consequence of
+    the political conditions at that time, as of the frequent failures
+    of the harvest so that corn [wheat] for bread had to be imported
+    from Luebeck, Rostock, Wismar and so forth. The Hansa Union undertook
+    the importation and obtained political power by its economic
+    influence. The Norwegian land-owners were forced to lower their
+    rents. The population decreased and became impoverished. The revenue
+    sank 60 to 70 per cent. Even the income from Church property
+    decreased. In 1367 corn was imported from Luebeck to a value of
+    one-half million kroner. The trade balance inclined to the
+    disadvantage of Norway whose sole article of export at that time
+    was dried fish. (The production of fish increased enormously in
+    the Baltic regions off south Sweden because of the same changes
+    which were influencing the lands, but this did not benefit
+    Norway.) Dr. Bull draws a comparison with the conditions described
+    in the Sagas when Nordland [at the Arctic Circle] produced enough
+    corn to feed the inhabitants of the country. At the time of
+    Asbjoern Selsbane the chieftains in Trondhenaes [still farther north
+    in latitude 69 deg.] grew so much corn that they did not need to go
+    southward to buy corn unless three successive years of dearth had
+    occurred. The province of Trondheim exported wheat to Iceland and
+    so forth. Probably the turbulent political state of Scandinavia at
+    the end of the Middle Ages was in a great measure due to
+    unfavorable climatic conditions, which lowered the standard of
+    life, and not entirely to misgovernment and political strife as
+    has hitherto been taken for granted.
+
+During this same unfortunate first half of the fourteenth century
+England also suffered from conditions which, if sufficiently
+intensified, might be those of a glacial period. According to Thorwald
+Rogers[33] the severest famine ever experienced in England was that of
+1315-1316, and the next worst was in 1321. In fact, from 1308 to 1322
+great scarcity of food prevailed most of the time. Other famines of less
+severity occurred in 1351 and 1369. "The same cause was at work in all
+these cases," says Rogers, "incessant rain, and cold, stormy summers. It
+is said that the inclemency of the seasons affected the cattle, and that
+numbers perished from disease and want." After the bad harvest of 1315
+the price of wheat, which was already high, rose rapidly, and in May,
+1316, was about five times the average. For a year or more thereafter it
+remained at three or four times the ordinary level. The severity of the
+famine may be judged from the fact that previous to the Great War the
+most notable scarcity of wheat in modern England and the highest
+relative price was in December, 1800. At that time wheat cost nearly
+three times the usual amount, instead of five as in 1316. During the
+famine of the early fourteenth century "it is said that people were
+reduced to subsist upon roots, upon horses and dogs, and stories are
+told of even more terrible acts by reason of the extreme famine." The
+number of deaths was so great that the price of labor suffered a
+permanent rise of at least 10 per cent. There simply were not people
+enough left among the peasants to do the work demanded by the more
+prosperous class who had not suffered so much.
+
+After the famine came drought. The year 1325 appears to have been
+peculiarly dry, and 1331, 1344, 1362, 1374, and 1377 were also dry. In
+general these conditions do little harm in England. They are of interest
+chiefly as showing how excessive rain and drought are apt to succeed one
+another.
+
+These facts regarding northern and central Europe during the fourteenth
+century are particularly significant when compared with the conclusions
+which we have drawn in _Earth and Sun_ from the growth of trees in
+Germany and from the distribution of storms. A careful study of all the
+facts shows that we are dealing with two distinct types of phenomena. In
+the first place, the climate of central Europe seems to have been
+peculiarly continental during the fourteenth century. The winters were
+so cold that the rivers froze, and the summers were so wet that there
+were floods every other year or oftener. This seems to be merely an
+intensification of the conditions which prevail at the present time
+during periods of many sunspots, as indicated by the growth of trees at
+Eberswalde in Germany and by the number of storms in winter as compared
+with summer. The prevalence of droughts, especially in the spring, is
+also not inconsistent with the existence of floods at other seasons, for
+one of the chief characteristics of a continental climate is that the
+variations from one season to another are more marked than in oceanic
+climates. Even the summer droughts are typically continental, for when
+continental conditions prevail, the difference between the same season
+in different years is extreme, as is well illustrated in Kansas. It must
+always be remembered that what causes famine is not so much absolute
+dryness as a temporary diminution of the rainfall.
+
+The second type of phenomena is peculiarly oceanic in character. It
+consists of two parts, both of which are precisely what would be
+expected if a highly continental climate prevailed over the land. In the
+first place, at certain times the cold area of high pressure, which is
+the predominating characteristic of a continent during the winter,
+apparently spread out over the neighboring oceans. Under such conditions
+an inland sea, such as the Baltic, would be frozen, so that horses could
+cross the ice even in the Far West. In the second place, because of the
+unusually high pressure over the continent, the barometric gradients
+apparently became intensified. Hence at the margin of the continental
+high-pressure area the winds were unusually strong and the storms of
+corresponding severity. Some of these storms may have passed entirely
+along oceanic tracks, while others invaded the borders of the land, and
+gave rise to the floods and to the wearing away of the coast described
+by Pettersson.
+
+Turning now to the east of Europe, Brueckner's[34] study of the Caspian
+Sea shows that that region as well as western Europe was subject to
+great climatic vicissitudes in the first half of the fourteenth century.
+In 1306-1307 the Caspian Sea, after rising rapidly for several years,
+stood thirty-seven feet above the present level and it probably rose
+still higher during the succeeding decades. At least it remained at a
+high level, for Hamdulla, the Persian, tells us that in 1325 a place
+called Aboskun was under water.[35]
+
+Still further east the inland lake of Lop Nor also rose at about this
+time. According to a Chinese account the Dragon Town on the shore of Lop
+Nor was destroyed by a flood. From Himley's translation it appears that
+the level of the lake rose so as to overwhelm the city completely. This
+would necessitate the expansion of the lake to a point eighty miles east
+of Lulan, and fully fifty from the present eastern end of the Kara
+Koshun marsh. The water would have to rise nearly, or quite, to a strand
+which is now clearly visible at a height of twelve feet above the modern
+lake or marsh.
+
+In India the fourteenth century was characterized by what appears to
+have been the most disastrous drought in all history. Apparently the
+decrease in rainfall here was as striking as the increase in other parts
+of the world. No statistics are available but we are told that in the
+great famine which began in 1344 even the Mogul emperor was unable to
+obtain the necessaries of life for his household. No rain worth
+mentioning fell for years. In some places the famine lasted three or
+four years, and in some twelve, and entire cities were left without an
+inhabitant. In a later famine, 1769-1770, which occurred in Bengal
+shortly after the foundation of British rule in India, but while the
+native officials were still in power, a third of the population, or ten
+out of thirty millions, perished. The famine in the first half of the
+fourteenth century seems to have been far worse. These Indian famines
+were apparently due to weak summer monsoons caused presumably by the
+failure of central Asia to warm up as much as usual. The heavier
+snowfall, and the greater cloudiness of the summer there, which probably
+accompanied increased storminess, may have been the reason.
+
+The New World as well as the Old appears to have been in a state of
+climatic stress during the first half of the fourteenth century.
+According to Pettersson, Greenland furnishes an example of this. At
+first the inhabitants of that northland were fairly prosperous and were
+able to approach from Iceland without much hindrance from the ice. Today
+the North Atlantic Ocean northeast of Iceland is full of drift ice much
+of the time. The border of the ice varies from season to season, but in
+general it extends westward from Iceland not far from the Arctic circle
+and then follows the coast of Greenland southward to Cape Farewell at
+the southern tip and around to the western side for fifty miles or more.
+Except under exceptional circumstances a ship cannot approach the coast
+until well northward on the comparatively ice-free west coast. In the
+old Sagas, however, nothing is said of ice in this region. The route
+from Iceland to Greenland is carefully described. In the earliest times
+it went from Iceland a trifle north of west so as to approach the coast
+of Greenland after as short an ocean passage as possible. Then it went
+down the coast in a region where approach is now practically impossible
+because of the ice. At that time this coast was icy close to the shore,
+but there is no sign that navigation was rendered difficult as is now
+the case. Today no navigator would think of keeping close inland. The
+old route also went _north_ of the island on which Cape Farewell is
+located, although the narrow channel between the island and the mainland
+is now so blocked with ice that no modern vessel has ever penetrated it.
+By the thirteenth century, however, there appears to have been a change.
+In the Kungaspegel or _Kings' Mirror_, written at that time, navigators
+are warned not to make the east coast too soon on account of ice, but no
+new route is recommended in the neighborhood of Cape Farewell or
+elsewhere. Finally, however, at the end of the fourteenth century,
+nearly 150 years after the Kungaspegel, the old sailing route was
+abandoned, and ships from Iceland sailed directly southwest to avoid the
+ice. As Pettersson says:
+
+    ... At the end of the thirteenth and the beginning of the fourteenth
+    century the European civilization in Greenland was wiped out by an
+    invasion of the aboriginal population. The colonists in the
+    Vesterbygd were driven from their homes and probably migrated to
+    America leaving behind their cattle in the fields. So they were
+    found by Ivar Bardsson, steward to the Bishop of Gardar, in his
+    official journey thither in 1342.
+
+    The Eskimo invasion must not be regarded as a common raid. It was
+    the transmigration of a people, and like other big movements of this
+    kind [was] impelled by altered conditions of nature, in this case
+    the alterations of climate caused by [or which caused?] the advance
+    of the ice. For their hunting and fishing the Eskimos require an at
+    least partially open arctic sea. The seal, their principal prey,
+    cannot live where the surface of the sea is entirely frozen over.
+    The cause of the favorable conditions in the Viking-age was,
+    according to my hypothesis, that the ice then melted at a higher
+    latitude in the arctic seas.
+
+    The Eskimos then lived further north in Greenland and North America.
+    When the climate deteriorated and the sea which gave them their
+    living was closed by ice the Eskimos had to find a more suitable
+    neighborhood. This they found in the land colonized by the Norsemen
+    whom they attacked and finally annihilated.
+
+Finally, far to the south in Yucatan the ancient Maya civilization made
+its last flickering effort at about this time. Not much is known of this
+but in earlier periods the history of the Mayas seems to have agreed
+quite closely with the fluctuations in climate.[36] Among the Mayas, as
+we have seen, relatively dry periods were the times of greatest
+progress.
+
+Let us turn now to Fig. 3 once more and compare the climatic conditions
+of the fourteenth century with those of periods of increasing rainfall.
+Southern England, Ireland, and Scandinavia, where the crops were ruined
+by extensive rain and storms in summer, are places where storminess and
+rainfall now increase when sunspots are numerous. Central Europe and the
+coasts of the North Sea, where flood and drought alternated, are regions
+which now have relatively less rain when sunspots increase than when
+they diminish. However, as appears from the trees measured by Douglass,
+the winters become more continental and hence cooler, thus corresponding
+to the cold winters of the fourteenth century when people walked on the
+ice from Scandinavia to Denmark. When such high pressure prevails in the
+winter, the total rainfall is diminished, but nevertheless the storms
+are more severe than usual, especially in the spring. In southeastern
+Europe, the part of the area whence the Caspian derives its water,
+appears to have less rainfall during times of increasing sunspots than
+when sunspots are few, but in an equally large area to the south, where
+the mountains are higher and the run-off of the rain is more rapid, the
+reverse is the case. This seems to mean that a slight diminution in the
+water poured in by the Volga would be more than compensated by the water
+derived from Persia and from the Oxus and Jaxartes rivers, which in the
+fourteenth century appear to have filled the Sea of Aral and overflowed
+in a large stream to the Caspian. Still farther east in central Asia, so
+far as the records go, most of the country receives more rain when
+sunspots are many than when they are few, which would agree with what
+happened when the Dragon Town was inundated. In India, on the contrary,
+there is a large area where the rainfall diminishes at times of many
+sunspots, thus agreeing with the terrible famine from which the Moguls
+suffered so severely. In the western hemisphere, Greenland, Arizona, and
+California are all parts of the area where the rain increases with many
+sunspots, while Yucatan seems to lie in an area of the opposite type.
+Thus all the evidence seems to show that at times of climatic stress,
+such as the fourteenth century, the conditions are essentially the same
+as those which now prevail at times of increasing sunspots.
+
+As to the number of sunspots, there is little evidence previous to about
+1750. Yet that little is both interesting and important. Although
+sunspots have been observed with care in Europe only a little more than
+three centuries, the Chinese have records which go back nearly to the
+beginning of the Christian era. Of course the records are far from
+perfect, for the work was done by individuals and not by any great
+organization which continued the same methods from generation to
+generation. The mere fact that a good observer happened to use his
+smoked glass to advantage may cause a particular period to appear to
+have an unusual number of spots. On the other hand, the fact that such
+an observer finds spots at some times and not at others tends to give a
+valuable check on his results, as does the comparison of one observer's
+work with that of another. Hence, in spite of many and obvious defects,
+most students of the problem agree that the Chinese record possesses
+much value, and that for a thousand years or more it gives a fairly true
+idea of the general aspect of the sun. In the Chinese records the years
+with many spots fall in groups, as would be expected, and are sometimes
+separated by long intervals. Certain centuries appear to have been
+marked by unusual spottedness. The most conspicuous of these is the
+fourteenth, when the years 1370 to 1385 were particularly noteworthy,
+for spots large enough to be visible to the naked eye covered the sun
+much of the time. Hence Wolf,[37] who has made an exhaustive study of
+the matter, concludes that there was an absolute maximum of spots about
+1372. While this date is avowedly open to question, the great abundance
+of sunspots at that time makes it probable that it cannot be far wrong.
+If this is so, it seems that the great climatic disturbances of which we
+have seen evidence in the fourteenth century occurred at a time when
+sunspots were increasing, or at least when solar activity was under some
+profoundly disturbing influence. Thus the evidence seems to show not
+merely that the climate of historic times has been subject to important
+pulsations, but that those pulsations were magnifications of the little
+climatic changes which now take place in sunspot cycles. The past and
+the present are apparently a unit except as to the intensity of the
+changes.
+
+FOOTNOTES:
+
+[Footnote 31: O. Pettersson: The connection between hydrographical and
+meteorological phenomena; Quarterly Journal of the Royal Meteorological
+Society, Vol. 38, pp. 174-175.]
+
+[Footnote 32: A. Norlind: Einige Bemerkungen ueber das Klima der
+historischen Zeit nebst einem Verzeichnis mittelaltlicher Witterungs
+erscheinungen; Lunds Univ. Arsskrift, N. F., Vol. 10, 1914, 53 pp.]
+
+[Footnote 33: Thorwald Rogers: A History of Agriculture and Prices in
+England.]
+
+[Footnote 34: E. Brueckner: Klimaschwankungen seit 1700, Vienna, 1891.]
+
+[Footnote 35: For a full discussion of the changes in the Caspian Sea,
+see The Pulse of Asia, pp. 329-358.]
+
+[Footnote 36: S. Q. Morley: The Inscriptions at Copan; Carnegie Inst. of
+Wash., No. 219, 1920.
+
+Ellsworth Huntington: The Red Man's Continent, 1919.]
+
+[Footnote 37: See summary of Wolf's work with additional information by
+H. Fritz; Zuerich Vierteljahrschrift, Vol. 38, 1893, pp. 77-107.]
+
+
+
+
+CHAPTER VII
+
+GLACIATION ACCORDING TO THE SOLAR-CYCLONIC HYPOTHESIS[38]
+
+
+The remarkable phenomena of glacial periods afford perhaps the best
+available test to which any climatic hypothesis can be subjected. In
+this chapter and the two that follow, we shall apply this test. Since
+much more is known about the recent Great Ice Age, or Pleistocene
+glaciation, than about the more ancient glaciations, the problems of the
+Pleistocene will receive especial attention. In the present chapter the
+oncoming of glaciation and the subsequent disappearance of the ice will
+be outlined in the light of what would be expected according to the
+solar-cyclonic hypothesis. Then in the next chapter several problems of
+especial climatic significance will be considered, such as the
+localization of ice sheets, the succession of severe glacial and mild
+inter-glacial epochs, the sudden commencement of glaciation and the
+peculiar variations in the height of the snow line. Other topics to be
+considered are the occurrence of pluvial or rainy climates in
+non-glaciated regions, and glaciation near sea level in subtropical
+latitudes during the Permian and Proterozoic. Then in Chapter IX we
+shall consider the development and distribution of the remarkable
+deposits of wind-blown material known as loess.
+
+Facts not considered at the time of framing an hypothesis are especially
+significant in testing it. In this particular case, the cyclonic
+hypothesis was framed to explain the historic changes of climate
+revealed by a study of ruins, tree rings, and the terraces of streams
+and lakes, without special thought of glaciation or other geologic
+changes. Indeed, the hypothesis had reached nearly its present form
+before much attention was given to geological phases of the problem.
+Nevertheless, it appears to meet even this severe test.
+
+According to the solar-cyclonic hypothesis, the Pleistocene glacial
+period was inaugurated at a time when certain terrestrial conditions
+tended to make the earth especially favorable for glaciation. How these
+conditions arose will be considered later. Here it is enough to state
+what they were. Chief among them was the fact that the continents stood
+unusually high and were unusually large. This, however, was not the
+primary cause of glaciation, for many of the areas which were soon to be
+glaciated were little above sea level. For example, it seems clear that
+New England stood less than a thousand feet higher than now. Indeed,
+Salisbury[39] estimates that eastern North America in general stood not
+more than a few hundred feet higher than now, and W. B. Wright[40]
+reaches the same conclusion in respect to the British Isles.
+Nevertheless, widespread lands, even if they are not all high, lead to
+climatic conditions which favor glaciation. For example, enlarged
+continents cause low temperature in high latitudes because they
+interfere with the ocean currents that carry heat polewards. Such
+continents also cause relatively cold winters, for lands cool much
+sooner than does the ocean. Another result is a diminution of water
+vapor, not only because cold air cannot hold much vapor, but also
+because the oceanic area from which evaporation takes place is reduced
+by the emergence of the continents. Again, when the continents are
+extensive the amount of carbonic acid gas in the atmosphere probably
+decreases, for the augmented erosion due to uplift exposes much igneous
+rock to the air, and weathering consumes the atmospheric carbon dioxide.
+When the supply of water vapor and of atmospheric carbon dioxide is
+small, an extreme type of climate usually prevails. The combined result
+of all these conditions is that continental emergence causes the climate
+to be somewhat cool and to be marked by relatively great contrasts from
+season to season and from latitude to latitude.
+
+When the terrestrial conditions thus permitted glaciation, unusual solar
+activity is supposed to have greatly increased the number and severity
+of storms and to have altered their location, just as now happens at
+times of many sunspots. If such a change in storminess had occurred when
+terrestrial conditions were unfavorable for glaciation, as, for example,
+when the lands were low and there were widespread epicontinental seas in
+middle and high latitudes, glaciation might not have resulted. In the
+Pleistocene, however, terrestrial conditions permitted glaciation, and
+therefore the supposed increase in storminess caused great ice sheets.
+
+The conditions which prevail at times of increased storminess have been
+discussed in detail in _Earth and Sun_. Those which apparently brought
+on glaciation seem to have acted as follows: In the first place the
+storminess lowered the temperature of the earth's surface in several
+ways. The most important of these was the rapid upward convection in the
+centers of cyclonic storms whereby abundant heat was carried to high
+levels where most of it was radiated away into space. The marked
+increase in the number of tropical cyclones which accompanies increased
+solar activity was probably important in this respect. Such cyclones
+carry vast quantities of heat and moisture out of the tropics. The
+moisture, to be sure, liberates heat upon condensing, but as
+condensation occurs above the earth's surface, much of the heat escapes
+into space. Another reason for low temperature was that under the
+influence of the supposedly numerous storms of Pleistocene times
+evaporation over the oceans must have increased. This is largely because
+the velocity of the winds is relatively great when storms are strong and
+such winds are powerful agents of evaporation. But evaporation requires
+heat, and hence the strong winds lower the temperature.[B]
+
+The second great condition which enabled increased storminess to bring
+on glaciation was the location of the storm tracks. Kullmer's maps, as
+illustrated in Fig. 2, suggest that a great increase in solar activity,
+such as is postulated in the Pleistocene, might shift the main storm
+track poleward even more than it is shifted by the milder solar changes
+during the twelve-year sunspot cycle. If this is so, the main track
+would tend to cross North America through the middle of Canada instead
+of near the southern border. Thus there would be an increase in
+precipitation in about the latitude of the Keewatin and Labradorean
+centers of glaciation. From what is known of storm tracks in Europe, the
+main increase in the intensity of storms would probably center in
+Scandinavia. Fig. 3 in Chapter V bears this out. That figure, it will be
+recalled, shows what happens to precipitation when solar activity is
+increasing. A high rate of precipitation is especially marked in the
+boreal storm track, that is, in the northern United States, southern
+Canada, and northwestern Europe.
+
+Another important condition in bringing on glaciation would be the fact
+that when storms are numerous the total precipitation appears to
+increase in spite of the slightly lower temperature. This is largely
+because of the greater evaporation. The excessive evaporation arises
+partly from the rapidity of the winds, as already stated, and partly
+from the fact that in areas where the air is clear the sun would
+presumably be able to act more effectively than now. It would do so
+because at times of abundant sunspots the sun in our own day has a
+higher solar constant than at times of milder activity. Our whole
+hypothesis is based on the supposition that what now happens at times of
+many sunspots was intensified in glacial periods.
+
+A fourth condition which would cause glaciation to result from great
+solar activity would be the fact that the portion of the yearly
+precipitation falling as snow would increase, while the proportion of
+rain would diminish in the main storm track. This would arise partly
+because the storms would be located farther north than now, and partly
+because of the diminution in temperature due to the increased
+convection. The snow in itself would still further lower the
+temperature, for snow is an excellent reflector of sunlight. The
+increased cloudiness which would accompany the more abundant storms
+would also cause an unusually great reflection of the sunlight and still
+further lower the temperature. Thus at times of many sunspots a strong
+tendency toward the accumulation of snow would arise from the rapid
+convection and consequent low temperature, from the northern location of
+storms, from the increased evaporation and precipitation, from the
+larger percentage of snowy rather than rainy precipitation, and from the
+great loss of heat due to reflection from clouds and snow.
+
+If events at the beginning of the last glacial period took place in
+accordance with the cyclonic hypothesis, as outlined above, one of the
+inevitable results would be the production of snowfields. The places
+where snow would accumulate in special quantities would be central
+Canada, the Labrador plateau, and Scandinavia, as well as certain
+mountain regions. As soon as a snowfield became somewhat extensive, it
+would begin to produce striking climatic alterations in addition to
+those to which it owed its origin.[41] For example, within a snowfield
+the summers remain relatively cold. Hence such a field is likely to be
+an area of high pressure at all seasons. The fact that the snowfield is
+always a place of relatively high pressure results in outblowing surface
+winds except when these are temporarily overcome by the passage of
+strong cyclonic storms. The storms, however, tend to be concentrated
+near the margins of the ice throughout the year instead of following
+different paths in each of the four seasons. This is partly because
+cyclonic lows always avoid places of high pressure and are thus pushed
+out of the areas where permanent snow has accumulated. On the other
+hand, at times of many sunspots, as Kullmer has shown, the main storm
+track tends to be drawn poleward, perhaps by electrical conditions.
+Hence when a snowfield is present in the north, the lows, instead of
+migrating much farther north in summer than in winter, as they now do,
+would merely crowd on to the snowfield a little farther in summer than
+in winter. Thus the heavy precipitation which is usual in humid climates
+near the centers of lows would take place near the advancing margin of
+the snowfield and cause the field to expand still farther southward.
+
+The tendency toward the accumulation of snow on the margins of the
+snowfields would be intensified not only by the actual storms
+themselves, but by other conditions. For example, the coldness of the
+snow would tend to cause prompt condensation of the moisture brought by
+the winds that blow toward the storm centers from low latitudes. Again,
+in spite of the general dryness of the air over a snowfield, the lower
+air contains some moisture due to evaporation from the snow by day
+during the clear sunny weather of anti-cyclones or highs. Where this is
+sufficient, the cold surface of the snowfields tends to produce a frozen
+fog whenever the snowfield is cooled by radiation, as happens at night
+and during the passage of highs. Such a frozen fog is an effective
+reflector of solar radiation. Moreover, because ice has only half the
+specific heat of water, and is much more transparent to heat, such a
+"radiation fog" composed of ice crystals is a much less effective
+retainer of heat than clouds or fog made of unfrozen water particles.
+Shallow fogs of this type are described by several polar expeditions.
+They clearly retard the melting of the snow and thus help the icefield
+to grow.
+
+For all these reasons, so long as storminess remained great, the
+Pleistocene snowfields, according to the solar hypothesis, must have
+deepened and expanded. In due time some of the snow was converted into
+glacial ice. When that occurred, the growth of the snowfield as well as
+of the ice cap must have been accelerated by glacial movement. Under
+such circumstances, as the ice crowded southward toward the source of
+the moisture by which it grew, the area of high pressure produced by its
+low temperature would expand. This would force the storm track southward
+in spite of the contrary tendency due to the sun. When the ice sheet had
+become very extensive, the track would be crowded relatively near to the
+northern margin of the trade-wind belt. Indeed, the Pleistocene ice
+sheets, at the time of their maximum extension, reached almost as far
+south as the latitude now marking the northern limit of the trade-wind
+belt in summer. As the storm track with its frequent low pressure and
+the subtropical belt with its high pressure were forced nearer and
+nearer together, the barometric gradient between the two presumably
+became greater, winds became stronger, and the storms more intense.
+
+This zonal crowding would be of special importance in summer, at which
+time it would also be most pronounced. In the first place, the storms
+would be crowded far upon the ice cap which would then be protected from
+the sun by a cover of fog and cloud more fully than at any other season.
+Furthermore, the close approach of the trade-wind belt to the storm belt
+would result in a great increase in the amount of moisture drawn from
+the belt of evaporation which the trade winds dominate. In the
+trade-wind belt, clear skies and high temperature make evaporation
+especially rapid. Indeed, in spite of the vast deserts it is probable
+that more than three-fourths of the total evaporation now taking place
+on the earth occurs in the belt of trades, an area which includes about
+one-half of the earth's surface.
+
+The agency which could produce this increased drawing northward of
+moisture from the trade-wind belt would be the winds blowing into the
+lows. According to the cyclonic hypothesis, many of these lows would be
+so strong that they would temporarily break down the subtropical belt of
+high pressure which now usually prevails between the trades and the zone
+of westerly winds. This belt is even now often broken by tropical
+cyclones. If the storms of more northerly regions temporarily destroyed
+the subtropical high-pressure belt, even though they still remained on
+its northern side, they would divert part of the trade winds. Hence the
+air which now is carried obliquely equatorward by those winds would be
+carried spirally northward into the cyclonic lows. Precipitation in the
+storm track on the margin of the relatively cold ice sheet would thus be
+much increased, for most winds from low latitudes carry abundant
+moisture. Such a diversion of moisture from low latitudes probably
+explains the deficiency of precipitation along the heat equator at times
+of solar activity, as shown in Fig. 3. Taken as a whole, the summer
+conditions, according to the cyclonic hypothesis, would be such that
+increased evaporation in low latitudes would cooeperate with increased
+storminess, cloudiness, and fog in higher latitudes to preserve and
+increase the accumulation of ice upon the borders of the ice sheet. The
+greater the storminess, the more this would be true and the more the ice
+sheet would be able to hold its own against melting in summer. Such a
+combination of precipitation and of protection from the sun is
+especially important if an ice sheet is to grow.
+
+The meteorologist needs no geologic evidence that the storm track was
+shoved equatorward by the growth of the ice sheet, for he observes a
+similar shifting whenever a winter's snow cap occupies part of the
+normal storm tract. The geologist, however, may welcome geologic
+evidence that such an extreme shift of the storm track actually occurred
+during the Pleistocene. Harmer, in 1901, first pointed out the evidence
+which was repeated with approval by Wright of the Ireland Geological
+Survey in 1914.[42] According to these authorities, numerous boulders of
+a distinctive chalk were deposited by Pleistocene icebergs along the
+coast of Ireland. Their distribution shows that at the time of maximum
+glaciation the strong winds along the south coast of Ireland were from
+the northeast while today they are from the southwest. Such a reversal
+could apparently be produced only by a southward shift of the center of
+the main storm track from its present position in northern Ireland,
+Scotland, and Norway to a position across northern France, central
+Germany, and middle Russia. This would mean that while now the centers
+of the lows commonly move northeastward a short distance north of
+southern Ireland, they formerly moved eastward a short distance south of
+Ireland. It will be recalled that in the northern hemisphere the winds
+spiral into a low counter-clockwise and that they are strongest near the
+center. When the centers pass not far north of a given point, the strong
+winds therefore blow from the west or southwest, while when the centers
+pass just south of that point, the strong winds come from the east or
+northeast.
+
+In addition to the consequences of the crowding of the storm track
+toward the trade-wind belt, several other conditions presumably operated
+to favor the growth of the ice sheet. For example, the lowering of the
+sea level by the removal of water to form the snowfields and glaciers
+interfered with warm currents. It also increased the rate of erosion,
+for it was equivalent to an uplift of all the land. One consequence of
+erosion and weathering was presumably a diminution of the carbon dioxide
+in the atmosphere, for although the ice covered perhaps a tenth of the
+lands and interfered with carbonation to that extent, the removal of
+large quantities of soil by accelerated erosion on the other nine-tenths
+perhaps more than counterbalanced the protective effect of the ice. At
+the same time, the general lowering of the temperature of the ocean as
+well as the lands increased the ocean's capacity for carbon dioxide and
+thus facilitated absorption. At a temperature of 50 deg.F. water absorbs 32
+per cent more carbon dioxide than at 68 deg.. The high waves produced by the
+severe storms must have had a similar effect on a small scale. Thus the
+percentage of carbon dioxide in the atmosphere was presumably
+diminished. Of less significance than these changes in the lands and the
+air, but perhaps not negligible, was the increased salinity of the ocean
+which accompanied the removal of water to form snow, and the increase of
+the dissolved mineral load of the rejuvenated streams. Increased
+salinity slows up the deep-sea circulation, as we shall see in a later
+chapter. This increases the contrasts from zone to zone.
+
+At times of great solar activity the agencies mentioned above would
+apparently cooeperate to cause an advance of ice sheets into lower
+latitudes. The degree of solar activity would have much to do with the
+final extent of the ice sheets. Nevertheless, certain terrestrial
+conditions would tend to set limits beyond which the ice would not
+greatly advance unless the storminess were extraordinarily severe. The
+most obvious of these conditions is the location of oceans and of
+deserts or semi-arid regions. The southwestward advance of the European
+ice sheet and the southeastward advance of the Labradorean sheet in
+America were stopped by the Atlantic. The semi-aridity of the Great
+Plains, produced by their position in the lee of the Rocky Mountains,
+stopped the advance of the Keewatin ice sheet toward the southwest. The
+advance of the European ice sheet southeast seems to have been stopped
+for similar reasons. The cessation of the advance would be brought about
+in such an area not alone by the light precipitation and abundant
+sunshine, but by the dryness of the air, and also by the power of dust
+to absorb the sun's heat. Much dust would presumably be drawn in from
+the dry regions by passing cyclonic storms and would be scattered over
+the ice.
+
+The advance of the ice is also slowed up by a rugged topography, as
+among the Appalachians in northern Pennsylvania. Such a topography
+besides opposing a physical obstruction to the movement of the ice
+provides bare south-facing slopes which the sun warms effectively. Such
+warm slopes are unfavorable to glacial advance. The rugged topography
+was perhaps quite as effective as the altitude of the Appalachians in
+causing the conspicuous northward dent in the glacial margin in
+Pennsylvania. Where glaciers lie in mountain valleys the advance beyond
+a certain point is often interfered with by the deployment of the ice at
+the mouths of gorges. Evaporation and melting are more rapid where a
+glacier is broad and thin than where it is narrow and thick, as in a
+gorge. Again, where the topography or the location of oceans or dry
+areas causes the glacial lobes to be long and narrow, the elongation of
+the lobe is apparently checked in several ways. Toward the end of the
+lobe, melting and evaporation increase rapidly because the planetary
+westerly winds are more likely to overcome the glacial winds and sweep
+across a long, narrow lobe than across a broad one. As they cross the
+lobe, they accelerate evaporation, and probably lessen cloudiness, with
+a consequent augmentation of melting. Moreover, although lows rarely
+cross a broad ice sheet, they do cross a narrow lobe. For example,
+Nansen records that strong lows occasionally cross the narrow southern
+part of the Greenland ice sheet. The longer the lobe, the more likely it
+is that lows will cross it, instead of following its margin. Lows which
+cross a lobe do not yield so much snow to the tip as do those which
+follow the margin. Hence elongation is retarded and finally stopped even
+without a change in the earth's general climate.
+
+Because of these various reasons the advances of the ice during the
+several epochs of a glacial period might be approximately equal, even if
+the durations of the periods of storminess and low temperature were
+different. Indeed, they might be sub-equal, even if the periods differed
+in intensity as well as length. Differences in the periods would
+apparently be manifested less in the extent of the ice than in the depth
+of glacial erosion and in the thickness of the terminal moraines,
+outwash plains, and other glacial or glacio-fluvial formations.
+
+Having completed the consideration of the conditions leading to the
+advance of the ice, let us now consider the condition of North America
+at the time of maximum glaciation.[43] Over an area of nearly four
+million square miles, occupying practically all the northern half of the
+continent and part of the southern half, as appears in Fig. 6, the
+surface was a monotonous and almost level plain of ice covered with
+snow. When viewed from a high altitude, all parts except the margins
+must have presented a uniformly white and sparkling appearance. Along
+the margins, however, except to the north, the whiteness was irregular,
+for the view must have included not only fresh snow, but moving clouds
+and dirty snow or ice. Along the borders where melting was in progress
+there was presumably more or less spottedness due to morainal material
+or glacial debris brought to the surface by ice shearage and wastage.
+Along the dry southwestern border it is also possible that there were
+numerous dark spots due to dust blown onto the ice by the wind.
+
+[Illustration: _Fig. 6. Distribution of Pleistocene ice sheets._
+(_After Schuchert._)]
+
+The great white sheet with its ragged border was roughly circular in
+form, with its center in central Canada. Yet there were many departures
+from a perfectly circular form. Some were due to the oceans, for, except
+in northern Alaska, the ice extended into the ocean all the way from New
+Jersey around by the north to Washington. On the south, topographic
+conditions made the margin depart from a simple arc. From New Jersey to
+Ohio it swung northward. In the Mississippi Valley it reached far south;
+indeed most of the broad wedge between the Ohio and the Missouri rivers
+was occupied by ice. From latitude 37 deg. near the junction of the Missouri
+and the Mississippi, however, the ice margin extended almost due north
+along the Missouri to central North Dakota. It then stretched westward
+to the Rockies. Farther west lowland glaciation was abundant as far
+south as western Washington. In the Rockies, the Cascades, and the
+Sierra Nevadas glaciation was common as far south as Colorado and
+southern California, respectively, and snowfields were doubtless
+extensive enough to make these ranges ribbons of white. Between these
+lofty ranges lay a great unglaciated region, but even in the Great Basin
+itself, in spite of its present aridity, certain ranges carried
+glaciers, while great lakes expanded widely.
+
+In this vast field of snow the glacial ice slowly crept outward,
+possibly at an average speed of half a foot a day, but varying from
+almost nothing in winter at the north, to several feet a day in summer
+at the south.[44] The force which caused the movement was the presence
+of the ice piled up not far from the margins. Almost certainly, however,
+there was no great dome from the center in Canada outward, as some early
+writers assumed. Such a dome would require that the ice be many
+thousands of feet thick near its center. This is impossible because of
+the fact that ice is more voluminous than water (about 9 per cent near
+the freezing point). Hence when subjected to sufficient pressure it
+changes to the liquid form. As friction and internal heat tend to keep
+the bottom of a glacier warm, even in cold regions, the probabilities
+are that only under very special conditions was a continental ice sheet
+much thicker than about 2500 feet. In Antarctica, where the temperature
+is much lower than was probably attained in the United States, the ice
+sheet is nearly level, several expeditions having traveled hundreds of
+miles with practically no change in altitude. In Shackleton's trip
+almost to the South Pole, he encountered a general rise of 3000 feet in
+1200 miles. Mountains, however, projected through the ice even near the
+pole and the geologists conclude that the ice is not very thick even at
+the world's coldest point, the South Pole.
+
+Along the margin of the ice there were two sorts of movement, much more
+rapid than the slow creep of the ice. One was produced by the outward
+drift of snow carried by the outblowing dry winds and the other and more
+important was due to the passage of cyclonic storms. Along the border of
+the ice sheet, except at the north, storm presumably closely followed
+storm. Their movement, we judge, was relatively slow until near the
+southern end of the Mississippi lobe, but when this point was passed
+they moved much more rapidly, for then they could go toward instead of
+away from the far northern path which the sun prescribes when solar
+activity is great. The storms brought much snow to the icefield, perhaps
+sometimes in favored places as much as the hundred feet a year which is
+recorded for some winters in the Sierras at present. Even the
+unglaciated intermontane Great Basin presumably received considerable
+precipitation, perhaps twice as much as its present scanty supply. The
+rainfall was enough to support many lakes, one of which was ten times as
+large as Great Salt Lake; and grass was doubtless abundant upon many
+slopes which are now dry and barren. The relatively heavy precipitation
+in the Great Basin was probably due primarily to the increased number of
+storms, but may also have been much influenced by their slow eastward
+movement. The lows presumably moved slowly in that general region not
+only because they were retarded and turned from their normal path by the
+cold ice to the east, but because during the summer the area between the
+Sierra snowfields on the west and the Rocky Mountain and Mississippi
+Valley snowfields on the east was relatively warm. Hence it was normally
+a place of low pressure and therefore of inblowing winds. Slow-moving
+lows are much more effective than fast-moving ones in drawing moisture
+northwestward from the Gulf of Mexico, for they give the moisture more
+time to move spirally first northeast, under the influence of the normal
+southwesterly winds, then northwest and finally southwest as it
+approaches the storm center. In the case of the present lows, before
+much moisture-laden air can describe such a circuit, first eastward and
+then westward, the storm center has nearly always moved eastward across
+the Rockies and even across the Great Plains. A result of this is the
+regular decrease in precipitation northward, northwestward, and westward
+from the Gulf of Mexico.
+
+Along the part of the glacial margins where for more than 3000 miles the
+North American ice entered the Atlantic and the Pacific oceans, myriads
+of great blocks broke off and floated away as stately icebergs, to
+scatter boulders far over the ocean floor and to melt in warmer climes.
+Where the margin lay upon the lands numerous streams issued from beneath
+the ice, milk-white with rock flour, and built up great outwash plains
+and valley trains of gravel and sand. Here and there, just beyond the
+ice, marginal lakes of strange shapes occupied valleys which had been
+dammed by the advancing ice. In many of them the water level rose until
+it reached some low point in the divide and then overflowed, forming
+rapids and waterfalls. Indeed, many of the waterfalls of the eastern
+United States and Canada were formed in just this way and not a few
+streams now occupy courses through ridges instead of parallel to them,
+as in pre-glacial times.
+
+In the zone to the south of the continental ice sheet, the plant and
+animal life of boreal, cool temperate, and warm temperate regions
+commingled curiously. Heather and Arctic willow crowded out elm and oak;
+musk ox, hairy mammoth, and marmot contested with deer, chipmunk, and
+skunk for a chance to live. Near the ice on slopes exposed to the cold
+glacial gales, the immigrant boreal species were dominant, but not far
+away in more protected areas the species that had formerly lived there
+held their own. In Europe during the last two advances of the great ice
+sheet the caveman also struggled with fierce animals and a fiercer
+climate to maintain life in an area whose habitability had long been
+decreasing.
+
+The next step in our history of glaciation is to outline the
+disappearance of the ice sheets. When a decrease in solar activity
+produced a corresponding decrease in storminess, several influences
+presumably combined to cause the disappearance of the ice. Most of their
+results are the reverse of those which brought on glaciation. A few
+special aspects, however, some of which have been discussed in _Earth
+and Sun_, ought to be brought to mind. A diminution in storminess
+lessens upward convection, wind velocity, and evaporation, and these
+changes, if they occurred, must have united to raise the temperature of
+the lower air by reducing the escape of heat. Again a decrease in the
+number and intensity of tropical cyclones presumably lessened the amount
+of moisture carried into mid-latitudes, and thus diminished the
+precipitation. The diminution of snowfall on the ice sheets when
+storminess diminished was probably highly important. The amount of
+precipitation on the sheets was presumably lessened still further by
+changes in the storminess of middle latitudes. When storminess
+diminishes, the lows follow a less definite path, as Kullmer's maps
+show, and on the average a more southerly path. Thus, instead of all the
+lows contributing snow to the ice sheet, a large fraction of the
+relatively few remaining lows would bring rain to areas south of the ice
+sheet. As storminess decreased, the trades and westerlies probably
+became steadier, and thus carried to high latitudes more warm water than
+when often interrupted by storms. Steadier southwesterly winds must have
+produced a greater movement of atmospheric as well as oceanic heat to
+high latitudes. The warming due to these two causes was probably the
+chief reason for the disappearance of the European ice sheet and of
+those on the Pacific coast of North America. The two greater American
+ice sheets, however, and the glaciers elsewhere in the lee of high
+mountain ranges, probably disappeared chiefly because of lessened
+precipitation. If there were no cyclonic storms to draw moisture
+northward from the Gulf of Mexico, most of North America east of the
+Rocky Mountain barrier would be arid. Therefore a diminution of
+storminess would be particularly effective in causing the disappearance
+of ice sheets in these regions.
+
+That evaporation was an especially important factor in causing the ice
+from the Keewatin center to disappear, is suggested by the relatively
+small amount of water-sorted material in its drift. In South Dakota, for
+example, less than 10 per cent of the drift is stratified.[45] On the
+other hand, Salisbury estimates that perhaps a third of the Labradorean
+drift in eastern Wisconsin is crudely stratified, about half of that in
+New Jersey, and more than half of the drift in western Europe.
+
+When the sun's activity began to diminish, all these conditions, as well
+as several others, would cooeperate to cause the ice sheets to disappear.
+Step by step with their disappearance, the amelioration of the climate
+would progress so long as the period of solar inactivity continued and
+storms were rare. If the inactivity continued long enough, it would
+result in a fairly mild climate in high latitudes, though so long as the
+continents were emergent this mildness would not be of the extreme type.
+The inauguration of another cycle of increased disturbance of the sun,
+with a marked increase in storminess, would inaugurate another glacial
+epoch. Thus a succession of glacial and inter-glacial epochs might
+continue so long as the sun was repeatedly disturbed.
+
+FOOTNOTES:
+
+[Footnote 38: This chapter is an amplification and revision of the
+sketch of the glacial period contained in The Solar Hypothesis of
+Climatic Changes; Bull. Geol. Soc. Am., Vol. 25, 1914.]
+
+[Footnote 39: R. D. Salisbury: Physical Geography of the Pleistocene, in
+Outlines of Geologic History, by Willis, Salisbury, and others, 1910, p.
+265.]
+
+[Footnote 40: The Quaternary Ice Age, 1914, p. 364.]
+
+[Footnote B: For fuller discussion of climatic controls see S. S.
+Visher: Seventy Laws of Climate, Annals Assoc. Am. Geographers, 1922.]
+
+[Footnote 41: Many of these alterations are implied or discussed in the
+following papers:
+
+1. F. W. Harmer: Influence of Winds upon the Climate of the Pleistocene;
+Quart. Jour. Geol. Soc., Vol. 57, 1901, p. 405.
+
+2. C. E. P. Brooks: Meteorological Conditions of an Ice Sheet; Quart.
+Jour. Royal Meteorol. Soc., Vol. 40, 1914, pp. 53-70, and The Evolution
+of Climate in Northwest Europe; _op. cit._, Vol. 47, 1921, pp. 173-194.
+
+3. W. H. Hobbs: The Role of the Glacial Anticyclone in the Air
+Circulation of the Globe; Proc. Am. Phil. Soc., Vol. 54, 1915, pp.
+185-225.]
+
+[Footnote 42: W. B. Wright: The Quaternary Ice Age, 1914, p. 100.]
+
+[Footnote 43: The description of the distribution of the ice sheet is
+based on T. C. Chamberlin's wall map of North America at the maximum of
+glaciation, 1913.]
+
+[Footnote 44: Chamberlin and Salisbury: Geology, 1906, Vol. 3, and W. H.
+Hobbs: Characteristics of Existing Glaciers, 1911.]
+
+[Footnote 45: S. S. Visher: The Geography of South Dakota; S. D. Geol.
+Surv., 1918.]
+
+
+
+
+CHAPTER VIII
+
+SOME PROBLEMS OF GLACIAL PERIODS
+
+
+Having outlined in general terms the coming of the ice sheets and their
+disappearance, we are now ready to discuss certain problems of
+compelling climatic interest. The discussion will be grouped under five
+heads: (I) the localization of glaciation; (II) the sudden coming of
+glaciation; (III) peculiar variations in the height of the snow line and
+of glaciation; (IV) lakes and other evidences of humidity in unglaciated
+regions during the glacial epochs; (V) glaciation at sea level and in
+low latitudes in the Permian and Proterozoic eras. The discussion of
+perhaps the most difficult of all climatic problems of glaciation, that
+of the succession of cold glacial and mild inter-glacial epochs, has
+been postponed to the next to the final chapter of this book. It cannot
+be properly considered until we take up the history of solar
+disturbances.
+
+I. The first problem, the localization of the ice sheets, arises from
+the fact that in both the Pleistocene and the Permian periods glaciation
+was remarkably limited. In neither period were all parts of high
+latitudes glaciated; yet in both cases glaciation occurred in large
+regions in lower latitudes. Many explanations of this localization have
+been offered, but most are entirely inadequate. Even hypotheses with
+something of proven worth, such as those of variations in volcanic dust
+and in atmospheric carbon dioxide, fail to account for localization. The
+cyclonic form of the solar hypothesis, however, seems to afford a
+satisfactory explanation.
+
+The distribution of the ice in the last glacial period is well known,
+and is shown in Fig. 6. Four-fifths of the ice-covered area, which was
+eight million square miles, more or less, was near the borders of the
+North Atlantic in eastern North America and northwestern Europe. The ice
+spread out from two great centers in North America, the Labradorean east
+of Hudson Bay, and the Keewatin west of the bay. There were also many
+glaciers in the western mountains, especially in Canada, while
+subordinate centers occurred in Newfoundland, the Adirondacks, and the
+White Mountains. The main ice sheet at its maximum extension reached as
+far south as latitude 39 deg. in Kansas and Kentucky, and 37 deg. in Illinois.
