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diff --git a/18562.txt b/18562.txt new file mode 100644 index 0000000..d1a55e5 --- /dev/null +++ b/18562.txt @@ -0,0 +1,14656 @@ +The Project Gutenberg EBook of Outlines of the Earth's History, by +Nathaniel Southgate Shaler + +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: Outlines of the Earth's History + A Popular Study in Physiography + +Author: Nathaniel Southgate Shaler + +Release Date: June 12, 2006 [EBook #18562] + +Language: English + +Character set encoding: ASCII + +*** START OF THIS PROJECT GUTENBERG EBOOK OUTLINES OF THE EARTH'S HISTORY *** + + + + +Produced by Brendan Lane, Riikka Talonpoika, Jeroen van +Luin and the Online Distributed Proofreading Team at +http://www.pgdp.net + + + + + +[Illustration: _Dunes at Ipswich Light, Massachusetts. Note the +effect of bushes in arresting the movement of the wind-blown sand._] + + + + + OUTLINES OF THE + EARTH'S HISTORY + + + A POPULAR STUDY + IN PHYSIOGRAPHY + + BY + + NATHANIEL SOUTHGATE SHALER + + PROFESSOR OF GEOLOGY IN HARVARD UNIVERSITY + DEAN OF LAWRENCE SCIENTIFIC SCHOOL + + ILLUSTRATED WITH INDEX + + NEW YORK AND LONDON + D. APPLETON AND COMPANY + + 1898, 1910 + + + + + + PREFACE. + + +The object of this book is to provide the beginner in the study of the +earth's history with a general account of those actions which can be +readily understood and which will afford him clear understandings as +to the nature of the processes which have made this and other +celestial spheres. It has been the writer's purpose to select those +series of facts which serve to show the continuous operations of +energy, so that the reader might be helped to a truer conception of +the nature of this sphere than he can obtain from ordinary text-books. + +In the usual method of presenting the elements of the earth's history +the facts are set forth in a manner which leads the student to +conceive that history as in a way completed. The natural prepossession +to the effect that the visible universe represents something done, +rather than something endlessly doing, is thus re-enforced, with the +result that one may fail to gain the largest and most educative +impression which physical science can afford him in the sense of the +swift and unending procession of events. + +It is well known to all who are acquainted with the history of geology +that the static conception of the earth--the idea that its existing +condition is the finished product of forces no longer in action--led +to prejudices which have long retarded, and indeed still retard, the +progress of that science. This fact indicates that at the outset of a +student's work in this field he should be guarded against such +misconceptions. The only way to attain the end is by bringing to the +understanding of the beginner a clear idea of successions of events +which are caused by the forces operating in and on this sphere. Of all +the chapters of this great story, that which relates to the history of +the work done by the heat of the sun is the most interesting and +awakening. Therefore an effort has been made to present the great +successive steps by which the solar energy acts in the processes of +the air and the waters. + +The interest of the beginner in geology is sure to be aroused when he +comes to see how very far the history of the earth has influenced the +fate of men. Therefore the aim has been, where possible, to show the +ways in which geological processes and results are related to +ourselves; how, in a word, this earth has been the well-appointed +nursery of our kind. + +All those who are engaged in teaching elementary science learn the +need of limiting the story they have to tell to those truths which can +be easily understood by beginners. It is sometimes best, as in stating +such difficult matters as those concerning the tides, to give +explanations which are far from complete, and which, as to their mode +of presentation, would be open to criticism were it not for the fact +that any more elaborate statements would most likely be +incomprehensible to the novice, thus defeating the teacher's aim. + +It will be observed that no account is here given of the geological +ages or of the successions of organic life. Chapters on these subjects +were prepared, but were omitted for the reason that they made the +story too long, and also because they carried the reader into a field +of much greater difficulty than that which is found in the physical +history of the earth. + + N.S.S. +_March, 1898._ + + + + + CONTENTS. + + + CHAPTER PAGE + + I.--INTRODUCTION TO THE STUDY OF NATURE 1 + II.--WAYS AND MEANS OF STUDYING NATURE 9 + III.--THE STELLAR REALM 31 + IV.--THE EARTH 81 + V.--THE ATMOSPHERE 97 + VI.--GLACIERS 207 + VII.--THE WORK OF UNDERGROUND WATER 250 + VIII.--THE SOIL 313 + IX.--THE ROCKS AND THEIR ORDER 349 + + + + + LIST OF FULL-PAGE ILLUSTRATIONS. + + FACING PAGE + + Dunes at Ipswich Light, Massachusetts _Frontispiece_ + Seal Rocks near San Francisco, California 33 + Lava stream, in Hawaiian Islands, flowing into the sea 72 + Waterfall near Gadsden, Alabama 90 + South shore, Martha's Vineyard, Massachusetts 121 + Pocket Creek, Cape Ann, Massachusetts 163 + Muir Glacier, Alaska 207 + Front of Muir Glacier 240 + Mount AEtna, seen from near Catania 201 + Mountain gorge, Himalayas, India 330 + + + + + + OUTLINES OF THE EARTH'S HISTORY. + + + CHAPTER I. + + AN INTRODUCTION TO THE STUDY OF NATURE. + + +The object of this book is to give the student who is about to enter +on the study of natural science some general idea as to the conditions +of the natural realm. As this field of inquiry is vast, it will be +possible only to give the merest outline of its subject-matter, noting +those features alone which are of surpassing interest, which are +demanded for a large understanding of man's place in this world, or +which pertain to his duties in life. + +In entering on any field of inquiry, it is most desirable that the +student should obtain some idea as to the ways in which men have been +led to the knowledge which they possess concerning the world about +them. Therefore it will be well briefly to sketch the steps by which +natural science has come to be what it is. By so doing we shall +perceive how much we owe to the students of other generations; and by +noting the difficulties which they encountered, and how they avoided +them, we shall more easily find our own way to knowledge. + +The primitive savages, who were the ancestors of all men, however +civilized they may be, were students of Nature. The remnants of these +lowly people who were left in different parts of the world show us +that man was not long in existence before he began to devise some +explanation concerning the course of events in the outer world. +Seeing the sun rise and set, the changes of the moon, the alternation +of the seasons, the incessant movement of the streams and sea, and the +other more or less orderly successions of events, our primitive +forefathers were driven to invent some explanation of them. This, +independently, and in many different times and places, they did in a +simple and natural way by supposing that the world was controlled by a +host of intelligent beings, each of which had some part in ordering +material things. Sometimes these invisible powers were believed to be +the spirits of great chieftains, who were active when on earth, and +who after death continued to exercise their power in the larger realms +of Nature. Again, and perhaps more commonly, these movements of Nature +were supposed to be due to the action of great though invisible +beasts, much like those which the savage found about him. Thus among +our North American Indians the winds are explained by the supposition +that the air is fanned by the wings of a great unseen bird, whose duty +it is to set the atmosphere into motion. That no one has ever seen the +bird doing the work, or that the task is too great for any conceivable +bird, is to the simple, uncultivated man no objection to this view. It +is long, indeed, before education brings men to the point where they +can criticise their first explanations of Nature. + +As men in their advance come to see how much nobler are their own +natures than those of the lower animals, they gradually put aside the +explanation of events by the actions of beasts, and account for the +order of the world by the supposition that each and every important +detail is controlled by some immortal creature essentially like a man, +though much more powerful than those of their own kind. This stage of +understanding is perhaps best shown by the mythology of the Greeks, +where there was a great god over all, very powerful but not +omnipotent; and beneath him, in endless successions of command, +subordinate powers, each with a less range of duties and capacities +than those of higher estate, until at the bottom of the system there +were minor deities and demigods charged with the management of the +trees, the flowers, and the springs--creatures differing little from +man, except that they were immortal, and generally invisible, though +they, like all the other deities, might at their will display +themselves to the human beings over whom they watched, and whose path +in life they guided. + +Among only one people do we find that the process of advance led +beyond this early and simple method of accounting for the processes of +Nature, bringing men to an understanding such as we now possess. This +great task was accomplished by the Greeks alone. About twenty-five +hundred years ago the philosophers of Greece began to perceive that +the early notion as to the guidance of the world by creatures +essentially like men could not be accepted, and must be replaced by +some other view which would more effectively account for the facts. +This end they attained by steps which can not well be related here, +but which led them to suppose separate powers behind each of the +natural series--powers having no relation to the qualities of mankind, +but ever acting to a definite end. Thus Plato, who represents most +clearly this advance in the interpretation of facts, imagined that +each particular kind of plant or animal had its shape inevitably +determined by something which he termed an idea, a shape-giving power +which existed before the object was created, and which would remain +after it had been destroyed, ever ready again to bring matter to the +particular form. From this stage of understanding it was but a short +step to the modern view of natural law. This last important advance +was made by the great philosopher Aristotle, who, though he died about +twenty-two hundred years ago, deserves to be accounted the first and +in many ways the greatest of the ancient men of science who were +informed with the modern spirit. + +With Aristotle, as with all his intellectual successors, the +operations of Nature were conceived as to be accounted for by the +action of forces which we commonly designate as natural laws, of which +perhaps the most familiar and universal is that of gravitation, which +impels all bodies to move toward each other with a degree of intensity +which is measured by their weight and the distance by which they are +separated. + +For many centuries students used the term law in somewhat the same way +as the more philosophical believers in polytheism spoke of their gods, +or as Plato of the ideas which he conceived to control Nature. We see +by this instance how hard it is to get rid of old ways of thinking. +Even when the new have been adopted we very often find that something +of the ancient and discarded notions cling in our phrases. The more +advanced of our modern philosophers are clear in their mind that all +we know as to the order of Nature is that, given certain conditions, +certain consequences inevitably follow. + +Although the limitations which modern men of science perceive to be +put upon their labours may seem at first sight calculated to confine +our understanding within a narrow field of things which can be seen, +or in some way distinctly proved to exist, the effect of this +limitation has been to make science what it is--a realm of things +known as distinct from things which may be imagined. All the +difference between ancient science and modern consists in the fact +that in modern science inquirers demand a businesslike method in the +interpretation of Nature. Among the Greeks the philosopher who taught +explanations of any feature in the material world which interested him +was content if he could imagine some way which would account for the +facts. It is the modern custom now to term the supposition of an +explanation a _working hypothesis_, and only to give it the name of +theory after a very careful search has shown that all the facts which +can be gathered are in accordance with the view. Thus when Newton made +his great suggestion concerning the law of gravitation, which was to +the effect that all bodies attracted each other in proportion to +their masses, and inversely as the square of their distance from each +other, he did not rest content, as the old Greeks would have done, +with the probable truth of the explanation, but carefully explored the +movements of the planets and satellites of the solar system to see if +the facts accorded with the hypothesis. Even the perfect +correspondence which he found did not entirely content inquirers, and +in this century very important experiments have been made which have +served to show that a ball suspended in front of a precipice will be +attracted toward the steep, and that even a mass of lead some tons in +weight will attract toward itself a small body suspended in the manner +of a pendulum. + +It is this incessant revision of the facts, in order to see if they +accord with the assumed rule or law, which has given modern science +the sound footing that it lacked in earlier days, and which has +permitted our learning to go on step by step in a safe way up the +heights to which it has climbed. All explanations of Nature begin with +the work of the imagination. In common phrase, they all are guesses +which have at first but little value, and only attain importance in +proportion as they are verified by long-continued criticism, which has +for its object to see whether the facts accord with the theory. It is +in this effort to secure proof that modern science has gathered the +enormous store of well-ascertained facts which constitutes its true +wealth, and which distinguishes it from the earlier imaginative and to +a great extent unproved views. + +In the original state of learning, natural science was confounded with +political and social tradition, with the precepts of duty which +constitute the law of the people, as well as with their religion, the +whole being in the possession of the priests or wise men. So long as +natural action was supposed to be in the immediate control of numerous +gods and demigods, so long, in a word, as the explanation of Nature +was what we term polytheistic, this association of science with other +forms of learning was not only natural but inevitable. Gradually, +however, as the conception of natural law replaced the earlier idea as +to the intervention of a spirit, science departed from other forms of +lore and came to possess a field to itself. At first it was one body +of learning. The naturalists of Aristotle's time, and from his day +down to near our own, generally concerned themselves with the whole +field of Nature. For a time it was possible for any one able and +laborious man to know all which had been ascertained concerning +astronomy, chemistry, geology, as well as the facts relating to living +beings. The more, however, as observation accumulated, and the store +of facts increased, it became difficult for any one man to know the +whole. Hence it has come about that in our own time natural learning +is divided into many distinct provinces, each of which demands a +lifetime of labour from those who would know what has already been +done in the field, and what it is now important to do in the way of +new inquiries. + +The large divisions which naturalists have usually made of their tasks +rest in the main on the natural partitions which we may readily +observe in the phenomenal world. First of all comes astronomy, +including the phenomena exhibited in the heavens, beyond the limits of +the earth's atmosphere. Second, geology, which takes account of all +those actions which in process of time have been developed in our own +sphere. Third, physics, which is concerned with the laws of energy, or +those conditions which affect the motion of bodies, and the changes +which are impressed upon them by the different natural forces. Fourth, +chemistry, which seeks to interpret the principles which determine the +combination of atoms and the molecules which are built of them under +the influence of the chemical affinities. Fifth, biology, or the laws +of life, a study which pertains to the forms and structures of animals +and plants, and their wonderful successions in the history of the +world. Sixth, mathematics, or the science of space and number, that +deals with the principles which underlie the order of Nature as +expressed at once in the human understanding and in the material +universe. By its use men were made able to calculate, as in +arithmetic, the problems which concern their ordinary business, as +well as to compute the movements of the celestial bodies, and a host +of actions which take place on the earth that would be inexplicable +except by the aid of this science. Last of all among the primary +sciences we may name that of psychology, which takes account of mental +operations among man and his lower kindred, the animals. + +In addition to the seven sciences above mentioned, which rest in a +great measure on the natural divisions of phenomena, there are many, +indeed, indefinitely numerous, subdivisions which have been made to +suit the convenience of students. Thus astronomy is often separated +into physical and mathematical divisions, which take account either of +the physical phenomena exhibited by the heavenly bodies or of their +motions. In geology there are half a dozen divisions relating to +particular branches of that subject. In the realm of organic life, in +chemistry, and in physics there are many parts of these sciences which +have received particular names. + +It must not be supposed that these sciences have the independence of +each other which their separate names would imply. In fact, the +student of each, however, far he may succeed in separating his field +from that of the other naturalists, as we may fitly term all students +of Nature, is compelled from time to time to call in the aid of his +brethren who cultivate other branches of learning. The modern +astronomer needs to know much of chemistry, or else he can not +understand many of his observations on the sun. The geologists have to +share their work with the student of animal and vegetable life, with +the physicists; they must, moreover, know something of the celestial +spheres in order to interpret the history of the earth. In fact, day +by day, with the advance of learning, we come more clearly to +perceive that all the processes of Nature are in a way related to each +other, and that in proportion as we understand any part of the great +mechanism, we are forced in a manner to comprehend the whole. In other +words, we are coming to understand that these divisions of the field +of science depend upon the limitations of our knowledge, and not upon +the order of Nature itself. For the purposes of education it is +important that every one should know something of the great truths +which each science has disclosed. No mortal man can compass the whole +realm of this knowledge, but every one can gain some idea of the +larger truths which may help him to understand the beauty and grandeur +of the sphere in which he dwells, which will enable him the better to +meet the ordinary duties of life, that in almost all cases are related +to the facts of the world about us. It has been of late the custom to +term this body of general knowledge which takes account of the more +evident facts and important series of terrestrial actions +physiography, or, as the term implies, a description of Nature, with +the understanding that the knowledge chosen for the account is that +which most intimately concerns the student who seeks information that +is at once general and important. Therefore, in this book the effort +is made first to give an account as to the ways and means which have +led to our understanding of scientific problems, the methods by which +each person may make himself an inquirer, and the outline of the +knowledge that has been gathered since men first began to observe and +criticise the revelations the universe may afford them. + + + + + CHAPTER II. + + WAYS AND MEANS OF STUDYING NATURE. + + +It is desirable that the student of Nature keep well in mind the means +whereby he is able to perceive what goes on in the world about him. He +should understand something as to the nature of his senses, and the +extent to which these capacities enable him to discern the operations +of Nature. Man, in common with his lower kindred, is, by the mechanism +of the body, provided with five somewhat different ways by which he +may learn something of the things about him. The simplest of these +capacities is that of touch, a faculty that is common to the general +surface of the body, and which informs us when the surface is affected +by contact with some external object. It also enables us to discern +differences of temperature. Next is the sense of taste, which is +limited to the mouth and the parts about it. This sense is in a way +related to that of touch, for the reason that it depends on the +contact of our body with material things. Third is the sense of smell, +so closely related to that of taste that it is difficult to draw the +line between the two. Yet through the apparatus of the nose we can +perceive the microscopically small parts of matter borne to us through +the air, which could not be appreciated by the nerves of the mouth. +Fourth in order of scope comes the hearing, which gives us an account +of those waves of matter that we understand as sound. This power is +much more far ranging than those before noted; in some cases, as in +that of the volcanic explosions from the island of Krakatoa, in the +eruption of 1883, the convulsions were audible at the distance of +more than a thousand miles away. The greater cannon of modern days may +be heard at the distance of more than a hundred miles, so that while +the sense of touch, taste, and smell demand contact with the bodies +which we appreciate, hearing gives us information concerning objects +at a considerable distance. Last and highest of the senses, vastly the +most important in all that relates to our understanding of Nature, is +sight, or the capacity which enables us to appreciate the movement of +those very small waves of ether which constitute light. The eminent +peculiarity of sight is that it may give us information concerning +things which are inconceivably far away; it enables us to discern the +light of suns probably millions of times as remote from us as is the +centre of our own solar system. + +Although much of the pleasure which the world affords us comes through +the other senses, the basis of almost all our accurate knowledge is +reported by sight. It is true that what we have observed with our eyes +may be set forth in words, and thus find its way to the understanding +through the ears; also that in many instances the sense of touch +conveys information which extends our perceptions in many important +ways; but science rests practically on sight, and on the insight that +comes from the training of the mind which the eyes make possible. + +The early inquirers had no resources except those their bodies +afforded; but man is a tool-making creature, and in very early days he +began to invent instruments which helped him in inquiry. The earliest +deliberate study was of the stars. Science began with astronomy, and +the first instruments which men contrived for the purpose of +investigation were astronomical. In the beginning of this search the +stars were studied in order to measure the length of the year, and +also for the reason that they were supposed in some way to control the +fate of men. So far as we know, the first pieces of apparatus for this +purpose were invented in Egypt, perhaps about four thousand years +before the Christian era. These instruments were of a simple nature, +for the magnifying glass was not yet contrived, and so the telescope +was impossible. They consisted of arrangements of straight edges and +divided circles, so that the observers, by sighting along the +instruments, could in a rough way determine the changes in distance +between certain stars, or the height of the sun above the horizon at +the various seasons of the year. It is likely that each of the great +pyramids of Egypt was at first used as an observatory, where the +priests, who had some knowledge of astronomy, found a station for the +apparatus by which they made the observations that served as a basis +for casting the horoscope of the king. + +In the progress of science and of the mechanical invention attending +its growth, a great number of inventions have been contrived which +vastly increase our vision and add inconceivably to the precision it +may attain. In fact, something like as much skill and labour has been +given to the development of those inventions which add to our learning +as to those which serve an immediate economic end. By far the greatest +of these scientific inventions are those which depend upon the lens. +By combining shaped bits of glass so as to control the direction in +which the light waves move through them, naturalists have been able to +create the telescope, which in effect may bring distant objects some +thousand times nearer to view than they are to the naked eye; and the +microscope, which so enlarges minute objects as to make them visible, +as they were not before. The result has been enormously to increase +our power of vision when applied to distant or to small objects. In +fact, for purposes of learning, it is safe to say that those tools +have altogether changed man's relation to the visible universe. The +naked eye can see at best in the part of the heavens visible from any +one point not more than thirty thousand stars. With the telescope +somewhere near a hundred million are brought within the limits of +vision. Without the help of the microscope an object a thousandth of +an inch in diameter appears as a mere point, the existence of which we +can determine only under favourable circumstances. With that +instrument the object may reveal an extended and complicated structure +which it may require a vast labour for the observer fully to explore. + +Next in importance to the aid of vision above noted come the +scientific tools which are used in weighing and measuring. These +balances and gauges have attained such precision that intervals so +small as to be quite invisible, and weights as slight as a +ten-thousandth of a grain, can be accurately measured. From these +instruments have come all those precise examinations on which the +accuracy of modern science intimately depends. All these instruments +of precision are the inventions of modern days. The simplest +telescopes were made only about two hundred and fifty years ago, and +the earlier compound microscopes at a yet later date. Accurate +balances and other forms of gauges of space, as well as good means of +dividing time, such as our accurate astronomical clocks and +chronometers, are only about a century old. The instruments have made +science accurate, and have immensely extended its powers in nearly all +the fields of inquiry. + +Although the most striking modern discoveries are in the field which +was opened to us by the lens in its manifold applications, it is in +the chemist's laboratory that we find that branch of science, long +cultivated, but rapidly advanced only within the last two centuries, +which has done the most for the needs of man. The ancients guessed +that the substances which make up the visible world were more +complicated in their organization than they appear to our vision. They +even suggested the great truth that matter of all kinds is made up of +inconceivably small indivisible bits which they and we term atoms. It +is likely that in the classic days of Greece men began to make simple +experiments of a chemical nature. A century or two after the time of +Mohammed, the Arabians of his faith, a people who had acquired Greek +science from the libraries which their conquests gave them, conducted +extensive experiments, and named a good many familiar chemical +products, such as alcohol, which still bears its Arabic name. + +These chemical studies were continued in Europe by the alchemists, a +name also of Arabic origin, a set of inquirers who were to a great +extent drawn away from scientific studies by vain though unending +efforts to change the baser metals into gold and silver, as well as to +find a compound which would make men immortal in the body. By the +invention of the accurate balance, and by patient weighing of the +matters which they submitted to experiment, by the invention of +hypotheses or guesses at truth, which were carefully tested by +experiment, the majestic science of modern chemistry has come forth +from the confused and mystical studies of the alchemists. We have +learned to know that there are seventy or more primitive or apparently +unchangeable elements which make up the mass of this world, and +probably constitute all the celestial spheres, and that these elements +in the form of their separate atoms may group themselves in almost +inconceivably varied combinations. In the inanimate realm these +associations, composed of the atoms of the different substances, +forming what are termed molecules, are generally composed of but few +units. Thus carbonic-acid gas, as it is commonly called, is made up of +an aggregation of molecules, each composed of one atom of carbon and +two of oxygen; water, of two atoms of hydrogen and one of oxygen; +ordinary iron oxide, of two atoms of iron and three of oxygen. In the +realm of organic life, however, these combinations become vastly more +complicated, and with each of them the properties of the substance +thus produced differ from all others. A distinguished chemist has +estimated that in one group of chemical compounds, that of carbon, it +would be possible to make such an array of substances that it would +require a library of many thousand ordinary volumes to contain their +names alone. + +It is characteristic of chemical science that it takes account of +actions which are almost entirely invisible. No contrivances have been +or are likely to be invented which will show the observer what takes +place when the atoms of any substance depart from their previous +combination and enter on new arrangements. We only know that under +certain conditions the old atomic associations break up, and new ones +are formed. But though the processes are hidden, the results are +manifest in the changes which are brought about upon the masses of +material which are subjected to the altering conditions. Gradually the +chemists of our day are learning to build up in their laboratories +more and more complicated compounds; already they have succeeded in +producing many of the materials which of old could only be obtained by +extracting them from plants. Thus a number of the perfumes of flowers, +and many of the dye-stuffs which a century ago were extracted from +vegetables, and were then supposed to be only obtainable in that way, +are now readily manufactured. In time it seems likely that important +articles of food, for which we now depend upon the seeds of plants, +may be directly built up from the mineral kingdom. Thus the result of +chemical inquiry has been not only to show us much of the vast realm +of actions which go on in the earth, but to give us control of many of +these movements so that we may turn them to the needs of man. + +Animals and plants were at an early day very naturally the subjects of +inquiry. The ancients perceived that there were differences of kind +among these creatures, and even in Aristotle's time the sciences of +zooelogy and botany had attained the point where there were +considerable treatises on those subjects. It was not, however, until a +little more than a century ago that men began accurately to describe +and classify these species of the organic world. Since the time of +Linnaeus the growth of our knowledge has gone forward with amazing +swiftness. Within a century we have come to know perhaps a hundred +times as much concerning these creatures as was learned in all the +earlier ages. This knowledge is divisible into two main branches: in +one the inquirers have taken account of the different species, genera, +families, orders, and classes of living forms with such effect that +they have shown the existence at the present time of many hundred +thousand distinct species, the vast assemblage being arranged in a +classification which shows something as to the relationship which the +forms bear to each other, and furthermore that the kinds now living +have not been long in existence, but that at each stage in the history +of the earth another assemblage of species peopled the waters and the +lands. + +At first naturalists concerned themselves only with the external forms +of living creatures; but they soon came to perceive that the way in +which these organisms worked, their physiology, in a word, afforded +matters for extended inquiry. These researches have developed the +science of physiology, or the laws of bodily action, on many accounts +the most modern and extensive of our new acquisitions of natural +learning. Through these studies we have come to know something of the +laws or principles by which life is handed on from generation to +generation, and by which the gradations of structure have been +advanced from the simple creatures which appear like bits of animated +jelly to the body and mind of man. + +The greatest contribution which modern naturalists have made to +knowledge concerns the origin of organic species. The students of a +century ago believed that all these different kinds had been suddenly +created either through natural law or by the immediate will of God. We +now know that from the beginning of organic life in the remote past to +the present day one kind of animal or plant has been in a natural and +essentially gradual way converted into the species which was to be its +successor, so that all the vast and complicated assemblage of kinds +which now exists has been derived by a process of change from the +forms which in earlier ages dwelt upon this planet. The exact manner +in which these alterations were produced is not yet determined, but in +large part it has evidently been brought about by the method indicated +by Mr. Darwin, through the survival of the fittest individuals in the +struggle for existence. + +Until men came to have a clear conception as to the spherical form of +the earth, it was impossible for them to begin any intelligent +inquiries concerning its structure or history. The Greeks knew the +earth to be a sphere, but this knowledge was lost among the early +Christian people, and it was not until about four hundred years ago +that men again came to see that they dwelt upon a globe. On the basis +of this understanding the science of geology, which had in a way been +founded by the Greeks, was revived. As this science depends upon the +knowledge which we have gained of astronomy, physics, chemistry, and +biology, all of which branches of learning have to be used in +explaining the history of the earth, the advance which has been made +has been relatively slow. Geology as a whole is the least perfectly +organized of all the divisions of learning. A special difficulty +peculiar to this science has also served to hinder its development. +All the other branches of learning deal mainly, if not altogether, +with the conditions of Nature as they now exist. In this alone is it +necessary at every step to take account of actions which have been +performed in the remote past. + +It is an easy matter for the students of to-day to imagine that the +earth has long endured; but to our forefathers, who were educated in +the view that it had been brought from nothingness into existence +about seven thousand years ago, it was most difficult and for a time +impossible to believe in its real antiquity. Endeavouring, as they +naturally did, to account for all the wonderful revolutions, the +history of which is written in the pages of the great stone book, the +early geologists supposed this planet to have been the seat of +frequent and violent changes, each of which revolutionized its shape +and destroyed its living tenants. It was only very gradually that +they became convinced that a hundred million years or more have +elapsed since the dawn of life on the earth, and that in this vast +period the march of events has been steadfast, the changes taking +place at about the same rate in which they are now going on. As yet +this conception as to the history of our sphere has not become the +general property of the people, but the fact of it is recognised by +all those who have attentively studied the matter. It is now as well +ascertained as any of the other truths which science has disclosed to +us. + +It is instructive to note the historic outlines of scientific +development. The most conspicuous truth which this history discloses +is that all science has had its origin and almost all its development +among the peoples belonging to the Aryan race. This body of folk +appears to have taken on its race characteristics, acquired its +original language, its modes of action, and the foundations of its +religion in that part of northern Europe which is about the Baltic +Sea. Thence the body of this people appear to have wandered toward +central Asia, where after ages of pastoral life in the high table +lands and mountains of their country it sent forth branches to India, +Asia Minor and Greece, to Persia, and to western Europe. It seems ever +to have been a characteristic of these Aryan peoples that they had an +extreme love for Nature; moreover, they clearly perceived the need of +accounting for the things that happened in the world about them. In +general they inclined to what is called the pantheistic explanation of +the universe. They believed a supreme God in many different forms to +be embodied in all the things they saw. Even their own minds and +bodies they conceived as manifestations of this supreme power. Among +the Aryans who came to dwell in Europe and along the eastern +Mediterranean this method of explaining Nature was in time changed to +one in which humanlike gods were supposed to control the visible and +invisible worlds. In that marvellous centre of culture which was +developed among the Greeks this conception of humanlike deities was in +time replaced by that of natural law, and in their best days the +Greeks were men of science essentially like those of to-day, except +that they had not learned by experience how important it was to +criticise their theories by patiently comparing them with the facts +which they sought to explain. The last of the important Greek men of +science, Strabo, who was alive when Christ was born, has left us +writings which in quality are essentially like many of the able works +of to-day. But for the interruption in the development of Greek +learning, natural science would probably have been fifteen hundred +years ahead of its present stage. This interruption came in two ways. +In one, through the conquest of Greece and the destruction of its +intellectual life by the Romans, a people who were singularly +incapable of appreciating natural science, and who had no other +interest in it except now and then a vacant and unprofitable curiosity +as to the processes of the natural world. A second destructive +influence came through the fact that Christianity, in its energetic +protest against the sins of the pagan civilization, absolutely +neglected and in a way despised all forms of science. + +The early indifference of Christians to natural learning is partly to +be explained by the fact that their religion was developed among the +Hebrews, a people remarkable for their lack of interest in the +scientific aspects of Nature. To them it was a sufficient explanation +that one omnipotent God ruled all things at his will, the heavens and +the earth alike being held in the hollow of his hand. + +Finding the centre of its development among the Romans, Christianity +came mainly into the control of a people who, as we have before +remarked, had no scientific interest in the natural world. This +condition prolonged the separation of our faith from science for +fifteen hundred years after its beginning. In this time the records of +Greek scientific learning mostly disappeared. The writings of +Aristotle were preserved in part for the reason that the Church +adopted many of his views concerning questions in moral philosophy and +in politics. The rest of Greek learning was, so far as Europe was +concerned, quite neglected. + +A large part of Greek science which has come down to us owes its +preservation to a very singular incident in the history of learning. +In the ninth century, after the Arabs had been converted to +Mohammedanism, and on the basis of that faith had swiftly organized a +great and cultivated empire, the scholars of that folk became deeply +interested in the remnants of Greek learning which had survived in the +monastic and other libraries about the eastern Mediterranean. So +greatly did they prize these records, which were contemned by the +Christians, that it was their frequent custom to weigh the old +manuscripts in payment against the coin of their realm. In astronomy, +mathematics, chemistry, and geology the Arabian students, building on +the ancient foundations, made notable and for a time most important +advances. In the tenth century of our era they seemed fairly in the +way to do for science what western Europe began five centuries later +to accomplish. In the fourteenth century the centre of Mohammedan +strength was transferred from the Arabians to the Turks, from a people +naturally given to learning to a folk of another race, who despised +all such culture. Thenceforth in place of the men who had treasured +and deciphered with infinite pains all the records of earlier +learning, the followers of Mohammed zealously destroyed all the +records of the olden days. Some of these records, however, survived +among the Arabs of Spain, and others were preserved by the Christian +scholars who dwelt in Byzantium, or Constantinople, and were brought +into western Europe when that city was captured by the Turks in the +fifteenth century. + +Already the advance of the fine arts in Italy and the general tendency +toward the study of Nature, such as painting and sculpture indicate, +had made a beginning, or rather a proper field for a beginning, of +scientific inquiry. The result was a new interest in Greek learning in +all its branches, and a very rapid awakening of the scientific spirit. +At first the Roman Church made no opposition to this new interest +which developed among its followers, but in the course of a few years, +animated with the fear that science would lead men to doubt many of +the dogmas of the Church, it undertook sternly to repress the work of +all inquirers. + +The conflict between those of the Roman faith and the men of science +continued for above two hundred years. In general, the part which the +Church took was one of remonstrance, but in a few cases the spirit of +fanaticism led to the persecution of the men who did not obey its +mandates and disavow all belief in the new opinions which were deemed +contrary to the teachings of Scripture. The last instance of such +oppression occurred in France in the year 1756, when the great Buffon +was required to recant certain opinions concerning the antiquity of +the earth which he had published in his work on Natural History. This +he promptly did, and in almost servile language withdrew all the +opinions to which the fathers had objected. A like conflict between +the followers of science and the clerical authorities occurred in +Protestant countries. Although in no case were the men of science +physically tortured or executed for their opinions, they were +nevertheless subjected to great religious and social pressure: they +were almost as effectively disciplined as were those who fell under +the ban of the Roman Church. + +Some historians have criticised the action of the clerical authorities +toward science as if the evil which was done had been performed in our +own day. It should be remembered, however, that in the earlier +centuries the churches regarded themselves as bound to protect all men +from the dangers of heresy. For centuries in the early history of +Christianity the defenders of the faith had been engaged in a +life-and-death struggle with paganism, the followers of which held all +that was known of Nature. Quite naturally the priestly class feared +that the revival of scientific inquiry would bring with it the evils +from which the world had suffered in pagan times. There is no doubt +that these persecutions of science were done under what seemed the +obligations of duty. They may properly be explained particularly by +men of science as one of the symptoms of development in the day in +which they were done. It is well for those who harshly criticise the +relations of the Church to science to remember that in our own +country, about two centuries ago, among the most enlightened and +religious people of the time, Quakers were grievously persecuted, and +witches hanged, all in the most dutiful and God-fearing way. In +considering these relations of science to our faith, the matter should +be dealt with in a philosophical way, and with a sense of the +differences between our own and earlier ages. + +To the student of the relations between Christianity and science it +must appear doubtful whether the criticism or the other consequences +which the men of science had to meet from the Church was harmful to +their work. The early naturalists, like the Greeks whom they followed, +were greatly given to speculations concerning the processes of Nature, +which, though interesting, were unprofitable. They also showed a +curious tendency to mingle their scientific speculations with ancient +and base superstitions. They were often given to the absurdity +commonly known as the "black art," or witchcraft, and held to the +preposterous notions of the astrologists. Even the immortal astronomer +Kepler, who lived in the sixteenth century, was a professional +astrologer, and still held to the notion that the stars determined the +destiny of men. Many other of the famous inquirers in those years +which ushered in modern science believed in witchcraft. Thus for a +time natural learning was in a way associated with ancient and +pernicious beliefs which the Church was seeking to overthrow. One +result of the clerical opposition to the advancement of science was +that its votaries were driven to prove every step which led to their +conclusions. They were forced to abandon the loose speculation of +their intellectual guides, the Greeks, and to betake themselves to +observation. Thus a part of the laborious fact-gathering habit on +which the modern advance of science has absolutely depended was due to +the care which men had to exercise in face of the religious +authorities. + +In our own time, in the latter part of the nineteenth century, the +conflict between the religious authority and the men of science has +practically ceased. Even the Roman Church permits almost everywhere an +untrammelled teaching of the established learning to which it was at +one time opposed. Men have come to see that all truth is accordant, +and that religion has nothing to fear from the faithful and devoted +study of Nature. + +The advance of science in general in modern times has been greatly due +to the development of mechanical inventions. Among the ancients, the +tools which served in the arts were few in number, and these of +exceeding simplicity. So far as we can ascertain, in the five hundred +years during which the Greeks were in their intellectual vigour, not +more than half a dozen new machines were invented, and these were +exceedingly simple. The fact seems to be that a talent for mechanical +invention is mainly limited to the peoples of France, Germany, and of +the English-speaking folk. The first advances in these contrivances +were made in those countries, and all our considerable gains have come +from their people. Thus, while the spirit of science in general is +clearly limited to the Aryan folk, that particular part of the motive +which leads to the invention of tools is restricted to western and +northern Europe, to the people to whom we give the name of Teutonic. + +Mechanical inventions have aided the development of our sciences in +several ways. They have furnished inquirers with instruments of +precision; they have helped to develop accuracy of observation; best +of all, they have served ever to bring before the attention of men a +spectacle of the conditions in Nature which we term cause and effect. +The influence of these inventions on the development of learning has +been particularly great where the machines, such as our wind and water +mills, and our steam engine, make use of the forces of Nature, +subjugating them to the needs of man. Such instruments give an +unending illustration as to the presence in Nature of energy. They +have helped men to understand that the machinery of the universe is +propelled by the unending application of power. It was, in fact, +through such machines that men of science first came to understand +that energy, manifested in the natural forces, is something that +eternally endures; that we may change its form in our arts as its form +is changed in the operations of Nature, but the power endures forever. + +It is interesting to note that the first observation which led to this +most important scientific conclusion that energy is indestructible +however much it may change its form, was made by an American, Benjamin +Thompson, who left this country at the time of the Revolution, and +after a curious life became the executive officer, and in effect king, +of Bavaria. While engaged in superintending the manufacture of cannon, +he observed that in boring out the barrel of the gun an amount of heat +was produced which evaporated a certain amount of water. He therefore +concluded that the energy required to do the boring of the metal +passed into the state of heat, and thus only changed its state, in no +wise disappearing from the earth. Other students pursuing the same +line of inquiry have clearly demonstrated what is called the law of +the conservation of energy, which more than anything has helped us to +understand the large operations of Nature. Through these studies we +have come to see that, while the universe is a place of ceaseless +change, the quantities of energy and of matter remain unaltered. + +The foregoing brief sketch, which sets forth some of the important +conditions which have affected the development of science, may in a +way serve to show the student how he can himself become an interpreter +of Nature. The evidence indicates that the people of our race have +been in a way chosen among all the varieties of mankind to lead in +this great task of comprehending the visible universe. The facts, +moreover, show that discovery usually begins with the interest which +men feel in the world immediately about them, or which is presented to +their senses in a daily spectacle. Thus Benjamin Franklin, in the +midst of a busy life, became deeply interested in the phenomena of +lightning, and by a very simple experiment proved that this wonder of +the air was due to electrical action such as we may arouse by rubbing +a stick of sealing-wax or a piece of amber with a cloth. All +discoveries, in a word, have had their necessary beginnings in an +interest in the facts which daily experience discloses. This desire to +know something more than the first sight exhibits concerning the +actions in the world about us is native in every human soul--at least, +in all those who are born with the heritage of our race. It is +commonly strong in childhood; if cultivated by use, it will grow +throughout a lifetime, and, like other faculties, becomes the stronger +and more effective by the exertions which it inspires. It is therefore +most important that every one should obey this instinctive command to +inquiry, and organize his life and work so that he may not lose but +gain more and more as time goes on of this noble capacity to +interrogate and understand the world about him. + +It is best that all study of Nature should begin not in laboratories, +nor with the things which are remote from us, but in the field of +Nature which is immediately about us. The student, even if he dwell in +the unfavourable conditions of a great city, is surrounded by the +world which has yielded immeasurable riches in the way of learning, +which he can appropriate by a little study. He can readily come to +know something of the movements of the air; the buildings will give +him access to a great many different kinds of stone; the smallest +park, a little garden, or even a few potted plants and captive +animals, may tell him much concerning the forms and actions of living +beings. By studying in this way he can come to know something of the +differences between things and their relations to each other. He will +thus have a standard by which he can measure and make