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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: 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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