+Huge boulders were transferred more than one thousand miles from their
+source in Canada. The northward extension was somewhat less. Indeed, the
+northern margin of the continent was apparently relatively little
+glaciated and much of Alaska unglaciated. Why should northern Kentucky
+be glaciated when northern Alaska was not?
+
+In Europe the chief center from which the continental glacier moved was
+the Scandinavian highlands. It pushed across the depression now occupied
+by the Baltic to southern Russia and across the North Sea depression to
+England and Belgium. The Alps formed a center of considerable
+importance, and there were minor centers in Scotland, Ireland, the
+Pyrenees, Apennines, Caucasus, and Urals. In Asia numerous ranges also
+contained large glaciers, but practically all the glaciation was of the
+alpine type and very little of the vast northern lowland was covered
+with ice.
+
+In the southern hemisphere glaciation at low latitudes was less striking
+than in the northern hemisphere. Most of the increase in the areas of
+ice was confined to mountains which today receive heavy precipitation
+and still contain small glaciers. Indeed, except for relatively slight
+glaciation in the Australian Alps and in Tasmania, most of the
+Pleistocene glaciation in the southern hemisphere was merely an
+extension of existing glaciers, such as those of south Chile, New
+Zealand, and the Andes. Nevertheless, fairly extensive glaciation
+existed much nearer the equator than is now the case.
+
+In considering the localization of Pleistocene glaciation, three main
+factors must be taken into account, namely, temperature, topography, and
+precipitation. The absence of glaciation in large parts of the Arctic
+regions of North America and of Asia makes it certain that low
+temperature was not the controlling factor. Aside from Antarctica, the
+coldest place in the world is northeastern Siberia. There for seven
+months the average temperature is below 0 deg.C., while the mean for the
+whole year is below -10 deg.C. If the temperature during a glacial period
+averaged 6 deg.C. lower than now, as is commonly supposed, this part of
+Siberia would have had a temperature below freezing for at least nine
+months out of the twelve even if there were no snowfield to keep the
+summers cold. Yet even under such conditions no glaciation occurred,
+although in other places, such as parts of Canada and northwestern
+Europe, intense glaciation occurred where the mean temperature is much
+higher.
+
+The topography of the lands apparently had much more influence upon the
+localization of glaciation than did temperature. Its effect, however,
+was always to cause glaciation exactly where it would be expected and
+not in unexpected places as actually occurred. For example, in North
+America the western side of the Canadian Rockies suffered intense
+glaciation, for there precipitation was heavy because the westerly winds
+from the Pacific are forced to give up their moisture as they rise. In
+the same way the western side of the Sierra Nevadas was much more
+heavily glaciated than the eastern side. In similar fashion the windward
+slopes of the Alps, the Caucasus, the Himalayas, and many other mountain
+ranges suffered extensive glaciation. Low temperature does not seem to
+have been the cause of this glaciation, for in that case it is hard to
+see why both sides of the various ranges did not show an equal
+percentage of increase in the size of their icefields.
+
+From what has been said as to temperature and topography, it is evident
+that variations in precipitation have had much more to do with
+glaciation than have variations in temperature. In the Arctic lowlands
+and on the leeward side of mountains, the slight development of
+glaciation appears to have been due to scarcity of precipitation. On the
+windward side of mountains, on the other hand, a notable increase in
+precipitation seems to have led to abundant glaciation. Such an increase
+in precipitation must be dependent on increased evaporation and this
+could arise either from relatively high temperature or strong winds.
+Since the temperature in the glacial period was lower than now, we seem
+forced to attribute the increased precipitation to a strengthening of
+the winds. If the westerly winds from the Pacific should increase in
+strength and waft more moisture to the western side of the Canadian
+Rockies, or if similar winds increased the snowfall on the upper slopes
+of the Alps or the Tian-Shan Mountains, the glaciers would extend lower
+than now without any change in temperature.
+
+Although the incompetence of low temperature to cause glaciation, and
+the relative unimportance of the mountains in northeastern Canada and
+northwestern Europe throw most glacial hypotheses out of court, they are
+in harmony with the cyclonic hypothesis. The answer of that hypothesis
+to the problem of the localization of ice sheets seems to be found in
+certain maps of storminess and rainfall in relation to solar activity.
+In Fig. 2 a marked belt of increased storminess at times of many
+sunspots is seen in southern Canada. A comparison of this with a series
+of maps given in _Earth and Sun_ shows that the stormy belt tends to
+migrate northward in harmony with an increase in the activity of the
+sun's atmosphere. If the sun were sufficiently active the belt of
+maximum storminess would apparently pass through the Keewatin and
+Labradorean centers of glaciation instead of well to the south of them,
+as at present. It would presumably cross another center in Greenland,
+and then would traverse the fourth of the great centers of Pleistocene
+glaciation in Scandinavia. It would not succeed in traversing northern
+Asia, however, any more than it does now, because of the great
+high-pressure area which develops there in winter. When the ice sheets
+expanded from the main centers of glaciation, the belt of storms would
+be pushed southward and outward. Thus it might give rise to minor
+centers of glaciers such as the Patrician between Hudson Bay and Lake
+Superior, or the centers in Ireland, Cornwall, Wales, and the northern
+Ural Mountains. As the main ice sheets advanced, however, the minor
+centers would be overridden and the entire mass of ice would be merged
+into one vast expanse in the Atlantic portion of each of the two
+continents.
+
+In this connection it may be well to consider briefly the most recent
+hypothesis as to the growth and hence the localization of glaciation. In
+1911 and more fully in 1915, Hobbs,[46] advanced the anti-cyclonic
+hypothesis of the origin of ice sheets. This hypothesis has the great
+merit of focusing attention upon the fact that ice sheets are pronounced
+anti-cyclonic regions of high pressure. This is proved by the strong
+outblowing winds which prevail along their margins. Such winds must, of
+course, be balanced by inward-moving winds at high levels. Abundant
+observations prove that such is the case. For example, balloons sent up
+by Barkow near the margin of the Antarctic ice sheet reveal the
+occurrence of inblowing winds, although they rarely occur below a height
+of 9000 meters. The abundant data gathered by Guervain on the coast of
+Greenland indicate that outblowing winds prevail up to a height of about
+4000 meters. At that height inblowing winds commence and increase in
+frequency until at an altitude of over 5000 meters they become more
+common than outblowing winds. It should be noted, however, that in both
+Antarctica and Greenland, although the winds at an elevation of less
+than a thousand meters generally blow outward, there are frequent and
+decided departures from this rule, so that "variable winds" are quite
+commonly mentioned in the reports of expeditions and balloon soundings.
+
+The undoubted anti-cyclonic conditions which Hobbs thus calls to the
+attention of scientists seem to him to necessitate a peculiar mechanism
+in order to produce the snow which feeds the glaciers. He assumes that
+the winds which blow toward the centers of the ice sheets at high levels
+carry the necessary moisture by which the glaciers grow. When the air
+descends in the centers of the highs, it is supposed to be chilled on
+reaching the surface of the ice, and hence to give up its moisture in
+the form of minute crystals. This conclusion is doubtful for several
+reasons. In the first place, Hobbs does not seem to appreciate the
+importance of the variable winds which he quotes Arctic and Antarctic
+explorers as describing quite frequently on the edges of the ice sheets.
+They are one of many signs that cyclonic storms are fairly frequent on
+the borders of the ice though not in its interior. Thus there is a
+distinct and sufficient form of precipitation actually at work near the
+margin of the ice, or exactly where the thickness of the ice sheet would
+lead us to expect.
+
+Another consideration which throws grave doubt on the anti-cyclonic
+hypothesis of ice sheets is the small amount of moisture possible in the
+highs because of their low temperature. Suppose, for the sake of
+argument, that the temperature in the middle of an ice sheet averages
+20 deg.F. This is probably much higher than the actual fact and therefore
+unduly favorable to the anti-cyclonic hypothesis. Suppose also that the
+decrease in temperature from the earth's surface upward proceeds at the
+rate of 1 deg.F. for each 300 feet, which is 50 per cent less than the
+actual rate for air with only a slight amount of moisture, such as is
+found in cold regions. Then at a height of 10,000 feet, where the
+inblowing winds begin to be felt, the temperature would be -20 deg.F. At
+that temperature the air is able to hold approximately 0.166 grain of
+moisture per cubic foot when fully saturated. This is an exceedingly
+small amount of moisture and even if it were all precipitated could
+scarcely build a glacier. However, it apparently would not be
+precipitated because when such air descends in the center of the
+anti-cyclone it is warmed adiabatically, that is, by compression. On
+reaching the surface it would have a temperature of 20 deg. and would be
+able to hold 0.898 grain of water vapor per cubic foot; in other words,
+it would have a relative humidity of about 18 per cent. Under no
+reasonable assumption does the upper air at the center of an ice sheet
+appear to reach the surface with a relative humidity of more than 20 or
+25 per cent. Such air cannot give up moisture. On the contrary, it
+absorbs it and tends to diminish rather than increase the thickness of
+the sheet of ice and snow. But after the surplus heat gained by descent
+has been lost by radiation, conduction, and evaporation, the air may
+become super-saturated with the moisture picked up while warm. Hobbs
+reports that explorers in Antarctica and Greenland have frequently
+observed condensation on their clothing. If such moisture is not derived
+directly from the men's own bodies, it is apparently picked up from the
+ice sheet by the descending air, and not added to the ice sheet by air
+from aloft.
+
+The relation of all this to the localization of ice sheets is this. If
+Hobbs' anti-cyclonic hypothesis of glacial growth is correct, it would
+appear that ice sheets should grow up where the temperature is lowest
+and the high-pressure areas most persistent; for instance, in northern
+Siberia. It would also appear that so far as the topography permitted,
+the ice sheets ought to move out uniformly in all directions; hence the
+ice sheet ought to be as prominent to the north of the Keewatin and
+Labradorean centers as to the south, which is by no means the case.
+Again, in mountainous regions, such as the glacial areas of Alaska and
+Chile, the glaciation ought not to be confined to the windward slope of
+the mountains so closely as is actually the fact. In each of these cases
+the glaciated region was large enough so that there was probably a true
+anti-cyclonic area comparable with that now prevailing over southern
+Greenland. In both places the correlation between glaciation and
+mountain ranges seems much too close to support the anti-cyclonic
+hypothesis, for the inblowing winds which on that hypothesis bring the
+moisture are shown by observation to occur at heights far greater than
+that of all but the loftiest ranges.
+
+II. The sudden coming of glaciation is another problem which has been a
+stumbling-block in the way of every glacial hypothesis. In his _Climates
+of Geologic Times_, Schuchert states that the fossils give almost no
+warning of an approaching catastrophe. If glaciation were solely due to
+uplift, or other terrestrial changes aside from vulcanism, Schuchert
+holds that it would have come slowly and the stages preceding glaciation
+would have affected life sufficiently to be recorded in the rocks. He
+considers that the suddenness of the coming of glaciation is one of the
+strongest arguments against the carbon dioxide hypothesis of glaciation.
+
+According to the cyclonic hypothesis, however, the suddenness of the
+oncoming of glaciation is merely what would be expected on the basis of
+what happens today. Changes in the sun occur suddenly. The sunspot cycle
+is only eleven or twelve years long, and even this short period of
+activity is inaugurated more suddenly than it declines. Again the
+climatic record derived from the growth of trees, as given in Figs. 4
+and 5, also shows that marked changes in climate are initiated more
+rapidly than they disappear. In this connection, however, it must be
+remembered that solar activity may arise in various ways, as will appear
+more fully later. Under certain conditions storminess may increase and
+decrease slowly.
+
+III. The height of the snow line and of glaciation furnishes another
+means of testing glacial hypotheses. It is well established that in
+times of glaciation the snow line was depressed everywhere, but least
+near the equator. For example, according to Penck, permanent snow
+extended 4000 feet lower than now in the Alps, whereas it stood only
+1500 feet below the present level near the equator in Venezuela. This
+unequal depression is not readily accounted for by any hypothesis
+depending solely upon the lowering of temperature. By the carbon dioxide
+and the volcanic dust hypotheses, the temperature presumably was lowered
+almost equally in all latitudes, but a little more at the equator than
+elsewhere. If glaciation were due to a temporary lessening of the
+radiation received from the sun, such as is demanded by the thermal
+solar hypothesis, and by the longer periods of Croll's hypothesis, the
+lowering would be distinctly greatest at the equator. Thus, according to
+all these hypotheses, the snow line should have been depressed most at
+the equator, instead of least.
+
+The cyclonic hypothesis explains the lesser depression of the snow line
+at the equator as due to a diminution of precipitation. The
+effectiveness of precipitation in this respect is illustrated by the
+present great difference in the height of the snow line on the humid and
+dry sides of mountains. On the wet eastern side of the Andes near the
+equator, the snow line lies at 16,000 feet; on the dry western side, at
+18,500 feet. Again, although the humid side of the Himalayas lies toward
+the south, the snow line has a level of 15,000 feet, while farther
+north, on the dry side, it is 16,700 feet.[47] The fact that the snow
+line is lower near the margin of the Alps than toward the center points
+in the same direction. The bearing of all this on the glacial period may
+be judged by looking again at Fig. 3 in Chapter V. This shows that at
+times of sunspot activity and hence of augmented storminess, the
+precipitation diminishes near the heat equator, that is, where the
+average temperature for the whole year is highest. At present the great
+size of the northern continents and their consequent high temperature in
+summer, cause the heat equator to lie north of the "real" equator,
+except where Australia draws it to the southward.[48] When large parts
+of the northern continents were covered with ice, however, the heat
+equator and the true equator were probably much closer than now, for the
+continents could not become so hot. If so, the diminution in equatorial
+precipitation, which accompanies increased storminess throughout the
+world as a whole, would take place more nearly along the true equator
+than appears in Fig. 3. Hence so far as precipitation alone is
+concerned, we should actually expect that the snow line near the equator
+would rise a little during glacial periods. Another factor, however,
+must be considered. Koeppen's data, it will be remembered, show that at
+times of solar activity the earth's temperature falls more at the
+equator than in higher latitudes. If this effect were magnified it would
+lower the snow line. The actual position of the snow line at the equator
+during glacial periods thus appears to be the combined effect of
+diminished precipitation, which would raise the line, and of lower
+temperature, which would bring it down.
+
+Before leaving this subject it may be well to recall that the relative
+lessening of precipitation in equatorial latitudes during the glacial
+epochs was probably caused by the diversion of moisture from the
+trade-wind belt. This diversion was presumably due to the great number
+of tropical cyclones and to the fact that the cyclonic storms of middle
+latitudes also drew much moisture from the trade-wind belt in summer
+when the northern position of the sun drew that belt near the storm
+track which was forced to remain south of the ice sheet. Such diversion
+of moisture out of the trade-wind belt must diminish the amount of water
+vapor that is carried by the trades to equatorial regions; hence it
+would lessen precipitation in the belt of so-called equatorial calms,
+which lies along the heat equator rather than along the geographical
+equator.
+
+Another phase of the vertical distribution of glaciation has been the
+subject of considerable discussion. In the Alps and in many other
+mountains the glaciation of the Pleistocene period appears to have had
+its upper limit no higher than today. This has been variously
+interpreted. It seems, however, to be adequately explained as due to
+decreased precipitation at high altitudes during the cold periods. This
+is in spite of the fact that precipitation in general increased with
+increased storminess. The low temperature of glacial times presumably
+induced condensation at lower altitudes than now, and most of the
+precipitation occurred upon the lower slopes of the mountains,
+contributing to the lower glaciers, while little of it fell upon the
+highest glaciers. Above a moderate altitude in all lofty mountains the
+decrease in the amount of precipitation is rapid. In most cases the
+decrease begins at a height of less than 3000 feet above the base of the
+main slope, provided the slope is steep. The colder the air, the lower
+the altitude at which this occurs. For example, it is much lower in
+winter than in summer. Indeed, the higher altitudes in the Alps are
+sunny in winter even where there are abundant clouds lower down.
+
+IV. The presence of extensive lakes and other evidences of a pluvial
+climate during glacial periods in non-glaciated regions which are
+normally dry is another of the facts which most glacial hypotheses fail
+to explain satisfactorily. Beyond the ice sheets many regions appear to
+have enjoyed an unusually heavy precipitation during the glacial epochs.
+The evidence of this is abundant, including numerous abandoned strand
+lines of salt lakes and an abundance of coarse material in deltas and
+flood plains. J. D. Whitney,[49] in an interesting but neglected volume,
+was one of the first to marshal the evidence of this sort. More recently
+Free[50] has amplified this. According to him in the Great Basin region
+of the United States sixty-two basins either contain unmistakable
+evidence of lakes, or belong to one of the three great lake groups named
+below. Two of these, the Lake Lahontan and the Lake Bonneville groups,
+comprise twenty-nine present basins, while the third, the Owens-Searles
+chain, contained at least five large lakes, the lowest being in Death
+Valley. In western and central Asia a far greater series of salt lakes
+is found and most of these are surrounded by strands at high levels.
+Many of these are described in _Explorations in Turkestan_, _The Pulse
+of Asia_, and _Palestine and Its Transformation_. There has been a good
+deal of debate as to whether these lakes actually date from the glacial
+period, as is claimed by C. E. P. Brooks, for example, or from some
+other period. The evidence, however, seems to be convincing that the
+lakes expanded when the ice also expanded.
+
+According to the older glacial hypotheses the lower temperature which is
+postulated as the cause of glaciation would almost certainly mean less
+evaporation over the oceans and hence less precipitation during glacial
+periods. To counteract this the only way in which the level of the lakes
+could be raised would be because the lower temperature would cause less
+evaporation from their surfaces. It seems quite impossible, however,
+that the lowering of temperature, which is commonly taken to have been
+not more than 10 deg.C., could counteract the lessened precipitation and
+also cause an enormous expansion of most of the lakes. For example,
+ancient Lake Bonneville was more than ten times as large as its modern
+remnant, Great Salt Lake, and its average depth more than forty times as
+great.[51] Many small lakes in the Old World expanded still more.[52]
+For example, in eastern Persia many basins which now contain no lake
+whatever are floored with vast deposits of lacustrine salt and are
+surrounded by old lake bluffs and beaches. In northern Africa similar
+conditions prevail.[53] Other, but less obvious, evidence of more
+abundant rainfall in regions that are now dry is found in thick strata
+of gravel, sand, and fine silt in the alluvial deposits of flood plains
+and deltas.[54]
+
+The cyclonic hypothesis supposes that increased storminess accounts for
+pluvial climates in regions that are now dry just as it accounts for
+glaciation in the regions of the ice sheets. Figs. 2 and 3, it will be
+remembered, illustrate what happens when the sun is active. Solar
+activity is accompanied by an increase in storminess in the southwestern
+United States in exactly the region where elevated strands of diminished
+salt lakes are most numerous. In Fig. 3, the same condition is seen in
+the region of salt lakes in the Old World. Judging by these maps, which
+illustrate what has happened since careful meteorological records were
+kept, an increase in solar activity is accompanied by increased rainfall
+in large parts of what are now semi-arid and desert regions. Such
+precipitation would at once cause the level of the lakes to rise. Later,
+when ice sheets had developed in Europe and America, the high-pressure
+areas thus caused might force the main storm belt so far south that it
+would lie over these same arid regions. The increase in tropical
+hurricanes at times of abundant sunspots may also have a bearing on the
+climate of regions that are now arid. During the glacial period some of
+the hurricanes probably swept far over the lands. The numerous tropical
+cyclones of Australia, for example, are the chief source of
+precipitation for that continent.[55] Some of the stronger cyclones
+locally yield more rain in a day or two than other sources yield in a
+year.
+
+V. The occurrence of widespread glaciation near the tropics during the
+Permian, as shown in Fig. 7, has given rise to much discussion. The
+recent discovery of glaciation in latitudes as low as 30 deg. in the
+Proterozoic is correspondingly significant. In all cases the occurrence
+of glaciation in low and middle latitudes is probably due to the same
+general causes. Doubtless the position and altitude of the mountains had
+something to do with the matter. Yet taken by itself this seems
+insufficient. Today the loftiest range in the world, the Himalayas, is
+almost unglaciated, although its southern slope may seem at first
+thought to be almost ideally located in this respect. Some parts rise
+over 20,000 feet and certain lower slopes receive 400 inches of rain per
+year. The small size of the Himalayan glaciers in spite of these
+favorable conditions is apparently due largely to the seasonal character
+of the monsoon winds. The strong outblowing monsoons of winter cause
+about half the year to be very dry with clear skies and dry winds from
+the interior of Asia. In all low latitudes the sun rides high in the
+heavens at midday, even in winter, and thus melts snow fairly
+effectively in clear weather. This is highly unfavorable to glaciation.
+The inblowing southern monsoons bring all their moisture in midsummer at
+just the time when it is least effective in producing snow. Conditions
+similar to those now prevailing in the Himalayas must accompany any
+great uplift of the lands which produces high mountains and large
+continents in subtropical and middle latitudes. Hence, uplift alone
+cannot account for extensive glaciation in subtropical latitudes during
+the Permian and Proterozoic.
+
+[Illustration: _Fig. 7. Permian geography and glaciation._
+(_After Schuchert._)]
+
+The assumption of a great general lowering of temperature is also not
+adequate to explain glaciation in subtropical latitudes. In the first
+place this would require a lowering of many degrees,--far more than in
+the Pleistocene glacial period. The marine fossils of the Permian,
+however, do not indicate any such condition. In the second place, if the
+lands were widespread as they appear to have been in the Permian, a
+general lowering of temperature would diminish rather than increase the
+present slight efficiency of the monsoons in producing glaciation.
+Monsoons depend upon the difference between the temperatures of land and
+water. If the general temperature were lowered, the reduction would be
+much less pronounced on the oceans than on the lands, for water tends to
+preserve a uniform temperature, not only because of its mobility, but
+because of the large amount of heat given out when freezing takes place,
+or consumed in evaporation. Hence the general lowering of temperature
+would make the contrast between continents and oceans less than at
+present in summer, for the land temperature would be brought toward that
+of the ocean. This would diminish the strength of the inblowing summer
+monsoons and thus cut off part of the supply of moisture. Evidence that
+this actually happened in the cold fourteenth century has already been
+given in Chapter VI. On the other hand, in winter the lands would be
+much colder than now and the oceans only a little colder, so that the
+dry outblowing monsoons of the cold season would increase in strength
+and would also last longer than at present. In addition to all this, the
+mere fact of low temperature would mean a general reduction in the
+amount of water vapor in the air. Thus, from almost every point of view
+a mere lowering of temperature seems to be ruled out as a cause of
+Permian glaciation. Moreover, if the Permian or Proterozoic glacial
+periods were so cold that the lands above latitude 30 deg. were snow-covered
+most of the time, the normal surface winds in subtropical latitudes
+would be largely equatorward, just as the winter monsoons now are. Hence
+little or no moisture would be available to feed the snowfields which
+give rise to the glaciers.
+
+It has been assumed by Marsden Manson and others that increased general
+cloudiness would account for the subtropical glaciation of the Permian
+and Proterozoic. Granting for the moment that there could be universal
+persistent cloudiness, this would not prevent or counteract the
+outblowing anti-cyclonic winds so characteristic of great snowfields.
+Therefore, under the hypothesis of general cloudiness there would be no
+supply of moisture to cause glaciation in low latitudes. Indeed,
+persistent cloudiness in all higher latitudes would apparently deprive
+the Himalayas of most of their present moisture, for the interior of
+Asia would not become hot in summer and no inblowing monsoons would
+develop. In fact, winds of all kinds would seemingly be scarce, for they
+arise almost wholly from contrasts of temperature and hence of
+atmospheric pressure. The only way to get winds and hence precipitation
+would be to invoke some other agency, such as cyclonic storms, but that
+would be a departure from the supposition that glaciation arose from
+cloudiness.
+
+Let us now inquire how the cyclonic hypothesis accounts for glaciation
+in low latitudes. We will first consider the terrestrial conditions in
+the early Permian, the last period of glaciation in such latitudes.
+Geologists are almost universally agreed that the lands were
+exceptionally extensive and also high, especially in low latitudes. One
+evidence of this is the presence of abundant conglomerates composed of
+great boulders. It is also probable that the carbon dioxide in the air
+during the early Permian had been reduced to a minimum by the
+extraordinary amount of coal formed during the preceding period. This
+would tend to produce low temperature and thus make the conditions
+favorable for glaciation as soon as an accentuation of solar activity
+caused unusual storminess. If the storminess became extreme when
+terrestrial conditions were thus universally favorable to glaciation, it
+would presumably produce glaciation in low latitudes. Numerous and
+intense tropical cyclones would carry a vast amount of moisture out of
+the tropics, just as now happens when the sun is active, but on a far
+larger scale. The moisture would be precipitated on the equatorward
+slopes of the subtropical mountain ranges. At high elevations this
+precipitation would be in the form of snow even in summer. Tropical
+cyclones, however, as is shown in _Earth and Sun_, occur in the autumn
+and winter as well as in summer. For example, in the Bay of Bengal the
+number recorded in October is fifty, the largest for any month; while in
+November it is thirty-four, and December fourteen as compared with an
+average of forty-two for the months of July to September. From January
+to March, when sunspot numbers averaged more than forty, the number of
+tropical hurricanes was 143 per cent greater than when the sunspot
+numbers averaged below forty. During the months from April to June,
+which also would be times of considerable snowy precipitation, tropical
+hurricanes averaged 58 per cent more numerous with sunspot numbers above
+forty than with numbers below forty, while from July to September the
+difference amounted to 23 per cent. Even at this season some snow falls
+on the higher slopes, while the increased cloudiness due to numerous
+storms also tends to preserve the snow. Thus a great increase in the
+frequency of sunspots is accompanied by increased intensity of tropical
+hurricanes, especially in the cooler autumn and spring months, and
+results not only in a greater accumulation of snow but in a decrease in
+the melting of the snow because of more abundant clouds. At such times
+as the Permian, the general low temperature due to rapid convection and
+to the scarcity of carbon dioxide presumably joined with the extension
+of the lands in producing great high-pressure areas over the lands in
+middle latitudes during the winters, and thus caused the more northern,
+or mid-latitude type of cyclonic storms to be shifted to the equatorward
+side of the continents at that season. This would cause an increase of
+precipitation in winter as well as during the months when tropical
+hurricanes abound. Many other circumstances would cooeperate to produce a
+similar result. For example, the general low temperature would cause the
+sea to be covered with ice in lower latitudes than now, and would help
+to create high-pressure areas in middle latitudes, thus driving the
+storms far south. If the sea water were fresher than now, as it probably
+was to a notable extent in the Proterozoic and perhaps to some slight
+extent in the Permian, the higher freezing point would also further the
+extension of the ice and help to keep the storms away from high
+latitudes. If to this there is added a distribution of land and sea such
+that the volume of the warm ocean currents flowing from low to high
+latitudes was diminished, as appears to have been the case, there seems
+to be no difficulty in explaining the subtropical location of the main
+glaciation in both the Permian and the Proterozoic. An increase of
+storminess seems to be the key to the whole situation.
+
+One other possibility may be mentioned, although little stress should be
+laid on it. In _Earth and Sun_ it has been shown that the main storm
+track in both the northern and southern hemispheres is not concentric
+with the geographical poles. Both tracks are roughly concentric with the
+corresponding magnetic poles, a fact which may be important in
+connection with the hypothesis of an electrical effect of the sun upon
+terrestrial storminess. The magnetic poles are known to wander
+considerably. Such wandering gives rise to variations in the direction
+of the magnetic needle from year to year. In 1815 the compass in England
+pointed 24-1/2 deg. W. of N. and in 1906 17 deg. 45' W. Such a variation seems
+to mean a change of many miles in the location of the north magnetic
+pole. Certain changes in the daily march of electromagnetic phenomena
+over the oceans have led Bauer and his associates to suggest that the
+magnetic poles may even be subject to a slight daily movement in
+response to the changes in the relative positions of the earth and sun.
+Thus there seems to be a possibility that a pronounced change in the
+location of the magnetic pole in Permian times, for example, may have
+had some connection with a shifting in the location of the belt of
+storms. It must be clearly understood that there is as yet no evidence
+of any such change, and the matter is introduced merely to call
+attention to a possible line of investigation.
+
+Any hypothesis of Permian and Proterozoic glaciation must explain not
+only the glaciation of low latitudes but the lack of glaciation and the
+accumulation of red desert beds in high latitudes. The facts already
+presented seem to explain this. Glaciation could not occur extensively
+in high latitudes partly because during most of the year the air was too
+cold to hold much moisture, but still more because the winds for the
+most part must have blown outward from the cold northern areas and the
+cyclonic storm belt was pushed out of high latitudes. Because of these
+conditions precipitation was apparently limited to a relatively small
+number of storms during the summer. Hence great desert areas must have
+prevailed at high latitudes. Great aridity now prevails north of the
+Himalayas and related ranges, and red beds are accumulating in the
+centers of the great deserts, such as those of the Tarim Basin and the
+Transcaspian. The redness is not due to the original character of the
+rock, but to intense oxidation, as appears from the fact that along the
+edges of the desert and wherever occasional floods carry sediment far
+out into the midst of the sand, the material has the ordinary brownish
+shades. As soon as one goes out into the places where the sand has been
+exposed to the air for a long time, however, it becomes pink, and then
+red. Such conditions may have given rise to the high degree of oxidation
+in the famous Permian red beds. If the air of the early Permian
+contained an unusual percentage of oxygen because of the release of that
+gas by the great plant beds which formed coal in the preceding era, as
+Chamberlin has thought probable, the tendency to produce red beds would
+be still further increased.
+
+It must not be supposed, however, that these conditions would absolutely
+limit glaciation to subtropical latitudes. The presence of early Permian
+glaciation in North America at Boston and in Alaska and in the Falkland
+Islands of the South Atlantic Ocean proves that at least locally there
+was sufficient moisture to form glaciers near the coast in relatively
+high latitudes. The possibility of this would depend entirely upon the
+form of the lands and the consequent course of ocean currents. Even in
+those high latitudes cyclonic storms would occur unless they were kept
+out by conditions of pressure such as have been described above.
+
+The marine faunas of Permian age in high latitudes have been interpreted
+as indicating mild oceanic temperatures. This is a point which requires
+further investigation. Warm oceans during times of slight solar activity
+are a necessary consequence of the cyclonic hypothesis, as will appear
+later. The present cold oceans seem to be the expectable result of the
+Pleistocene glaciation and of the present relatively disturbed condition
+of the sun. If a sudden disturbance threw the solar atmosphere into
+violent commotion within a few thousand years during Permian times,
+glaciation might occur as described above, while the oceans were still
+warm. In fact their warmth would increase evaporation while the violent
+cyclonic storms and high winds would cause heavy rain and keep the air
+cool by constantly raising it to high levels where it would rapidly
+radiate its heat into space.
+
+Nevertheless it is not yet possible to determine how warm the oceans
+were at the actual time of the Permian glaciation. Some faunas formerly
+reported as Permian are now known to be considerably older. Moreover,
+others of undoubted Permian age are probably not strictly
+contemporaneous with the glaciation. So far back in the geological
+record it is very doubtful whether we can date fossils within the limits
+of say 100,000 years. Yet a difference of 100,000 years would be more
+than enough to allow the fossils to have lived either before or after
+the glaciation, or in an inter-glacial epoch. One such epoch is known to
+have occurred and nine others are suggested by the inter-stratification
+of glacial till and marine sediments in eastern Australia. The warm
+currents which would flow poleward in inter-glacial epochs must have
+favored a prompt reintroduction of marine faunas driven out during times
+of glaciation. Taken all and all, the Permian glaciation seems to be
+accounted for by the cyclonic hypothesis quite as well as does the
+Pleistocene. In both these cases, as well as in the various pulsations
+of historic times, it seems to be necessary merely to magnify what is
+happening today in order to reproduce the conditions which prevailed in
+the past. If the conditions which now prevail at times of sunspot minima
+were magnified, they would give the mild conditions of inter-glacial
+epochs and similar periods. If the conditions which now prevail at times
+of sunspot maxima are magnified a little they seem to produce periods of
+climatic stress such as those of the fourteenth century. If they are
+magnified still more the result is apparently glacial epochs like those
+of the Pleistocene, and if they are magnified to a still greater extent,
+the result is Permian or Proterozoic glaciation. Other factors must
+indeed be favorable, for climatic changes are highly complex and are
+unquestionably due to a combination of circumstances. The point which is
+chiefly emphasized in this book is that among those several
+circumstances, changes in cyclonic storms due apparently to activity of
+the sun's atmosphere must always be reckoned.
+
+FOOTNOTES:
+
+[Footnote 46: W. H. Hobbs: Characteristics of Existing Glaciers, 1911.
+The Role of the Glacial Anticyclones in the Air Circulation of the
+Globe; Proc. Am. Phil. Soc., Vol. 54, 1915, pp. 185-225.]
+
+[Footnote 47: R. D. Salisbury: Physiography, 1919.]
+
+[Footnote 48: Griffith Taylor: Australian Meteorology, 1920, p. 283.]
+
+[Footnote 49: J. D. Whitney: Climatic Changes of the Later Geological
+Times, 1882.]
+
+[Footnote 50: E. E. Free: U. S. Dept. of Agriculture, Bull. 54, 1914.
+Mr. Free has prepared a summary of this Bulletin which appears in The
+Solar Hypothesis, Bull. Geol. Sec. of Am., Vol. 25, pp. 559-562.]
+
+[Footnote 51: G. K. Gilbert: Lake Bonneville; Monograph 1, U. S. Geol.
+Surv.]
+
+[Footnote 52: C. E. P. Brooks: Quart. Jour. Royal Meteorol. Soc., 1914,
+pp. 63-66.]
+
+[Footnote 53: H. J. L. Beadnell: A. Egyptian Oasis, London, 1909.
+Ellsworth Huntington: The Libyan Oasis of Kharga; Bull. Am. Geog. Soc.,
+Vol. 42, Sept., 1910, pp. 641-661.]
+
+[Footnote 54: S. S. Visher: The Bajada of the Tucson Bolson of Southern
+Arizona; Science, N. S., Mar. 23, 1913.
+
+Ellsworth Huntington: The Basins of Eastern Persia and Seistan, in
+Explorations in Turkestan.]
+
+[Footnote 55: Griffith Taylor: Australian Meteorology, 1920, p. 189.]
+
+
+
+
+CHAPTER IX
+
+THE ORIGIN OF LOESS
+
+
+One of the most remarkable formations associated with glacial deposits
+consists of vast sheets of the fine-grained, yellowish, wind-blown
+material called loess. Somewhat peculiar climatic conditions evidently
+prevailed when it was formed. At present similar deposits are being laid
+down only near the leeward margin of great deserts. The famous loess
+deposits of China in the lee of the Desert of Gobi are examples. During
+the Pleistocene period, however, loess accumulated in a broad zone along
+the margin of the ice sheet at its maximum extent. In the Old World it
+extended from France across Germany and through the Black Earth region
+of Russia into Siberia. In the New World a still larger area is
+loess-covered. In the Mississippi Valley, tens of thousands of square
+miles are mantled by a layer exceeding twenty feet in thickness and in
+many places approaching a hundred feet. Neither the North American nor
+the European deposits are associated with a desert. Indeed, loess is
+lacking in the western and drier parts of the great plains and is best
+developed in the well-watered states of Iowa, Illinois, and Missouri.
+Part of the loess overlies the non-glacial materials of the great
+central plain, but the northern portions overlie the drift deposits of
+the first three glaciations. A few traces of loess are associated with
+the Kansan and Illinoian, the second and third glaciations, but most of
+the America loess appears to have been formed at approximately the time
+of the Iowan or fourth glaciation, while only a little overlies the
+drift sheets of the Wisconsin age. The loess is thickest near the margin
+of the Iowan till sheet and thins progressively both north and south.
+The thinning southward is abrupt along the stream divides, but very
+gradual along the larger valleys. Indeed, loess is abundant along the
+bluffs of the Mississippi, especially the east bluff, almost to the Gulf
+of Mexico.[56]
+
+It is now generally agreed that all typical loess is wind blown. There
+is still much question, however, as to its time of origin, and thus
+indirectly as to its climatic implications. Several American and
+European students have thought that the loess dates from inter-glacial
+times. On the other hand, Penck has concluded that the loess was formed
+shortly before the commencement of the glacial epochs; while many
+American geologists hold that the loess accumulated while the ice sheets
+were at approximately their maximum size. W. J. McGee, Chamberlin and
+Salisbury, Keyes, and others lean toward this view. In this chapter the
+hypothesis is advanced that it was formed at the one other possible
+time, namely, immediately following the retreat of the ice.
+
+These four hypotheses as to the time of origin of loess imply the
+following differences in its climatic relations. If loess was formed
+during typical inter-glacial epochs, or toward the close of such epochs,
+profound general aridity must seemingly have prevailed in order to kill
+off the vegetation and thus enable the wind to pick up sufficient dust.
+If the loess was formed during times of extreme glaciation when the
+glaciers were supplying large quantities of fine material to outflowing
+streams, less aridity would be required, but there must have been sharp
+contrasts between wet seasons in summer when the snow was melting and
+dry seasons in winter when the storms were forced far south by the
+glacial high pressure. Alternate floods and droughts would thus affect
+broad areas along the streams. Hence arises the hypothesis that the wind
+obtained the loess from the flood plains of streams at times of maximum
+glaciation. If the loess was formed during the rapid retreat of the ice,
+alternate summer floods and winter droughts would still prevail, but
+much material could also be obtained by the winds not only from flood
+plains, but also from the deposits exposed by the melting of the ice and
+not yet covered by vegetation.
+
+The evidence for and against the several hypotheses may be stated
+briefly. In support of the hypothesis of the inter-glacial origin of
+loess, Shimek and others state that the glacial drift which lies beneath
+the loess commonly gives evidence that some time elapsed between the
+disappearance of the ice and the deposition of the loess. For example,
+abundant shells of land snails in the loess are not of the sort now
+found in colder regions, but resemble those found in the drier regions.
+It is probable that if they represented a glacial epoch they would be
+depauperated by the cold as are the snails of far northern regions. The
+gravel pavement discussed below seems to be strong evidence of erosion
+between the retreat of the ice and the deposition of the loess.
+
+Turning to the second hypothesis, namely, that the loess accumulated
+near the close of the inter-glacial epoch rather than in the midst of
+it, we may follow Penck. The mammalian fossils seem to him to prove that
+the loess was formed while boreal animals occupied the region, for they
+include remains of the hairy mammoth, woolly rhinoceros, and reindeer.
+On the other hand, the typical inter-glacial beds not far away yield
+remains of species characteristic of milder climates, such as the
+elephant, the smaller rhinoceros, and the deer. In connection with these
+facts it should be noted that occasional remains of tundra vegetation
+and of trees are found beneath the loess, while in the loess itself
+certain steppe animals, such as the common gopher or spermaphyl, are
+found. Penck interprets this as indicating a progressive desiccation
+culminating just before the oncoming of the next ice sheet.
+
+The evidence advanced in favor of the hypothesis that the loess was
+formed when glaciation was near its maximum includes the fact that if
+the loess does not represent the outwash from the Iowan ice, there is
+little else that does, and presumably there must have been outwash. Also
+the distribution of loess along the margins of streams suggests that
+much of the material came from the flood plains of overloaded streams
+flowing from the melting ice.
+
+Although there are some points in favor of the hypothesis that the loess
+originated (1) in strictly inter-glacial times, (2) at the end of
+inter-glacial epochs, and (3) at times of full glaciation, each
+hypothesis is much weakened by evidence that supports the others. The
+evidence of boreal animals seems to disprove the hypothesis that the
+loess was formed in the middle of a mild inter-glacial epoch. On the
+other hand, Penck's hypothesis as to loess at the end of inter-glacial
+times fails to account for certain characteristics of the lowest part of
+the loess deposits and of the underlying topography. Instead of normal
+valleys and consequent prompt drainage such as ought to have developed
+before the end of a long inter-glacial epoch, the surface on which the
+loess lies shows many undrained depressions. Some of these can be seen
+in exposed banks, while many more are inferred from the presence of
+shells of pond snails here and there in the overlying loess. The pond
+snails presumably lived in shallow pools occupying depressions in the
+uneven surface left by the ice. Another reason for questioning whether
+the loess was formed at the end of an inter-glacial epoch is that this
+hypothesis does not provide a reasonable origin for the material which
+composes the loess. Near the Alps where the loess deposits are small and
+where glaciers probably persisted in the inter-glacial epochs and thus
+supplied flood plain material in large quantities, this does not appear
+important. In the broad upper Mississippi Basin, however, and also in
+the Black Earth region of Russia there seems to be no way to get the
+large body of material composing the loess except by assuming the
+existence of great deserts to windward. But there seems to be little or
+no evidence of such deserts where they could be effective. The
+mineralogical character of the loess of Iowan age proves that the
+material came from granitic rocks, such as formed a large part of the
+drift. The nearest extensive outcrops of granite are in the southwestern
+part of the United States, nearly a thousand miles from Iowa and
+Illinois. But the loess is thickest near the ice margin and thins toward
+the southwest and in other directions, whereas if its source were the
+southwestern desert, its maximum thickness would probably be near the
+margin of the desert.
+
+The evidence cited above seems inconsistent not only with the hypothesis
+that the loess was formed at the end of an inter-glacial epoch, but also
+with the idea that it originated at times of maximum glaciation either
+from river-borne sediments or from any other source. A further and more
+convincing reason for this last conclusion is the probability and almost
+the certainty that when the ice advanced, its front lay close to areas
+where the vegetation was not much thinner than that which today prevails
+under similar climatic conditions. If the average temperature of glacial
+maxima was only 6 deg.C. lower than that of today, the conditions just
+beyond the ice front when it was in the loess region from southern
+Illinois to Minnesota would have been like those now prevailing in
+Canada from New Brunswick to Winnipeg. The vegetation there is quite
+different from the grassy, semi-arid vegetation of which evidence is
+found in the loess. The roots and stalks of such grassy vegetation are
+generally agreed to have helped produce the columnar structure which
+enables the loess to stand with almost vertical surfaces.
+
+We are now ready to consider the probability that loess accumulated
+mainly during the retreat of the ice. Such a retreat exposed a zone of
+drift to the outflowing glacial winds. Most glacial hypotheses, such as
+that of uplift, or depleted carbon dioxide, call for a gradual retreat
+of the ice scarcely faster than the vegetation could advance into the
+abandoned area. Under the solar-cyclonic hypothesis, on the other hand,
+the climatic changes may have been sudden and hence the retreat of the
+ice may have been much more rapid than the advance of vegetation. Now
+wind-blown materials are derived from places where vegetation is scanty.
+Scanty vegetation on good soil, it is true, is usually due to aridity,
+but may also result because the time since the soil was exposed to the
+air has not been long enough for the soil to be sufficiently weathered
+to support vegetation. Even when weathering has had full opportunity, as
+when sand bars, mud flats, and flood plains are exposed, vegetation
+takes root only slowly. Moreover, storms and violent winds may prevent
+the spread of vegetation, as is seen on sandy beaches even in distinctly
+humid regions like New Jersey and Denmark. Thus it appears that unless
+the retreat of the ice were as slow as the advance of vegetation, a
+barren area of more or less width must have bordered the retreating ice
+and formed an ideal source of loess.
+
+Several other lines of evidence seemingly support the conclusion that
+the loess was formed during the retreat of the ice. For example, Shimek,
+who has made almost a lifelong study of the Iowan loess, emphasizes the
+fact that there is often an accumulation of stones and pebbles at its
+base. This suggests that the underlying till was eroded before the loess
+was deposited upon it. The first reaction of most students is to assume
+that of course this was due to running water. That is possible in many
+cases, but by no means in all. So widespread a sheet of gravel could not
+be deposited by streams without destroying the irregular basins and
+hollows of which we have seen evidence where the loess lies on glacial
+deposits. On the other hand, the wind is competent to produce a similar
+gravel pavement without disturbing the old topography. "Desert
+pavements" are a notable feature in most deserts. On the edges of an ice
+sheet, as Hobbs has made us realize, the commonest winds are outward.
+They often attain a velocity of eighty miles an hour in Antarctica and
+Greenland. Such winds, however, usually decline rapidly in velocity only
+a few score miles from the ice. Thus their effect would be to produce
+rapid erosion of the freshly bared surface near the retreating ice. The
+pebbles would be left behind as a pavement, while sand and then loess
+would be deposited farther from the ice where the winds were weaker and
+where vegetation was beginning to take root. Such a decrease in wind
+velocity may explain the occasional vertical gradation from gravel
+through sand to coarse loess and then to normal fine loess. As the ice
+sheet retreated the wind in any given place would gradually become less
+violent. As the ice continued to retreat the area where loess was
+deposited would follow at a distance, and thus each part of the gravel
+pavement would in turn be covered with the loess.
+
+The hypothesis that loess is deposited while the ice is retreating is in
+accord with many other lines of evidence. For example, it accords with
+the boreal character of the mammal remains as described above. Again,
+the advance of vegetation into the barren zone along the front of the
+ice would be delayed by the strong outblowing winds. The common pioneer
+plants depend largely on the wind for the distribution of their seeds,
+but the glacial winds would carry them away from the ice rather than
+toward it. The glacial winds discourage the advance of vegetation in
+another way, for they are drying winds, as are almost all winds blowing
+from a colder to a warmer region. The fact that remains of trees
+sometimes occur at the bottom of the loess probably means that the
+deposition of loess extended into the forests which almost certainly
+persisted not far from the ice. This seems more likely than that a
+period of severe aridity before the advance of the ice killed the trees
+and made a steppe or desert. Penck's chief argument in favor of the
+formation of loess before the advance of the ice rather than after, is
+that since loess is lacking upon the youngest drift sheet in Europe it
+must have been formed before rather than after the last or Wuerm advance
+of the ice. This breaks down on two counts. First, on the corresponding
+(Wisconsin) drift sheet in America, loess is present,--in small
+quantities to be sure, but unmistakably present. Second, there is no
+reason to assume that conditions were identical at each advance and
+retreat of the ice. Indeed, the fact that in Europe, as in the United
+States, nearly all the loess was formed at one time, and only a little
+is associated with the other ice advances, points clearly against
+Penck's fundamental assumption that the accumulation of loess was due to
+the approach of a cold climate.