familiar the +body of learning concerning Nature which he may find in books. From +printed pages alone, however well they be written, he can never hope +to catch the spirit that animates the real inquirer, the true lover of +Nature. + +On many accounts the most attractive way of beginning to form the +habit of the naturalist is by the study of living animals and plants. +To all of us life adds interest, and growth has a charm. Therefore it +is well for the student to start on the way of inquiry by watching the +actions of birds and insects or by rearing plants. It is fortunate if +he can do both these agreeable things. When the habit of taking an +account of that most important part of the world which is immediately +about him has been developed in the student, he may profitably proceed +to acquire the knowledge of the invisible universe which has been +gathered by the host of inquirers of his race. However far he +journeys, he should return to the home world that lies immediately and +familiarly about him, for there alone can he acquire and preserve that +personal acquaintance with things which is at once the inspiration and +the test of all knowledge. + +Along with this study of the familiar objects about us the student may +well combine some reading which may serve to show him how others have +been successful in thus dealing with Nature at first hand. For this +purpose there are, unfortunately, but few works which are well +calculated to serve the needs of the beginner. Perhaps the best +naturalist book, though its form is somewhat ancient, is White's +Natural History of Selborne. Hugh Miller's works, particularly his Old +Red Sandstone and My Schools and Schoolmasters, show well how a man +may become a naturalist under difficulties. Sir John Lubbock's studies +on Wasps, and Darwin's work on Animals and Plants under Domestication +are also admirable to show how observation should be made. Dr. Asa +Gray's little treatise on How Plants Grow will also be useful to the +beginner who wishes to approach botany from its most attractive +side--that of the development of the creature from the seed to seed. + +There is another kind of training which every beginner in the art of +observing Nature should obtain, and which many naturalists of repute +would do well to give themselves--namely, an education in what we may +call the art of distance and geographical forms. With the primitive +savage the capacity to remember and to picture to the eye the shape of +a country which he knows is native and instinctive. Accustomed to +range the woods, and to trust to his recollection to guide him through +the wilderness to his home, the primitive man develops an important +art which among civilized people is generally dormant. In fact, in our +well-trodden ways people may go for many generations without ever +being called upon to use this natural sense of geography. The easiest +way to cultivate the geographic sense is by practising the art of +making sketch maps. This the student, however untrained, can readily +do by taking first his own dwelling house, on which he should practise +until he can readily from memory make a tolerably correct and +proportional plan of all its rooms. Then on a smaller scale he should +begin to make also from recollection a map showing the distribution of +the roads, streams, and hills with which his daily life makes him +familiar. From time to time this work from memory should be compared +with the facts. At first the record will be found to be very poor, but +with a few months of occasional endeavour the observer will find that +his mind takes account of geographic features in a way it did not +before, and, moreover, that his mind becomes enriched with +impressions of the country which are clear and distinct, in place of +the shadowy recollections which he at first possessed. + +When the student has attained the point where, after walking or riding +over a country, he can readily recall its physical features of the +simpler sort, he will find it profitable to undertake the method of +mapping with contour lines--that is, by pencilling in indications to +show the exact shape of the elevations and depressions. The principle +of contour lines is that each of them represents where water would +come against the slope if the area were sunk step by step below the +sea level--in other words, each contour line marks the intersection of +a horizontal plane with the elevation of the country. Practice on this +somewhat difficult task will soon give the student some idea as to the +complication of the surface of a region, and afford him the basis for +a better understanding of what geography means than all the reading he +can do will effect. It is most desirable that training such as has +been described should be a part of our ordinary school education. + +Very few people have clear ideas of distances. Even the men whose +trade requires some such knowledge are often without that which a +little training could give them. Without some capacity in this +direction, the student is always at a disadvantage in his contact with +Nature. He can not make a record of what he sees as long as the +element of horizontal and vertical distance is not clearly in mind. To +attain this end the student should begin by pacing some length of road +where the distances are well known. In this way he will learn the +length of his step, which with a grown man generally ranges between +two and a half and three feet. Learning the average length of his +stride by frequent counting, it is easy to repeat the trial until one +can almost unconsciously keep the count as he walks. Properly to +secure the training of this sort the observer should first attentively +look across the distance which is to be determined. He should notice +how houses, fences, people, and trees appear at that distance. He will +quickly perceive that each hundred feet of additional interval +somewhat changes their aspect. In training soldiers to measure with +the eye the distances which they have to know in order effectively to +use the modern weapons of war, a common device is to take a squad of +men, or sometimes a company, under the command of an officer, who +halts one man at each hundred yards until the detachment is strung out +with that interval as far as the eye can see them. The men then walk +to and fro so that the troops who are watching them may note the +effects of increased distance on their appearance, whether standing or +in motion. At three thousand yards a man appears as a mere dot, which +is not readily distinguishable. Schoolboys may find this experiment +amusing and instructive. + +After the student has gained, as he readily may, some sense of the +divisions of distance within the range of ordinary vision, he should +try to form some notion of greater intervals, as of ten, a hundred, +and perhaps a thousand miles. The task becomes more difficult as the +length of the line increases, but most persons can with a little +address manage to bring before their eyes a tolerably clear image of a +hundred miles of distance by looking from some elevation which +commands a great landscape. It is doubtful, however, whether the +best-trained man can get any clear notion of a thousand miles--that +is, can present it to himself in imagination as he may readily do with +shorter intervals. + +The most difficult part of the general education which the student has +to give himself is begun when he undertakes to picture long intervals +of time. Space we have opportunities to measure, and we come in a way +to appreciate it, but the longest lived of men experiences at most a +century of life, and this is too small a measure to give any notion as +to the duration of such great events as are involved in the history of +the earth, where the periods are to be reckoned by the millions of +years. The only way in which we can get any aid in picturing to +ourselves great lapses of time is by expressing them in units of +distance. Let a student walk away on a straight road for the distance +of a mile; let him call each step a year; when he has won the first +milestone, he may consider that he has gone backward in time to the +period of Christ's birth. Two miles more will take him to the station +which will represent the age when the oldest pyramids were built. He +is still, however, in the later days of man's history on this planet. +To attain on the scale the time when man began, he might well have to +walk fifty miles away, while a journey which would thus by successive +steps describe the years of the earth's history since life appeared +upon its surface would probably require him to circle the earth at +least four times. We may accept it as impossible for any one to deal +with such vast durations save with figures which are never really +comprehended. It is well, however, to enlarge our view as to the age +of the earth by such efforts as have just been indicated. + +When we go beyond the earth into the realm of the stars all efforts +toward understanding the ranges of space or the durations of time are +quite beyond the efforts of man. Even the distance of about two +hundred and forty thousand miles which separates us from the moon can +not be grasped by even the greater minds. No human intelligence, +however cultivated, can conceive the distance of about ninety-five +million miles which separates us from the sun. In the celestial realm +we can only deal with relations of space and time in a general and +comparative way. We can state the distances if we please in millions +of miles, or we can reckon the ampler spaces by using the interval +which separates the earth from the sun as we do a foot rule in our +ordinary work, but the depths of the starry spaces can only be sounded +by the winged imagination. + +Although the student has been advised to begin his studies of Nature +on the field whereon he dwells, making that study the basis of his +most valuable communications with Nature, it is desirable that he +should at the same time gain some idea as to the range and scope of +our knowledge concerning the visible universe. As an aid toward this +end the following chapters of this book will give a very brief survey +of some of the most important truths concerning the heavens and the +earth which have rewarded the studies of scientific men. Of remoter +things, such as the bodies in the stellar spaces, the account will be +brief, for that which is known and important to the general student +can be briefly told. So, too, of the earlier ages of the earth's +history, although a vast deal is known, the greater part of the +knowledge is of interest and value mainly to geologists who cultivate +that field. That which is most striking and most important to the mass +of mankind is to be found in the existing state of our earth, the +conditions which make it a fit abode for our kind, and replete with +lessons which he may study with his own eyes without having to travel +the difficult paths of the higher sciences. + +Although physiography necessarily takes some account of the things +which have been, even in the remote past, and this for the reason that +everything in this day of the world depends on the events of earlier +days, the accent of its teaching is on the immediate, visible, as we +may say, living world, which is a part of the life of all its +inhabitants. + + + + + CHAPTER III. + + THE STELLAR REALM. + + +Even before men came to take any careful account of the Nature +immediately about them they began to conjecture and in a way to +inquire concerning the stars and the other heavenly bodies. It is +difficult for us to imagine how hard it was for students to gain any +adequate idea of what those lights in the sky really are. At first men +imagined the celestial bodies to be, as they seemed, small objects not +very far away. Among the Greeks the view grew up that the heavens were +formed of crystal spheres in which the lights were placed, much as +lanterns may be hung upon a ceiling. These spheres were conceived to +be one above the other; the planets were on the lower of them, and the +fixed stars on the higher, the several crystal roofs revolving about +the earth. So long as the earth was supposed to be a flat and +limitless expanse, forming the centre of the universe, it was +impossible for the students of the heavens to attain any more rational +view as to their plan. + +The fact that the earth was globular in form was understood by the +Greek men of science. They may, indeed, have derived the opinion from +the Egyptian philosophers. The discovery rested upon the readily +observed fact that on a given day the shadow of objects of a certain +height was longer in high latitude than in low. Within the tropics, +when the sun was vertical, there would be no shadow, while as far +north as Athens it would be of considerable length. The conclusion +that the earth was a sphere appears to have been the first large +discovery made by our race. It was, indeed, one of the most important +intellectual acquisitions of man. + +Understanding the globular form of the earth, the next and most +natural step was to learn that the earth was not the centre of the +planetary system, much less of the universe, but that that centre was +the sun, around which the earth and the other planets revolved. The +Greeks appear to have had some idea that this was the case, and their +spirit of inquiry would probably have led them to the whole truth but +for the overthrow of their thought by the Roman conquest and the +spread of Christianity. It was therefore not until after the revival +of learning that astronomers won their way to our modern understanding +concerning the relation of the planets to the sun. With Galileo this +opinion was affirmed. Although for a time the Church, resting its +opposition on the interpretation of certain passages of Scripture, +resisted this view, and even punished the men who held it, it +steadfastly made its way, and for more than two centuries has been the +foundation of all the great discoveries in the stellar realm. Yet long +after the fact that the sun was the centre of the solar system was +well established no one understood why the planets should move in +their ceaseless, orderly procession around the central mass. To Newton +we owe the studies on the law of gravitation which brought us to our +present large conception as to the origin of this order. Starting with +the view that bodies attracted each other in proportion to their +weight, and in diminishing proportion as they are removed from each +other, Newton proceeded by most laborious studies to criticise this +view, and in the end definitely proved it by finding that the motions +of the moon about the earth, as well as the paths of the planets, +exactly agreed with the supposition. + +The last great path-breaking discovery which has helped us in our +understanding of the stars was made by Fraunhofer and other +physicists, who showed us that substances when in a heated, gaseous, +or vaporous state produced, in a way which it is not easy to explain +in a work such as this, certain dark lines in the spectrum, or streak +of divided light which we may make by means of a glass prism, or, as +in the rainbow, by drops of water. Carefully studying these very +numerous lines, those naturalists found that they could with singular +accuracy determine what substances there were in the flame which gave +the light. So accurate is this determination that it has been made to +serve in certain arts where there is no better means of ascertaining +the conditions of a flaming substance except by the lines which its +light exhibits under this kind of analysis. Thus, in the manufacture +of iron by what is called the Bessemer process, it has been found very +convenient to judge as to the state of the molten metal by such an +analysis of the flame which comes forth from it. + +[Illustration: _Seal Rocks near San Francisco, California, showing +slight effect of waves where there is no beach._] + +No sooner was the spectroscope invented than astronomers hastened by +its aid to explore the chemical constitution of the sun. These studies +have made it plain that the light of our solar centre comes forth from +an atmosphere composed of highly heated substances, all of which are +known among the materials forming the earth. Although for various +reasons we have not been able to recognise in the sun all the elements +which are found in our sphere, it is certain that in general the two +bodies are alike in composition. An extension of the same method of +inquiry to the fixed stars was gradually though with difficulty +attained, and we now know that many of the elements common to the sun +and earth exist in those distant spheres. Still further, this method +of inquiry has shown us, in a way which it is not worth while here to +describe, that among these remoter suns there are many aggregations of +matter which are not consolidated as are the spheres of our own solar +system, but remain in the gaseous state, receiving the name of nebulae. + +Along with the growth of observational astronomy which has taken place +since the discoveries of Galileo, there has been developed a view +concerning the physical history of the stellar world, known as the +nebular hypothesis, which, though not yet fully proved, is believed by +most astronomers and physicists to give us a tolerably correct notion +as to the way in which the heavenly spheres were formed from an +earlier condition of matter. This majestic conception was first +advanced, in modern times at least, by the German philosopher Immanuel +Kant. It was developed by the French astronomer Laplace, and is often +known by his name. The essence of this view rests upon the fact +previously noted that in the realm of the fixed stars there are many +faintly shining aggregations of matter which are evidently not solid +after the manner of the bodies in our solar system, but are in the +state where their substances are in the condition of dustlike +particles, as are the bits of carbon in flame or the elements which +compose the atmosphere. The view held by Laplace was to the effect +that not only our own solar system, but the centres of all the other +similar systems, the fixed stars, were originally in this gaseous +state, the material being disseminated throughout all parts of the +heavenly realm, or at least in that portion of the universe of which +we are permitted to know something. In this ancient state of matter we +have to suppose that the particles of it were more separated from each +other than are the atoms of the atmospheric gases in the most perfect +vacuum which we can produce with the air-pump. Still we have to +suppose that each of these particles attract the other in the +gravitative way, as in the present state of the universe they +inevitably do. + +Under the influence of the gravitative attraction the materials of +this realm of vapour inevitably tended to fall in toward the centre. +If the process had been perfectly simple, the result would have been +the formation of one vast mass, including all the matter which was in +the original body. In some way, no one has yet been able to make a +reasonable suggestion of just how, there were developed in the +process of concentration a great many separate centres of aggregation, +each of which became the beginning of a solar system. The student may +form some idea of how readily local centres may be produced in +materials disseminated in the vaporous state by watching how fog or +the thin, even misty clouds of the sunrise often gather into the +separate shapes which make what we term a "mackerel" sky. It is +difficult to imagine what makes centres of attraction, but we readily +perceive by this instance how they might have occurred. + +When the materials of each solar system were thus set apart from the +original mass of star dust or vapour, they began an independent +development which led step by step, in the case of our own solar +system at least, and presumably also in the case of the other suns, +the fixed stars, to the formation of planets and their moons or +satellites, all moving around the central sun. At this stage of the +explanation the nebular hypothesis is more difficult to conceive than +in the parts of it which have already been described, for we have now +to understand how the planets and satellites had their matter +separated from each other and from the solar centre, and why they came +to revolve around that central body. These problems are best +understood by noting some familiar instances connected with the +movement of fluids and gases toward a centre. First let us take the +case of a basin in which the water is allowed to flow out through a +hole in its centre. When we lift the stopper the fluid for a moment +falls straight down through the opening. Very quickly, however, all +the particles of the water start to move toward the centre, and almost +at once the mass begins to whirl round with such speed that, although +it is working toward the middle, it is by its movement pushed away +from the centre and forms a conical depression. As often as we try the +experiment, the effect is always the same. We thus see that there is +some principle which makes particles of fluid that tend toward a +centre fail directly to attain it, but win their way thereto in a +devious, spinning movement. + +Although the fact is not so readily made visible to the eye, the same +principle is illustrated in whirling storms, in which, as we shall +hereafter note with more detail, the air next the surface of the earth +is moving in toward a kind of chimney by which it escapes to the upper +regions of the atmosphere. A study of cyclones and tornadoes, or even +of the little air-whirls which in hot weather lift the dust of our +streets, shows that the particles of the atmosphere in rushing in +toward the centre of upward movement take on the same whirling motion +as do the molecules of water in the basin--in fact, the two actions +are perfectly comparable in all essential regards, except that the +fluid is moving downward, while the air flows upward. Briefly stated, +the reason for the movement of fluid and gas in the whirling way is as +follows: If every particle on its way to the centre moved on a +perfectly straight line toward the point of escape, the flow would be +directly converging, and the paths followed would resemble the spokes +of a wheel. But when by chance one of the particles sways ever so +little to one side of the direct way, a slight lateral motion would +necessarily be established. This movement would be due to the fact +that the particle which pursued the curved line would press against +the particles on the out-curved side of its path--or, in other words, +shove them a little in that direction--to the extent that they +departed from the direct line they would in turn communicate the +shoving to the next beyond. When two particles are thus shoving on one +side of their paths, the action which makes for revolution is doubled, +and, as we readily see, the whole mass may in this way become quickly +affected, the particles driven out of their path, moving in a curve +toward the centre. We also see that the action is accumulative: the +more curved the path of each particle, the more effectively it shoves; +and so, in the case of the basin, we see the whirling rapidly +developed before our eyes. + +In falling in toward the centre the particles of star dust or vapour +would no more have been able one and all to pursue a perfectly +straight line than the particles of water in the basin. If a man +should spend his lifetime in filling and emptying such a vessel, it is +safe to say that he would never fail to observe the whirling movement. +As the particles of matter in the nebular mass which was to become a +solar system are inconceivably greater than those of water in the +basin, or those of air in the atmospheric whirl, the chance of the +whirling taking place in the heavenly bodies is so great that we may +assume that it would inevitably occur. + +As the vapours in the olden day tended in toward the centre of our +solar system, and the mass revolved, there is reason to believe that +ringlike separations took place in it. Whirling in the manner +indicated, the mass of vapour or dust would flatten into a disk or a +body of circular shape, with much the greater diameter in the plane of +its whirling. As the process of concentration went on, this disk is +supposed to have divided into ringlike masses, some approach to which +we can discern in the existing nebulae, which here and there among the +farther fixed stars appear to be undergoing such stages of development +toward solar systems. It is reasonably supposed that after these rings +had been developed they would break to pieces, the matter in them +gathering into a sphere, which in time was to become a planet. The +outermost of these rings led to the formation of the planet farthest +from the sun, and was probably the first to separate from the parent +mass. Then in succession rings were formed inwardly, each leading in +turn to the creation of another planet, the sun itself being the +remnant, by far the greater part of the whole mass of matter, which +did not separate in the manner described, but concentrated on its +centre. Each of these planetary aggregations of vapour tended to +develop, as it whirled upon its centre, rings of its own, which in +turn formed, by breaking and concentrating, the satellites or moons +which attend the earth, as they do all the planets which lie farther +away from the sun than our sphere. + +[Illustration: Fig. 1.--Saturn, Jan. 26, 1889 (Antoniadi).] + +As if to prove that the planets and moons of the solar system were +formed somewhat in the manner in which we have described it, one of +these spheres, Saturn, retains a ring, or rather a band which appears +to be divided obscurely into several rings which lie between its group +of satellites and the main sphere. How this ring has been preserved +when all the others have disappeared, and what is the exact +constitution of the mass, is not yet well ascertained. It seems clear, +however, that it can not be composed of solid matter. It is either in +the form of dust or of small spheres, which are free to move on each +other; otherwise, as computation shows, the strains due to the +attraction which Saturn itself and its moons exercise upon it would +serve to break it in pieces. Although this ring theory of the +formation of the planets and satellites is not completely proved, the +occurrence of such a structure as that which girdles Saturn affords +presumptive evidence that it is true. Taken in connection with what we +know of the nebulae, the proof of Laplace's nebular hypothesis may +fairly be regarded as complete. + +It should be said that some of the fixed stars are not isolated suns +like our own, but are composed of two great spheres revolving about +one another; hence they are termed double stars. The motions of these +bodies are very peculiar, and their conditions show us that it is not +well to suppose that the solar system in which we dwell is the only +type of order which prevails in the celestial families; there may, +indeed, be other variations as yet undetected. Still, these +differences throw no doubt on the essential truth of the theory as to +the process of development of the celestial systems. Though there is +much room for debate as to the details of the work there, the general +truth of the theory is accepted by nearly all the students of the +problem. + +A peculiar advantage of the nebular hypothesis is that it serves to +account for the energy which appears as light and heat in the sun and +the fixed stars, as well as that which still abides in the mass of our +earth, and doubtless also in the other large planets. When the matter +of which these spheres were composed was disseminated through the +realms of space, it is supposed to have had no positive temperature, +and to have been dark, realizing the conception which appears in the +first chapter of Genesis, "without form, and void." With each stage of +the falling in toward the solar centres what is called the "energy of +position" of this original matter became converted into light and +heat. To understand how this took place, the reader should consider +certain simple yet noble generalizations of physics. We readily +recognise the fact that when a hammer falls often on an anvil it heats +itself and the metal on which it strikes. Those who have been able to +observe the descent of meteoric stones from the heavens have remarked +that when they came to the earth they were, on their surfaces at +least, exceedingly hot. Any one may observe shining meteors now and +then flashing in the sky. These are known commonly to be very small +bits of matter, probably not larger than grains of sand, which, +rushing into our atmosphere, are so heated by the friction which they +encounter that they burn to a gas or vapour before they attain the +earth. As we know that these particles come from the starry spaces, +where the temperature is somewhere near 500 deg. below 0 deg. Fahr., it is +evident that the light and heat are not brought with them into the +atmosphere; it can only be explained by the fact that when they enter +the air they are moving at an average speed of about twenty miles a +second, and that the energy which this motion represents is by the +resistance which the body encounters converted into heat. This fact +will help us to understand how, as the original star dust fell in +toward the centre of attraction, it was able to convert what we have +termed the energy of position into temperature. We see clearly that +every such particle of dust or larger bit of matter which falls upon +the earth brings about the development of heat, even though it does +not actually strike upon the solid mass of our sphere. The conception +of what took place in the consolidation of the originally disseminated +materials of the sun and planets can be somewhat helped by a simple +experiment. If we fit a piston closely into a cylinder, and then +suddenly drive it down with a heavy blow, the compressed air is so +heated that it may be made to communicate fire. If the piston should +be slowly moved, the same amount of heat would be generated, or, as we +may better say, liberated by the compression, though the effect would +not be so striking. A host of experiments show that when a given mass +of matter is brought to occupy a less space the effect is in +practically all cases to increase the temperature. The energy which +kept the particles apart is, when they are driven together, converted +into heat. These two classes of actions are somewhat different in +their nature; in the case of the meteors, or the equivalent star dust, +the coming together of the particles is due to gravitation. In the +experiment with the cylinder above described, the compression is due +to mechanical energy, a force of another nature. + +There is reason for believing that all our planets, as well as the sun +itself, and also the myriad other orbs of space, have all passed +through the stages of a transition in which a continually +concentrating vapour, drawn together by gravitation, became +progressively hotter and more dense until it assumed the condition of +a fluid. This fluid gradually parted with its heat to the cold spaces +of the heavens, and became more and more concentrated and of a lower +temperature until in the end, as in the case of our earth and of other +planets, it ceased to glow on the outside, though it remained +intensely heated in the inner parts. It is easy to see that the rate +of this cooling would be in some proportion to the size of the sphere. +Thus the earth, which is relatively small, has become relatively cold, +while the sun itself, because of its vastly greater mass, still +retains an exceedingly high temperature. The reason for this can +readily be conceived by making a comparison of the rate of cooling +which occurs in many of our ordinary experiences. Thus a vial of hot +water will quickly come down to the temperature of the air, while a +large jug filled with the fluid at the same temperature will retain +its heat many times as long. The reason for this rests upon the simple +principle that the contents of a sphere increase with its enlargement +more rapidly than the surface through which the cooling takes place. + +The modern studies on the physical history of the sun and other +celestial bodies show that their original store of heat is constantly +flowing away into the empty realms of space. The rate at which this +form of energy goes away from the sun is vast beyond the powers of the +imagination to conceive; thus, in the case of our earth, which viewed +from the sun would appear no more than a small star, the amount of +heat which falls upon it from the great centre is enough each day to +melt, if it all could be put to such work, about eight thousand cubic +miles of ice. Yet the earth receives only 1/2,170,000,000 part of the +solar radiation. The greater part of this solar heat--in fact, we may +say nearly all of it--slips by the few and relatively small planets +and disappears in the great void. + +The destiny of all the celestial spheres seems in time to be that +they shall become cooled down to a temperature far below anything +which is now experienced on this earth. Even the sun, though its heat +will doubtless endure for millions of years to come, must in time, so +far as we can see, become dark and cold. So far as we know, we can +perceive no certain method by which the life of the slowly decaying +suns can be restored. It has, however, been suggested that in many +cases a planetary system which has attained the lifeless and lightless +stage may by collision with some other association of spheres be by +the blow restored to its previous state of vapour, the joint mass of +the colliding systems once again to resume the process of +concentration through which it had gone before. Now and then stars +have been seen to flash suddenly into great brilliancy in a way which +suggests that possibly their heat had been refreshed by a collision +with some great mass which had fallen into them from the celestial +spaces. There is room for much speculation in this field, but no +certainty appears to be attainable. + +The ancients believed that light and heat were emanations which were +given off from the bodies that yielded them substantially as odours +are given forth by many substances. Since the days of Newton inquiry +has forced us to the conviction that these effects of temperature are +produced by vibrations having the general character of waves, which +are sent through the spaces with great celerity. When a ray of light +departs from the sun or other luminous body, it does not convey any +part of the mass; it transmits only motion. A conception of the action +can perhaps best be formed by suspending a number of balls of ivory, +stone, or other hard substance each by a cord, the series so arranged +that they touch each other. Then striking a blow against one end of +the line, we observe that the ball at the farther end of the line is +set in motion, swinging a little away from the place it occupied +before. The movement of the intermediate balls may be so slight as to +escape attention. We thus perceive that energy can be transmitted +from one to another of these little spheres. Close observation shows +us that under the impulse which the blow gives each separate body is +made to sway within itself much in the manner of a bell when it is +rung, and that the movement is transmitted to the object with which it +is in contact. In passing from the sun to the earth, the light and +heat traverse a space which we know to be substantially destitute of +any such materials as make up the mass of the earth or the sun. Judged +by the standards which we can apply, this space must be essentially +empty. Yet because motions go through it, we have to believe that it +is occupied by something which has certain of the properties of +matter. It has, indeed, one of the most important properties of all +substances, in that it can vibrate. This practically unknown thing is +called ether. + +The first important observational work done by the ancients led them +to perceive that there was a very characteristic difference between +the planets and the fixed stars. They noted the fact that the planets +wandered in a ceaseless way across the heavens, while the fixed stars +showed little trace of changing position in relation to one another. +For a long time it was believed that these, as well as the remoter +fixed stars, revolved about the earth. This error, though great, is +perfectly comprehensible, for the evident appearance of the movement +is substantially what would be brought about if they really coursed +around our sphere. It was only when the true nature of the earth and +its relations to the sun were understood that men could correct this +first view. It was not, indeed, until relatively modern times that the +solar system came to be perceived as something independent and widely +detached from the fixed stars system; that the spaces which separate +the members of our own solar family, inconceivably great as they are, +are but trifling as compared with the intervals which part us from the +nearer fixed stars. At this stage of our knowledge men came to the +noble suggestion that each of the fixed stars was itself a sun, each +of the myriad probably attended by planetary bodies such as exist +about our own luminary. + +It will be well for the student to take an imaginary journey from the +sun forth into space, along the plane in which extends that vast +aggregation of stars which we term the Milky Way. Let him suppose that +his journey could be made with something like the speed of light, or, +say, at the rate of about two hundred thousand miles a second. It is +fit that the imagination, which is free to go through all things, +should essay such excursions. On the fancied outgoing, the observer +would pass the interval between the sun and the earth in about eight +minutes. It would require some hours before he attained to the outer +limit of the solar system. On his direct way he would pass the orbits +of the several planets. Some would have their courses on one side or +the other of his path; we should say above or below, but for the fact +that we leave these terms behind in the celestial realm. On the margin +of the solar system the sun would appear shrunken to the state where +it was hardly greater than the more brilliant of the other fixed +stars. The onward path would then lead through a void which it would +require years to traverse. Gradually the sun which happened to lie +most directly in his path would grow larger; with nearer approach, it +would disclose its planets. Supposing that the way led through this +solar system, there would doubtless be revealed planets and satellites +in their order somewhat resembling those of our own solar family, yet +there would doubtless be many surprises in the view. Arriving near the +first sun to be visited, though the heavens would have changed their +shape, all the existing constellations having altered with the change +in the point of view, there would still be one familiar element in +that the new-found planets would be near by, and the nearest fixed +stars far away in the firmament. + +With the speed of light a stellar voyage could be taken along the path +of the Milky Way, which would endure for thousands of years. Through +all the course the journeyer would perceive the same vast girdle of +stars, faint because they were far away, which gives the dim light of +our galaxy. At no point is it probable that he would find the separate +suns much more aggregated or greatly farther apart than they are in +that part of the Milky Way which our sun now occupies. Looking forth +on either side of the "galactic plane," there would be the same +scattering of stars which we now behold when we gaze at right angles +to the way we are supposing the spirit to traverse. + +As the form of the Milky Way is irregular, the mass, indeed, having +certain curious divisions and branches, it well might be that the +supposed path would occasionally pass on one or the other side of the +vast star layer. In such positions the eye would look forth into an +empty firmament, except that there might be in the far away, tens of +thousands of years perhaps at the rate that light travels away from +the observer, other galaxies or Milky Ways essentially like that which +he was traversing. At some point the journeyer would attain the margin +of our star stratum, whence again he would look forth into the +unpeopled heavens, though even there he might discern other remote +star groups separated from his own by great void intervals. + + * * * * * + +The revelations of the telescope show us certain features in the +constitution and movements of the fixed stars which now demand our +attention. In the first place, it is plain that not all of these +bodies are in the same physical condition. Though the greater part of +these distant luminous masses are evidently in the state of +aggregation displayed by our own sun, many of them retain more or less +of that vaporous, it may be dustlike, character which we suppose to +have been the ancient state of all the matter in the universe. Some of +these masses appear as faint, almost indistinguishable clouds, which +even to the greatest telescope and the best-trained vision show no +distinct features of structure. In other cases the nebulous +appearance is hardly more than a mist about a tolerably distinct +central star. Yet again, and most beautifully in the great nebula of +the constellation of Orion, the cloudy mass, though hardly visible to +the naked eye, shows a division into many separate parts, the whole +appearing as if in process of concentration about many distinct +centres. + +The nebulas are reasonably believed by many astronomers to be examples +of the ancient condition of the physical universe, masses of matter +which for some reason as yet unknown have not progressed in their +consolidation to the point where they have taken on the +characteristics of suns and their attendant planets. + +Many of the fixed stars, the incomplete list of which now amounts to +several hundred, are curiously variable in the amount of light which +they send out to the earth. Sometimes these variations are apparently +irregular, but in the greater number of cases they have fixed periods, +the star waxing and waning at intervals varying from a few months to a +few years. Although some of the sudden flashings forth of stars from +apparent small size to near the greatest brilliancy may be due to +catastrophes such as might be brought about by the sudden falling in +of masses of matter upon the luminous spheres, it is more likely that +the changes which we observe are due to the fact that two suns +revolving around a common centre are in different stages of +extinction. It may well be that one of these orbs, presumably the +smaller, has so far lost temperature that it has ceased to glow. If in +its revolution it regularly comes between the earth and its luminous +companion, the effect would be to give about such a change in the +amount of light as we observe. + +The supposition that a bright sun and a relatively dark sun might +revolve around a common centre of gravity may at first sight seem +improbable. The fact is, however, that imperfect as our observations +on the stars really are, we know many instances in which this kind of +revolution of one star about another takes place. In some cases these +stars are of the same brilliancy, but in others one of the lights is +much brighter than the other. From this condition to the state where +one of the stars is so nearly dark as to be invisible, the transition +is but slight. In a word, the evidence goes to show that while we see +only the luminous orbs of space, the dark bodies which people the +heavens are perhaps as numerous as those which send us light, and +therefore appear as stars. + +Besides the greater spheres of space, there is a vast host of lesser +bodies, the meteorites and comets, which appear to be in part members +of our solar system, and perhaps of other similar systems, and in part +wanderers in the vast realm which intervenes between the solar +systems. Of these we will first consider the meteors, of which we know +by far the most; though even of them, as we shall see, our knowledge +is limited. + +From time to time on any starry night, and particularly in certain +periods of the year, we may behold, at the distance of fifty or more +miles above the surface of the earth, what are commonly called +"shooting stars." The most of these flashing meteors are evidently +very small, probably not larger than tiny sand grains, possibly no +greater than the fragments which would be termed dust. They enter the +air at a speed of about thirty miles a second. They are so small that +they burn to vapour in the very great heat arising from their friction +on the air, and do not attain the surface of the earth. These are so +numerous that, on the average, some hundreds of thousands probably +strike the earth's atmosphere each day. From time to time larger +bodies fall--bodies which are of sufficient bulk not to be burned up +in the air, but which descend to the ground. These may be from the +smallest size which may be observed to masses of many hundred pounds +in weight. These are far less numerous than the dust meteorites; it is +probable, however, that several hundred fragments each year attain the +earth's surface. They come from various directions of space, and +there is as yet no means of determining whether they were formed in +some manner within our planetary system or whether they wander to us +from remoter realms. We know that they are in part composed of +metallic iron commingled with nickel and carbon (sometimes as very +small diamonds) in a way rarely if ever found on the surface of our +sphere, and having a structure substantially unknown in its deposits. +In part they are composed of materials which somewhat resemble certain +lavas. It is possible that these fragments of iron and stone which +constitute the meteorites have been thrown into the planetary spaces +by the volcanic eruption of our own and other planets. If hurled forth +with a sufficient energy, the fragments would escape from the control +of the attraction of the sphere whence they came, and would become +independent wanderers in space, moving around the sun in varied orbits +until they were again drawn in by some of the greater planets. + +As they come to us these meteorites often break up in the atmosphere, +the bits being scattered sometimes over a wide area of country. Thus, +in the case of the Cocke County meteorite of Tennessee, one of the +iron species, the fragments, perhaps thousands in number, which came +from the explosion of the body were scattered over an area of some +thousand square miles. When they reach the surface in their natural +form, these meteors always have a curious wasted and indented +appearance, which makes it seem likely that they have been subject to +frequent collisions in their journeys after they were formed by some +violent rending action. + +In some apparent kinship with the meteorites may be classed the +comets. The peculiarity of these bodies is that they appear in most +cases to be more or less completely vaporous. Rushing down from the +depths of the heavens, these bodies commonly appear as faintly +shining, cloudlike masses. As they move in toward the sun long trails +of vapour stream back from the somewhat consolidated head. Swinging +around that centre, they journey again into the outer realm. As they +retreat, their tail-like streamers appear to gather again upon their +centres, and when they fade from view they are again consolidated. In +some cases it has been suspected that a part at least of the cometary +mass was solid. The evidence goes to show, however, that the matter is +in a dustlike or vaporous condition, and that the weight of these +bodies is relatively very small. + +[Illustration: Fig. 2.--The Great Comet of 1811, one of the many +varied forms of these bodies.] + +Owing to their strange appearance, comets were to the ancients omens +of calamity. Sometimes they were conceived as flaming swords; their +forms, indeed, lend themselves to this imagining. They were thought to +presage war, famine, and the death of kings. Again, in more modern +times, when they were not regarded as portents of calamity, it was +feared that these wanderers moving vagariously through our solar +system might by chance come in contact with the earth with disastrous +results. Such collisions are not impossible, for the reason that the +planets would tend to draw these errant bodies toward them if they +came near their spheres; yet the chance of such collisions happening +to the earth is so small that they may be disregarded. + + + MOTIONS OF THE SPHERES. + +Although little is known of the motions which occur among the +celestial bodies beyond the sphere of our solar family, that which has +been ascertained is of great importance, and serves to make it likely +that all the suns in space are upon swift journeys which in their +speed equal, if they do not exceed, the rate of motion among the +planetary spheres, which may, in general, be reckoned at about twenty +miles a second. Our whole solar system is journeying away from certain +stars, and in the direction of others which are situated in the +opposite part of the heavens. The proof of this fact is found in the +observations which show that on one side of us the stars are +apparently coming closer together, while on the other side they are +going farther apart. The phenomenon, in a word, is one of perspective, +and may be made real to the understanding by noting what takes place +when we travel down a street along which there are lights. We readily +note that these lights appear to close in behind us, and widen their +intervals in the direction in which we journey. By such evidence +astronomers have become convinced that our sphere, along with the sun +which controls it, is each second a score of miles away from the point +where it was before. + +There is yet other and most curious evidence which serves to show that +certain of the stars are journeying toward our part of the heavens at +great speed, while others are moving away from us by their own proper +motion. These indications are derived from the study of the lines in +the light which the spectrum reveals to us when critically examined. +The position of these cross lines is, as we know, affected by the +motion of the body whence the light comes, and by close analysis of +the facts it has been pretty well determined that the distortion in +their positions is due to very swift motions of the several stars. It +is not yet certain whether these movements of our sun and of other +solar bodies are in straight lines or in great circles. + +It should be noted that, although the evidence from the spectroscope +serves to show that the matter in the stars is akin to that of our own +earth, there is reason to believe that those great spheres differ much +from each other in magnitude. + +We have now set forth some of the important facts exhibited by the +stellar universe. The body of details concerning that realm is vast, +and the conclusions drawn from it important; only a part, however, of +the matter with which it deals is of a nature to be apprehended by the +student who does not approach it in a somewhat professional way. We +shall therefore now turn to a description of the portion of the starry +world which is found in the limits of our solar system. There the +influences of the several spheres upon our planet are matters of vital +importance; they in a way affect, if they do not control, all the +operations which go on upon the surface of the earth. + + + THE SOLAR SYSTEM. + +We have seen that the matter in the visible universe everywhere tends +to gather into vast associations which appear to us as stars, and that +these orbs are engaged in ceaseless motion in journeys through space. +In only one of these aggregations--that which makes our own solar +system--are the bodies sufficiently near to our eyes for us, even with +the resources of our telescopes and other instruments, to divine +something of the details which they exhibit. In studying what we may +concerning the family of the sun, the planets, and their satellites, +we may reasonably be assured that we are tracing a history which with +many differences is in general repeated in the development of each +star in the firmament. Therefore the inquiry is one of vast range and +import. + +Following, as we may reasonably do, the nebular hypothesis--a view +which, though not wholly proved, is eminently probable--we may regard +our solar system as having begun when the matter of which it is +composed, then in a finely divided, cloudy state, was separated from +the similar material which went to make the neighbouring fixed stars. +The period when our solar system began its individual life was remote +beyond the possibility of conception. Naturalists are pretty well +agreed that living beings began to exist upon the earth at least a +hundred million years ago; but the beginnings of our solar system must +be placed at a date very many times as remote from the present day.[1] + +[Footnote 1: Some astronomers, particularly the distinguished Professor +Newcomb, hold that the sun can not have been supplying heat as at +present for more than about ten million years, and that all geological +time must be thus limited. The geologist believes that this reckoning is +far too short.] + +According to the nebular theory, the original vapour of the solar +system began to fall in toward its centre and to whirl about that +point at a time long before the mass had shrunk to the present limits +of the solar system as defined by the path of the outermost planets. +At successive stages of the concentration, rings after the manner of +those of Saturn separated from the disklike mass, each breaking up and +consolidating into a body of nebulous matter which followed in the +same path, generally forming rings which became by the same process +the moons or satellites of the sphere. In this way the sun produced +eight planets which are known, and possibly others of small size on +the outer verge of the system which have eluded discovery. According +to this view, the planetary masses were born in succession, the +farthest away being the oldest. It is, however, held by an able +authority that the mass of the solar system would first form a rather +flat disk, the several rings forming and breaking into planets at +about the same time. The conditions in Saturn, where the inner ring +remains parted, favours the view just stated. + +Before making a brief statement of the several planets, the asteroids, +and the satellites, it will be well to consider in a general way the +motions of these bodies about their centres and about the sun. The +most characteristic and invariable of these movements is that by which +each of the planetary spheres, as well as the satellites, describes an +orbit around the gravitative centre which has the most influence upon +it--the sun. To conceive the nature of this movement, it will be well +to imagine a single planet revolving around the sun, each of these +bodies being perfect spheres, and the two the only members of the +solar system. In this condition the attraction of the two bodies would +cause them to circle around a common centre of gravity, which, if the +planet were not larger or the sun smaller than is the case in our +solar system, would lie within the mass of the sun. In proportion as +the two bodies might approach each other in size, the centre of +gravity would come the nearer to the middle point in a line connecting +the two spheres. In this condition of a sun with a single planet, +whatever were the relative size of sun and planet, the orbits which +they traverse would be circular. In this state of affairs it should be +noted that each of the two bodies would have its plane of rotation +permanently in the same position. Even if the spheres were more or +less flattened about the poles of their axes, as is the case with all +the planets which we have been able carefully to measure, as well as +with the sun, provided the axes of rotation were precisely parallel to +each other, the mutual attraction of the masses would cause no +disturbance of the spheres. The same would be the case if the polar +axis of one sphere stood precisely at right angles to that of the +other. If, however, the spheres were somewhat flattened at the poles, +and the axes inclined to each other, then the pull of one mass on the +other would cause the polar axes to keep up a constant movement which +is called nutation, or nodding. + +The reason why this nodding movement of the polar axes would occur +when these lines were inclined to each other is not difficult to see +if we remember that the attraction of masses upon each other is +inversely as the square of the distance; each sphere, pulling on the +equatorial bulging of the other, pulls most effectively on the part of +it which is nearest, and tends to draw it down toward its centre. The +result is that the axes of the attracted spheres are given a wobbling +movement, such as we may note in the spinning top, though in the toy +the cause of the motion is not that which we are considering. + +If, now, in that excellent field for the experiment we are essaying, +the mind's eye, we add a second planet outside of the single sphere +which we have so far supposed to journey about the sun, or rather +about the common centre of gravity, we perceive at once that we have +introduced an element which leads to a complication of much +importance. The new sphere would, of course, pull upon the others in +the measure of its gravitative value--i.e., its weight. The centre of +gravity of the system would now be determined not by two distinct +bodies, but by three. If we conceive the second planet to journey +around the sun at such a rate that a straight line always connected +the centres of the three orbs, then the only effect on their +gravitative centre would be to draw the first-mentioned planet a +little farther away from the centre of the sun; but in our own solar +system, and probably in all others, this supposition is inadmissible, +because the planets have longer journeys to go and also move slower, +the farther they are from the sun. Thus Mercury completes the circle +of its year in eighty-eight of our days, while the outermost planet +requires sixty thousand days (more than one hundred and sixty-four +years) for the same task. The result is not only that the centre of +gravity of the system is somewhat displaced--itself a matter of no +great account--but also that the orbit of the original planet ceases +to be circled and becomes elliptical, and this for the evident reason +that the sphere will be drawn somewhat away from the sun when the +second planet happens to lie in the part of its orbit immediately +outside of its position, in which case the pull is away from the solar +centre; while, on the other hand, when the new planet was on the other +side of the sun, its pull would serve to intensify the attraction +which drew the first sphere toward the centre of gravity. As the +pulling action of the three bodies upon each other, as well as upon +their equatorial protuberances, would vary with every change in their +relative position, however slight, the variations in the form of their +orbits, even if the spheres were but three in number, would be very +important. The consequences of these perturbations will appear in the +sequel. + +In our solar system, though there are but eight great planets, the +group of asteroids, and perhaps a score of satellites, the variety of +orbital and axial movement which is developed taxes the computing +genius of the ablest astronomer. The path which our earth follows +around the sun, though it may in general and for convenience be +described as a variable ellipse, is, in fact, a line of such +complication that if we should essay a diagram of it on the scale of +this page it would not be possible to represent any considerable part +of its deviations. These, in fact, would elude depiction, even if the +draughtsman had a sheet for his drawing as large as the orbit itself, +for every particle of matter in space, even if it be lodged beyond the +limits of the farthest stars revealed to us by the telescope, +exercises a certain attraction, which, however small, is effective on +the mass of the earth. Science has to render its conclusions in +general terms, and we can safely take them as such; but in this, as in +other instances, it is well to qualify our acceptance of the +statements by the memory that all things are infinitely more +complicated than we can possibly conceive or represent them to be. + +We have next to consider the rotations of the planetary spheres upon +their axes, together with the similar movement, or lack of it, in the +case of their satellites. This rotation, according to the nebular +hypothesis, may be explained by the movements which would set up in +the share of matter which was at first a ring of the solar nebula, and +which afterward gathered into the planetary aggregation. The way of it +may be briefly set forth as follows: Such a ring doubtless had a +diameter of some million miles; we readily perceive that the particles +of matter in the outer part of the belt would have a swifter movement +around the sun than those on the inside. When by some disturbance, as +possibly by the passage of a great meteoric body of a considerable +gravitative power, this ring was broken in two, the particles +composing it on either side would, because of their mutual attraction, +tend to draw away from the breach, widening that gap until the matter +of the broken ring was aggregated into a sphere of the star dust or +vapour. When the nebulous matter originally in the ring became +aggregated into a spherical form, it would, on account of the +different rates at which the particles were moving when they came +together, be the surer to fall in toward the centre, not in straight +lines, but in curves--in other words, the mass would necessarily take +on a movement of rotation essentially like that which we have +described in setting forth the nebular hypothesis. + +In the stages of concentration the planetary nebulae might well repeat +those through which the greater solar mass proceeded. If the volume of +the material were great, subordinate rings would be formed, which when +they broke and concentrated would constitute secondary planets or +satellites, such as our moon. For some reason as yet unknown the outer +planets--in fact, all those in the solar system except the two inner, +Venus and Mercury and the asteroids--formed such attendants. All these +satellite-forming rings have broken and concentrated except the inner +of Saturn, which remains as an intellectual treasure of the solar +system to show the history of its development. + +To the student who is not seeking the fulness of knowledge which +astronomy has to offer, but desires only to acquaint himself with the +more critical and important of the heavenly phenomena which help to +explain the earth, these features of planetary movement should prove +especially interesting for the reason that they shape the history of +the spheres. As we shall hereafter see, the machinery of the earth's +surface, all the life which it bears, its winds and rains--everything, +indeed, save the actions which go on in the depths of the sphere--is +determined by the heat and light which come from the sun. The +conditions under which this vivifying tide is received have their +origin in the planetary motion. If our earth's path around the centre +of the system was a perfect circle, and if its polar axis lay at right +angles to the plane of its journey, the share of light and heat which +would fall upon any one point on the sphere would be perfectly +uniform. There would be no variations in the length of day or night; +no changes in the seasons; the winds everywhere would blow with +exceeding steadiness--in fact, the present atmospheric confusion would +be reduced to something like order. From age to age, except so far as +the sun itself might vary in the amount of energy which it radiated, +or lands rose up into the air or sunk down toward the sea level, the +climate of each region would be perfectly stable. In the existing +conditions the influences bring about unending variety. First of all, +the inclined position of the polar axis causes the sun apparently to +move across the heavens, so that it comes in an overhead position once +or twice in the year in quite half the area of the lands and seas. +This apparent swaying to and fro of the sun, due to the inclination of +the axis of rotation, also affects the width of the climatal belts on +either side of the equator, so that all parts of the earth receive a +considerable share of the sun's influence. If the axis of the earth's +rotation were at right angles to the plane of its orbit, there would +be a narrow belt of high temperature about the equator, north and +south of which the heat would grade off until at about the parallels +of fifty degrees we should find a cold so considerable and uniform +that life would probably fade away; and from those parallels to the +poles the conditions would be those of permanent frost, and of days +which would darken into the enduring night or twilight in the realm +of the far north and south. Thus the wide habitability of the earth is +an effect arising from the inclination of its polar axis. + +[Illustration: Fig. 3.--Inclination of Planetary Orbits (from +Chambers).] + +As the most valuable impression which the student can receive from his +study of Nature is that sense of the order which has made possible all +life, including his own, it will be well for him to imagine, as he may +readily do, what would be the effect arising from changes in relations +of earth and sun. Bringing the earth's axis in imagination into a +position at right angles to the plane of the orbit, he will see that +the effect would be to intensify the equatorial heat, and to rob the +high latitudes of the share which they now have. On moving the axis +gradually to positions where it approaches the plane of the orbit, he +will note that each stage of the change widens the tropic belt. +Bringing the polar axis down to the plane of the orbit, one hemisphere +would receive unbroken sunshine, the other remaining in perpetual +darkness and cold. In this condition, in place of an equatorial line +we should have an equatorial point at the pole nearest the sun; thence +the temperatures would grade away to the present equator, beyond which +half the earth would be in more refrigerating condition than are the +poles at the present day. In considering the movements of our planet, +we shall see that no great changes in the position of the polar axis +can have taken place. On this account the suggested alterations of the +axis should not be taken as other than imaginary changes. + +It is easy to see that with a circular orbit and with an inclined axis +winter and summer would normally come always at the same point in the +orbit, and that these seasons would be of perfectly even length. But, +as we have before noted, the earth's path around the sun is in its +form greatly affected by the attractions which are exercised by the +neighbouring planets, principally by those great spheres which lie in +the realm without its orbit, Jupiter and Saturn. When these attracting +bodies, as is the case from time to time, though at long intervals, +are brought together somewhere near to that part of the solar system +in which the earth is moving around the sun, they draw our planet +toward them, and so make its path very elliptical. When, however, they +are so distributed that their pulling actions neutralize each other, +the orbit returns more nearly to a circular form. The range in its +eccentricity which can be brought about by these alterations is very +great. When the path is most nearly circular, the difference in the +major and minor axis may amount to as little as about five hundred +thousand miles, or about one one hundred and eighty-sixth of its +average diameter. When the variation is greatest the difference in +these measurements may be as much as near thirteen million miles, or +about one seventh of the mean width of the orbit. + +The first and most evident effect arising from these changes of the +orbit comes from the difference in the amount of heat which the earth +may receive according as it is nearer or farther from the sun. As in +the case of other fires, the nearer a body is to it the larger the +share of light and heat which it will receive. In an orbit made +elliptical by the planetary attraction the sun necessarily occupies +one of the foci of the ellipse. The result is, of course, that the +side of the earth which is toward the sun, while it is thus brought +the nearer to the luminary, receives more energy in the form of light +and heat than come to any part which is exposed when the spheres are +farther away from each other in the other part of the orbit. +Computations clearly show that the total amount of heat and the +attendant light which the earth receives in a year is not affected by +these changes in the form of its path. While it is true that it +receives heat more rapidly in the half of the ellipse which is nearest +the source of the inundation, it obtains less while it is farther +away, and these two variations just balance each other. + +Although the alterations in the eccentricity of its orbit do not vary +the annual supply of heat which the earth receives, they are capable +of changing the character of the seasons, and this in the way which we +will now endeavour to set forth, though we must do it at the cost of +considerable attention on the part of the reader, for the facts are +somewhat complicated. In the first place, we must note that the +ellipticity of the earth's orbit is not developed on fixed lines, but +is endlessly varied, as we can readily imagine it would be for the +reason that its form depends upon the wandering of the outer planetary +spheres which pull the earth about. The longer axis of the ellipse is +itself in constant motion in the direction in which the earth travels. +This movement is slow, and at an irregular rate. It is easy to see +that the effect of this action, which is called the revolution of the +apsides, or, as the word means, the movement of the poles of the +ellipse, is to bring the earth, when a given hemisphere is turned +toward the sun, sometimes in the part of the orbit which is nearest +the source of light and heat, and sometimes farther away. It may thus +well come about that at one time the summer season of a hemisphere +arrives when it is nearest the sun, so that the season, though hot, +will be very short, while at another time the same season will arrive +when the earth is farthest from the sun, and receives much less heat, +which would tend to make a long and relatively cool summer. The reason +for the difference in length of the seasons is to be found in the +relative swiftness of the earth's revolution when it is nearest the +sun, and the slowness when it is farther away. + +There is a further complication arising from that curious phenomenon +called the precession of the equinoxes, which has to be taken into +account before we can sufficiently comprehend the effect of the +varying eccentricity of the orbit on the earth's seasons. To +understand this feature of precession we should first note that it +means that each year the change from the winter to the summer--or, as +we phrase it, the passage of the equinoctial line--occurs a little +sooner than the year before. The cause of this is to be found in the +attraction which the heavenly bodies, practically altogether the moon, +exercises on the equatorial protuberance of the earth. We know that +the diameter of our sphere at the equator is, on the average, +something more than twenty-six miles greater than it is through the +poles. We know, furthermore, that the position of the moon in relation +to the earth is such that it causes the attraction on one half of this +protuberance to be greater than it is upon the other. We readily +perceive that this action will cause the polar axis to make a certain +revolution, or, what comes to the same thing, that the plane of the +equator will constantly be altering its position. Now, as the +equinoctial points in the orbit depend for their position upon the +attitude of the equatorial plane, we can conceive that the effect is a +change in position of the place in that orbit where summer and winter +begin. The actual result is to bring the seasonal points backward, +step by step, through the orbit in a regular measure until in +twenty-two thousand five hundred years they return to the place where +they were before. This cycle of change was of old called the Annus +Magnus, or great year. + +If the earth's orbit were an ellipse, the major axis of which remained +in the same position, we could readily reckon all the effects which +arise from the variations of the great year. But this ellipse is ever +changing in form, and in the measure of its departure from a circle +the effects on the seasons distributed over a great period of time are +exceedingly irregular. Now and then, at intervals of hundreds of +thousands or millions of years, the orbit becomes very elliptical; +then again for long periods it may in form approach a circle. When in +the state of extreme ellipticity, the precession of the equinoxes will +cause the hemispheres in turn each to have their winter and summer +alternately near and far from the sun. It is easily seen that when the +summer season comes to a hemisphere in the part of the orbit which is +then nearest the sun the period will be very hot. When the summer +came farthest from the sun that part of the year would have the +temperature mitigated by its removal to a greater distance from the +source of heat. A corresponding effect would be produced in the winter +season. As long as the orbit remained eccentric the tendency would be +to give alternately intense seasons to each hemisphere through periods +of about twelve thousand years, the other hemisphere having at the +same time a relatively slight variation in the summer and winter. + +At first sight it may seem to the reader that these studies we have +just been making in matters concerning the shape of the orbit and the +attendant circumstances which regulate the seasons were of no very +great consequence; but, in the opinion of some students of climate, we +are to look to these processes for an explanation of certain climatal +changes on the earth, including the Glacial periods, accidents which +have had the utmost importance in the history of man, as well as of +all the other life of the planet. + +It is now time to give some account as to what is known concerning the +general conditions of the solar bodies--the planets and satellites of +our own celestial group. For our purpose we need attend only to the +general physical state of these orbs so far as it is known to us by +the studies of astronomers. The nearest planet to the sun is Mercury. +This little sphere, less than half the diameter of our earth, is so +close to the sun that even when most favourably placed for observation +it is visible for but a few minutes before sunrise and after sunset. +Although it may without much difficulty be found by the ordinary eye, +very few people have ever seen it. To the telescope when it is in the +_full moon_ state it appears as a brilliant disk; it is held by most +astronomers that the surface which we see is made up altogether of +clouds, but this, as most else that has been stated concerning this +planet, is doubtful. The sphere is so near to the sun that if it were +possessed of water it would inevitably bear an atmosphere full of +vapour. Under any conceivable conditions of a planet placed as +Mercury is, provided it had an atmosphere to retain the heat, its +temperature would necessarily be very high. Life as we know it could +not well exist upon such a sphere. + +Next beyond Mercury is Venus, a sphere only a little less in diameter +than the earth. Of this sphere we know more than we do of Mercury, for +the reason that it is farther from the sun and so appears in the +darkened sky. Most astronomers hold that the surface of this planet +apparently is almost completely and continually hidden from us by what +appears to be a dense cloud envelope, through which from time to time +certain spots appear of a dark colour. These, it is claimed, retain +their place in a permanent way; it is, indeed, by observing them that +the rotation period of the planet has, according to some observers, +been determined. It therefore seems likely that these spots are the +summits of mountains, which, like many of our own earth, rise above +the cloud level. + +Recent observations on Venus made by Mr. Percival Lowell appear to +show that the previous determinations of the rotation of that planet, +as well as regards its cloud wrap, are in error. According to these +observations, the sphere moves about the sun, always keeping the same +side turned toward the solar centre, just as the moon does in its +motion around the earth. Moreover, Mr. Lowell has failed to discover +any traces of clouds upon the surface of the planet. As yet these +results have not been verified by the work of other astronomers; +resting, however, as they do on studies made with an excellent +telescope and in the very translucent and steady air of the Flagstaff +Station, they are more likely to be correct than those obtained by +other students. If it be true that Venus does not turn upon its axis, +such is likely to be the case also with the planet Mercury. + +Next in the series of the planets is our own earth. As the details of +this planet are to occupy us during nearly all the remainder of this +work, we shall for the present pass it by. + +Beyond the earth we pass first to the planet Mars, a sphere which has +already revealed to us much concerning its peculiarities of form and +physical state, and which is likely in the future to give more +information than we shall obtain from any other of our companions in +space, except perhaps the moon. Mars is not only nearer to us than any +other planet, but it is so placed that it receives the light of the +sun under favourable conditions for our vision. Moreover, its sky +appears to be generally almost cloudless, so that when in its orbital +course the sphere is nearest our earth it is under favourable +conditions for telescopic observation. At such times there is revealed +to the astronomer a surface which is covered with an amazing number of +shadings and markings which as yet have been incompletely interpreted. +The faint nature of these indications has led to very contradictory +statements as to their form; no two maps which have been drawn agree +except in their generalities. There is reason to believe that Mars has +an atmosphere; this is shown by the fact that in the appropriate +season the region about either pole is covered by a white coating, +presumably snow. This covering extends rather less far toward the +planet's equator than does the snow sheet on our continents. Taking +into account the colour of the coating, and the fact that it +disappears when the summer season comes to the hemisphere in which it +was formed, we are, in fact, forced to believe that the deposit is +frozen water, though it has been suggested that it may be frozen +carbonic acid. Taken in connection with what we have shortly to note +concerning the apparent seas of this sphere, the presumption is +overwhelmingly to the effect that Mars has seasons not unlike our own. + +The existence of snow on any sphere may safely be taken as evidence +that there is an atmosphere. In the case of Mars, this supposition is +borne out by the appearance of its surface. The ruddy light which it +sends back to us, and the appearance on the margin of the sphere, +which is somewhat dim, appears to indicate that its atmosphere is +dense. In fact, the existence of an atmosphere much denser than that +of our own earth appears to be demanded by the fact that the +temperatures are such as to permit the coming and going of snow. It is +well known that the temperature of any point on the earth, other +things being equal, is proportionate to the depth of atmosphere above +its surface. If Mars had no more air over its surface than has an +equal area of the earth, it would remain at a temperature so low that +such seasonal changes as we have observed could not take place. The +planet receives one third less heat than an equal area of the earth, +and its likeness to our own temperature, if such exists, is doubtless +brought about by the greater density of its atmosphere, that serves to +retain the heat which comes upon its surface. The manner in which this +is effected will be set forth in the study of the earth's atmosphere. + +[Illustration: Fig. 4.--Mars, August 27, 1892 (Guiot), the white patch +is the supposed Polar Snow Cap.] + +As is shown by the maps of Mars, the surface is occupied by shadings +which seem to indicate the existence of water and lands. Those +portions of the area which are taken to be land are very much divided +by what appear to be narrow seas. The general geographic conditions +differ much from those of our own sphere in that the parts of the +planet about the water level are not grouped in great continents, and +there are no large oceans. The only likeness to the conditions of our +earth which we can perceive is in a general pointing of the somewhat +triangular masses of what appears to be land toward one pole. As a +whole, the conditions of the Martial lands and seas as regards their +form, at least, is more like that of Europe than that of any other +part of the earth's surface. Europe in the early Tertiary times had a +configuration even more like that of Mars than it exhibits at present, +for in that period the land was very much more divided than it now is. + +If the lands of Mars are framed as are those of our own earth, there +should be ridges of mountains constituting what we may term the +backbones of the continent. As yet such have not been discerned, which +may be due to the fact that they have not been carefully looked for. +The only peculiar physical features which have as yet been discerned +on the lands of Mars are certain long, straight, rather narrow +crevicelike openings, which have received the name of "canals." These +features are very indistinct, and are just on the limit of visibility. +As yet they have been carefully observed by but few students, so that +their features are not yet well recorded; as far as we know them, +these fissures have no likeness in the existing conditions of our +earth. It is difficult to understand how they are formed or preserved +on a surface which is evidently subjected to rainfalls. + +It will require much more efficient telescopes than we now have before +it will be possible to begin any satisfactory study on the geography +of this marvellous planet. We can not hope as yet to obtain any +indications as to the details of its structure; we can not see closely +enough to determine whether rivers exist, or whether there is a +coating which we may interpret as vegetation, changing its hues in the +different seasons of the year. An advance in our instruments of +research during the coming century, if made with the same speed as +during the last, will perhaps enable us to interpret the nature of +this neighbour, and thereby to extend the conception of planetary +histories which we derive from our own earth. + +[Illustration: Fig. 5.--Comparative Sizes of the Planets (Chambers).] + +Beyond Mars we find one of the most singular features of our solar +system in a group of small planetary bodies, the number of which now +known amounts to some two hundred, and the total may be far greater. +These bodies are evidently all small; it is doubtful if the largest is +three hundred and the smaller more than twenty miles in diameter. So +far as it has been determined by the effect of their aggregate mass in +attracting the other spheres, they would, if put together, make a +sphere far less in diameter than our earth, perhaps not more than five +hundred miles through. The forms of these asteroids is as yet unknown; +we therefore can not determine whether their shapes are spheroidal, as +are those of the other planets, or whether they are angular bits like +the meteorites. We are thus not in a position to conjecture whether +their independence began when the nebulous matter of the ring to which +they belonged was in process of consolidation, or whether, after the +aggregation of the sphere was accomplished, and the matter solidified, +the mass was broken into bits in some way which we can not yet +conceive. It has been conjectured that such a solid sphere might have +been driven asunder by a collision with some wandering celestial body; +but all we can conceive of such actions leads us to suppose that a +blow of this nature would tend to melt or convert materials subjected +to it into the state of vapour, rather than to drive them asunder in +the manner of an explosion. + +The four planets which lie beyond the asteroids give us relatively +little information concerning their physical condition, though they +afford a wide field for the philosophic imagination. From this point +of view the reader is advised to consult the writings of the late R.A. +Proctor, who has brought to the task of interpreting the planetary +conditions the skill of a well-trained astronomer and a remarkable +constructive imagination. + +The planet Jupiter, by far the largest of the children of the sun, +appears to be still in a state where its internal heat has not so far +escaped that the surface has cooled down in the manner of our earth. +What appear to be good observations show that the equatorial part of +its area, at least, still glows from its own heat. The sphere is +cloud-wrapped, but it is doubtful whether the envelope be of watery +vapour; it is, indeed, quite possible that besides such vapour it may +contain some part of the many substances which occupy the atmosphere +of the sun. If the Jovian sphere were no larger than the earth, it +would, on account of its greater age, long ago have parted with its +heat; but on account of its great size it has been able, +notwithstanding its antiquity, to retain a measure of temperature +which has long since passed away from our earth. + +In the case of Saturn, the cloud bands are somewhat less visible than +on Jupiter, but there is reason to suppose in this, as in the +last-named planet, that we do not behold the more solid surface of the +sphere, but see only a cloud wrap, which is probably due rather to the +heat of the sphere itself than to that which comes to it from the sun. +At the distance of Saturn from the centre of the solar system a given +area of surface receives less than one ninetieth of the sun's heat as +compared with the earth; therefore we can not conceive that any +density of the atmosphere whatever would suffice to hold in enough +temperature to produce ordinary clouds. Moreover, from time to time +bright spots appear on the surface of the planet, which must be due to +some form of eruptions from its interior. + +Beyond Saturn the two planets Uranus and Neptune, which occupy the +outer part of the solar system, are so remote that even our best +telescopes discern little more than their presence, and the fact that +they have attendant moons. + +From the point of view of astronomical science, the outermost planet +Neptune, of peculiar interest for the reason that it was, as we may +say, discovered by computation. Astronomers had for many years +remarked the fact that the next inner planetary sphere exhibited +peculiarities in its orbit which could only be accounted for on the +supposition that it was subjected to the attraction of another +wandering body which had escaped observation. By skilful computation +the place in the heavens in which this disturbing element lay was so +accurately determined that when the telescope was turned to the given +field a brief study revealed the planet. Nothing else in the history +of the science of astronomy, unless it be the computation of eclipses, +so clearly and popularly shows the accuracy of the methods by which +the work of that science may be done. + +As we shall see hereafter, in the chapters which are devoted to +terrestrial phenomena, the physical condition of the sun determines +the course of all the more important events which take place on the +surface of the earth. It is therefore fit that in this preliminary +study of the celestial bodies, which is especially designed to make +the earth more interpretable to us, we should give a somewhat special +attention to what is known under the title of "Solar Physics." + +The reader has already been told that the sun is one of many million +similar bodies which exist in space, and, furthermore, that these +aggregations of matter have been developed from an original nebulous +condition. The facts indicate that the natural history of the sun, as +well as that of its attendant spheres, exhibits three momentous +stages: First, that of vapour; second, that of igneous fluidity; +third, that in which the sphere is so far congealed that it becomes +dark. Neither of these states is sharply separated from the other; a +mass may be partly nebulous and partly fluid; even when it has been +converted into fluid, or possibly into the solid state, it may still +retain on the exterior some share of its original vaporous condition. +In our sun the concentration has long since passed beyond the limits +of the nebulous state; the last of the successively developed rings +has broken, and has formed itself into the smallest of the planets, +which by its distance from the sun seems to indicate that the process +of division by rings long ago attained in our solar system its end, +the remainder of its nebulous material concentrating on its centre +without sign of any remaining tendency to produce these planet-making +circles. + + + THE CONSTITUTION OF THE SUN. + +Before the use of the telescope in astronomical work, which was begun +by the illustrious Galileo in 1608, astronomers were unable to +approach the problem of the structure of the sun. They could discern +no more than can be seen by any one who looks at the great sphere +through a bit of smoked glass, as we know this reveals a disklike body +of very uniform appearance. The only variation in this simple aspect +occurs at the time of a total eclipse, when for a minute or two the +moon hides the whole body of the sun. On such occasions even the +unaided eye can see that there is about the sphere a broad, rather +bright field, of an aspect like a very thin cloud or fog, which rises +in streamer like projections at points to a quarter of a million miles +or more above the surface of the sphere. The appearance of this +shining field, which is called the corona, reminds one of the aurora +which glows in the region about either pole of the earth. + +One of the first results of the invention of the telescope was the +revelation of the curious dark objects on the sun's disk, known by the +name of spots from the time of their discovery, or, at least, from the +time when it was clearly perceived that they were not planets, but +really on the solar body. The interest in the constitution of the +sphere has increased during the last fifty years. This interest has +rapidly grown until at the present time a vast body of learning has +been gathered for the solution of the many problems concerning the +centre of our system. As yet there is great divergence in the views of +astronomers as to the interpretation of their observations, but +certain points of great general interest have been tolerably well +determined. These may be briefly set forth by an account of what would +meet the eye if an observer were able to pass from the surface of the +earth to the central part of the sun. + +[Illustration: _Lava stream, in Hawaiian Islands, flowing into the +sea. Note the "ropy" character of the half-frozen rock on the sides of +the nearest rivulet of the lava._] + +In passing from the earth to a point about a quarter of a million +miles from the sun's surface--a distance about that of the moon from +our sphere--the observer would traverse the uniformly empty spaces of +the heavens, where, but for the rare chance of a passing meteorite or +comet, there would be nothing that we term matter. Arriving at a point +some two or three hundred thousand miles from the body of the sun, he +would enter the realm of the corona; here he would find scattered +particles of matter, the bits so far apart that there would perhaps be +not more than one or two in the cubic mile; yet, as they would glow +intensely in the central light, they would be sufficient to give the +illumination which is visible in an eclipse. These particles are most +likely driven up from the sun by some electrical action, and are +constantly in motion, much as are the streamers of the aurora. + +Below the corona and sharply separated from it the observer finds +another body of very dense vapour, which is termed the chromosphere, +and which has been regarded as the atmosphere of the sun. This layer +is probably several thousand miles thick. From the manner in which it +moves, in the way the air of our own planet does in great storms, it +is not easy to believe that it is a fluid, yet its sharply defined +upper surface leads us to suppose that it can not well be a mere mass +of vapour. The spectroscope shows us that this chromosphere contains +in the state of vapour a number of metallic substances, such as iron +and magnesium. To an observer who could behold this envelope of the +sun from the distance at which we see the moon, the spectacle would be +more magnificent than the imagination, guided by the sight of all the +relatively trifling fractures of our earth, can possibly conceive. +From the surface of the fiery sea vast uprushes of heated matter rise +to the height of two or three hundred thousand miles, and then fall +back upon its surface. These jets of heated matter have the aspect of +flames, but they would not be such in fact, for the materials are not +burning, but merely kept at a high temperature by the heat of the +great sphere beneath. They spring up with such energy that they at +times move with a speed of one hundred and fifty miles a second, or at +a rate which is attained by no other matter in the visible universe, +except that strange, wandering star known to astronomers as +"Grombridge, 1830," which is traversing the firmament with a speed of +not less than two hundred miles a second. + +Below the chromosphere is the photosphere, the lower envelope of the +sun, if it be not indeed the body of the sphere itself; from this +comes the light and heat of the mass. This, too, can not well be a +firm-set mass, for the reason that the spots appear to form in and +move over it. It may be regarded as an extremely dense mass of gas, so +weighed down by the vast attraction of the great sphere below it that +it is in effect a fluid. The near-at-hand observer would doubtless +find this photosphere, as it appears in the telescope, to be sharply +separated from the thinner and more vaporous envelopes--the +chromosphere and the corona--which are, indeed, so thin that they are +invisible even with the telescope, except when the full blaze of the +sun is cut off in a total eclipse. The fact that the photosphere, +except when broken by the so-called spots, lies like a great smooth +sea, with no parts which lie above the general line, shows that it has +a very different structure from the envelope which lies upon it. If +they were both vaporous, there would be a gradation between them. + +On the surface of the photosphere, almost altogether within thirty +degrees of the equator of the sun, a field corresponding approximately +to the tropical belt of the earth, there appear from time to time the +curious disturbances which are termed spots. These appear to be +uprushes of matter in the gaseous state, the upward movement being +upon the margins of the field and a downward motion taking place in +the middle of the irregular opening, which is darkened in its central +part, thus giving it, when seen by an ordinary telescope, the aspect +of a black patch on the glowing surface. These spots, which are from +some hundred to some thousand miles in diameter, may endure for +months before they fade away. It is clear that they are most abundant +at intervals of about eleven years, the last period of abundance being +in 1893. The next to come may thus be expected in 1904. In the times +of least spotting more than half the days of a year may pass without +the surface of the photosphere being broken, while in periods of +plenty no day in the year is likely to fail to show them. + +[Illustration: Fig. 6.--Ordinary Sun-spot, June 22, 1885.] + +It is doubtful if the closest seeing would reveal the cause of the +solar spots. The studies of the physicists who have devoted the most +skill to the matter show little more than that they are tumults in the +photosphere, attended by an uprush of vapours, in which iron and other +metals exist; but whether these movements are due to outbreaks from +the deeper parts of the sun or to some action like the whirling storms +of the earth's atmosphere is uncertain. It is also uncertain what +effect these convulsions of the sun have on the amount of the heat and +light which is poured forth from the orb. The common opinion that the +sun-spot years are the hottest is not yet fully verified. + +Below the photosphere lies the vast unknown mass of the unseen solar +realm. It was at one time supposed that the dark colour of the spots +was due to the fact that the photosphere was broken through in those +spaces, and that we looked down through them upon the surface of the +slightly illuminated central part of the sphere. This view is +untenable, and in its place we have to assume that for the eight +hundred and sixty thousand miles of its diameter the sun is composed +of matter such as is found in our earth, but throughout in a state of +heat which vastly exceeds that known on or in our planet. Owing to its +heat, this matter is possibly not in either the solid or the fluid +state, but in that of very compressed gases, which are kept from +becoming solid or even fluid by the very high temperature which exists +in them. This view is apparently supported by the fact that, while the +pressure upon its matter is twenty-seven times greater in the sun than +it is in the earth, the weight of the whole mass is less than we +should expect under these conditions. + +As for the temperature of the sun, we only know that it is hot enough +to turn the metals into gases in the manner in which this is done in a +strong electric arc, but no satisfactory method of reckoning the scale +of this heat has been devised. The probabilities are to the effect +that the heat is to be counted by the tens of thousands of degrees +Fahrenheit, and it may amount to hundreds of thousands; it has, +indeed, been reckoned as high as a million degrees. This vast +discharge is not due to any kind of burning action--i.e., to the +combustion of substances, as in a fire. It must be produced by the +gradual falling in of the materials, due to the gravitation of the +mass toward its centre, each particle converting its energy of +position into heat, as does the meteorite when it comes into the air. + +It is well to close this very imperfect account of the learning which +relates to the sun with a brief tabular statement showing the relative +masses of the several bodies of the solar system. It should be +understood that by mass is meant not the bulk of the object, but the +actual amount of matter in it as determined by the gravitative +attraction which it exercises on other celestial bodies. In this test +the sun is taken as the measure, and its mass is for convenience +reckoned at 1,000,000,000. + + + TABLE OF RELATIVE MASSES OF SUN AND PLANETS.