+
+Having seen that the loess was probably formed during the retreat of the
+ice, we are now ready to inquire what conditions the cyclonic hypothesis
+would postulate in the loess areas during the various stages of a
+glacial cycle. Fig. 2, in Chapter IV, gives the best idea of what would
+apparently happen in North America, and events in Europe would
+presumably be similar. During the nine maximum years on which Fig. 2 is
+based the sunspot numbers averaged seventy, while during the nine
+minimum years they averaged less than five. It seems fair to suppose
+that the maximum years represent the average conditions which prevailed
+in the past at times when the sun was in a median stage between the full
+activity which led to glaciation and the mild activity of the minimum
+years which appear to represent inter-glacial conditions. This would
+mean that when a glacial period was approaching, but before an ice sheet
+had accumulated to any great extent, a crescent-shaped strip from
+Montana through Illinois to Maine would suffer a diminution in
+storminess ranging up to 60 per cent as compared with inter-glacial
+conditions. This is in strong contrast with an increase in storminess
+amounting to 75 or even 100 per cent both in the boreal storm belt in
+Canada and in the subtropical belt in the Southwest. Such a decrease in
+storminess in the central United States would apparently be most
+noticeable in summer, as is shown in _Earth and Sun_. Hence it would
+have a maximum effect in producing aridity. This would favor the
+formation of loess, but it is doubtful whether the aridity would become
+extreme enough to explain such vast deposits as are found throughout
+large parts of the Mississippi Basin. That would demand that hundreds of
+thousands of square miles should become almost absolute desert, and it
+is not probable that any such thing occurred. Nevertheless, according to
+the cyclonic hypothesis the period immediately before the advent of the
+ice would be relatively dry in the central United States, and to that
+extent favorable to the work of the wind.
+
+As the climatic conditions became more severe and the ice sheet
+expanded, the dryness and lack of storms would apparently diminish. The
+reason, as has been explained, would be the gradual pushing of the
+storms southward by the high-pressure area which would develop over the
+ice sheet. Thus at the height of a glacial epoch there would apparently
+be great storminess in the area where the loess is found, especially in
+summer. Hence the cyclonic hypothesis does not accord with the idea of
+great deposition of loess at the time of maximum glaciation.
+
+Finally we come to the time when the ice was retreating. We have already
+seen that not only the river flood plains, but also vast areas of fresh
+glacial deposits would be exposed to the winds, and would remain without
+vegetation for a long time. At that very time the retreat of the ice
+sheet would tend to permit the storms to follow paths determined by the
+degree of solar activity, in place of the far southerly paths to which
+the high atmospheric pressure over the expanded ice sheet had previously
+forced them. In other words, the conditions shown in Fig. 2 would tend
+to reappear when the sun's activity was diminishing and the ice sheet
+was retreating, just as they had appeared when the sun was becoming more
+active and the ice sheet was advancing. This time, however, the
+semi-arid conditions arising from the scarcity of storms would prevail
+in a region of glacial deposits and widely spreading river deposits, few
+or none of which would be covered with vegetation. The conditions would
+be almost ideal for eolian erosion and for the transportation of loess
+by the wind to areas a little more remote from the ice where grassy
+vegetation had made a start.
+
+The cyclonic hypothesis also seems to offer a satisfactory explanation
+of variations in the amount of loess associated with the several glacial
+epochs. It attributes these to differences in the rate of disappearance
+of the ice, which in turn varied with the rate of decline of solar
+activity and storminess. This is supposed to be the reason why the Iowan
+loess deposits are much more extensive than those of the other epochs,
+for the Iowan ice sheet presumably accomplished part of its retreat much
+more suddenly than the other ice sheets.[57] The more sudden the
+retreat, the greater the barren area where the winds could gather fine
+bits of dust. Temporary readvances may also have been so distributed and
+of such intensity that they frequently accentuated the condition shown
+in Fig. 2, thus making the central United States dry soon after the
+exposure of great amounts of glacial debris. The closeness with which
+the cyclonic hypothesis accords with the facts as to the loess is one of
+the pleasant surprises of the hypothesis. The first draft of Fig. 2 and
+the first outlines of the hypothesis were framed without thought of the
+loess. Yet so far as can now be seen, both agree closely with the
+conditions of loess formation.
+
+FOOTNOTES:
+
+[Footnote 56: Chamberlin and Salisbury: Geology, 1906, Vol. III, pp.
+405-412.]
+
+[Footnote 57: It may have retreated soon after reaching its maximum. If
+so, the general lack of thick terminal moraines would be explained. See
+page 122.]
+
+
+
+
+CHAPTER X
+
+CAUSES OF MILD GEOLOGICAL CLIMATES
+
+
+In discussions of climate, as of most subjects, a peculiar psychological
+phenomenon is observable. Everyone sees the necessity of explaining
+conditions different from those that now exist, but few realize that
+present conditions may be abnormal, and that they need explanation just
+as much as do others. Because of this tendency glaciation has been
+discussed with the greatest fullness, while there has been much neglect
+not only of the periods when the climate of the earth resembled that of
+the present, but also of the vastly longer periods when it was even
+milder than now.
+
+How important the periods of mild climate have been in geological times
+may be judged from the relative length of glacial compared with
+inter-glacial epochs, and still more from the far greater relative
+length of the mild parts of periods and eras when compared with the
+severe parts. Recent estimates by R. T. Chamberlin[58] indicate that
+according to the consensus of opinion among geologists the average
+inter-glacial epoch during the Pleistocene was about five times as long
+as the average glacial epoch, while the whole of a given glacial epoch
+averaged five times as long as the period when the ice was at a maximum.
+Climatic periods far milder, longer, and more monotonous than any
+inter-glacial epoch appear repeatedly during the course of geological
+history. Our task in this chapter is to explain them.
+
+Knowlton[59] has done geology a great service by collecting the evidence
+as to the mild type of climate which has again and again prevailed in
+the past. He lays special stress on botanical evidence since that
+pertains to the variable atmosphere of the lands, and hence furnishes a
+better guide than does the evidence of animals that lived in the
+relatively unchanging water of the oceans. The nature of the evidence
+has already been indicated in various parts of this book. It includes
+palms, tree ferns, and a host of other plants which once grew in regions
+which are now much too cold to support them. With this must be placed
+the abundant reef-building corals and other warmth-loving marine
+creatures in latitudes now much too cold for them. Of a piece with this
+are the conditions of inter-glacial epochs in Europe, for example, when
+elephants and hippopotamuses, as well as many species of plants from low
+latitudes, were abundant. These conditions indicate not only that the
+climate was warmer than now, but that the contrast from season to season
+was much less. Indeed, Knowlton goes so far as to say that "relative
+uniformity, mildness, and comparative equability of climate, accompanied
+by high humidity, have prevailed over the greater part of the earth,
+extending to, or into, polar circles, during the greater part of
+geologic time--since, at least, the Middle Paleozoic. This is the
+regular, the ordinary, the normal condition." ... "By many it is thought
+that one of the strongest arguments against a gradually cooling globe
+and a humid, non-zonally disposed climate in the ages before the
+Pleistocene is the discovery of evidences of glacial action practically
+throughout the entire geologic column. Hardly less than a dozen of these
+are now known, ranging in age from Huronian to Eocene. It seems to be a
+very general assumption by those who hold this view that these evidences
+of glacial activities are to be classed as ice ages, largely comparable
+in effect and extent to the Pleistocene refrigeration, but as a matter
+of fact only three are apparently of a magnitude to warrant such
+designation. These are the Huronian glaciation, that of the
+'Permo-Carboniferous,' and that of the Pleistocene. The others, so far
+as available data go, appear to be explainable as more or less local
+manifestations that had no widespread effect on, for instance, ocean
+temperatures, distribution of life, et cetera. They might well have been
+of the type of ordinary mountain glaciers, due entirely to local
+elevation and precipitation." ... "If the sun had been the principal
+source of heat in pre-Pleistocene time, terrestrial temperatures would
+of necessity have been disposed in zones, whereas the whole trend of
+this paper has been the presentation of proof that these temperatures
+were distinctly non-zonal. Therefore it seems to follow that the sun--at
+least the present small-angle sun--could not have been the sole or even
+the principal source of heat that warmed the early oceans."
+
+Knowlton is so strongly impressed by the widespread fossil floras that
+usually occur in the middle parts of the geological periods, that as
+Schuchert[3] puts it, he neglects the evidence of other kinds. In the
+middle of the periods and eras the expansion of the warm oceans over the
+continents was greatest, while the lands were small and hence had more
+or less insular climates of the oceanic type. At such times, the marine
+fauna agrees with the flora in indicating a mild climate. Large
+colony-forming foraminifera, stony corals, shelled cephalopods,
+gastropods and thick-shelled bivalves, generally the cemented forms,
+were common in the Far North and even in the Arctic. This occurred in
+the Silurian, Devonian, Pennsylvanian, and Jurassic periods, yet at
+other times, such as the Cretaceous and Eocene, such forms were very
+greatly reduced in variety in the northern regions or else wholly
+absent. These things, as Schuchert[60] says, can only mean that Knowlton
+is right when he states that "climatic zoning such as we have had since
+the beginning of the Pleistocene did not obtain in the geologic ages
+prior to the Pleistocene." It does not mean, however, that there was a
+"non-zonal arrangement" and that the temperature of the oceans was
+everywhere the same and "without widespread effect on the distribution
+of life."
+
+Students of paleontology hold that as far back as we can go in the study
+of plants, there are evidences of seasons and of relatively cool
+climates in high latitudes. The cycads, for instance, are one of the
+types most often used as evidence of a warm climate. Yet Wieland,[61]
+who has made a lifelong study of these plants, says that many of them
+"might well grow in temperate to cool climates. Until far more is
+learned about them they should at least be held as valueless as indices
+of tropic climates." The inference is "that either they or their close
+relatives had the capacity to live in every clime. There is also a
+suspicion that study of the associated ferns may compel revision of the
+long-accepted view of the universality of tropic climates throughout the
+Mesozoic." Nathorst is quoted by Wieland as saying, "I think ... that
+during the time when the Gingkophytes and Cycadophytes dominated, many
+of them must have adapted themselves for living in cold climates also.
+Of this I have not the least doubt."
+
+Another important line of evidence which Knowlton and others have cited
+as a proof of the non-zonal arrangement of climate in the past, is the
+vast red beds which are found in the Proterozoic, late Silurian,
+Devonian, Permian, and Triassic, and in some Tertiary formations. These
+are believed to resemble laterite, a red and highly oxidized soil which
+is found in great abundance in equatorial regions. Knowlton does not
+attempt to show that the red beds present equatorial characteristics in
+other respects, but bases his conclusion on the statement that "red beds
+are not being formed at the present time in any desert region." This is
+certainly an error. As has already been said, in both the Transcaspian
+and Takla Makan deserts, the color of the sand regularly changes from
+brown on the borders to pale red far out in the desert. Kuzzil Kum, or
+Red Sand, is the native name. The sands in the center of the desert
+apparently were originally washed down from the same mountains as those
+on the borders, and time has turned them red. Since the same condition
+is reported from the Arabian Desert, it seems that redness is
+characteristic of some of the world's greatest deserts. Moreover, beds
+of salt and gypsum are regularly found in red beds, and they can
+scarcely originate except in deserts, or in shallow almost landlocked
+bays on the coasts of deserts, as appears to have happened in the
+Silurian where marine fossils are found interbedded with gypsum.
+
+Again, Knowlton says that red beds cannot indicate deserts because the
+plants found in them are not "pinched or depauperate, nor do they
+indicate xerophytic adaptations. Moreover, very considerable deposits of
+coal are found in red beds in many parts of the world, which implies the
+presence of swamps but little above sea-level."
+
+Students of desert botany are likely to doubt the force of these
+considerations. As MacDougal[62] has shown, the variety of plants in
+deserts is greater than in moist regions. Not only do xerophytic desert
+species prevail, but halophytes are present in the salty areas, and
+hygrophytes in the wet swampy areas, while ordinary mesophytes prevail
+along the water courses and are washed down from the mountains. The
+ordinary plants, not the xerophytes, are the ones that are chiefly
+preserved since they occur in most abundance near streams where
+deposition is taking place. So far as swamps are concerned, few are of
+larger size than those of Seistan in Persia, Lop Nor in Chinese
+Turkestan, and certain others in the midst of the Asiatic deserts.
+Streams flowing from the mountains into deserts are almost sure to form
+large swamps, such as those along the Tarim River in central Asia. Lake
+Chad in Africa is another example. In it, too, reeds are very numerous.
+
+Putting together the evidence on both sides in this disputed question,
+it appears that throughout most of geological time there is some
+evidence of a zonal arrangement of climate. The evidence takes the form
+of traces of cool climates, of seasons, and of deserts. Nevertheless,
+there is also strong evidence that these conditions were in general less
+intense than at present and that times of relatively warm, moist climate
+without great seasonal extremes have prevailed very widely during
+periods much longer than those when a zonal arrangement as marked as
+that of today prevailed. As Schuchert[63] puts it: "Today the variation
+on land between the tropics and the poles is roughly between 110 deg. and
+-60 deg.F., in the oceans between 85 deg. and 31 deg.F. In the geologic past the
+temperature of the oceans for the greater parts of the periods probably
+was most often between 85 deg. and 55 deg.F., while on land it may have varied
+between 90 deg. and 0 deg.F. At rare intervals the extremes were undoubtedly as
+great as they are today. The conclusion is therefore that at all times
+the earth had temperature zones, varying between the present-day
+intensity and times which were almost without such belts, and at these
+latter times the greater part of the earth had an almost uniformly mild
+climate, without winters."
+
+It is these mild climates which we must now attempt to explain. This
+leads us to inquire what would happen to the climate of the earth as a
+whole if the conditions which now prevail at times of few sunspots were
+to become intensified. That they could become greatly intensified seems
+highly probable, for there is good reason to think that aside from the
+sunspot cycle the sun's atmosphere is in a disturbed condition. The
+prominences which sometimes shoot out hundreds of thousands of miles
+seem to be good evidence of this. Suppose that the sun's atmosphere
+should become very quiet. This would apparently mean that cyclonic
+storms would be much less numerous and less severe than during the
+present times of sunspot minima. The storms would also apparently follow
+paths in middle latitudes somewhat as they do now when sunspots are
+fewest. The first effect of such a condition, if we can judge from what
+happens at present, would be a rise in the general temperature of the
+earth, because less heat would be carried aloft by storms. Today, as is
+shown in _Earth and Sun_, a difference of perhaps 10 per cent in the
+average storminess during periods of sunspot maxima and minima is
+correlated with a difference of 3 deg.C. in the temperature at the earth's
+surface. This includes not only an actual lowering of 0.6 deg.C. at times of
+sunspot maxima, but the overcoming of the effect of increased insolation
+at such times, an effect which Abbot calculates as about 2.5 deg.C. If the
+storminess were to be reduced to one-half or one-quarter its present
+amount at sunspot minima, not only would the loss of heat by upward
+convection in storms be diminished, but the area covered by clouds would
+diminish so that the sun would have more chance to warm the lower air.
+Hence the average rise of temperature might amount to as much at 5 deg. or
+10 deg.C.
+
+Another effect of the decrease in storminess would be to make the
+so-called westerly winds, which are chiefly southwesterly in the
+northern hemisphere and northwesterly in the southern hemisphere, more
+strong and steady than at present. They would not continually suffer
+interruption by cyclonic winds from other directions, as is now the
+case, and would have a regularity like that of the trades. This
+conclusion is strongly reenforced in a paper by Clayton[64] which came
+to hand after this chapter had been completed. From his studies of the
+solar constant and the temperature of the earth which are described in
+_Earth and Sun_, he reaches the following conclusion: "The results of
+these researches have led me to believe: 1. That if there were no
+variation in solar radiation the atmospheric motions would establish a
+stable system with exchanges of air between equator and pole and between
+ocean and land, in which the only variations would be daily and annual
+changes set in operation by the relative motions of the earth and sun.
+2. The existing abnormal changes, which we call weather, have their
+origins chiefly, if not entirely, in the variations of solar radiation."
+
+If cyclonic storms and "weather" were largely eliminated and if the
+planetary system of winds with its steady trades and southwesterlies
+became everywhere dominant, the regularity and volume of the
+poleward-flowing currents, such as the Gulf Stream and the Atlantic
+Drift in one ocean, and the Japanese Current in another, would be
+greatly increased. How important this is may be judged from the work of
+Helland-Hansen and Nansen.[65] These authors find that with the passage
+of each cyclonic storm there is a change in the temperature of the
+surface water of the Atlantic Ocean. Winds at right angles to the course
+of the Drift drive the water first in one direction and then in the
+other but do not advance it in its course. Winds with an easterly
+component, on the other hand, not only check the Drift but reverse it,
+driving the warm water back toward the southwest and allowing cold water
+to well up in its stead. The driving force in the Atlantic Drift is
+merely the excess of the winds with a westerly component over those with
+an easterly component.
+
+Suppose that the numbers in Fig. 8 represent the strength of the winds
+in a certain part of the North Atlantic or North Pacific, that is, the
+total number of miles moved by the air per year. In quadrant A of the
+left-hand part all the winds move from a more or less southwesterly
+direction and produce a total movement of the air amounting to thirty
+units per year. Those coming from points between north and west move
+twenty-five units; those between north and east, twenty units; and those
+between east and south, twenty-five units. Since the movement of the
+winds in quadrants B and D is the same, these winds have no effect in
+producing currents. They merely move the water back and forth, and thus
+give it time to lose whatever heat it has brought from more southerly
+latitudes. On the other hand, since the easterly winds in quadrant C do
+not wholly check the currents caused by the westerly winds of quadrant
+A, the effective force of the westerly winds amounts to ten, or the
+difference between a force of thirty in quadrant A and of twenty in
+quadrant C. Hence the water is moved forward toward the northeast, as
+shown by the thick part of arrow A.
+
+[Illustration: _Fig. 8. Effect of diminution of storms on movement of
+water._]
+
+Now suppose that cyclonic storms should be greatly reduced in number so
+that in the zone of prevailing westerlies they were scarcely more
+numerous than tropical hurricanes now are in the trade-wind belt. Then
+the more or less southwesterly winds in quadrant A' in the right-hand
+part of Fig. 8 would not only become more frequent but would be stronger
+than at present. The total movement from that quarter might rise to
+sixty units, as indicated in the figure. In quadrants B' and D' the
+movement would fall to fifteen and in quadrant C' to ten. B' and D'
+would balance one another as before. The movement in A', however, would
+exceed that in C' by fifty instead of ten. In other words, the
+current-making force would become five times as great as now. The actual
+effect would be increased still more, for the winds from the southwest
+would be stronger as well as steadier if there were no storms. A strong
+wind which causes whitecaps has much more power to drive the water
+forward than a weaker wind which does not cause whitecaps. In a wave
+without a whitecap the water returns to practically the original point
+after completing a circle beneath the surface. In a wave with a
+whitecap, however, the cap moves forward. Any increase in velocity
+beyond the rate at which whitecaps are formed has a great influence upon
+the amount of water which is blown forward. Several times as much water
+is drifted forward by a persistent wind of twenty miles an hour as by a
+ten-mile wind.[66]
+
+In this connection a suggestion which is elaborated in Chapter XIII may
+be mentioned. At present the salinity of the oceans checks the general
+deep-sea circulation and thereby increases the contrasts from zone to
+zone. In the past, however, the ocean must have been fresher than now.
+Hence the circulation was presumably less impeded, and the transfer of
+heat from low latitudes to high was facilitated.
+
+Consider now the magnitude of the probable effect of a diminution in
+storms. Today off the coast of Norway in latitude 65 deg.N. and longitude
+10 deg.E., the mean temperature in January is 2 deg.C. and in July 12 deg.C. This
+represents a plus anomaly of about 22 deg. in January and 2 deg. in July; that
+is, the Norwegian coast is warmer than the normal for its latitude by
+these amounts. Suppose that in some past time the present distribution
+of lands and seas prevailed, but Norway was a lowland where extensive
+deposits could accumulate in great flood plains. Suppose, also, that the
+sun's atmosphere was so inactive that few cyclonic storms occurred,
+steady winds from the west-southwest prevailed, and strong,
+uninterrupted ocean currents brought from the Caribbean Sea and Gulf of
+Mexico much greater supplies of warm water than at present. The
+Norwegian winters would then be warmer than now not only because of the
+general increase in temperature which the earth regularly experiences at
+sunspot minima, but because the currents would accentuate this
+condition. In summer similar conditions would prevail except that the
+warming effect of the winds and currents would presumably be less than
+in winter, but this might be more than balanced by the increased heat of
+the sun during the long summer days, for storms and clouds would be
+rare.
+
+If such conditions raised the winter temperature only 8 deg.C. and the
+summer temperature 4 deg.C., the climate would be as warm as that of the
+northern island of New Zealand (latitude 35 deg.-43 deg.S.). The flora of that
+part of New Zealand is subtropical and includes not only pines and
+beeches, but palms and tree ferns. A climate scarcely warmer than that
+of New Zealand would foster a flora like that which existed in far
+northern latitudes during some of the milder geological periods. If,
+however, the general temperature of the earth's surface were raised 5 deg.
+because of the scarcity of storms, if the currents were strong enough so
+that they increased the present anomaly by 50 per cent, and if more
+persistent sunshine in summer raised the temperature at that season
+about 4 deg.C., the January temperature would be 18 deg.C. and the July
+temperature 22 deg.C. These figures perhaps make summer and winter more
+nearly alike than was ever really the case in such latitudes.
+Nevertheless, they show that a diminution of storms and a consequent
+strengthening and steadying of the southwesterlies might easily raise
+the temperature of the Norwegian coast so high that corals could
+flourish within the Arctic Circle.
+
+Another factor would cooeperate in producing mild temperatures in high
+latitudes during the winter, namely, the fogs which would presumably
+accumulate. It is well known that when saturated air from a warm ocean
+is blown over the lands in winter, as happens so often in the British
+Islands and around the North Sea, fog is formed. The effect of such a
+fog is indeed to shut out the sun's radiation, but in high latitudes
+during the winter when the sun is low, this is of little importance.
+Another effect is to retain the heat of the earth itself. When a
+constant supply of warm water is being brought from low latitudes this
+blanketing of the heat by the fog becomes of great importance. In the
+past, whenever cyclonic storms were weak and westerly winds were
+correspondingly strong, winter fogs in high latitudes must have been
+much more widespread and persistent than now.
+
+The bearing of fogs on vegetation is another interesting point. If a
+region in high latitudes is constantly protected by fog in winter, it
+can support types of vegetation characteristic of fairly low latitudes,
+for plants are oftener killed by dry cold than by moist cold. Indeed,
+excessive evaporation from the plant induced by dry cold when the
+evaporated water cannot be rapidly replaced by the movement of sap is a
+chief reason why large plants are winterkilled. The growing of
+transplanted palms on the coast of southwestern Ireland, in spite of its
+location in latitude 50 deg.N., is possible only because of the great
+fogginess in winter due to the marine climate. The fogs prevent the
+escape of heat and ward off killing frosts. The tree ferns in latitude
+46 deg.S. in New Zealand, already referred to, are often similarly protected
+in winter. Therefore, the relative frequency of fogs in high latitudes
+when storms were at a minimum would apparently tend not merely to
+produce mild winters but to promote tropical vegetation.
+
+The strong steady trades and southwesterlies which would prevail at
+times of slight solar activity, according to our hypothesis, would have
+a pronounced effect on the water of the deep seas as well as upon that
+of the surface. In the first place, the deep-sea circulation would be
+hastened. For convenience let us speak of the northern hemisphere. In
+the past, whenever the southwesterly winds were steadier than now, as
+was probably the case when cyclonic storms were relatively rare, more
+surface water than at present was presumably driven from low latitudes
+and carried to high latitudes. This, of course, means that a greater
+volume of water had to flow back toward the equator in the lower parts
+of the ocean, or else as a cool surface current. The steady
+southwesterly winds, however, would interfere with south-flowing surface
+currents, thus compelling the polar waters to find their way equatorward
+beneath the surface. In low latitudes the polar waters would rise and
+their tendency would be to lower the temperature. Hence steadier
+westerlies would make for lessened latitudinal contrasts in climate not
+only by driving more warm water poleward but by causing more polar water
+to reach low latitudes.
+
+At this point a second important consideration must be faced. Not only
+would the deep-sea circulation be hastened, but the ocean depths might
+be warmed. The deep parts of the ocean are today cold because they
+receive their water from high latitudes where it sinks because of low
+temperature. Suppose, however, that a diminution in storminess combined
+with other conditions should permit corals to grow in latitude 70 deg.N. The
+ocean temperature would then have to average scarcely lower than 20 deg.C.
+and even in the coldest month the water could scarcely fall below about
+15 deg.C. Under such conditions, if the polar ocean were freely connected
+with the rest of the oceans, no part of it would probably have a
+temperature much below 10 deg.C., for there would be no such thing as ice
+caps and snowfields to reflect the scanty sunlight and radiate into
+space what little heat there was. On the contrary, during the winter an
+almost constant state of dense fogginess would prevail. So great would
+be the blanketing effect of this that a minimum monthly temperature of
+10 deg.C. for the coldest part of the ocean may perhaps be too low for a
+time when corals thrived in latitude 70 deg..
+
+The temperature of the ocean depths cannot permanently remain lower than
+that of the coldest parts of the surface. Temporarily this might indeed
+happen when a solar change first reduced the storminess and strengthened
+the westerlies and the surface currents. Gradually, however, the
+persistent deep-sea circulation would bring up the colder water in low
+latitudes and carry downward the water of medium temperature at the
+coldest part of the surface. Thus in time the whole body of the ocean
+would become warm. The heat which at present is carried away from the
+earth's surface in storms would slowly accumulate in the oceans. As the
+process went on, all parts of the ocean's surface would become warmer,
+for equatorial latitudes would be less and less cooled by cold water
+from below, while the water blown from low latitudes to high would be
+correspondingly warmer. The warming of the ocean would come to an end
+only with the attainment of a state of equilibrium in which the loss of
+heat by radiation and evaporation from the ocean's surface equaled the
+loss which under other circumstances would arise from the rise of warm
+air in cyclonic storms. When once the oceans were warmed, they would
+form an extremely strong conservative force tending to preserve an
+equable climate in all latitudes and at all seasons. According to the
+solar cyclonic hypothesis such conditions ought to have prevailed
+throughout most of geological time. Only after a strong and prolonged
+solar disturbance with its consequent storminess would conditions like
+those of today be expected.
+
+In this connection another possibility may be mentioned. It is commonly
+assumed that the earth's axis is held steadily in one direction by the
+fact that the rotating earth is a great gyroscope. Having been tilted to
+a certain position, perhaps by some extraneous force, the axis is
+supposed to maintain that position until some other force intervenes.
+Cordeiro,[67] however, maintains that this is true only of an absolutely
+rigid gyroscope. He believes that it is mathematically demonstrable that
+if an elastic gyroscope be gradually tilted by some extraneous force,
+and if that force then ceases to act, the gyroscope as a whole will
+oscillate back and forth. The earth appears to be slightly elastic.
+Cordeiro therefore applies his formulae to it, on the following
+assumptions: (1) That the original position of the axis was nearly
+vertical to the plane of the ecliptic in which the earth revolves around
+the sun; (2) that at certain times the inclination has been even greater
+than now; and (3) that the position of the axis with reference to the
+earth has not changed to any great extent, that is, the earth's poles
+have remained essentially stationary with reference to the earth,
+although the whole earth has been gyroscopically tilted back and forth
+repeatedly.
+
+With a vertical axis the daylight and darkness in all parts of the earth
+would be of equal duration, being always twelve hours. There would be no
+seasons, and the climate would approach the average condition now
+experienced at the two equinoxes. On the whole the climate of high
+latitudes would give the impression of being milder than now, for there
+would be less opportunity for the accumulation of snow and ice with
+their strong cooling effect. On the other hand, if the axis were tilted
+more than now, the winter nights would be longer and the winters more
+severe than at present, and there would be a tendency toward glaciation.
+Thus Cordeiro accounts for alternating mild and glacial epochs. The
+entire swing from the vertical position to the maximum inclination and
+back to the vertical may last millions of years depending on the earth's
+degree of elasticity. The swing beyond the vertical position in the
+other direction would be equally prolonged. Since the axis is now
+supposed to be much nearer its maximum than its minimum degree of
+tilting, the duration of epochs having a climate more severe than that
+of the present would be relatively short, while the mild epochs would be
+long.
+
+Cordeiro's hypothesis has been almost completely ignored. One reason is
+that his treatment of geological facts, and especially his method of
+riding rough-shod over widely accepted conclusions, has not commended
+his work to geologists. Therefore they have not deemed it worth while to
+urge mathematicians to test the assumptions and methods by which he
+reached his results. It is perhaps unfair to test Cordeiro by geology,
+for he lays no claim to being a geologist. In mathematics he labors
+under the disadvantage of having worked outside the usual professional
+channels, so that his work does not seem to have been subjected to
+sufficiently critical analysis.
+
+Without expressing any opinion as to the value of Cordeiro's results we
+feel that the subject of the earth's gyroscopic motion and of a possible
+secular change in the direction of the axis deserves investigation for
+two chief reasons. In the first place, evidences of seasonal changes and
+of seasonal uniformity seem to occur more or less alternately in the
+geological record. Second, the remarkable discoveries of Garner and
+Allard[68] show that the duration of daylight has a pronounced effect
+upon the reproduction of plants. We have referred repeatedly to the tree
+ferns, corals, and other forms of life which now live in relatively low
+latitudes and which cannot endure strong seasonal contrasts, but which
+once lived far to the north. On the other hand, Sayles,[69] for example,
+finds that microscopical examination of the banding of ancient shales
+and slates indicates distinct seasonal banding like that of recent
+Pleistocene clays or of the Squantum slate formed during or near the
+Permian glacial period. Such seasonal banding is found in rocks of
+various ages: (a) Huronian, in cobalt shales previously reported by
+Coleman; (b) late Proterozoic or early Cambrian in Hiwassee slate; (c)
+lower Cambrian, in Georgian slates of Vermont; (d) lower Ordovician, in
+Georgia (Rockmart slate), Tennessee (Athens shale), Vermont (slates),
+and Quebec (Beekmantown formation); and (e) Permian in Massachusetts
+(Squantum slate). How far the periods during which such evidence of
+seasons was recorded really alternated with mild periods, when tropical
+species lived in high latitudes and the contrast of seasons was almost
+or wholly lacking, we have as yet no means of knowing. If periods
+characterized by marked seasonal changes should be found to have
+alternated with those when the seasons were of little importance, the
+fact would be of great geological significance.
+
+The discoveries of Garner and Allard as to the effect of light on
+reproduction began with a peculiar tobacco plant which appeared in some
+experiments at Washington. The plant grew to unusual size, and seemed to
+promise a valuable new variety. It formed no seeds, however, before the
+approach of cold weather. It was therefore removed to a greenhouse where
+it flowered and produced seed. In succeeding years the flowering was
+likewise delayed till early winter, but finally it was discovered that
+if small plants were started in the greenhouse in the early fall they
+flowered at the same time as the large ones. Experiments soon
+demonstrated that the time of flowering depends largely upon the length
+of the daily period when the plants are exposed to light. The same is
+true of many other plants, and there is great variety in the conditions
+which lead to flowering. Some plants, such as witch hazel, appear to be
+stimulated to bloom by very short days, while others, such as evening
+primrose, appear to require relatively long days. So sensitive are
+plants in this respect that Garner and Allard, by changing the length of
+the period of light, have caused a flowerbud in its early stages not
+only to stop developing but to return once more to a vegetative shoot.
+
+    Common iris, which flowers in May and June, will not blossom under
+    ordinary conditions when grown in the greenhouse in winter, even
+    under the same temperature conditions that prevail in early summer.
+    Again, one variety of soy beans will regularly begin to flower in
+    June of each year, a second variety in July, and a third in August,
+    when all are planted on the same date. There are no temperature
+    differences during the summer months which could explain these
+    differences in time of flowering; and, since "internal causes" alone
+    cannot be accepted as furnishing a satisfactory explanation, some
+    external factor other than temperature must be responsible.
+
+    The ordinary varieties of cosmos regularly flower in the fall in
+    northern latitudes if they are planted in the spring or summer. If
+    grown in a warm greenhouse during the winter months the plants also
+    flower readily, so that the cooler weather of fall is not a
+    necessary condition. If successive plantings of cosmos are made in
+    the greenhouse during the late winter and early spring months,
+    maintaining a uniform temperature throughout, the plantings made
+    after a certain date will fail to blossom promptly, but, on the
+    contrary, will continue to grow till the following fall, thus
+    flowering at the usual season for this species. This curious
+    reversal of behavior with advance of the season cannot be attributed
+    to change in temperature. Some other factor is responsible for the
+    failure of cosmos to blossom during the summer months. In this
+    respect the behavior of cosmos is just the opposite of that observed
+    in iris.
+
+    Certain varieties of soy beans change their behavior in a peculiar
+    manner with advance of the summer season. The variety known as
+    Biloxi, for example, when planted early in the spring in the
+    latitude of Washington, D. C., continues to grow throughout the
+    summer, flowering in September. The plants maintain growth without
+    flowering for fifteen to eighteen weeks, attaining a height of five
+    feet or more. As the dates of successive plantings are moved forward
+    through the months of June and July, however there is a marked
+    tendency for the plants to cut short the period of growth which
+    precedes flowering. This means, of course, that there is a tendency
+    to flower at approximately the same time of year regardless of the
+    date of planting. As a necessary consequence, the size of the plants
+    at the time of flowering is reduced in proportion to the delay in
+    planting.
+
+The bearing of this on geological problems lies in a query which it
+raises as to the ability of a genus or family of plants to adapt itself
+to days of very different length from those to which it is wonted. Could
+tree ferns, ginkgos, cycads, and other plants whose usual range of
+location never subjects them to daylight for more than perhaps fourteen
+hours or less than ten, thrive and reproduce themselves if subjected to
+periods of daylight ranging all the way from nothing up to about
+twenty-four hours? No answer to this is yet possible, but the question
+raises most interesting opportunities of investigation. If Cordeiro is
+right as to the earth's elastic gyroscopic motion, there may have been
+certain periods when a vertical or almost vertical axis permitted the
+days to be of almost equal length at all seasons in all latitudes. If
+such an absence of seasons occurred when the lands were low, when the
+oceans were extensive and widely open toward the poles, and when storms
+were relatively inactive, the result might be great mildness of climate
+such as appears sometimes to have prevailed in the middle of geological
+eras. Suppose on the other hand that the axis should be tilted more than
+now, and that the lands should be widely emergent and the storm belt
+highly active in low latitudes, perhaps because of the activity of the
+sun. The conditions might be favorable for glaciation at latitudes as
+low as those where the Permo-Carboniferous ice sheets appear to have
+centered. The possibilities thus suggested by Cordeiro's hypothesis are
+so interesting that the gyroscopic motion of the earth ought to be
+investigated more thoroughly. Even if no such gyroscopic motion takes
+place, however, the other causes of mild climate discussed in this
+chapter may be enough to explain all the observed phenomena.
+
+Many important biological consequences might be drawn from this study of
+mild geological climates, but this book is not the place for them. In
+the first chapter we saw that one of the most remarkable features of the
+climate of the earth is its wonderful uniformity through hundreds of
+millions of years. As we come down through the vista of years the mild
+geological periods appear to represent a return as nearly as possible to
+this standard condition of uniformity. Certain changes of the earth
+itself, as we shall see in the next chapter, may in the long run tend
+slightly to change the exact conditions of this climatic standard, as we
+might perhaps call it. Yet they act so slowly that their effect during
+hundreds of millions of years is still open to question. At most they
+seem merely to have produced a slight increase in diversity from season
+to season and from zone to zone. The normal climate appears still to be
+of a milder type than that which happens to prevail at present. Some
+solar condition, whose possible nature will be discussed later, seems
+even now to cause the number of cyclonic storms to be greater than
+normal. Hence the earth's climate still shows something of the great
+diversity of seasons and of zones which is so marked a characteristic of
+glacial epochs.
+
+FOOTNOTES:
+
+[Footnote 58: Rollin T. Chamberlin: Personal Communication.]
+
+[Footnote 59: F. H. Knowlton: Evolution of Geologic Climates; Bull.
+Geol. Soc. Am., Vol. 30, 1919, pp. 499-566.]
+
+[Footnote 60: Chas. Schuchert: Review of Knowlton's Evolution of
+Geological Climates, in Am. Jour. Sci., 1921.]
+
+[Footnote 61: G. R. Wieland: Distribution and Relationships of the
+Cycadeoids; Am. Jour. Bot., Vol. 7, 1920, pp. 125-145.]
+
+[Footnote 62: D. T. MacDougal: Botanical Features of North American
+Deserts; Carnegie Instit. of Wash., No. 99, 1908.]
+
+[Footnote 63: _Loc. cit._]
+
+[Footnote 64: H. H. Clayton: Variation in Solar Radiation and the
+Weather; Smiths. Misc. Coll., Vol. 71, No. 3, Washington, 1920.]
+
+[Footnote 65: B. Helland Hansen and F. Nansen: Temperature Variations in
+the North Atlantic Ocean and in the Atmosphere; Misc. Coll., Smiths.
+Inst., Vol. 70, No. 4, Washington, 1920.]
+
+[Footnote 66: The climatic significance of ocean currents is well
+discussed in Croll's Climate and Time, 1875, and his Climate and
+Cosmogony, 1889.]
+
+[Footnote 67: F. J. B. Cordeiro: The Gyroscope, 1913.]
+
+[Footnote 68: W. W. Garner and H. A. Allard: Flowering and Fruition of
+Plants as Controlled by Length of Day; Yearbook Dept. Agri., 1920, pp.
+377-400.]
+
+[Footnote 69: Report of Committee on Sedimentation, National Research
+Council, April, 1922.]
+
+
+
+
+CHAPTER XI
+
+TERRESTRIAL CAUSES OF CLIMATIC CHANGES
+
+
+The major portion of this book has been concerned with the explanation
+of the more abrupt and extreme changes of climate. This chapter and the
+next consider two other sorts of climatic changes, the slight secular
+progression during the hundreds of millions of years of recorded earth
+history, and especially the long slow geologic oscillations of millions
+or tens of millions of years. It is generally agreed among geologists
+that the progressive change has tended toward greater extremes of
+climate; that is, greater seasonal contrasts, and greater contrasts from
+place to place and from zone to zone.[70] The slow cyclic changes have
+been those that favored widespread glaciation at one extreme near the
+ends of geologic periods and eras, and mild temperatures even in
+subpolar regions at the other extreme during the medial portions of the
+periods.
+
+As has been pointed out in an earlier chapter, it has often been assumed
+that all climatic changes are due to terrestrial causes. We have seen,
+however, that there is strong evidence that solar variations play a
+large part in modifying the earth's climate. We have also seen that no
+known terrestrial agency appears to be able to produce the abrupt
+changes noted in recent years, the longer cycles of historical times, or
+geological changes of the shorter type, such as glaciation.
+Nevertheless, terrestrial changes doubtless have assisted in producing
+both the progressive change and the slow cyclic changes recorded in the
+rocks, and it is the purpose of this chapter and the two that follow to
+consider what terrestrial changes have taken place and the probable
+effect of such changes.
+
+The terrestrial changes that have a climatic significance are numerous.
+Some, such as variations in the amount of volcanic dust in the higher
+air, have been considered in an earlier chapter. Others are too
+imperfectly known to warrant discussion, and in addition there are
+presumably others which are entirely unknown. Doubtless some of these
+little known or unknown changes have been of importance in modifying
+climate. For example, the climatic influence of vegetation, animals, and
+man may be appreciable. Here, however, we shall confine ourselves to
+purely physical causes, which will be treated in the following order:
+First, those concerned with the solid parts of the earth, namely: (I)
+amount of land; (II) distribution of land; (III) height of land; (IV)
+lava flows; and (V) internal heat. Second, those which arise from the
+salinity of oceans, and third, those depending on the composition and
+amount of atmosphere.
+
+The terrestrial change which appears indirectly to have caused the
+greatest change in climate is the contraction of the earth. The problem
+of contraction is highly complex and is as yet only imperfectly
+understood. Since only its results and not its processes influence
+climate, the following section as far as page 196 is not necessary to
+the general reader. It is inserted in order to explain why we assume
+that there have been oscillations between certain types of distribution
+of the lands.
+
+The extent of the earth's contraction may be judged from the shrinkage
+indicated by the shortening of the rock formations in folded mountains
+such as the Alps, Juras, Appalachians, and Caucasus. Geologists are
+continually discovering new evidence of thrust faults of great magnitude
+where masses of rock are thrust bodily over other rocks, sometimes for
+many miles. Therefore, the estimates of the amount of shrinkage based on
+the measurements of folds and faults need constant revision upward.
+Nevertheless, they have already reached a considerable figure. For
+example, in 1919, Professor A. Heim estimated the shortening of the
+meridian passing through the modern Alps and the ancient Hercynian and
+Caledonian mountains as fully a thousand miles in Europe, and over five
+hundred miles for the rest of this meridian.[71] This is a radial
+shortening of about 250 miles. Possibly the shrinkage has been even
+greater than this. Chamberlin[72] has compared the density of the earth,
+moon, Mars, and Venus with one another, and found it probable that the
+radial shrinkage of the earth may be as much as 570 miles. This result
+is not so different from Heim's as appears at first sight, for Heim made
+no allowance for unrecognized thrust faults and for the contraction
+incident to metamorphism. Moreover, Heim did not include shrinkage
+during the first half of geological time before the above-mentioned
+mountain systems were upheaved.
+
+According to a well-established law of physics, contraction of a
+rotating body results in more rapid rotation and greater centrifugal
+force. These conditions must increase the earth's equatorial bulge and
+thereby cause changes in the distribution of land and water. Opposed to
+the rearrangement of the land due to increased rotation caused by
+contraction, there has presumably been another rearrangement due to
+tidal retardation of the earth's rotation and a consequent lessening of
+the equatorial bulge. G. H. Darwin long ago deduced a relatively large
+retardation due to lunar tides. A few years ago W. D. MacMillan, on
+other assumptions, deduced only a negligible retardation. Still more
+recently Taylor[73] has studied the tides of the Irish Sea, and his work
+has led Jeffreys[74] and Brown[75] to conclude that there has been
+considerable retardation, perhaps enough, according to Brown, to equal
+the acceleration due to the earth's contraction. From a prolonged and
+exhaustive study of the motions of the moon Brown concludes that tidal
+friction or some other cause is now lengthening the day at the rate of
+one second per thousand years, or an hour in almost four million years
+if the present rate continues. He makes it clear that the retardation
+due to tides would not correspond in point of time with the acceleration
+due to contraction. The retardation would occur slowly, and would take
+place chiefly during the long quiet periods of geologic history, while
+the acceleration would occur rapidly at times of diastrophic
+deformation. As a consequence, the equatorial bulge would alternately be
+reduced at a slow rate, and then somewhat suddenly augmented.
+
+The less rigid any part of the earth is, the more quickly it responds to
+the forces which lead to bulging or which tend to lessen the bulge.
+Since water is more fluid than land, the contraction of the earth and
+the tidal retardation presumably tend alternately to increase and
+decrease the amount of water near the equator more than the amount of
+land. Thus, throughout geological history we should look for cyclic
+changes in the relative area of the lands within the tropics and similar
+changes of opposite phase in higher latitudes. The extent of the change
+would depend upon (a) the amount of alteration in the speed of rotation,
+and (b) the extent of low land in low latitudes and of shallow sea in
+high latitudes. According to Slichter's tables, if the earth should
+rotate in twenty-three hours instead of twenty-four, the great Amazon
+lowland would be submerged by the inflow of oceanic water, while wide
+areas in Hudson Bay, the North Sea, and other northern regions, would
+become land because the ocean water would flow away from them.[76]
+
+Following the prompt equatorward movement of water which would occur as
+the speed of rotation increased, there must also be a gradual movement
+or creepage of the solid rocks toward the equator, that is, a bulging of
+the ocean floor and of the lands in low latitudes, with a consequent
+emergence of the lands there and a relative rise of sea level in higher
+latitudes. Tidal retardation would have a similar effect. Suess[77] has
+described widespread elevated strand lines in the tropics which he
+interprets as indicating a relatively sudden change in sea level, though
+he does not suggest a cause of the change. However, in speaking of
+recent geological times, Suess reports that a movement more recent than
+the old strands "was an accumulation of water toward the equator, a
+diminution toward the poles, and (it appears) as though this last
+movement were only one of the many oscillations which succeed each other
+with the same tendency, i.e., with a positive excess at the equator, a
+negative excess at the poles." (Vol. II, p. 551.) This creepage of the
+rocks equatorward seemingly might favor the growth of mountains in
+tropical and subtropical regions, because it is highly improbable that
+the increase in the bulge would go on in all longitudes with perfect
+uniformity. Where it went on most rapidly mountains would arise. That
+such irregularity of movement has actually occurred is suggested not
+only by the fact that many Cenozoic and older mountain ranges extend
+east and west, but by the further fact that these include some of our
+greatest ranges, many of which are in fairly low latitudes. The
+Himalayas, the Javanese ranges, and the half-submerged Caribbean chains
+are examples. Such mountains suggest a thrust in a north and south
+direction which is just what would happen if the solid mass of the earth
+were creeping first equatorward and then poleward.
+
+A fact which is in accord with the idea of a periodic increase in the
+oceans in low latitudes because of renewed bulging at the equator is the
+exposure in moderately high latitudes of the greatest extent of ancient
+rocks. This seems to mean that in low latitudes the frequent deepening
+of the oceans has caused the old rocks to be largely covered by
+sediments, while the old lands in higher latitudes have been left more
+fully exposed to erosion.
+
+Another suggestion of such periodic equatorward movements of the ocean
+water is found in the reported contrast between the relative stability
+with which the northern part of North America has remained slightly
+above sea level except at times of widespread submergence, while the
+southern parts have suffered repeated submergence alternating with great
+emergence.[78] Furthermore, although the northern part of North America
+has been generally exposed to erosion since the Proterozoic, it has
+supplied much less sediment than have the more southern land areas.[79]
+This apparently means that much of Canada has stood relatively low,
+while repeated and profound uplift alternating with depression has
+occurred in subtropical latitudes, apparently in adjustment to changes
+in the earth's speed of rotation. The uplifts generally followed the
+times of submergence due to equatorward movement of the water, though
+the buckling of the crust which accompanies shrinkage doubtless caused
+some of the submergence. The evidence that northern North America stood
+relatively low throughout much of geological time depends not only on
+the fact that little sediment came to the south from the north, but also
+on the fact that at times of especially widespread epicontinental seas,
+the submergence was initiated at the north.[80] This is especially true
+for Ordovician, Silurian, Devonian, and Jurassic times in North America.