[2] + +------------------------------------------------------------+ + | The sun 1,000,000,000 | + | Mercury 200 | + | Venus 2,353 | + | Earth 3,060 | + | Mars 339 | + | Asteroids ? | + | Saturn 285,580 | + | Jupiter 954,305 | + | Uranus 44,250 | + | Neptune 51,600 | + | Combined mass of the four inner planets 5,952 | + | Combined mass of all the planets 1,341,687 | + +------------------------------------------------------------+ + +[Footnote 2: See Newcomb's Popular Astronomy, p. 234. Harper Brothers, +New York.] + + +It thus appears that the mass of all the planets is about one seven +hundredth that of the sun. + +Those who wish to make a close study of celestial geography will do +well to procure the interesting set of diagrams prepared by the late +James Freeman Clarke, in which transparencies placed in a convenient +lantern show the grouping of the important stars in each +constellation. The advantage of this arrangement is that the little +maps can be consulted at night and in the open air in a very +convenient manner. After the student has learned the position of a +dozen of the constellations visible in the northern hemisphere, he can +rapidly advance his knowledge in the admirable method invented by Dr. +Clarke. + +Having learned the constellations, the student may well proceed to +find the several planets, and to trace them in their apparent path +across the fixed stars. It will be well for him here to gain if he can +the conception that their apparent movement is compounded of their +motion around the sun and that of our own sphere; that it would be +very different if our earth stood still in the heavens. At this stage +he may well begin to take in mind the evidence which the planetary +motion supplies that the earth really moves round the sun, and not +the sun and planets round the earth. This discovery was one of the +great feats of the human mind; it baffled the wits of the best men for +thousands of years. Therefore the inquirer who works over the evidence +is treading one of the famous paths by which his race climbed the +steeps of science. + +The student must not expect to find the evidence that the sun is the +centre of the solar system very easy to interpret; and yet any youth +of moderate curiosity, and that interest in the world about him which +is the foundation of scientific insight, can see through the matter. +He will best begin his inquiries by getting a clear notion of the fact +that the moon goes round the earth. This is the simplest case of +movements of this nature which he can see in the solar system. Noting +that the moon occupies a different place at a given hour in the +twenty-four, but is evidently at all times at about the same distance +from the earth, he readily perceives that it circles about our sphere. +This the people knew of old, but they made of it an evidence that the +sun also went around our sphere. Here, then, is the critical point. +Why does the sun not behave in the same manner as the moon? At this +stage of his inquiry the student best notes what takes place in the +motions of the planets between the earth and the sun. He observes that +those so-called inferior planets Mercury and Venus are never very far +away from the central body; that they appear to rise up from it, and +then to go back to it, and that they have phases like the moon. Now +and then Venus may be observed as a black spot crossing the disk of +the sun. A little consideration will show that on the theory that +bodies revolve round each other in the solar system these movements of +the inner planets can only be explained on the supposition that they +at least travel around the great central fire. Now, taking up the +outer planets, we observe that they occasionally appear very bright, +and that they are then at a place in the heavens where we see that +they are far from the solar centre. Gradually they move down toward +the sunset and disappear from view. Here, too, the movement, though +less clearly so, is best reconcilable with the idea that these bodies +travel in orbits, such as those which are traversed by the inner +planets. The wonder is that with these simple facts before them, and +with ample time to think the matter over, the early astronomers did +not learn the great truth about the solar system--namely, that the sun +is the centre about which the planets circled. Their difficulty lay +mainly in the fact that they did not conceive the earth as a sphere, +and even after they attained that conception they believed that our +globe was vastly larger than the planets, or even than the sun. This +misconception kept even the thoughtful Greeks, who knew that the earth +was spherical in form, from a clear notion as to the structure of our +system. It was not, indeed, until mathematical astronomy attained a +considerable advance, and men began to measure the distances in the +solar system, and until the Newtonian theory of gravitation was +developed, that the planetary orbits and the relation of the various +bodies in the solar system to each other could be perfectly discerned. + +Care has been taken in the above statements to give the student +indices which may assist him in working out for himself the evidence +which may properly lead a person, even without mathematical +considerations of a formal kind, to construct a theory as to the +relation of the planets to the sun. It is not likely that he can go +through all the steps of this argument at once, but it will be most +useful to him to ponder upon the problem, and gradually win his way to +a full understanding of it. With that purpose in mind, he should avoid +reading what astronomers have to say on the matter until he is +satisfied that he has done as much as he can with the matter on his +own account. He should, however, state his observations, and as far as +possible draw the results in his note-book in a diagrammatic form. He +should endeavour to see if the facts are reconcilable with any other +supposition than that the earth and the other planets move around the +sun. When he has done his task, he will have passed over one of the +most difficult roads which his predecessors had to traverse on their +way to an understanding of the heavens. Even if he fail he will have +helped himself to some large understandings. + +The student will find it useful to make a map of the heavens, or +rather make several representing their condition at different times in +the year. On this plot he should put down only the stars whose places +and names he has learned, but he should plot the position of the +planets at different times. In this way, though at first his efforts +will be very awkward, he will soon come to know the general geography +of the heavens. + +Although the possession or at least the use of a small astronomical +telescope is a great advantage to a student after he has made a +certain advance in his work, such an instrument is not at all +necessary, or, indeed, desirable at the outset of his studies. An +ordinary opera-glass, however, will help him in picking out the stars +in the constellations, in identifying the planets, and in getting a +better idea as to the form of the moon's surface--a matter which will +be treated in this work in connection with the structure of the earth. + + + + + CHAPTER IV. + + THE EARTH. + + +In beginning the study of the earth it is important that the student +should at once form the habit of keeping in mind the spherical form of +the planet. Many persons, while they may blindly accept the fact that +the earth is a sphere, do not think of it as having that form. Perhaps +the simplest way of securing the correct image of the shape is to +imagine how the earth would appear as seen from the moon. In its full +condition the moon is apt to appear as a disk. When it is new, and +also when in its waning stages it is visible in the daytime, the +spherical form is very apparent. Imagining himself on the surface of +the moon, the student can well perceive how the earth would appear as +a vast body in the heavens; its eight thousand miles of diameter, +about four times that of the satellite, would give an area sixteen +times the size which the moon presents to us. On this scale the +continents and oceans would appear very much more plain than do the +relatively slight irregularities on the lunar surface. + +With the terrestrial globe in hand, the student can readily construct +an image which will represent, at least in outline, the appearance +which the sphere he inhabits would present when seen from a distance +of about a quarter of a million miles away. The continent of +Europe-Asia would of itself appear larger than all the lunar surface +which is visible to us. Every continent and all the greater islands +would be clearly indicated. The snow covering which in the winter of +the northern hemisphere wraps so much of the land would be seen to +come and go in the changes of the seasons; even the permanent ice +about either pole, and the greater regions of glaciers, such as those +of the Alps and the Himalayas, would appear as brilliant patches of +white amid fields of darker hue. Even the changes in the aspect of the +vegetation which at one season clothes the wide land with a green +mantle, and at another assumes the dun hue of winter, would be, to the +unaided eye, very distinct. It is probable that all the greater rivers +would be traceable as lines of light across the relatively dark +surface of the continents. By such exercises of the constructive +imagination--indeed, in no other way--the student can acquire the +habit of considering the earth as a vast whole. From time to time as +he studies the earth from near by he should endeavour to assemble the +phenomena in the general way which we have indicated. + +The reader has doubtless already learned that the earth is a slightly +flattened sphere, having an average diameter of about eight thousand +miles, the average section at the equator being about twenty-six miles +greater than that from pole to pole. In a body of such large +proportions this difference in measurement appears not important; it +is, however, most significant, for it throws light upon the history of +the earth's mass. Computation shows that the measure of flattening at +the poles is just what would occur if the earth were or had been at +the time when it assumed its present form in a fluid condition. We +readily conceive that a soft body revolving in space, while all its +particles by gravitation tended to the centre, would in turning +around, as our earth does upon its axis, tend to bulge out in those +parts which were remote from the line upon which the turning took +place. Thus the flattening of our sphere at the poles corroborates the +opinion that its mass was once molten--in a word, that its ancient +history was such as the nebular theory suggests. + +Although we have for convenience termed the earth a flattened +spheroid, it is only such in a very general sense. It has an infinite +number of minor irregularities which it is the province of the +geographer to trace and that of the geologist to account for. In the +first place, its surface is occupied by a great array of ridges and +hollows. The larger of these, the oceans and continents, first deserve +our attention. The difference in altitude of the earth's surface from +the height of the continents to the deepest part of the sea is +probably between ten and eleven miles, thus amounting to about two +fifths of the polar flattening before noted. The average difference +between the ocean floor and the summits of the neighbouring continents +is probably rather less than four miles. It happens, most fortunately +for the history of the earth, that the water upon its surface fills +its great concavities on the average to about four fifths of their +total depth, leaving only about one fifth of the relief projecting +above the ocean level. We have termed this arrangement fortunate, for +it insures that rainfall visits almost all the land areas, and thereby +makes those realms fit for the uses of life. If the ocean had only +half its existing area, the lands would be so wide that only their +fringes would be fertile. If it were one fifth greater than it is, the +dry areas would be reduced to a few scattered islands. + +From all points of view the most important feature of the earth's +surface arises from its division into land and water areas, and this +for the reason that the physical and vital work of our sphere is +inevitably determined by this distribution. The shape of the seas and +lands is fixed by the positions at which the upper level of the great +water comes against the ridges which fret the earth's surface. These +elevations are so disposed that about two thirds of the hard mass is +at the present time covered with water, and only one third exposed to +the atmosphere. This proportion is inconstant. Owing to the endless +up-and-down goings of the earth's surface, the place of the shore +lines varies from year to year, and in the geological ages great +revolutions in the forms and relative area of water and land are +brought about. + +Noting the greater divisions of land and water as they are shown on a +globe, we readily perceive that those parts of the continental ridges +which rise above the sea level are mainly accumulated in the northern +hemisphere--in fact, far more than half the dry realm is in that part +of the world. We furthermore perceive that all the continents more or +less distinctly point to the southward; they are, in a word, +triangles, with their bases to the northward, and their apices, +usually rather acute, directed to the southward. This form is very +well indicated in three of the great lands, North and South America +and Africa; it is more indistinctly shown in Asia and in Australia. As +yet we do not clearly understand the reason why the continents are +triangular, why they point toward the south pole, or why they are +mainly accumulated in the northern hemisphere. As stated in the +chapter on astronomy, some trace of the triangular form appears in the +land masses of the planet Mars. There, too, these triangles appear to +point toward one pole. + +Besides the greater lands, the seas are fretted by a host of smaller +dry areas, termed islands. These, as inquiry has shown, are of two +very diverse natures. Near the continents, practically never more than +a thousand miles from their shores, we find isles, often of great +size, such as Madagascar, which in their structure are essentially +like the continents--that is, they are built in part or in whole of +non-volcanic rocks, sandstones, limestones, etc. In most cases these +islands, to which we may apply the term continental, have at some time +been connected with the neighbouring mainland, and afterward separated +from it by a depression of the surface which permitted the sea to flow +over the lowlands. Geologists have traced many cases where in the past +elevations which are now parts of a continent were once islands next +its shore. In the deeper seas far removed from the margins of the +continents the islands are made up of volcanic ejections of lava, +pumice, and dust, which has been thrown up from craters and fallen +around their margin or are formed of coral and other organic remains. + +Next after this general statement as to the division of sea and land +we should note the peculiarities which the earth's surface exhibits +where it is bathed by the air, and where it is covered by the water. +Beginning with the best-known region, that of the dry land, we observe +that the surface is normally made up of continuous slopes of varying +declivity, which lead down from the high points to the sea. Here and +there, though rarely, these slopes centre in a basin which is occupied +by a lake or a dead sea. On the deeper ocean floors, so far as we may +judge with the defective information which the plumb line gives us, +there is no such continuity in the downward sloping of the surface, +the area being cast into numerous basins, each of great extent. + +When we examine in some detail the shape of the land surface, we +readily perceive that the continuous down slopes are due to the +cutting action of rivers. In the basin of a stream the waters act to +wear away the original heights, filling them into the hollows, until +the whole area has a continuous down grade to the point where the +waters discharge into the ocean or perhaps into a lake. On the bottom +of the sea, except near the margin of the continent, where the floor +may in recent geological times have been elevated into the air, and +thus exposed to river action, there is no such agent working to +produce continuous down grades. + +Looking upon a map of a continent which shows the differences in +altitude of the land, we readily perceive that the area is rather +clearly divided into two kinds of surface, mountains and plains, each +kind being sharply distinguished from the other by many important +peculiarities. Mountains are characteristically made up of distinct, +more or less parallel ridges and valleys, which are grouped in very +elongated belts, which, in the case of the American Cordilleras, +extend from the Arctic to the Antarctic Circle. Only in rare instances +do we find mountains occupying an area which is not very distinctly +elongated, and in such cases the elevations are usually of no great +height. Plains, on the other hand, commonly occupy the larger part of +the continent, and are distributed around the flanks of the mountain +systems. There is no rule as to their shape; they normally grade away +from the bases of the mountains toward the sea, and are often +prolonged below the level of the water for a considerable distance +beyond the shore, forming what is commonly known as the continental +shelf or belt of shallows along the coast line. We will now consider +some details concerning the form and structure of mountains. + +In almost any mountain region a glance over the surface of the country +will give the reader a clew to the principal factor which has +determined the existence of these elevations. Wherever the bed rocks +are revealed he will recognise the fact that they have been much +disturbed. Almost everywhere the strata are turned at high angles; +often their slopes are steeper than those of house roofs, and not +infrequently they stand in attitudes where they appear vertical. Under +the surface of plains bedded rocks generally retain the nearly +horizontal position in which all such deposits are most likely to be +found. If the observer will attentively study the details of position +of these tilted rocks of mountainous districts, he will in most cases +be able to perceive that the beds have been flexed or folded in the +manner indicated by the diagram. Sometimes, though rarely, the tops of +these foldings or arches have been preserved, so that the nature of +the movement can be clearly discerned. More commonly the upper parts +of the upward-arching strata have been cut off by the action of the +decay-bringing forces--frost, flowing water, or creeping ice in +glaciers--so that only the downward pointing folds which were formed +in the mountain-making are well preserved, and these are almost +invariably hidden within the earth. + +[Illustration: Fig. 7.--Section of mountains. Rockbridge and Bath +counties, Va. (from Dana). The numbers indicate the several +formations.] + +By walking across any considerable mountain chain, as, for instance, +that of the Alleghanies, it is generally possible to trace a number of +these parallel up-and-down folds of the strata, so that we readily +perceive that the original beds had been packed together into a much +less space than they at first occupied. In some cases we could prove +that the shortening of the line has amounted to a hundred miles or +more--in other words, points on the plain lands on either side of the +mountain range which now exists may have been brought a hundred miles +or so nearer together than they were before the elevations were +produced. The reader can make for himself a convenient diagram showing +what occurred by pressing a number of leaves of this book so that the +sheets of paper are thrown into ridges and furrows. By this experiment +he also will see that the easiest way to account for such foldings as +we observe in mountains is by the supposition that some force residing +in the earth tends to shove the beds into a smaller space than they +originally occupied. Not only are the rocks composing the mountains +much folded, but they are often broken through after the manner of +masonry which has been subjected to earthquake shocks, or of ice which +has been strained by the expansion that affects it as it becomes +warmed before it is melted. In fact, many of our small lakes in New +England and in other countries of a long winter show in a miniature +way during times of thawing ice folds which much resemble mountain +arches. + +At first geologists were disposed to attribute all the phenomena of +mountain-folding to the progressive cooling of the earth. Although +this sphere has already lost a large part of the heat with which it +was in the beginning endowed, it is still very hot in its deeper +parts, as is shown by the phenomena of volcanoes. This internal heat, +which to the present day at the depth of a hundred miles below the +surface is probably greater than that of molten iron, is constantly +flowing away into space; probably enough of it goes away on the +average each day to melt a hundred cubic miles or more of ice, or, in +more scientific phrase, the amount of heat rendered latent by melting +that volume of frozen water. J.R. Meyer, an eminent physicist, +estimated the quantity of heat so escaping each day of the year to be +sufficient to melt two hundred and forty cubic miles of ice. The +effect of this loss of heat is constantly to shrink the volume of the +earth; it has, indeed, been estimated that the sphere on this account +contracts on the average to the amount of some inches each thousand +years. For the reason that almost all this heat goes from the depths +of the earth, the cool outer portion losing no considerable part of +it, the contraction that is brought about affects the interior +portions of the sphere alone. The inner mass constantly shrinking as +it loses heat, the outer, cold part is by its weight forced to settle +down, and can only accomplish this result by wrinkling. An analogous +action may be seen where an apple or a potato becomes dried; in this +case the hard outer rind is forced to wrinkle, because, losing no +water, it does not diminish in its extent, and can only accommodate +itself to the interior by a wrinkling process. In one case it is water +which escapes, in the other heat; but in both contraction of the part +which suffers the loss leads to the folding of the outside of the +spheroid. + +Although this loss of heat on the part of the earth accounts in some +measure for the development of mountains, it is not of itself +sufficient to explain the phenomena, and this for the reason that +mountains appear in no case to develop on the floors of the wide sea. +The average depth of the ocean is only fifteen thousand feet, while +there are hundreds, if not thousands, of mountain crests which exceed +that height above the sea. Therefore if mountains grew on the sea +floor as they do upon the land, there should be thousands of peaks +rising above the plain of the waters, while, in fact, all of the +islands except those near the shores of continents are of volcanic +origin--that is, are lands of totally different nature. + +Whenever a considerable mountain chain is formed, although the actual +folding of the beds is limited to the usually narrow field occupied by +these disturbances, the elevation takes place over a wide belt of +country on one or both sides of the range. Thus if we approach the +Rocky Mountains from the Mississippi Valley, we begin to mount up an +inclined plane from the time we pass westward from the Mississippi +River. The beds of rock as well as the surface rises gradually until +at the foot of the mountain; though the rocks are still without +foldings, they are at a height of four or five thousand feet above the +sea. It seems probable--indeed, we may say almost certain--that when +the crust is broken, as it is in mountain-building, by extensive folds +and faults, the matter which lies a few score miles below the crust +creeps in toward those fractures, and so lifts up the country on which +they lie. When we examine the forms of any of our continents, we find +that these elevated portions of the earth's crust appear to be made up +of mountains and the table-lands which fringe those elevations. There +is not, as some of our writers suppose, two different kinds of +elevation in our great lands--the continents and the mountains which +they bear--but one process of elevation by which the foldings and the +massive uplifts which constitute the table-lands are simultaneously +and by one process formed. + +Looking upon continents as the result of mountain growth, we may say +that here and there on the earth's crust these dislocations have +occurred in such association and of such magnitude that great areas +have been uplifted above the plain of the sea. In general, we find +these groups of elevations so arranged that they produce the +triangular form which is characteristic of the great lands. It will be +observed, for instance, that the form of North America is in general +determined by the position of the Appalachian and Cordilleran systems +on its eastern and western margins, though there are a number of +smaller chains, such as the Laurentians in Canada and the ice-covered +mountains of Greenland, which have a measure of influence in fixing +its shore lines. + +[Illustration: _Waterfall near Gadsden, Alabama. The upper shelf of +rock is a hard sandstone, the lower beds are soft shale. The +conditions are those of most waterfalls, such as Niagara._] + +The history of plains, as well as that of mountains, will have further +light thrown upon it when in the next chapter we come to consider the +effect of rain water on the land. We may here note the fact that the +level surfaces which are above the seashores are divisible into two +main groups--those which have been recently lifted above the sea +level, composed of materials laid down in the shallows next the shore, +and which have not yet shared in mountain-building disturbances, and +those which have been slightly tilted in the manner before indicated +in the case of the plains which border the Rocky Mountains on the +east. The great southern plain of eastern and southern United States, +extending from near New York to Mexico, is a good specimen of the +level lands common on all the continents which have recently emerged +from the sea. The table-lands on either side of the Mississippi +Valley, sloping from the Alleghanies and the Cordilleras, represent +the more ancient type of plain which has already shared in the +elevation which mountain-building brings about. In rarer cases plains +of small area are formed where mountains formerly existed by the +complete moving down of the original ridges. + +There is a common opinion that the continents are liable in the course +of the geologic ages to very great changes of position; that what is +now sea may give place to new great lands, and that those already +existing may utterly disappear. This opinion was indeed generally held +by geologists not more than thirty years ago. Further study of the +problem has shown us that while parts of each continent may at any +time be depressed beneath the sea, the whole of its surface rarely if +ever goes below the water level. Thus, in the case of North America, +we can readily note very great changes in its form since the land +began to rise above the water. But always, from that ancient day to +our own, some portion of the area has been above the level of the sea, +thus providing an ark of refuge for the land life when it was +disturbed by inundations. The strongest evidence in favour of the +opinion that the existing continents have endured for many million +years is found in the fact that each of the great lands preserves many +distinct groups of animals and plants which have descended from +ancient forms dwelling upon the same territory. If at any time the +relatively small continent of Australia had gone beneath the sea, all +of the curious pouched animals akin to the opossum and kangaroo which +abound in that country--creatures belonging in the ancient life of the +world--would have been overwhelmed. + +We have already noted the fact that the uplifting of mountains and of +the table-lands about them, which appears to have been the basis of +continental growth, has been due to strains in the rocks sufficiently +strong to disturb the beds. At each stage of the mountain-building +movement these compressive strains have had to contend with the very +great weight of the rocks which they had to move. These lands are not +to be regarded as firm set or rigid arches, but as highly elastic +structures, the shapes of which may be determined by any actions which +put on or take off burden. We see a proof of this fact from numerous +observations which geologists are now engaged in making. Thus during +the last ice epoch, when almost all the northern part of this +continent, as well as the northern part of Europe, was covered by an +ice sheet several thousand feet thick, the lands sank down under their +load, and to an extent roughly proportional to the depth of the icy +covering. While the northern regions were thus tilted down by the +weight which was upon them, the southern section of this land, the +region about the Gulf of Mexico, was elevated much above its present +level; it seems likely, indeed, that the peninsula of Florida rose to +the height of several hundred feet above its present shore line. After +the ice passed away the movements were reversed, the northern region +rising and the southern sinking down. These movements are attested by +the position of the old shore lines formed during the later stages of +the Glacial epoch. Thus around Lake Ontario, as well as the other +Great Lakes, the beaches which mark the higher positions of those +inland seas during the closing stages of the ice time, and which, of +course, were when formed horizontal, now rise to the northward at the +rate of from two to five feet for each mile of distance. Recent +studies by Mr. G.K. Gilbert show that this movement is still in +progress. + +Other evidence going to show the extent to which the movements of the +earth's crust are affected by the weight of materials are found in the +fact that wherever along the shores thick deposits of sediments are +accumulated the tendency of the region where they lie is gradually to +sink downward, so that strata having an aggregate thickness of ten +thousand feet or more may be accumulated in a sea which was always +shallow. The ocean floor, in general, is the part of the earth's +surface where strata are constantly being laid down. In the great +reservoir of the waters the _debris_ washed from the land, the dust +from volcanoes, and that from the stellar spaces, along with the vast +accumulation of organic remains, almost everywhere lead to the +steadfast accumulation of sedimentary deposits. On the other hand, the +realms of the surface above the ocean level are constantly being worn +away by the action of the rivers and glaciers, of the waves which beat +against the shores, and of the winds which blow over desert regions. +The result is that the lands are wearing down at the geologically +rapid average rate of somewhere about one foot in five thousand years. +All this heavy matter goes to the sea bottoms. Probably to this cause +we owe in part the fact that in the wrinklings of the crust due to the +contraction of the interior the lands exhibit a prevailing tendency to +uprise, while the ocean floors sink down. In this way the continents +are maintained above the level of the sea despite the powerful forces +which are constantly wearing their substance away, while the seas +remain deep, although they are continually being burdened with +imported materials. + +[Illustration: Fig. 8.--Diagram showing the effect of the position of +the fulcrum point in the movement of the land masses. In diagrams I +and II, the lines _a b_ represent the land before the movement, and +_a' b'_ its position after the movement; _s_, _s_, the position of the +shore line; _p_, _p_, the pivotal points; _l_, _s_, the sea line. In +diagram III, the curved line designates a shore; the line _a b_, +connecting the pivotal points _p_, _p_, is partly under the land and +partly under the sea.] + +It is easy to see that if the sea floors tend to sink downward, while +the continental lands uprise, the movements which take place may be +compared with those which occur in a lever about a fulcrum point. In +this case the sea end of the bar is descending and the land end +ascending. Now, it is evident that the fulcrum point may fall to the +seaward or to the landward of the shore; only by chance and here and +there would it lie exactly at the coast line. By reference to the +diagram (Fig. 8), it will be seen that, while the point of rotation is +just at the shore, a considerable movement may take place without +altering the position of the coast line. Where the point of no +movement is inland of the coast, the sea will gain on the continent; +where, however, the point is to seaward, beneath the water, the land +will gain on the ocean. In this way we can, in part at least, account +for the endless changes in the attitude of the land along the coastal +belt without having to suppose that the continents cease to rise or +the sea floors to sink downward. It is evident that the bar or section +of the rocks from the interior of the land to the bottoms of the seas +is not rigid; it is also probable that the matter in the depths of the +earth, which moves with the motions of this bar, would change the +position of the fulcrum point from time to time. Thus it may well come +about that our coast lines are swaying up and down in ceaseless +variation. + +In very recent geological times, probably since the beginning of the +last Glacial period, the region about the Dismal Swamp in Virginia has +swayed up and down through four alternating movements to the extent of +from fifty to one hundred feet. The coast of New Jersey is now sinking +at the rate of about two feet in a hundred years. The coast of New +England, though recently elevated to the extent of a hundred feet or +more, at a yet later time sank down, so that at some score of points +between New York and Eastport, Me., we find the remains of forests +with the roots of their trees still standing below high-tide mark in +positions where the trees could not have grown. Along all the marine +coasts of the world which have been carefully studied from this point +of view there are similar evidences of slight or great modern changes +in the level of the lands. At some points, particularly on the coast +of Alaska and along the coast of Peru, these uplifts of the land have +amounted to a thousand feet or more. In the peninsular district of +Scandinavia the swayings, sometimes up and sometimes down, which are +now going on have considerably changed the position of the shore lines +since the beginning of the historical period. + +There are other causes which serve to modify the shapes and sizes of +the continents which may best be considered in the sequel; for the +present we may pass from this subject with the statement that our +great lands are relatively permanent features; their forms change from +age to age, but they have remained for millions of years habitable to +the hosts of animals and plants which have adapted their life to the +conditions which these fields afford them. + + + + + CHAPTER V. + + THE ATMOSPHERE. + + +The firm-set portion of the earth, composed of materials which became +solid when the heat so far disappeared from the sphere that rocky +matter could pass from its previous fluid condition to the solid or +frozen state, is wrapped about by two great envelopes, the atmosphere +and the waters. Of these we shall first consider the lighter and more +universal air; in taking account of its peculiarities we shall have to +make some mention of the water with which it is greatly involved; +afterward we shall consider the structure and functions of that fluid. + +Atmospheric envelopes appear to be common features about the celestial +spheres. In the sun there is, as we have noted, a very deep envelope +of this sort which is in part composed of the elements which form our +own air; but, owing to the high temperature of the sphere, these are +commingled with many substances which in our earth--at least in its +outer parts--have entered in the solid state. Some of the planets, so +far as we can discern their conditions, seem also to have gaseous +wraps; this is certainly the case with the planet Mars, and even the +little we know of the other like spheres justifies the supposition +that Jupiter and Saturn, at least, have a like constitution. We may +regard an atmosphere, in a word, as representing a normal and +long-continued state in the development of the heavenly orbs. In only +one of these considerable bodies of the solar system, the moon, do we +find tolerably clear evidence that there is no atmosphere. + +The atmosphere of the earth is composed mainly of very volatile +elements, known as nitrogen and argon. This is commingled with oxygen, +also a volatile element. Into this mass a number of other substances +enter in varying but always relatively very small proportions. Of +these the most considerable are watery vapour and carbon dioxide; the +former of these rarely amounts to one per cent of the weight of the +air, considering the atmosphere as a whole, and the latter is never +more than a small fraction of one per cent in amount. As a whole, the +air envelope of the earth should be regarded as a mass of nitrogen and +argon, which only rarely, under the influence of conditions which +exist in the soil, enters into combinations with other elements by +which it assumes a solid form. The oxygen, though a permanent element +in the atmosphere, tends constantly to enter into combinations which +fix it temporarily or permanently in the earth, in which it forms, +indeed, in its combined state about one half the weight of all the +mineral substances we know. The carbon dioxide, or carbonic-acid gas, +as it is commonly termed, is a most important substance, as it affords +plants all that part of their bodies which disappear on burning. It is +constantly returned to the atmosphere by the decay of organic matter, +as well as by volcanic action. + +In addition to the above-noted materials composing the air, all of +which are imperatively necessary to the wonderful work accomplished by +that envelope, we find a host of other substances which are +accidentally, variably, and always in small quantities contained in +this realm. Thus near the seashores, and indeed for a considerable +distance into the continent, we find the air contains a certain amount +of salt so finely divided that it floats in the atmosphere. So, too, +we find the air, even on the mountain tops amid eternal snows, charged +with small particles of dust, which, though not evident to the +unassisted eye, become at once visible when we permit a slender ray of +light to enter a dark chamber. + +It is commonly asserted that the atmosphere does not effectively +extend above the height of forty-five miles; we know that it is +densest on the surface of the earth, the most so in those depressions +which lie below the level of the sea. This is proved to us by the +weight which the air imposes upon the mercury at the open end of a +barometric tube. If we could deepen these cavities to the extent of a +thousand miles, the pressure would become so great that if the pit +were kept free from the heat of the earth the gaseous materials would +become liquefied. Upward from the earth's surface at the sea level the +atoms and molecules of the air become farther apart until, at the +height of somewhere between forty and fifty miles, the quantity of +them contained in the ether is so small that we can trace little +effect from them on the rays of light which at lower levels are +somewhat bent by their action. At yet higher levels, however, meteors +appear to inflame by friction against the particles of air, and even +at the height of eighty miles very faint clouds have at times been +discerned, which are possibly composed of volcanic dust floating in +the very rarefied medium, such as must exist at this great elevation. + +The air not only exists in the region where we distinctly recognise +it; it also occupies the waters and the under earth. In the waters it +occurs as a mechanical mixture which is brought about as the rain +forms and falls in the air, as the streams flow to the sea, and as the +waves roll over the deep and beat against the shores. In the realm of +the waters, as well as on the land, the air is necessary for the +maintenance of all animal forms; but for its presence such life would +vanish from the earth. + +Owing to certain peculiarities in its constitution, the atmosphere of +our earth, and that doubtless of myriad other spheres, serves as a +medium of communication between different regions. It is, as we know, +in ceaseless motion at rates which may vary from the speed in the +greatest tempests, which may move at the rate of somewhere a hundred +and fifty miles an hour, to the very slow movements which occur in +caverns, where the transfer is sometimes effected at an almost +microscopic rate in the space of a day. The motion of the atmosphere +is brought about by the action of heat here and there, and in a +trifling way, by the heat from the interior of the earth escaping +through hot springs or volcanoes, but almost altogether by the heat of +the sun. If we can imagine the earth cut off from the solar radiation, +the air would cease to move. We often note how the variable winds fall +away in the nighttime. Those who in seeking for the North Pole have +spent winters in the long-continued dark of that region have noted +that the winds almost cease to blow, the air being disturbed only when +a storm originated in the sunlit realm forced its way into the +circumpolar darkness. + +The sun's heat does not directly disturb the atmosphere; if we could +take the solid sphere of the world away, leaving the air, the rays +would go straight through, and there would be no winds produced. This +is due to the fact that the air permits the direct rays of heat, such +as come from the sun, to pass through it with very slight resistance. +In an aerial globe such as we have imagined, the rays impinging upon +its surface would be slightly thrown out of their path as they are in +passing through a lens, but they would journey on in space without in +any considerable measure warming the mass. Coming, however, upon the +solid earth, the heat rays warm the materials on which they are +arrested, bringing them to a higher temperature than the air. Then +these heated materials radiate the energy into the air; it happens, +however, that this radiant heat can not journey back into space as +easily as it came in; therefore the particles of air next the surface +acquire a relatively high temperature. Thus a thermometer next the +ground may rise to over a hundred degrees Fahrenheit, while at the +same time the fleecy clouds which we may observe floating at the +height of five or six miles above the surface are composed of frozen +water. + +The effect of the heated air which acquires its temperature by +radiation from the earth's surface is to produce the winds. This it +brings about in a very simple manner, though the details of the +process have a certain complication. The best illustration of the mode +in which the winds are produced is obtained by watching what takes +place about an ordinary fire at the bottom of a chimney. As soon as +the fire is lit, we observe that the air about it, so far as it is +heated, tends upward, drawing the smoke with it. If the air in the +chimney be cold, it may not draw well at first; but in a few minutes +the draught is established, or, in other words, the heated lower air +breaks its way up the shaft, gradually pushing the cooler matter out +at the top. In still air we may observe the column from the flue +extending about the chimney-top, sometimes to the height of a hundred +feet or more before it is broken to pieces. It is well here to note +the fact that the energy of the draught in a chimney is, with a given +heat of fire and amount of air which is permitted to enter the shaft, +directly proportionate to the height; thus in very tall flues, between +two and three hundred feet high, which are sometimes constructed, the +uprush is at the speed of a gale. + +Whenever the air next the surface is so far heated that it may +overcome the inertia of the cooler air above, it forces its way up +through it in the general manner indicated in the chimney flue. When +such a place of uprush is established, the hot air next the surface +flows in all directions toward the shaft, joining the expedition to +the heights of the atmosphere. Owing to the conditions of the earth's +surface, which we shall now proceed to trace, these ascents of heated +air belong in two distinct classes--those which move upward through +more or less cylindrical chimneys in the atmosphere, shafts which are +impermanent, which vary in diameter from a few feet to fifty or +perhaps a hundred miles, and which move over the surface of the earth; +and another which consists of a broad, beltlike shaft in the +equatorial regions, which in a way girdles the earth, remains in +about the same place, continually endures, and has a width of hundreds +of miles. Of these two classes of uprushes we shall first consider the +greatest, which occurs in the central portions of the tropical realm. + +Under the equator, owing to the fact that the sun for a considerable +belt of land and sea maintains the earth at a high temperature, there +is a general updraught which began many million years ago, probably +before the origin of life, in the age when our atmosphere assumed its +present conditions. Into this region the cooler air from the north and +south necessarily flows, in part pressed in by the weight of the cold +air which overlies it, but aided in its motion by the fact that the +particles which ascend leave place for others to occupy. Over the +surfaces of the land within the tropical region this draught toward +what we may term the equatorial chimney is perturbed by the +irregularities of the surface and many local accidents. But on the +sea, where the conditions are uniform, the air moving toward the point +of ascent is marked in the trade winds, which blow with a steadfast +sweep down toward the equator. Many slight actions, such as the +movement of the hot and cold currents of the sea, the local air +movements from the lands or from detached islands, somewhat perturb +the trade winds, but they remain among the most permanent features in +this changeable world. It is doubtful if anything on this sphere +except the atoms and molecules of matter have varied as little as the +trade winds in the centre of the wide ocean. So steadfast and uniform +are they that it is said that the helm and sails of a ship may be set +near the west coast of South America and be left unchanged for a +voyage which will carry the navigator in their belt across the width +of the Pacific. + +Rising up from the earth in the tropical belt, the air attains the +height of several thousand feet; it then begins to curve off toward +the north and south, and at the height of somewhere about three to +five miles above the surface is again moving horizontally toward +either pole; attaining a distance on that journey, it gradually +settles down to the surface of the earth, and ceases to move toward +higher latitudes. If the earth did not revolve upon its axis the +course of these winds along the surface toward the equator, and in the +upper air back toward the poles, would be made in what we may call a +square manner--that is, the particles of air would move toward the +point where they begin to rise upward in due north and south lines, +according as they came from the southern or northern hemisphere, and +the upper currents or counter trades would retrace their paths also +parallel with the meridians or longitude lines. But because the earth +revolves from west to east, the course of the trade winds is oblique +to the equator, those in the northern hemisphere blowing from +northeast to southwest, those in the southern from southeast to +northwest. The way in which the motion of the earth affects the +direction of these currents is not difficult to understand. It is as +follows: + +Let us conceive a particle of air situated immediately over the +earth's polar axis. Such an atom would by the rotation of the sphere +accomplish no motion except, indeed, that it might turn round on its +own centre. It would acquire no velocity whatever by virtue of the +earth's movement. Then let us imagine the particle moving toward the +equator with the speed of an ordinary wind. At every step of its +journey toward lower latitudes it would come into regions having a +greater movement than those which it had just left. Owing to its +inertia, it would thus tend continually to lag behind the particles of +matter about it. It would thus fall off to the westward, and, in place +of moving due south, would in the northern hemisphere drift to the +southwest, and in the southern hemisphere toward the northwest. A good +illustration of this action may be obtained from an ordinary +turn-table such as is used about railway stations to reverse the +position of a locomotive. If the observer will stand in the centre of +such a table while it is being turned round he will perceive that his +body is not swayed to the right or left. If he will then try to walk +toward the periphery of the rotating disk, he will readily note that +it is very difficult, if not impossible, to walk along the radius of +the circle; he naturally falls behind in the movement, so that his +path is a curved line exactly such as is followed by the winds which +move toward the equator in the trades. If now he rests a moment on the +periphery of the table, so that his body acquires the velocity of the +disk at that point, and then endeavours to walk toward the centre, he +will find that again he can not go directly; his path deviates in the +opposite direction--in other words, the body continually going to a +place having a less rate of movement by virtue of the rotation of the +earth, on account of its momentum is ever moving faster than the +surface over which it passes. This experiment can readily be tried on +any small rotating disk, such as a potter's wheel, or by rolling a +marble or a shot from the centre to the circumference and from the +circumference to the centre. A little reflection will show the +inquirer how these illustrations clearly account for the oblique +though opposite sets of the trade winds in the upper and lower parts +of the air. + +The dominating effect of the tropical heat in controlling the +movements of the air currents extends, on the ocean surface, in +general about as far north and south as the parallels of forty +degrees, considerably exceeding the limits of the tropics, those lines +where the sun, because of the inclination of the earth's axis, at some +time of the year comes just overhead. Between these belts of trade +winds there is a strip or belt under the region where the atmosphere +is rising from the earth, in which the winds are irregular and have +little energy. This region of the "doldrums" or frequent calms is one +of much trouble to sailing ships on their voyages from one hemisphere +to another. In passing through it their sails are filled only by the +airs of local storms, or winds which make their way into that part of +the sea from the neighbouring continents. Beyond the trade-wind belt, +toward the poles, the movements of the atmosphere are dependent in +part on the counter trades which descend to the surface of the earth +in latitudes higher than that in which the surface or trade winds +flow. Thus along our Atlantic coast, and even in the body of the +continent, at times when the air is not controlled by some local +storm, the counter trade blows with considerable regularity. + +The effect of the trade and counter-trade movements of the air on the +distribution of temperature over the earth's surface is momentous. In +part their influence is due to the direct heat-carrying power of the +atmosphere; in larger measure it is brought about by the movement of +the ocean waters which they induce. Atmospheric air, when deprived of +the water which it ordinarily contains, has very little +heat-containing capacity. Practically nearly all the power of +conveying heat which it possesses is due to the vapour of water which +it contains. By virtue of this moisture the winds do a good deal to +transfer heat from the tropical or superheated portion of the earth's +surface to the circumpolar or underheated realms. At first, the +relatively cool air which journeys toward the equator along the +surface of the sea constantly gains in heat, and in that process takes +up more and more water, for precisely the same reason that causes +anything to dry more rapidly in air which has been warmed next a fire. +The result is that before it begins to ascend in the tropical +updraught, being much moisture-laden, the atmosphere stores a good +deal of heat. As it rises, rarefies, and cools, the moisture descends +in the torrential rains which ordinarily fall when the sun is nearly +vertical in the tropical belt. + +Here comes in a very interesting principle which is of importance in +understanding the nature of great storms, either the continuous storm +of the tropics or the local and irregular whirlings which occur in +various parts of the earth. When the moisture-laden air starts on its +upward journey from the earth it has, by virtue of the watery vapour +which it contains, a store of energy which becomes applied to +promoting the updraught. As it rises, the moisture in the air gathers +together or condenses, and in so doing parts with the heat which +caused it to evaporate from the ocean surface. For a given weight of +water, the amount of heat required to effect the evaporation is very +great; this we may roughly judge by observing what a continuous fire +is required to send a pint of water into the state of steam. This +energy, when it is released by the condensation of water into rain or +snow, becomes again heat, and tends somewhat, as does the fire in the +chimney, to accelerate the upward passage of the air. The result is +that the water which ascends in the equatorial updraught becomes what +we may term fuel to promote this important element in the earth's +aerial circulation. Trades and counter trades would doubtless exist +but for the efficiency of this updraught, which is caused by the +condensation of watery vapour, but the movement would be much less +than it is. + + + WHIRLING STORMS. + +In the region near the equator, or near the line of highest +temperature, which for various reasons does not exactly follow the +equator, there is, as we have noticed, a somewhat continuous uprushing +current where the air passes upward through an ascending chimney, +which in a way girdles the sea-covered part of the earth. In this +region the movements of the air are to a great extent under the +control of the great continuous updraught. As we go to the north and +south we enter realms where the air at the surface of the earth is, by +the heat which it acquires from contact with that surface, more or +less impelled upward; but there being no permanent updraught for its +escape, it from time to time breaks through the roof of cold air which +overlies it and makes a temporary channel of passage. Going polarward +from the equator, we first encounter these local and temporary +upcastings of the air near the margin of the tropical belt. In these +districts, at least over the warmer seas, during the time of the year +when it is midsummer, and in the regions where the trade winds are not +strong enough to sweep the warm and moisture-laden air down to the +equatorial belt, the upward tending strain of the atmosphere next the +earth often becomes so strong that the overlying air is displaced, +forming a channel through which the air swiftly passes. As the +moisture condenses in the way before noted, the energy set free serves +to accelerate the updraught, and a hurricane is begun. At first the +movement is small and of no great speed, but as the amount of air +tending upward is likely to be great, as is also the amount of +moisture which it contains, the aerial chimney is rapidly enlarged, +and the speed of the rising air increased. The atmosphere next the +surface of the sea flows in toward the channel of escape; its passage +is marked by winds which are blowing toward the centre. On the +periphery of the movement the particles move slowly, but as they win +their way toward the centre they travel with accelerating velocity. On +the principle which determines the whirling movement of the water +escaping through a hole in the bottom of a basin, the particles of the +air do not move on straight lines toward the centre, but journey in +spiral paths, at first along the surface, and then ascending. + +We have noted the fact that in a basin of water the direction of the +whirling is what we may term accidental--that is, dependent on +conditions so slight that they elude our observation--but in +hurricanes a certain fact determines in an arbitrary way the direction +in which the spin shall take place. As soon as such a movement of the +air attains any considerable diameter, although in its beginning it +may have spun in a direction brought about by local accidents, it will +be affected by the diverse rates of travel, by virtue of the earth's +rotation, of the air on its equatorial and polar sides. On the +equatorial side this air is moving more rapidly than it is on the +polar side. By observing the water passing from a basin this +principle, with a few experiments, can be made plain. The result is to +cause these great whirlwinds of the hurricanes of higher latitudes to +whirl round from right to left in the northern hemisphere and in the +reverse way in the southern. The general system of the air currents +still further affects these, as other whirling storms, by driving +their centres or chimneys over the surface of the earth. The principle +on which this is done may be readily understood by observing how the +air shaft above a chimney, through which we may observe the smoke to +rise during a time of calm, is drawn off to one side by the slight +current which exists even when we feel no wind; it may also be +discerned in the little dust whirls which form in the streets on a +summer day when the air is not much disturbed. While they spin they +move on in the direction of the air drift. In this way a hurricane +originating in the Gulf of Mexico may gradually journey under the +influence of the counter trades across the Antilles, or over southern +Florida, and thence pursue a devious northerly course, generally near +the Atlantic coast and in the path of the Gulf Stream, until it has +travelled a thousand miles or more toward the North Atlantic. The +farther it goes northward the less effectively it is fed with warm and +moisture-laden air, the feebler its movement becomes, until at length +it is broken up by the variable winds which it encounters. + +A very interesting and, from the point of view of the navigator, +important peculiarity of these whirls is that at their centre there is +a calm, similar in origin and nature to the calm under the equator +between the trade-wind belts. Both these areas are in the field where +the air is ascending, and therefore at the surface of the earth does +not affect the sails of ships, though if men ever come to use flying +machines and sail through the tropics at a good height above the sea +it will be sensible enough. The difference between the doldrum of the +equator and that of the hurricane, besides their relative areas, is +that one is a belt and the other a disk. If the seafarer happens to +sail on a path which leads him through the hurricane centre, he will +first discern, as from the untroubled air and sea he approaches the +periphery of the storm, the horizon toward the disturbance beset by +troubled clouds, all moving in one direction. Entering beneath this +pall, he finds a steadily increasing wind, which in twenty miles of +sailing may, and in a hundred miles surely will, compel him to take in +all but his storm sails, and is likely to bring his ship into grave +peril. The most furious winds the mariner knows are those which he +encounters as he approaches the still centre. These trials are made +the more appalling by the fact that in the furious part of the whirl +the rain, condensing from the ascending air, falls in torrents, and +the electricity generated in the condensation gives rise to vivid +lightning. If the storm-beset ship can maintain her way, in a score or +two of miles of journey toward the centre, generally very quickly, it +passes into the calm disk, where the winds, blowing upward, cease to +be felt. In this area the ship is not out of danger, for the waves, +rolling in from the disturbed areas on either side, make a torment of +cross seas, where it is hard to control the movements of a sailing +vessel because the impulse of the winds is lost. Passing through this +disk of calm, the ship re-encounters in reverse order the furious +portion of the whirl, afterward the lessening winds, until it escapes +again into the airs which are not involved in the great torment. + +In the old days, before Dove's studies of storms had shown the laws of +hurricane movement, unhappy shipmasters were likely to be caught and +retained in hurricanes, and to battle with them for weeks until their +vessels were beaten to pieces. Now the "Sailing Directions," which are +the mariner's guide, enable him, from the direction of the winds and +the known laws of motion of the storm centre, to sail out of the +danger, so that in most cases he may escape calamity. It is otherwise +with the people who dwell upon the land over which these atmospheric +convulsions sweep. Fortunately, where these great whirlwinds trespass +on the continent, they quickly die out, because of the relative lack +of moisture which serves to stimulate the uprush which creates them. +Thus in their more violent forms hurricanes are only felt near the +sea, and generally on islands and peninsulas. There the hurricane +winds, by the swiftness of their movement, which often attains a speed +of a hundred miles or more, apply a great deal of energy to all +obstacles in their path. The pressure thus produced is only less +destructive than that which is brought about by the tornadoes, which +are next to be described. + +There is another effect from hurricanes which is even more destructive +to life than that caused by the direct action of the wind. In these +whirlings great differences in atmospheric pressure are brought about +in contiguous areas of sea. The result is a sudden elevation in the +level of one part of the water. These disturbances, where the shore +lands are low and thickly peopled, as is the case along the western +coast of the Bay of Bengal, may produce inundations which are terribly +destructive to life and property. They are known also in southern +Florida and along the islands of the Caribbean, but in that region are +not so often damaging to mankind. + +Fortunately, hurricanes are limited to a very small part of the +tropical district. They occur only in those regions, on the eastern +faces of tropical lands, where the general westerly set of the winds +favours the accumulation of great bodies of very warm, moist air next +the surface of the sea. The western portion of the Gulf of Mexico and +the Caribbean, the Bay of Bengal, and the southeastern portion of Asia +are especially liable to their visitations. They sometimes develop, +though with less fury, in other parts of the tropics. On the western +coast of South America and Africa, where the oceans are visited by the +dry land winds, and where the waters are cooled by currents setting +in from high latitudes, they are unknown. + +Only less in order of magnitude than the hurricanes are the circular +storms known as cyclones. These occur on the continents, especially +where they afford broad plains little interrupted by mountain ranges. +They are particularly well exhibited in that part of North America +north of Mexico and south of Hudson Bay. Like the hurricanes, they +appear to be due to the inrush of relatively warm air entering an +updraught which had been formed in the overlying, cooler portions of +the atmosphere. They are, however, much less energetic, and often of +greater size than the hurricane whirl. The lack of energy is probably +due to the comparative dryness of the air. The greater width of the +ascending column may perhaps be accounted for by the fact that, +originating at a considerable height above the sea, they have a less +thickness of air to break through, and so the upward setting column is +readily made broad. + +The cyclones of North America appear generally to originate in the +region of the Rocky Mountains, though it is probable that in some +instances, perhaps in many, the upward set of the air which begins the +storm originates in the ocean along the Pacific coast. They gather +energy as they descend the great sloping plain leading eastward from +the Rocky Mountains to the central portion of the great continental +valley. Thence they move on across the country to the Atlantic coast. +Not infrequently they continue on over the ocean to the European +continent. The eastward passage of the storm centre is due to the +prevailing eastward movement of the air in its upper part throughout +that portion of the northern hemisphere. Commonly they incline +somewhat to the northward of east in their journey. In all cases the +winds appear to blow spirally into the common storm centre. There is +the same doldrum area or calm field in the centre of the storm that we +note between the trade winds and in the middle of a hurricane disk, +though this area is less defined than in the other instances, and the +forward motion of the storm at a considerable speed is in most cases +characteristic of the disturbance. On the front of one of these storms +in North America the winds commonly begin in the northeast, thence +they veer by the east to the southwest. At this stage in the movement +the storm centre has passed by, the rainfall commonly ceases, and +cold, dry winds setting to the northwestward set in. This is caused by +the fact that the ascending air, having attained a height above the +earth, settles down behind the storm, forming an anticyclone or mass +of dry air, which presses against the retreating side of the great +whirlwind. + +In front of the storm the warm and generally moist relatively warm +air, pressing in toward the point of uprise and overlaid by the upper +cold air, is brought into a condition where it tends to form small +subordinate shafts up through which it whirls on the same principle, +but with far greater intensity than the main ascending column. The +reason for the violence of this movement is that the difference in +temperature of the air next the surface and that at the height of a +few thousand feet is great. As might be expected, these local +spinnings are most apt to occur in the season when the air next the +earth is relatively warm, and they are aptest to take place in the +half of the advancing front lying between the east and south, for the +reason that there the highest temperatures and the greatest humidity +are likely to coexist. In that part of the field, during the time when +the storm is advancing from the Rocky Mountains to the Atlantic, a +dozen or more of these spinning uprushes may be produced, though few +of them are likely to be of large size or of great intensity. + +The secondary storms of cyclones, such as are above noted, receive the +name of tornadoes. They are frequent and terrible visitations of the +country from northern Texas, Florida, and Alabama to about the line of +the Great Lakes; they are rarely developed in the region west of +central Kansas, and only occasionally do they exhibit much energy in +the region east of the plain-lands of the Ohio Valley. Although known +in other lands, they nowhere, so far as our observations go, exhibit +the paroxysmal intensity which they show in the central portion of the +North American continent. There the air which they affect acquires a +speed of movement and a fury of action unknown in any other +atmospheric disturbances, even in those of the hurricanes. + +The observer who has a chance to note from an advantageous position +the development of a tornado observes that in a tolerably still air, +or at least an air unaffected by violent winds--generally in what is +termed a "sultry" state of the atmosphere--the storm clouds in the +distance begin to form a kind of funnel-shaped dependence, which +gradually extends until it appears to touch the earth. As the clouds +are low, this downward-growing column probably in no case is observed +for the height of more than three or four thousand feet. As the funnel +descends, the clouds above and about it may be seen to take on a +whirling movement around the centre, and under favourable +circumstances an uprush of vapours may be noted in the centre of the +swaying shaft. As the whirl comes nearer, the roar of the disturbance, +which at a distance is often compared to the sound made by a threshing +machine or to that of distant musketry, increases in loudness until it +becomes overwhelming. When a storm such as this strikes a building, it +is not only likely to be razed by the force of the wind, but it may be +exploded, as by the action of gunpowder fired within its walls, +through the sudden expansion of the air which it contains. In the +centre of the column, although it rarely has a diameter of more than a +few hundred feet, the uprush is so swift that it makes a partial +vacuum. The air, striving to get into the space which it is eager to +occupy, is whirling about at such a rate that the centrifugal motion +which it thus acquires restrains its entrance. In this way there may +be, as the column rapidly moves by, a difference of pressure +amounting probably to what the mercury of a barometer would indicate +by four or five inches of fall. Unless the structure is small and its +walls strong, its roof and sides are apt to be blown apart by this +difference of pressure and the consequent expansion of the contained +air. In some cases where wooden buildings have withstood this curious +action the outer clapboards have been blown off by the expansion of +the small amount of air contained in the interspaces between that +covering and the lath and plaster within (see Fig. 9). + +[Illustration: Fig. 9.--Showing effect of expansion of air contained +in a hollow wall during the passage of the storm.] + +The blow of the air due to its rotative whirling has in several cases +proved sufficient to throw a heavy locomotive from the track of a +well-constructed railway. In all cases where it is intense it will +overturn the strongest trees. The ascending wind in the centre of the +column may sometimes lift the bodies of men and of animals, as well as +the branches and trunks of trees and the timber of houses, to the +height of hundreds of feet above the surface. One of the most striking +exhibitions of the upsucking action in a tornado is afforded by the +effect which it produces when it crosses a small sheet of water. In +certain cases where, in the Northwestern States of this country, the +path of the storm lay over the pool, the whole of the water from a +basin acres in extent has been entirely carried away, leaving the +surface, as described by an observer, apparently dry enough to plough. + +Fortunately for the interests of man, as well as those of the lower +organic life, the paths of these storms, or at least the portion of +their track where the violence of the air movement makes them very +destructive, often does not exceed five hundred feet in width, and is +rarely as great as half a mile in diameter. In most cases the length +of the journey of an individual tornado does not exceed thirty miles. +It rarely if ever amounts to twice that distance. + +In every regard except their small size and their violence these +tornadoes closely resemble hurricanes. There is the same broad disk of +air next the surface spirally revolving toward the ascending centre, +where its motion is rapidly changed from a horizontal to a vertical +direction. The energy of the uprush in both cases is increased by the +energy set free through the condensation of the water, which tends +further to heat and thus to expand the air. The smaller size of the +tornado may be accounted for by the fact that we have in their +originating conditions a relatively thin layer of warm, moist air next +the earth and a relatively very cold layer immediately overlying it. +Thus the tension which serves to start the movement is intense, though +the masses involved are not very great. The short life of a tornado +may be explained by the fact that, though it apparently tends to grow +in width and energy, the central spout is small, and is apt to be +broken by the movements of the atmosphere, which in the front of a +cyclone are in all cases irregular. + +On the warmer seas, but often beyond the limits of the tropics, +another class of spinning storms, known as waterspouts, may often be +observed. In general appearance these air whirls resemble tornadoes, +except that they are in all cases smaller than that group of +whirlings. As in the tornadoes, the waterspout begins with a funnel, +which descends from the sky to the surface of the sea. Up the tube +vapours may be seen ascending at great speed, the whole appearing like +a gigantic pillar of swiftly revolving smoke. When the whirl reaches +the water, it is said that the fluid leaps up into the tube in the +form of dense spray, an assertion which, in view of the fact of the +action of a tornado on a lake as before described, may well be +believed. Like the tornadoes and dust whirls, the life of a waterspout +appears to be brief. They rarely endure for more than a few minutes, +or journey over the sea for more than two or three miles before the +column appears to be broken by some swaying of the atmosphere. As +these peculiar storms are likely to damage ships, the old-fashioned +sailors were accustomed to fire at them with cannon. It has been +claimed that a shot would break the tube and end the little +convulsion. This, in view of the fact that they appear to be easily +broken up by relatively trifling air currents, may readily be +believed. The danger which these disturbances bring to ships is +probably not very serious. + +The special atmospheric conditions which bring about the formation of +waterspouts are not well known; they doubtless include, however, warm, +moist air next the surface of the sea and cold air above. Just why +these storms never attain greater size or endurance is not yet known. +These disturbances have been seen for centuries, but as yet they have +not been, in the scientific sense, observed. Their picturesqueness +attracts all beholders; it is interesting to note the fact that +perhaps the earliest description of their phenomena--one which takes +account in the scientific spirit of all the features which they +present--was written by the poet Camoens in the Lusiad, in which he +strangely mingles fancy and observation in his account of the great +voyage of Vasco da Gama. The poet even notes that the water which +falls when the spout is broken is not salt, but fresh--a point which +clearly proves that not much of the water which the tube contains is +derived from the sea. It is, in fact, watery vapour drawn from the air +next the surface of the ocean, and condensed in its ascent through the +tube. In this and other descriptions of Nature Camoens shows more of +the scientific spirit than any other poet of his time. He was in this +regard the first of modern writers to combine a spiritual admiration +for Nature with some sense of its scientific meaning. + +In treating of the atmosphere, meteorologists base their studies +largely on changes in the weight of that medium, which they determine +by barometric observations. In fact, the science of the air had its +beginning in Pascal's admirable observation on the changes in the +height of a column of mercury contained in a bent tube as he ascended +the volcanic peak known as Puy de Dome, in central France. As before +noted, it is to the disturbances in the weight of the air, brought +about mainly by variations in temperature, that we owe all its +currents, and it is upon these winds that the features we term climate +in largest measure depend. Every movement of the winds is not only +brought about by changes in the relative weight of the air at certain +points, but the winds themselves, owing to the momentum which the air +attains by them, serve to bring about alterations in the quantity of +air over different parts of the earth, which are marked most +distinctly by barometric variations. These changes are exceedingly +complicated; a full account of them would demand the space of this +volume. A few of the facts, however, should be presented here. In the +first place, we note that each day there is normally a range in the +pressure which causes the barometer to be at the lowest at about four +o'clock in the morning and four o'clock in the afternoon, and highest +at about ten o'clock in those divisions of the day. This change is +supposed to be due to the fact that the motes of dust in the +atmosphere in the night, becoming cooled, condense the water vapour +upon their surfaces, thus diminishing the volume of the air. When the +sun rises the water evaporated by the heat returns from these little +storehouses into the body of the atmosphere. Again in the evening the +condensation sets in; at the same time the air tends to drift in from +the region to the westward, where the sun is still high, toward the +field where the barometer has been thus lowered; the current gradually +attains a certain volume, and so brings about the rise of the +barometer about ten o'clock at night. + +In the winter time, particularly on the well-detached continent of +North America, we find a prevailing high barometer in the interior of +the country and a corresponding low state of pressure on the Atlantic +Ocean. In the summer season these conditions are on the whole +reversed. + +Under the tropics, in the doldrum belt, there is a zone of low +barometer connected to the ascending currents which take place along +that line. This is a continuous manifestation of the same action which +gives a large area of a disklike form in the centre or eye of the +hurricane and in the middle portion of the tornado's whirl. In +general, it may be said that the weight of the air is greatest in the +regions from which it is blowing toward the points of upward escape, +and least in and about those places where the superincumbent air is +rising through a temporary or permanent line of escape. In other +words, ascending air means generally a relatively low barometer, while +descending air is accompanied by greater pressure in the field upon +which it falls. + +In almost every part of the earth which is affected by a particular +physiography we find that the movements of the atmosphere next the +surface are qualified by the condition which it encounters. In fact, +if a person were possessed of all the knowledge which could be +obtained concerning winds, he could probably determine as by a map the +place where he might chance to find himself, provided he could extend +his observations over a term of years. In other words, the regimen of +the winds--at least those of a superficial nature--is almost as +characteristic of the field over which they go as is a map of the +country. Of these special winds a number of the more important have +been noted, only a few of which we can advert to. First among these +may well come the land and sea breezes which are remarked about all +islands which are not continuously swept by permanent winds. One of +the most characteristic instances of these alternate winds is perhaps +that afforded on the island of Jamaica. + +The island of Jamaica is so situated within the basin of the Caribbean +that it does not feel the full influence of the trades. It has a range +of high mountains through its middle part. In the daytime the surface +of the land, which has the sun overhead twice each year, and is always +exposed to nearly vertical radiation, becomes intensely hot, so that +an upcurrent is formed. The formation of this current is favoured by +the mountains, which apply a part of the heat at the height of about a +mile above the surface of the sea. This action is parallel to that we +notice when, in order to create a draught in the air of a chimney, we +put a torch some distance up above the fireplace, thus diminishing the +height of the column of air which has to be set in motion. It is +further shown by the fact that when miners sought to make an upcurrent +in a shaft, in order to lead pure air into the workings through other +openings, they found after much experience that it was better to have +the fire near the top of the shaft rather than at the bottom. + +The ascending current being induced up the mountain sides of Jamaica, +the air is forced in from the sea to the relatively free space. Before +noon the current, aided in its speed by a certain amount of the +condensation of the watery vapour before described, attains the +proportions of a strong wind. As the sun begins to sink, the earth's +surface pours forth its heat; the radiation being assisted by the +extended surfaces of the plants, cooling rapidly takes place. +Meanwhile the sea, because of the great heat-storing power of water, +is very little cooled, the ascent of the air ceases, the temporary +chimney with its updraught is replaced by a downward current, and the +winds blow from the land until the sun comes again to reverse the +current. In many cases these movements of the daily winds flowing into +and from islands induce a certain precipitation of moisture in the +form of rain. Generally, however, their effect is merely to ameliorate +the heat by bringing alternately currents from the relatively cool sea +and from the upper atmosphere to lessen the otherwise excessive +temperature of the fields which they traverse. + +Although characteristic sea and land winds are limited to regions +where the sun's heat is great, they are traceable even in high +latitudes during the periods of long-continued calm attended with +clear skies. Thus on the island of Martha's Vineyard, in +Massachusetts, the writer has noted, when the atmosphere was in such a +state, distinct night and day, or sea and land, breezes coming in +their regular alternation. During the night when these alternate winds +prevail the central portion of the island, at the distance of three +miles from the sea, is remarkably cold, the low temperature being due +to the descending air current. To the same physical cause may be +attributed the frequent insets of the sea winds toward midday along +the continental shores of various countries. Thus along the coast of +New England in the summer season a clear, still, hot day is certain to +lead to the creation of an ingoing tide of air, which reaches some +miles into the interior. This stream from the sea enters as a thin +wedge, it often being possible to note next the shore when the +movement begins a difference of ten degrees of temperature between the +surface of the ground to which the point of the wedge has attained, +and a position twenty feet higher in the air. This is a beautiful +example to show at once how the relative weight of the atmosphere, +even when the differences are slight, may bring about motion, and also +how masses of the atmosphere may move by or through the rest of the +medium in a way which we do not readily conceive from our observations +on the transparent mass. Very few people have any idea how general is +the truth that the air, even in continuous winds, tends to move in +more or less individualized masses. This, however, is made very +evident by watching the gusts of a storm or the wandering patches of +wind which disturb the surface of an otherwise smooth sea. + +[Illustration: _South shore, Martha's Vineyard, Massachusetts, showing +a characteristic sand beach with long slope and low dunes. Note the +three lines of breakers and the splash flows cutting little bays in +the sand._] + +Among the notable local winds are those which from their likeness to +the Foehn of the Swiss valleys receive that name. Foehns are produced +where a body of air blowing against the slope of a continuous mountain +range is lifted to a considerable height, and, on passing over the +crest, falls again to a low position. In its ascent the air is cooled, +rarefied, and to a great extent deprived of its moisture. In +descending it is recondensed, and by the process by which its atoms +are brought together its latent heat is made sensible. There being but +little watery vapour in the mass, this heat is not much called for by +that heat-storing fluid, and so the air is warmed. So far Foehn winds +have only been remarked as conspicuous features in Switzerland and on +the eastern face of the Rocky Mountains. In the region about the head +waters of the Missouri and to the northward their influence in what +are called the Chinook winds is distinctly to ameliorate the severe +winter climate of the country. + +In almost all great desert regions, particularly in the typical +Sahara, we find a variety of storm belonging to the whirlwind group, +which, owing to the nature of the country, take on special +characteristics. These desert storms take up from the verdureless +earth great quantities of sand and other fine _debris_, which often so +clouds the air as to bring the darkness of night at midday. Their +whirlings appear in size to be greater than those which produce +tornadoes or waterspouts, but less than hurricanes or cyclones. +Little, however, is known about them. They have not been well +observed by meteorologists. In some ways they are important, for the +reason that they serve to carry the desert sand into regions +previously verdure-clad, and thus to extend the bounds of the desolate +fields in which they originate. Where they blow off to the seaward, +they convey large quantities of dust into the ocean, and thus serve to +wear down the surface of the land in regions where there are no rivers +to effect that action in the normal way. + +Notwithstanding its swift motion when impelled by differences in +weight, the movements of the air have had but little direct and +immediate influence on the surface of the earth. The greater part of +the work which it does, as we shall see hereafter, is done through the +waters which it impels and bears about. Yet where winds blow over +verdureless surfaces the effect of the sand which they sweep before +them is often considerable. In regions of arid mountains the winds +often drive trains of sand through the valleys, where the sharp +particles cut the rocks almost as effectively as torrents of water +would, distributing the wearing over the width of the valley. The dust +thus blown, from a desert region may, when it attains a country +covered with vegetation, gradually accumulate on its surface, forming +very thick deposits. Thus in northwestern China there is a wide area +where dust accumulations blown from the arid districts of central Asia +have gradually heaped up in the course of ages to the depth of +thousands of feet, and this although much of the _debris_ is +continually being borne away by the action of the rain waters as they +journey toward the sea. Such dust accumulations occur in other parts +of the world, particularly in the districts about the upper +Mississippi and in the valleys of the Rocky Mountains, but nowhere are +they so conspicuous as in the region first mentioned. + +Where prevailing winds from the sea, from great lakes, and even from +considerable rivers, blow against sandy shores or cliffs of the same +nature, large quantities of sand and dust are often driven inland +from the coast line. In most cases these wind-borne materials take on +the form of dunes, or heaps of sand, varying from a few feet to +several hundred feet in height. It is characteristic of these hills of +blown sand that they move across the face of the country. Under +favourable conditions they may journey scores of miles from the shore. +The marching of a dune is effected through the rolling up of the sand +on the windward side of the elevation, when it is impelled by the +current of air to the crest where it falls into the lee or shelter +which the hill makes to the wind. In this way in the course of a day +the centre of the dune, if the wind be blowing furiously, may advance +a measurable distance from the place it occupied before. By fits and +starts this ongoing may be indefinitely continued. A notable and +picturesque instance of the march of a great dune may be had from the +case in which one of them overwhelmed in the last century the village +of Eccles in southeastern England. The advancing sand gradually crept +into the hamlet, and in the course of a decade dispossessed the people +by burying their houses. In time the summit of the church spire +disappeared from view, and for many years thereafter all trace of the +hamlet was lost. Of late years, however, the onward march of the sands +has disclosed the church spire, and in the course of another century +the place may be revealed on its original site, unchanged except that +the marching hill will be on its other side. + +In the region about the head of the Bay of Biscay the quantity of +these marching sands is so great that at one time they jeopardized the +agriculture of a large district. The French Government has now +succeeded, by carefully planting the surface of the country with +grasses and other herbs which will grow in such places, in checking +the movement of the wind-blown materials. By so doing they have merely +hastened the process by which Nature arrests the march of dunes. As +these heaps creep away from the sea, they generally come into regions +where a greater variety of plants flourish; moreover, their sand +grains become decayed, so that they afford a better soil. Gradually +the mat of vegetation binds them down, and in time covers them over so +that only the expert eye can recognise their true nature. Only in +desert regions can the march of these heaps be maintained for great +distances. + +Characteristic dunes occur from point to point all along the Atlantic +coast from the State of Maine to the northern coast of Florida. They +also occur along the coasts of our Great Lakes, being particularly +well developed at the southern end of Lake Michigan, where they form, +perhaps, the most notable accumulations within the limits of the +United States. + +When blown sands invade a forest and the deposit is rapidly +accumulated, the trees are often buried in an undecayed condition. In +this state, with certain chemical reactions which may take place in +the mass, the woody matter is apt to become replaced by silex +dissolved from the sand, which penetrates the tissues of the plants. +In this way salicified forests are produced, such as are found in the +region of the Rocky Mountains, where the trunks of the trees, now very +hard stone, so perfectly preserve their original structure that when +cut and polished they may be used for decorative purposes. Conspicuous +as is this work of the dunes, it is in a geological way much less +important than that accomplished by the finer dust which drifts from +one region of land to another or into the sea. Because of their +weight, the sand grains journey over the surface of the earth, except, +indeed, where they are uplifted by whirl storms. They thus can not +travel very fast or far. Dust, however, rises into the air, and +journeys for indefinite distances. We thus see how slight differences +in the weight of substances may profoundly affect the conditions of +their deportation. + + + THE SYSTEM OF WATERS. + +The envelope of air wraps the earth completely about, and, though +varying in thickness, is everywhere present over its surface. That of +the waters is much less equally distributed. Because of its weight, it +is mainly gathered in the depths of the earth, where it lies in the +interstices of the rocks and in the great realm of the seas. Only a +very small portion of the fluid is in the atmosphere or on the land. +Perhaps less than a ten thousandth part of the whole is at any one +time on this round from the seas through the air to the land and back +to the great reservoir. + +The great water store of the earth is contained in two distinct +realms--in the oceans, where the fluid is concentrated in a quantity +which fills something like nine tenths of the hollows formed by the +corrugations of the earth's surface; and in the rocks, where it is +stored in a finely divided form, partly between the grains of the +stony matter and partly in the substance of its crystals, where it +exists in a combination, the precise nature of which is not well +known, but is called water of crystallization. On the average, it +seems likely that the materials of the earth, whether under the sea or +on the land, have several per cent of their mass of the fluid. + +It is not yet known to what depth the water-bearing section of the +earth extends; but, as we shall see more particularly hereafter when +we come to consider volcanoes, the lavas which they send up to the +surface are full of contained water, which passes from them in the +form of steam. The very high temperature of these volcanic ejections +makes it necessary for us to suppose that they come from a great +depth. It is difficult to believe that they originate at less than a +hundred miles below the earth's surface. If, then, the rocks contain +an average of even five per cent of water to the depth of one hundred +miles, the quantity of the fluid stored within the earth is greater +than that which is contained in the reservoir of the ocean. The +oceans, on the average, are not more than three miles deep; spread +evenly over the surface of the whole earth, their depth would be less +than two miles, while the water in the rocks, if it could be added to +the seas, would make the total depth seven miles or more. As we shall +note hereafter, the processes of formation of strata tend to imprison +water in the beds, which in time is returned to the earth's surface by +the forces which operate within the crust. + +Although the water in the seas is, as we have seen, probably less than +one half of the store which the earth possesses, the part it plays in +the economy of the planet is in the highest measure important. The +underground water operates solely to promote certain changes which +take place in the mineral realm. Its effect, except in volcanic +processes, are brought about but slowly, and are limited in their +action. The movements of this buried water are exceedingly gradual; +the forces which impel it about and which bring it to do its work +originate in the earth. In the seas the fluid has an exceeding freedom +of motion; it can obey the varied impulses which the solar energy +imposes upon it. The role of these wonderful actions which we are +about to trace includes almost everything which goes on upon the +surface of the planet--that which relates to the development of animal +and vegetable life, as well as to the vast geological changes which +the earth is undergoing. + +If the surface of the earth were uniformly covered with water to the +depth of ten thousand feet or more, every particle of fluid would, in +a measure, obey the attraction of the sun, of the moon, and, +theoretically, also of all the other bodies in space, on the principle +that every particle of matter in the universe exercises a gravitative +effect on every other. As it is, owing to the divided condition of the +water on the earth's surface, only that which is in the ocean and +larger seas exhibits any measurable influence from these distant +attractions. In fact, only the tides produced by the moon and sun are +of determinable magnitude, and of these the lunar is of greater +importance, the reason being the near position of our satellite to our +own sphere. The solar tide is four tenths as great as the lunar. The +water doubtless obeys in a slight way the attraction of the other +celestial bodies, but the motions thus imparted are too small to be +discerned; they are lost in the great variety of influences which +affect all the matter on the earth. + +Although the tides are due to the attraction of the solar bodies, +mainly to that of the moon, the mode in which the result is brought +about is somewhat complicated. It may briefly and somewhat +incompletely be stated as follows: Owing to the fact that the +attracting power of the earth is about eighty times greater than that +of the moon, the centre of gravity of the two bodies lies within the +earth. About this centre the spheres revolve, each in a way swinging +around the other. At this point there is no centrifugal motion arising +from the revolution of the pair of spheres, but on the side of the +earth opposite the moon, some six thousand miles away, the centrifugal +force is considerable, becoming constantly greater as we pass away +from the turning point. At the same time the attraction of the moon on +the water becomes less. Thus the tide opposite the satellite is +formed. On the side toward the moon the same centrifugal action +operates, though less effectively than in the other case, for the +reason that the turning point is nearer the surface; but this action +is re-enforced by the greater attraction of the moon, due to the fact +that the water is much nearer that body. + +In the existing conditions of the earth, what we may call the normal +run of the tides is greatly interrupted. Only in the southern ocean +can the waters obey the lunar and solar attraction in anything like a +normal way. In that part of the earth two sets of tides are +discernible, the one and greater due to the moon, the other, much +smaller, to the sun. As these tides travel round at different rates, +the movements which they produce are sometimes added to each other +and sometimes subtracted--that is, at times they come together, while +again the elevation of one falls in the hollow of the other. Once +again supposing the earth to be all ocean covered, computation shows +that the tides in such a sea would be very broad waves, having, +indeed, a diameter of half the earth's circumference. Those produced +by the moon would have an altitude of about one foot, and those by the +sun of about three inches. The geological effects of these swayings +would be very slight; the water would pass over the bottom to and fro +twice each day, with a maximum journey of a hundred or two feet each +way from a fixed point. This movement would be so slow that it could +not stir the fine sediment; its only influence would perhaps be to +help feed the animals which were fixed upon the bottom by drawing the +nurture-bringing water by their mouths. + +Although the divided condition of the ocean perturbs the action of the +tides, so that except by chance their waves are rarely with their +centres where the attracting bodies tend to make them, the influence +of these divisions is greatly to increase the geological or +change-bringing influences arising from these movements. When from the +southern ocean the tides start to the northward up the bays of the +Atlantic, the Pacific, or the Indian Ocean, they have, as before +noted, a height of perhaps less than two feet. As they pass up the +narrowing spaces the waves become compressed--that is, an equal volume +of moving water has less horizontal room for its passage, and is +forced to rise higher. We see a tolerably good illustration of the +same principle when we observe a wind-made wave enter a small recess +of the shore, the sides of which converge in the direction of the +motion. With the diminished room, the wave gains in height. It thus +comes about that the tide throughout the Atlantic basin is much higher +than in the southern ocean. On the same principle, when the tide rolls +in against the shores every embayment of a distinct kind, whose sides +converge toward the head, packs up the tidal wave, often increasing +its height in a remarkable way. When these bays are wide-mouthed and +of elongate triangular form, with deep bottoms, the tides which on +their outer parts have a height of ten or fifteen feet may attain an +altitude of forty or fifty feet at the apex of the triangle. + +We have already noted the fact that the tide, such as runs in the +southern ocean, exercises little or no influence upon the bottom of +the sea over which it moves. As the height of the confined waters +increases, the range of their journey over the bottom as the wave +comes and goes rapidly increases. When they have an elevation of ten +feet they can probably stir the finer mud on the ocean floor, and in +shallow water move yet heavier particles. In the embayments of the +land, where a great body of water journeys like an alternating river +into extensive basins, the tidal action becomes intense; the current +may be able to sweep along large stones quite as effectively as a +mountain torrent. Thus near Eastport, Me., where the tides have a +maximum rise and fall of over twenty feet, the waters rush in places +so swiftly that at certain stages of the movement they are as much +troubled as those at the rapids of the St. Lawrence. In such portions +of the shore the tides do important work in carving channels into the +lands. + +Along the shores of the continents about the North Atlantic, where the +tides act in a vigorous manner, we almost everywhere find an +underwater shelf extending from the shore with a declivity of only +five to ten feet to the mile toward the centre of the sea, until the +depth of about five hundred feet is attained; from this point the +bottom descends more steeply into the ocean's depth. It is probable +that the larger part of the material composing these continental +shelves has been brought to its position by tidal action. Each time +the tidal wave sweeps in toward the shore it urges the finer particles +of sediment along with it. When it moves out it drags them on the +return journey toward the depths of the sea. If this shelf were +perfectly horizontal, the two journeys of the sand and mud grains +would be of the same length; but as the movement takes place up and +down a slope, the bits will travel farther under the impulse which +leads them downward than under that which impels them up. The result +will be that the particles will travel a little farther out from the +shore each time it is swung to and fro in the alternating movement of +the tide. + +The effect of tidal movement in nurturing marine life is very great. +It aids the animals fixed on the bottoms of the deep seas to obtain +their provision of food and their share of oxygen by drawing the water +by their bodies. All regions which are visited by strong tides +commonly have in the shallows near the shores a thick growth of +seaweed which furnishes an ample provision of food for the fishes and +other forms of animal life. + +A peculiar effect arising from tidal action is believed by students of +the phenomena to be found in the slowing of the earth's rotation on +its axis. The tides rotate around the earth from east to west, or +rather, we should say, the solid mass of the earth rubs against them +as it spins from west to east. As they move over the bottom and as +they strike against the shores this push of the great waves tends in a +slight measure to use up the original spinning impulse which causes +the earth's rotation. Computation shows that the amount of this action +should be great enough gradually to lengthen the day, or the time +occupied by the earth in making a complete revolution on the polar +axis. The effect ought to be great enough to be measurable by +astronomers in the course of a thousand years. On the other hand, the +records of ancient eclipses appear pretty clearly to show that the +length of the day has not changed by as much as a second in the course +of three thousand years. This evidence does not require us to abandon +the supposition that the tides tend to diminish the earth's rate of +rotation. It is more likely that the effect of the reduction in the +earth's diameter due to the loss of heat which is continually going on +counterbalances the influence of the tidal friction. As the diameter +of a rotating body diminishes, the tendency is for the mass to spin +more rapidly; if it expands, to turn more slowly, provided in each +case the amount of the impulse which leads to the turning remains the +same. This can be directly observed by whirling a small weight +attached to a string in such a manner that the cord winds around the +finger with each revolution; it will be noted that as the line +shortens the revolution is more quickly accomplished. We can readily +conceive that the earth is made up of weights essentially like that +used in the experiment, each being drawn toward the centre by the +gravitative stress, which is like that applied to the weight by the +cord. + +The fact that the days remain of the same length through vast periods +of time is probably due to this balance between the effects of tidal +action and those arising from the loss of heat--in other words, we +have here one of those delicate arrangements in the way of +counterpoise which serve to maintain the balanced conditions of the +earth's surface amid the great conflicts of diverse energies which are +at work in and upon the sphere. + +It should be understood that the effects of the attraction which +produces tides are much more extensive than they are seen to be in the +movements of the sea. So long as the solar and planetary spheres +remain fluid, the whole of their masses partake of the movement. It is +a consequence of this action, as the computations of Prof. George +Darwin has shown, that the moon, once nearer the earth than it is at +present, has by a curious action of the tidal force been pushed away +from the centre of our sphere, or rather the two bodies have repelled +each other. An American student of the problem, Mr. T.J.J. See, has +shown that the same action has served to give to the double stars the +exceeding eccentricity of their orbits. + +Although these recent studies of tidal action in the celestial sphere +are of the utmost importance to the theory of the universe, for they +may lead to changes in the nebular hypotheses, they are as yet too +incomplete and are, moreover, too mathematical to be presented in an +elementary treatise such as this. + + * * * * * + +We now turn to another class of waves which are of even more +importance than those of the tides--to the undulations which are +produced by the action of the wind on the surface of the water. While +the tide waves are limited to the open ocean, and to the seas and bays +which afford them free entrance, wind waves are produced everywhere +where water is subjected to the friction of air which flows over it. +While tidal waves come upon the shores but twice each day, the wind +waves of ordinary size which roll in from the ocean deliver their +blows at intervals of from three to ten seconds. Although the tidal +waves sometimes, by a packing-up process, attain the height of fifty +feet, their average altitude where they come in contact with the shore +probably does not much exceed four feet; usually they come in gently. +It is likely that in a general way the ocean surges which beat against +the coast are of greater altitude. + +Wind waves are produced and perform their work in a manner which we +shall now describe. When the air blows over any resisting surface, it +tends, in a way which we can hardly afford here to describe, to +produce motions. If the particle is free to move under the impulse +which it communicates, it bears it along; if it is linked together in +the manner of large masses, which the wind can not transport, it tends +to set it in motion in an alternating way. The sounds of our musical +instruments which act by wind are due to these alternating vibrations, +such as all air currents tend to produce. An AEolian harp illustrates +the action which we are considering. Moving over matter which has the +qualities that we denote by the term fluid, the swayings which the air +produces are of a peculiar sort, though they much resemble those of +the fiddle string. The surface of the liquid rises and falls in what +we term waves, the size of which is determined by the measure of +fluidity, and by the energy of the wind. Thus, because fresh water is +considerably lighter than salt, a given wind will produce in a given +distance for the run of the waves heavier surges in a lake than it +will in the sea. For this reason the surges in a great storm which +roll on the ocean shore, because of the wide water over which they +have gathered their impetus, are in size very much greater than those +of the largest lakes, which do not afford room for the development of +great undulations. + +To the eye, a wave in the water appears to indicate that the fluid is +borne on before the wind. Examination, however, shows that the amount +of motion in the direction in which the wind is blowing is very +slight. We may say, indeed, that the essential feature of a wave is +found in the transmission of impulse rather than in the movement of +the fluid matter. A strip of carpet when shaken sends through its +length undulations which are almost exactly like water waves. If we +imagine ourselves placed in a particle of water, moving in the +swayings of a wave in the open and deep sea, we may conceive ourselves +carried around in an ellipse, in each revolution returning through +nearly the same orbit. Now and then, when the particle came to the +surface, it would experience the slight drift which the continual +friction of the wind imposes on the water. If the wave in which the +journey was made lay in the trade winds, where the long-continued, +steadfast blowing had set the water in motion to great depths, the +orbit traversed would be moving forward with some rapidity; where also +the wind was strong enough to blow the tops of the waves over, forming +white-caps, the advance of the particle very near the surface would be +speedy. Notwithstanding these corrections, waves are to be regarded +each as a store of energy, urging the water to sway much in the manner +of a carpet strip, and by the swaying conveying the energy in the +direction of the wave movement. + +The rate of movement of wind waves increases with their height. +Slight undulations go forward at the rate of less than half a mile an +hour. The greater surges of the deeps when swept by the strongest +winds move with the speed which, though not accurately determined, has +been estimated by the present writer as exceeding forty miles an hour. +As these surges often have a length transverse to the wind of a mile +or more, a width of about an eighth of a mile, and a height of from +thirty-five to forty-five feet, the amount of energy which they +transmit is very great. If it could be effectively applied to the +shores in the manner in which the energy of exploding gunpowder is +applied by cannon shot, it is doubtful whether the lands could have +maintained their position against the assaults of the sea. But there +are reasons stated below why the ocean waves can use only a very small +part of their energy in rending the rocks against which they strike on +the coast line. + +In the first place, we should note that wind waves have very little +influence on the bottom of the deep sea. If an observer could stand on +the sea floor at the depth of a mile below a point over which the +greatest waves were rolling, he could not with his unaided senses +discern that the water was troubled. He would, indeed, require +instruments of some delicacy to find out that it moved at all. Making +the same observations at the depth of a thousand feet, it is possible +that he would note a slight swaying motion in the water, enough +sensibly to affect his body. At five hundred feet in depth the +movement would probably be sufficient to disturb fine mud. At two +hundred feet, the rasping of the surge on the bottom would doubtless +be sufficient to push particles of coarse sand to and fro. At one +hundred feet in depth, the passage of the surge would be strong enough +to urge considerable pebbles before it. Thence up the slope the +driving action would become more and more intense until we attained +the point where the wave broke. It should furthermore be noted that, +while the movement of the water on the floor of the deep sea as the +wave passes overhead would be to and fro, with every advance in the +shallowing and consequent increased friction on the bottom, the +forward element in the movement would rapidly increase. Near the coast +line the effect of the waves is continually to shove the detritus up +the slopes of the continental shelf. Here we should note the fact that +on this shelf the waves play a part exactly the opposite of that +effected by the tides. The tides, as we have noted, tend to drag the +particles down the slope, while the waves operate to roll them up the +declivity. + +As the wave in advancing toward the shore ordinarily comes into +continually shallowing water, the friction on the bottom is +ever-increasing, and serves to diminish the energy the surge contains, +and therefore to reduce its proportions. If this action operated +alone, the subtraction which the friction makes would cause the surf +waves which roll in over a continental shelf to be very low, probably +in height less than half that which they now attain. In fact, however, +there is an influence at work to increase the height of the waves at +the expense of its width. Noting that the friction rapidly increases +with the shallowing, it is easy to see that this resistance is +greatest on the advancing front of the wave, and least on its seaward +side. The result is that the front moves more slowly than the rear, so +that the wave is forced to gain in height; but for the fact that the +total friction which the wave encounters takes away most of its +impetus, we might have combers a hundred feet high rolling upon the +shelving shores which almost everywhere face the seas. + +As the wave shortens its width and gains in relative height, though +not in actual elevation, another action is introduced which has +momentous consequences. The water in the bottom of the wave is greatly +retarded in its ongoing by its friction over