+General submergence of this kind is supposed to be due chiefly to the
+overflowing of the ocean when its level is slowly raised by the
+deposition of sediment derived from the erosion of what once were
+continental highlands but later are peneplains. The fact that such
+submergence began in high latitudes, however, seems to need a further
+explanation. The bulging of the rock sphere at the equator and the
+consequent displacement of some of the water in low latitudes would
+furnish such an explanation, as would also a decrease in the speed of
+rotation induced by tidal retardation, if that retardation were great
+enough and rapid enough to be geologically effective.
+
+The climatic effects of the earth's contraction, which we shall shortly
+discuss, are greatly complicated by the fact that contraction has taken
+place irregularly. Such irregularity has occurred in spite of the fact
+that the processes which cause contraction have probably gone on quite
+steadily throughout geological history. These processes include the
+chemical reorganization of the minerals of the crust, a process which is
+illustrated by the metamorphism of sedimentary rocks into crystalline
+forms. The escape of gases through volcanic action or otherwise has been
+another important process.
+
+Although the processes which cause contraction probably go on steadily,
+their effect, as Chamberlin[81] and others have pointed out, is probably
+delayed by inertia. Thus the settling of the crust or its movement on a
+large scale is delayed. Perhaps the delay continues until the stresses
+become so great that of themselves they overcome the inertia, or
+possibly some outside agency, whose nature we shall consider later,
+reenforces the stresses and gives the slight impulse which is enough to
+release them and allow the earth's crust to settle into a new state of
+equilibrium. When contraction proceeds actively, the ocean segments,
+being largest and heaviest, are likely to settle most, resulting in a
+deepening of the oceans and an emergence of the lands. Following each
+considerable contraction there would be an increase in the speed of
+rotation. The repeated contractions with consequent growth of the
+equatorial bulge would alternate with long quiet periods during which
+tidal retardation would again decrease the speed of rotation and hence
+lessen the bulge. The result would be repeated changes of distribution
+of land and water, with consequent changes in climate.
+
+I. We shall now consider the climatic effect of the repeated changes in
+the relative amounts of land and water which appear to have resulted
+from the earth's contraction and from changes in its speed of rotation.
+During many geologic epochs a larger portion of the earth was covered
+with water than at present. For example, during at least twelve out of
+about twenty epochs, North America has suffered extensive
+inundations,[82] and in general the extensive submergence of Europe, the
+other area well known geologically, has coincided with that of North
+America. At other times, the ocean has been less extensive than now, as
+for example during the recent glacial period, and probably during
+several of the glacial periods of earlier date. Each of the numerous
+changes in the relative extent of the lands must have resulted in a
+modification of climate.[83] This modification would occur chiefly
+because water becomes warm far more slowly than land, and cools off far
+more slowly.
+
+An increase in the lands would cause changes in several climatic
+conditions. (a) The range of temperature between day and night and
+between summer and winter would increase, for lands become warmer by day
+and in summer than do oceans, and cooler at night and in winter. The
+higher summer temperature when the lands are widespread is due chiefly
+to the fact that the land, if not snow-covered, absorbs more of the
+sun's radiant energy than does the ocean, for its reflecting power is
+low. The lower winter temperature when lands are widespread occurs not
+only because they cool off rapidly but because the reduced oceans cannot
+give them so much heat. Moreover, the larger the land, the more
+generally do the winds blow outward from it in winter and thus prevent
+the ocean heat from being carried inland. So long as the ocean is not
+frozen in high latitudes, it is generally the chief source of heat in
+winter, for the nights are several months long near the poles, and even
+when the sun does shine its angle is so low that reflection from the
+snow is very great. Furthermore, although on the average there is more
+reflection from water than from land, the opposite is true in high
+latitudes in winter when the land is snow-covered while the ocean is
+relatively dark and is roughened by the waves. Another factor in causing
+large lands to have extremely low temperature in winter is the fact that
+in proportion to their size they are less protected by fog and cloud
+than are smaller areas. The belt of cloud and fog which is usually
+formed when the wind blows from the ocean to the relatively cold land is
+restricted to the coastal zone. Thus the larger the land, the smaller
+the fraction in which loss of heat by radiation is reduced by clouds and
+fogs. Hence an increase in the land area is accompanied by an increase
+in the contrasts in temperature between land and water.
+
+(b) The contrasts in temperature thus produced must cause similar
+contrasts in atmospheric pressure, and hence stronger barometric
+gradients. (c) The strong gradients would mean strong winds, flowing
+from land to sea or from sea to land. (d) Local convection would also be
+strengthened in harmony with the expansion of the lands, for the more
+rapid heating of land than of water favors active convection.
+
+(e) As the extent of the ocean diminished, there would normally be a
+decrease in the amount of water vapor for three reasons: (1) Evaporation
+from the ocean is the great source of water vapor. Other conditions
+being equal, the smaller the ocean becomes, the less the evaporation.
+(2) The amount of water vapor in the air diminishes as convection
+increases, since upward convection is a chief method by which
+condensation and precipitation are produced, and water vapor removed
+from the atmosphere. (3) Nocturnal cooling sufficient to produce dew and
+frost is very much more common upon land than upon the ocean. The
+formation of dew and frost diminishes the amount of water vapor at least
+temporarily. (f) Any diminution in water vapor produced in these ways,
+or otherwise, is significant because water vapor is the most essential
+part of the atmosphere so far as regulation of temperature is concerned.
+It tends to keep the days from becoming hot or the nights cold.
+Therefore any decrease in water vapor would increase the diurnal and
+seasonal range of temperature, making the climate more extreme and
+severe. Thus a periodic increase in the area of the continents would
+clearly make for periodic increased climatic contrasts, with great
+extremes, a type of climatic change which has recurred again and again.
+Indeed, each great glaciation accompanied or followed extensive
+emergence of the lands.[84]
+
+Whether or not there has been a _progressive_ increase from era to era
+in the area of the lands is uncertain. Good authorities disagree widely.
+There is no doubt, however, that at present the lands are more extensive
+than at most times in the past, though smaller, perhaps, than at certain
+periods. The wide expanse of lands helps explain the prominence of
+seasons at present as compared with the past.
+
+II. The contraction of the earth, as we have seen, has produced great
+changes in the distribution as well as in the extent of land and water.
+Large parts of the present continents have been covered repeatedly by
+the sea, and extensive areas now covered with water have been land. In
+recent geological times, that is, during the Pliocene and Pleistocene,
+much of the present continental shelf, the zone less than 600 feet below
+sea level, was land. If the whole shelf had been exposed, the lands
+would have been greater than at present by an area larger than North
+America. When the lands were most elevated, or a little earlier, North
+America was probably connected with Asia and almost with Europe. Asia in
+turn was apparently connected with the larger East Indian islands. In
+much earlier times land occupied regions where now the ocean is fairly
+deep. Groups of islands, such as the East Indies and Malaysia and
+perhaps the West Indies, were united into widespreading land masses.
+Figs. 7 and 9, illustrating the paleography of the Permian and the
+Cretaceous periods, respectively, indicate a land distribution radically
+different from that of today.
+
+So far as appears from the scattered facts of geological history, the
+changes in the distribution of land seem to have been marked by the
+following characteristics: (1) Accompanying the differentiation of
+continental and oceanic segments of the earth's crust, the oceans have
+become somewhat deeper, and their basins perhaps larger, while the
+continents, on the average, have been more elevated and less subject to
+submergence. Hence there have been less radical departures from the
+present distribution during the relatively recent Cenozoic era than in
+the ancient Paleozoic because the submergence of continental areas has
+become less general and less frequent. For example, the last extensive
+epeiric or interior sea in North America was in the Cretaceous, at least
+ten million years ago, and according to Barrell perhaps fifty million,
+while in Europe, according to de Lapparent,[85] a smaller share of the
+present continent has been submerged since the Cretaceous than before.
+Indeed, as in North America, the submergence has decreased on the
+average since the Paleozoic era. (2) The changes in distribution of land
+which have taken place during earth history have been cyclic.
+Repeatedly, at the close of each of the score or so of geologic periods,
+the continents emerged more or less, while at the close of the groups of
+periods known as eras, the lands were especially large and emergent.
+After each emergence, a gradual encroachment of the sea took place, and
+toward the close of several of the earlier periods, the sea appears to
+have covered a large fraction of the present land areas. (3) On the
+whole, the amount of land in the middle and high latitudes of the
+northern hemisphere appears to have increased during geologic time. Such
+an increase does not require a growth of the continents, however, in the
+broader sense of the term, but merely that a smaller fraction of the
+continent and its shelf should be submerged. (4) In tropical latitudes,
+on the other hand, the extent of the lands seems to have decreased,
+apparently by the growth of the ocean basins. South America and Africa
+are thought by many students to have been connected, and Africa was
+united with India via Madagascar, as is suggested in Fig. 9. The most
+radical cyclic as well as the most radical progressive changes in land
+distribution also seem to have taken place in tropical regions.[86]
+
+[Illustration: _Fig. 9. Cretaceous Paleogeography._
+(_After Schuchert._)]
+
+Although there is much evidence of periodic increase of the sea in
+equatorial latitudes and of land in high latitudes, it has remained for
+the zooelogist Metcalf to present a very pretty bit of evidence that at
+certain times submergence along the equator coincided with emergence in
+high latitudes, and vice versa. Certain fresh water frogs which carry
+the same internal parasite are confined to two widely separated areas in
+tropical and south temperate America and in Australia. The extreme
+improbability that both the frogs and the parasites could have
+originated independently in two unconnected areas and could have
+developed by convergent evolution so that they are almost identical in
+the two continents makes it almost certain that there must have been a
+land connection between South America and Australia, presumably by way
+of Antarctica. The facts as to the parasites seem also to prove that
+while the land connection existed there was a sea across South America
+in equatorial latitudes. The parasite infests not only the frogs but the
+American toads known as Bufo. Now Bufo originated north of the equator
+in America and differs from the frogs which originated in southern South
+America in not being found in Australia. This raises the question of how
+the frogs could go to Australia via Antarctica carrying the parasite
+with them, while the toads could not go. Metcalf's answer is that the
+toads were cut off from the southern part of South America by an
+equatorial sea until after the Antarctic connection between the Old
+World and the New was severed.
+
+    As Patagonia let go of Antarctica by subsidence of the intervening
+    land area, there was a probable concomitant rise of land through
+    what is now middle South America and the northern and southern
+    portions of this continent came together.[87]
+
+These various changes in the earth's crust have given rise to certain
+specific types of distribution of the lands, which will now be
+considered. We shall inquire what climatic conditions would arise from
+changes in (a) the continuity of the lands from north to south, (b) the
+amount of land in tropical latitudes, and (c) the amount of land in
+middle and high latitudes.
+
+(a) At present the westward drift of warm waters, set in motion by the
+trade winds, is interrupted by land masses and turned poleward,
+producing the important Gulf Stream Drift and Japan Current in the
+northern hemisphere, and corresponding, though less important, currents
+in the southern hemisphere. During the past, quite different sets of
+ocean currents doubtless have existed in response to a different
+distribution of land. Repeatedly, in the mid-Cretaceous (Fig. 9) and
+several other periods, the present American barrier to the westward
+moving tropical current was broken in Central America. Even if the
+supposed continent of "Gondwana Land" extended from Africa to South
+America in equatorial latitudes, strong currents must still have flowed
+westward along its northern shore under the impulse of the peculiarly
+strong trade winds which the equatorial land would create. Nevertheless
+at such times relatively little warm tropical water presumably entered
+the North Atlantic, for it escaped into the Pacific. At several other
+times, such as the late Ordovician and mid-Devonian, when the isthmian
+barrier existed, it probably turned an important current northward into
+what is now the Mississippi Basin instead of into the Atlantic. There it
+traversed an epeiric, or mid-continental sea open to both north and
+south. Hence its effectiveness in warming Arctic regions must have been
+quite different from that of the present Gulf Stream.
+
+(b) We will next consider the influences of changes in the amount of
+equatorial and tropical land. As such lands are much hotter than the
+corresponding seas, the intensity and width of the equatorial belt of
+low pressure must be great when they are extensive. Hence the trade
+winds must have been stronger than now whenever tropical lands were more
+extensive than at present. This is because the trades are produced by
+the convection due to excessive heat along the heat equator. There the
+air expands upward and flows poleward at high altitudes. The trade wind
+consists of air moving toward the heat equator to take the place of the
+air which there rises. When the lands in low latitudes were wide the
+trade winds must also have dominated a wide belt. The greater width of
+the trade-wind belt today over Africa than over the Atlantic illustrates
+the matter. The belt must have been still wider when Gondwana Land was
+large, as it is believed to have been during the Paleozoic era and the
+early Mesozoic.
+
+An increase in the width of the equatorial belt of low pressure under
+the influence of broad tropical lands would be accompanied not only by
+stronger and more widespread trade winds, but by a corresponding
+strengthening of the subtropical belts of high pressure. The chief
+reason would be the greater expansion of the air in the equatorial low
+pressure belt and the consequent more abundant outflow of air at high
+altitudes in the form of anti-trades or winds returning poleward above
+the trades. Such winds would pile up the air in the region of the
+high-pressure belt. Moreover, since the meridians converge as one
+proceeds away from the equator, the air of the poleward-moving
+anti-trades tends to be crowded as it reaches higher latitudes, thus
+increasing the pressure. Unless there were a corresponding increase in
+tropical cyclones, one of the most prominent results of the strengthened
+trades and the intensified subtropical high-pressure belt at times of
+broad lands in low latitudes would be great deserts. It will be recalled
+that the trade-wind lowlands and the extra-tropical belt of highs are
+the great desert belts at present. The trade-wind lowlands are desert
+because air moving into warmer latitudes takes up water except where it
+is cooled by rising on mountain-sides. The belt of highs is arid because
+there, too, air is being warmed, but in this case by descending from
+aloft.
+
+Again, if the atmospheric pressure in the subtropical belt should be
+intensified, the winds flowing poleward from this belt would necessarily
+become stronger. These would begin as southwesterlies in the northern
+hemisphere and northwesterlies in the southern. In the preceding chapter
+we have seen that such winds, especially when cyclonic storms are few
+and mild, are a powerful agent in transferring subtropical heat
+poleward. If the strength of the westerlies were increased because of
+broad lands in low latitudes, their efficacy in transferring heat would
+be correspondingly augmented. It is thus evident that any change in the
+extent of tropical lands during the geologic past must have had
+important climatic consequences in changing the velocity of the
+atmospheric circulation and in altering the transfer of heat from low
+latitudes to high. When the equatorial and tropical lands were broad the
+winds and currents must have been strong, much heat must have been
+carried away from low latitudes, and the contrast between low and high
+latitudes must have been relatively slight. As we have already remarked,
+leading paleogeographers believe that changes in the extent of the lands
+have been especially marked in low latitudes, and that on the average
+there has been a decrease in the extent of land within the tropics.
+Gondwana Land is the greatest illustration of this. In the same way, on
+the numerous paleogeographic maps of North America, most
+paleogeographers have shown fairly extensive lands south of the latitude
+of the United States during most of the geologic epochs.[88]
+
+(c) There is evidence that during geologic history the area of the lands
+in middle and high latitudes, as well as in low latitudes, has changed
+radically. An increase in such lands would cause the winters to grow
+colder. This would be partly because of the loss of heat by radiation
+into the cold dry air over the continents in winter, and partly because
+of increased reflection from snow and frost, which gather much more
+widely upon the land than upon the ocean. Furthermore, in winter when
+the continents are relatively cold, there is a strong tendency for winds
+to blow out from the continent toward the ocean. The larger the land the
+stronger this tendency. In Asia it gives rise to strong winter monsoons.
+The effect of such winds is illustrated by the way in which the
+westerlies prevent the Gulf Stream from warming the eastern United
+States in winter. The Gulf Stream warms northwestern Europe much more
+than the United States because, in Europe, the prevailing winds are
+onshore.
+
+Another effect of an increase in the area of the lands in middle and
+high latitudes would be to interpose barriers to oceanic circulation and
+thus lower the temperature of polar regions. This would not mean
+glaciation in high latitudes, however, even when the lands were
+widespread as in the Mesozoic and early Tertiary. Students of glaciology
+are more and more thoroughly convinced that glaciation depends on the
+availability of moisture even more than upon low temperature.
+
+In conclusion it may be noted that each of the several climatic
+influences of increased land area in the high latitudes would tend to
+increase the contrasts between land and sea, between winter and summer,
+and between low latitudes and high. In other words, so far as the effect
+upon high latitudes themselves is concerned, an expansion of the lands
+there would tend in the same direction as a diminution in low latitudes.
+In so far as the general trend of geological evolution has been toward
+more land in high latitudes and less in low, it would help to produce a
+progressive increase in climatic diversity such as is faintly indicated
+in the rock strata. On the other hand, the oscillations in the
+distribution of the lands, of which geology affords so much evidence,
+must certainly have played an important part in producing the periodic
+changes of climate which the earth has undergone.
+
+III. Throughout geological history there is abundant evidence that the
+process of contraction has led to marked differences not only in the
+distribution and area of the lands, but in their height. On the whole
+the lands have presumably increased in height since the Proterozoic,
+somewhat in proportion to the increased differentiation of continents
+and oceans.[89] If there has been such an increase, the contrast between
+the climate of ocean and land must have been accentuated, for highlands
+have a greater diurnal and seasonal range of temperature than do
+lowlands. The ocean has very little range of either sort. The large
+range at high altitudes is due chiefly to the small quantity of water
+vapor, for this declines steadily with increased altitude. A diminution
+in the density of the other constituents of the air also decreases the
+blanketing effect of the atmosphere. In conformity with the great
+seasonal range in temperature at times when the lands stand high, the
+direction of the wind would be altered. When the lands are notably
+warmer than the oceans, the winds commonly flow from land to sea, and
+when the continents are much colder than the oceans, the direction is
+reversed. The monsoons of Asia are examples. Strong seasonal winds
+disturb the normal planetary circulation of the trade winds in low
+latitudes and of the westerlies in middle latitudes. They also interfere
+with the ocean currents set in motion by the planetary winds. The net
+result is to hinder the transfer of heat from low latitudes to high, and
+thus to increase the contrasts between the zones. Local as well as zonal
+contrasts are also intensified. The higher the land, the greater,
+relatively speaking, are the cloudiness and precipitation on seaward
+slopes, and the drier the interior. Indeed, most highlands are arid.
+Henry's[90] recent study of the vertical distribution of rainfall on
+mountain-sides indicates that a decrease sets in at about 3500 feet in
+the tropics and only a little higher in mid-latitudes.
+
+In addition to the main effects upon atmospheric circulation and
+precipitation, each of the many upheavals of the lands must have been
+accompanied by many minor conditions which tended toward diversity. For
+example, the streams were rejuvenated, and instead of meandering perhaps
+over vast flood plains they intrenched their channels and in many cases
+dug deep gorges. The water table was lowered, soil was removed from
+considerable areas, the bare rock was exposed, and the type of dominant
+vegetation altered in many places. An almost barren ridge may represent
+all that remains of what was once a vast forested flood plain. Thus,
+increased elevation of the land produces contrasted conditions of slope,
+vegetation, availability of ground water, exposure to wind and so forth,
+and these unite in diversifying climate. Where mountains are formed,
+strong contrasts are sure to occur. The windward slopes may be very
+rainy, while neighboring leeward slopes are parched by a dry foehn wind.
+At the same time the tops may be snow-covered. Increased local contrasts
+in climatic conditions are known to influence the intensity of cyclonic
+storms,[91] and these affect the climatic conditions of all middle and
+high latitudes, if not of the entire earth. The paths followed by
+cyclonic storms are also altered by increased contrast between land and
+water. When the continents are notably colder than the neighboring
+oceans, high atmospheric pressure develops on the lands and interferes
+with the passage of lows, which are therefore either deflected around
+the continent or forced to move slowly.
+
+The distribution of lofty mountains has an even more striking climatic
+effect than the general uplift of a region. In Proterozoic times there
+was a great range in the Lake Superior region; in the late Devonian the
+Acadian mountains of New England and the Maritime Provinces of Canada
+possibly attained a height equal to the present Rockies. Subsequently,
+in the late Paleozoic a significant range stood where the Ouachitas now
+are. Accompanying the uplift of each of these ranges, and all others,
+the climate of the surrounding area, especially to leeward, must have
+been altered greatly. Many extensive salt deposits found now in fairly
+humid regions, for example, the Pennsylvanian and Permian deposits of
+Kansas and Oklahoma, were probably laid down in times of local aridity
+due to the cutting off of moisture-bearing winds by the mountains of
+Llanoria in Louisiana and Texas. Hence such deposits do not necessarily
+indicate periods of widespread and profound aridity.
+
+When the causes of ancient glaciation were first considered by
+geologists, about the middle of the nineteenth century, it was usually
+assumed that the glaciated areas had been elevated to great heights, and
+thus rendered cold enough to permit the accumulation of glaciers. The
+many glaciers occurring in the Alps of central Europe where glaciology
+arose doubtless suggested this explanation. However, it is now known
+that most of the ancient glaciation was not of the alpine type, and
+there is adequate proof that the glacial periods cannot be explained as
+due directly and solely to uplift. Nevertheless, upheavals of the lands
+are among the most important factors in controlling climate, and
+variations in the height of the lands have doubtless assisted in
+producing climate oscillations, especially those of long duration.
+Moreover, the progressive increase in the height of the lands has
+presumably played a part in fostering local and zonal diversity in
+contrast with the relative uniformity of earlier geological times.
+
+IV. The contraction of the earth has been accompanied by volcanic
+activity as well as by changes in the extent, distribution, and altitude
+of the lands. The probable part played by volcanic dust as a
+contributory factor in producing short sudden climatic variations has
+already been discussed. There is, however, another though probably less
+important respect in which volcanic activity may have had at least a
+slight climatic significance. The oldest known rocks, those of the
+Archean era, contain so much igneous matter that many students have
+assumed that they show that the entire earth was once liquid. It is now
+considered that they merely indicate igneous activity of great
+magnitude. In the later part of Proterozoic time, during the second
+quarter of the earth's history according to Schuchert's estimate, there
+were again vast outflowings of lava. In the Lake Superior district, for
+example, a thickness of more than a mile accumulated over a large area,
+and lavas are common in many areas where rocks of this age are known.
+The next quarter of the earth's history elapsed without any
+correspondingly great outflows so far as is known, though several lesser
+ones occurred. Toward the end of the last quarter, and hence quite
+recently from the geological standpoint, another period of outflows,
+perhaps as noteworthy as that of the Proterozoic, occurred in the
+Cretaceous and Tertiary.
+
+The climatic effects of such extensive lava flows would be essentially
+as follows: In the first place so long as the lavas were hot they would
+set up a local system of convection with inflowing winds. This would
+interfere at least a little with the general winds of the area. Again,
+where the lava flowed out into water, or where rain fell upon hot lava,
+there would be rapid evaporation which would increase the rainfall. Then
+after the lava had cooled, it would still influence climate a trifle in
+so far as its color was notably darker or lighter than that of the
+average surface. Dark surfaces absorb solar heat and become relatively
+warm when the sun shines upon them. Dark objects likewise radiate heat
+more rapidly than light-colored objects. Hence they cool more rapidly at
+night, and in the winter. As most lavas are relatively dark they
+increase the average diurnal range of temperature. Hence even after they
+are cool they increase the climatic diversity of the land.
+
+The amount of heat given to the atmosphere by an extensive lava flow,
+though large according to human standards, is small compared with the
+amount received from the sun by a like area, except during the first few
+weeks or months before the lava has formed a thick crust. Furthermore,
+probably only a small fraction of any large series of flows occurred in
+a given century or millennium. Moreover, even the largest lava flows
+covered an area of only a few hundredths of one per cent of the earth's
+surface. Nevertheless, the conditions which modify climate are so
+complicated that it would be rash to state that this amount of
+additional heat has been of no climatic significance. Like the
+proverbial "straw that broke the camel's back," the changes it would
+surely produce in local convection, atmospheric pressure, and the
+direction of the wind may have helped to shift the paths of storms and
+to produce other complications which were of appreciable climatic
+significance.
+
+V. The last point which we shall consider in connection with the effect
+of the earth's interior upon climate is internal heat. The heat given
+off by lavas is merely a small part of that which is emitted by the
+earth as a whole. In the earliest part of geological history enough heat
+may have escaped from the interior of the earth to exert a profound
+influence on the climate. Knowlton,[92] as we have seen, has recently
+built up an elaborate theory on this assumption. At present, however,
+accurate measurements show that the escape of heat is so slight that it
+has no appreciable influence except in a few volcanic areas. It is
+estimated to raise the average temperature of the earth's surface less
+than 0.1 deg.C.[93]
+
+In order to contribute enough heat to raise the surface temperature
+1 deg.C., the temperature gradient from the interior of the earth to the
+surface would need to be ten times as great as now, for the rate of
+conduction varies directly with the gradient. If the gradient were ten
+times as great as now, the rocks at a depth of two and one-half miles
+would be so hot as to be almost liquid according to Barrell's[94]
+estimates. The thick strata of unmetamorphosed Paleozoic rocks indicate
+that such high temperatures have not prevailed at such slight depths
+since the Proterozoic. Furthermore, the fact that the climate was cold
+enough to permit glaciation early in the Proterozoic era and at from one
+to three other times before the opening of the Paleozoic suggests that
+the rate of escape of heat was not rapid even in the first half of the
+earth's recorded history. Yet even if the general escape of heat has
+never been large since the beginning of the better-known part of
+geological history, it was presumably greater in early times than at
+present.
+
+If there actually has been an appreciable decrease in the amount of heat
+given out by the earth's interior, its effects would agree with the
+observed conditions of the geological record. It would help to explain
+the relative mildness of zonal, seasonal, and local contrasts of climate
+in early geological times, but it would not help to explain the long
+oscillations from era to era which appear to have been of much greater
+importance. Those oscillations, so far as we can yet judge, may have
+been due in part to solar changes, but in large measure they seem to be
+explained by variations in the extent, distribution, and altitude of the
+lands. Such variations appear to be the inevitable result of the earth's
+contraction.
+
+FOOTNOTES:
+
+[Footnote 70: Chas. Schuchert: The Earth's Changing Surface and Climate
+during Geologic Time; in Lull: The Evolution of the Earth and Its
+Inhabitants, 1918, p. 55.]
+
+[Footnote 71: Quoted by J. Cornet: Cours de Geologie, 1920, p. 330.]
+
+[Footnote 72: T. C. Chamberlin: The Order of Magnitude of the Shrinkage
+of the Earth; Jour. Geol., Vol. 28, 1920, pp. 1-17, 126-157.]
+
+[Footnote 73: G. I. Taylor: Philosophical Transactions, A. 220, 1919,
+pp. 1-33; Monthly Notices Royal Astron. Soc., Jan., 1920, Vol. 80,
+p. 308.]
+
+[Footnote 74: J. Jeffreys: Monthly Notices Royal Astron. Soc., Jan.,
+1920, Vol. 80, p. 309.]
+
+[Footnote 75: E. W. Brown: personal communication.]
+
+[Footnote 76: C. S. Slichter: The Rotational Period of a Heterogeneous
+Spheroid; in Contributions to the Fundamental Problems of Geology, by
+T. C. Chamberlin, _et al._, Carnegie Inst. of Wash., No. 107, 1909.]
+
+[Footnote 77: E. Suess: The Face of the Earth, Vol. II, p. 553, 1901.]
+
+[Footnote 78: Chas. Schuchert: The Earth's Changing Surface and Climate;
+in Lull: The Evolution of the Earth and Its Inhabitants, 1918, p. 78.]
+
+[Footnote 79: J. Barren: Rhythms and the Measurement of Geologic Time;
+Bull. Geol. Soc. Am., Vol. 28, 1917, p. 838.]
+
+[Footnote 80: Chas. Schuchert: _loc. cit._, p. 78.]
+
+[Footnote 81: T. C. Chamberlin: Diastrophism, the Ultimate Basis of
+Correlation; Jour. Geol., Vol. 16, 1909; Chas. Schuchert: _loc. cit._]
+
+[Footnote 82: Pirsson-Schuchert: Textbook of Geology, 1915, Vol. II, p.
+982; Chas. Schuchert: Paleogeography of North America; Bull. Geol. Soc.
+Am., Vol. 20, pp. 427-606; reference on p. 499.]
+
+[Footnote 83: The general subject of the climatic significance of
+continentality is discussed by C. E. P. Brooks: continentality and
+Temperature; Quart. Jour. Royal Meteorol. Soc., April, 1917, and Oct.,
+1918.]
+
+[Footnote 84: Chas. Schuchert: Climates of Geologic Time; in The
+Climatic Factor; Carnegie Institution, 1914, p. 286.]
+
+[Footnote 85: A. de Lapparent: Traite de Geologie, 1906.]
+
+[Footnote 86: Chas. Schuchert: Historical Geology, 1915, p. 464.]
+
+[Footnote 87: M. M. Metcalf: Upon an important method of studying
+problems of relationship and of geographical distribution; Proceedings
+National Academy of Sciences, Vol. 6, July, 1920, pp. 432-433.]
+
+[Footnote 88: Chas. Schuchert: Paleogeography of North America; Bull.
+Geol. Soc. Am., Vol. 20, 1910; and Willis, Salisbury, and others:
+Outlines of Geologic History, 1910.]
+
+[Footnote 89: Chas. Schuchert: The Earth's Changing Surface and Climate;
+in Lull: The Evolution of the Earth and Its Inhabitants, 1918, p. 50.]
+
+[Footnote 90: A. J. Henry: The Decrease of Precipitation with Altitude;
+Monthly Weather Review, Vol. 47, 1919, pp. 33-41.]
+
+[Footnote 91: Chas. F. Brooks: Monthly Weather Review, Vol. 46, 1918, p.
+511; and also A. J. Henry and others: Weather Forecasting in the United
+States, 1913.]
+
+[Footnote 92: F. H. Knowlton: Evolution of Geologic Climates; Bull.
+Geol. Soc. Am., Vol. 30, Dec., 1919, pp. 499-566.]
+
+[Footnote 93: Talbert, quoted by I. Bowman: Forest Physiography, 1911,
+p. 63.]
+
+[Footnote 94: J. Barrell: Rhythms and the Measurement of Geologic Time;
+Bull. Geol. Soc. Am., Vol. 28, 1917, pp. 745-904.]
+
+
+
+
+CHAPTER XII
+
+POST-GLACIAL CRUSTAL MOVEMENTS AND CLIMATIC CHANGES
+
+
+An interesting practical application of some of the preceding
+generalizations is found in an attempt by C. E. P. Brooks[95] to
+interpret post-glacial climatic changes almost entirely in terms of
+crustal movement. We believe that he carries the matter much too far,
+but his discussion is worthy of rather full recapitulation, not only
+for its theoretical value but because it gives a good summary of
+post-glacial changes. His climatic table for northwest Europe as
+reprinted from the annual report of the Smithsonian Institution for
+1917, p. 366, is as follows:
+
+      _Phase_                      _Climate_                _Date_
+
+  1. The Last Great            Arctic climate.      30,000-18,000 B. C.
+       Glaciation.
+  2. The Retreat of the        Severe continental   18,000-6000 B. C.
+       Glaciers.                 climate.
+  3. The Continental Phase.    Continental climate.  6000-4000 B. C.
+  4. The Maritime Phase.       Warm and moist.       4000-3000 B. C.
+  5. The Later Forest Phase.   Warm and dry.         3000-1800 B. C.
+  6. The Peat-Bog Phase.       Cooler and moister.  1800 B. C.-300 A. D.
+  7. The Recent Phase.         Becoming drier.       300 A. D.-
+
+Brooks bases his chronology largely on De Geer's measurements of the
+annual layers of clay in lake bottoms but makes much use of other
+evidence. According to Brooks the last glacial epoch lasted roughly from
+30,000 to 18,000 B. C., but this includes a slight amelioration of
+climate followed by a readvance of the ice, known as the Buhl stage.
+During the time of maximum glaciation the British Isles stood twenty or
+thirty feet higher than now and Scandinavia was "considerably" more
+elevated. The author believes that this caused a fall of 1 deg.C. in the
+temperature of the British Isles and of 2 deg.C. in Scandinavia. By an
+ingenious though not wholly convincing method of calculation he
+concludes that this lowering of temperature, aided by an increase in the
+area of the lands, sufficed to start an ice sheet in Scandinavia. The
+relatively small area of ice cooled the air and gave rise to an area of
+high barometric pressure. This in turn is supposed to have caused
+further expansion of the ice and to have led to full-fledged glaciation.
+
+About 18,000 B. C. the retreat of the ice began in good earnest. Even
+though no evidence has yet been found, Brooks believes there must have
+been a change in the distribution of land and sea to account for the
+diminution of the ice. The ensuing millenniums formed the Magdalenian
+period in human history, the last stage of the Paleolithic, when man
+lived in caves and reindeer were abundant in central Europe.[96] At
+first the ice retreated very slowly and there were periods when for
+scores of years the ice edge remained stationary or even readvanced.
+About 10,000 B. C. the edge of the ice lay along the southern coast of
+Sweden. During the next 2000 years it withdrew more rapidly to about
+59 deg.N. Then came the Fennoscandian pause, or Gschnitz stage, when for
+about 200 years the ice edge remained in one position, forming a great
+moraine. Brooks suggests that this pause about 8000 B. C. was due to the
+closing of the connection between the Atlantic Ocean and the Baltic Sea
+and the synchronous opening of a connection between the Baltic and the
+White Seas, whereby cold Arctic waters replaced the warmer Atlantic
+waters. He notes, however, that about 7500 B. C. the obliquity of the
+ecliptic was probably nearly 1 deg. greater than at present. This he
+calculates to have caused the climate of Germany and Sweden to be 1 deg.F.
+colder than at present in winter and 1 deg.F. warmer in summer.
+
+The next climatic stage was marked by a rise of temperature till about
+6000 B. C. During this period the ice at first retreated, presumably
+because the climate was ameliorating, although no cause of such
+amelioration is assigned. At length the ice lay far enough north to
+allow a connection between the Baltic and the Atlantic by way of Lakes
+Wener and Wetter in southern Sweden. This is supposed to have warmed the
+Baltic Sea and to have caused the climate to become distinctly milder.
+Next the land rose once more so that the Baltic was separated from the
+Atlantic and was converted into the Ancylus lake of fresh water. The
+southwest Baltic region then stood 400 feet higher than now. The result
+was the Daun stage, about 5000 B. C., when the ice halted or perhaps
+readvanced a little, its front being then near Ragunda in about latitude
+63 deg.. Why such an elevation did not cause renewed glaciation instead of
+merely the slight Daun pause, Brooks does not explain, although his
+calculations as to the effect of a slight elevation of the land during
+the main period of glaciation from 30,000 to 18,000 B. C. would seem to
+demand a marked readvance.
+
+After 5000 B. C. there ensued a period when the climate, although still
+distinctly continental, was relatively mild. The winters, to be sure,
+were still cold but the summers were increasingly warm. In Sweden, for
+example, the types of vegetation indicate that the summer temperature
+was 7 deg.F. higher than now. Storms, Brooks assumes, were comparatively
+rare except on the outer fringe of Great Britain. There they were
+sufficiently abundant so that in the Northwest they gave rise to the
+first Peat-Bog period, during which swamps replaced forests of birch and
+pine. Southern and eastern England, however, probably had a dry
+continental climate. Even in northwest Norway storms were rare as is
+indicated by remains of forests on islands now barren because of the
+strong winds and fierce storms. Farther east most parts of central and
+northern Europe were relatively dry. This was the early Neolithic period
+when man advanced from the use of unpolished to polished stone
+implements.
+
+Not far from 4000 B. C. the period of continental climate was replaced
+by a comparatively moist maritime climate. Brooks believes that this was
+because submergence opened the mouth of the Baltic and caused the fresh
+Ancylus lake to give place to the so-called Litorina sea. The
+temperature in Sweden averaged about 3 deg.F. higher than at present and in
+southwestern Norway 2 deg.. More important than this was the small annual
+range of temperature due to the fact that the summers were cool while
+the winters were mild. Because of the presence of a large expanse of
+water in the Baltic region, storms, as our author states, then crossed
+Great Britain and followed the Baltic depression, carrying the moisture
+far inland. In spite of the additional moisture thus available the snow
+line in southern Norway was higher than now.
+
+At this point Brooks turns to other parts of the world. He states that
+not far from 4000 B. C., a submergence of the lands, rarely amounting to
+more than twenty-five feet, took place not only in the Baltic region but
+in Ireland, Iceland, Spitzbergen, and other parts of the Arctic Ocean,
+as well as in the White Sea, Greenland, and the eastern part of North
+America. Evidences of a mild climate are found in all those places.
+Similar evidence of a mild warm climate is found in East Africa, East
+Australia, Tierra del Fuego, and Antarctica. The dates are not
+established with certainty but they at least fall in the period
+immediately preceding the present epoch. In explanation of these
+conditions Brooks assumes a universal change of sea level. He suggests
+with some hesitation that this may have been due to one of Pettersson's
+periods of maximum "tide-generating force." According to Pettersson the
+varying positions of the moon, earth, and sun cause the tides to vary in
+cycles of about 9, 90, and 1800 years, though the length of the periods
+is not constant. When tides are high there is great movement of ocean
+waters and hence a great mixture of the water at different latitudes.
+This is supposed to cause an amelioration of climate. The periods of
+maximum and minimum tide-generating force are as follows:
+
+  Maxima 3500 B. C.--------2100 B. C.--------350 B. C.-------A. D. 1434
+  Minima ---------2800 B. C.--------1200 B. C.-------A. D. 530---------
+
+Brooks thinks that the big trees in California and the Norse sagas and
+Germanic myths indicate a rough agreement of climatic phenomena with
+Pettersson's last three dates, while the mild climate of 4000 B. C. may
+really belong to 3500 B. C. He gives no evidence confirming Pettersson's
+view at the other three dates.
+
+To return to Brooks' sketch of the relation of climatic pulsations to
+the altitude of the lands, by 3000 B. C., that is, toward the close of
+the Neolithic period, further elevation is supposed to have taken place
+over the central latitudes of western Europe. Southern Britain, which
+had remained constantly above its present level ever since 30,000 B. C.,
+was perhaps ninety feet higher than now. Ireland was somewhat enlarged
+by elevation, the Straits of Dover were almost closed, and parts of the
+present North Sea were land. To these conditions Brooks ascribes the
+prevalence of a dry continental climate. The storms shifted northward
+once more, the winds were mild, as seems to be proved by remains of
+trees in exposed places; and forests replaced fields of peat and heath
+in Britain and Germany. The summers were perhaps warmer than now but the
+winters were severe. The relatively dry climate prevailed as far west as
+Ireland. For example, in Drumkelin Bog in Donegal County a corded oak
+road and a two-story log cabin appear to belong to this time. Fourteen
+feet of bog lie below the floor and twenty-six above. This period,
+perhaps 3000-2000 B. C., was the legendary heroic age of Ireland when
+"the vigour of the Irish reached a level not since attained." This, as
+Brooks points out, may have been a result of the relatively dry climate,
+for today the extreme moisture of Ireland seems to be a distinct
+handicap. In Scandinavia, civilization, or at least the stage of
+relative progress, was also high at this time.
+
+By 1600 B. C. the land had assumed nearly its present level in the
+British Isles and the southern Baltic region, while northern Scandinavia
+still stood lower than now. The climate of Britain and Germany was so
+humid that there was an extensive formation of peat even on high ground
+not before covered. This moist stage seems to have lasted almost to the
+time of Christ, and may have been the reason why the Romans described
+Britain as peculiarly wet and damp. At this point Brooks again departs
+from northwest Europe to a wider field:
+
+    It is possible that we have to attribute this damp period in
+    Northwest Europe to some more general cause, for Ellsworth
+    Huntington's curves of tree-growth in California and climate in
+    Western Asia both show moister conditions from about 1000 B. C. to
+    A. D. 200, and the same author believes that the Mediterranean lands
+    had a heavier rainfall about 500 B. C. to A. D. 200. It seems that
+    the phase was marked by a general increase of the storminess of the
+    temperate regions of the northern hemisphere at least, with a
+    maximum between Ireland and North Germany, indicating probably that
+    the Baltic again became the favourite track of depressions from the
+    Atlantic.
+
+Brooks ends his paper with a brief resume of glacial changes in North
+America, but as the means of dating events are unreliable the degree of
+synchronism with Europe is not clear. He sums up his conclusions as
+follows:
+
+    On the whole it appears that though there is a general similarity in
+    the climatic history of the two sides of the North Atlantic, the
+    changes are not really contemporaneous, and such relationship as
+    appears is due mainly to the natural similarity in the geographical
+    history of two regions both recovering from an Ice Age, and only
+    very partially to world-wide pulsations of climate. Additional
+    evidence on this head will be available when Baron de Geer publishes
+    the results of his recent investigations of the seasonal glacial
+    clays of North America, especially if, as he hopes, he is able to
+    correlate the banding of these clays with the growth-rings of the
+    big trees.
+
+    When we turn to the northwest of North America, this is brought out
+    very markedly. For in Yukon and Alaska the Ice Age was a very mild
+    affair compared with its severity in eastern America and
+    Scandinavia. As the land had not a heavy ice-load to recover from,
+    there were no complicated geographical changes. Also, there were no
+    fluctuations of climate, but simply a gradual passage to present
+    conditions. The latter circumstance especially seems to show that
+    the emphasis laid on geographical rather than astronomical factors
+    of _great_ climatic changes is not misplaced.
+
+Brooks' painstaking discussion of post-glacial climatic changes is of
+great value because of the large body of material which he has so
+carefully wrought together. His strong belief in the importance of
+changes in the level of the lands deserves serious consideration. It is
+difficult, however, to accept his final conclusion that such changes are
+the main factors in recent climatic changes. It is almost impossible,
+for example, to believe that movements of the land could produce almost
+the same series of climatic changes in Europe, Central Asia, the western
+and eastern parts of North America, and the southern hemisphere. Yet
+such changes appear to have occurred during and since the glacial
+period. Again there is no evidence whatever that movements of the land
+have anything to do with the historic cycles of climate or with the
+cycles of weather in our own day, which seem to be the same as glacial
+cycles on a small scale. Also, as Dr. Simpson points out in discussing
+Brooks' paper, there appears "no solution along these lines of the
+problem connected with rich vegetation in both polar circles and the
+ice-age which produced the ice-sheet at sea-level in Northern India."
+Nevertheless, we may well believe that Brooks is right in holding that
+changes in the relative level and relative area of land and sea have had
+important local effects. While they are only one of the factors involved
+in climatic changes, they are certainly one that must constantly be kept
+in mind.
+
+FOOTNOTES:
+
+[Footnote 95: C. E. P. Brooks: The Evolution of Climate in Northwest
+Europe. Quart. Jour. Royal Meteorol. Soc., Vol. 47, 1921, pp. 173-194.]
+
+[Footnote 96: H. F. Osborn: Men of the Old Stone Age, N. Y., 1915; J. M.
+Tyler: The New Stone Age in Northwestern Europe, N. Y., 1920.]
+
+
+
+
+CHAPTER XIII
+
+THE CHANGING COMPOSITION OF OCEANS AND ATMOSPHERE
+
+
+Having discussed the climatic effect of movements of the earth's crust
+during the course of geological time, we are now ready to consider the
+corresponding effects due to changes in the movable envelopes--the
+oceans and the atmosphere. Variations in the composition of sea water
+and of air and in the amount of air must almost certainly have occurred,
+and must have produced at least slight climatic consequences. It should
+be pointed out at once that such variations appear to be far less
+important climatically than do movements of the earth's crust and
+changes in the activity of the sun. Moreover, in most cases, they are
+not reversible as are the crustal and solar phenomena. Hence, while most
+of them appear to have been unimportant so far as climatic oscillations
+and fluctuations are concerned, they seemingly have aided in producing
+the slight secular progression to which we have so often referred.
+
+There is general agreement among geologists that the ocean has become
+increasingly saline throughout the ages. Indeed, calculations of the
+rate of accumulation of salt have been a favorite method of arriving at
+estimates of the age of the ocean, and hence of the earliest marine
+sediments. So far as known, however, no geologist or climatologist has
+discussed the probable climatic effects of increased salinity. Yet it
+seems clear that an increase in salinity must have a slight effect upon
+climate.
+
+Salinity affects climate in four ways: (1) It appreciably influences the
+rate of evaporation; (2) it alters the freezing point; (3) it produces
+certain indirect effects through changes in the absorption of carbon
+dioxide; and (4) it has an effect on oceanic circulation.
+
+(1) According to the experiments of Mazelle and Okada, as reported by
+Kruemmel,[97] evaporation from ordinary sea water is from 9 to 30 per
+cent less rapid than from fresh water under similar conditions. The
+variation from 9 to 30 per cent found in the experiments depends,
+perhaps, upon the wind velocity. When salt water is stagnant, rapid
+evaporation tends to result in the development of a film of salt on the
+top of the water, especially where it is sheltered from the wind. Such a
+film necessarily reduces evaporation. Hence the relatively low salinity
+of the oceans in the past probably had a tendency to increase the amount
+of water vapor in the air. Even a little water vapor augments slightly
+the blanketing effect of the air and to that extent diminishes the
+diurnal and seasonal range of temperature and the contrast from zone to
+zone.
+
+(2) Increased salinity means a lower freezing temperature of the oceans
+and hence would have an effect during cold periods such as the present
+and the Pleistocene ice age. It would not, however, be of importance
+during the long warm periods which form most of geologic time. A
+salinity of about 3.5 per cent at present lowers the freezing point of
+the ocean roughly 2 deg.C. below that of fresh water. If the ocean were
+fresh and our winters as cold as now, all the harbors of New England and
+the Middle Atlantic States would be icebound. The Baltic Sea would also
+be frozen each winter, and even the eastern harbors of the British Isles
+would be frequently locked in ice. At high latitudes the area of
+permanently frozen oceans would be much enlarged. The effect of such a
+condition upon marine life in high latitudes would be like that of a
+change to a warmer climate. It would protect the life on the continental
+shelf from the severe battering of winter storms. It would also lessen
+the severity of the winter temperature in the water for when water
+freezes it gives up much latent heat,--eighty calories per cubic
+centimeter. Part of this raises the temperature of the underlying water.
+
+The expansion of the ice near northern shores would influence the life
+of the lands quite differently from that of the oceans. It would act
+like an addition of land to the continents and would, therefore,
+increase the atmospheric contrasts from zone to zone and from
+continental interior to ocean. In summer the ice upon the sea would tend
+to keep the coastal lands cool, very much as happens now near the Arctic
+Ocean, where the ice floes have a great effect through their reflection
+of light and their absorption of heat in melting. In winter the virtual
+enlargement of the continents by the addition of an ice fringe would
+decrease the snowfall upon the lands. Still more important would be the
+effect in intensifying the anti-cyclonic conditions which normally
+prevail in winter not only over continents but over ice-covered oceans.
+Hence the outblowing cold winds would he strengthened.[98] The net
+effect of all these conditions would apparently be a diminution of
+snowfall in high latitudes upon the lands even though the summer
+snowfall upon the ocean and the coasts may have increased. This
+condition may have been one reason why widespread glaciation does not
+appear to have prevailed in high latitudes during the Proterozoic and
+Permian glaciations, even though it occurred farther south. If the ocean
+during those early glacial epochs were ice-covered down to middle
+latitudes, a lack of extensive glaciation in high latitudes would be no
+more surprising than is the lack of Pleistocene glaciation in the
+northern parts of Alaska and Asia. Great ice sheets are impossible
+without a large supply of moisture.
+
+(3) Among the indirect effects of salinity one of the chief appears to
+be that the low salinity of the water in the past and the greater ease
+with which it froze presumably allowed the temperature of the entire
+ocean to be slightly higher than now. This is because ice serves as a
+blanket and hinders the radiation of heat from the underlying water. The
+temperature of the ocean has a climatic significance not only directly,
+but indirectly through its influence on the amount of carbon dioxide
+held by the oceans. A change of even 1 deg.C. from the present mean
+temperature of 2 deg.C. would alter the ability of the entire ocean to
+absorb carbon dioxide by about 4 per cent. This, according to F. W.
+Clarke,[99] is because the oceans contain from eighteen to twenty-seven
+times as much carbon dioxide as the air when only the free carbon
+dioxide is considered, and about seventy times as much according to
+Johnson and Williamson[100] when the partially combined carbon dioxide
+is also considered. Moreover, the capacity of water for carbon dioxide
+varies sharply with the temperature.[101] Hence a rise in temperature of
+only 1 deg.C. would theoretically cause the oceans to give up from 30 to 280
+times as much carbon dioxide as the air now holds. This, however, is on
+the unfounded assumption that the oceans are completely saturated. The
+important point is merely that a slight change in ocean temperature
+would cause a disproportionately large change in the amount of carbon
+dioxide in the air with all that this implies in respect to blanketing
+the earth, and thus altering temperature.
+
+(4) Another and perhaps the most important effect of salinity upon
+climate depends upon the rapidity of the deep-sea circulation. The
+circulation is induced by differences of temperature, but its speed is
+affected at least slightly by salinity. The vertical circulation is now
+dominated by cold water from subpolar latitudes. Except in closed seas
+like the Mediterranean the lower portions of the ocean are near the
+freezing point. This is because cold water sinks in high latitudes by
+reason of its superior density, and then "creeps" to low latitudes.
+There it finally rises and replaces either the water driven poleward by
+the winds, or that which has evaporated from the Surface.[102]
+
+During past ages, when the sea water was less salty, the circulation was
+presumably more rapid than now. This was because, in tropical regions,
+the rise of cold water is hindered by the sinking of warm surface water
+which is relatively dense because evaporation has removed part of the
+water and caused an accumulation of salt. According to Kruemmel and
+Mill,[103] the surface salinity of the subtropical belt of the North
+Atlantic commonly exceeds 3.7 per cent and sometimes reaches 3.77 per
+cent, whereas the underlying waters have a salinity of less than 3.5 per
+cent and locally as little as 3.44 per cent. The other oceans are
+slightly less saline than the North Atlantic at all depths, but the
+vertical salinity gradients along the tropics are similar. According to
+the Smithsonian Physical Tables, the difference in salinity between the
+surface water and that lying below is equivalent to a difference of .003
+in density, where the density of fresh water is taken as 1.000. Since
+the decrease in density produced by warming water from the temperature
+of its greatest density (4 deg.C.) to the highest temperatures which ever
+prevail in the ocean (30 deg.C. or 86 deg.F.) is only .004, the more saline
+surface waters of the dry tropics are at most times almost as dense as
+the less saline but colder waters beneath the surface, which have come
+from higher latitudes. During days of especially great evaporation,
+however, the most saline portions of the surface waters in the dry
+tropics are denser than the underlying waters and therefore sink, and
+produce a temporary local stagnation in the general circulation. Such a
+sinking of the warm surface waters is reported by Kruemmel, who detected
+it by means of the rise in temperature which it produces at considerable
+depths. If such a hindrance to the circulation did not exist, the
+velocity of the deep-sea movements would be greater.
+
+If in earlier times a more rapid circulation occurred, low latitudes
+must have been cooled more than now by the rise of cold waters. At the
+same time higher latitudes were presumably warmed by a greater flow of
+warm water from tropical regions because less of the surface heat sank
+in low latitudes. Such conditions would tend to lessen the climatic
+contrast between the different latitudes. Hence, in so far as the rate
+of deep-sea circulation depends upon salinity, the slowly increasing
+amount of salt in the oceans must have tended to increase the contrasts
+between low and high latitudes. Thus for several reasons, the increase
+of salinity during geologic history seems to deserve a place among the
+minor agencies which help to explain the apparent tendency toward a
+secular progression of climate in the direction of greater contrasts
+between tropical and subpolar latitudes.
+
+Changes in the composition and amount of the atmosphere have presumably
+had a climatic importance greater than that of changes in the salinity
+of the oceans. The atmospheric changes may have been either progressive
+or cyclic, or both. In early times, according to the nebular hypothesis,
+the atmosphere was much more dense than now and contained a larger
+percentage of certain constituents, notably carbon dioxide and water.
+The planetesimal hypothesis, on the other hand, postulates an increase
+in the density of the atmosphere, for according to this hypothesis the
+density of the atmosphere depends upon the power of the earth to hold
+gases, and this power increases as the earth grows bigger with the
+infall of material from without.[104]
+
+Whichever hypothesis may be correct, it seems probable that when life
+first appeared on the land the atmosphere resembled that of today in
+certain fundamental respects. It contained the elements essential to
+life, and its blanketing effect was such as to maintain temperatures not
+greatly different from those of the present. The evidence of this
+depends largely upon the narrow limits of temperature within which the
+activities of modern life are possible, and upon the cumulative evidence
+that ancient life was essentially similar to the types now living. The
+resemblance between some of the oldest forms and those of today is
+striking. For example, according to Professor Schuchert:[105] "Many of
+the living genera of forest trees had their origin in the Cretaceous,
+and the giant sequoias of California go back to the Triassic, while
+Ginkgo is known in the Permian. Some of the fresh-water molluscs
+certainly were living in the early periods of the Mesozoic, and the
+lung-fish of today (Ceratodus) is known as far back as the Triassic and
+is not very unlike other lung-fishes of the Devonian. The higher
+vertebrates and insects, on the other hand, are very sensitive to their
+environment, and therefore do not extend back generically beyond the
+Cenozoic, and only in a few instances even as far as the Oligocene. Of
+marine invertebrates the story is very different, for it is well known
+that the horseshoe crab (Limulus) lived in the Upper Jurassic, and
+Nautilus in the Triassic, with forms in the Devonian not far removed
+from this genus. Still longer-ranging genera occur among the
+brachiopods, for living Lingula and Crania have specific representatives
+as far back as the early Ordovician. Among living foraminifers, Lagena,
+Globigerina, and Nodosaria are known in the later Cambrian or early
+Ordovician. In the Middle Cambrian near Field, British Columbia, Walcott
+has found a most varied array of invertebrates among which are
+crustaceans not far removed from living forms. Zooelogists who see these
+wonderful fossils are at once struck with their modernity and the little
+change that has taken place in certain stocks since that far remote
+time. Back of the Paleozoic, little can be said of life from the generic
+standpoint, since so few fossils have been recovered, but what is at
+hand suggests that the marine environment was similar to that of today."
+
+At present, as we have repeatedly seen, little growth takes place either
+among animals or plants at temperatures below 0 deg.C. or above 40 deg.C., and
+for most species the limiting temperatures are about 10 deg. and 30 deg.. The
+maintenance of so narrow a scale of temperature is a function of the
+atmosphere, as well as of the sun. Without an atmosphere, the
+temperature by day would mount fatally wherever the sun rides high in
+the sky. By night it would fall everywhere to a temperature approaching
+absolute zero, that is -273 deg.C. Some such temperature prevails a few
+miles above the earth's surface, beyond the effective atmosphere.
+Indeed, even if the atmosphere were almost as it is now, but only lacked
+one of the minor constituents, a constituent which is often actually
+ignored in statements of the composition of the air, life would be
+impossible. Tyndall concludes that if water vapor were entirely removed
+from the atmosphere for a single day and night, all life--except that
+which is dormant in the form of seeds, eggs, or spores--would be
+exterminated. Part would be killed by the high temperature developed by
+day when the sun was high, and part, by the cold night.
+
+The testimony of ancient glaciation as to the slight difference in the
+climate and therefore in the atmosphere of early and late geological
+times is almost as clear as that of life. Just as life proves that the
+earth can never have been extremely cold during hundreds of millions of
+years, so glaciation in moderately low latitudes near the dawn of earth
+history and at several later times, proves that the earth was not
+particularly hot even in those early days. The gentle progressive change
+of climate which is recorded in the rocks appears to have been only in
+slight measure a change in the mean temperature of the earth as a whole,
+and almost entirely a change in the distribution of temperature from
+place to place and season to season. Hence it seems probable that
+neither the earth's own emission of heat, nor the supply of solar heat,
+nor the power of the atmosphere to retain heat can have been much
+greater a few hundred million years ago than now. It is indeed possible
+that these three factors may have varied in such a way that any
+variation in one has been offset by variations of the others in the
+opposite direction. This, however, is so highly improbable that it seems
+advisable to assume that all three have remained relatively constant.
+This conclusion together with a realization of the climatic significance
+of carbon dioxide has forced most of the adherents of the nebular
+hypothesis to abandon their assumption that carbon dioxide, the heaviest
+gas in the air, was very abundant until taken out by coal-forming plants
+or combined with the calcium oxide of igneous rocks to form the
+limestone secreted by animals. In the same way the presence of sun
+cracks in sedimentary rocks of all ages suggests that the air cannot
+have contained vast quantities of water vapor such as have been assumed
+by Knowlton and others in order to account for the former lack of sharp
+climatic contrast between the zones. Such a large amount of water vapor
+would almost certainly be accompanied by well-nigh universal and
+continual cloudiness so that there would be little chance for the pools
+on the earth's water-soaked surface to dry up. Furthermore, there is
+only one way in which such cloudiness could be maintained and that is by
+keeping the air at an almost constant temperature night and day. This
+would require that the chief source of warmth be the interior of the
+earth, a condition which the Proterozoic, Permian, and other widespread
+glaciations seem to disprove.
+
+Thus there appears to be strong evidence against the radical changes in
+the atmosphere which are sometimes postulated. Yet some changes must
+have taken place, and even minor changes would be accompanied by some
+sort of climatic effect. The changes would take the form of either an
+increase or a decrease in the atmosphere as a whole, or in its
+constituent elements. The chief means by which the atmosphere has
+increased appear to be as follows: (a) By contributions from the
+interior of the earth via volcanoes and springs and by the weathering of
+igneous rocks with the consequent release of their enclosed gases;[106]
+(b) by the escape of some of the abundant gases which the ocean holds in
+solution; (c) by the arrival on the earth of gases from space, either
+enclosed in meteors or as free-flying molecules; (d) by the release of
+gases from organic compounds by oxidation, or by exhalation from animals
+and plants. On the other hand, one or another of the constituents of the
+atmosphere has presumably decreased (a) by being locked up in newly
+formed rocks or organic compounds; (b) by being dissolved in the ocean;
+(c) by the escape of molecules into space; and (d) by the condensation
+of water vapor.
+
+The combined effect of the various means of increase and decrease
+depends partly on the amount of each constituent received from the
+earth's interior or from space, and partly on the fact that the agencies
+which tend to deplete the atmosphere are highly selective in their
+action. Our knowledge of how large a quantity of new gases the air has
+received is very scanty, but judging by present conditions the general
+tendency is toward a slow increase chiefly because of meteorites,
+volcanic action, and the work of deep-seated springs. As to decrease,
+the case is clearer. This is because the chemically active gases,
+oxygen, CO_{2}, and water vapor, tend to be locked up in the rocks,
+while the chemically inert gases, nitrogen and argon, show almost no
+such tendency. Though oxygen is by far the most abundant element in the
+earth's crust, making up more than 50 per cent of the total, it forms
+only about one-fifth of the air. Nitrogen, on the other hand, is very
+rare in the rocks, but makes up nearly four-fifths of the air. It would,
+therefore, seem probable that throughout the earth's history, there has
+been a progressive increase in the amount of atmospheric nitrogen, and
+presumably a somewhat corresponding increase in the mass of the air. On
+the other hand, it is not clear what changes have occurred in the amount
+of atmospheric oxygen. It may have increased somewhat or perhaps even
+notably. Nevertheless, because of the greater increase in nitrogen, it
+may form no greater percentage of the air now than in the distant past.
+
+As to the absolute amounts of oxygen, Barrell[107] thought that
+atmospheric oxygen began to be present only after plants had appeared.
+It will be recalled that plants absorb carbon dioxide and separate the
+carbon from the oxygen, using the carbon in their tissues and setting
+free the oxygen. As evidence of a paucity of oxygen in the air in early
+Proterozoic times, Barrell cites the fact that the sedimentary rocks of
+that remote time commonly are somewhat greyish or greenish-grey wackes,
+or other types, indicating incomplete oxidation. He admits, however,
+that the stupendous thicknesses of red sandstones, quartzite, and
+hematitic iron ores of the later Proterozoic prove that by that date
+there was an abundance of atmospheric oxygen. If so, the change from
+paucity to abundance must have occurred before fossils were numerous
+enough to give much clue to climate. However, Barrell's evidence as to
+a former paucity of atmospheric oxygen is not altogether convincing. In
+the first place, it does not seem justifiable to assume that there could
+be no oxygen until plants appeared to break down the carbon dioxide, for
+some oxygen is contributed by volcanoes,[108] and lightning decomposes
+water into its elements. Part of the hydrogen thus set free escapes into
+space, for the earth's gravitative force does not appear great enough to
+hold this lightest of gases, but the oxygen remains. Thus electrolysis
+of water results in the accumulation of oxygen. In the second place,
+there is no proof that the ancient greywackes are not deoxidized
+sediments. Light colored rock formations do not necessarily indicate
+a paucity of atmospheric oxygen, for such rocks are abundant
+even in recent times. For example, the Tertiary formations are
+characteristically light colored, a result, however, of deoxidation.
+Finally, the fact that sedimentary rocks, irrespective of their age,
+contain an average of about 1.5 per cent more oxygen than do igneous
+rocks,[109] suggests that oxygen was present in the air in quantity even
+when the earliest shales and sandstones were formed, for atmospheric
+oxygen seems to be the probable source of the extra oxygen they contain.
+The formation of these particular sedimentary rocks by weathering of
+igneous rocks involves only a little carbon dioxide and water. Although
+it seems probable that oxygen was present in the atmosphere even at the
+beginning of the geological record, it may have been far less abundant
+then than now. It may have been removed from the atmosphere by animals
+or by the oxidation of the rocks almost as rapidly as it was added by
+volcanoes, plants, and other agencies.
+
+After this chapter was in type, St. John[C] announced his interesting
+discovery that oxygen is apparently lacking in the atmosphere of Venus.
+He considers that this proves that Venus has no life. Furthermore he
+concludes that so active an element as oxygen cannot be abundant in the
+atmosphere of a planet unless plants continually supply large quantities
+by breaking down carbon dioxide.
+
+But even if the earth has experienced a notable increase in atmospheric
+oxygen since the appearance of life, this does not necessarily involve
+important climatic changes except those due to increased atmospheric
+density. This is because oxygen has very little effect upon the passage
+of light or heat, being transparent to all but a few wave lengths. Those
+absorbed are chiefly in the ultra violet.
+
+The distinct possibility that oxygen has increased in amount, makes it
+the more likely that there has been an increase in the total atmosphere,
+for the oxygen would supplement the increase in the relatively inert
+nitrogen and argon, which has presumably taken place. The climatic
+effects of an increase in the atmosphere include, in the first place, an
+increased scattering of light as it approaches the earth. Nitrogen,
+argon, and oxygen all scatter the short waves of light and thus
+interfere with their reaching the earth. Abbot and Fowle,[110] who have
+carefully studied the matter, believe that at present the scattering is
+quantitatively important in lessening insolation. Hence our supposed
+general increase in the volume of the air during part of geological
+times would tend to reduce the amount of solar energy reaching the
+earth's surface. On the other hand, nitrogen and argon do not appear to
+absorb the long wave lengths known as heat, and oxygen absorbs so little
+as to be almost a non-absorber. Therefore the reduced penetration of the
+air by solar radiation due to the scattering of light would apparently
+not be neutralized by any direct increase in the blanketing effect of
+the atmosphere, and the temperature near the earth's surface would be
+slightly lowered by a thicker atmosphere. This would diminish the amount
+of water vapor which would be held in the air, and thereby lower the
+temperature a trifle more.
+
+In the second place, the higher atmospheric pressure which would result
+from the addition of gases to the air would cause a lessening of the
+rate of evaporation, for that rate declines as pressure increases.
+Decreased evaporation would presumably still further diminish the vapor
+content of the atmosphere. This would mean a greater daily and seasonal
+range of temperature, as is very obvious when we compare clear weather
+with cloudy. Cloudy nights are relatively warm while clear nights are
+cool, because water vapor is an almost perfect absorber of radiant heat,
+and there is enough of it in the air on moist nights to interfere
+greatly with the escape of the heat accumulated during the day.
+Therefore, if atmospheric moisture were formerly much more abundant than
+now, the temperature must have been much more uniform. The tendency
+toward climatic severity as time went on would be still further
+increased by the cooling which would result from the increased wind
+velocity discussed below; for cooling by convection increases with the
+velocity of the wind, as does cooling by conduction.
+
+Any persistent lowering of the general temperature of the air would
+affect not only its ability to hold water vapor, but would produce a
+lessening in the amount of atmospheric carbon dioxide, for the colder
+the ocean becomes the more carbon dioxide it can hold in solution. When
+the oceanic temperature falls, part of the atmospheric carbon dioxide is
+dissolved in the ocean. This minor constituent of the air is important
+because although it forms only 0.003 per cent of the earth's atmosphere,
+Abbot and Fowle's[111] calculations indicate that it absorbs over 10 per
+cent of the heat radiated outward from the earth. Hence variations in
+the amount of carbon dioxide may have caused an appreciable variation in
+temperature and thus in other climatic conditions. Humphreys, as we have
+seen, has calculated that a doubling of the carbon dioxide in the air
+would directly raise the earth's temperature to the extent of 1.3 deg.C.,
+and a halving would lower it a like amount. The indirect results of such
+an increase or decrease might be greater than the direct results, for
+the change in temperature due to variations in carbon dioxide would
+alter the capacity of the air to hold moisture.
+
+Two conditions would especially help in this respect; first, changes in
+nocturnal cooling, and second, changes in local convection. The presence
+of carbon dioxide diminishes nocturnal cooling because it absorbs the
+heat radiated by the earth, and re-radiates part of it back again. Hence
+with increased carbon dioxide and with the consequent warmer nights
+there would be less nocturnal condensation of water vapor to form dew
+and frost. Local convection is influenced by carbon dioxide because this
+gas lessens the temperature gradient. In general, the less the gradient,
+that is, the less the contrast between the temperature at the surface
+and higher up, the less convection takes place. This is illustrated by
+the seasonal variation in convection. In summer, when the gradient is
+steepest, convection reaches its maximum. It will be recalled that when
+air rises it is cooled by expansion, and if it ascends far the moisture
+is soon condensed and precipitated. Indeed, local convection is
+considered by C. P. Day to be the chief agency which keeps the lower air
+from being continually saturated with moisture. The presence of carbon
+dioxide lessens convection because it increases the absorption of heat
+in the zone above the level in which water vapor is abundant, thus
+warming these higher layers. The lower air may not be warmed
+correspondingly by an increase in carbon dioxide if Abbot and Fowle are
+right in stating that near the earth's surface there is enough water
+vapor to absorb practically all the wave lengths which carbon dioxide is
+capable of absorbing. Hence carbon dioxide is chiefly effective at
+heights to which the low temperature prevents water vapor from
+ascending. Carbon dioxide is also effective in cold winters and in high
+latitudes when even the lower air is too cold to contain much water
+vapor. Moreover, carbon dioxide, by altering the amount of atmospheric
+water vapor, exerts an indirect as well as a direct effect upon
+temperature.
+
+Other effects of the increase in air pressure which we are here assuming
+during at least the early part of geological times are corresponding
+changes in barometric contrasts, in the strength of winds, and in the
+mass of air carried by the winds along the earth's surface. The increase
+in the mass of the air would reenforce the greater velocity of the winds
+in their action as eroding and transporting agencies. Because of the
+greater weight of the air, the winds would be capable of picking up more
+dust and of carrying it farther and higher; while the increased
+atmospheric friction would keep it aloft a longer time. The significance
+of dust at high levels and its relation to solar radiation have already
+been discussed in connection with volcanoes. It will be recalled that on
+the average it lowers the surface temperature. At lower levels, since
+dust absorbs heat quickly and gives it out quickly, its presence raises
+the temperature of the air by day and lowers it by night. Hence an
+increase in dustiness tends toward greater extremes.
+
+From all these considerations it appears that if the atmosphere has
+actually evolved according to the supposition which is here tentatively
+entertained, the general tendency of the resultant climatic changes must
+have been partly toward long geological oscillations and partly toward a
+general though very slight increase in climatic severity and in the
+contrasts between the zones. This seems to agree with the geological
+record, although the fact that we are living in an age of relative
+climatic severity may lead us astray.
+
+The significant fact about the whole matter is that the three great
+types of terrestrial agencies, namely, those of the earth's interior,
+those of the oceans, and those of the air, all seem to have suffered
+changes which lead to slow variations of climate. Many reversals have
+doubtless taken place, and the geologic oscillations thus induced are
+presumably of much greater importance than the progressive change, yet
+so far as we can tell the purely terrestrial changes throughout the
+hundreds of millions of years of geological time have tended toward
+complexity and toward increased contrasts from continent to ocean, from
+latitude to latitude, from season to season, and from day to night.
+
+Throughout geological history the slow and almost imperceptible
+differentiation of the earth's surface has been one of the most
+noteworthy of all changes. It has been opposed by the extraordinary
+conservatism of the universe which causes the average temperature today
+to be so like that of hundreds of millions of years ago that many types
+of life are almost identical. Nevertheless, the differentiation has gone
+on. Often, to be sure, it has presumably been completely masked by the
+disturbances of the solar atmosphere which appear to have been the cause
+of the sharper, shorter climatic pulsations. But regardless of cosmic
+conservatism and of solar impulses toward change, the slow
+differentiation of the earth's surface has apparently given to the world
+of today much of the geographical complexity which is so stimulating a
+factor in organic evolution. Such complexity--such diversity from place
+to place--appears to be largely accounted for by purely terrestrial
+causes. It may be regarded as the great terrestrial contribution to the
+climatic environment which guides the development of life.
+
+FOOTNOTES:
+
+[Footnote 97: Encyclopaedia Britannica, 11th edition: article "Ocean."]
+
+[Footnote 98: C. E. P. Brooks: The Meteorological Conditions of an Ice
+sheet and Their Bearing on the Desiccation of the Globe; Quart. Jour.
+Royal Meteorol. Soc., Vol. 40, 1914, pp. 53-70.]
+
+[Footnote 99: Data of Geochemistry, Fourth Ed., 1920; Bull. No. 695, U.
+S. Geol. Survey.]
+
+[Footnote 100: Quoted by Schuchert in The Evolution of the Earth.]
+
+[Footnote 101: Smithsonian Physical Tables, Sixth Revision, 1914, p.
+142.]
+
+[Footnote 102: Chamberlin, in a very suggestive article "On a possible
+reversal of oceanic circulation" (Jour. of Geol., Vol. 14, pp. 363-373,
+1906), discusses the probable climatic consequences of a reversal in the
+direction of deep-sea circulation. It is not wholly beyond the bounds of
+possibility that, in the course of ages the increasing drainage of salt
+from the lands not only by nature but by man's activities in agriculture
+and drainage, may ultimately cause such a reversal by increasing the
+ocean's salinity until the more saline tropical portion is heavier than
+the cooler but fresher subpolar waters. If that should happen,
+Greenland, Antarctica, and the northern shores of America and Asia would
+be warmed by the tropical heat which had been transferred poleward
+beneath the surface of the ocean, without loss _en route_. Subpolar
+regions, under such a condition of reversed deep-sea circulation, might
+have a mild climate. Indeed, they might be among the world's most
+favorable regions climatically.]
+
+[Footnote 103: Encyclopaedia Britannica: article "Ocean."]
+
+[Footnote 104: Chamberlin and Salisbury: Geology, Vol. II, pp. 1-132,
+1906; and T. C. Chamberlin: The Origin of the Earth, 1916.]
+
+[Footnote 105: Personal communication.]
+
+[Footnote 106: R. T. Chamberlin: Gases in Rocks, Carnegie Inst. of
+Wash., No. 106, 1908.]
+
+[Footnote 107: J. Barrell: The Origin of the Earth, in Evolution of the
+Earth and Its Inhabitants, 1918, p. 44, and more fully in an unpublished
+manuscript.]
+
+[Footnote 108: F. W. Clarke: Data of Geochemistry, Fourth Ed., 1920,
+Bull. No. 695, U. S. Geol. Survey, p. 256.]
+
+[Footnote 109: F. W. Clarke: _loc. cit._, pp. 27-34 et al.]
+
+[Footnote C: Chas. E. St. John: Science Service Press Reports from the
+Mt. Wilson Observatory, May, 1922.]
+
+[Footnote 110: Abbot and Fowle: Annals Astrophysical Observatory;
+Smiths. Inst., Vol. II, 1908, p. 163.
+
+F. E. Fowle: Atmospheric Scattering of Light; Misc. Coll. Smiths. Inst.,
+Vol. 69, 1918.]
+
+[Footnote 111: Abbot and Fowle: _loc. cit._, p. 172.]
+
+
+
+
+CHAPTER XIV
+
+THE EFFECT OF OTHER BODIES ON THE SUN
+
+
+If solar activity is really an important factor in causing climatic
+changes, it behooves us to subject the sun to the same kind of inquiry
+to which we have subjected the earth. We have inquired into the nature
+of the changes through which the earth's crust, the oceans, and the
+atmosphere have influenced the climate of geological times. It has not
+been necessary, however, to study the origin of the earth, nor to trace
+its earlier stages. Our study of the geological record begins only when
+the earth had attained practically its present mass, essentially its
+present shape, and a climate so similar to that of today that life as we
+know it was possible. In other words, the earth had passed the stages of
+infancy, childhood, youth, and early maturity, and had reached full
+maturity. As it still seems to be indefinitely far from old age, we
+infer that during geological times its relative changes have been no
+greater than those which a man experiences between the ages of perhaps
+twenty-five and forty.
+
+Similar reasoning applies with equal or greater force to the sun.
+Because of its vast size it presumably passes through its stages of
+development much more slowly than the earth. In the first chapter of
+this book we saw that the earth's relative uniformity of climate for
+hundreds of millions of years seems to imply a similar uniformity in
+solar activity. This accords with a recent tendency among astronomers
+who are more and more recognizing that the stars and the solar system
+possess an extraordinary degree of conservatism. Changes that once were
+supposed to take place in thousands of years are now thought to have
+required millions. Hence in this chapter we shall assume that throughout
+geological times the condition of the sun has been almost as at present.
+It may have been somewhat larger, or different in other ways, but it was
+essentially a hot, gaseous body such as we see today and it gave out
+essentially the same amount of energy. This assumption will affect the
+general validity of what follows only if it departs widely from the
+truth. With this assumption, then, let us inquire into the degree to
+which the sun's atmosphere has probably been disturbed throughout
+geological times.
+
+In _Earth and Sun_, as already explained, a detailed study has led to
+the conclusion that cyclonic storms are influenced by the electrical
+action of the sun. Such action appears to be most intense in sunspots,
+but apparently pertains also to other disturbed areas in the sun's
+atmosphere. A study of sunspots suggests that their true periodicity is
+almost if not exactly identical with that of the orbital revolution of
+Jupiter, 11.8 years. Other investigations show numerous remarkable
+coincidences between sunspots and the orbital revolution of the other
+planets, including especially Saturn and Mercury. This seems to indicate
+that there is some truth in the hypothesis that sunspots and other
+related disturbances of the solar atmosphere owe their periodicity to
+the varying effects of the planets as they approach and recede from the
+sun in their eccentric orbits and as they combine or oppose their
+effects according to their relative positions. This does not mean that
+the energy of the solar disturbances is supposed to come from the
+planets, but merely that their variations act like the turning of a
+switch to determine when and how violently the internal forces of the
+sun shall throw the solar atmosphere into commotion. This hypothesis is
+by no means new, for in one form or another it has been advocated by
+Wolfer, Birkeland, E. W. Brown, Schuster, Arctowski, and others.
+
+The agency through which the planets influence the solar atmosphere is
+not yet clear. The suggested agencies are the direct pull of
+gravitation, the tidal effect of the planets, and an electro-magnetic
+effect. In _Earth and Sun_ the conclusion is reached that the first two
+are out of the question, a conclusion in which E. W. Brown acquiesces.
+Unless some unknown cause is appealed to, this leaves an
+electro-magnetic hypothesis as the only one which has a reasonable
+foundation. Schuster inclines to this view. The conclusions set forth in
+_Earth and Sun_ as to the electrical nature of the sun's influence on
+the earth point somewhat in the same direction. Hence in this chapter we
+shall inquire what would happen to the sun, and hence to the earth, on
+their journey through space, if the solar atmosphere is actually subject
+to disturbance by the electrical or other effects of other heavenly
+bodies. It need hardly be pointed out that we are here venturing into
+highly speculative ground, and that the verity or falsity of the
+conclusions reached in this chapter has nothing to do with the validity
+of the reasoning in previous chapters. Those chapters are based on the
+assumption that terrestrial causes of climatic changes are supplemented
+by solar disturbances which produce their effect partly through
+variations in temperature but also through variations in the intensity
+and paths of cyclonic storms. The present chapter seeks to shed some
+light on the possible causes and sequence of solar disturbances.
+
+Let us begin by scanning the available evidence as to solar disturbances
+previous to the time when accurate sunspot records are available. Two
+rather slender bits of evidence point to cycles of solar activity
+lasting hundreds of years. One of these has already been discussed in
+Chapter VI, where the climatic stress of the fourteenth century was
+described. At that time sunspots are known to have been unusually
+numerous, and there were great climatic extremes. Lakes overflowed in
+Central Asia; storms, droughts, floods, and cold winters were unusually
+severe in Europe; the Caspian Sea rose with great rapidity; the trees of
+California grew with a vigor unknown for centuries; the most terrible of
+recorded famines occurred in England and India; the Eskimos were
+probably driven south by increasing snowiness in Greenland; and the
+Mayas of Yucatan appear to have made their last weak attempt at a
+revival of civilization under the stimulus of greater storminess and
+less constant rainfall.
+
+The second bit of evidence is found in recent exhaustive studies of
+periodicities by Turner[112] and other astronomers. They have sought
+every possible natural occurrence for which a numerical record is
+available for a long period. The most valuable records appear to be
+those of tree growth, Nile floods, Chinese earthquakes, and sunspots.
+Turner reaches the conclusion that all four types of phenomena show the
+same periodicity, namely, cycles with an average length of about 260 to
+280 years. He suggests that if this is true, the cycles in tree growth
+and in floods, both of which are climatic, are probably due to a
+non-terrestrial cause. The fact that the sunspots show similar cycles
+suggests that the sun's variations are the cause.
+
+These two bits of evidence are far too slight to form the foundation of
+any theory as to changes in solar activity in the geological past.
+Nevertheless it may be helpful to set forth certain possibilities as a
+stimulus to further research. For example, it has been suggested that
+meteoric bodies may have fallen into the sun and caused it suddenly to
+flare up, as it were. This is not impossible, although it does not
+appear to have taken place since men became advanced enough to make
+careful observations. Moreover, the meteorites which now fall on the
+earth are extremely small, the average size being computed as no larger
+than a grain of wheat. The largest ever found on the earth's surface, at
+Bacubirito in Mexico, weighs only about fifty tons, while within the
+rocks the evidences of meteorites are extremely scanty and
+insignificant. If meteorites had fallen into the sun often enough and of
+sufficient size to cause glacial fluctuations and historic pulsations of
+climate, it seems highly probable that the earth would show much more
+evidence of having been similarly disturbed. And even if the sun should
+be bombarded by large meteors the result would probably not be sudden
+cold periods, which are the most notable phenomena of the earth's
+climatic history, but sudden warm periods followed by slow cooling.
+Nevertheless, the disturbance of the sun by collision with meteoric
+matter can by no means be excluded as a possible cause of climatic
+variations.
+
+Allied to the preceding hypothesis is Shapley's[113] nebular hypothesis.
+At frequent intervals, averaging about once a year during the last
+thirty years, astronomers have discovered what are known as novae. These
+are stars which were previously faint or even invisible, but which flash
+suddenly into brilliancy. Often their light-giving power rises seven or
+eight magnitudes--a thousand-fold. In addition to the spectacular novae
+there are numerous irregular variables whose brilliancy changes in every
+ratio from a few per cent up to several magnitudes. Most of them are
+located in the vicinity of nebulae, as is also the case with novae. This,
+as well as other facts, makes it probable that all these stars are
+"friction variables," as Shapley calls them. Apparently as they pass
+through the nebulae they come in contact with its highly diffuse matter
+and thereby become bright much as the earth would become bright if its
+atmosphere were filled with millions of almost infinitesimally small
+meteorites. A star may also lose brilliancy if nebulous matter
+intervenes between it and the observer. If our sun has been subjected to
+any of these changes some sort of climatic effect must have been
+produced.
+
+In a personal communication Shapley amplifies the nebular climatic
+hypothesis as follows:
+
+    Within 700 light years of the sun in many directions (Taurus,
+    Cygnus, Ophiuchus, Scorpio) are great diffuse clouds of nebulosity,
+    some bright, most of them dark. The probability that stars moving in
+    the general region of such clouds will encounter this material is
+    very high, for the clouds fill enormous volumes of space,--e.g.,
+    probably more than a hundred thousand cubic light years in the Orion
+    region, and are presumably composed of rarefied gases or of dust
+    particles. Probably throughout all our part of space such nebulosity
+    exists (it is all around us, we are sure), but only in certain
+    regions is it dense enough to affect conspicuously the stars
+    involved in it. If a star moving at high velocity should collide
+    with a dense part of such a nebulous cloud, we should probably have
+    a typical nova. If the relative velocity of nebulous material and
+    star were low or moderate, or if the material were rare, we should
+    not expect a conspicuous effect on the star's light.
+
+    In the nebulous region of Orion, which is probably of unusually high
+    density, there are about 100 known stars, varying between 20% and
+    80% of their total light--all of them irregularly--some slowly, some
+    suddenly. Apparently they are "friction variables." Some of the
+    variables suddenly lose 40% of their light as if blanketed by
+    nebulous matter. In the Trifid Nebula there are variables like those
+    of Orion, in Messier 8 also, and probably many of the 100 or so
+    around the Rho Ophiuchi region belong to this kind.
+
+    I believe that our sun could not have been a typical nova, at least
+    not since the Archeozoic, that is for perhaps a billion years. I
+    believe we have in geological climates final proof of this, because
+    an increase in the amount of solar radiation by 1000 times as in the
+    typical nova, would certainly punctuate emphatically the life cycle
+    on the earth, even if the cause of the nova would not at the same
+    time eliminate the smaller planets. But the sun may have been one of
+    these miniature novae or friction variables; and I believe it very
+    probable that its wanderings through this part of space could not
+    long leave its mean temperature unaffected to the amount of a few
+    per cent.
+
+    One reason we have not had this proposal insisted upon before is
+    that the data back of it are mostly new--the Orion variables have
+    been only recently discovered and studied, the distribution and
+    content of the dark nebulae are hardly as yet generally known.
+
+This interesting hypothesis cannot be hastily dismissed. If the sun
+should pass through a nebula it seems inevitable that there would be at
+least slight climatic effects and perhaps catastrophic effects through
+the action of the gaseous matter not only on the sun but on the earth's
+own atmosphere. As an explanation of the general climatic conditions of
+the past, however, Shapley points out that the hypothesis has the
+objection of being vague, and that nebulosity should not be regarded as
+more than "a possible factor." One of the chief difficulties seems to be
+the enormously wide distribution of as yet undiscovered nebulous matter
+which must be assumed if any large share of the earth's repeated
+climatic changes is to be ascribed to such matter. If such matter is
+actually abundant in space, it is hard to see how any but the nearest
+stars would be visible. Another objection is that there is no known
+nebulosity near at hand with which to connect the climatic vicissitudes
+of the last glacial period. Moreover, the known nebulae are so much less
+numerous than stars that the chances that the sun will encounter one of
+them are extremely slight. This, however, is not an objection, for
+Shapley points out that during geological times the sun can never have
+varied as much as do the novae, or even as most of the friction
+variables. Thus the hypothesis stands as one that is worth
+investigating, but that cannot be finally rejected or accepted until it
+is made more definite and until more information is available.
+
+Another suggested cause of solar variations is the relatively sudden
+contraction of the sun such as that which sometimes occurs on the earth
+when continents are uplifted and mountains upheaved. It seems improbable
+that this could have occurred in a gaseous body like the sun. Lacking,
+as it does, any solid crust which resists a change of form, the sun
+probably shrinks steadily. Hence any climatic effects thus produced must
+be extremely gradual and must tend steadily in one direction for
+millions of years.
+
+Still another suggestion is that the tidal action of the stars and other
+bodies which may chance to approach the sun's path may cause
+disturbances of the solar atmosphere. The vast kaleidoscope of space is
+never quiet. The sun, the stars, and all the other heavenly bodies are
+moving, often with enormous speed. Hence the effect of gravitation upon
+the sun must vary constantly and irregularly, as befits the geological
+requirements. In the case of the planets, however, the tidal effect does
+not seem competent to produce the movements of the solar atmosphere
+which appear to be concerned in the inception of sunspots. Moreover,
+there is only the most remote probability that a star and the sun will
+approach near enough to one another to produce a pronounced
+gravitational disturbance in the solar atmosphere. For instance, if it
+be assumed that changes in Jupiter's tidal effect on the sun are the
+main factor in regulating the present difference between sunspot maxima
+and sunspot minima, the chances that a star or some non-luminous body of
+similar mass will approach near enough to stimulate solar activity and
+thereby bring on glaciation are only one in twelve billion years, as
+will be explained below. This seems to make a gravitational hypothesis
+impossible.
+
+Another possible cause of solar disturbances is that the stars in their
+flight through space may exert an electrical influence which upsets the
+equilibrium of the solar atmosphere. At first thought this seems even
+more impossible than a gravitational effect. Electrostatic effects,
+however, differ greatly from those of tides. They vary as the diameter
+of a body instead of as its mass; their differentials also vary
+inversely as the square of the distance instead of as the cube.
+Electrostatic effects also increase as the fourth power of the
+temperature or at least would do so if they followed the law of black
+bodies; they are stimulated by the approach of one body to another; and
+they are cumulative, for if ions arrive from space they must accumulate
+until the body to which they have come begins to discharge them. Hence,
+on the basis of assumptions such as those used in the preceding
+paragraph, the chances of an electrical disturbance of the solar
+atmosphere sufficient to cause glaciation on the earth may be as high as
+one in twenty or thirty million years. This seems to put an electrical
+hypothesis within the bounds of possibility. Further than that we cannot
+now go. There may be other hypotheses which fit the facts much better,
+but none seems yet to have been suggested.
+
+In the rest of this chapter the tidal and electrical hypotheses of
+stellar action on the sun will be taken up in detail. The tidal
+hypothesis is considered because in discussions of the effect of the
+planets it has hitherto held almost the entire field. The electrical
+hypothesis will be considered because it appears to be the best yet
+suggested, although it still seems doubtful whether electrical effects
+can be of appreciable importance over such vast distances as are
+inevitably involved. The discussion of both hypotheses will necessarily
+be somewhat technical, and will appeal to the astronomer more than to
+the layman. It does not form a necessary part of this book, for it has
+no bearing on our main thesis of the effect of the sun on the earth. It
+is given here because ultimately the question of changes in solar
+activity during geological times must be faced.
+
+In the astronomical portion of the following discussion we shall follow
+Jeans[114] in his admirable attempt at a mathematical analysis of the
+motions of the universe. Jeans divides the heavenly bodies into five
+main types. (1) Spiral nebulae, which are thought by some astronomers to
+be systems like our own in the making, and by others to be independent
+universes lying at vast distances beyond the limits of our Galactic
+universe, as it is called from the Galaxy or Milky Way. (2) Nebulae of a
+smaller type, called planetary. These lie within the Galactic portion of
+the universe and seem to be early stages of what may some day be stars
+or solar systems. (3) Binary or multiple stars, which are
+extraordinarily numerous. In some parts of the heavens they form 50 or
+even 60 per cent of the stars and in the galaxy as a whole they seem to
+form "fully one third." (4) Star clusters. These consist of about a
+hundred groups of stars in each of which the stars move together in the
+same direction with approximately the same velocity. These, like the
+spiral nebulae, are thought by some astronomers to lie outside the limits
+of the galaxy, but this is far from certain. (5) The solar system.
+According to Jeans this seems to be unique. It does not fit into the
+general mathematical theory by which he explains spiral nebulae,
+planetary nebulae, binary stars, and star clusters. It seems to demand a
+special explanation, such as is furnished by tidal disruption due to the
+passage of the sun close to another star.
+
+The part of Jeans' work which specially concerns us is his study of the
+probability that some other star will approach the sun closely enough to
+have an appreciable gravitative or electrical effect, and thus cause
+disturbances in the solar atmosphere. Of course both the star and the
+sun are moving, but to avoid circumlocution we shall speak of such
+mutual approaches simply as approaches of the sun. For our present
+purpose the most fundamental fact may be summed up in a quotation from
+Jeans in which he says that most stars "show evidence of having
+experienced considerable disturbance by other systems; there is no
+reason why our solar system should be expected to have escaped the
+common fate." Jeans gives a careful calculation from which it is
+possible to derive some idea of the probability of any given degree of
+approach of the sun and some other star. Of course all such calculations
+must be based on certain assumptions. The assumptions made by Jeans are
+such as to make the probability of close approaches as great as
+possible. For example, he allows only 560 million years for the entire
+evolution of the sun, whereas some astronomers and geologists would put
+the figure ten or more times as high. Nevertheless, Jeans' assumptions
+at least show the order of magnitude which we may expect on the basis of
+reasonable astronomical conclusions.
+
+According to the planetary hypothesis of sunspots, the difference in the
+effect of Jupiter when it is nearest and farthest from the sun is the
+main factor in starting the sunspot cycle and hence the corresponding
+terrestrial cycle. The climatic difference between sunspot maxima and
+minima, as measured by temperature, apparently amounts to at least a
+twentieth and perhaps a tenth of the difference between the climate of
+the last glacial epoch and the present. We may suppose, then, that a
+body which introduced a gravitative or electrical factor twenty times as
+great as the difference in Jupiter's effect at its maximum and minimum
+distances from the sun would cause a glacial epoch if the effect lasted
+long enough. Of course the other planets combine their effects with that
+of Jupiter, but for the sake of simplicity we will leave the others out
+of account. The difference between Jupiter's maximum and minimum tidal
+effect on the sun amounts to 29 per cent of the planet's average effect.
+The corresponding difference, according to the electrical hypothesis, is
+about 19 per cent, for electrostatic action varies as the square of the
+distance instead of as the cube. Let us assume that a body exerting four
+times Jupiter's present tidal effect and placed at the average distance
+of Jupiter from the sun would disturb the sun's atmosphere twenty times
+as much as the present difference between sunspot maxima and minima, and
+thus, perhaps, cause a glacial period on the earth.
+
+On the basis of this assumption our first problem is to estimate the
+frequency with which a star, visible or dark, is likely to approach near
+enough to the sun to produce a _tidal_ effect four times that of
+Jupiter. The number of visible stars is known or at least well
+estimated. As to dark stars, which have grown cool, Arrhenius believed
+that they are a hundred times as numerous as bright stars; few
+astronomers believe that there are less than three or four times as
+many. Dr. Shapley of the Harvard Observatory states that a new
+investigation of the matter suggests that eight or ten is probably a
+maximum figure. Let us assume that nine is correct. The average visible
+star, so far as measured, has a mass about twice that of the sun, or
+about 2100 times that of Jupiter. The distances of the stars have been
+measured in hundreds of cases and thus we can estimate how many stars,
+both visible and invisible, are on an average contained in a given
+volume of space. On this basis Jeans estimates that there is only one
+chance in thirty billion years that a visible star will approach within
+2.8 times the distance of Neptune from the sun, that is, within about
+eight billion miles. If we include the invisible stars the chances
+become one in three billion years. In order to produce four times the
+tidal effect of Jupiter, however, the average star would have to
+approach within about four billion miles of the sun, and the chances of
+that are only one in twelve billion years. The disturbing star would be
+only 40 per cent farther from the sun than Neptune, and would almost
+pass within the solar system.
+
+Even though Jeans holds that the frequency of the mutual approach of the
+sun and a star was probably much greater in the distant past than at
+present, the figures just given lend little support to the tidal
+hypothesis. In fact, they apparently throw it out of court. It will be
+remembered that Jeans has made assumptions which give as high a
+frequency of stellar encounters as is consistent with the astronomical
+facts. We have assumed nine dark stars for every bright one, which may
+be a liberal estimate. Also, although we have assumed that a disturbance
+of the sun's atmosphere sufficient to cause a glacial period would arise
+from a tidal effect only twenty times as great as the difference in
+Jupiter's effect when nearest the sun and farthest away, in our
+computations this has actually been reduced to thirteen. With all these
+favorable assumptions the chances of a stellar approach of the sort here
+described are now only one in twelve billion years. Yet within a hundred
+million years, according to many estimates of geological time, and
+almost certainly within a billion, there have been at least half a dozen
+glaciations.
+
+Our use of Jeans' data interposes another and equally insuperable
+difficulty to any tidal hypothesis. Four billion miles is a very short
+distance in the eyes of an astronomer. At that distance a star twice the
+size of the sun would attract the outer planets more strongly than the
+sun itself, and might capture them. If a star should come within four
+billion miles of the sun, its effect in distorting the orbits of all the
+planets would be great. If this had happened often enough to cause all
+the glaciations known to geologists, the planetary orbits would be
+strongly elliptical instead of almost circular. The consideration here
+advanced militate so strongly against the tidal hypothesis of solar
+disturbances that it seems scarcely worth while to consider it further.