the sea floor, while the +upper part of the surge is much less affected in this way. The result +is that at a certain point in the advance, the place of which is +determined by the depth, the size, and the speed of the undulation, +the front swiftly steepens until it is vertical, and the top shoots +forward to a point where it is no longer supported by underlying +water, when it plunges down in what is called the surf or breaker. In +this part of the wave's work the application of the energy which it +transmits differs strikingly from the work previously done. Before the +wave breaks, the only geological task which it accomplishes is +effected by forcing materials up the slope, in which movement they are +slightly ground over each other until they come within the battering +zone of the shore, where they may be further divided by the action of +the mill which is there in operation. + +When the wave breaks on the shore it operates in the following manner: +First, the overturning of its crest sends a great mass of water, it +may be from the height of ten or more feet, down upon the shore. Thus +falling water has not only the force due to its drop from the summit +of the wave, but it has a share of the impulse due to the velocity +with which the surge moved against the shore. It acts, in a word, like +a hammer swung down by a strong arm, where the blow represents not +only the force with which the weight would fall of itself, but the +impelling power of the man's muscles. Any one who will expose his body +to this blow of the surf will recognise how violent it is; he may, if +the beach be pebbly, note how it drives the stones about; fragments +the size of a man's head may be hurled by the stroke to the distance +of twenty feet or more; those as large as the fist may be thrown clear +beyond the limits of the wave. So vigorous is this stroke that the +sound of it may sometimes be heard ten miles inland from the coast +where it is delivered. + +Moving forward up the slope of a gently inclined beach, the fragments +of the wave are likely to be of sufficient volume to permit them to +regather into a secondary surge, which, like the first, though much +smaller, again rises into a wall, forming another breaker. Under +favourable conditions as many as four or five of these successive +diminishing surf lines may be seen. The present writer has seen in +certain cases as many as a dozen in the great procession, the lowest +and innermost only a few inches high, the outer of all with a height +of perhaps twenty feet. + +Along with the direct bearing action of the surf goes a to-and-fro +movement, due to the rushing up and down of the water on the beach. +This swashing affects not only the broken part of the waves, but all +the water between the outer breaker and the shore. These swayings in +the surf belt often swing the _debris_ on the inner margin over a +range of a hundred feet or more, the movement taking place with great +swiftness, affecting the pebbles to the depth of several inches, and +grinding the bits together in a violent way. Listening to the turmoil +of a storm, we can on a pebbly beach distinctly hear the sound of the +downward stroke, a crashing tone, and the roar of the rolling stones. + +As waves are among the interesting things in the world, partly on +account of their living quality and partly because of their immediate +and often exceeding interest to man, we may here note one or two +peculiar features in their action. In the first place, as the reader +who has gained a sense of the changes in form of action may readily +perceive, the beating of waves on the shore converts the energy which +they possess into heat. This probably warms the water during great +storms, so that by the hand we may note the difference in temperature +next the coast line and in the open waters. This relative warmth of +the surf water is perhaps a matter of some importance in limiting the +development of ice along the shore line; it may also favour the +protection of the coast life against the severe cold of the winter +season. + +The waves which successively come against the shore in any given time, +particularly if a violent wind is blowing on to the coast, are usually +of about the same size. When, however, in times of calm an old sea, as +it is called, is rolling in, the surges may occasionally undergo very +great variations in magnitude. Not infrequently these occasional waves +are great enough to overwhelm persons who are upon the rocks next the +shore. These greater surges are probably to be accounted for by the +fact that in the open sea waves produced by winds blowing in different +directions may run on with their diverse courses and varied intervals +until they come near the shore. Running in together, it very well +happens that two of the surges belonging to different sets may combine +their forces, thus doubling the swell. The danger which these +conjoined waves bring is obviously greatest on cliff shores, where, on +account of the depth of water, the waves do not break until they +strike the steep. + + * * * * * + +Having considered in a general way the action of waves as they roll in +to the shore, bearing with them the solar energy which was contributed +to them by the winds, we shall now take up in some detail the work +which goes on along the coast line--work which is mainly accomplished +by wave action. + +On most coast lines the observer readily notes that the shore is +divided into two different kinds of faces--those where the inner +margin of the wave-swept belt comes against rocky steeps, and those +bordered by a strand altogether composed of materials which the surges +have thrown up. These may be termed for convenience cliff shores and +wall-beach shores. We shall begin our inquiry with cliff shores, for +in those sections of the coast line the sea is doing its most +characteristic and important work of assaulting the land. If the +student has an opportunity to approach a set of cliffs of hard rock in +time of heavy storm, when the waves have somewhere their maximum +height, he should seek some headland which may offer him safe foothold +whence he can behold the movements which are taking place. If he is so +fortunate as to have in view, as well may be the case, cliffs which +extend down into deep water, and others which are bordered by rude +and generally steeply sloping beaches covered with large stones, he +may perceive that the waves come in against the cliffs which plunge +into deep water without taking on the breaker form. In this case the +undulation strikes but a moderate blow; the wave is not greatly +broken. The part next the rock may shoot up as a thin sheet to a +considerable height; it is evident that while the ongoing wave applies +a good deal of pressure to the steep, it does not deliver its energy +in the effective form of a blow as when the wave overturns, or in the +consequent rush of the water up a beach slope. It is easy to perceive +that firm-set rock cliffs, with no beaches at their bases, can almost +indefinitely withstand the assaults. On the steep and stony beach, +because of its relatively great declivity, the breaker or surf forms +far in, and even in its first plunge often attains the base of the +precipice. The blow of the overfalling as well as that of the inrush +moves about stones of great size; those three feet or more in diameter +are often hurled by the action against the base of the steep, striking +blows, the sharp note of which can often be heard above the general +roar which the commotion produces. The needlelike crags forming isles +standing at a distance from the shore, such as are often found along +hard rock coasts, are singularly protected from the action of +effective waves. The surges which strike against them are unarmed with +stones, and the water at their bases is so deep that it does not sway +with the motion with sufficient energy to move them on the bottom. +Where a cliff is in this condition, it may endure until an elevation +of the coast line brings its base near the level of the sea, or until +the process of decay has detached a sufficient quantity of stone to +form a talus or inclined plane reaching near to the water level. + +As before noted, it is the presence of a sloping beach reaching to +about the base of the cliff which makes it possible for the waves to +strike at with a hammer instead of with a soft hand. Battering at the +base of the cliff, the surges cut a crease along the strip on which +they strike, which gradually enters so far that the overhanging rock +falls of its own weight. The fragments thus delivered to the sea are +in turn broken up and used as battering instruments until they are +worn to pieces. We may note that in a few months of heavy weather the +stones of such a fall have all been reduced to rudely spherical forms. +Observations made on the eastern face of Cape Ann, Mass., where the +seas are only moderately heavy, show that the storms of a single +winter reduce the fragments thrown into the sea from the granite +quarries to spheroidal shapes, more than half of their weight commonly +being removed in the form of sand and small pebbles which have been +worn from their surfaces. + +We can best perceive the effect of battering action which the sea +applies to the cliffs by noting the points where, owing to some chance +features in the structure in the rock, it has proved most effective. +Where a joint or a dike, or perhaps a softer layer, if the rocks be +bedded, causes the wear to go on more rapidly, the waves soon excavate +a recess in which the pebbles are retained, except in stormy weather, +in an unmoved condition. When the surges are heavy, these stones are +kept in continuous motion, receding as the wave goes back, and rushing +forward with its impulse until they strike against the firm-set rock +at the end of the chasm. In this way they may drive in a cut having +the length of a hundred feet or more from the face of the precipice. +In most cases the roofs over these sea caves fall in, so that the +structure is known as a chasm. Occasionally these roofs remain, in +which case, for the reason that the floor of the cutting inclines +upward, an opening is made to the surface at their upper end, forming +what is called in New England a "spouting horn"; from the inland end +of the tunnel the spray may be thrown far into the air. As long as the +cave is closed at this inner end, and is not so high but that it may +be buried beneath a heavy wave, the inrushing water compresses the +air in the rear parts of the opening. When the wave begins to retreat +this air blows out, sending a gust of spray before it, the action +resembling the discharge of a great gun from the face of a +fortification. It often happens that two chasms converging separate a +rock from the cliff. Then a lowering of the coast may bring the mass +to the state of a columnar island, such as abound in the Hebrides and +along various other shores. + +If a cliff shore retreats rapidly, it may be driven back into the +shore, and its face assumes the curve of a small bay. With every step +in this change the bottom is sure to become shallower, so that the +waves lose more and more of their energy in friction over the bottom. +Moreover, in entering a bay the friction which the waves encounter in +running along the sides is greater than that which they meet in +coming in upon a headland or a straight shore. The result is, with the +inward retreat of the steep it enters on conditions which diminish the +effectiveness of the wave stroke. The embayment also is apt to hold +detritus, and so forms in time a beach at the foot of the cliff, over +which the waves rarely are able to mount with such energy as will +enable them to strike the wall in an effective manner. With this +sketch of the conditions of a cliff shore, we will now consider the +fate of the broken-tip rock which the waves have produced on that +section of the coast land. + +By observation of sea-beaten cliffs the student readily perceives that +a great amount of rocky matter has been removed from most cliff-faced +shores. Not uncommonly it can be shown that such sea faces have +retreated for several miles. The question now arises, What becomes of +the matter which has been broken up by the wave action? In some part +the rock, when pulverized by the pounding to which it is subjected, +has dissolved in the water. Probably ninety per cent of it, however, +retains the visible state, and has a fate determined by the size of +the fragments of which it is composed. If these be as fine as mud, so +that they may float in the water, they are readily borne away by the +currents which are always created along a storm-swept shore, +particularly by the undertow or bottom outcurrent--the "sea-puss," as +it is sometimes called--that sweeps along the bottom from every shore, +against which the waves form a surf. If as coarse as sand grains, or +even very small pebbles, they are likely to be drawn out, rolling over +the bottom to an indefinite distance from the sea margin. The coarser +stones, however, either remain at the foot of the cliff until they are +beaten to pieces, or are driven along the shore until they find some +embayment into which they enter. The journey of such fragments may, +when the wind strikes obliquely to the shore, continue for many miles; +the waves, running with the wind, drive the fragments in oscillating +journeys up and down the beach, sometimes at the rate of a mile or +more a day. The effect of this action can often be seen where a vessel +loaded with brick or coal is wrecked on the coast. In a month +fragments of the materials may be stretched along for the distance of +many miles on either side of the point where the cargo came ashore. +Entering an embayment deep enough to restrain their further journey, +the fragments of rock form a boulder beach, where the bits roll to and +fro whenever they are struck by heavy surges. The greater portion of +them remain in this mill until they are ground to the state of sand +and mud. Now and then one of the fragments is tossed up beyond the +reach of the waves, and is contributed to the wall of the beach. In +very heavy storms these pebbles which are thrown inland may amount in +weight to many tons for each mile of shore. + +The study of a pebbly beach, drawn from crest to the deep water +outside, will give an idea as to the history of its work. On either +horn of the crescent by which the pebbles are imported into the pocket +we find the largest fragments. If the shore of the bay be long, the +innermost part of the recess may show even only very small pebbles, or +perhaps only fine sand, the coarser material having been worn out in +the journey. On the bottom of the bay, near low tide, we begin to find +some sand produced by the grinding action. Yet farther out, below +high-tide mark, there is commonly a layer of mud which represents the +finer products of the mill. + +Boulder beaches are so quick in answering to every slight change in +the conditions which affect them that they seem almost alive. If by +any chance the supply of detritus is increased, they fill in between +the horns, diminish the incurve of the bay, and so cause its beach to +be more exposed to heavy waves. If, on the other hand, the supply of +grist to the mill is diminished, the beach becomes more deeply +incurved, and the wave action is proportionately reduced. We may say, +in general, that the curve of these beaches represents a balance +between the consumption and supply of the pebbles which they grind up. +The supply of pebbles brought along the shore by the waves is in many +cases greatly added to by a curious action of seaweeds. If the bottom +of the water off the coast is covered by these fragments, as is the +case along many coast lines within the old glaciated districts, the +spores of algae are prone to take root upon them. Fastening themselves +in those positions, and growing upward, the seaweeds may attain +considerable size. Being provided with floats, the plant exercises a +certain lifting power on the stone, and finally the tugging action of +the waves on the fronds may detach the fragments from the bottom, +making them free to journey toward the shore. Observing from near at +hand the straight wall of the wave in times of heavy storm, the +present writer has seen in one view as many as a dozen of these +plant-borne stones, sometimes six inches in diameter, hanging in the +walls of water as it was about to topple over. As soon as they strike +the wave-beaten part of the shore these stones are apt to become +separated from the plants, though we can often notice the remains or +prints of the attachments adhering to the surface of the rock. Where +the pebbles off the shore are plenty, a rocky beach may be produced +by this process of importation through the agency of seaweeds without +any supply being brought by the waves along the coast line. + +Returning to sand beaches, we enter the most interesting field of +contact between seas and lands. Probably nine tenths of all the coast +lines of the open ocean are formed of arenaceous material. In general, +sand consists of finely broken crystals of silica or quartz. These +bits are commonly distinctly faceted; they rarely have a spherical +form. Not only do accumulations of sand border most of the shore line, +but they protect the land against the assaults of the sea, and this in +the following curious manner: When shore waves beat pebbles against +each other, they rapidly wear to bits; we can hear the sound of the +wearing action as the wave goes to and fro. We can often see that the +water is discoloured by the mud or powdered rock. When, however, the +waves tumble on a sandy coast, they make but a muffled sound, and +produce no mud. In fact, the particles of sand do not touch each other +when they receive the blow. Between them there lies a thin film of +water, drawn in by the attraction known as capillarity, which sucks +the fluid into a sponge or between plates of glass placed near +together. The stroke of the waves slightly compresses this capillary +water, but the faces of the grains are kept apart as sheets of glass +may be observed to be restrained from contact when water is between +them. If the reader would convince himself as to the condition of the +sand grains and the water which is between them, he may do so by +pressing his foot on the wet beach which the wave has just left. He +will observe that it whitens and sinks a little under the pressure, +but returns in good part to its original form when the foot is lifted. +In the experiment he has pushed a part of the contained water aside, +but he has not brought the grains together; they do not make the sound +which he will often hear when the sand is dry. The result is that the +sand on the seashore may wear more in going the distance of a mile in +the dry sand dune than in travelling for hundreds along the wet shore. + +If the rock matter in the state of sand wore as rapidly under the +heating of the waves as it does in the state of pebbles, the +continents would doubtless be much smaller than they are. Those coasts +which have no other protection than is afforded by a low sand beach +are often better guarded against the inroads of the sea than the +rock-girt parts of the continents. It is on account of this remarkable +endurance of sand of the action of the waves that the stratified rocks +which make up the crust of the earth are so thick and are to such an +extent composed of sand grains. + +The tendency of the _debris_-making influences along the coast line is +to fill in the irregularities which normally exist there; to batter +off the headlands, close up the bays and harbours, and generally to +reduce the shores to straight lines. Where the tide has access to +these inlets, it is constantly at work in dragging out the detritus +which the waves make and thrust into the recesses. These two actions +contend with each other, and determine the conditions of the coast +line, whether they afford ports for commerce or are sealed in by sand +bars, as are many coast lines which are not tide-swept, as that of +northern Africa, which faces the Mediterranean, a nearly tideless sea. +The same is the case with the fresh-water lakes; even the greater of +them are often singularly destitute of shelters which can serve the +use of ships, and this because there are no tides to keep the bays and +harbours open. + + + THE OCEAN CURRENTS. + +The system of ocean currents, though it exhibits much complication in +detail, is in the main and primarily dependent on the action of the +constant air streams known as the trade winds. With the breath from +the lips over a basin of water we can readily make an experiment which +shows in a general way the method in which the winds operate in +producing the circulation of the sea. Blowing upon the surface of the +water in the basin, we find that even this slight impulse at once sets +the upper part in motion, the movement being of two kinds--pulsating +movements or waves are produced, and at the same time the friction of +the air on the surface causes its upper part to slide over the under. +With little floats we can shortly note that the stream which forms +passes to the farther side of the vessel, there divides, and returns +to the point of beginning, forming a double circle, or rather two +ellipses, the longer sides of which are parallel with the line of the +air current. Watching more closely, aiding the sight by the particles +which float at various distances below the surface, we note the fact +that the motion which was at first imparted to the surface gradually +extends downward until it affects the water to the depth of some +inches. + +In the trade-wind belt the ocean waters to the depth of some hundreds +of feet acquire a continuous movement in the direction in which they +are impelled by those winds. This motion is most rapid at the surface +and near the tropics. It diminishes downwardly in the water, and also +toward the polar sides of the trade-wind districts. Thus the trades +produce in the sea two broad, slow-moving, deep currents, flowing in +the northern hemisphere toward the southwest, and in the southern +hemisphere toward the northwest. Coming down upon each other +obliquely, these broad streams meet about the middle of the tropical +belt. Here, as before noted, the air of the trade winds leaves the +surface and rises upward. The waters being retained on their level, +form a current which moves toward the west. If the earth within the +tropics were covered by a universal sea, the result of this movement +would be the institution of a current which, flowing under the +equator, would girdle the sphere. + +With a girdling equatorial current, because of the intense heat of the +tropics and the extreme cold of the parallels beyond the fortieth +degree of latitude, the earth would be essentially uninhabitable to +man, and hardly so to any forms of life. Its surface would be visited +by fierce winds induced by the very great differences of temperature +which would then prevail. Owing, however, to the barriers which the +continents interpose to the motions of these windward-setting tropical +currents, all the water which they bear, when it strikes the opposing +shores, is diverted to the right and left, as was the stream in the +experiment with the basin and the breath, the divided currents seeking +ways toward high latitudes, conveying their store of heat to the +circumpolar lands. So effective is this transfer of temperature that a +very large part of the heat which enters the waters in the tropical +region is taken out of that division of the earth's surface and +distributed over the realms of sea and land which lie beyond the +limits of the vertical sun. Thus the Gulf Stream, the northern branch +of the Atlantic tropical current, by flowing into the North Atlantic, +contributes to the temperature of the region within the Arctic Circle +more heat than actually comes to that district by the direct influx +from the sun. + +The above statements as to the climatal effect of the ocean streams +show us how important it is to obtain a sufficient conception as to +the way in which these currents now move and what we can of their +history during the geologic ages. This task can not yet be adequately +done. The fields of the sea are yet too imperfectly explored to afford +us all the facts required to make out the whole story. Only in the +case of our Gulf Stream can we form a full conception as to the +journey which the waters undergo and the consequence of their motion. +In the case of this current, observations clearly show that it arises +from the junction near the equatorial line of the broad stream created +by the two trade-wind belts. Uniting at the equator, these produce a +westerly setting current, having the width of some hundred miles and a +depth of several hundred feet. Its velocity is somewhat greater than a +mile an hour. The centre of the current, because of the greater +strength of the northern as compared with the southern trades, is +considerably south of the equator. When this great slow-moving stream +comes against the coast of South America, it encounters the projecting +shoulder of that land which terminates at Cape St. Roque. There it +divides, as does a current on the bows of an anchored ship, a +part--rather more than one half--of the stream turning to the +northward, the remainder passing toward the southern pole; this +northerly portion becomes what is afterward known as the Gulf Stream, +the history of which we shall now briefly follow. + +Flowing by the northwesterly coast of South America, the northern +share of the tropical current, being pressed in against the land by +the trade winds, is narrowed, and therefore acquires at once a swifter +flow, the increased speed being due to conditions like those which add +to the velocity of the water flowing through a hose when it comes to +the constriction of the nozzle. Attaining the line of the southeastern +or Lesser Antilles, often known as the Windward Islands, a part of +this current slips through the interspaces between these isles and +enters the Gulf of Mexico. Another portion, failing to find sufficient +room through these passages, skirts the Antilles on their eastern and +northern sides, passes by and among the Bahama Islands, there to +rejoin the part of the stream which entered the Caribbean. This +Caribbean portion of the tide spreads widely in that broad sea, is +constricted again between Cuba and Yucatan, again expands in the Gulf +of Mexico, and is finally poured forth through the Straits of Florida +as a stream having the width of forty or fifty miles, a depth of a +thousand feet or more, and a speed of from three to five miles an +hour, exceeding in its rate of flow the average of the greatest +rivers, and conveying more water than do all the land streams of the +earth. In this part of its course the deep and swift stream from the +Gulf of Mexico, afterward to be named the Gulf Stream, receives the +contribution of slower moving and shallower currents which skirted the +Antilles on their eastern verge. The conjoined waters then move +northward, veering toward the east, at first as a swift river of the +sea having a width of less than a hundred miles and of great depth; +with each step toward the pole this stream widens, diminishing +proportionately in depth; the speed of its current decreases as the +original impetus is lost, and the baffling winds set its surface +waters to and fro in an irregular way. Where it passes Cape Hatteras +it has already lost a large share of its momentum and much of its +heat, and is greatly widened. + +Although the current of the Gulf Stream becomes more languid as we go +northward, it for a very long time retains its distinction from the +waters of the sea through which it flows. Sailing eastward from the +mouth of the Chesapeake, the navigator can often observe the moment +when he enters the waters of this current. This is notable not only in +the temperature, but in the hue of the sea. North of that line the +sharpness of the parting wall becomes less distinct, the stream +spreads out broadly over the surface of the Atlantic, yet its +thermometric effects are distinctly traceable to Iceland and Nova +Zembla, and the tropical driftwood which it carries affords the +principal timber supply of the inhabitants of the first-named isle. +Attaining this circumpolar realm, and finally losing the impulse which +bore it on, the water of the Gulf Stream partly returns to the +southward in a relatively slight current which bears the fluid along +the coast of Europe until it re-enters the system of tropical winds +and the currents which they produce. A larger portion stagnates in the +circumpolar region, in time slowly to return to the tropical district +in a manner afterward to be described. Although the Gulf Stream in the +region north of Cape Hatteras is so indistinct that its presence was +not distinctly recognised until the facts were subjected to the keen +eye of Benjamin Franklin, its effects in the way of climate are so +great that we must attribute the fitness of northern Europe for the +uses of civilized man to its action. But for the heat which this +stream brings to the realm of the North Atlantic, Great Britain would +be as sterile as Labrador, and the Scandinavian region, the +cradle-land of our race, as uninhabitable as the bleakest parts of +Siberia. + +It is a noteworthy fact that when the equatorial current divides on +the continents against which it flows, the separate streams, although +they may follow the shores for a certain distance toward the poles, +soon diverge from them, just as the Gulf Stream passes to the seaward +from the eastern coast of the United States. The reason for this +movement is readily found in the same principle which explains the +oblique flow of the trades and counter trades in their passage to and +from the equatorial belt. The particle of water under the equator, +though it flows to the west, has, by virtue of the earth's rotation, +an eastward-setting velocity of a thousand miles an hour. Starting +toward the poles, the particle is ever coming into regions of the sea +where the fluid has a less easterly movement, due to the earth's +rotation on its axis. Consequently the journeying water by its +momentum tends to move off in an easterly course. Attaining high +latitudes and losing its momentum, it abides in the realm long enough +to become cooled. + +We have already noted the fact that only a portion of the waters sent +northward in the Gulf Stream and the other currents which flow from +the equator to the poles is returned by the surface flow which sets +toward the equator along the eastern side of the basins. The largest +share of the tide effects its return journey in other ways. Some +portion of this remainder sets equatorward in local cold streams, such +as that which pours forth through Davis Strait into Baffin Bay, +flowing under the Gulf Stream waters for an unknown distance toward +the tropics. There are several of these local as yet little known +streams, which doubtless bring about a certain amount of circulation +between the polar regions and the tropical districts. Their effect is, +however, probably small as compared with that massive drift which we +have now to note. + +The tropical waters when they attain high latitudes are constantly +cooled, and are overlaid by the warmer contributions of that tide, and +are thus brought lower and lower in the sea. When they start downward +they have, as observations show, a temperature not much above the +freezing point of salt water. They do not congeal for the reason that +the salt of the ocean lowers the point at which the water solidifies +to near 28 deg. Fahr. The effect of this action is gradually to press down +the surface cold water until it attains the very bottom in all the +circumpolar regions. At the same time this descending water drifts +along the bottom of the ocean troughs toward the equatorial realm. As +this cold water is heavier than that which is of higher temperature +and nearer the surface, it has no tendency to rise. Being below the +disturbing influences of any current save its own, it does not tend, +except in a very small measure, to mingle with the warmer overlying +fluid. The result is that it continues its journey until it may come +within the tropics without having gained a temperature of more than +35 deg. Fahr., the increase in heat being due in small measure to that +which it receives from the earth's interior and that which it acquires +from the overlying warmer water. Attaining the region of the tropical +current, this drift water from the poles gradually rises, to take the +place of that which goes poleward, becomes warm, and again starts on +its surface journey toward the arctic and antarctic regions. + +Nothing is known as to the rate of this bottom drift from the polar +districts toward the equator, but, from some computation which he has +made, the writer is of the opinion that several centuries is doubtless +required for the journey from the Arctic Circle to the tropics. The +speed of the movement probably varies; it may at times require some +thousand years for its accomplishment. The effect of the bottom drift +is to withdraw from seas in high latitudes the very cold water which +there forms, and to convey it beneath the seas of middle latitudes to +a realm where it is well placed for the reheating process. If all the +cold water of circumpolar regions had to journey over the surface to +the equator, the perturbing effect of its flow on the climates of +various lands would be far greater than it is at present. Where such +cold currents exist the effect is to chill the air without adding much +to the rainfall; while the currents setting northward not only warm +the regions near which they flow, but by so doing send from the water +surfaces large quantities of moisture which fall as snow or rain. Thus +the Gulf Stream, directly and indirectly, probably contributes more +than half the rainfall about the Atlantic basin. The lack of this +influence on the northern part of North America and Asia causes those +lands to be sterilized by cold, although destitute of permanent ice +and snow upon their surfaces. + +We readily perceive that the effect of the oceanic circulation upon +the temperatures of different regions is not only great but widely +contrasted. By taking from the equatorial belt a large part of the +heat which falls within that realm, it lowers the temperature to the +point which makes the district fit for the occupancy of man, perhaps, +indeed, tenable to all the higher forms of life. This same heat +removed to high latitudes tempers the winter's cold, and thus makes a +vast realm inhabitable which otherwise would be locked in almost +enduring frosts. Furthermore, this distribution of temperatures tends +to reduce the total wind energy by diminishing the trades and counter +trades which are due to the variations of heat which are encountered +in passing polarward from the equator. Still further, but for this +circulation of water in the sea, the oceans about the poles would be +frozen to their very bottom, and this vast sheet of ice might be +extended southward to within the parallels of fifty degrees north and +south latitude, although the waters under the equator might at the +same time be unendurably hot and unfit for the occupancy of living +beings. + +A large part of the difficulties which geologists encounter in +endeavouring to account for the changes of the past arise from the +evidences of great climatal revolutions which the earth has undergone. +In some chapters of the great stone book, whose leaves are the strata +of the earth, we find it plainly written in the impressions made by +fossils that all the lands beyond the equatorial belt have undergone +changes which can only be explained by the supposition that the heat +and moisture of the countries have been subjected to sudden and +remarkable changes. Thus in relatively recent times thick-leaved +plants which retained their vegetation in a rather tender state +throughout the year have flourished near to the poles, while shortly +afterward an ice sheet, such as now covers the greater part of +Greenland, extended down to the line of the Ohio River at Cincinnati. +Although these changes of climate are, as we shall hereafter note, +probably due to entangled causes, we must look upon the modifications +of the ocean streams as one of the most important elements in the +causation. We can the more readily imagine such changes to be due to +the alterations in the course and volume of the ocean current when we +note how trifling peculiarities in the geography of the +shores--features which are likely to be altered by the endless changes +which occur in the form of a continent--affect the run of these +currents. Thus the growth of coral reefs in southern Florida, and, in +general, the formation of that peninsula, by narrowing the exit of the +great current from the Gulf of Mexico, has probably increased its +velocity. If Florida should again sink down, that current would go +forth into the North Atlantic with the speed of about a mile an hour, +and would not have momentum enough to carry its waters over half the +vast region which they now traverse. If the lands about the western +border of the Caribbean Sea, particularly the Isthmus of Darien, +should be depressed to a considerable depth below the ocean level, +the tropical current would enter the Pacific Ocean, adding to the +temperature of its waters all the precious heat which now vitalizes +the North Atlantic region. Such a geographic accident would not only +profoundly alter the life conditions of that part of the world, but it +would make an end of European civilization. + +In the chapter on climatal changes further attention will be given to +the action of ocean currents from the point of view of their influence +on the heat and moisture of different parts of the world. We now have +to consider the last important influence of ocean currents--that which +they directly exercise on the development of organic life. The most +striking effect of this nature which the sea streams bring about is +caused by the ceaseless transportation to which they subject the eggs +and seeds of animals and plants, as well as the bodies of the mature +form which are moved about by the flowing waters. But for the +existence of these north and south flowing currents, due to the +presence of the continental barriers, the living tenants of the seas +would be borne along around the earth, always in the same latitude, +and therefore exposed to the same conditions of temperature. In this +state of affairs the influences which now make for change in organic +species would be far less than they are. Journeying in the great +whirlpools which the continental barriers make out of the westward +setting tropical currents, these organic species are ever being +exposed to alterations in their temperature conditions which we know +to be favourable to the creation of those variations on which the +advance of organic life so intimately depends. Thus the ocean currents +not only help to vary the earth by producing changes in the climate of +both sea and land, breaking up the uniformity which would otherwise +characterize regions at the same distance from the equator, but they +induce, by the consequences of the migrations which they enforce, +changes in the organic tenants of the sea. + +Another immediate effect of ocean streams arises where their currents +of warm water come against shores or shallows of the sea. At these +points, if the water have a tropical temperature, we invariably find a +vast and rapid development of marine animals and plants, of which the +coral-making polyps are the most important. In such positions the +growth of forms which secrete solid skeletons is so rapid that great +walls of their remains accumulate next the shore, the mass being built +outwardly by successive growths until the realm of the land may be +extended for scores of miles into the deep. In other cases vast mounds +of this organic _debris_ may be accumulated in mid ocean until its +surface is interspersed with myriads of islands, all of which mark the +work due to the combined action of currents and the marine life which +they nourish. Probably more than four fifths of all the islands in the +tropical belt are due in this way to the life-sustaining action of the +currents which the trade winds create. + +There are many secondary influences of a less important nature which +are due to the ocean streams. The reader will find on most wall-maps +of the world certain areas in the central part of the oceans which are +noted as Sargassum seas, of which that of the North Atlantic, west and +south of the Azore Islands, is one of the most conspicuous. In these +tracts, which in extent may almost be compared with the continents, we +find great quantities of floating seaweed, the entangled fronds of +which often form a mass sufficiently dense to slightly restrain the +speed of ships. When the men on the caravels of Columbus entered this +tangle, they were alarmed lest they should be unable to escape from +its toils. It is a curious fact that these weeds of the sea while +floating do not reproduce by spores the structures which answer to the +seeds of higher plants, but grow only by budding. It seems certain +that they could not maintain their place in the ocean but for the +action of the currents which convey the bits rent off from the shores +where the plant is truly at home. This vast growth of plant life in +the Sargassum basins doubtless contributed considerable and important +deposits of sediment to the sea floors beneath the waters which it +inhabits. Certain ancient strata, known as the Devonian black shale, +occupying the Ohio valley and the neighbouring parts of North America +to the east and north of that basin, appear to be accumulations which +were made beneath an ancient Sargassum sea. + +The ocean currents have greatly favoured and in many instances +determined the migrations not only of marine forms, but of land +creatures as well. Floating timber may bear the eggs and seeds of many +forms of life to great distances until the rafts are cast ashore in a +realm where, if the conditions favour, the creatures may find a new +seat for their life. Seeds of plants incased in their often dense +envelopes may, because they float, be independently carried great +distances. So it comes about that no sooner does a coral or other +island rise above the waters of the sea than it becomes occupied by a +varied array of plants. The migrations of people, even down to the +time of the voyages which discovered America, have in large measure +been controlled by the run of the ocean streams. The tropical set of +the waters to the westward helped Columbus on his way, and enabled him +to make a journey which but for their assistance could hardly have +been accomplished. This same current in the northern part of the Gulf +Stream opposed the passage of ships from northern Europe to the +westward, and to this day affects the speed with which their voyages +are made. + + + THE CIRCUIT OF THE RAIN. + +We have now to consider those movements of the water which depend upon +the fact that at ordinary temperatures the sea yields to the air a +continued and large supply of vapour, a contribution which is made in +lessened proportion by water in all stages of coldness, and even by +ice when it is exposed to dry air. This evaporation of the sea water +is proportional to the temperature and to the dryness of the air where +it rests upon the ocean. It probably amounts on the average to +somewhere about three feet per annum; in regions favourably situated +for the process, as on the west coast of northern Africa, it may be +three or four times as much, while in the cold and humid air about the +poles it may be as little as one foot. When contributed to the air, +the water enters on the state of vapour, in which state it tends to +diffuse itself freely through the atmosphere by virtue of the motion +which is developed in particles when in the vaporous or gaseous state. + +The greater part of the water evaporated from the seas probably finds +its way as rain at once back into the deep, yet a considerable portion +is borne away horizontally until it encounters the land. The +precipitation of the water from the air is primarily due to the +cooling to which it is subjected as it rises in the atmosphere. Over +the sea the ascent is accomplished by the simple diffusion of the +vapour or by the uprise through the aerial shaft, such as that near +the equator or over the centres of the whirling storms. It is when the +air strikes the slopes of the land that we find it brought into a +condition which most decidedly tends to precipitate its moisture. +Lifted upward, the air as it ascends the slopes is brought into cooler +and more rarefied conditions. Losing temperature and expanding, it +parts with its water for the same reason that it does in the ascending +current in the equatorial belt or in the chimneys of the whirl storms. +A general consequence of this is that wherever moisture-laden winds +from the sea impinge upon a continent they lay down a considerable +part of the water which they contain. + +If all the lands were of the same height, the rain would generally +come in largest proportion upon their coastal belt, or those portions +of the shore-line districts over which the sea winds swept. But as +these winds vary in the amount of the watery vapour which they +contain, and as the surface of the land is very irregular, the +rainfall is the most variable feature in the climatal conditions of +our sphere. Near the coasts it ranges from two or three inches in arid +regions--such as the western part of the Sahara and portions of the +coast regions of Chili and Peru--to eight hundred inches about the +head waters of the Brahmapootra River in northern India, where the +high mountains are swept over by the moisture-laden airs from the +neighbouring sea. Here and there detached mountainous masses produce a +singular local increase in the amount of the rainfall. Thus in the +lake district in northwestern England the rainfall on the seaward side +of mountains, not over four thousand feet high, is very much greater +than it is on the other slope, less than a score of miles away. These +local variations are common all over the world, though they are but +little observed. + +In general, the central parts of continents are likely to receive much +less rainfall than their peripheral portions. Thus the central +districts of North America, Asia, and Australia--three out of the five +continental masses--have what we may call interior deserts. Africa has +one such, though it is north of the centre, and extends to the shores +of the Mediterranean and the Atlantic. The only continent without this +central nearly rainless field is South America, where the sole +characteristic arid district is situated on the western slope of the +Cordilleran range. In this case the peculiarity is due to the fact +that the strong westerly setting winds which sweep over the country +encounter no high mountains until they strike the Andean chain. They +journey up a long and rather gradual slope, where the precipitation is +gradually induced, the process being completed when they strike the +mountain wall. Passing over its summit, they appear as dry winds on +the Pacific coast. + +Even while the winds frequently blow in from the sea, as along the +western coast of the Americas, they may come over water which is +prevailingly colder than the land. This is characteristically the case +on the western faces of the American continent, where the sea is +cooled by the currents setting toward the equator from high latitudes. +Such cool sea air encountering the warm land has its temperature +raised, and therefore does not tend to lay down its burden of +moisture, but seeks to take up more. On this account the rainfall in +countries placed under such conditions is commonly small. + +By no means all the moisture which comes upon the earth from the +atmosphere descends in the form of rain or snow. A variable, large, +though yet undetermined amount falls in the form of dew. Dew is a +precipitation of moisture which has not entered the peculiar state +which we term fog or cloud, but has remained invisible in the air. It +is brought to the earth through the radiation of heat which +continually takes place, but which is most effective during the +darkened half of the day, when the action is not counterbalanced by +the sun's rays. While the sun is high and the air is warm there is a +constant absorption of moisture in large part from the ground or from +the neighbouring water areas, probably in some part from those +suspended stores of water, the clouds, if such there be in the +neighbourhood. We can readily notice how clouds drifting in from the +sea often melt into the dry air which they encounter. Late in the +afternoon, even before the sun has sunk, the radiation of heat from +the earth, which has been going on all the while, but has been less +considerable than the incurrent of temperature, in a way overtakes +that influx. The air next the surface becomes cooled from its contact +with the refrigerating earth, and parts with its moisture, forming a +coating of water over everything it touches. At the same time the +moisture escaping from the warmed under earth likewise drops back upon +its cooled surface almost as soon as it has escaped. The thin sheet of +water precipitated by this method is quickly returned to the air when +it becomes warmed by the morning sunshine, but during the night +quantities of it are absorbed by the plants; very often, indeed, with +the lowlier vegetation it trickles down the leaves and enters the +earth about the base of the stem, so that the roots may appropriate +it. Our maize, or Indian corn, affords an excellent example of a plant +which, having developed in a land of droughts, is well contrived, +through its capacities for gathering dew, to protect itself against +arid conditions. In an ordinary dew-making night the leaves of a +single stem may gather as much as half a pint of water, which flows +down their surfaces to the roots. So efficient is this dew supply, +this nocturnal cloudless rain, that on the western coast of South +America and elsewhere, where the ordinary supply of moisture is almost +wanting, many important plants are able to obtain from it much of the +water which they need. The effect is particularly striking along +seashores, where the air, although it may not have the humidity +necessary for the formation of rain, still contains enough to form +dew. + +It is interesting to note that the quantity of dew which falls upon an +area is generally proportioned to the amount of living vegetation +which it bears. The surfaces of leaves are very efficient agents of +radiation, and the tangle which they make offers an amount of +heat-radiating area many times as great as that afforded by a surface +of bared earth. Moreover, the ground itself can not well cool down to +the point where it will wring the moisture out of the air, while the +thin membranes of the plants readily become so cooled. Thus vegetation +by its own structure provides itself with means whereby it may be in a +measure independent of the accidental rainfall. We should also note +the fact that the dewfall is a concomitant of cloudless skies. The +quantity which is precipitated in a cloudy night is very small, and +this for the reason that when the heavens are covered the heat from +the earth can not readily fly off into space. Under these conditions +the temperature of the air rarely descends low enough to favour the +precipitation of dew. + +Having noted the process by which in the rain circuit the water +leaves the sea and the conditions of distribution when it returns to +the earth, we may now trace in more detail the steps in this great +round. First, we should take note of the fact that the water after it +enters the air may come back to the surface of the earth in either of +two ways--directly in the manner of dewfall, or in a longer circuit +which leads it through the state of clouds. As yet we are not very +well informed as to the law of the cloud-making, but certain features +in this picturesque and most important process have been tolerably +well ascertained. + +Rising upward from the sea, the vapour of water commonly remains +transparent and invisible until it attains a considerable height above +the surface, where the cooling tends to make it assume again the +visible state of cloud particles. The formation of these cloud +particles is now believed to depend on the fact that the air is full +of small dust motes, exceedingly small bits of matter derived from the +many actions which tend to bring comminuted solid matter into the air, +as, for instance, the combustion of meteoric stones, which are greatly +heated by friction in their swift course through the air, the +ejections of volcanoes, the smoke of forest and other fires, etc. +These tiny bits, floating in the air, because of their solid nature +radiate their heat, cool the air which lies against them, and thereby +precipitate the water in the manner of dew, exactly as do the leaves +and other structures on the surface of the earth. In fact, dew +formation is essentially like cloud formation, except that in the one +case the water is gathered on fixed bodies, and in the other on +floating objects. Each little dust raft with its cargo of condensed +water tends, of course, to fall downward toward the earth's surface, +and, except for the winds which may blow upward, does so fall, though +with exceeding slowness. Its rate of descent may be only a few feet a +day. It was falling before it took on the load of water; it will fall +a little more rapidly with the added