+
+Let us turn now to the electrical hypothesis. Here the conditions are
+fundamentally different from those of the tidal hypothesis. In the first
+place the electrostatic effect of a body has nothing to do with its
+mass, but depends on the area of its surface; that is, it varies as the
+square of the radius. Second, the emission of electrons varies
+exponentially. If hot glowing stars follow the same law as black bodies
+at lower temperatures, the emission of electrons, like the emission of
+other kinds of energy, varies as the fourth power of the absolute
+temperature. In other words, suppose there are two black bodies,
+otherwise alike, but one with a temperature of 27 deg. C. or 300 deg. on the
+absolute scale, and the other with 600 deg. on the absolute scale. The
+temperature of one is twice as high as that of the other, but the
+electrostatic effect will be sixteen times as great.[115] Third, the
+number of electrons that reach a given body varies inversely as the
+square of the distance, instead of as the cube which is the case with
+tide-making forces.
+
+In order to use these three principles in calculating the effect of the
+stars we must know the diameters, distances, temperature, and number of
+the stars. The distances and number may safely be taken as given by
+Jeans in the calculations already cited. As to the diameters, the
+measurements of the stars thus far made indicate that the average mass
+is about twice that of the sun. The average density, as deduced by
+Shapley[116] from the movements of double stars, is about one-eighth the
+solar density. This would give an average diameter about two and a half
+times that of the sun. For the dark stars, we shall assume for
+convenience that they are ten times as numerous as the bright ones. We
+shall also assume that their diameter is half that of the sun, for being
+cool they must be relatively dense, and that their temperature is the
+same as that which we shall assume for Jupiter.
+
+As to Jupiter we shall continue our former assumption that a body with
+four times the effectiveness of that planet, which here means with twice
+as great a radius, would disturb the sun enough to cause glaciation. It
+would produce about twenty times the electrostatic effect which now
+appears to be associated with the difference in Jupiter's effect at
+maximum and minimum. The temperature of Jupiter must also be taken into
+account. The planet is supposed to be hot because its density is low,
+being only about 1.25 that of water. Nevertheless, it is probably not
+luminous, for as Moulton[117] puts it, shadows upon it are black and its
+moons show no sign of illumination except from the sun. Hence a
+temperature of about 600 deg.C., or approximately 900 deg. on the absolute
+scale, seems to be the highest that can reasonably be assigned to the
+cold outer layer whence electrons are emitted. As to the temperature of
+the sun, we shall adopt the common estimate of about 6300 deg.C. on the
+absolute scale. The other stars will be taken as averaging the same,
+although of course they vary greatly.
+
+When Jeans' method of calculating the probability of a mutual approach
+of the sun and a star is applied to the assumptions given above, the
+results are as shown in Table 5. On that basis the dark stars seem to be
+of negligible importance so far as the electrical hypothesis is
+concerned. Even though they may be ten times as numerous as the bright
+ones there appears to be only one chance in 130 billion years that one
+of them will approach the sun closely enough to cause the assumed
+disturbance of the solar atmosphere. On the other hand, if all the
+visible stars were the size of the sun, and as hot as that body, their
+electrical effect would be fourfold that of our assumed dark star
+because of their size, and 2401 times as great because of their
+temperature, or approximately 10,000 times as great. Under such
+conditions the theoretical chance of an approach that would cause
+glaciation is one in 130 million years. If the average visible star is
+somewhat cooler than the sun and has a radius about two and one-half
+times as great, as appears to be the fact, the chances rise to one in
+thirty-eight million years. A slight and wholly reasonable change in our
+assumptions would reduce this last figure to only five or ten million.
+For instance, the earth's mean temperature during the glacial period has
+been assumed as 10 deg.C. lower than now, but the difference may have been
+only 6 deg.. Again, the temperature of the outer atmosphere of Jupiter where
+the electrons are shot out may be only 500 deg. or 700 deg. absolute, instead of
+900 deg.. Or the diameter of the average star may be five or ten times that
+of the sun, instead of only two and one-half times as great. All this,
+however, may for the present be disregarded. The essential point is that
+even when the assumptions err on the side of conservatism, the results
+are of an order of magnitude which puts the electrical hypothesis within
+the bounds of possibility, whereas similar assumptions put the tidal
+hypothesis, with its single approach in twelve billion years, far beyond
+those limits.
+
+The figures for Betelgeuse in Table 5 are interesting. At a meeting of
+the American Association for the Advancement of Science in December,
+1920, Michelson reported that by measurements of the interference of
+light coming from the two sides of that bright star in Orion, the
+observers at Mount Wilson had confirmed the recent estimates of three
+other authorities that the star's diameter is about 218 million miles,
+or 250 times that of the sun. If other stars so much surpass the
+estimates of only a decade or two ago, the average diameter of all the
+visible stars must be many times that of the sun. The low figure for
+Betelgeuse in section D of the table means that if all the stars were as
+large as Betelgeuse, several might often be near enough to cause
+profound disturbances of the solar atmosphere. Nevertheless, because of
+the low temperature of the giant red stars of the Betelgeuse type, the
+distance at which one of them would produce a given electrical effect is
+only about five times the distance at which our assumed average star
+would produce the same effect. This, to be sure, is on the assumption
+that the radiation of energy from incandescent bodies varies according
+to temperature in the same ratio as the radiation from black bodies.
+Even if this assumption departs somewhat from the truth, it still seems
+almost certain that the lower temperature of the red compared with the
+high temperature of the white stars must to a considerable degree reduce
+the difference in electrical effect which would otherwise arise from
+their size.
+
+TABLE 5
+
+THEORETICAL PROBABILITY OF STELLAR APPROACHES
+
+  ---------------------------------------------------------------------
+                   |      1        |     2     |    3     |     4      |
+                   |               |           |_Average  |            |
+                   |  _Dark Stars_ |   _Sun_   |  Star_   |_Betelgeuse_|
+  ---------------------------------------------------------------------
+  A. Approximate   |               |           |          |            |
+  radius in miles  |    430,000    |  860,000  | 2,150,000|218,000,000 |
+                   |               |           |          |            |
+  B. Assumed       |               |           |          |            |
+  temperature above|               |           |          |            |
+  absolute zero.   |    900 deg. C.    |   6300 deg. C.|  5400 deg. C.|  3150 deg. C.  |
+                   |               |           |          |            |
+  C. Approximate   |               |           |          |            |
+  theoretical      |               |           |          |            |
+  distance at which|               |           |          |            |
+  star would cause |               |           |          |            |
+  solar disturbance|               |           |          |            |
+  great enough to  |               |           |          |            |
+  cause glaciation |               |           |          |            |
+  (billions[118]   |               |           |          |            |
+  of miles).       |      1.2      |    120    |    220   |   3200     |
+                   |               |           |          |            |
+  D. Average       |               |           |          |            |
+  interval between |               |           |          |            |
+  approaches       |               |           |          |            |
+  close enough to  |               |           |          |            |
+  cause glaciation |               |           |          |            |
+  if all stars     |130,000,000,000|           |          |            |
+  were of given    |[119]          |           |          |            |
+  type. Years.     |               |130,000,000|38,000,000|  700,000   |
+  ---------------------------------------------------------------------
+
+Thus far in our attempt to estimate the distance at which a star might
+disturb the sun enough to cause glaciation on the earth, we have
+considered only the star's size and temperature. No account has been
+taken of the degree to which its atmosphere is disturbed. Yet in the
+case of the sun this seems to be one of the most important factors. The
+magnetic field of sunspots is sometimes 50 or 100 times as strong as
+that of the sun in general. The strength of the magnetic field appears
+to depend on the strength of the electrical currents in the solar
+atmosphere. But the intensity of the sunspots and, by inference, of the
+electrical currents, may depend on the electrical action of Jupiter and
+the other planets. If we apply a similar line of reasoning to the stars,
+we are at once led to question whether the electrical activity of double
+stars may not be enormously greater than that of isolated stars like the
+sun.
+
+If this line of reasoning is correct, the atmosphere of every double
+star must be in a state of commotion vastly greater than that of the
+sun's atmosphere even when it is most disturbed. For example, suppose
+the sun were accompanied by a companion of equal size at a distance of
+one million miles, which would make it much like many known double
+stars. Suppose also that in accordance with the general laws of physics
+the electrical effect of the two suns upon one another is proportional
+to the fourth power of the temperature, the square of the radius, and
+the inverse square of the distance. Then the effect of each sun upon the
+other would be sixty billion (6 x 10^{10}) times as great as the present
+electrical effect of Jupiter upon the sun. Just what this would mean as
+to the net effect of a pair of such suns upon the electrical potential
+of other bodies at a distance we can only conjecture. The outstanding
+fact is that the electrical conditions of a double star must be
+radically different and vastly more intense than those of a single star
+like the sun.
+
+This conclusion carries weighty consequences. At present twenty or more
+stars are known to be located within about 100 trillion miles of the sun
+(five parsecs, as the astronomers say), or 16.5 light years. According
+to the assumptions employed in Table 5 an average single star would
+influence the sun enough to cause glaciation if it came within
+approximately 200 billion miles. If the star were double, however, it
+might have an electrical capacity enormously greater than that of the
+sun. Then it would be able to cause glaciation at a correspondingly
+great distance. Today Alpha Centauri, the nearest known star about
+twenty-five trillion miles, or 4.3 light years from the sun, and Sirius,
+the brightest star in the heavens, is about fifty trillion miles away,
+or 8.5 light years. If these stars were single and had a diameter three
+times that of the sun, and if they were of the same temperature as has
+been assumed for Betelgeuse, which is about fifty times as far away as
+Alpha Centauri, the relative effects of the three stars upon the sun
+would be, approximately, Betelgeuse 700, Alpha Centauri 250, Sirius 1.
+But Alpha Centauri is triple and Sirius double, and both are much hotter
+than Betelgeuse. Hence Alpha Centauri and even Sirius may be far more
+effective than Betelgeuse.
+
+The two main components of Alpha Centauri are separated by an average
+distance of about 2,200,000,000 miles, or somewhat less than that of
+Neptune from the sun. A third and far fainter star, one of the faintest
+yet measured, revolves around them at a great distance. In mass and
+brightness the two main components are about like the sun, and we will
+assume that the same is true of their radius. Then, according to the
+assumptions made above, their effect in disturbing one another
+electrically would be about 10,000 times the total effect of Jupiter
+upon the sun, or 2500 times the effect that we have assumed to be
+necessary to produce a glacial period. We have already seen in Table 5
+that, according to our assumptions, a single star like the sun would
+have to approach within 120 billion miles of the solar system, or within
+2 per cent of a light year, in order to cause glaciation. By a similar
+process of reasoning it appears that if the mutual electrical excitation
+of the two main parts of Alpha Centauri, regardless of the third part,
+is proportional to the apparent excitation of the sun by Jupiter, Alpha
+Centauri would be 5000 times as effective as the sun. In other words, if
+it came within 8,500,000,000,000 miles of the sun, or 1.4 light years,
+it would so change the electrical conditions as to produce a glacial
+epoch. In that case Alpha Centauri is now so near that it introduces a
+disturbing effect equal to about one-sixth of the effect needed to cause
+glaciation on the earth. Sirius and perhaps others of the nearer and
+brighter or larger stars may also create appreciable disturbances in the
+electrical condition of the sun's atmosphere, and may have done so to a
+much greater degree in the past, or be destined to do so in the future.
+Thus an electrical hypothesis of solar disturbances seems to indicate
+that the position of the sun in respect to other stars may be a factor
+of great importance in determining the earth's climate.
+
+FOOTNOTES:
+
+[Footnote 112: H. H. Turner: On a Long Period in Chinese Earthquake
+Records; Mon. Not. Royal Astron. Soc., Vol. 79, 1919, pp. 531-539; Vol.
+80, 1920, pp. 617-619; Long Period Terms in the Growth of Trees; _idem_,
+pp.793-808.]
+
+[Footnote 113: Harlow Shapley: Note on a Possible Factor in Geologic
+Climates; Jour. Geol., Vol. 29, No. 4, May, 1921; Novae and Variable
+Stars, Pub. Astron. Soc. Pac., No. 194, Aug., 1921.]
+
+[Footnote 114: J. H. Jeans: Problems of Cosmogony and Stellar Dynamics,
+Cambridge, 1919.]
+
+[Footnote 115: This fact is so important and at the same time so
+surprising to the layman, that a quotation from The Electron Theory of
+Matter by O. W. Richardson, 1914, pp. 326 and 334 is here added.
+
+"It is a very familiar fact that when material bodies are heated they
+emit electromagnetic radiations, in the form of thermal, luminous, and
+actinic rays, in appreciable quantities. Such an effect is a natural
+consequence of the electron and kinetic theories of matter. On the
+kinetic theory, temperature is a measure of the violence of the motion
+of the ultimate particles; and we have seen that on the electron theory,
+electromagnetic radiation is a consequence of their acceleration. The
+calculation of this emission from the standpoint of the electron theory
+alone is a very complex problem which takes us deeply into the structure
+of matter and which has probably not yet been satisfactorily resolved.
+Fortunately, we can find out a great deal about these phenomena by the
+application of general principles like the conservation of energy and
+the second law of thermodynamics without considering special assumptions
+about the ultimate constitution of matter. It is to be borne in mind
+that the emission under consideration occurs at all temperatures
+although it is more marked the higher the temperature.... The energy per
+unit volume, _in vacuo_, of the radiation in equilibrium in an enclosure
+at the absolute temperature, T, is equal to a universal constant, A,
+multiplied by the fourth power of the absolute temperature. Since the
+intensity of the radiation is equal to the energy per unit volume
+multiplied by the velocity of light, it follows that the former must
+also be proportional to the fourth power of the absolute temperature.
+Moreover, if E is the total emission from unit area of a perfectly black
+body, we see from p. 330 that E=A'T^{4}, where A' is a new universal
+constant. This result is usually known as Stefan's Law. It was suggested
+by Stefan in the inaccurate form that the total radiant energy of
+emission from bodies varies as the fourth power of the absolute
+temperature, as a generalization from the results of experiments. The
+credit for showing that it is a consequence of the existence of
+radiation pressure combined with the principles of thermodynamics is due
+to Bartoli and Boltzmann."]
+
+[Footnote 116: Quoted by Moulton in his Introduction to Astronomy.]
+
+[Footnote 117: Introduction to Astronomy.]
+
+[Footnote 118: The term billions, here and elsewhere, is used in the
+American sense, 10^{9}.]
+
+[Footnote 119: The assumed number of stars here is ten times as great as
+in the other parts of this line.]
+
+
+
+
+CHAPTER XV
+
+THE SUN'S JOURNEY THROUGH SPACE
+
+
+Having gained some idea of the nature of the electrical hypothesis of
+solar disturbances and of the possible effect of other bodies upon the
+sun's atmosphere, let us now compare the astronomical data with those of
+geology. Let us take up five chief points for which the geologist
+demands an explanation, and which any hypothesis must meet if it is to
+be permanently accepted. These are (1) the irregular intervals at which
+glacial periods occur; (2) the division of glacial periods into epochs
+separated sometimes by hundreds of thousands of years; (3) the length of
+glacial periods and epochs; (4) the occurrence of glacial stages and
+historic pulsations in the form of small climatic waves superposed upon
+the larger waves of glacial epochs; (5) the occurrence of climatic
+conditions much milder than those of today, not only in the middle
+portion of the great geological eras, but even in some of the recent
+inter-glacial epochs.
+
+1. The irregular duration of the interval from one glacial epoch to
+another corresponds with the irregular distribution of the stars. If
+glaciation is indirectly due to stellar influences, the epochs might
+fall close together, or might be far apart. If the average interval were
+ten million years, one interval might be thirty million or more and the
+next only one or two hundred thousand. According to Schuchert, the known
+periods of glacial or semi-glacial climate have been approximately as
+follows:
+
+LIST OF GLACIAL PERIODS
+
+  1. Archeozoic.
+     (1/4 of geological time or perhaps much more)
+
+      No known glacial periods.
+
+  2. Proterozoic.
+     (1/4 of geological time)
+
+      a. Oldest known glacial period near base of Proterozoic in
+         Canada. Evidence widely distributed.
+
+      b. Indian glacial period; time unknown.
+
+      c. African glacial period; time unknown.
+
+      d. Glaciation near end of Proterozoic in Australia, Norway,
+         and China.
+
+  3. Paleozoic.
+     (1/4 of geological time)
+
+      a. Late Ordovician(?). Local in Arctic Norway.
+
+      b. Silurian. Local in Alaska.
+
+      c. Early Devonian. Local in South Africa.
+
+      d. Early Permian. World-wide and very severe.
+
+  4. Mesozoic and Cenozoic.
+     (1/4 of geological time)
+
+      a-b. None definitely determined during Mesozoic, although
+        there appears to have been periods of cooling (a) in the
+        late Triassic, and (b) in the late Cretacic, with at least
+        local glaciation in early Eocene.
+
+      c. Severe glacial period during Pleistocene.
+
+This table suggests an interesting inquiry. During the last few decades
+there has been great interest in ancient glaciation and geologists have
+carefully examined rocks of all ages for signs of glacial deposits. In
+spite of the large parts of the earth which are covered with deposits
+belonging to the Mesozoic and Cenozoic, which form the last quarter of
+geological time, the only signs of actual glaciation are those of the
+great Pleistocene period and a few local occurrences at the end of the
+Mesozoic or beginning of the Cenozoic. Late in the Triassic and early in
+the Jurassic, the climate appears to have been rigorous, although no
+tillites have been found to demonstrate glaciation. In the preceding
+quarter, that is, the Paleozoic, the Permian glaciation was more severe
+than that of the Pleistocene, and the Devonian than that of the Eocene,
+while the Ordovician evidences of low temperature are stronger than
+those at the end of the Triassic. In view of the fact that rocks of
+Paleozoic age cover much smaller areas than do those of later age, the
+three Paleozoic glaciations seem to indicate a relative frequency of
+glaciation. Going back to the Proterozoic, it is astonishing to find
+that evidence of two highly developed glacial periods, and possibly
+four, has been discovered. Since the Indian and the African glaciations
+of Proterozoic times are as yet undated, we cannot be sure that they are
+not of the same date as the others. Nevertheless, even two is a
+surprising number, for not only are most Proterozoic rocks so
+metamorphosed that possible evidences of glacial origin are destroyed,
+but rocks of that age occupy far smaller areas than either those of
+Paleozoic or, still more, Mesozoic and Cenozoic age. Thus the record of
+the last three-quarters of geological time suggests that if rocks of all
+ages were as abundant and as easily studied as those of the later
+periods, the frequency of glacial periods would be found to increase as
+one goes backward toward the beginnings of the earth's history. This is
+interesting, for Jeans holds that the chances that the stars would
+approach one another were probably greater in the past than at present.
+This conclusion is based on the assumption that our universe is like the
+spiral nebulae in which the orbits of the various members are nearly
+circular during the younger stages. Jeans considers it certain that in
+such cases the orbits will gradually become larger and more elliptical
+because of the attraction of one body for another. Thus as time goes on
+the stars will be more widely distributed and the chances of approach
+will diminish. If this is correct, the agreement between astronomical
+theory and geological conclusions suggests that the two are at least not
+in opposition.
+
+The first quarter of geological time as well as the last three must be
+considered in this connection. During the Archeozoic, no evidence of
+glaciation has yet been discovered. This suggests that the geological
+facts disprove the astronomical theory. But our knowledge of early
+geological times is extremely limited, so limited that lack of evidence
+of glaciation in the Archeozoic may have no significance. Archeozoic
+rocks have been studied minutely over a very small percentage of the
+earth's land surface. Moreover, they are highly metamorphosed so that,
+even if glacial tills existed, it would be hard to recognize them.
+Third, according to both the nebular and the planetesimal hypotheses, it
+seems possible that during the earliest stages of geological history the
+earth's interior was somewhat warmer than now, and the surface may have
+been warmed more than at present by conduction, by lava flows, and by
+the fall of meteorites. If the earth during the Archeozoic period
+emitted enough heat to raise its surface temperature a few degrees, the
+heat would not prevent the development of low forms of life but might
+effectively prevent all glaciation. This does not mean that it would
+prevent changes of climate, but merely changes so extreme that their
+record would be preserved by means of ice. It will be most interesting
+to see whether future investigations in geology and astronomy indicate
+either a semi-uniform distribution of glacial periods throughout the
+past, or a more or less regular decrease in frequency from early times
+down to the present.
+
+2. The Pleistocene glacial period was divided into at least four epochs,
+while in the Permian at least one inter-glacial epoch seems certain, and
+in some places the alternation between glacial and non-glacial beds
+suggests no less than nine. In the other glaciations the evidence is not
+yet clear. The question of periodicity is so important that it
+overthrows most glacial hypotheses. Indeed, had their authors known the
+facts as established in recent years, most of the hypotheses would never
+have been advanced. The carbon dioxide hypothesis is the only one which
+was framed with geologically rapid climatic alternations in mind. It
+certainly explains the facts of periodicity better than does any of its
+predecessors, but even so it does not account for the intimate way in
+which variations of all degrees from those of the weather up to glacial
+epochs seem to grade into one another.
+
+According to our stellar hypothesis, occasional groups of glacial epochs
+would be expected to occur close together and to form long glacial
+periods. This is because many of the stars belong to groups or clusters
+in which the stars move in parallel paths. A good example is the cluster
+in the Hyades, where Boss has studied thirty-nine stars with special
+care.[120] The stars are grouped about a center about 130 light years
+from the sun. The stars themselves are scattered over an area about
+thirty light years in diameter. They average about the same distance
+apart as do those near the sun, but toward the center of the group they
+are somewhat closer together. The whole thirty-nine sweep forward in
+essentially parallel paths. Boss estimates that 800,000 years ago the
+cluster was only half as far from the sun as at present, but probably
+that was as near as it has been during recent geological times. All of
+the thirty-nine stars of this cluster, as Moulton[121] puts it, "are
+much greater in light-giving power than the sun. The luminosities of
+even the five smallest are from five to ten times that of the sun, while
+the largest are one hundred times greater in light-giving power than our
+own luminary. Their masses are probably much greater than that of the
+sun." If the sun were to pass through such a cluster, first one star and
+then another might come so near as to cause a profound disturbance in
+the sun's atmosphere.
+
+3. Another important point upon which a glacial hypothesis may come to
+grief is the length of the periods or rather of the epochs which compose
+the periods. During the last or Pleistocene glacial period the evidence
+in America and Europe indicates that the inter-glacial epochs varied in
+length and that the later ones were shorter than the earlier. Chamberlin
+and Salisbury, from a comparison of various authorities, estimate that
+the intervals from one glacial epoch to another form a declining series,
+which may be roughly expressed as follows: 16-8-4-2-1, where unity is
+the interval from the climax of the late Wisconsin, or last glacial
+epoch, to the present. Most authorities estimate the culmination of the
+late Wisconsin glaciation as twenty or thirty thousand years ago. Penck
+estimates the length of the last inter-glacial period as 60,000 years
+and the preceding one as 240,000.[122] R. T. Chamberlin, as already
+stated, finds that the consensus of opinion is that inter-glacial epochs
+have averaged five times as long as glacial epochs. The actual duration
+of the various glaciations probably did not vary in so great a ratio as
+did the intervals from one glaciation to another. The main point,
+however, is the irregularity of the various periods.
+
+The relation of the stellar electrical hypothesis to the length of
+glacial epochs may be estimated from column C, in Table 5. There we see
+that the distances at which a star might possibly disturb the sun enough
+to cause glaciation range all the way from 120 billion miles in the case
+of a small star like the sun, to 3200 billion in the case of Betelgeuse,
+while for double stars the figure may rise a hundred times higher. From
+this we can calculate how long it would take a star to pass from a point
+where its influence would first amount to a quarter of the assumed
+maximum to a similar point on the other side of the sun. In making these
+calculations we will assume that the relative rate at which the star and
+the sun approach each other is about twenty-two miles per second, or 700
+million miles per year, which is the average rate of motion of all the
+known stars. According to the distances in Table 5 this gives a range
+from about 500 years up to about 10,000, which might rise to a million
+in the case of double stars. Of course the time might be relatively
+short if the sun and a rapidly moving star were approaching one another
+almost directly, or extremely long if the sun and the star were moving
+in almost the same direction and at somewhat similar rates,--a condition
+more common than the other. Here, as in so many other cases, the
+essential point is that the figures which we thus obtain seem to be of
+the right order of magnitude.
+
+4. Post-glacial climatic stages are so well known that in Europe they
+have definite names. Their sequence has already been discussed in
+Chapter XII. Fossils found in the peat bogs of Denmark and Scandinavia,
+for example, prove that since the final disappearance of the continental
+ice cap at the close of the Wisconsin there has been at least one period
+when the climate of Europe was distinctly milder than now. Directly
+overlying the sheets of glacial drift laid down by the ice there is a
+flora corresponding to that of the present tundras. Next come remains of
+a forest vegetation dominated by birches and poplars, showing that the
+climate was growing a little warmer. Third, there follow evidences of a
+still more favorable climate in the form of a forest dominated by pines;
+fourth, one where oak predominates; and fifth, a flora similar to that
+of the Black Forest of Germany, indicating that in Scandinavia the
+temperature was then decidedly higher than today. This fifth flora has
+retreated southward once more, having been driven back to its present
+latitude by a slight recurrence of a cool stormy climate.[123] In
+central Asia evidence of post-glacial stages is found not only in five
+distinct moraines but in a corresponding series of elevated strands
+surrounding salt lakes and of river terraces in non-glaciated arid
+regions.[124]
+
+In historic as well as prehistoric times, as we have already seen, there
+have been climatic fluctuations. For instance, the twelfth or thirteenth
+century B. C. appears to have been almost as mild as now, as does the
+seventh century B. C. On the other hand about 1000 B. C., at the time of
+Christ, and in the fourteenth century there were times of relative
+severity. Thus it appears that both on a large and on a small scale
+pulsations of climate are the rule. Any hypothesis of climatic changes
+must satisfy the periods of these pulsations. These conditions furnish a
+problem which makes difficulty for almost all hypotheses of climatic
+change. According to the present hypothesis, earth movements such as are
+discussed in Chapter XII may cooeperate with two astronomical factors.
+One is the constant change in the positions of the stars, a change which
+we have already called kaleidoscopic, and the other is the fact that a
+large proportion of the stars are double or multiple. When one star in a
+group approaches the sun closely enough to cause a great solar
+disturbance, numerous others may approach or recede and have a minor
+effect. Thus, whenever the sun is near groups of stars we should expect
+that the earth would show many minor climatic pulsations and stages
+which might or might not be connected with glaciation. The historic
+pulsations shown in the curve of tree growth in California, Fig. 4, are
+the sort of changes that would be expected if movements of the stars
+have an effect on the solar atmosphere.
+
+Not only are fully a third of all the visible stars double, as we have
+already seen, but at least a tenth of these are known to be triple or
+multiple. In many of the double stars the two bodies are close together
+and revolve so rapidly that whatever periodicity they might create in
+the sun's atmosphere would be very short. In the triplets, however, the
+third star is ordinarily at least ten times as far from the other two as
+they are from each other, and its period of rotation sometimes runs into
+hundreds or thousands of years. An actual multiple star in the
+constellation Polaris will serve as an example. The main star is
+believed by Jeans to consist of two parts which are almost in contact
+and whirl around each other with extraordinary speed in four days. If
+this is true they must keep each other's atmospheres in a state of
+intense commotion. Much farther away a third star revolves around this
+pair in twelve years. At a much greater distance a fourth star revolves
+around the common center of gravity of itself and the other three in a
+period which may be 20,000 years. Still more complicated cases probably
+exist. Suppose such a system were to traverse a path where it would
+exert a perceptible influence on the sun for thirty or forty thousand
+years. The varying movements of its members would produce an intricate
+series of cycles which might show all sorts of major and minor
+variations in length and intensity. Thus the varied and irregular stages
+of glaciation and the pulsations of historic times might be accounted
+for on the hypothesis of the proximity of the sun to a multiple star, as
+well as on that of the less pronounced approach and recession of a
+number of stars. In addition to all this, an almost infinitely complex
+series of climatic changes of long and short duration might arise if the
+sun passed through a nebula.
+
+5. We have seen in Chapter VIII that the contrast between the somewhat
+severe climate of the present and the generally mild climate of the past
+is one of the great geological problems. The glacial period is not a
+thing of the distant past. Geologists generally recognize that it is
+still with us. Greenland and Antarctica are both shrouded in ice sheets
+in latitudes where fossil floras prove that at other periods the climate
+was as mild as in England or even New Zealand. The present glaciated
+regions, be it noted, are on the polar borders of the world's two most
+stormy oceanic areas, just where ice would be expected to last longest
+according to the solar cyclonic hypothesis. In contrast with the
+semi-glacial conditions of the present, the last inter-glacial epoch was
+so mild that not only men but elephants and hippopotamuses flourished in
+central Europe, while at earlier times in the middle of long eras, such
+as the Paleozoic and Mesozoic, corals, cycads, and tree ferns flourished
+within the Arctic circle.
+
+If the electro-stellar hypothesis of solar disturbances proves well
+founded, it may explain these peculiarities. Periods of mild climate
+would represent a return of the sun and the earth to their normal
+conditions of quiet. At such times the atmosphere of the sun is assumed
+to be little disturbed by sunspots, faculae, prominences, and other
+allied evidences of movements; and the rice-grain structure is perhaps
+the most prominent of the solar markings. The earth at such times is
+supposed to be correspondingly free from cyclonic storms. Its winds are
+then largely of the purely planetary type, such as trade winds and
+westerlies. Its rainfall also is largely planetary rather than cyclonic.
+It falls in places such as the heat equator where the air rises under
+the influence of heat, or on the windward slopes of mountains, or in
+regions where warm winds blow from the ocean over cold lands.
+
+According to the electro-stellar hypothesis, the conditions which
+prevailed during hundreds of millions of years of mild climate mean
+merely that the solar system was then in parts of the heavens where
+stars--especially double stars--were rare or small, and electrical
+disturbances correspondingly weak. Today, on the other hand, the sun is
+fairly near a number of stars, many of which are large doubles. Hence it
+is supposed to be disturbed, although not so much as at the height of
+the last glacial epoch.
+
+After the preceding parts of this book had been written, the assistance
+of Dr. Schlesinger made it possible to test the electro-stellar
+hypothesis by comparing actual astronomical dates with the dates of
+climatic or solar phenomena. In order to make this possible, Dr.
+Schlesinger and his assistants have prepared Table 6, giving the
+position, magnitude, and motions of the thirty-eight nearest stars, and
+especially the date at which each was nearest the sun. In column 10
+where the dates are given, a minus sign indicates the past and a plus
+sign the future. Dr. Shapley has kindly added column 12, giving the
+absolute magnitudes of the stars, that of the sun being 4.8, and column
+13, showing their luminosity or absolute radiation, that of the sun
+being unity. Finally, column 14 shows the effective radiation received
+by the sun from each star when the star is at a minimum distance. Unity
+in this case is the effect of a star like the sun at a distance of one
+light year.
+
+It is well known that radiation of all kinds, including light, heat, and
+electrical emissions, varies in direct proportion to the exposed
+surface, that is, as the square of the radius of a sphere, and inversely
+as the square of the distance. From black bodies, as we have seen, the
+total radiation varies as the fourth power of the absolute temperature.
+It is not certain that either light or electrical emissions from
+incandescent bodies vary in quite this same proportion, nor is it yet
+certain whether luminous and electrical emissions vary exactly together.
+Nevertheless they are closely related. Since the light coming from each
+star is accurately measured, while no information is available as to
+electrical emissions, we have followed Dr. Shapley's suggestion and used
+the luminosity of the stars as the best available measure of total
+radiation. This is presumably an approximate measure of electrical
+activity, provided some allowance be made for disturbances by outside
+bodies such as companion stars. Hence the inclusion of column 14.
+
+TABLE 6
+
+THIRTY-EIGHT STARS HAVING LARGEST KNOWN PARALLAXES
+
+   Star
+   Code
+   1 Groombr. 34
+   2 ++[Greek: e] Cassiop.
+   3
+   4 ++[Greek: k] Tucanae
+   5 [Greek: t] Ceti
+   6 [Greek: d]_2 Eridani
+   7 ++[Greek: e] Eridani
+   8 ++40(0)^2 Eridani
+   9  Cordoba Z. 243
+   10 Weisse 592
+   11 ++[Greek: a] Can. Maj. (Sirius)
+   12 ++[Greek: a] Can. Min. (Procyon)
+   13 ++Fedorenko 1457-8
+   14  Groombr. 1618
+   15  Weisse 234
+   16  Lalande 21185
+   17  Lalande 21258
+   18
+   19  Lalande 25372
+   20 ++[Greek: a] Centauri
+   21 ++[Greek: x] Bootes
+   22 ++Lalande 27173
+   23  Weisse 1259
+   24  Lacaille 7194
+   25 ++[Greek: b] 416
+   26 Argel -0.17415-6
+   27 Barnard's star
+   28 ++70p Ophiuchi
+   29 ++[Greek: S] 2398
+   30 [Greek: s] Draconis
+   31 ++[Greek: a] Aquilae (Altair)
+   32 ++61 Cygni
+   33 Lacaille 8760
+   34 [Greek: e] Indi
+   35 ++Krueger 60
+   36 Lacaille 9352
+   37 Lalande 46650
+   38 C. G. A. 32416
+
+    (++ Double star.)
+
+            (1)          (2)        (3)      (4)      (5)        (6)
+           Right     Declination  Visual  Spectrum  Proper   Radial
+  Star   Ascension   [Greek: d]   Mag. m            Motion  Velocity
+  code   [Greek: a]       1900                              km. per
+               1900                                           sec.
+   ------------------------------------------------------------------
+   1     0^h 12^m.7    +43 deg.27'       8.1      Ma      2".89     +  3
+   2         43 .0     +57 17        3.6      F8      1 .24     + 10
+   3         43 .9      +4 55       12.3      F0      3 .01     .....
+   4       1 12 .4     -69 24        5.0      F8        .39      + 12
+   5         39 .4     -16 28        3.6      K0      1 .92      - 16
+   ------------------------------------------------------------------
+   6       3 15 .9     -43 27        4.3      G5      3 .16      + 87
+   7         28 .2     - 9 48        3.8      K0        .97      + 16
+   8       4 10 .7     - 7 49        4.5      G5      4 .08      - 42
+   9       5  7 .7     -44 59        9.2      K2      8 .75      +242
+   10        26 .4     - 3 42        8.8      K2      2 .22     .....
+   ------------------------------------------------------------------
+   11     6 40 .7      -16 35      -1.6       A0      1 .32      -  8
+   12     7 34 .1      + 5 29       0.5       F5      1 .24      -  4
+   13     9  7 .6      +53  7       7.9       Ma      1 .68      + 10
+   14    10  5 .3      +49 58       6.8       K5p     1 .45      - 30
+   15       14 .2      +20 22       9.0       ...       .49     .....
+   ------------------------------------------------------------------
+   16       57 .9      +36 38       7.6       Mb      4 .78      - 87
+   17    11  0 .5      +44  2       8.5       K5      4 .52      + 65
+   18       12 .0      -57  2      12.0       ...     2 .69     .....
+   19    13 40 .7      +15 26       8.5       K5      2 .30     .....
+   20    14 32 .8      -60 25       0.2       G       3 .68      + 22
+   ------------------------------------------------------------------
+   21    14 46 .8      +19 31       4.6       K5p       .17      +  4
+   22       51 .6      -20 58       5.8       Kp      1 .96      + 20
+   23    16 41 .4      +33 41       8.4       ...       .37     .....
+   24    17 11 .5      -46 32       5.7       K         .97     .....
+   25       12 .1      -34 53       5.9       K5      1 .19      -  4
+   ------------------------------------------------------------------
+   26       37 .0      +68 26       9.1       K       1 .33     .....
+   27       52 .9      + 4 25       9.7       Mb     10 .30      - 80
+   28    18  0 .4      + 2 31       4.3       K       1 .13     .....
+   29       41 .7      +59 29       8.8       K       2 .31     .....
+   30    19 32 .5      +69 29       4.8       G5      1 .84      + 26
+   ------------------------------------------------------------------
+   31       45 .9      + 8 36       1.2       A5        .66      - 33
+   32    21  2 .4      +38 15       5.6       K5      5 .20      - 64
+   33       11 .4      -39 15       6.6       G       3 .53      + 13
+   34       55 .7      -57 12       4.8       K5      4 .70      - 39
+   35    22 24 .4      +57 12       9.2       ...       .87     .....
+   ------------------------------------------------------------------
+   36       59 .4      -36 26       7.1       K       6 .90      + 12
+   37    23 44 .0      + 1 52       8.7       Ma      1 .39     .....
+   38       59 .5      -37 51       8.2       G       6 .05      + 26
+
+
+         (7)          (9)           (11)             (13)       (14)
+       Present      Minimum       Magnitude       Luminosity  Effective
+       Parallax    Distance      at Min. Dist.          |     radiation
+      [Greek: p]   Light Yrs.          |                |        at
+         |            |                |                |      minimum
+         |     (8)    |       (10)     |     (12)       |     distance
+ Star    |   Maximum  |     Time of    |   Absolute     |     from sun
+ Code    |  Parallax  |     Minimum    |   Magnitude    |         |
+         |     |      |     Distance   |      |         |         |
+ -----------------------------------------------------------------------
+ 1   ".28    ".28    11.6     -4000   8.1   10.3      0.0063    0.000051
+ 2    .18     .19    17.1    -47000   3.5    4.9      0.91      0.003110
+ 3    .24    ....    ....    ......  ....   14.2      0.00017   ........
+ 4    .16     .23    14.2   -264000   4.2    6.0      0.33      0.001610
+ 5    .32     .37     8.8    +46000   3.3    6.1      0.30      0.003840
+ -----------------------------------------------------------------------
+ 6    .16     .22    14.8    -33000   3.6    5.3      0.63      0.002960
+ 7    .31     .46     7.1   -106000   3.0    6.3      0.25      0.004970
+ 8    .21     .23    14.2    +19000   4.3    6.1      0.30      0.001470
+ 9    .32     .68     4.8    -10000   7.6   11.7      0.0017    0.000074
+ 10   .17    ....    ....    ......   ....   9.9      0.009     ........
+ -----------------------------------------------------------------------
+ 11   .37     .41     8.0    +65000  -1.8    1.2     27.50      0.429000
+ 12   .31     .32    10.2    +34000   0.5    3.0      5.25      0.051300
+ 13   .16     .16    20.4    -24000   7.9    8.9      0.023     0.000055
+ 14   .18     .23    14.2    +69000   6.3    8.1      0.048     0.000238
+ 15   .19    ....    ....    ......   ....  10.4      0.0057    ........
+ -----------------------------------------------------------------------
+ 16   .41     .76     4.3    +20000   6.2   10.7      0.0044    0.000238
+ 17   .19     .22    14.8    -20000   8.2    9.9      0.009     0.000041
+ 18   .34    ....    ....    ......   ....  14.7      0.00011   ........
+ 19   .19    ....    ....    ......   ....   9.9      0.009     ........
+ 20   .76    1.03     3.2    -28000  -0.5    4.6      1.20      0.117500
+ -----------------------------------------------------------------------
+ 21   .17     .22    14.8   -598000   4.0    5.8      0.40      0.001815
+ 22   .18     .19    17.1    -36000   5.6    7.1      0.12      0.000412
+ 23   .18    ....    ....    ......   ....   9.7      0.011     ........
+ 24   .19    ....    ....    ......   ....   7.1      0.12      ........
+ 25   .17     .17    19.2    +21000   5.7    7.1      0.12      0.000329
+ -----------------------------------------------------------------------
+ 26   .22    ....    ....    ......   ....  10.8      0.004     ........
+ 27   .53     .70     4.7    +10000   9.1   13.3      0.0025    0.000114
+ 28   .19    ....    ....    ......   ....   5.7      0.44      ........
+ 29   .29    ....    ....    ......   ....  11.1      0.0030    ........
+ 30   .20     .23    14.2    -49000   4.5    6.3      0.25      0.001238
+ -----------------------------------------------------------------------
+ 31   .21     .51     6.4   +117000  -0.7    2.8      6.30      0.153600
+ 32   .30     .38     8.6    +19000   5.1    8.0      0.053     0.000715
+ 33   .25     .26    12.6    -11000   6.6    8.6      0.030     0.000189
+ 34   .28     .31    10.5    +17000   4.6    7.0      0.13      0.001230
+ 35   .26    ....    ....   .......  ....   11.3      0.0025    ........
+ -----------------------------------------------------------------------
+ 36   .29     .29    11.2     -3000   7.1    9.4      0.014     0.000111
+ 37   .17    ....    ....   .......  ....    9.9      0.009     ........
+ 38   .22     .22    14.8     -7000   8.2    9.9      0.009     0.000041
+ -----------------------------------------------------------------------
+
+On the basis of column 14 and of the movements and distances of the
+stars as given in the other columns Fig. 10 has been prepared. This
+gives an estimate of the approximate electrical energy received by the
+sun from the nearest stars for 70,000 years before and after the
+present. It is based on the twenty-six stars for which complete data are
+available in Table 6. The inclusion of the other twelve would not alter
+the form of the curve, for even the largest of them would not change any
+part by more than about half of 1 per cent, if as much. Nor would the
+curve be visibly altered by the omission of all except four of the
+twenty-six stars actually used. The four that are important, and their
+relative luminosity when nearest the sun, are Sirius 429,000, Altair
+153,000, Alpha Centauri 117,500, and Procyon 51,300. The figure for the
+next star is only 4970, while for this star combined with the other
+twenty-one that are unimportant it is only 24,850.
+
+Figure 10 is not carried more than 70,000 years into the past or into
+the future because the stars near the sun at more remote times are not
+included among the thirty-eight having the largest known parallaxes.
+That is, they have either moved away or are not yet near enough to be
+included. Indeed, as Dr. Schlesinger strongly emphasizes, there may be
+swiftly moving, bright or gigantic stars which are now quite far away,
+but whose inclusion would alter Fig. 10 even within the limits of the
+140,000 years there shown. It is almost certain, however, that the most
+that these would do would be to raise, but not obliterate, the minima on
+either side of the main maximum.
+
+[Illustration: _Fig. 10. Climatic changes of 140,000 years as inferred
+from the stars._]
+
+In preparing Fig. 10 it has been necessary to make allowance for double
+stars. Passing by the twenty-two unimportant stars, it appears that the
+companion of Sirius is eight or ten magnitudes smaller than that star,
+while the companions of Procyon and Altair are five or more magnitudes
+smaller than their bright comrades. This means that the luminosity of
+the faint components is at most only 1 per cent of that of their bright
+companions and in the case of Sirius not a hundredth of 1 per cent.
+Hence their inclusion would have no visible effect on Fig. 10. In Alpha
+Centauri, on the other hand, the two components are of almost the same
+magnitude. For this reason the effective radiation of that star as given
+in column 14 is doubled in Fig. 10, while for another reason it is
+raised still more. The other reason is that if our inferences as to the
+electrical effect of the sun on the earth and of the planets on the sun
+are correct, double stars, as we have seen, must be much more effective
+electrically than single stars. By the same reasoning two bright stars
+close together must excite one another much more than a bright star and
+a very faint one, even if the distances in both cases are the same. So,
+too, other things being equal, a triple star must be more excited
+electrically than a double star. Hence in preparing Fig. 10 all double
+stars receive double weight and each part of Alpha Centauri receives an
+additional 50 per cent because both parts are bright and because they
+have a third companion to help in exciting them.
+
+According to the electro-stellar hypothesis, Alpha Centauri is more
+important climatically than any other star in the heavens not only
+because it is triple and bright, but because it is the nearest of all
+stars, and moves fairly rapidly. Sirius and Procyon move slowly in
+respect to the sun, only about eleven and eight kilometers per second
+respectively, and their distances at minimum are fairly large, that is,
+8 and 10.2 light years. Hence their effect on the sun changes slowly.
+Altair moves faster, about twenty-six kilometers per second, and its
+minimum distance is 6.4 light years, so that its effect changes fairly
+rapidly. Alpha Centauri moves about twenty-four kilometers per second,
+and its minimum distance is only 3.2 light years. Hence its effect
+changes very rapidly, the change in its apparent luminosity as seen from
+the sun amounting at maximum to about 30 per cent in 10,000 years
+against 14 per cent for Altair, 4 for Sirius, and 2 for Procyon. The
+vast majority of the stars change so much more slowly than even Procyon
+that their effect is almost uniform. All the stars at a distance of more
+than perhaps twenty or thirty light years may be regarded as sending to
+the sun a practically unchanging amount of radiation. It is the bright
+stars within this limit which are important, and their importance
+increases with their proximity, their speed of motion, and the
+brightness and number of their companions. Hence Alpha Centauri causes
+the main maximum in Fig. 10, while Sirius, Altair, and Procyon combine
+to cause a general rise of the curve from the past to the future.
+
+Let us now interpret Fig. 10 geologically. The low position of the curve
+fifty to seventy thousand years ago suggests a mild inter-glacial
+climate distinctly less severe than that of the present. Geologists say
+that such was the case. The curve suggests a glacial epoch culminating
+about 28,000 years ago. The best authorities put the climax of the last
+glacial epoch between twenty-five and thirty thousand years ago. The
+curve shows an amelioration of climate since that time, although it
+suggests that there is still considerable severity. The retreat of the
+ice from North America and Europe, and its persistence in Greenland and
+Antarctica agree with this. And the curve indicates that the change of
+climate is still persisting, a conclusion in harmony with the evidence
+as to historic changes.
+
+If Alpha Centauri is really so important, the effect of its variations,
+provided it has any, ought perhaps to be evident in the sun. The
+activity of the star's atmosphere presumably varies, for the orbits of
+the two components have an eccentricity of 0.51. Hence during their
+period of revolution, 81.2 years, the distance between them ranges from
+1,100,000,000 to 3,300,000,000 miles. They were at a minimum distance in
+1388, 1459, 1550, 1631, 1713, 1794, 1875, and will be again in 1956. In
+Fig. 11, showing sunspot variations, it is noticeable that the years
+1794 and 1875 come just at the ends of periods of unusual solar
+activity, as indicated by the heavy horizontal line. A similar period of
+great activity seems to have begun about 1914. If its duration equals
+the average of its two predecessors, it will end about 1950. Back in the
+fourteenth century a period of excessive solar activity, which has
+already been described, culminated from 1370 to 1385, or just before the
+two parts of Alpha Centauri were at a minimum distance. Thus in three
+and perhaps four cases the sun has been unusually active during a time
+when the two parts of the star were most rapidly approaching each other
+and when their atmospheres were presumably most disturbed and their
+electrical emanations strongest.
+
+[Illustration: _Fig. 11. Sunspot curve showing cycles, 1750 to 1920._
+
+_Note._ The asterisks indicate two absolute minima of sunspots in 1810
+and 1913, and the middle years (1780 and 1854) of two periods when the
+sunspot maxima never fell below 95. If Alpha Centauri has an effect on
+the sun's atmosphere, the end of another such period would be expected
+not far from 1957.]
+
+The fact that Alpha Centauri, the star which would be expected most
+strongly to influence the sun, and hence the earth, was nearest the sun
+at the climax of the last glacial epoch, and that today the solar
+atmosphere is most active when the star is presumably most disturbed may
+be of no significance. It is given for what it is worth. Its importance
+lies not in the fact that it proves anything but that no contradiction
+is found when we test the electro-stellar hypothesis by facts which were
+not thought of when the hypothesis was framed. A vast amount of
+astronomical work is still needed before the matter can be brought to
+any definite conclusion. In case the hypothesis stands firm, it may be
+possible to use the stars as a help in determining the exact chronology
+of the later part of geological times. If the hypothesis is disproved,
+it will merely leave the question of solar variations where it is today.
+It will not influence the main conclusions of this book as to the causes
+and nature of climatic changes. Its value lies in the fact that it calls
+attention to new lines of research.
+
+FOOTNOTES:
+
+[Footnote 120: Lewis Boss: Convergent of a Moving Cluster in Taurus;
+Astronom. Jour., Vol. 26, No. 4, 1908, pp. 31-36.]
+
+[Footnote 121: F. R. Moulton: in Introduction to Astronomy, 1916.]
+
+[Footnote 122: A. Penck: Die Alpen im Eiszeitalter, Leipzig, 1909.]
+
+[Footnote 123: R. D. Salisbury: Physical Geography of the Pleistocene,
+in Outlines of Geologic History, by Willis and Salisbury, 1910, pp.
+273-274.]
+
+[Footnote 124: Davis, Pumpelly, and Huntington: Explorations in
+Turkestan, Carnegie Inst. of Wash., No. 26, 1905.
+
+In North America the stages have been the subject of intensive studies
+on the part of Taylor, Leverett, Goldthwait, and many others.]
+
+
+
+
+CHAPTER XVI
+
+THE EARTH'S CRUST AND THE SUN
+
+
+Although the problems of this book may lead far afield, they ultimately
+bring us back to the earth and to the present. Several times in the
+preceding pages there has been mention of the fact that periods of
+extreme climatic fluctuations are closely associated with great
+movements of the earth's crust whereby mountains are uplifted and
+continents upheaved. In attempting to explain this association the
+general tendency has been to look largely at the past instead of the
+present. Hence it has been almost impossible to choose among three
+possibilities, all beset with difficulties. First, the movements of the
+crust may have caused the climatic fluctuations; second, climatic
+changes may cause crustal movements; and third, variations in solar
+activity or in some other outside agency may give rise to both types of
+terrestrial phenomena.
+
+The idea that movements of the earth's crust are the main cause of
+geological changes of climate is becoming increasingly untenable as the
+complexity and rapidity of climatic changes become more clear,
+especially during post-glacial times. It implies that the earth's
+surface moves up and down with a speed and facility which appear to be
+out of the question. If volcanic activity be invoked the problem becomes
+no clearer. Even if volcanic dust should fill the air frequently and
+completely, neither its presence nor absence would produce such peculiar
+features as the localization of glaciers, the distribution of loess, and
+the mild climate of most parts of geological time. Nevertheless, because
+of the great difficulties presented by the other two possibilities many
+geologists still hold that directly or indirectly the greater climatic
+changes have been mainly due to movements of the earth's crust and to
+the reaction of the crustal movements on the atmosphere.
+
+The possibility that climatic changes are in themselves a cause of
+movements of the earth's crust seems so improbable that no one appears
+to have investigated it with any seriousness. Nevertheless, it is worth
+while to raise the question whether climatic extremes may cooeperate with
+other agencies in setting the time when the earth's crust shall be
+deformed.
+
+As to the third possibility, it is perfectly logical to ascribe both
+climatic changes and crustal deformation to some outside agency, solar
+or otherwise, but hitherto there has been so little evidence on this
+point that such an ascription has merely begged the question. If
+heavenly bodies should approach the earth closely enough so that their
+gravitational stresses caused crustal deformation, all life would
+presumably be destroyed. As to the sun, there has hitherto been no
+conclusive evidence that it is related to crustal movements, although
+various writers have made suggestions along this line. In this chapter
+we shall carry these suggestions further and shall see that they are at
+least worthy of study.
+
+As a preliminary to this study it may be well to note that the
+coincidence between movements of the earth's crust and climatic changes
+is not so absolute as is sometimes supposed. For example, the profound
+crustal changes at the end of the Mesozoic were not accompanied by
+widespread glaciation so far as is yet known, although the temperature
+appears to have been lowered. Nor was the violent volcanic and
+diastrophic activity in the Miocene associated with extreme climates.
+Indeed, there appears to have been little contrast from zone to zone,
+for figs, bread fruit trees, tree ferns, and other plants of low
+latitudes grew in Greenland. Nevertheless, both at the end of the
+Mesozoic and in the Miocene the climate may possibly have been severe
+for a time, although the record is lost. On the other hand, Kirk's
+recent discovery of glacial till in Alaska between beds carrying an
+undoubted Middle Silurian fauna indicates glaciation at a time when
+there was little movement of the crust so far as yet appears.[125] Thus
+we conclude that while climatic changes and crustal movements usually
+occur together, they may occur separately.
+
+According to the solar-cyclonic hypothesis such a condition is to be
+expected. If the sun were especially active when the terrestrial
+conditions prohibited glaciation, changes of climate would still occur,
+but they would be milder than under other circumstances, and would leave
+little record in the rocks. Or there might be glaciation in high
+latitudes, such as that of southern Alaska in the Middle Silurian, and
+none elsewhere. On the other hand, when the sun was so inactive that no
+great storminess occurred, the upheaval of continents and the building
+of mountains might go on without the formation of ice sheets, as
+apparently happened at the end of the Mesozoic. The lack of absolute
+coincidence between glaciation and periods of widespread emergence of
+the lands is evident even today, for there is no reason to suppose that
+the lands are notably lower or less extensive now than they were during
+the Pleistocene glaciation. In fact, there is much evidence that many
+areas have risen since that time. Yet glaciation is now far less
+extensive than in the Pleistocene. Any attempt to explain this
+difference on the basis of terrestrial changes is extremely difficult,
+for the shape and altitude of continents and mountains have not changed
+much in twenty or thirty thousand years. Yet the present moderately mild
+epoch, like the puzzling inter-glacial epochs of earlier times, is
+easily explicable on the assumption that the sun's atmosphere may
+sometimes vary in harmony with crustal activity, but does not
+necessarily do so at all times.
+
+Turning now to the main problem of how climatic changes may be connected
+with movements of the earth's crust, let us follow our usual method and
+examine what is happening today. Let us first inquire whether
+earthquakes, which are one of the chief evidences that crustal movements
+are actually taking place in our own times, show any connection with
+sunspots. In order to test this, we have compared _Milne's Catalogue of
+Destructive Earthquakes_ from 1800 to 1899, with Wolf's sunspot numbers
+for the same period month by month. The earthquake catalogue, as its
+compiler describes it, "is an attempt to give a list of earthquakes
+which have announced changes of geological importance in the earth's
+crust; movements which have probably resulted in the creation or the
+extension of a line of fault, the vibrations accompanying which could,
+with proper instruments, have been recorded over a continent or the
+whole surface of our world. Small earthquakes have been excluded, while
+the number of large earthquakes both for ancient and modern times has
+been extended. As an illustration of exclusion, I may mention that
+between 1800 and 1808, which are years taken at random, I find in
+Mallet's catalogue 407 entries. Only thirty-seven of these, which were
+accompanied by structural damage, have been retained. Other catalogues
+such as those of Perry and Fuchs have been treated similarly."[126]
+
+If the earthquakes in such a carefully selected list bear a distinct
+relation to sunspots, it is at least possible and perhaps probable that
+a similar relation may exist between solar activity and geological
+changes in the earth's crust. The result of the comparison of
+earthquakes and sunspots is shown in Table 7. The first column gives the
+sunspot numbers; the second, the number of months that had the
+respective spot numbers during the century from 1800 to 1899. Column C
+shows the total number of earthquakes during the months having any
+particular degree of spottedness; while D, which is the significant
+column, gives the average number of destructive earthquakes per month
+under each of the six conditions of solar spottedness. The regularity of
+column D is so great as to make it almost certain that we are here
+dealing with a real relationship. Column F, which shows the average
+number of earthquakes in the month succeeding any given condition of the
+sun, is still more regular except for the last entry.
+
+TABLE 7
+
+DESTRUCTIVE EARTHQUAKES FROM 1800 TO 1899 COMPARED WITH SUNSPOTS
+
+  A: _Sunspot numbers_
+  B: _Number of months per Wolf's Table_
+  C: _Number of earthquakes_
+  D: _Average number of earthquakes per month_
+  E: _Number of earthquakes in succeeding month_
+  F: _Average number of earthquakes in succeeding month_
+
+        A       B          C            D            E           F
+
+       0-15   344        522          1.52         512          1.49
+      15-30   194        306          1.58         310          1.60
+      30-50   237        433          1.83         439          1.85
+      50-70   195        402          2.06         390          2.00
+     70-100   135        286          2.12         310          2.30
+   over 100    95        218          2.30         175          1.84
+
+The chance that six numbers taken at random will arrange themselves in
+any given order is one in 720. In other words, there is one chance in
+720 that the regularity of column D is accidental. But column F is as
+regular as column D except for the last entry. If columns D and E were
+independent there would be one chance in about 500,000 that the six
+numbers in both columns would fall in the same order, and one chance in
+14,400 that five numbers in each would fall in the same order. But the
+two columns are somewhat related, for although the after-shocks of a
+great earthquake are never included in Milne's table, a world-shaking
+earthquake in one region during a given month probably creates
+conditions that favor similar earthquakes elsewhere during the next
+month. Hence the probability that we are dealing with a purely
+accidental arrangement in Table 7 is less than one in 14,400 and greater
+than one in 500,000. It may be one in 20,000 or 100,000. In any event it
+is so slight that there is high probability that directly or indirectly
+sunspots and earthquakes are somehow connected.
+
+In ascertaining the relation between sunspots and earthquakes it would
+be well if we could employ the strict method of correlation
+coefficients. This, however, is impossible for the entire century, for
+the record is by no means homogeneous. The earlier decades are
+represented by only about one-fourth as many earthquakes as the later
+ones, a condition which is presumably due to lack of information. This
+makes no difference with the method employed in Table 7, since years
+with many and few sunspots are distributed almost equally throughout the
+entire nineteenth century, but it renders the method of correlation
+coefficients inapplicable. During the period from 1850 onward the record
+is much more nearly homogeneous, though not completely so. Even in these
+later decades, however, allowance must be made for the fact that there
+are more earthquakes in winter than in summer, the average number per
+month for the fifty years being as follows:
+
+    Jan. 2.8              May  2.4              Sept. 2.5
+    Feb. 2.4              June 2.3              Oct.  2.6
+    Mar. 2.5              July 2.4              Nov.  2.7
+    Apr. 2.4              Aug. 2.4              Dec.  2.8
+
+The correlation coefficient between the departures from these monthly
+averages and the corresponding departures from the monthly averages of
+the sunspots for the same period, 1850-1899, are as follows:
+
+    Sunspots and earthquakes of same month: +0.042, or 1.5 times the
+    probable error.
+
+    Sunspots of a given month and earthquakes of that month and the
+    next: +0.084, or 3.1 times the probable error.
+
+    Sunspots of three consecutive months and earthquakes of three
+    consecutive months allowing a lag of one month, i.e., sunspots of
+    January, February, and March compared with earthquakes of February,
+    March, and April; sunspots of February, March, and April with
+    earthquakes of March, April, and May, etc.; +0.112, or 4.1 times the
+    probable error.
+
+These coefficients are all small, but the number of individual cases,
+600 months, is so large that the probable error is greatly reduced,
+being only +-0.027 or +-0.028. Moreover, the nature of our data is such
+that even if there is a strong connection between solar changes and
+earth movements, we should not expect a large correlation coefficient.
+In the first place, as already mentioned, the earthquake data are not
+strictly homogeneous. Second, an average of about two and one-half
+strong earthquakes per month is at best only a most imperfect indication
+of the actual movement of the earth's crust. Third, the sunspots are
+only a partial and imperfect measure of the activity of the sun's
+atmosphere. Fourth, the relation between solar activity and earthquakes
+is almost certainly indirect. In view of all these conditions, the
+regularity of Table 7 and the fact that the most important correlation
+coefficient rises to more than four times the probable error makes it
+almost certain that the solar and terrestrial phenomena are really
+connected.
+
+We are now confronted by the perplexing question of how this connection
+can take place. Thus far only three possibilities present themselves,
+and each is open to objections. The chief agencies concerned in these
+three possibilities are heat, electricity, and atmospheric pressure.
+Heat may be dismissed very briefly. We have seen that the earth's
+surface becomes relatively cool when the sun is active. Theoretically
+even the slightest change in the temperature of the earth's surface must
+influence the thermal gradient far into the interior and hence cause a
+change of volume which might cause movements of the crust. Practically
+the heat of the surface ceases to be of appreciable importance at a
+depth of perhaps twenty feet, and even at that depth it does not act
+quickly enough to cause the relatively prompt response which seems to be
+characteristic of earthquakes in respect to the sun.
+
+The second possibility is based on the relationship between solar and
+terrestrial electricity. When the sun is active the earth's atmospheric
+electrical potential is subject to slight variations. It is well known
+that when two opposing points of an ionized solution are oppositely
+charged electrically, a current passes through the liquid and sets up
+electrolysis whereby there is a segregation of materials, and a
+consequent change in the volume of the parts near the respective
+electrical poles. The same process takes place, although less freely, in
+a hot mass such as forms the interior of the earth. The question arises
+whether internal electrical currents may not pass between the two
+oppositely charged poles of the earth, or even between the great
+continental masses and the regions of heavier rock which underlie the
+oceans. Could this lead to electrolysis, hence to differentiation in
+volume, and thus to movements of the earth's crust? Could the results
+vary in harmony with the sun? Bowie[127] has shown that numerous
+measurements of the strength and direction of the earth's gravitative
+pull are explicable only on the assumption that the upheaval of a
+continent or a mountain range is due in part not merely to pressure, or
+even to flowage of the rocks beneath the crust, but also to an actual
+change in volume whereby the rocks beneath the continent attain
+relatively great volume and those under the oceans a small volume in
+proportion to their weight. The query arises whether this change of
+volume may be related to electrical currents at some depth below the
+earth's surface.
+
+The objections to this hypothesis are numerous. First, there is little
+evidence of electrolytic differentiation in the rocks. Second, the outer
+part of the earth's crust is a very poor conductor so that it is
+doubtful whether even a high degree of electrification of the surface
+would have much effect on the interior. Third, electrolysis due to any
+such mild causes as we have here postulated must be an extremely slow
+process, too slow, presumably, to have any appreciable result within a
+month or two. Other objections join with these three in making it seem
+improbable that the sun's electrical activity has any direct effect upon
+movements of the earth's crust.
+
+The third, or meteorological hypothesis, which makes barometric pressure
+the main intermediary between solar activity and earthquakes, seems at
+first sight almost as improbable as the thermal and electrical
+hypotheses. Nevertheless, it has a certain degree of observational
+support of a kind which is wholly lacking in the other two cases. Among
+the extensive writings on the periodicity of earthquakes one main fact
+stands out with great distinctness: earthquakes vary in number according
+to the season. This fact has already been shown incidentally in the
+table of earthquake frequency by months. If allowance is made for the
+fact that February is a short month, there is a regular decrease in the
+frequency of severe earthquakes from December and January to June. Since
+most of Milne's earthquakes occurred in the northern hemisphere, this
+means that severe earthquakes occur in winter about 20 per cent oftener
+than in summer.
+
+The most thorough investigation of this subject seems to have been that
+of Davisson.[128] His results have been worked over and amplified by
+Knott,[129] who has tested them by Schuster's exact mathematical
+methods. His results are given in Table 8.[130] Here the northern
+hemisphere is placed first; then come the East Indies and the Malay
+Archipelago lying close to the equator; and finally the southern
+hemisphere. In the northern hemisphere practically all the maxima come
+in the winter, for the month of December appears in fifteen cases out of
+the twenty-five in column D, while January, February, or November
+appears in six others. It is also noticeable that in sixteen cases out
+of twenty-five the ratio of the actual to the expected amplitude in
+column G is four or more, so that a real relationship is indicated,
+while the ratio falls below three only in Japan and Zante. The
+equatorial data, unlike those of the northern hemisphere, are
+indefinite, for in the East Indies no month shows a marked maximum and
+the expected amplitude exceeds the actual amplitude. Even in the Malay
+Archipelago, which shows a maximum in May, the ratio of actual to
+expected amplitude is only 2.6. Turning to the southern hemisphere, the
+winter months of that hemisphere are as strongly marked by a maximum as
+are the winter months of the northern hemisphere. July or August appears
+in five out of six cases. Here the ratio between the actual and expected
+amplitudes is not so great as in the northern hemisphere. Nevertheless,
+it is practically four in Chile, and exceeds five in Peru and Bolivia,
+and in the data for the entire southern hemisphere.
+
+TABLE 8
+
+SEASONAL MARCH OF EARTHQUAKES
+
+AFTER DAVISSON AND KNOTT
+
+  A: _Region_
+  B: _Limiting Dates_
+  C: _Number of Shocks_
+  D: _Maximum Month_
+  E: _Amplitude_
+  F: _Expected Amplitude_
+  G: _Ratio of Actual to Expected Amplitude_
+
+           A              B       C        D        E      F        G
+
+  Northern Hemisphere  223-1850  5879     Dec.    0.110   0.023    4.8
+  Northern Hemisphere 1865-1884  8133     Dec.    0.290   0.020   14.5
+  Europe              1865-1884  5499     Dec.    0.350   0.024   14.6
+  Europe               306-1843  1961     Dec.    0.220   0.040    5.5
+  Southeast Europe    1859-1887  3470     Dec.    0.210   0.030    7.0
+  Vesuvius District   1865-1883   513     Dec.    0.250   0.078    3.2
+  Italy:
+    Old Tromometre    1872-1887 61732     Dec.    0.490   0.007   70.0
+    Old Tromometre    1876-1887 38546     Dec.    0.460   0.009   49.5
+    Normal Tromometre 1876-1887 38546     Dec.    0.490   0.009   52.8
+  Balkan, etc.        1865-1884   624     Dec.    0.270   0.071    3.8
+  Hungary, etc.       1865-1884   384     Dec.    0.310   0.090    3.4
+  Italy               1865-1883  2350  Dec.(Sept.)0.140   0.037    3.8
+  Grecian Archip.     1859-1881  3578   Dec.-Jan. 0.164   0.030    5.5
+  Austria             1865-1884   461     Jan.    0.370   0.083    4.4
+  Switzerland, etc.   1865-1883   524     Jan.    0.560   0.077    7.3
+  Asia                1865-1884   458     Feb.    0.330   0.083    4.0
+  North America       1865-1884   552     Nov.    0.350   0.075    4.7
+  California          1850-1886   949     Oct.    0.300   0.058    5.2
+  Japan               1878-1881   246     Dec.    0.460   0.113    4.1
+  Japan               1872-1880   367  Dec.-Jan.  0.256   0.093    2.8
+  Japan               1876-1891  1104     Feb.    0.190   0.053    3.6
+  Japan               1885-1889  2997     Oct.    0.080   0.032    2.5
+  Zante               1825-1863  1326     Aug.    0.100   0.049    2.0
+  Italy, North        1865-1883  1513 Sept.(Nov.) 0.210   0.046    4.6
+        of Naples
+  East Indies         1873-1881   515 Aug., Oct., 0.071?  0.078    0.9
+                                        or Dec.?
+  Malay Archip.       1865-1884   598     May     0.190   0.072    2.6
+  New Zealand         1869-1879   585  Aug.-Sept. 0.203   0.073    2.8
+  Chile               1873-1881   212     July    0.480   0.122    3.9
+  Southern Hemisphere 1865-1884   751     July    0.370   0.065    5.7
+  New Zealand         1868-1890   641  March, May 0.050   0.070    0.7
+  Chile               1865-1883?  316  July, Dec. 0.270   0.100    2.7
+  Peru, Bolivia       1865-1884   350     July    0.480   0.095    5.1
+
+The whole relationship between earthquakes and the seasons in the
+northern and southern hemispheres is summed up in Fig. 12 taken from
+Knott. The northern hemisphere shows a regular diminution in earthquake
+frequency from December until June, and an increase the rest of the
+year. In the southern hemisphere the course of events is the same so far
+as summer and winter are concerned, for August with its maximum comes in
+winter, while February with its minimum comes in summer. In the southern
+hemisphere the winter month of greatest seismic activity has over 100
+per cent more earthquakes than the summer month of least activity. In
+the northern hemisphere this difference is about 80 per cent, but this
+smaller figure occurs partly because the northern data include certain
+interesting and significant regions like Japan and China where the usual
+conditions are reversed.[131] If equatorial regions were included in
+Fig. 12, they would give an almost straight line.
+
+The connection between earthquakes and the seasons is so strong that
+almost no students of seismology question it, although they do not agree
+as to its cause. A meteorological hypothesis seems to be the only
+logical explanation.[132] Wherever sufficient data are available,
+earthquakes appear to be most numerous when climatic conditions cause
+the earth's surface to be most heavily loaded or to change its load most
+rapidly. The main factor in the loading is apparently atmospheric
+pressure. This acts in two ways. First, when the continents become cold
+in winter the pressure increases. On an average the air at sea level
+presses upon the earth's surface at the rate of 14.7 pounds per square
+inch, or over a ton per square foot, and only a little short of thirty
+million tons per square mile. An average difference of one inch between
+the atmospheric pressure of summer and winter over ten million square
+miles of the continent of Asia, for example, means that the continent's
+load in winter is about ten million million tons heavier than in summer.
+Second, the changes in atmospheric pressure due to the passage of storms
+are relatively sharp and sudden. Hence they are probably more effective
+than the variations in the load from season to season. This is suggested
+by the rapidity with which the terrestrial response seems to follow the
+supposed solar cause of earthquakes. It is also suggested by the fact
+that violent storms are frequently followed by violent earthquakes.
+"Earthquake weather," as Dr. Schlesinger suggests, is a common phrase in
+the typhoon region of Japan, China, and the East Indies. During tropical
+hurricanes a change of pressure amounting to half an inch in two hours
+is common. On September 22, 1885, at False Point Lighthouse on the Bay
+of Bengal, the barometer fell about an inch in six hours, then nearly an
+inch and a half in not much over two hours, and finally rose fully two
+inches inside of two hours. A drop of two inches in barometric pressure
+means that a load of about two million tons is removed from each square
+mile of land; the corresponding rise of pressure means the addition of a
+similar load. Such a storm, and to a less degree every other storm,
+strikes a blow upon the earth's surface, first by removing millions of
+tons of pressure and then by putting them on again.[133] Such storms, as
+we have seen, are much more frequent and severe when sunspots are
+numerous than at other times. Moreover, as Veeder[134] long ago showed,
+one of the most noteworthy evidences of a connection between sunspots
+and the weather is a sudden increase of pressure in certain widely
+separated high pressure areas. In most parts of the world winter is not
+only the season of highest pressure and of most frequent changes of
+Veeder's type, but also of severest storms. Hence a meteorological
+hypothesis would lead to the expectation that earthquakes would occur
+more frequently in winter than in summer. On the Chinese coast, however,
+and also on the oceanic side of Japan, as well as in some more tropical
+regions, the chief storms come in summer in the form of typhoons. These
+are the places where earthquakes also are most abundant in summer. Thus,
+wherever we turn, storms and the related barometric changes seem to be
+most frequent and severe at the very times when earthquakes are also
+most frequent.
+
+[Illustration: _Fig. 12. Seasonal distribution of earthquakes. (After
+Davisson and Knott.)_
+
+  solid line   ----  Northern Hemisphere.
+  dashed line  ....  Southern Hemisphere.]
+
+Other meteorological factors, such as rain, snow, winds, and currents,
+probably have some effect on earthquakes through their ability to load
+the earth's crust. The coming of vegetation may also help. These
+agencies, however, appear to be of small importance compared with the
+storms. In high latitudes and in regions of abundant storminess most of
+these factors generally combine with barometric pressure to produce
+frequent changes in the load of the earth's crust, especially in winter.
+In low latitudes, on the other hand, there are few severe storms, and
+relatively little contrast in pressure and vegetation from season to
+season; there is no snow; and the amount of ground water changes little.
+With this goes the twofold fact that there is no marked seasonal
+distribution of earthquakes, and that except in certain local volcanic
+areas, earthquakes appear to be rare. In proportion to the areas
+concerned, for example, there is little evidence of earthquakes in
+equatorial Africa and South America.
+
+The question of the reality of the connection between meteorological
+conditions and crustal movements is so important that every possible
+test should be applied. At the suggestion of Professor Schlesinger we
+have looked up a very ingenious line of inquiry. During the last decades
+of the nineteenth century, a long series of extremely accurate
+observations of latitude disclosed a fact which had previously been
+suspected but not demonstrated, namely, that the earth wabbles a little
+about its axis. The axis itself always points in the same direction, and
+since the earth slides irregularly around it the latitude of all parts
+of the earth keeps changing. Chandler has shown that the wabbling thus
+induced consists of two parts. The first is a movement in a circle with
+a radius of about fifteen feet which is described in approximately 430
+days. This so-called Eulerian movement is a normal gyroscopic motion
+like the slow gyration of a spinning top. This depends on purely
+astronomical causes, and no terrestrial cause can stop it or eliminate
+it. The period appears to be constant, but there are certain puzzling
+irregularities. The usual amplitude of this movement, as
+Schlesinger[135] puts it, "is about 0".27, but twice in recent years it
+has jumped to 0".40. Such a change could be accounted for by supposing
+that the earth had received a severe blow or a series of milder blows
+tending in the same direction." These blows, which were originally
+suggested by Helmert are most interesting in view of our suggestion as
+to the blows struck by storms.
+
+The second movement of the pole has a period of a year, and is roughly
+an ellipse whose longest radius is fourteen feet and the shortest, four
+feet; or, to put it technically, there is an annual term with a maximum
+amplitude of about 0".20. This, however, varies irregularly. The result
+is that the pole seems to wander over the earth's surface in the spiral
+fashion illustrated in Fig. 13. It was early suggested that this
+peculiar wandering of the pole in an annual period must be due to
+meteorological causes. Jeffreys[136] has investigated the matter
+exhaustively. He assumes certain reasonable values for the weight of air
+added or subtracted from different parts of the earth's surface
+according to the seasons. He also considers the effect of precipitation,
+vegetation, and polar ice, and of variations of temperature and
+atmospheric pressure in their relation to movements of the ocean. Then
+he proceeds to compare all these with the actual wandering of the pole
+from 1907 to 1913. While it is as yet too early to say that any special
+movement of the pole was due to the specific meteorological conditions
+of any particular year, Jeffreys' work makes it clear that
+meteorological causes, especially atmospheric pressure, are sufficient
+to cause the observed irregular wanderings. Slight wanderings may arise
+from various other sources such as movements of the rocks when
+geological faults occur or the rush of a great wave due to a submarine
+earthquake. So far as known, however, all these other agencies cause
+insignificant displacements compared with those arising from movements
+of the air. This fact coupled with the mathematical certainty that
+meteorological phenomena must produce some wandering of the pole, has
+caused most astronomers to accept Jeffreys' conclusion. If we follow
+their example we are led to conclude that changes in atmospheric
+pressure and in the other meteorological conditions strike blows which
+sometimes shift the earth several feet from its normal position in
+respect to the axis.
+
+[Illustration: _Fig. 13. Wandering of the pole from 1890 to 1898._
+(_After Moulton._)]
+
+If the foregoing reasoning is correct, the great and especially the
+sudden departures from the smooth gyroscopic circle described by the
+pole in the Eulerian motion would be expected to occur at about the same
+time as unusual earthquake activity. This brings us to an interesting
+inquiry carried out by Milne[137] and amplified by Knott.[138] Taking
+Albrecht's representation of the irregular spiral-like motion of the
+pole, as given in Fig. 13, they show that there is a preponderance of
+severe earthquakes at times when the direction of motion of the earth in
+reference to its axis departs from the smooth Eulerian curve. A summary
+of their results is given in Table 9. The table indicates that during
+the period from 1892 to 1905 there were nine different times when the
+curve of Fig. 13 changed its direction or was deflected by less than 10 deg.
+during a tenth of a year. In other words, during those periods it did
+not curve as much as it ought according to the Eulerian movement. At
+such times there were 179 world-shaking earthquakes, or an average of
+about 19.9 per tenth of a year. According to the other lines of Table 9,
+in thirty-two cases the deflection during a tenth of a year was between
+10 deg. and 25 deg., while in fifty-six cases it was from 25 deg. to 40 deg.. During
+these periods the curve remained close to the Eulerian path and the
+world-shaking earthquakes averaged only 8.2 and 12.9. Then, when the
+deflection was high, that is, when meteorological conditions threw the
+earth far out of its Eulerian course, the earthquakes were again
+numerous, the number rising to 23.4 when the deflection amounted to more
+than 55 deg..
+
+TABLE 9
+
+DEFLECTION OF PATH OF POLE COMPARED WITH EARTHQUAKES
+
+                   _No. of      _No. of        _Average No.
+    _Deflection_  Deflections_  Earthquakes_  of Earthquakes_
+       0-10 deg.          9          179            19.9
+      10-25 deg.         32          263             8.2
+      25-40 deg.         56          722            12.9
+      40-55 deg.         19          366            19.3
+    over 55 deg.          7          164            23.4
+
+In order to test this conclusion in another way we have followed a
+suggestion of Professor Schlesinger. Under his advice the Eulerian
+motion has been eliminated and a new series of earthquake records has
+been compared with the remaining motions of the poles which presumably
+arise largely from meteorological causes. For this purpose use has been
+made of the very full records of earthquakes published under the
+auspices of the International Seismological Commission for the years
+1903 to 1908, the only years for which they are available. These include
+every known shock of every description which was either recorded by
+seismographs or by direct observation in any part of the world. Each
+shock is given the same weight, no matter what its violence or how
+closely it follows another. The angle of deflection has been measured as
+Milne measured it, but since the Eulerian motion is eliminated, our zero
+is approximately the normal condition which would prevail if there were
+no meteorological complications. Dividing the deflections into six equal
+groups according to the size of the angle, we get the result shown in
+Table 10.
+
+TABLE 10
+
+EARTHQUAKES IN 1903-1908 COMPARED WITH DEPARTURES OF THE PROJECTED
+CURVE OF THE EARTH'S AXIS FROM THE EULERIAN POSITION
+
+    _Average angle of deflection_     _Average daily number
+   (_10 periods of 1/10 year each_)      of earthquakes_
+              -10.5 deg.                         8.31
+               11.5 deg.                         8.35
+               25.8 deg.                         8.23
+               40.2 deg.                         8.14
+               54.7 deg.                         8.86
+               90.3 deg.                        11.81
+
+Here where some twenty thousand earthquakes are employed the result
+agrees closely with that of Milne for a different series of years and
+for a much smaller number of earthquakes. So long as the path of the
+pole departs less than about 45 deg. from the smooth gyroscopic Eulerian
+path, the number of earthquakes is almost constant, about eight and a
+quarter per day. When the angle becomes large, however, the number
+increases by nearly 50 per cent. Thus the work of Milne, Knott, and
+Jeffreys is confirmed by a new investigation. Apparently earthquakes and
+crustal movements are somehow related to sudden changes in the load
+imposed on the earth's crust by meteorological conditions.
+
+This conclusion is quite as surprising to the authors as to the
+reader--perhaps more so. At the beginning of this investigation we had
+no faith whatever in any important relation between climate and
+earthquakes. At its end we are inclined to believe that the relation is
+close and important.
+
+It must not be supposed, however, that meteorological conditions are the
+_cause_ of earthquakes and of movements of the earth's crust. Even
+though the load that the climatic agencies can impose upon the earth's
+crust runs into millions of tons per square mile, it is a trifle
+compared with what the crust is able to support. There is, however, a
+great difference between the cause and the occasion of a phenomenon.
+Suppose that a thick sheet of glass is placed under an increasing
+strain. If the strain is applied slowly enough, even so rigid a material
+as glass will ultimately bend rather than break. But suppose that while
+the tension is high the glass is tapped. A gentle tap may be followed by
+a tiny crack. A series of little taps may be the signal for small cracks
+to spread in every direction. A few slightly harder taps may cause the
+whole sheet to break suddenly into many pieces. Yet even the hardest tap
+may be the merest trifle compared with the strong force which is keeping
+the glass in a state of strain and which would ultimately bend it if
+given time.
+
+The earth as a whole appears to stand between steel and glass in
+rigidity. It is a matter of common observation that rocks stand high in
+this respect and in the consequent difficulty with which they can be
+bent without breaking. Because of the earth's contraction the crust
+endures a constant strain, which must gradually become enormous. This
+strain is increased by the fact that sediment is transferred from the
+lands to the borders of the sea and there forms areas of thick
+accumulation. From this has arisen the doctrine of isostasy, or of the
+equalization of crustal pressure. An important illustration of this is
+the oceanward and equatorial creep which has been described in Chapter
+XI. There we saw that when the lands have once been raised to high
+levels or when a shortening of the earth's axis by contraction has
+increased the oceanic bulge at the equator, or when the reverse has
+happened because of tidal retardation, the outer part of the earth
+appears to creep slowly back toward a position of perfect isostatic
+adjustment. If the sun had no influence upon the earth, either direct or
+indirect, isostasy and other terrestrial processes might flex the
+earth's crust so gradually that changes in the form and height of the
+lands would always take place slowly, even from the geological point of
+view. Thus erosion would usually be able to remove the rocks as rapidly
+as they were domed above the general level. If this happened, mountains
+would be rare or unknown, and hence climatic contrasts would be far less
+marked than is actually the case on our earth where crustal movements
+have repeatedly been rapid enough to produce mountains.
+
+Nature's methods rarely allow so gradual an adjustment to the forces of
+isostasy. While the crust is under a strain, not only because of
+contraction, but because of changes in its load through the transference
+of sediments and the slow increase or decrease in the bulge at the
+equator, the atmosphere more or less persistently carries on the tapping
+process. The violence of that process varies greatly, and the variations
+depend largely on the severity of the climatic contrasts. If the main
+outlines of the cyclonic hypothesis are reliable, one of the first
+effects of a disturbance of the sun's atmosphere is increased storminess
+upon the earth. This is accompanied by increased intensity in almost
+every meteorological process. The most important effect, however, so far
+as the earth's crust is concerned would apparently be the rapid and
+intense changes of atmospheric pressure which would arise from the swift
+passage of one severe storm after another. Each storm would be a little
+tap on the tensely strained crust. Any single tap might be of little
+consequence, even though it involved a change of a billion tons in the
+pressure on an area no larger than the state of Rhode Island. Yet a
+rapid and irregular succession of such taps might possibly cause the
+crust to crack, and finally to collapse in response to stresses arising
+from the shrinkage of the earth.
+
+Another and perhaps more important effect of variations in storminess
+and especially in the location of the stormy areas would be an
+acceleration of erosion in some places and a retardation elsewhere. A
+great increase in rainfall may almost denude the slopes of soil, while a
+diminution to the point where much of the vegetation dies off has a
+similar effect. If such changes should take place rapidly, great
+thicknesses of sediment might be concentrated in certain areas in a
+short time, thus disturbing the isostatic adjustment of the earth's
+crust. This might set up a state of strain which would ultimately have
+to be relieved, thus perhaps initiating profound crustal movements.
+Changes in the load of the earth's crust due to erosion and the
+deposition of sediment, no matter how rapid they may be from the
+geological standpoint, are slow compared with those due to changes in
+barometric pressure. A drop of an inch in barometric pressure is
+equivalent to the removal of about five inches of solid rock. Even under
+the most favorable circumstances, the removal of an average depth of
+five inches of rock or its equivalent in soil over millions of square
+miles would probably take several hundred years, while the removal of a
+similar load of air might occur in half a day or even a few hours. Thus
+the erosion and deposition due to climatic variations presumably play
+their part in crustal deformation chiefly by producing crustal stresses,
+while the storms, as it were, strike sharp, sudden blows.
+
+Suppose now that a prolonged period of world-wide mild climate, such as
+is described in Chapter X, should permit an enormous accumulation of
+stresses due to contraction and tidal retardation. Suppose that then a
+sudden change of climate should produce a rapid shifting of the deep
+soil that had accumulated on the lands, with a corresponding
+localization and increase in strains. Suppose also that frequent and
+severe storms play their part, whether great or small, by producing an
+intensive tapping of the crust. In such a case the ultimate collapse
+would be correspondingly great, as would be evident in the succeeding
+geological epoch. The sea floor might sink lower, the continents might
+be elevated, and mountain ranges might be shoved up along lines of
+special weakness. This is the story of the geological period as known to
+historical geology. The force that causes such movements would be the
+pull of gravity upon the crust surrounding the earth's shrinking
+interior. Nevertheless climatic changes might occasionally set the date
+when the gravitative pull would finally overcome inertia, and thus usher
+in the crustal movements that close old geologic periods and inaugurate
+new ones. This, however, could occur only if the crust were under
+sufficient strain. As Lawson[139] says in his discussion of the "elastic
+rebound theory," the sudden shifts of the crust which seem to be the
+underlying cause of earthquakes "can occur only after the accumulation
+of strain to a limit and ... this accumulation involves a slow creep of
+the region affected. In the long periods between great earthquakes the
+energy necessary for such shocks is being stored up in the rocks as
+elastic compression."
+
+If a period of intense storminess should occur when the earth as a whole
+was in such a state of strain, the sudden release of the strains might
+lead to terrestrial changes which would alter the climate still further,
+making it more extreme, and perhaps permitting the storminess due to the
+solar disturbances to bring about glaciation. At the same time if
+volcanic activity should increase it would add its quota to the tendency
+toward glaciation. Nevertheless, it might easily happen that a very
+considerable amount of crustal movement would take place without causing
+a continental ice sheet or even a marked alpine ice sheet. Or again, if
+the strains in the earth's crust had already been largely released
+through other agencies before the stormy period began, the climate might
+become severe enough to cause glaciation in high latitudes without
+leading to any very marked movements of the earth's crust, as apparently
+happened in the Mid-Silurian period.
+
+FOOTNOTES:
+
+[Footnote 125: E. Kirk: Paleozoic Glaciation in Alaska; Am. Jour. Sci.,
+1918, p. 511.]
+
+[Footnote 126: J. Milne: Catalogue of Destructive Earthquakes; Rep.
+Brit. Asso. Adv. Sci., 1911.]
+
+[Footnote 127: Wm. Bowie: Lecture before the Geological Club of Yale
+University. See Am. Jour. Sci., 1921.]
+
+[Footnote 128: Chas. Davisson: On the Annual and Semi-annual Seismic
+Periods; Roy. Soc. of London, Philosophical Transactions, Vol. 184,
+1893, 1107 _ff._]
+
+[Footnote 129: C. G. Knott: The Physics of Earthquake Phenomena, Oxford,
+1908.]
+
+[Footnote 130: In Table 8 the first column indicates the region; the
+second, the dates; and the third, the number of shocks. The fourth
+column gives the month in which the annual maximum occurs when the crude
+figures are smoothed by the use of overlapping six-monthly means. In
+other words, the average for each successive six months has been placed
+in the middle of the period. Thus the average of January to June,
+inclusive, is placed between March and April, that for February to July
+between April and May, and so on. This method eliminates the minor
+fluctuations and also all periodicities having a duration of less than a
+year. If there were no annual periodicity the smoothing would result in
+practically the same figure for each month. The column marked
+"Amplitude" gives the range from the highest month to the lowest divided
+by the number of earthquakes and then corrected according to Schuster's
+method which is well known to mathematicians, but which is so confusing
+to the layman that it will not be described. Next, in the column marked
+"Expected Amplitude," we have the amplitude that would be expected if a
+series of numbers corresponding to the earthquake numbers and having a
+similar range were arranged in accidental order throughout the year.
+This also is calculated by Schuster's method in which the expected
+amplitude is equal to the square root of "pi" divided by the number of
+shocks. When the actual amplitude is four or more times the expected
+amplitude, the probability that there is a real periodicity in the
+observed phenomena becomes so great that we may regard it as practically
+certain. If there is no periodicity the two are equal. The last column
+gives the number of times by which the actual exceeds the expected
+amplitude, and thus is a measure of the probability that earthquakes
+vary systematically in a period of a year.]
+
+[Footnote 131: N. F. Drake: Destructive Earthquakes in China; Bull.
+Seism. Soc. Am., Vol. 2, 1912, pp. 40-91, 124-133.]
+
+[Footnote 132: The only other explanation that seems to have any
+standing is the psychological hypothesis of Montessus de Ballore as
+given in Les Tremblements de Terre. He attributes the apparent seasonal
+variation in earthquakes to the fact that in winter people are within
+doors, and hence notice movements of the earth much more than in summer
+when they are out of doors. There is a similar difference between
+people's habits in high latitudes and low. Undoubtedly this does have a
+marked effect upon the degree to which minor earthquake shocks are
+noticed. Nevertheless, de Ballore's contention, as well as any other
+psychological explanation, is completely upset by two facts: First,
+instrumental records show the same seasonal distribution as do records
+based on direct observation, and instruments certainly are not
+influenced by the seasons. Second, in some places, notably China, as
+Drake has shown, the summer rather than the winter is very decidedly the
+time when earthquakes are most frequent.]
+
+[Footnote 133: A comparison of tropical hurricanes with earthquakes is
+interesting. Taking all the hurricanes recorded in August, September,
+and October, from 1880 to 1899, and the corresponding earthquakes in
+Milne's catalogue, the correlation coefficient between hurricanes and
+earthquakes is +0.236, with a probable error of +-0.082, the month being
+used as the unit. This is not a large correlation, yet when it is
+remembered that the hurricanes represent only a small part of the
+atmospheric disturbances in any given month, it suggests that with
+fuller data the correlation might be large.]
+
+[Footnote 134: Ellsworth Huntington: The Geographic Work of Dr. M. A.
+Veeder; Geog. Rev., Vol. 3, March and April, 1917, Nos. 3 and 4.]
+
+[Footnote 135: Frank Schlesinger: Variations of Latitude; Their Bearing
+upon Our Knowledge of the Interior of the Earth; Proc. Am. Phil. Soc.,
+Vol. 54, 1915, pp. 351-358. Also Smithsonian Report for 1916, pp.
+248-254.]
+
+[Footnote 136: Harold Jeffreys: Causes Contributory to the Annual
+Variations of Latitude; Monthly Notices, Royal Astronomical Soc., Vol.
+76, 1916, pp. 499-525.]
+
+[Footnote 137: John Milne: British Association Reports for 1903 and
+1906.]
+
+[Footnote 138: C. G. Knott: The Physics of Earthquake Phenomena, Oxford,
+1908.]
+
+[Footnote 139: A. C. Lawson: The Mobility of the Coast Ranges of
+California; Univ. of Calif. Pub., Geology, Vol. 12, No. 7, pp. 431-473.]
+
+
+
+
+CONCLUSION
+
+
+Here we must bring this study of the earth's evolution to a close. Its
+fundamental principle has been that the present, if rightly understood,
+affords a full key to the past. With this as a guide we have touched on
+many hypotheses, some essential and some unessential to the general line
+of thought. The first main hypothesis is that the earth's present
+climatic variations are correlated with changes in the solar atmosphere.
+This is the keynote of the whole book. It is so well established,
+however, that it ranks as a theory rather than as an hypothesis. Next
+comes the hypothesis that variations in the solar atmosphere influence
+the earth's climate chiefly by causing variations not only in
+temperature but also in atmospheric pressure and thus in storminess,
+wind, and rainfall. This, too, is one of the essential foundations on
+which the rest of the book is built, but though this cyclonic hypothesis
+is still a matter of discussion, it seems to be based on strong
+evidence. These two hypotheses might lead us astray were they not
+balanced by another. This other is that many climatic conditions are due
+to purely terrestrial causes, such as the form and altitude of the
+lands, the degree to which the continents are united, the movement of
+ocean currents, the activity of volcanoes, and the composition of the
+atmosphere and the ocean. Only by combining the solar and the
+terrestrial can the truth be perceived. Finally, the last main
+hypothesis of this book holds that if the climatic conditions which now
+prevail at times of solar activity were magnified sufficiently and if
+they occurred in conjunction with certain important terrestrial
+conditions of which there is good evidence, they would produce most of
+the notable phenomena of glacial periods. For example, they would
+explain such puzzling conditions as the localization and periodicity of
+glaciation, the formation of loess, and the occurrence of glaciation in
+low latitudes during Permian and Proterozoic times. The converse of this
+is that if the conditions which now prevail at times when the sun is
+relatively inactive should be intensified, that is, if the sun's
+atmosphere should become calmer than now, and if the proper terrestrial
+conditions of topographic form and atmospheric composition should
+prevail, there would arise the mild climatic conditions which appear to
+have prevailed during the greater part of geological time. In short,
+there seems thus far to be no phase of the climate of the past which is
+not in harmony with an hypothesis which combines into a single unit the
+three main hypotheses of this book, solar, cyclonic, and terrestrial.