burden, but even in a still air +it might be months or years before it would come to the ground. The +reason for this slow descent may not at first sight be plain, though a +little consideration will make it so. + +If we take a shot of small size and a feather of the same weight, we +readily note that their rate of falling through the air may vary in +the proportion of ten to one or more. It is easy to conceive that this +difference is due to the very much less friction which the smaller +body encounters in its motion by the particles of air. With this point +in mind, the student should observe that the surface presented by +solid bodies in relation to their solid contents is the greater the +smaller the diameter. A rough, though not very satisfactory, instance +of this principle may be had by comparing the surface and interior +contents of two boxes, one ten feet square and the other one foot +square. The larger has six hundred feet of surface to one thousand +cubic feet of interior, or about half a square foot of outer surface +to the cubic foot of contents; while the smaller box has six feet of +surface for the single cubic foot of interior, or about ten times the +proportion of exterior to contents. The result is that the smaller +particles encounter more friction in moving toward the earth, until, +in the case of finely divided matter, such as the particles of carbon +in the smoke from an ordinary fire, the rate of down-falling may be so +small as to have little effect in the turbulent conditions of +atmospheric motion. + +[Illustration: _Pocket Creek, Cape Ann, Massachusetts. Note the +relatively even size of the pebbles, and the splash wave which sets +them in motion._] + +The little drops of water which gather round dust motes, falling but +slowly toward the earth, are free to obey the attractions which they +exercise upon each other--impulses which are partly gravitative and +partly electrical. We have no precise knowledge concerning these +movements, further than that they serve to aggregate the myriad little +floats into cloud forms, in which the rafts are brought near together, +but do not actually touch each other. They are possibly kept apart by +electrical repulsion. In this state of association without union the +divided water may undergo the curiously modified aggregations which +give us the varied forms of clouds. As yet we know little as to the +cause of cloud shapes. We remark the fact that in the higher of these +agglomerations of condensed vapour, the clouds which float at an +elevation of from twenty to thirty thousand feet or more, the masses +are generally thin, and arranged more or less in a leaflike form, +though even here a tendency to produce spherical clouds is apparent. +In this high realm floating water is probably in the frozen state, +answering to the form of dew, which we call hoar frost. The lower +clouds, gathering in the still air, show very plainly the tendency to +agglomerate into spheres, which appears to be characteristic of all +vaporous material which is free to move by its own impulses. It is +probable that the spherical shape of clouds is more or less due to the +same conditions as gathered the stellar matter from the ancient +nebular chaos into the celestial spheres. Upon these spherical +aggregations of the clouds the winds act in extremely varied ways. The +cloud may be rubbed between opposite currents, and so flattened out +into a long streamer; it may take the same form by being carried off +by a current in the manner of smoke from a fire; the spheres may be +kept together, so as to form the patchwork which we call "mackerel" +sky; or they may be actually confounded with each other in a vast +common cloud-heap. In general, where the process of aggregation of two +cloud bodies occurs, changes of temperature are induced in the masses +which are mixed together. If the temperature resulting from this +association of cloud masses is an average increase, the cloud may +become lighter, and in the manner of a balloon move upward. Each of +the motes in the cloud with its charge of vapour may be compared with +the ballast of the balloon; if they are warmed, they send forth a part +of their load of condensed water again to the state of invisible +vapour. Rising to a point where it cools, the vapour gathers back on +the rafts and tends again to weight the cloud downward. The ballast of +an ordinary balloon has to be thrown away from its car; but if some +arrangement for condensing the moisture from the air could be +contrived, a balloon might be brought into the adjustable state of a +cloud, going up or down according as it was heated or cooled. + +When the formation of the drop of water or snowflake begins, the mass +is very small. If in descending it encounters great thickness of +cloud, the bit may grow by further condensation until it becomes +relatively large. Generally in this way we may account for the +diversities in the size of raindrops or snowflakes. It often happens +that the particles after taking on the form of snowflakes encounter in +their descent air so warm that they melt into raindrops, or, if only +partly melted, reach the surface as sleet. Or, starting as raindrops, +they may freeze, and in this simple state may reach the earth, or +after freezing they may gather other frozen water about them, so that +the hailstone has a complicated structure which, from the point of +view of classification, is between a raindrop and a snowflake. + +In the process of condensation--indeed, in the steps which precede the +formation of rain and snow--there is often more or less trace of +electrical action; in fact, a part of the energy which was involved in +the vapourization of water, on its condensation, even on the dust +motes appears to be converted into electrical action, which probably +operates in part to keep the little aggregates of water asunder. When +they coalesce in drops or flakes, this electricity often assumes the +form of lightning, which represents the swift passage of the electric +store from a region where it is most abundant to one where it is less +so. The variations in this process of conveying the electricity are +probably great. In general, it probably passes, much as an electric +current is conveyed, through a wire from the battery which produces +the force. In other cases, where the tension is high, or, in other +words, where the discharge has to be hastened, we have the phenomena +of lightning in which the current burns its way along its path, as it +may traverse a slender wire, vapourizing it as it goes. In general, +the lightning flash expends its force on the air conductors, or lines +of the moist atmosphere along which it breaks its path, its energy +returning into the vapour which it forms or the heat which it produces +in the other parts of the air. In some cases, probably not one in the +thousand of the flashes, the charge is so heavy that it is not used up +in its descent toward the earth, and so electrifies, or, as we say, +strikes, some object attached to the earth, through which it passes to +the underlying moisture, where it finds a convenient place to take on +a quiet form. Almost all these hurried movements of electrical energy +which intensely heat and light the air which they traverse fly from +one part of a cloud to another, or cross from cloud sphere to cloud +sphere; of those which start toward the earth, many are exhausted +before they reach its surface, and even those that strike convey but a +portion of their original impulse to the ground. + +The wearing-out effect of lightning in its journey along the air +conductors in its flaming passages is well illustrated by what happens +when the charge strikes a wire which is not large enough freely to +convey it. The wire is heated, generally made white hot, often melted, +and perhaps scattered in the form of vapour. In doing this work the +electricity may, and often is, utterly dissipated--that is, changed +into heat. It has been proposed to take advantage of this principle in +protecting buildings from lightning by placing in them many thin +wires, along which the current will try to make its way, being +exhausted in melting or vaporizing the metal through which it passes. + +There are certain other forms of lightning, or at least of electrical +discharges, which produce light and which may best be described in +this connection. It occasionally happens that the earth becomes so +charged that the current proceeds from its surface to the clouds. More +rarely, and under conditions which we do not understand, the electric +energy is gathered into a ball-like form, which may move slowly along +the surface until it suddenly explodes. It is a common feature of all +these forms of lightning which we have noted that they ordinarily make +in their movement considerable noise. This is due to the sudden +displacement of the air which they traverse--displacement due to the +action of heat in separating the particles. It is in all essential +regards similar to the sounds made by projectiles, such as meteors or +swift cannon shots, as they fly through the air. It is even more +comparable to the sound produced by exploding gunpowder. The first +sound effect from the lightning stroke is a single rending note, which +endures no longer--indeed, not as long--as the explosion of a cannon. +Heard near by, this note is very sharp, reminding one of the sound +made by the breaking of glass. The rolling, continuous sound which we +commonly hear in thunder is, as in the case of the noise produced by +cannon, due to echo from the clouds and the earth. Thunder is +ordinarily much more prolonged and impressive in a mountainous country +than in a region of plains, because the steeps about the hearer +reverberate the original single crash. + +The distribution of thunderstorms is as yet not well understood, but +it appears in many cases that they are attendants on the advancing +face of cyclones and hurricanes, the area in front of these great +whirlstorms being subjected to the condensation and irregular air +movements which lead to the development of much electrical energy. +There are, however, certain parts of the earth which are particularly +subjected to lightning flashes. They are common in the region near the +equator, where the ascending currents bring about heavy rains, which +mean a rapid condensation and consequent liberation of electrical +energy. They diminish in frequency toward the arctic regions. An +observer at the pole would probably fail ever to perceive strong +flashes. For the same reason thunderstorms are more frequent in +summer, the time when the difference in temperature between the +surface and the upper air is greatest, when, therefore, the uprushes +of air are likely to be most violent. They appear to be more common in +the night than in the daytime, for the reason that condensation is +favoured by the cooling which occurs in the dark half of the day. It +is rare, indeed, that a thunderstorm occurs near midday, a period when +the air is in most cases taking up moisture on account of the swiftly +increasing heat. + +There are other forms of electrical discharges not distinctly +connected with the then existing condensation of moisture. What the +sailors call St. Elmo's fire--a brush of electric light from the mast +tops and other projections of the ship--indicates the passage of +electrical energy between the vessel and the atmosphere. Similar +lights are said sometimes to be seen rising from the surface of the +water. Such phenomena are at present not satisfactorily explained. +Perhaps in the same group of actions comes the so-called +"Jack-o'-lantern" or "Will-o'-the-wisp" fires flashing from the earth +in marshy places, which are often described by the common people, but +have never been observed by a naturalist. If this class of +illuminations really exists, we have to afford them some other +explanation than that they are emanations of self-inflamed +phosphoretted hydrogen, a method of accounting for them which +illogically finds a place in many treatises on atmospheric phenomena. +A gas of any kind would disperse itself in the air; it could not dance +about as these lights are said to do, and there is no chemical means +known whereby it could be produced in sufficient purity and quantity +from the earth to produce the effects which are described.[3] + +[Footnote 3: The present writer has made an extended and careful study +of marsh and swamp phenomena, and is very familiar with the aspect of +these fields in the nighttime. He has never been able to see any sign of +the Jack-o'-lantern light. Looking fixedly into any darkness, such as is +afforded by the depths of a wood, the eye is apt to imagine the +appearance of faint lights. Those who have had to do with outpost duty +in an army know how the anxious sentry, particularly if he is new to the +soldier's trade, will often imagine that he sees lights before him. +Sometimes the pickets will be so convinced of the fact that they see +lights that they will fire upon the fiction of the imaginations. These +facts make it seem probable that the Jack-o'-lantern and his companion, +the Will-o'-the-wisp, are stories of the overcredulous.] + +In the upper air, or perhaps even beyond the limits of the field +which deserves the name, in the regions extending from the poles to +near the tropics, there occur electric glowings commonly known as the +aurora borealis. This phenomenon occurs in both hemispheres. These +illuminations, though in some way akin to those of lightning, and +though doubtless due to some form of electrical action, are peculiar +in that they are often attended by glows as if from clouds, and by +pulsations which indicate movements not at electric speed. As yet but +little is known as to the precise nature of these curious storms. It +has been claimed, however, that they are related to the sun spots; +those periods when the solar spots are plenty, at intervals of about +eleven years, are the times of auroral discharges. Still further, it +seems probable that the magnetic currents of the earth, that circling +energy which encompasses the sphere, moving round in a general way +parallel to the equator, are intensified during these illuminations of +the circumpolar skies. + + + GEOLOGICAL WORK OF WATER. + +We turn now to the geological work which is performed by falling +water. Where the rain or snow returns from the clouds to the sea, the +energy of position given to the water by its elevation above the earth +through the heat which it acquired from the sun is returned to the air +through which it falls or to the ocean surface on which it strikes. In +this case the circuit of the rain is short and without geological +consequence which it is worth while to consider, except to note that +the heat thus returned is likely to be delivered in another realm than +that in which the falling water acquired the store, thus in a small +way modifying the climate. When, however, the precipitation occurs on +the surface of the land, the drops of frozen or fluid water apply a +part of their energy in important geological work, the like of which +is not done where they return at once to the sea. + +[Illustration: Fig. 10.--Showing the diverse action of rain on wooded +and cleared fields, _a_, wooded area; _b_, tilled ground.] + +We shall first consider what takes place when the water in the form of +drops of rain comes to the surface of the land. Descending as they do +with a considerable speed, these raindrops apply a certain amount of +energy to the surface on which they fall. Although the beat of a +raindrop is proverbially light, the stroke is not ineffective. +Observing what happens where the action takes place on the surface of +bare rock, we may notice that the grains of sand or small pebbles +which generally abound on such surfaces, if they be not too steeply +inclined, dance about under the blows which they receive. If we could +cover hard plate glass, a much firmer material than ordinary stone, +with such bits, we should soon find that its surface would become +scratched all over by the friction. Moreover, the raindrops +perceptibly urge the small detached bits of stone down the slopes +toward the streams. + +If all the earth's surface were bare rocks, the blow of the raindrops +would deserve to be reckoned among the important influences which lead +to the wearing of land. As it is, when a country is in a state of +Nature, only a small part of its surface is exposed to this kind of +wearing. Where there is rain enough to effect any damage, there is +sure to be sufficient vegetation to interpose a living and +self-renewed covering between the rocks and the rain. Even the lichens +which coat what at first sight often seems to be bare rock afford an +ample covering for this purpose. It is only where man bares the field +by stripping away and overturning this protecting vegetation that the +raindrops cut away the earth. The effect of their action can often be +noted by observing how on ploughed ground a flat stone or a potsherd +comes after a rain to cap a little column. The geologist sometimes +finds in soft sandstones that the same action is repeated in a larger +way where a thin fragment of hard rock has protected a column many +feet in height against the rain work which has shorn down the +surrounding rock. + +When water strikes the moistened surface it at once loses the droplike +form which all fluids assume when they fall through the air.[4] + +[Footnote 4: This principle of the spheroidal form in falling fluids is +used in making ordinary bird shot. The melted lead drops through +sievelike openings, the resulting spheres of the metal being allowed to +fall into water which chills them. Iron shot, used in cutting stone, +where they are placed between the saw and the surface of the rock, are +also made in the same manner. The descending fluid divides into drops +because it is drawn out by the ever-increasing speed of the falling +particles, which soon make the stream so thin that it can not hold +together.] + +When the raindrops coalesce on the surface of the earth, the role of +what we may call land water begins. Thenceforward until the fluid +arrives at the surface of the sea it is continually at work in +effecting a great range of geological changes, only a few of which can +well be traced by the general student. The work of land water is due +to three classes of properties--to the energy with which it is endowed +by virtue of its height above the sea, a power due to the heat of the +sun; to the capacity it has for taking substances into solution; and +to its property of giving some part of its own substance to other +materials with which it comes in contact. The first of these groups of +properties may be called dynamical; the others, chemical. + +The dynamic value of water when it falls upon the land is the amount +of energy it can apply in going down the slope which separates it from +the sea. A ton of the fluid, such as may gather in an ordinary rain on +a thousand square feet of ground in the highlands of a country--say at +an elevation of a thousand feet above the sea--expends before it comes +to rest in the great reservoir as much energy as would be required to +lift that weight from the ocean's surface to the same height. The ways +in which this energy may be expended we shall now proceed in a general +way to trace. + +As soon as the water has been gathered, from its drop to its sheet +state--a process which takes place as soon as it falls--the fluid +begins its downward journey. On this way it is at once parted into two +distinct divisions, the surface water and the ground water: the former +courses more or less swiftly, generally at the rate of a mile or more +an hour, in the light of day; the latter enters the interstices of the +earth, slowly descends therein to a greater or less depth, and +finally, journeying perhaps at the rate of a mile a year, rejoins the +surface water, escaping through the springs. The proportion of these +two classes, the surface and the ground water, varies greatly, and an +intermixture of them is continually going on. Thus on the surface of +bare rock or frozen earth all the rain may go away without entering +the ground. On very sandy fields the heaviest rainfall may be taken +up by the porous earth, so that no streams are found. On such surfaces +the present writer has observed that a rainfall amounting to six +inches in depth in two hours produced no streams whatever. We shall +first follow the history of the surface water, afterward considering +the work which the underground movements effect. + +If the student will observe what takes place on a level ploughed +field--which, after all, will not be perfectly level, for all fields +are more or less undulating--he will note that, though the surface may +have been smoothed by a roller until it appears like a floor, the +first rain, where the fall takes place rapidly enough to produce +surface streams, will create a series of little channels which grow +larger as they conjoin, the whole appearing to the eye like a very +detailed map, or rather model, of a river system; it is, indeed, such +a system in miniature. If he will watch the process by which these +streamlet beds are carved, he will obtain a tolerably clear idea as to +that most important work which the greater streams do in carving the +face of the lands. The water is no sooner gathered into a sheet than, +guided by the slightest irregularities which it encounters, it begins +to flow. At first the motion is so slow that it does not disturb its +bed, but at some points in the bottom of the sheet the movement soon +becomes swift enough to drag the grains of sand and clay from their +adhesions, bearing them onward. As soon as this beginning of a channel +is formed the water moves more swiftly in the clearer way; it +therefore cuts more rapidly, deepening and enlarging its channel, and +making its motion yet more free. The tiny rills join the greater, all +their channels sway to and fro as directed this way and that by chance +irregularities, until something like river basins are carved out, +those gentle slopes which form broad valleys where the carving has +been due to the wanderings of many streams. If the field be large, +considerable though temporary brooks may be created, which cut +channels perhaps a foot in depth. At the end of this miniature stream +system we always find some part of the waste which has been carved +out. If the streamlet discharges into a pool, we find the tiny +representative of deltas, which form such an important feature on the +coast line where large rivers enter seas or lakes. Along the lines of +the stream we may observe here and there little benches, which are the +equivalent in all save size of the terraces that are generally to be +observed along the greater streams. In fact, these accidents of an +acre help in a most effective way the student to understand the +greater and more complicated processes of continental erosion. + +A normal river--in fact, all the greater streams of the +earth--originates in high country, generally in a region of mountains. +Here, because of the elevation of the region, the streams have cut +deep gorges or extensive valleys, all of which have slopes leading +steeply downward to torrent beds. Down these inclined surfaces the +particles worn off from the hard rock by frost and by chemical decay +gradually work their way until they attain the bed of the stream. The +agents which assist gravitation in bearing this detritus downward are +many, but they all work together for the same end. The stroke of the +raindrop accomplishes something, though but little; the direct washing +action of the brooklets which form during times of heavy rain, but dry +out at the close of the storm, do a good deal of the work; thawing and +freezing of the water contained in the mass of detritus help the +movement, for, although the thrust is in both directions, it is most +effective downhill; the wedges of tree roots, which often penetrate +between and under the stones, and there expand in their process of +growth, likewise assist the downward motion. The result is that on +ordinary mountain slopes the layer of fragments constituting the rude +soil is often creeping at the rate of from some inches to some feet a +year toward the torrent bed. If there be cliffs at the top of the +slope, as is often the case, very extensive falls of rock may take +place from it, the masses descending with such speed that they +directly attain the stream. If the steeps be low and the rock divided +into vertical joints, especially where there is a soft layer at the +base of the steep, detached masses from the precipice may move slowly +and steadfastly down the slope, so little disturbed in their journey +that trees growing upon their summits may continue to develop for the +thousands of years before the mass enters the stream bed. + +Although the fall of rocks from precipices does not often take place +in a conspicuously large way, all great mountain regions which have +long been inhabited by man abound in traditions and histories of such +accidents. Within a century or two there have been a dozen or more +catastrophes of this nature in the inhabited valleys of the Alps. As +these accidents are at once instructive and picturesque, it is well to +note certain of them in some detail. At Yvorgne, a little parish on +the north shore of the Rhone, just above the lake of Geneva, tradition +tells that an ancient village of the name was overwhelmed by the fall +of a great cliff. The vast _debris_ forming the steep slope which was +thus produced now bears famous vineyards, but the vintners fancy that +they from time to time hear deep in the earth the ringing of the bells +which belonged to the overwhelmed church. In 1806 the district of +Goldau, just north of Lake Lucerne, was buried beneath the ruins of a +peak which, resting upon a layer of clay, slipped away like a +launching ship on the surface of the soft material. The _debris_ +overwhelmed a village and many detached houses, and partly filled a +considerable lake. The wind produced by this vast rush of falling rock +was so great that people were blown away by it; some, indeed, were +killed in this singular manner. + +The most interesting field of these Swiss mountain falls is a high +mountain valley of amphitheatrical form, known as the Diablerets, or +the devil's own district. This great circus, which lies at the height +of about four thousand feet above the sea, is walled around on its +northern side by a precipice, above which rest, or rather once +rested, a number of mountain peaks of great bulk. The region has long +been valued for the excellent pasturage which the head of the valley +affords. Two costly roads, indeed, have been built into it to afford +footpaths for the flocks and herds and their keepers in the summer +season. Through this human experience with the valley, we have a +record of what has gone on in this part of the mountain wilderness. +Within the period of history and tradition, three very great mountain +falls have occurred in this field, each having made its memory good by +widespread disaster which it brought to the people of the _chalets_. +The last of these was brought about by the fall of a great peak which +spread itself out in a vast field of ruins in the valley below. The +belt of destruction was about half a mile wide and three miles long. +When the present writer last saw it, a quarter of a century ago, it +was still a wilderness of great rocks, but here and there the process +of their decay was giving a foothold for herbage, and in a few +centuries the field will doubtless be so verdure-clad that its story +will not be told on its face. It is likely, however, to be preserved +in the memory of the people, and this through a singular and pathetic +tradition which has grown up about the place, one which, if not true, +comes at least among the legends which we should like to believe. + +As told the present writer by a native of the district, it happened +when, in the nighttime the mountain came down, the herdsmen and their +cows gathered in the _chalets_--stout buildings which are prepared to +resist avalanches of snow. In one of these, which was protected from +crushing by the position of the stones which covered it, a solitary +herdsman found himself alive in his unharmed dwelling. With him in the +darkness were the cows, a store of food and water, and his provisions +for the long summer season. With nothing but hope to animate him, he +set to work burrowing upward among the rocks, storing the _debris_ in +the room of the _chalet_. He toiled for some months, but finally +emerged to the light of day, blanched by his long imprisonment in the +darkness, but with the strength to bear him to his home. In place of +the expected warm welcome, the unhappy man found himself received as a +ghost. He was exorcised by the priest and driven away to the distance. +It was only when long afterward his path of escape was discovered that +his history became known. + +Returning to the account of the _debris_ which descends at varied +speed into the torrents, we find that when the detritus encounters the +action of these vigorous streams it is rapidly ground to pieces while +it is pushed down the steep channels to the lower country. Where the +stones are of such size that the stream can urge them on, they move +rapidly; at least in times when the torrent is raging. They beat over +each other and against the firm-set rocks; the more they wear, the +smaller they become, and the more readily they are urged forward. +Where the masses are too large to be stirred by the violent current, +they lie unmoved until the pounding of the rolling stones reduces them +to the proportions where they may join the great procession. +Ordinarily those who visit mountains behold their torrents only in +their shrunken state, when the waters stir no stones, and fail even to +bear a charge of mud, all detachable materials having been swept away +when the streams course with more vigour. In storm seasons the +conditions are quite otherwise; then the swollen torrents, their +waters filled with clay and sand, bear with them great quantities of +boulders, the collisions of which are audible above the muffled roar +of the waters, attesting the very great energy of the action. + +When the waste on a mountain slope lies at a steep angle, particularly +where the accumulation is due to the action of ancient glaciers, it +not infrequently happens that when the ground is softened with frost +great masses of the material rush down the slope in the manner of +landslides. The observer readily notes that in many mountain regions, +as, for instance, in the White Mountains of New Hampshire, the steep +slopes are often seamed by the paths of these great landslides. Their +movement, indeed, is often begun by sliding snow, which gives an +impulse to the rocks and earth which it encounters in its descent. At +a place known as the Wylie Notch, in the White Mountains, in the early +part of this century, a family of that name was buried beneath a mass +of glacial waste which had hung on the mountain slope from the ancient +days until a heavy rain, following on a period of thaw, impelled the +mass down the slope. Although there have been few such catastrophes +noted in this country, it is because our mountains have not been much +dwelt in. As they become thickly inhabited as the Alps are, men are +sure to suffer from these accidents. + +As the volume of a mountain torrent increases through the junction of +many tributaries, the energy of its moving waters becomes sufficient +to sweep away the fragments which come to its bed. Before this stage +is attained the stream rarely touches the solid under rock of the +mountain, the base of the current resting upon the larger loose stones +which it was unable to stir. In this pebble-paved section, because the +stream could not attack the foundation rock, we find no gorges--in +fact, the whole of this upper section of the torrent system is +peculiarly conditioned by the fact that the streams are dealing not +with bed-rock, but with boulders or smaller loose fragments. If they +cut a little channel, the materials from either side slip the faster, +and soon repave the bed. But when the streams have by a junction +gained strength, and can keep their beds clear, they soon carve down a +gorge through which they descend from the upper mountain realm to the +larger valleys, where their conjoined waters take on a riverlike +aspect. It should be noted here that the cutting power of the water +moving in the torrent or in the wave, the capacity it has for abrading +rock, resides altogether in the bits of stone or cutting tools with +which it is armed. Pure water, because of its fluidity, may move over +or against firm-set stones for ages without wearing them; but in +proportion as it moves rocky particles of any size, the larger they +are, the more effective the work, it wears the rock over which it +flows. A capital instance of this may be found where a stream from a +hose is used in washing windows. If the water be pure, there is no +effect upon the glass; but if it be turbid, containing bits of sand, +in a little while the surface will appear cloudy from the multitude of +line scratches which the hard bits impelled by the water have +inflicted upon it. A somewhat similar case occurs where the wind bears +sand against window panes or a bottle which has long lain on the +shore. The glass will soon be deeply carved by the action, assuming +the appearance which we term "ground." This principle is made use of +in the arts. Glass vessels or sheets are prepared for carving by +pasting paper cut into figures on their surfaces. The material is then +exposed to a jet of air or steam-impelling sand grains; in a short +time all the surface which has not been protected by paper has its +polish destroyed and is no longer translucent. + +The passage from the torrent to the river, though not in a +geographical way distinct, is indicated to the observant eye by a +simple feature--namely, the appearance of alluvial terraces, those +more or less level heaps of water-borne _debris_ which accumulate +along the banks of rivers, which, indeed, constitute the difference +between those streams and torrents. Where the mountain waters move +swiftly, they manage to bear onward the waste which they receive. Even +where the blocks of stone cling in the bed, it is only a short time +before they are again set in motion or ground to pieces. If by chance +the detritus accumulates rapidly, the slope is steepened and the work +of the torrent made more efficient. As the torrent comes toward the +base of the mountains, where it neither finds nor can create steep +slopes over which to flow, its speed necessarily diminishes. With each +reduction in this feature its carrying power very rapidly diminishes. +Thus water flowing at the rate of ten miles an hour can urge stones +four times the mass that it can move when its speed is reduced to half +that rate. The result is that on the lowlands, with their relatively +gentle slopes, the combined torrents, despite the increase in the +volume of the stream arising from their confluence, have to lay down a +large part of their load of detritus. + +If we watch where a torrent enters a mountain river, we observe that +the main stream in a way sorts over the waste contributed to it, +bearing on only those portions which its rate of flow will permit it +to carry, leaving the remainder to be built into the bank in the form +of a rude terrace. This accumulation may not extend far below the +point where the torrent which imported the _debris_ joins the main +stream; a little farther down, however, we are sure to find another +such junction and a second accumulation of terrace material. As these +contributions increase, the terrace accumulations soon become +continuous, lying on one side or the other of the river, sometimes +bordering both banks of the stream. In general, it can be said that so +long as the rate of fall of the torrent exceeds one hundred feet to +the mile it does not usually exhibit these shelves of detritus. Below +that rate of descent they are apt to be formed. Much, however, depends +upon the amount of detritus which the stream bears and the coarseness +of it; moreover, where the water goes through a gorge in the manner of +a flume with steep rocky sides, it can urge a larger amount before it +than when it traverses a wide valley, through which it passes, it may +be, in a winding way. + +At first sight it may seem rather a fine distinction to separate +torrents from rivers by the presence or absence of terraces. As we +follow down the stream, however, and study its action in relation to +these terraces, and the peculiar history of the detritus of which they +are composed, we perceive that these latter accumulations are very +important features. Beginning at first with small and imperfect +alluvial plains, the river, as it descends toward the sea, gaining in +store of water and in the amount of _debris_ which comes with that +water from the hills, while the rate of fall and consequent speed of +the current are diminished, soon comes to a stage where it is engaged +in an endless struggle with the terrace materials. In times of flood, +the walls of the terraces compel the tide to flow over the tops of +these accumulations. Owing to the relative thinness of the water +beyond the bed, and to the growth of vegetation there, the current +moves more slowly, and therefore lays down a considerable deposit of +the silt and sand which it contains. This may result during a single +flood in lifting the level of the terrace by some inches in height, +still further serving to restrict the channel. Along the banks of the +Mississippi and other large rivers the most of this detritus falls +near the stream; a little of it penetrates to the farther side of the +plains, which often have a width of ten miles or more. The result is +that a broad elevation is constructed, a sort of natural mole or +levee, in a measure damming the flood waters, which can now only enter +the "back swamps" through the channels of the tributary streams. Each +of these back swamps normally discharges into the main stream through +a little river of its own, along the banks of which the natural levees +do not develop. + +We have now to note a curious swinging movement of rivers which was +first well observed by the skilful engineers of British India. This +movement can best be illustrated by its effects. If on any river which +winds through alluvial plains a jetty is so constructed as to deflect +the stream at any point, the course which it follows will be altered +during its subsequent flow, it may be, for the distance of hundreds of +miles. It will be perceived that in its movements a river normally +strikes first against one shore and then against the other. Its water +in a general way moves as does a billiard ball when it flies from one +cushion to another. It is true that in a torrent we have the same +conditions of motion; but there the banks are either of hard rock or, +if of detritus, they are continually moving into the stream in the +manner before described. In the case of the river, however, its points +of collision are often on soft banks, which are readily undermined by +the washing action of the stream. In the ordinary course of events, +the river beginning, we may imagine, with a straight channel, had its +current deflected by some obstacle, it may be even by the slight +pressure of a tributary stream, is driven against one bank; thence it +rebounds and strikes the other. At each point of impinge it cuts the +alluvium away. It can bear on only a small portion of that which it +thus obtains; the greater part of the material is deposited on the +opposite side of the stream, but a little lower down, where it makes a +shallow. On these shallows water-loving plants and even certain trees, +such as the willows and poplars, find a foothold. When the stream +rises, the sediment settles in this tangle, and soon extends the +alluvial plain from the neighbouring bank, or in rarer cases the river +comes to flow on either side of an island of its own construction. The +natural result of this billiard-ball movement of the waters is that +the path of the stream is sinuous. The less its rate of fall and the +greater the amount of silt it obtains from its tributaries, the more +winding its course becomes. This gain in those parts of the river's +curvings where deposition tends to take place may be accelerated by +tree-planting. Thus a skilful owner of a tract of land on the south +bank of the Ohio River, by assiduously planting willow trees on the +front of his property, gained in the course of thirty years more than +an acre in the width of his arable land. When told by the present +writer that he was robbing his neighbours on the other side of the +stream, he claimed that their ignorance of the laws of river motion +was sufficient evidence that they did not deserve to own land. + +In the primitive state of a country the water-loving plants, +particularly the trees which flourish in excessively humid conditions, +generally make a certain defence against these incursions of the +streams. But when a river has gained an opening in the bank it can, +during a flood, extend its width often to the distance of hundreds of +feet. During the inundations of the Mississippi the river may at times +be seen to eat away acres of land in a single day along one of the +outcurves of its banks. The undermined forests falling into the flood +join the great procession of drift timber, composed of trees which +have been similarly uprooted, which occupies the middle part of the +stream. This driftwood belt often has a width of three or four hundred +feet, the entangled stems and branches making it difficult for a boat +to pass from one side of the river to the other. + +[Illustration: Fig. 11.--Oxbows and cut-off. Showing the changes in +the course of a river in its alluvial plain.] + +When the curves of a river have been developed to a certain point (see +Fig. 11), when they have attained what is called the "oxbow" form, it +often happens that the stream breaks through the isthmus which +connects one of the peninsulas with the mainland. Where, as is not +infrequently the case, the bend has a length of ten miles or more, the +water just above and below the new-made opening is apt to differ in +height by some feet. Plunging down the declivity, the stream, flowing +with great velocity, soon enlarges the channel so that its whole tide +may take the easier way. When this result is accomplished, the old +curve is deserted, sand bars are formed across their mouths, which may +gradually grow to broad alluvial plains, so that the long-surviving, +crescent-shaped lake, the remnant of the river bed, may be seen far +from the present course of the ever-changing stream. Gradually the +accumulations of vegetable matter and the silt brought in by floods +efface this moat or oxbow cut-off, as it is so commonly termed. + +As soon as the river breaks through the neck of a peninsula in the +manner above described, the current of the stream becomes much swifter +for many miles below and above the opening. Slowly, however, the +slopes are rearranged throughout its whole course, yet for a time the +stream near the seat of the change becomes straighter than before, and +this for the reason that its swifter current is better able to dispose +of the _debris_ which is supplied to it. The effect of a change in the +current produced by such new channels as we have described as forming +across the isthmuses of bends is to perturb the course of the stream +in all its subsequent downward length. Thus an oxbow cut-off formed +near the junction of the Ohio and Mississippi may tend more or less to +alter the swings of the Mississippi all the way to the Gulf of Mexico. + +Although the swayings of the streams to and fro in their alluvial +plains will give the reader some idea as to the struggle which the +greater rivers have with the _debris_ which is committed to them, the +full measure of the work and its consequences can only be appreciated +by those who have studied the phenomena on the ground. A river such +as the Mississippi is endlessly endeavouring to bear its burden to the +sea. If its slope were a uniform inclined plane, the task might +readily be accomplished; but in this, as in almost all other large +water ways, the slope of the bed is ever diminishing with its onward +course. The same water which in the mountain torrent of the +Appalachians or Cordilleras rolled along stones several feet in +diameter down slopes of a hundred feet or more to the mile can in the +lower reaches of the stream move no pebbles which are more than one +fourth of an inch in diameter over slopes which descend on the average +about half a foot in a mile. Thus at every stage from the torrent to +the sea the detritus has from time to time to rest within the alluvial +banks, there awaiting the decay which slowly comes, and which may +bring it to the state where it may be dissolved in the water, or +divided into fragments so small that the stream may bear them on. A +computation which the present writer has made shows that, on the +average, it requires about forty thousand years for a particle of +stone to make its way down the Mississippi to the sea after it has +been detached from its original bed. Of course, some bits may make the +journey straightforwardly; others may require a far greater time to +accomplish the course which the water itself makes at most in a few +weeks. This long delay in the journey of the detritus--a delay caused +by its frequent rests in the alluvial plain--brings about important +consequences which we will now consider. + +As an alluvial plain is constructed, we generally find at the base +pebbly material which fell to the bottom in the current of the main +stream as the shores grew outward. Above this level we find the +deposits laid down by the flood waters containing no pebbles, and this +for the reason that those weightier bits remained in the stream bed +when the tide flowed over the plain. As the alluvial deposit is laid +down, a good deal of vegetable matter was built into it. Generally +this has decayed and disappeared. On the surface of the plain there +has always been growing abundant vegetation, the remains of which +decayed on the surface in the manner which we may observe at the +present day. This decomposing vegetable matter within and upon the +porous alluvial material produces large quantities of carbonic acid, a +gas which readily enters the rain water, and gives it a peculiar power +of breaking up rock matter. Acting on the _debris_, this gas-charged +water rapidly brings about a decay of the fragments. Much of the +material passes at once into solution in this water, and drains away +through the multitudinous springs which border the river. As this +matter is completely dissolved, as is sugar in water, it goes straight +away to the sea without ever again entering the alluvium. In many, if +not most, cases this dissolving work which is going on in alluvial +terraces is sufficient to render a large part of the materials which +they contain into the state where it disappears in an unseen manner; +thus while the annual floods are constantly laying down accumulations +on the surface of these plains, the springs are bearing it away from +below. + +In this way, through the decomposition which takes place in them, all +those river terraces where much vegetable matter is mingled with the +mineral substances, become laboratories in which substances are +brought into solution and committed to the seas. We find in the water +of the ocean a great array of dissolved mineral substances; it, +indeed, seems probable that the sea water contains some share, though +usually small, of all the materials which rivers encounter in their +journey over and under the lands. As the waters of the sea obtain but +little of this dissolved matter along the coast, it seems likely that +the greater share of it is brought into the state of solution in the +natural laboratories of the alluvial plains. + +Here and there along the sides of the valleys in which the rivers flow +we commonly find the remains of ancient plains lying at more or less +considerable heights above the level of the streams. Generally these +deposits, which from their form are called terraces, represent the +stages of down-wearing by which the stream has carved out its way +through the rocks. The greater part of these ancient alluvial plains +has been removed through the ceaseless swinging of the stream to and +fro in the valley which it has excavated. + +In all the states of alluvial plains, whether they be the fertile +deposits near the level of the streams which built them, or the poorer +and ruder surfaced higher terraces, they have a great value to +mankind. Men early learned that these lands were of singularly uniform +goodness for agricultural use. They are so light that they were easily +delved with the ancient pointed sticks or stone hoes, or turned by the +olden, wooden plough. They not only give a rich return when first +subjugated, but, owing to the depth of the soil and the frequency with +which they are visited by fertilizing inundations, they yield rich +harvests without fertilizing for thousands of years. It is therefore +not surprising that we find the peoples who depended upon tillage for +subsistence first developed on the great river plains. There, indeed, +were laid the foundations of our higher civilization; there alone +could the state which demands of its citizens fixed abodes and +continuous labour take rise. In the conditions which these fields of +abundance afforded, dense populations were possible, and all the arts +which lead toward culture were greatly favoured. Thus it is that the +civilization of China, India, Persia, and Egypt, the beginnings of +man's higher development, began near the mouths of the great river +valleys. These fields were, moreover, most favourably placed for the +institution of commerce, in that the arts of navigation, originating +in the sheltered reaches of the streams, readily found its way through +the estuaries to the open sea. + +Passing down the reaches of a great river as it approaches the sea, we +find that the alluvial plains usually widen and become lower. At +length we attain a point where the flood waters cover the surface for +so large a part of the year that the ground is swampy and untillable +unless it is artificially and at great expense of labour won to +agriculture in the manner in which this task has been effected in the +lower portion of the Rhine Valley. Still farther toward the sea, the +plain gradually dips downward until it passes below the level of the +waters. Through this mud-flat section the stream continues to cut +channels, but with the ever-progressive slowing of its motion the +burden of fine mud which it carries drops to the bottom, and +constantly closes the paths through which the water escapes. Every few +years they tend to break a new way on one side or the other of their +former path. Some of the greatest engineering work done in modern +times has been accomplished by the engineers engaged in controlling +the exits of large rivers to the sea. The outbreak of the Yellow River +in 1887, in which the stream, hindered by its own accumulations, +forced a new path across its alluvial plains, destroyed a vast deal of +life and property, and made the new exit seventy miles from the path +which it abandoned. + +Below the surface of the open water the alluvial deposits spread out +into a broad fan, which slopes gradually to a point where, in the +manner of the continental shelf, the bottom descends steeply into deep +water. + +It is the custom of naturalists to divide the lower section of river +deposits--that part of the accumulation which is near the sea--from +the other alluvial plains, terming the lower portion the delta. The +word originally came into use to describe that part of the alluvium +accumulated by the Nile near its mouth, which forms a fertile +territory shaped somewhat like the fourth letter of the Greek +alphabet. Although the definition is good in the Egyptian instance, +and has a certain use elsewhere, we best regard all the detritus in a +river valley which is in the state of repose along the stream to its +utmost branches as forming one great whole. It is, indeed, one of the +most united of the large features which the earth exhibits. The +student should consider it as a continuous inclined plane of +diminishing slope, extending from the base of the torrents to the +sea, and of course ramifying into the several branches of the river +system. He should further bear in mind the fact that it is a vast +laboratory where rock material is brought into the soluble state for +delivery to the seas. + +The diversity in the form of river valleys is exceedingly great. +Almost all the variety of the landscape is due to this impress of +water action which has operated on the surface in past ages. When +first elevated above the sea, the surface of the land is but little +varied; at this stage in the development the rivers have but shallow +valleys, which generally cut rather straight away over the plain +toward the sea. It is when the surface has been uplifted to a +considerable height, and especially when, as is usually the case, this +uplifting action has been associated with mountain-building, that +valleys take on their accented and picturesque form. The reason for +this is easily perceived: it lies in the fact that the rocks over +which the stream flows are guided in the cutting which they effect by +the diversities of hardness in the strata that they encounter. The +work which it does is performed by the hard substances that are +impelled by the current, principally by the sand and