+
+Outside the main line of thought lie several other hypotheses. Several
+of these, as well as some of the main hypotheses, are discussed chiefly
+in _Earth and Sun_, but as they are given a practical application in
+this book they deserve a place in this final summary. Each of these
+secondary hypotheses is in its way important. Yet any or all may prove
+untrue without altering our main conclusions. This point cannot be too
+strongly emphasized, for there is always danger that differences of
+opinion as to minor hypotheses and even as to details may divert
+attention from the main point. Among the non-essential hypotheses is the
+idea that the sun's atmosphere influences that of the earth electrically
+as well as thermally. This idea is still so new that it has only just
+entered the stage of active discussion, and naturally the weight of
+opinion is against it. Although not necessary to the main purpose of
+this book, it plays a minor role in the chapter dealing with the
+relation of the sun to other astronomical bodies. It also has a vital
+bearing on the further advance of the science of meteorology and the art
+of weather forecasting. Another secondary hypothesis holds that sunspots
+are set in motion by the planets. Whether the effect is gravitational or
+more probably electrical, or perhaps of some other sort, does not
+concern us at present, although the weight of evidence seems to point
+toward electronic emissions. This question, like that of the relative
+parts played by heat and electricity in terrestrial climatic changes,
+can be set aside for the moment. What does concern us is a third
+hypothesis, namely, that if the planets really determine the periodicity
+of sunspots, even though not supplying the energy, the sun in its flight
+through space must have been repeatedly and more strongly influenced in
+the same way by many other heavenly bodies. In that case, climatic
+changes like those of the present, but sometimes greatly magnified, have
+presumably arisen because of the constantly changing position of the
+solar system in respect to other parts of the universe. Finally, the
+fourth of our secondary hypotheses postulates that at present the date
+of movements of the earth's crust is often determined by the fact that
+storms and other meteorological conditions keep changing the load upon
+first one part of the earth's surface and then upon another. Thus
+stresses that have accumulated in the earth's isostatic shell during the
+preceding months are released. In somewhat the same way epochs of
+extreme storminess and rapid erosion in the past may possibly have set
+the date for great movements of the earth's crust. This hypothesis, like
+the other three in our secondary or non-essential group, is still so new
+that only the first steps have been taken in testing it. Yet it seems to
+deserve careful study.
+
+In testing all the hypotheses here discussed, primary and secondary
+alike, the first necessity is a far greater amount of quantitative work.
+In this book there has been a constant attempt to subject every
+hypothesis to the test of statistical facts of observation.
+Nevertheless, we have been breaking so much new ground that in many
+cases exact facts are not yet available, while in others they can be
+properly investigated only by specialists in physics, astronomy, or
+mathematics. In most cases the next great step is to ascertain whether
+the forces here called upon are actually great enough to produce the
+observed results. Even though they act only as a means of releasing the
+far greater forces due to the contraction of the earth and the sun, they
+need to be rigidly tested as to their ability to play even this minor
+role. Still another line of study that cries aloud for research is a
+fuller comparison between earthquakes on the one hand and meteorological
+conditions and the wandering of the poles on the other. Finally, an
+extremely interesting and hopeful quest is the determination of the
+positions and movements of additional stars and other celestial bodies,
+the faint and invisible as well as the bright, in order to ascertain the
+probable magnitude of their influence upon the sun and thus upon the
+earth at various times in the past and in the future. Perhaps we are
+even now approaching some star that will some day give rise to a period
+of climatic stress like that of the fourteenth century, or possibly to a
+glacial epoch. Or perhaps the variations in others of the nearer stars
+as well as Alpha Centauri may show a close relation to changes in the
+sun.
+
+Throughout this volume we have endeavored to discover new truth
+concerning the physical environment that has molded the evolution of all
+life. We have seen how delicate is the balance among the forces of
+nature, even though they be of the most stupendous magnitude. We have
+seen that a disturbance of this balance in one of the heavenly bodies
+may lead to profound changes in another far away. Yet during the billion
+years, more or less, of which we have knowledge, there appears never to
+have been a complete cataclysm involving the destruction of all life.
+One star after another, if our hypothesis is correct, has approached the
+solar system closely enough to set the atmosphere of the sun in such
+commotion that great changes of climate have occurred upon the earth.
+Yet never has the solar system passed so close to any other body or
+changed in any other way sufficiently to blot out all living things. The
+effect of climatic changes has always been to alter the environment and
+therefore to destroy part of the life of a given time, but with this
+there has invariably gone a stimulus to other organic types. New
+adaptations have occurred, new lines of evolutionary progress have been
+initiated, and the net result has been greater organic diversity and
+richness. Temporarily a great change of climate may seem to retard
+evolution, but only for a moment as the geologist counts time. Then it
+becomes evident that the march of progress has actually been more rapid
+than usual. Thus the main periods of climatic stress are the most
+conspicuous milestones upon the upward path toward more varied
+adaptation. The end of each such period of stress has found the life of
+the world nearer to the high mentality which reaches out to the utmost
+limits of space, of time, and of thought in the search for some
+explanation of the meaning of the universe. Each approach of the sun to
+other bodies, if such be the cause of the major climatic changes, has
+brought the organic world one step nearer to the solution of the
+greatest of all problems,--the problem of whether there is a psychic
+goal beyond the mental goal toward which we are moving with ever
+accelerating speed. Throughout the vast eons of geological time the
+adjustment of force to force, of one body of matter to another, and of
+the physical environment to the organic response has been so delicate,
+and has tended so steadily toward the one main line of mental progress
+that there seems to be a purpose in it all. If the cosmic uniformity of
+climate continues to prevail and if the uniformity is varied by changes
+as stimulating as those of the past, the imagination can scarcely
+picture the wonders of the future. In the course of millions or even
+billions of years the development of mind, and perhaps of soul, may
+excel that of today as far as the highest known type of mentality excels
+the primitive plasma from which all life appears to have arisen.
+
+
+
+
+INDEX
+
+* Indicates illustrations.
+
+
+  Abbot, C. G., cited, 45, 52, 237, 238, 239.
+
+  Aboskun, 104.
+
+  Africa, earthquakes, 301;
+    East, _see_ East Africa;
+    lakes, 143;
+    North, _see_ North Africa.
+
+  African glaciation, 266.
+
+  Air, _see_ Atmosphere.
+
+  Alaska, glacial till in, 287;
+    Ice Age in, 221.
+
+  Albrecht, cited, 304.
+
+  Alexander, march of, 88 f.
+
+  Allard, H. A., cited, 183, 184.
+
+  Alpha Centauri, companion of, 280;
+    distance from sun, 262;
+    luminosity, 278;
+    speed of, 281;
+    variations, 282.
+
+  Alps, loess in, 159;
+    precipitation in, 141;
+    snow level in, 139.
+
+  Altair, companion of, 280;
+    luminosity, 278;
+    speed of, 281.
+
+  Amazon forest, temperature, 17.
+
+  Ancylus lake, 217.
+
+  Andes, snow line, 139.
+
+  Animals, climate and, 1.
+
+  Antarctica, mild climate, 219;
+    thickness of ice in, 125;
+    winds, 135, 161.
+
+  Anti-cyclonic hypothesis, 135 ff.
+
+  Appalachians, effect on ice sheet, 121.
+
+  Arabia, civilization in, 67.
+
+  Aral, Sea of, 108.
+
+  Archean rocks, 211.
+
+  Archeozoic, 3 f.;
+    climate of, 267.
+
+  Arctic Ocean, submergence, 219.
+
+  Arctowski, H., cited, 29, 46, 244.
+
+  Argon, increase of, 236.
+
+  Arizona, rainfall, 89, 108;
+    trees measured in, 73.
+
+  Arrhenius, S., cited, 36, 254.
+
+  Arsis, of pulsation, 24.
+
+  Asbjoern Selsbane, corn of, 101.
+
+  Asia, atmospheric pressure, 298;
+    central, changes of climate, *75;
+    central, post-glacial climate, 271;
+    climate, 66;
+    glaciation in, 131;
+    storminess in, 60;
+    western, climate in, 84 f.
+
+  Atlantic Ocean, storminess, 57.
+
+  Atmosphere, changes, 19 f., 229;
+    composition of, 223-241;
+    effect on temperature, 231.
+
+  Atmospheric circulation, glaciation and, 42.
+
+  Atmospheric electricity, solar relations of, 56.
+
+  Atmospheric pressure, earthquakes and, 298;
+    evaporation and, 237;
+    increase in, 239;
+    redistribution of, 49;
+    variation, 53.
+
+  Australia, East, mild climate, 219;
+    precipitation, 144.
+
+  Axis, earth's, 48;
+    wabbling of, 301.
+
+
+  Bacon, Sir Francis, cited, 27.
+
+  Bacubirito, meteor at, 246.
+
+  Baltic Sea, as lake, 217;
+    freezing of, 100;
+    ice, 26;
+    storm-floods, 99;
+    submergence, 219.
+
+  Bardsson, Ivar, 106.
+
+  Barkow, cited, 135.
+
+  Barometric pressure, solar relations of, 56.
+
+  Barrell, J., cited, 3, 200, 213, 234.
+
+  Bartoli, A. G., cited, 257.
+
+  Bauer, L. A., cited, 150.
+
+  Beaches, under water, 97.
+
+  Beadnell, H. J. L., cited, 143.
+
+  Beluchistan, rainfall, 89.
+
+  Bengal, Bay of, cyclones in, 149.
+
+  Bengal, famine in, 104 f.
+
+  Berlin, rainfall and temperature, 93.
+
+  Betelgeuse, 259 f.;
+    distance from sun, 262.
+
+  Bible, climatic evidence in, 91 f.;
+    palms in, 92.
+
+  Binary stars, 252.
+
+  Birkeland, K., cited, 244.
+
+  Black Earth region, loess in, 159.
+
+  Boca, Cal., correlation coefficients, 83, 85.
+
+  Boltzmann, L., cited, 257.
+
+  Bonneville, Lake, 142, 143.
+
+  Borkum, storm-flood in, 99.
+
+  Boss, L. cited, 268, 269.
+
+  Botanical evidence of mild climates, 167 ff.
+
+  Boulders, on Irish coast, 119.
+
+  Bowie, W., cited, 293.
+
+  Bowman, I., cited, 213.
+
+  Britain, forests, 220;
+    level of land, 220.
+
+  British Isles, height of land, 111;
+    temperature, 216.
+
+  Brooks, C. E. P., cited, 115, 143, 196, 215, 225.
+
+  Brooks, C. F., cited, 209.
+
+  Brown, E. W., cited, 191, 244.
+
+  Brueckner, E., cited, 27.
+
+  Brueckner periods, 27 f.
+
+  Bufo, habitat of, 202.
+
+  Buhl stage, 216.
+
+  Bull, Dr., cited, 100, 101.
+
+  Butler, H. C., cited, 66, 67 ff., 70, 76.
+
+
+  California, changes of climate, *75;
+    correlations of rainfall, 86;
+    measurements of sequoias in, 73, 74 ff.;
+    rainfall, 108.
+
+  Cambrian period, 4 f.
+
+  Canada, storminess, 53 f., 57;
+    storm tracks in, 113.
+
+  Cape Farewell, shore ice at, 105.
+
+  Carbon dioxide, erosion and, 119 f.;
+    from volcanoes, 23;
+    hypothesis, 139;
+    importance of, 9, 11 f.;
+    in Permian, 148;
+    in atmosphere, 20, 96, 238;
+    in ocean, 226;
+    nebular hypothesis and, 232;
+    theory of glaciation, 36 ff.
+
+  Caribbean mountains, origin of, 193.
+
+  Carnegie Institution of Washington, 74.
+
+  Caspian Sea, climatic stress, 104;
+    rainfall, 107 f.;
+    rise and fall, 27;
+    ruins in, 71.
+
+  Cenozoic, climate, 266;
+    fossils, 21.
+
+  Central America, Maya ruins, 95.
+
+  Chad, Lake, swamps of, 171.
+
+  Chamberlin, R. T., cited, 166, 233, 269.
+
+  Chamberlin, T. C., cited, 19, 36, 38, 39, 42 f., 48, 122, 125,
+     152, 156, 190, 195, 227, 269.
+
+  Chandler, S. C., cited, 301.
+
+  Chinese earthquakes, periodicity of, 245.
+
+  Chinese, sunspot observations, 108 f.
+
+  Chinese Turkestan, desiccation in, 66.
+
+  Chronology, glacial, 215.
+
+  Clarke, F. W., cited, 226, 235.
+
+  Clayton, H. H., cited, 173 f.
+
+  Climate, effect of contraction, 189 ff.;
+    affect of salinity, 224;
+    in history, 64-97;
+    uniformity, 1-15;
+    variability, 16-32.
+
+  Climates, mild, causes of, 166-187;
+    mild, periods of, 274.
+
+  Climatic changes, and crustal movements, 285 ff.;
+    hypotheses of, 33-50;
+    mountain-building and, *25;
+    post-glacial crustal movements and, 215-222;
+    terrestrial causes of, 188-214.
+
+  Climatic sequence, 16 f.
+
+  Climatic stages, post-glacial, 270.
+
+  Climatic stress, in fourteenth century, 98-109.
+
+  Climatic uniformity, hypothesis of, 65, 71 f.
+
+  Climatic zoning, 169.
+
+  Cloudiness, glaciation and, 114, 147.
+
+  Clouds, as protection, 197.
+
+  Colfax, Cal., correlation coefficients, 83.
+
+  Cologne, flood at, 99.
+
+  Compass, variations, 150.
+
+  Continental climate, variations, 103.
+
+  Continents, effect on climate, 111 f.
+
+  Contraction, effect on climate, 189 ff., 199, 207;
+    effect on lands, 207;
+    heat of sun and, 13 f.;
+    irregular, 195;
+    of the earth, 18;
+    of the sun, 249;
+    stresses caused by, 310.
+
+  Convection, carbon dioxide and, 239.
+
+  Corals, in high latitudes, 21, 39, 167, 178.
+
+  Cordeiro, F. J. B., cited, 181, 183, 186.
+
+  Correlation coefficients, earthquakes and sunspots, 291;
+    Jerusalem rainfall and sequoia growth, 83 ff.;
+    rainfall and tree growth, 79 ff.
+
+  Cosmos, effect of light, 185.
+
+  Cressey, G. B., cited, 80.
+
+  Cretaceous, lava, 211;
+    mountain ranges, 44;
+    paleogeography, *201;
+    submergence of North America, 200.
+
+  Croll, J., cited, 34 ff., 176.
+
+  Croll's hypothesis, snow line, 139.
+
+  Crust, climate and movements of, 63, 287, 310;
+    movements of, 43;
+    strains in, 22.
+
+  Currents and planetary winds, 174.
+
+  Cycads, 169.
+
+  Cyclonic hypothesis, 97;
+    loess and, 163;
+    Permian glaciation and, 148;
+    snow line, 139.
+
+  Cyclonic storms, in glacial epochs, 140 f.;
+    solar electricity and, 243 (_see_ Storms, Storminess).
+
+  Cyclonic vacillations, 30 f.;
+    nature of, 57 ff.
+
+
+  Daily vibrations, 28 f.
+
+  Danube, frozen, 98.
+
+  Darwin, G. H., cited, 191.
+
+  Daun stage, 217.
+
+  Davis, W. M., cited, 271.
+
+  Davisson, C., cited, 294, 295, 299.
+
+  Day, C. P., cited, 239.
+
+  Day, length of, 18, 191.
+
+  Dead Sea, palms near, 92.
+
+  Death Valley, 142.
+
+  De Ballore, M., cited, 297, 298.
+
+  Deep-sea circulation, rapidity, 227;
+    salinity and, 176;
+    solar activity and, 179.
+
+  De Geer, S., cited, 215, 221.
+
+  De Lapparent, A., cited, 200.
+
+  Denmark, fossils, 271.
+
+  "Desert pavements," 161.
+
+  Deserts, abundant flora of, 171;
+    and pulsations theory, 88 ff.;
+    red beds of, 170.
+
+  Devonian, climate, 266;
+    mountains, 209.
+
+  Dog, climate and, 1.
+
+  Donegal County, Ireland, 220.
+
+  Double stars, 272, 280;
+    electrical effect of, 261.
+
+  Douglass, A. E., cited, 28, 73, 74 f., 84, 85, 107.
+
+  Dragon Town, destruction of, 104, 108.
+
+  Drake, N. F., cited, 297, 298.
+
+  Droughts, and pulsations theory, 87 f.;
+    in England, 102;
+    in India, 104 f.
+
+  Drumkelin Bog, Ireland, log cabin in, 220.
+
+  Dust, at high levels, 240.
+
+
+  Earth, crust of and the sun, 285-317;
+    internal heat, 212;
+    nature of mild climate, 274;
+    position of axis, 181;
+    rigidity of, 307;
+    temperature gradient, 213;
+    temperature of surface, 8.
+
+  Earthquakes, and seasons, 294, 297;
+    and sunspots, 288 f.;
+    and tropical hurricanes, 300;
+    and wandering of pole, 304 f.;
+    cause of, 307;
+    compared with departures from Eulerian position, 306;
+    seasonal distribution of, 299;
+    seasonal march, 295.
+
+  "Earthquake weather," 298.
+
+  East Africa, mild climate, 219.
+
+  East Indies, earthquakes of, 296.
+
+  Eberswalde, tree growth at, 102 f.
+
+  Ecliptic, obliquity of, 217.
+
+  Electrical currents, in solar atmosphere, 261.
+
+  Electrical emissions, variation of, 275.
+
+  Electrical hypothesis, 150, 250 f., 256 ff.
+
+  Electrical phenomena, storminess and, 56.
+
+  Electricity, and earthquakes, 292;
+    solar, 243.
+
+  Electro-magnetic hypothesis, 244.
+
+  Electrons, solar, 56;
+    variation of, 256.
+
+  Electro-stellar hypothesis, 274.
+
+  Elevation, climatic changes and, 39.
+
+  Engedi, palms in, 92.
+
+  England, climatic stress, 101 f.;
+    storminess and rainfall, 107.
+
+  Eocene, climate, 266.
+
+  Equinoxes, precession of, 96.
+
+  Erosion, storminess and, 309.
+
+  Eskimo, in Greenland, 106.
+
+  Eulerian movement, 301, 304.
+
+  Euphrates, 67.
+
+  Europe, climatic stress, 98 ff., 102 f.;
+    climatic table, 215;
+    glaciation in, 131;
+    ice sheet, 121;
+    inundations of rivers, 99;
+    post-glacial climate, 271;
+    rainfall, 107;
+    submergence, 196, 200.
+
+  Evaporation, and glaciation, 112, 114;
+    atmospheric pressure and, 237;
+    from plants, 179;
+    importance, 129;
+    in trade-wind belt, 117;
+    rapidity of, 224.
+
+  Evening primrose, effect of light, 184.
+
+  Evolution, climate and, 20;
+    geographical complexity and, 241;
+    glaciation and, 33;
+    of the earth, 311.
+
+
+  Faculae, cause of, 61.
+
+  False Point Lighthouse, barometric pressure at, 299.
+
+  Famine, cause of, 103;
+    in England, 101 f.;
+    in India, 104 f.;
+    pulsations theory and, 87 f.
+
+  Faunas, and mild climates, 168 f.;
+    in Permian, 152 f.
+
+  Fennoscandian pause, 216.
+
+  Flowering, light and, 184.
+
+  Fog, and glaciation, 116;
+    as protection, 197;
+    temperature and, 178.
+
+  Forests, climate and, 66.
+
+  Form of the land, 43 ff.
+
+  Fossil floras, and mild climates, 168;
+    in Antarctica, 273;
+    in Greenland, 273.
+
+  Fossils, 169, 230;
+    and loess, 158;
+    Archeozoic, 3 f.;
+    Cenozoic, 21;
+    dating of, 153;
+    glaciation and, 138;
+    in peat bogs, 271;
+    mild climate, 167;
+    Proterozoic, 4, 6 f.
+
+  Fourteenth century, climatic stress in, 98-109.
+
+  Fowle, F. E., cited, 45, 237, 238, 239.
+
+  Frech, F., cited, 36.
+
+  Free, E. E., cited, 142.
+
+  Freezing, salinity and, 224.
+
+  Fresno, rainfall record, 82.
+
+  "Friction variables," 247.
+
+  Frisian Islands, storm-flood, 99.
+
+  Fritz, H., cited, 109.
+
+  Frogs, distribution of, 202.
+
+  Fuchs, cited, 289.
+
+
+  Galaxy, 252.
+
+  Galveston, Tex., rainfall and temperature, 94.
+
+  Garner, W. W., cited, 183, 184.
+
+  Gasses, in air, 233.
+
+  Geographers, and climatic changes, 65 ff.
+
+  Geological time table, *5.
+
+  Geologic oscillations, 18 f., 21 ff., 188, 240.
+
+  Geologists, changes in ideas of, 64 f.
+
+  Germanic myths, 219.
+
+  Germany, forests, 220;
+    growth of trees in, 102;
+    storms in, 102.
+
+  Gilbert, G. K., cited, 143.
+
+  Glacial epochs, causes of, 268;
+    dates of, 216;
+    intervals between, 264 f.;
+    length of, 166 f.
+
+  Glacial fluctuations, 24 ff.;
+    nature of, 57 ff.
+
+  Glacial period, at present, 272;
+    ice in, 57 f.;
+    length of, 269;
+    list, 265;
+    temperature, 38.
+
+  Glaciation, and loess, 155 f.;
+    and movement of crust, 287;
+    conditions favorable for, 111;
+    extent of, 124;
+    hypotheses of, 33 ff.;
+    in southern Canada, 18;
+    localization of, 130 ff.;
+    Permian, *145;
+    solar-cyclonic hypothesis of, 110-129;
+    suddenness of, 138;
+    upper limit of, 141.
+
+  Goldthwait, J. W., cited, 271.
+
+  Gondwana land, 21, 204.
+
+  Gravitation, effect on sun, 250;
+    pull of, 244.
+
+  Great Basin, in glacial period, 126;
+    salt lakes in, 142.
+
+  Great Ice Age, see Pleistocene.
+
+  Great Plains, effect on ice sheet, 120.
+
+  Greenland, climatic stress, 105 ff.;
+    ice, 26;
+    rainfall, 108;
+    storminess, 57;
+    submergence, 219;
+    vegetation, 21, 37, 287;
+    winds, 135, 161.
+
+  Gregory, J. W., cited, 90 ff., 97.
+
+  Gschnitz stage, 216.
+
+  Guatemala, ruins in, 95.
+
+  Guervain, cited, 135.
+
+  Gyroscope, earth as, 181.
+
+
+  Hale, G. E., cited, 56, 62.
+
+  Hamdulla, cited, 104.
+
+  Hann, J., cited, 66.
+
+  Hansa Union, operations of, 100.
+
+  Harmer, F. W., cited, 115, 119.
+
+  Heat, and earthquakes, 292;
+    earth's internal, 18.
+
+  Hedin, S., cited, 88.
+
+  Heim, A., cited, 190.
+
+  Heligoland, flood in, 99.
+
+  Helland-Hansen, B., cited, 174.
+
+  Helmert, F. R., cited, 302.
+
+  Henderson, L. J., cited, 9, 10, 11, 12.
+
+  Henry, A. J., cited, 94, 208.
+
+  Hercynian Mountains, 45.
+
+  High pressure and glaciation, 115, 135.
+
+  Himalayas, glaciation, 144;
+    origin of, 193;
+    snow line, 139.
+
+  Himley, cited, 104.
+
+  Historic pulsations, 24 f.;
+    nature of, 57 ff.
+
+  History, climate of, 64-97;
+    climatic pulsations and, 26.
+
+  Hobbs, W. H., cited, 115, 125, 135, 161.
+
+  Hot springs, temperature of, 6.
+
+  Humphreys, W. J., cited, 2, 37 f., 45, 46, 50, 56, 238.
+
+  Hurricanes, in arid regions, 144;
+    sunspots and, 53.
+
+  Hyades, cluster in, 268.
+
+
+  Ice, accumulations, 57 f.;
+    advances of, 122;
+    distribution of, 131;
+    drift, 105.
+
+  Ice sheets, disappearance, 128;
+    limits, 120;
+    localization, 130 ff.;
+    rate of retreat, 165;
+    thickness, 125.
+
+  Iceland, submergence, 219.
+
+  Iowan ice sheet, rapid retreat, 165.
+
+  Iowan loess, 158.
+
+  India, drought, 104 f.;
+    famine, 104 f.;
+    rainfall, 108.
+
+  Indian glaciation, 266.
+
+  Inter-glacial epoch, Permian, 153.
+
+  Internal heat of earth, 212.
+
+  Ireland, Drumkelin Bog, 220;
+    in glacial period, 119;
+    level of land, 220;
+    storminess and rainfall, 107;
+    submergence, 219.
+
+  Irish Sea, tides, 191.
+
+  Irrigation ditches, abandoned, 97.
+
+  Isostasy, 307 ff.
+
+  Italy, southern, climate of, 86 f.
+
+
+  Japan, earthquakes of, 296.
+
+  Javanese mountains, origin of, 193.
+
+  Jaxartes, 108.
+
+  Jeans, J. H., cited, 251, 252, 253, 266, 272.
+
+  Jeffreys, H., cited, 302, 303, 306.
+
+  Jeffreys, J., cited, 191.
+
+  Jericho, palms in, 92.
+
+  Jerusalem, rainfall, 86;
+    rainfall and temperature, 94;
+    rainfall in, and sequoia growth, 83 ff.
+
+  Johnson, cited, 226.
+
+  Judea, palms in, 92.
+
+  Jupiter, and sunspots, 243;
+    effect of, 253;
+    periodicity of, 61 f.;
+    temperature of, 258;
+    tidal effect of, 250.
+
+  Jurassic, climate, 266;
+    mountain ranges, 44.
+
+
+  Kansas, variations of seasons, 103.
+
+  Kara Koshun marsh, Lop Nor, 104.
+
+  Keewatin center, 113;
+    evaporation in, 129.
+
+  Keewatin ice sheet, 121.
+
+  Kelvin, Lord, cited, 13 f.
+
+  Keyes, C. R., cited, 156.
+
+  Kirk, E., cited, 287.
+
+  Knott, C. G., cited, 294, 295, 297, 299, 304, 306.
+
+  Knowlton, F. H., cited 167, 169, 170, 212, 232.
+
+  Koeppen, W., 47, 52, 140.
+
+  Krakatoa, glaciation and, 48;
+    volcanic hypothesis and, 45.
+
+  Kruemmel, O., cited, 224, 228.
+
+  Kullmer, C. J., cited, 113, 115, 128;
+    map of storminess, *54.
+
+  _Kungaspegel_, sea routes described, 106.
+
+
+  Labor, price in England, 102.
+
+  Labradorean center of glaciation, 113.
+
+  Lahontan, Lake, 142.
+
+  Lake strands, _see_ Strands.
+
+  Lake Superior, lava, 211.
+
+  Lakes, during glacial periods, 141 f.;
+    in semi-arid regions, 60;
+    of Great Basin, 126;
+    ruins in, 97.
+
+  Land, and water, climatic effect of, 196 ff.;
+    distribution of, 200, form of, 43 ff.;
+    range of temperature and, 196.
+
+  Lavas, climatic effect of, 211.
+
+  Lawson, A. C., cited, 310.
+
+  Lebanon, cedars of, 83.
+
+  Leiter, H., cited, 71.
+
+  Leverett, F., cited, 271.
+
+  Life, atmosphere and, 229 f.;
+    chemical characteristic of, 12;
+    effect of salinity, 225;
+    of glacial period, 127;
+    persistence of forms, 230.
+
+  Light, effect of atmosphere on, 236;
+    effect on plants, 184 ff.;
+    ultra-violet, storminess and, 56;
+    variation of, 275.
+
+  Litorina sea, 218.
+
+  Loess, date of, 156 ff.;
+    origin of, 155, 165.
+
+  Lop Nor, rise of, 104;
+    swamps, 171.
+
+  Lows, and glacial lobes, 122;
+    movements of, 126;
+    see Storms and Cyclones.
+
+  Lulan, 104.
+
+  Lull, R. S., cited, 5, 188.
+
+
+  MacDougal, D. T., cited, 171.
+
+  McGee, W. J., cited, 156.
+
+  Macmillan, W. D., cited, 191.
+
+  Magdalenian period, 216.
+
+  Magnetic fields of sunspots, 56.
+
+  Magnetic poles, relation to storm tracks, 150.
+
+  Makran, climate, 89;
+    rainfall, 89.
+
+  Malay Archipelago, earthquakes of, 296.
+
+  Mallet, R., cited, 288.
+
+  Malta, rainfall, 86.
+
+  Manson, M., cited, 147.
+
+  Mayas, civilization, 26;
+    ruins, 95.
+
+  Mayence, flood at, 99.
+
+  Mazelle, E., cited, 224.
+
+  Mediterranean, climate of, 72;
+    rainfall records, 86;
+    storminess in, 60.
+
+  Mercury, and sunspots, 243.
+
+  Mesozoic, climate, 266;
+    crustal changes, 286;
+    emergence of lands, 287.
+
+  Messier, 8;
+    variables, 248.
+
+  Metcalf, M. M., cited, 202.
+
+  Meteorological factors and earthquakes, 300 f.
+
+  Meteorological hypothesis of crustal movements, 294.
+
+  Meteors, and sun's heat, 13, 246.
+
+  Michelson, A. A., cited, 259.
+
+  Middle Silurian, fauna in Alaska, 287.
+
+  Mild climates, _see_ Climates, mild.
+
+  Milky Way, 252.
+
+  Mill, H. R., cited, 228.
+
+  Milne, J., cited, 288, 290, 294, 304, 306.
+
+  Miocene, crustal changes, 287.
+
+  Mississippi Basin, loess in, 159.
+
+  Mogul emperor, and famine, 104.
+
+  Monsoons, character of, 146;
+    direction of, 208;
+    Indian famines and, 105.
+
+  Moulton, F. R., cited, 13, 258, 269.
+
+  Mountain building, climatic changes and, *25.
+
+  Mountains, folding of, 190;
+    rainfall, on, 208.
+
+  Multiple stars, 252.
+
+
+  Nansen, F., cited, 122, 174.
+
+  Naples, rainfall, 86.
+
+  Nathorst, cited, 169.
+
+  Nebulae, 247.
+
+  Nebular hypothesis, 232, 267.
+
+  Neolithic period, 218.
+
+  Nevada, correlations of rainfall, 86.
+
+  New England, height of land, 111.
+
+  New Mexico, rainfall, 89.
+
+  New Orleans, La., rainfall and temperature, 94.
+
+  New Zealand, climate, 177;
+    tree ferns, 179.
+
+  Newcomb, S., cited, 52.
+
+  Nile floods, periodicity in, 245.
+
+  Nitrogen, in atmosphere, 19.
+
+  Niya, Chinese Turkestan, desiccation at, 66.
+
+  Nocturnal cooling, changes in, 238 f.
+
+  Norlind, A., cited, 100.
+
+  Norsemen, route to Greenland, 26.
+
+  Norse sagas, 219.
+
+  North Africa, climate of, 71;
+    Roman aqueducts in, 71.
+
+  North America, at maximum glaciation, 122 ff.;
+    emergence of lands, 193;
+    glaciation in, 131;
+    height of land, 111;
+    interior sea in, 200;
+    inundations, 196;
+    loess in, 155;
+    submergence of lands, 19, 21.
+
+  North Atlantic Ocean, salinity, 228.
+
+  North Sea, climatic stress, 98 ff.;
+    floods around, 26, 99;
+    rainfall, 107;
+    storminess, 57.
+
+  Northern hemisphere, earthquakes of, 294.
+
+  Norway, decay, 100;
+    temperature, 177.
+
+  Novae, 247.
+
+
+  Oceanic circulation, carbon dioxide and, 39 ff.
+
+  Oceanic climate, characteristics, 103.
+
+  Oceanic currents, diversion, 44;
+    influence of land distribution, 203.
+
+  Oceans, age of, 223;
+    composition of, 223-241;
+    deepening of, 199;
+    salinity, 19, 223;
+    temperature, 6, 152, 180, 226.
+
+  Okada, T., cited, 224.
+
+  Old Testament, temperature, 92.
+
+  Orbital precessions, 27.
+
+  Ordovician, climate, 266.
+
+  Organic evolution, glacial fluctuations and, 26.
+
+  Orion, nebulosity near, 247;
+    stars near, 248.
+
+  Orontes, 67.
+
+  Osborn, H. F., cited, 216.
+
+  Owens-Searles, lakes, 142.
+
+  Oxus, 108.
+
+  Oxygen, in atmosphere, 20, 234;
+    in Permian, 152.
+
+  Ozone, cause of, 56.
+
+
+  Paleolithic, 216.
+
+  Paleozoic, climate, 266;
+    mountains in, 209.
+
+  Palestine, change of climate, 91 f.
+
+  Palms, climatic change and, 91 f.;
+    in Ireland, 179.
+
+  Palmyra, ruins of, 66.
+
+  Parallaxes of stars, 276 f.
+
+  Patrician center, 134.
+
+  Peat-bog period, first, 218.
+
+  Penck, A., cited, 139, 156, 157, 158, 269.
+
+  Pennsylvanian, life of, 26.
+
+  Periodicities, 245 f.
+
+  Periodicity, of climatic phenomena, 60 f.;
+    of glaciation, 268;
+    of sunspots, 243.
+
+  Permian, climate, 266;
+    distribution of glaciation, 152;
+    glaciation, 60, 144, *145, 226;
+    glaciation and mountains, 45;
+    life of, 26;
+    red beds, 151;
+    temperature, 146 f.
+
+  Perry, cited, 289.
+
+  Persia, lakes, 143;
+    rainfall, 89.
+
+  Pettersson, O., cited, 98 ff., 100 f., 103, 106, 219.
+
+  Pirsson, L. V., cited, 3, 196.
+
+  Planetary hypothesis, 253, 267.
+
+  Planetary nebulae, 252.
+
+  Planets, and sunspots, 243;
+    effect of star on, 255;
+    sunspot cycle and, 62;
+    temperatures, 8 f.
+
+  Plants, climate and, 1 f.;
+    effect of light, 184 ff.
+
+  Pleion, defined, 29.
+
+  Pleionian migrations, 29 f.
+
+  Pleistocene, climate, 266;
+    duration of, 48;
+    glaciation, 110 ff.;
+    ice sheets, *123.
+
+  Pluvial climate, causes of, 143;
+    during glacial periods, 141.
+
+  Po, frozen, 98.
+
+  Polaris, 272.
+
+  Polar wandering, hypothesis of, 48 f.
+
+  Pole and earthquakes, 305.
+
+  Post-glacial crustal movements and climatic changes, 215-222.
+
+  Poynting, J. H., cited, 8.
+
+  Precessional hypothesis, 34 f.
+
+  Precipitation, and glaciation, 114, 133;
+    during glacial period, 118;
+    snow line and, 139;
+    temperature and, 94.
+
+  Procyon, companion of, 280;
+    luminosity, 278;
+    speed of, 281.
+
+  Progressive change, 241.
+
+  Progressive desiccation, hypothesis of, 65 ff.
+
+  Proterozoic, 4 f.;
+    fossils, 6 f.;
+    glaciation, 18, 144, 226, 266;
+    lava, 211;
+    mountains in, 209;
+    oceanic salinity, 42 f.;
+    oxygen in air, 234;
+    red beds, 151;
+    temperature, 146 f.
+
+  Pulsations, hypothesis of, 65, 72 ff.
+
+  Pulsatory climatic changes, 72 ff.
+
+  Pulsatory hypothesis, 272.
+
+  Pumpelly, R., cited, 271.
+
+
+  Radiation, variation of, 275.
+
+  Radioactivity, heat of sun and, 14 f.
+
+  Rainfall, changes in, 93 f.;
+    glaciation and, 50;
+    sunspots and, 53, *58, 59;
+    tree growth and, 79.
+
+  Red beds, 151, 170.
+
+  Rhine, flood, 99;
+    frozen, 98.
+
+  Rho Ophiuchi, variables, 248.
+
+  "Rice grains," 61.
+
+  Richardson, O. W., cited, 256.
+
+  Rigidity, of earth, 307.
+
+  Roads, climate and, 66.
+
+  Rogers, Thorwald, cited, 101.
+
+  Romans, aqueduct of, 71.
+
+  Rome, history of, 87.
+
+  Rotation, of earth, 18 f.
+
+  Ruden, storm-flood, 99.
+
+  Rugen, storm-flood, 99.
+
+  Ruins, as climatic evidence, 66;
+    rainfall and, 60.
+
+
+  Sacramento, correlation coefficients, 82 f., 85;
+    rainfall, 86;
+    rainfall record, 79.
+
+  Sagas, cited, 105 f.
+
+  St. John, C. E., cited, 236.
+
+  Salinity, deep-sea circulation and, 176;
+    effect on climate, 224;
+    in North Atlantic, 228;
+    ocean temperature and, 226;
+    of ocean, 19, 120.
+
+  Salisbury, R. D., cited, 111, 125, 129, 139, 156, 206, 269, 271.
+
+  Salt, in ocean, 223.
+
+  San Bernardino, correlation of rainfall, 85.
+
+  Saturn, and sunspots, 243;
+    sunspot cycle and, 62.
+
+  Sayles, R. W., cited, 183.
+
+  Scandinavia, climatic stress, 100 f.;
+    fossils, 271;
+    post-glacial climate, 271;
+    rainfall, 107;
+    storminess, 57, 107;
+    temperature, 216.
+
+  Scandinavian center of glaciation, 113.
+
+  Schlesinger, F., cited, 275, 278, 298, 301, 305.
+
+  Schuchert, C., cited, 3, 5, 23, *25, *123, 138, *145, 168, 169,
+     172, 188, 193, 196, 198, 200, *201, 206, 211, 230, 265.
+
+  Schuster, A., cited, 61, 244, 294, 296.
+
+  Sculpture, Maya, 96.
+
+  Sea level and glaciation, 119.
+
+  Seasonal alternations, 28 f.
+
+  Seasonal banding, 183 f.
+
+  Seasonal changes, geological, 183.
+
+  Seasons, and earthquakes, 294, 295, 297, 299;
+    evidences of, 169.
+
+  Secular progression, 17 ff., 188.
+
+  Seistan, swamps, 171.
+
+  Sequoias, measurements of, 74 ff.;
+    rainfall record, 79.
+
+  Setchell, W. A., cited, 1.
+
+  Shackleton, E., cited, 125.
+
+  Shapley, H., cited, 246, 247, 254, 256, 275.
+
+  Shimek, E., cited, 157, 161.
+
+  Shreveport, La., rainfall and temperature, 93 f.
+
+  Shrinkage of the earth, 190.
+
+  Siberia, and glaciation, 132.
+
+  Sierras, rainfall records, 82.
+
+  Simpson, G. C., cited, 222.
+
+  Sirius, companion of, 280;
+    distance from sun, 262;
+    luminosity, 278;
+    speed of, 281.
+
+  Slichter, C. S., cited, 192.
+
+  Smith, J. W., cited, 73.
+
+  Snowfall, glaciation and, 50, 114.
+
+  Snowfield, climatic effects of, 115.
+
+  Snow line, height of, 138;
+    in Andes, 139;
+    in Himalayas, 139.
+
+  Solar activity, cycles of, 245;
+    deep-sea circulation and, 179;
+    ice and, 134.
+
+  Solar constant, 114.
+
+  Solar-cyclonic hypothesis, 51-63, 287;
+    glaciation and, 110-129.
+
+  Solar prominences, cause of, 61.
+
+  Solar system, 252;
+    conservation of, 243;
+    proximity to stars, 63.
+
+  Solar variations, storms and, 31.
+
+  South America, earthquakes, 301.
+
+  South Pole, thickness of ice at, 125.
+
+  Southern hemisphere, earthquakes, 296;
+    glaciation in, 131 f.
+
+  Southern Pacific railroad, rainfall records along, 82.
+
+  Soy beans, effect of light, 185 f.
+
+  Space, sun's journey through, 264-284.
+
+  Spiral nebulae, 251 f.;
+    universe of, 267.
+
+  Spitzbergen, submergence, 219.
+
+  Springs, climate and, 66.
+
+  Stars, approach to sun, 253;
+    binary, 252;
+    clusters, 252, 268;
+    effect on solar atmosphere, 63;
+    dark, 254;
+    parallaxes of, 276 f.;
+    tidal action of, 249.
+
+  Stefan's Law, 257.
+
+  Stein, M. A., cited, 78.
+
+  Stellar approaches, probability of, 260.
+
+  Storm belt in arid regions, 144.
+
+  Storm-floods, in fourteenth century, 99.
+
+  Storminess, and erosion, 309;
+    and ice, 134;
+    effect on glaciation, 112;
+    sunspots and, 163;
+    temperature and, 94, 173.
+
+  Storms, blows of, 300, 302;
+    increase, 60;
+    movement of, 125 f.;
+    movement of water and, *175;
+    origin of, 30 f.;
+    sunspots and, 28, 53;
+    _see_ Cyclones and Lows.
+
+  Storm tracks, during glacial period, 117;
+    location, 113;
+    relation to magnetic poles, 150;
+    shifting of, 119.
+
+  Strands, climate and, 66;
+    in semi-arid regions, 60;
+    of salt lakes, 142.
+
+  Suess, E., cited, 192.
+
+  Sun, and the earth's crust, 285-317;
+    approach to star, 253;
+    atmosphere of, 61, 274;
+    atmosphere of, and weather, 52;
+    cooling of, 49;
+    contraction of, 249;
+    disturbances of, 172;
+    effect of other bodies on, 242-263;
+    heat, 13;
+    journey through space, 264-284;
+    Knowlton's hypothesis of, 168.
+
+  Suncracks, 232.
+
+  Sunspot cycles, 27 f.
+
+  Sunspots, and earthquakes, 289;
+    causes of, 61;
+    magnetic field of, 261;
+    maximum of, 109;
+    mild climates and, 172;
+    number, 108 f.;
+    periodicity, 243;
+    planetary hypothesis of, 253;
+    records, 245;
+    storminess and, 163;
+    storms and, 300;
+    temperature of earth and, 52, 173.
+
+  Sunspot variations, 282.
+
+  Swamps, as desert phenomena, 171.
+
+  Sylt, storm-flood, 99.
+
+  Syria, civilization in, 67;
+    inscriptions in, 76;
+    Roman aqueducts in, 71.
+
+  Syrian Desert, ruins in, 66.
+
+
+  Talbert, cited, 213.
+
+  Tarim Basin, red beds, 151.
+
+  Tarim Desert, desiccation, 66.
+
+  Tarim River, swamps, 171.
+
+  Taylor, G., cited, 140, 144, 191, 271.
+
+  Temperature, change of in Atlantic, 174;
+    changes in, 93;
+    climatic change and, 49;
+    critical, 9;
+    geological time and, 3;
+    glacial period, 38;
+    glaciation and, 42, 132, 139;
+    gradient of earth, 213;
+    of ocean, 180;
+    in Norway, 177;
+    in Permian, 146 f.;
+    in Proterozoic, 146 f.;
+    limits, 6 ff.;
+    precipitation and, 94;
+    range of, 3, 8;
+    solar activity and, 140;
+    storminess and, 94, 112, 173;
+    sunspots and, 28, 173;
+    volcanic eruptions and, 46;
+    zones, 172.
+
+  Terrestial causes of climatic changes, 188-214.
+
+  Tertiary, lava, 211.
+
+  Thames, frozen, 98.
+
+  Thermal solar hypothesis, 49 f., 97.
+
+  Thermo-pleion, movements of, 30.
+
+  Thesis, of pulsations, 24.
+
+  Thiryu, storm-flood, 99.
+
+  Tian-Shan Mountains, irrigation in, 71.
+
+  Tidal action of stars, 249.
+
+  Tidal effect, of Jupiter, 253;
+    of planets, 244.
+
+  Tidal hypothesis, 251.
+
+  Tidal retardation, effect on land and sea, 191;
+    rotation of earth and, 18 f.;
+    stress caused by, 310.
+
+  Tides, cycles of, 219.
+
+  Time, geological, _see_ Geological time.
+
+  Toads, distribution of, 202.
+
+  Tobacco plant, effect of light, 184.
+
+  Topography, and glaciation, 132.
+
+  Transcaspian Basin, red beds, 151.
+
+  Tree ferns, in New Zealand, 179.
+
+  Tree growth, periodicity in, 245;
+    rainfall and, 79.
+
+  Trees, in California, 219;
+    measurement of, 73 ff.
+
+  Triassic, climate, 266.
+
+  Trifid Nebula, variables, 248.
+
+  Trondheim, wheat in, 101.
+
+  Trondhenaes, corn in, 101.
+
+  Tropical cyclones, in glacial epochs, 140 f.;
+    occurrence, 148;
+    solar activity and, 113.
+
+  Tropical hurricanes, earthquakes and, 300;
+    sunspots and, 149.
+
+  Turfan, temperature, 17.
+
+  Turner, H. H., cited, 245.
+
+  Tyler, J. M., cited, 216.
+
+  Tyndall, J., cited, 36, 37.
+
+  Typhoon region, "earthquake weather," 298.
+
+  Typhoons, occurrence, 300.
+
+
+  United States, rainfall and temperature in Gulf region, 93 f.;
+    salt lakes in, 142;
+    southwestern, climate, 66;
+    storminess, 53 f., 60.
+
+
+  Variables, 247.
+
+  Veeder, M. A., cited, 300.
+
+  Vegetation, theory of pulsations and, 90.
+
+  Venus, atmosphere of, 236.
+
+  Vesterbygd, invasion of, 106.
+
+  Vicksburg, Miss., rainfall and temperature, 93 f.
+
+  Volcanic activity, climate and, 210;
+    movement of the earth's crust and, 285;
+    times of uplifting lands and, 23.
+
+  Volcanic dust, climatic changes and, 97.
+
+  Volcanic hypothesis, climatic change and, 45 ff.;
+    snow line, 139.
+
+  Volcanoes, activity of, 96.
+
+  Volga, 108.
+
+
+  Walcott, C. D., cited, 4, 230.
+
+  Wandering of the pole, 302.
+
+  Water, importance, 9.
+
+  Water vapor, condensation of, 56;
+    effect on life, 231;
+    in atmosphere, 19.
+
+  Wave, effect on movement of water, 176.
+
+  Weather, changes of, 31 f.;
+    origin of, 174;
+    variations, 52.
+
+  Wells, H. G., cited, 35.
+
+  Wendingstadt, storm-flood, 99.
+
+  Westerlies, 21 f.
+
+  Wheat, price in England, 102.
+
+  White Sea, submergence, 219.
+
+  Whitney, J. D., cited, 142.
+
+  Wieland, G. R., cited, 169.
+
+  Williamson, E. D., cited, 226.
+
+  Willis, B., cited, 206.
+
+  Winds, at ice front, 162;
+    effect on currents, 174;
+    glaciation and, 133;
+    in Antarctica, 161;
+    in glacial period, 119;
+    in Greenland, 161;
+    planetary system of, 174;
+    velocity, 240.
+
+  Witch hazel, effect of light, 184.
+
+  Wolf, J. R., cited, 61, 109, 288.
+
+  Wolfer, cited, 244.
+
+  Wright, W. B., cited, 35, 111, 119.
+
+  Writing, among Mayas, 96.
+
+
+  Yucatan, Maya civilization, 26, 107;
+    rainfall, 108;
+    ruins, 95.
+
+  Yukon, Ice Age in, 221.
+
+
+  Zante, earthquakes of, 296.
+
+  Zonal crowding, 117.
+
+
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