pebbles. These +materials, driven along by the stream, become eroding tools of very +considerable energy. As will be seen when we shortly come to describe +waterfalls, the potholes formed at those points afford excellent +evidence as to the capacity of stream-impelled bits of stone to cut +away the firmest bed rocks. Naturally the ease with which this carving +work is done is proportionate to the energy of the currents, and also +to the relative hardness of the moving bits and the rocks over which +they are driven. + +So long as the rocks lie horizontally in their natural construction +attitude the course of the stream is not much influenced by the +variations in hardness which the bed exhibits. Where the strata are +very firm there is likely to be a narrow gorge, the steeps of which +rise on either side with but slight alluvial plains; where the beds +are soft the valley widens, perhaps again to contract where in the +course of its descent it encounters another hard layer. Where, +however, the beds have been subjected to mountain-building, and have +been thrown into very varied attitudes by folding and faulting, the +stream now here and now there encounters beds which either restrain +its flow or give it freedom. The stream is then forced to cut its way +according to the positions of the various underlying strata. This +effect upon its course is not only due to the peculiarities of +uplifted rocks, but to manifold accidents of other nature: veins and +dikes, which often interlace the beds with harder or softer partitions +than the country rock; local hardenings in the materials, due to +crystallization and other chemical processes, often create +indescribable variations which are more or less completely expressed +in the path of the stream. + +When a land has been newly elevated above the sea there is often--we +may say, indeed, generally--a very great difference between the height +of its head waters and the ocean level. In this condition of a country +the rivers have what we may call a new aspect; their valleys are +commonly narrow and rather steep, waterfalls are apt to abound, and +the alluvial terraces are relatively small in extent. Stage by stage +the torrents cut deeper; the waste which they make embarrasses the +course of the lower waters, where no great amount of down-cutting is +possible for the reason that the bed of the stream is near sea level. +At the same time the alluvial materials, building out to sea, thus +diminish the slope of the stream. In the extreme old age of the river +system the mountains are eaten down so that the torrent section +disappears, and the stream becomes of something like a uniform slope; +the higher alluvial plains gradually waste away, until in the end the +valley has no salient features. At this stage in the process, or even +before it is attained, the valley is likely to be submerged beneath +the sea, where it is buried beneath the deposits formed on the floor; +or a further uplift of the land may occur with the result that the +stream is rejuvenated; or once more endowed with the power to create +torrents, build alluvial plains, and do the other interesting work of +a normal river. + +It rarely, if ever, happens that a river valley attains old age before +it has sunk beneath the sea or been refreshed by further upliftings. +In the unstable conditions of the continents, one or the other of +these processes, sometimes in different places both together, is apt +to be going on. Thus if we take the case of the Mississippi and its +principal tributaries, the Ohio and Missouri, we find that for many +geological ages the mountains about their sources have frequently, if +not constantly, grown upward, so that their torrent sections, though +they have worn down tens of thousands of feet, are still high above +the sea level, perhaps on the average as high as they have ever been. +At the same time the slight up-and-down swayings of the shore lands, +amounting in general to less than five hundred feet, have greatly +affected the channels of the main river and its tributaries in their +lower parts. Not long ago the Mississippi between Cairo and the Gulf +flowed in a rather steep-sided valley probably some hundreds of feet +in depth, which had a width of many miles. Then at the close of the +last Glacial period the region sank down so that the sea flooded the +valley to a point above the present junction of the Ohio River with +the main stream. Since then alluvial plains have filled this estuary +to even beyond the original mouth. In many other of our Southern +rivers, as along the shore from the Mississippi to the Hudson, the +streams have not brought in enough detritus to fill their drowned +valleys, which have now the name of bays, of which the Delaware and +Chesapeake on the Atlantic coast, and Mobile Bay on the Gulf of +Mexico, are good examples. The failure of Chesapeake and Delaware Bays +to fill with _debris_ in the measure exhibited by the more southern +valleys is due to the fact that the streams which flow into them to a +great extent drain from a region thickly covered with glacial waste, a +mass which holds the flood waters, yielding the supply but slowly to +the torrents, which there have but a slight cutting power. + +In our sketch of river valleys no attention has been given to the +phenomena of waterfalls, those accidents of the flow which, as we have +noted, are particularly apt to characterize rivers which have not yet +cut down to near the sea level. Where the normal uniform descent which +is characteristic of a river's bed is interrupted by a sudden steep, +the fact always indicates the occurrence of one of a number of +geological actions. The commonest cause of waterfalls is due to a +sudden change in the character of horizontal or at least nearly level +beds over which the stream may flow. Where after coursing for a +distance over a hard layer the stream comes to its edge and drops on a +soft or easily eroded stratum, it will cut this latter bed away, and +create a more or less characteristic waterfall. Tumbling down the face +of the hard layer, the stream acquires velocity; the _debris_ which it +conveys is hurled against the bottom, and therefore cuts powerfully, +while before, being only rubbed over the stone as it moved along, it +cut but slightly. Masses of ice have the same effect as stones. Bits +dropping from the ledge are often swept round and round by the eddies, +so that they excavate an opening which prevents their chance escape. +In these confined spaces they work like augers, boring a deep, +well-like cavity. As the bits of stone wear out they are replaced by +others, which fall in from above. Working in this way, the fragments +often develop regular well-like depressions, the cavities of which +work back under the cliffs, and by the undermining process deprive the +face of the wall of its support, so that it tumbles in ruin to the +base, there to supply more material for the potholing action. + +Waterfalls of the type above described are by far the commonest of +those which occur out of the torrent districts of a great river +system. That of Niagara is an excellent specimen of the type, which, +though rarely manifested in anything like the dignity of the great +fall, is plentifully shown throughout the Mississippi Valley and the +basin of the Great Lakes. Within a hundred miles of Niagara there are +at least a hundred small waterfalls of the same type. Probably three +quarters of all the larger accidents of this nature are due to the +conditions of a hard bed overlying softer strata. + +Falls are also produced in very many instances by dikes which cross +the stream. So, too, though rarely, only one striking instance being +known, an ancient coral reef which has become buried in strata may +afford rock of such hardness that when the river comes to cross it it +forms a cascade, as at the Falls of the Ohio, at Louisville, Ky. It is +a characteristic of all other falls, except those first mentioned, +that they rarely plunge with a clean downward leap over the face of a +precipice which recedes at its base, but move downward over an +irregular sloping surface. + +In the torrent district of rivers waterfalls are commonly very +numerous, and are generally due to the varying hardness in the rocks +which the streams encounter. Here, where the cutting action is going +on with great rapidity, slight differences in the resistance which the +rocks make to the work will lead to great variations in the form of +the bed over which they flow, while on the more gently sloping bottoms +of the rivers, where the _debris_ moves slowly, such variations would +be unimportant in their effect. When the torrents escape into the main +river valleys, in regions where the great streams have cut deep +gorges, they often descend from a great vertical height, forming +wonderful waterfalls, such as those which occur in the famous +Lauterbrunnen Valley of Switzerland or in that of the Yosemite in +California. This group of cascades is peculiar in that the steep of +the fall is made not by the stream itself, but by the action of a +greater river or of a glacier which may have some time taken its +place. + +Waterfalls have an economic as well as a picturesque interest in that +they afford sources of power which may be a very great advantage to +manufacturers. Thus along the Atlantic coast the streams which come +from the Appalachian highlands, and which have hardly escaped from +their torrent section before they attain the sea, afford numerous +cataracts which have been developed so that they afford a vast amount +of power. Between the James on the south and the Ste. Croix on the +north more than a hundred of these Appalachian rivers have been turned +to economic use. The industrial arts of this part of the country +depend much upon them for the power which drives their machinery. The +whole of the United States, because of the considerable size of its +rivers and their relatively rapid fall, is richly endowed with this +source of energy, which, originating in the sun's heat and conveyed +through the rain, may be made to serve the needs of man. In view of +the fact that recent inventions have made it possible to convert this +energy of falling water into the form of electricity, which may be +conveyed to great distances, it seems likely that our rivers will in +the future be a great source of national wealth. + +We must turn again to river valleys, there to trace certain actions +less evident than those already noted, but of great importance in +determining these features of the land. First, we have to note that in +the valley or region drained by a river there is another degrading or +down-wearing action than that which is accomplished by the direct work +of the visible stream. All over such a valley the underground waters, +soaking through the soil and penetrating through the underlying rock, +are constantly removing a portion of the mineral matter which they +take into solution and bear away to the sea. In this way, deprived of +a part of their substance, the rocks are continually settling down by +underwear throughout the whole basin, while they are locally being cut +down by the action of the stream. Hence in part it comes about that in +a river basin we find two contrasted features--the general and often +slight slope of a country toward the main stream and its greater +tributaries, and the sharp indentation of the gorge in which the +streams flow, these latter caused by the immediate and recent action +of the streams. + +If now the reader will conceive himself standing at any point in a +river basin, preferably beyond the realms of the torrents, he may with +the guidance of the facts previously noted, with a little use of the +imagination, behold the vast perceptive which the history of the river +valley may unfold to him. He stands on the surface of the soil, that +_debris_ of the rocks which is just entering on its way to the ocean. +In the same region ten thousand years ago he would have stood upon a +surface from one to ten feet higher than the present soil covering. A +million years ago his station would have been perhaps five hundred +feet higher than the surface. Ten million years in the past, a period +less than the lifetime of certain rivers, such as the French Broad +River in North Carolina, the soil was probably five thousand feet or +more above its present plane. There are, indeed, cases where river +valleys appear to have worked down without interruption from the +subsidence of the land beneath the sea to the depth of at least two +miles. Looking upward through the space which the rocks once occupied, +we can conceive the action of the forces in their harmonious +co-operation which have brought the surface slowly downward. We can +imagine the ceaseless corrosion due to the ground water, bringing +about a constant though slow descent of the whole surface. Again and +again the streams, swinging to and fro under the guidance of the +underlying rock, or from the obstacles which the _debris_ they carried +imposed upon them, have crossed the surface. Now and then perhaps the +wearing was intensified by glacial action, for an ice sheet often cuts +with a speed many times as great as that which fluid water can +accomplish. On the whole, this exercise of the constructive +imagination in conceiving the history of a river valley is one of the +most enlarging tasks which the geologist can undertake. + +Where in a river valley there are many lateral streams, and especially +where the process of solution carried on by the underground waters is +most effective, as compared with erosive work done in the bed of the +main river, we commonly find the valley sloping gently toward its +centre, the rivers having but slight steeps near their banks. On the +other hand, where, as occasionally happens, a considerable stream fed +by the rain and snow fall in its torrent section courses for a great +distance over high, arid plains, on which the ground water and the +tributaries do but little work, the basin may slope with very slight +declivity to the river margins, and there descend to great depths, +forming very deep gorges, of which the Colorado Canon is the most +perfect type. As instances of these contrasted conditions, we may +take, on the one hand, the upper Mississippi, where the grades toward +the main stream are gentle and the valley gorge but slightly +exhibited; on the other, the above-mentioned Colorado, which bears a +great tide of waters drawn from the high and relatively rainy region +of the Rocky Mountains across the vast plateau lying in an almost +rainless country. In this section nearly all the down-wearing has been +brought about in the direct path of the stream, which has worn the +elevated plain into a deep gorge during the slow uprising of the +table-land to its present height. In this way a defile nearly a mile +in depth has been created in a prevailingly rather flat country. This +gorge has embranchments where the few great tributaries have done like +work, but, on the whole, this river flows in an almost unbroken +channel, the excavation of which has been due to its swift, +pebble-bearing waters. + +The tendency of a newly formed river is to cut a more or less distinct +canon. As the basin becomes ancient, this element of the gorge tends +to disappear, the reason for this being that, while the river bed is +high above the sea, the current is swift and the down-cutting rapid, +while the slow subsidence of the country on either side--a process +which goes on at a uniform rate--causes the surface of that region to +be left behind in the race for the sea level. As the stream bed comes +nearer the sea level its rate of descent is diminished, and so the +outlying country gradually overtakes it. + +In regions where the winters are very cold the effect of ice on the +development of the stream beds both in the torrent and river sections +of the valley is important. This work is accomplished in several +diverse ways. In the first place, where the stream is clear and the +current does not flow too swiftly, the stones on the bottom radiate +their heat through the water, and thus form ice on their surfaces, +which may attain considerable thickness. As ice is considerably +lighter than water, the effect is often to lift up the stones of the +bed if they be not too large; when thus detached from the bottom, they +are easily floated down stream until the ice melts away. The ice which +forms on the surface of the water likewise imprisons the pebbles along +the banks, and during the subsequent thaw may carry them hundreds of +miles toward the sea. It seems likely, from certain observations made +by the writer, that considerable stones may thus be carried from the +Alleghany River to the main Mississippi. + +Perhaps the most important effect of ice on river channels is +accomplished when in a time of flood the ice field which covered the +stream, perhaps to the depth of some feet, is broken up into vast +floes, which drift downward with the current. When, as on the Ohio, +these fields sometimes have the area of several hundred acres, they +often collide with the shores, especially where the stream makes a +sharp bend. Urged by their momentum, these ice floes pack into the +semblance of a dam, which may have a thickness of twenty, thirty, or +even fifty feet. Beginning on the shore, where the collision takes +place, the dam may swiftly develop clear across the stream, so that in +a few minutes the way of the waters is completely blocked. The +on-coming ice shoots up upon the accumulation, increases its height, +and extends it up stream, so that in an hour the mass completely bars +the current. The waters then heap up until they break their way over +the obstacle, washing its top away, until the whole is light enough +to be forced down the stream, where, by the friction it encounters on +the bottom and sides of the channel, it is broken to pieces. It is +easy to see that such moving dams of ice may sweep the bed of a river +as with a great broom. + +Sometimes where the gorges do not form a stationary dam large cakes of +ice become turned on edge and pack together so that they roll down the +stream like great wheels, grinding the bed rock as they go. + +In high northern countries, as in Siberia, the rivers, even the +deepest, often become so far frozen that their channels are entirely +obstructed. Where, as in the case of these Siberian rivers, the flow +is from south to north, it often happens that the spring thaw sets in +before the more northern beds of the main stream are released from +their bondage of frost. In this case the inundations have to find new +paths on either side of the obstructed way. The result is a type of +valleys characterized by very irregular and changeable stream beds, +the rivers having no chance to organize themselves into the shapely +curves which they ordinarily follow. + +The supply which finds its way to a river is composed, as has been +already incidentally noted, in part of the water which courses +underground for a greater or less distance before it emerges to the +surface, and in part of that which moves directly over the ground. +These two shares of water have somewhat different histories. On the +share of these two depends the stability of the flow. Where, as in New +England and other glaciated countries, the surface of the earth is +covered with a thick layer of sand and gravel, which, except when +frozen, readily admits the water; the rainfall is to a very great +extent absorbed by the earth, and only yielded slowly to the streams. +In these cases floods are rare and of no great destructive power. +Again, where also the river basin is covered by a dense mantle of +forests, the ground beneath which is coated, as is the case in +primeval woods, with a layer of decomposing vegetation a foot or more +in depth, this spongy mass retains the water even more effectively +than the open-textured glacial deposits above referred to. When the +woods, however, are removed from such an area, the rain may descend to +the streams almost as speedily as it finds its way to the gutters from +the house roofs. It thus comes about that all regions, when reduced to +tillage, and where the rainfall is enough to maintain a good +agriculture, are, except when they have a coating of glacial waste, +exceedingly liable to destructive inundations. + +Unhappily, the risk of river floods is peculiarly great in all the +regions of the United States lying much to the east of the Rocky +Mountains, except in the basin of the Great Lakes and in the district +of New England, where the prevalence of glacial sands and gravels +affords the protection which we have noted. Throughout this region the +rainfall is heavy, and the larger part of it is apt to come after the +ground has become deeply snow-covered. The result is a succession of +devastating floods which already are very damaging to the works of +man, and promise to become more destructive as time goes on. More than +in any other country, we need the protection which forests can give us +against these disastrous outgoings of our streams. + + + LAKES. + +In considering the journey of water from the hilltops to the sea, we +should take some account of those pauses which it makes on its way +when for a time it falls into the basin of a lake. These arrests in +the downward motion of water, which we term lakes, are exceedingly +numerous; their proper discussion would, indeed, require a +considerable volume. We shall here note only the more important of +their features, those which are of interest to the general student. + +The first and most noteworthy difference in lakes is that which +separates the group of dead seas from the living basins of fresh +water. When a stream attains a place where its waters have to expand +into the lakelike form, the current moves in a slow manner, and the +broad surface exposed to the air permits a large amount of +evaporation. If the basin be large in proportion to the amount of the +incurrent water, this evaporation may exceed the supply, and produce a +sea with no outlet, such as we find in the Dead Sea of Judea, in that +at Salt Lake, Utah, and in a host of other less important basins. If +the rate of evaporation be yet greater in proportion to the flow, the +lake may altogether dry away, and the river be evaporated before it +attains the basin where it might accumulate. In that case the river is +said to sink, but, in place of sinking into the earth, its waters +really rise into the air. Many such sinks occur in the central portion +of the Rocky Mountain district. It is important to note that the +process of evaporation we are describing takes place in the case of +all lakes, though only here and there is the air so dry that the +evaporation prevents the basin from overflowing at the lowest point on +its rim, forming a river which goes thence to the sea. Even in the +case of the Great Lakes of North America a considerable part of the +water which flows into them does not go to the St. Lawrence and thence +to the sea. As long as the lake finds an outlet to the sea its waters +contain but little more dissolved mineral matter than that we find in +the rivers. But because all water which has been in contact with the +earth has some dissolved mineral substances, while that which goes +away by evaporation is pure water, a lake without an outlet gradually +becomes so charged with these materials that it can hold no more in +solution, but proceeds to lay them down in deposits of that compound +substance which from its principal ingredient we name salt. The water +of dead seas, because of the additional weight of the substances which +it holds, is extraordinarily buoyant. The swimmer notes a difference +in this regard in the waters of rivers and fresh-water lakes and those +of the sea, due to this same cause. But in those of dead seas, +saturated with saline materials, the human body can not sink as it +does in the ordinary conditions of immersion. It is easy to understand +how the salt deposits which are mined in many parts of the world have +generally, if not in all cases, been formed in such dead seas.[5] + +[Footnote 5: In some relatively rare cases salt deposits are formed in +lagoons along the shores of arid lands, where the sea occasionally +breaks over the beach into the basin, affording waters which are +evaporated, leaving their salt behind them.] + +It is an interesting fact that almost all the known dead seas have in +recent geological times been living lakes--that is, they poured over +their brims. In the Cordilleras from the line between Canada and the +United States to central Mexico there are several of these basins. All +of those which have been studied show by their old shore lines that +they were once brimful, and have only shrunk away in modern times. +These conditions point to the conclusion that the rainfall in +different regions varies greatly in the course of the geologic ages. +Further confirmation of this is found in the fact that very great salt +deposits exist on the coast of Louisiana and in northern +Europe--regions in which the rainfall is now so great in proportion to +the evaporation that dead seas are impossible. + +Turning now to the question of how lake basins are formed, we note a +great variety in the conditions which may bring about their +construction. The greatest agent, or at least that which operates in +the construction of the largest basins, are the irregular movements of +the earth, due to the mountain-building forces. Where this work goes +on on a large scale, basin-shaped depressions are inevitably formed. +If all those which have existed remained, the large part of the lands +would be covered by them. In most cases, however, the cutting action +of the streams has been sufficient to bring the drainage channels down +to the bottom of the trough, while the influx of sediments has served +to further the work by filling up the cavities. Thus at the close of +the Cretaceous period there was a chain of lakes extending along the +eastern base of the Rocky Mountains, constituting fresh-water seas +probably as large as the so-called Great Lakes of North America. But +the rivers, by cutting down and tilling up, have long since +obliterated these water areas. In other cases the tiltings of the +continent, which sometimes oppose the flow of the streams, may for a +time convert the upper part of a river basin which originally sloped +gently toward the sea into a cavity. Several cases of this description +occurred in New England in the closing stages of the Glacial period, +when the ground rose up to the northward. + +We have already noted the fact that the basin of a dead sea becomes in +course of time the seat of extensive salt deposits. These may, indeed, +attain a thickness of many hundred feet. If now in the later history +of the country the tract of land with the salt beneath it were +traversed by a stream, its underground waters may dissolve out the +salt and in a way restore the basin to its original unfilled +condition, though in the second state that of a living lake. It seems +very probable that a portion at least of the areas of Lakes Ontario, +Erie, and Huron may be due to this removal of ancient salt deposits, +remains of which lie buried in the earth in the region bordering these +basins. + +By far the commonest cause of lake basins is found in the +irregularities of the surface which are produced by the occupation of +the country by glaciers. When these great sheets of ice lie over a +land, they are in motion down the slopes on which they rest; they wear +the bed rocks in a vigorous manner, cutting them down in proportion to +their hardness. As these rocks generally vary in the resistance which +they oppose to the ice, the result is that when the glacier passes +away the surface no longer exhibits the continued down slope which the +rivers develop, but is warped in a very complicated way. These +depressions afford natural basins in which lakes gather; they may vary +in extent from a few square feet to many square miles. When a glacier +occupies a country, the melting ice deposits on the surface of the +earth a vast quantity of rocky _debris_, which was contained in its +mass. This detritus is irregularly accumulated; in part it is disposed +in the form of moraines or rude mounds made at the margin of the +glacier, in part as an irregular sheet, now thick, now thin, which +covers the whole of the field over which the ice lay. The result of +this action is the formation of innumerable pools, which continue to +exist until the streams have cut channels through which their waters +may drain away, or the basins have become filled with detritus +imported from the surrounding country or by peat accumulations which +the plants form in such places. + +Doubtless more than nine tenths of all the lake basins, especially +those of small size, which exist in the world are due to +irregularities of the land surface which are brought about by glacial +action. Although the greater part of these small basins have been +obliterated since the ice left this country, the number still +remaining of sufficient size to be marked on a good map is +inconceivably great. In North America alone there are probably over a +hundred and fifty thousand of these glacial lakes, although by far the +greater part of those which existed when the glacial sheet disappeared +have been obliterated. + +Yet another interesting group of fresh-water lakes, or rather we +should call them lakelets from their small size, owes its origin to +the curious underground excavations or caverns which are formed in +limestone countries. The water enters these caverns through what are +termed "sink holes"--basins in the surface which slope gently toward a +central opening through which the water flows into the depths below. +The cups of the sink holes rarely exceed half a mile in diameter, and +are usually much smaller. Their basins have been excavated by the +solvent and cutting actions of the rain water which gathers in them to +be discharged into the cavern below. It often happens that after a +sink hole is formed some slight accident closes the downward-leading +shaft, so that the basin holds water; thus in parts of the United +States there are thousands of these nearly circular pools, which in +certain districts, as in southern Kentucky, serve to vary the +landscape in much the same manner as the glacial lakes of more +northern countries. + +Some of the most beautiful lakes in the world, though none more than a +few miles in diameter, occupy the craters of extinct volcanoes. When +for a time, or permanently, a volcano ceases to do its appointed work +of pouring forth steam and molten rock from the depths of the earth, +the pit in the centre of the cone gathers the rain water, forming a +deep circular lake, which is walled round by the precipitous faces of +the crater. If the volcano reawakens, the water which blocks its +passage may be blown out in a moment, the discharge spreading in some +cases to a great distance from the cone, to be accumulated again when +the vent ceases to be open. The most beautiful of these volcanic lakes +are to be found in the region to the north and south of Rome. The +original seat of the Latin state was on the shores of one of these +crater pools, south of the Eternal City. Lago Bolsena, which lies to +the northward, and is one of the largest known basins of this nature, +having a diameter of about eight miles, is a crater lake. The volcanic +cone to which it belongs, though low, is of great size, showing that +in its time of activity, which did not endure very long, this crater +was the seat of mighty ejections. The noblest specimen of this group +of basins is found in Crater Lake, Oregon, now contained in one of the +national parks of the United States. + +Inclosed bodies of water are formed in other ways than those +described; the list above given includes all the important classes of +action which produce these interesting features. We should now note +the fact that, unlike the seas, the lakes are to be regarded as +temporary features in the physiography of the land. One and all, they +endure for but brief geologic time, for the reason that the streams +work to destroy them by filling them with sediment and by carving out +channels through which their waters drain away. The nature of this +action can well be conceived by considering what will take place in +the course of time in the Great Lakes of North America. As Niagara +Falls cut back at the average rate of several feet a year, it will be +but a brief geologic period before they begin to lower the waters of +Lake Erie. It is very probable, indeed, that in twenty thousand years +the waters of that basin will be to a great extent drained away. When +this occurs, another fall or rapid will be produced in the channel +which leads from Lake Huron to Lake Erie. This in turn will go through +its process of retreat until the former expanse of waters disappears. +The action will then be continued at the outlets of Lakes Michigan and +Superior, and in time, but for the interposition of some actions which +recreate these basins, their floors will be converted into dry land. + +It is interesting to note that lakes owe in a manner the preservation +of their basins to an action which they bring about on the waters that +flow into them. These rivers or torrents commonly convey great +quantities of sediment, which serve to rasp their beds and thus to +lower their channels. In all but the smaller lakelets these turbid +waters lay down all their sediment before they attain the outlet of +the basin. Thus they flow away over the rim rock in a perfectly pure +state--a state in which, as we have noted before, water has no +capacity for abrading firm rock. Thus where the Niagara River passes +from Lake Erie its clean water hardly affects the stone over which it +flows. It only begins to do cutting work where it plunges down the +precipice of the Falls and sets in motion the fragments which are +constantly falling from that rocky face. These Falls could not have +begun as they did on the margin of Lake Ontario except for the fact +that when the Niagara River began to flow, as in relatively modern +times, it found an old precipice on the margin of Lake Ontario, formed +by the waves of the lake, down which the waters fell, and where they +obtained cutting tools with which to undermine the steep which forms +the Falls. + +Many great lakes, particularly those which we have just been +considering, have repeatedly changed their outlets, according as the +surface of the land on which they lie has swayed up and down in +various directions, or as glacial sheets have barred or unbarred the +original outlets of the basins. Thus in the Laurentian Lakes above +Ontario the geologist finds evidence that the drainage lines have +again and again been changed. For a time during the Glacial period, +when Lake Ontario and the valley of the St. Lawrence was possessed by +the ice, the discharge was southward into the upper Mississippi or the +Ohio. At a later stage channels were formed leading from Georgian Bay +to the eastern part of Ontario. Yet later, when the last-named lake +was bared, an ice dam appears to have remained in the St. Lawrence, +which held back the waters to such a height that they discharged +through the valley of the Mohawk into the Hudson. Furthermore, at some +time before the Glacial period, we do not know just when, there +appears to have been an old Niagara River, now filled with drift, +which ran from Lake Erie to Ontario, a different channel from that +occupied by the present stream. + +The effects of lakes on the river systems with which they are +connected is in many ways most important. Where they are of +considerable extent, or where even small they are very numerous, they +serve to retain the flood waters, delivering them slowly to the +excurrent streams. In rising one foot a lake may store away more water +than the river by its consequent rise at the point of outflow will +carry away in many months, and this for the simple reason that the +lake may be many hundred or even thousand times as wide as the stream. +Moreover, as before noted, the sediment gathered by the stream above +the level of the lake is deposited in its basin, and does not affect +the lower reaches of the river. The result is that great rivers, such +as drain from the Laurentian Lakes, flow clear water, are exempt from +floods, are essentially without alluvial plains or terraces, and form +no delta deposits. In all these features the St. Lawrence River +affords a wonderful contrast to the Mississippi. Moreover, owing to +the clear waters, though it has flowed for a long time, it has never +been able to cut away the slight obstructions which form its rapids, +barriers which probably would have been removed if its waters had been +charged with sediment. + +[Illustration: _Muir Glacier, Alaska, showing crevasses and dust +layer on surface of ice._] + + + + + CHAPTER VI. + + GLACIERS. + + +We have already noted the fact that the water in the clouds is very +commonly in the frozen state; a large part of that fluid which is +evaporated from the sea attains the solid form before it returns to +the earth. Nevertheless, in descending, at least nine tenths of the +precipitation returns to the fluid state, and does the kind of work +which we have noted in our account of water. Where, however, the water +arrives on the earth in the frozen condition, it enters on a role +totally different from that followed by the fluid material. + +Beginning its descent to the earth in a snowflake, the little mass +falls slowly, so that when it comes against the earth the blow which +it strikes is so slight that it does no effective work. In the state +of snow, even in the separate flakes, the frozen water contains a +relatively large amount of air. It is this air indeed, which, by +dividing the ice into many flakes that reflect the light, gives it the +white colour. This important point can be demonstrated by breaking +transparent ice into small bits, when we perceive that it has the hue +of snow. Much the same effect is given where glass is powdered, and +for the same reason. + +As the snowflakes accumulate layer on layer they imbed air between +them, so that when the material falls in a feathery shape--say to the +depth of a foot--more than nine tenths of the mass is taken up by the +air-containing spaces. As these cells are very small, the circulation +in them is slight, and so the layer becomes an admirable +non-conductor, having this quality for the same reason that feathers +have it--i.e., because the cells are small enough to prevent the +circulation of the air, so that the heat which passes has to go by +conduction, and all gases are very poor conductors. The result is that +a snow coating is in effect an admirable blanket. When the sun shines +upon it, much of the heat is reflected, and as the temperature does +not penetrate it to any depth, only the superficial part is melted. +This molten water takes up in the process of melting a great deal of +heat, so that when it trickles down into the mass it readily +refreezes. On the other hand, the heat going out from the earth, the +store accumulated in its superficial parts in the last warm season, +together with the small share which flows out from the earth's +interior, is held in by this blanket, which it melts but slowly. Thus +it comes about that in regions of long-enduring snowfall the ground, +though frozen to the depth of a foot or more at the time when the +accumulation took place, may be thawed out and so far warmed that the +vegetation begins to grow before the protecting envelope of snow has +melted away. Certain of the early flowers of high latitudes, indeed, +begin to blossom beneath the mantle of finely divided ice. + +In those parts of the earth which for the most part receive only a +temporary coating of snow the effect of this covering is +inconsiderable. The snow water is yielded to the earth, from which it +has helped to withdraw the frost, so that in the springtime, the +growing season of plants, the ground contains an ample store of +moisture for their development. Where the snowfall accumulates to a +great thickness, especially where it lodges in forests, the influence +of the icy covering is somewhat to protract the winter and thus to +abbreviate the growing season. + +Where snow rests upon a steep slope, and gathers to the depth of +several feet, it begins to creep slowly down the declivity in a manner +which we may often note on house roofs. This motion is favoured by the +gradual though incomplete melting of the flakes as the heat +penetrates the mass. Making a section through a mass of snow which has +accumulated in many successive falls, we note that the top may still +have the flaky character, but that as we go down the flakes are +replaced by adherent shotlike bodies, which have arisen from the +partial melting and gathering to their centres of the original +expanded crystalline bits. In this process of change the mass can move +particle by particle in the direction in which gravity impels it. The +energy of its motion, however, is slight, yet it can urge loose stones +and forest waste down hill. Sometimes, as in the cemetery at Augusta, +Me., where stone monuments or other structures, such as iron railings, +are entangled in the moving mass, it may break them off and convey +them a little distance down the slope. + +So long as the summer sun melts the winter's snow, even if the ground +be bare but for a day, the role of action accomplished by the snowfall +is of little geological consequence. When it happens that a portion of +the deposit holds through the summer, the region enters on the glacial +state, and its conditions undergo a great revolution, the consequences +of which are so momentous that we shall have to trace them in some +detail. Fortunately, the considerations which are necessary are not +recondite, and all the facts are of an extremely picturesque nature. + +Taking such a region as New England, where all the earth is +life-bearing in the summer season, and where the glacial period of the +winter continues but for a short time, we find that here and there on +the high mountains the snow endures throughout most of the summer, but +that all parts of the surface have a season when life springs into +activity. On the top of Mount Washington, in the White Mountains of +New Hampshire, in a cleft known as Tuckerman's Ravine, where the +deposit accumulates to a great depth, the snow-ice remains until +midsummer. It is, indeed, evident that a very slight change in the +climatal conditions of this locality would establish a permanent +accumulation of frozen water upon the summit of the mountain. If the +crest were lifted a thousand feet higher, without any general change +in the heat or rainfall of the district, this effect would be +produced. If with the same amount of rainfall as now comes to the +earth in that region more of it fell as snow, a like condition would +be established. Furthermore, with an increase of rainfall to something +like double that which now descends the snow bore the same proportion +to the precipitation which it does at present, we should almost +certainly have the peak above the permanent snow line, that level +below which all the winter's fall melts away. These propositions are +stated with some care, for the reason that the student should perceive +how delicate may be--indeed, commonly is--the balance of forces which +make the difference between a seasonal and a perennial snow covering. + +As soon as the snow outlasts the summer, the region which it occupies +is sterilized to life. From the time the snow begins to hold over the +warm period until it finally disappears, that field has to be reckoned +out of the habitable earth, not only to man, but to the lowliest +organisms.[6] + +[Footnote 6: In certain fields of permanent snow, particularly near their +boundaries, some very lowly forms of vegetable life may develop on a +frozen surface, drawing their sustenance from the air, and supplied with +water by the melting which takes place during the summertime. These +forms include the rare phenomenon termed red snow.] + +If the snow in a glaciated region lay where it fell, the result would +be a constant elevation of the deposit year by year in proportion to +the annual excess of deposition over the melting or evaporation of the +material. But no sooner does the deposit attain any considerable +thickness than it begins to move in the directions of least +resistance, in accordance with laws which the students of glaciers are +just beginning to discern. In small part this motion is accomplished +by avalanches or snow slides, phenomena which are in a way important, +and therefore merit description. Immediately after a heavy snowfall, +in regions where the slopes are steep, it often happens that the +deposit which at first clung to the surface on which it lay becomes so +heavy that it tends to slide down the slope; a trifling action, the +slipping, indeed, of a single flake, may begin the movement, which at +first is gradual and only involves a little of the snow. Gathering +velocity, and with the materials heaped together from the junction of +that already in motion with that about to be moved, the avalanche in +sliding a few hundred feet down the slope may become a deep stream of +snow-ice, moving with great celerity. At this stage it begins to break +off masses of ice from the glaciers over which it may flow, or even to +move large stones. Armed with these, it rends the underlying earth. +After it has flowed a mile it may have taken up so much earth and +material that it appears like a river of mud. Owing to the fact that +the energy which bears it downward is through friction converted into +heat, a partial melting of the mass may take place, which converts it +into what we call slush, or a mixture of snow and water. Finally, the +torrent is precipitated into the bottom of a valley, where in time the +frozen water melts away, leaving only the stony matter which it bore +as a monument to show the termination of its flow. + +It was the good fortune of the writer to see in the Swiss Oberland one +very great avalanche, which came from the high country through a +descent of several thousand feet to the surface of the Upper +Grindelwald Glacier. The first sign of the action was a vague tremor +of the air, like that of a great organ pipe when it begins to vibrate, +but before the pulsations come swiftly enough to make an audible note. +It was impossible to tell when this tremor came, but the wary guide, +noting it before his charge could perceive anything unusual, made +haste for the middle of the glacier. The vibration swelled to a roar, +but the seat of the sound amid the echoing cliffs was indeterminable. +Finally, from a valley high up on the southern face of the glacier, +there leaped forth first a great stone, which sprang with successive +rebounds to the floor of ice. Then in succession other stones and +masses of ice which had outrun the flood came thicker and thicker, +until at the end of about thirty seconds the steep front of the +avalanche appeared like a swift-moving wall. Attaining the cliffs, it +shot forth as a great cataract, which during the continuance of the +flow--which lasted for several minutes--heaped a great mound of +commingled stones and ice upon the surface of the glacier. The mass +thus brought down the steep was estimated at about three thousand +cubic yards, of which probably the fiftieth part was rock material. An +avalanche of this volume is unusual, and the proportion of stony +matter borne down exceptionally great; but by these sudden motions of +the frozen water a large part of the snow deposited above the zone of +complete melting is taken to the lower valleys, where it may disappear +in the summer season, and much of the erosion accomplished in the +mountains is brought about by these falls. + +In all Alpine regions avalanches are among the most dreaded accidents. +Their occurrence, however, being dependent upon the shape of the +surface, it is generally possible to determine in an accurate way the +liability of their happening in any particular field. The Swiss take +precaution to protect themselves from their ravages as other folk do +to procure immunity from floods. Thus the authorities of many of the +mountain hamlets maintain extensive forests on the sides of the +villages whence the downfall may be expected, experience having shown +that there is no other means so well calculated to break the blow +which these great snowfalls can deliver, as thick-set trees which, +though they are broken down for some distance, gradually arrest the +stream. + +As long as the region occupied by permanent snow is limited to sharp +mountain peaks, relief by the precipitation of large masses to the +level below the snow line is easily accomplished, but manifestly this +kind of a discharge can only be effective from a very small field. +Where the relief is not brought about by these tumbles of snow, +another mode of gravitative action accomplishes the result, though in +a more roundabout way, through the mechanism of glaciers. + +We have already noted the fact that the winter's snow upon our +hillsides undergoes a movement in the direction of the slope. What we +have now to describe in a rather long story concerning glaciers rests +upon movements of the same nature, though they are in certain features +peculiarly dependent on the continuity of the action from year to +year. It is desirable, however, that the student should see that there +is at the foundation no more mystery in glacial motion than there is +in the gradual descent of the snow after it has lain a week on a +hillside. It is only in the scale and continuity of the action that +the greatest glacial envelope exceeds those of our temporary +winters--in fact, whenever the snow falls the earth it covers enters +upon an ice period which differs only in degree from that from which +our hemisphere is just escaping. + +Where the reader is so fortunate as to be able to visit a region of +glaciers, he had best begin his study of their majestic phenomena by +ascending to those upper realms where the snow accumulates from year +to year. He will there find the natural irregularities of the rock +surface in a measure evened over by a vast sheet of snow, from which +only the summits of the greater mountains rise. He may soon satisfy +himself that this sheet is of great depth, for here and there it is +intersected by profound crevices. If the visit is made in the season +when snow falls, which is commonly during most of the year, he may +observe, as before noted in our winter's snow, that the deposit, +though at first flaky, attains at a short distance below the surface a +somewhat granular character, though the shotlike grains fall apart +when disturbed. Yet deeper, ordinarily a few feet below the surface, +these granules are more or less cemented together; the mass thus loses +the quality of snow, and begins to appear like a whitish ice. Looking +down one of the crevices, where the light penetrates to the depth of a +hundred feet or more, he may see that the bluish hue somewhat +increases with the depth. A trace of this colour is often visible even +in the surface snow on the glacier, and sometimes also in our ordinary +winter fields. In a hole made with a stick a foot or more in depth a +faint cerulean glimmer may generally be discerned; but the increased +blueness of the ice as we go down is conspicuous, and readily leads us +to the conclusion that the air, to which, as we before noted, the +whiteness of the snow is due, is working out of the mass as the +process of compaction goes on. In a glacial district this snow mass +above the melting line is called the _neve_. + +Remembering that the excess of snow beyond the melting in a _neve_ +district amounts, it may be, to some feet of material each year, we +easily come to the conclusion that the mass works down the slope in +the manner which it does even where the coating is impermanent. This +supposition is easily confirmed: by observing the field we find that +the sheet is everywhere drawing away from the cliffs, leaving a deep +fissure between the _neve_ and the precipices. This crevice is called +by the German-Swiss guides the _Bergschrund_. Passage over it is +often one of the most difficult feats to accomplish which the Alpine +explorer has to undertake. In fact, the very appearance of the +surface, which is that of a river with continuous down slopes, is +sufficient evidence that the mass is slowly flowing toward the +valleys. Following it down, we almost always come to a place where it +passes from the upper valleys to the deeper gorges which pierce the +skirts of the mountain. In going over this projection the mass of +snow-ice breaks to pieces, forming a crowd of blocks which march down +the slope with much more speed than they journeyed when united in the +higher-lying fields. In this condition and in this part of the +movement the snow-ice forms what are called the _seracs_, or curds, as +the word means in the French-Swiss dialect. Slipping and tumbling +down the steep slope on which the _seracs_ develop, the ice becomes +broken into bits, often of small size. These fragments are quickly +reknit into the body of ice, which we shall hereafter term the +glacier, and in this process the expulsion of the air goes on more +rapidly than before, and the mass assumes a more transparent icelike +quality. + +The action of the ice in the pressures and strains to which it is +subjected in joining the main glacier and in the further part of its +course demand for their understanding a revision of those notions as +to rigidity and plasticity which we derive from our common experience +with objects. It is hard to believe that ice can be moulded by +pressure into any shape without fracturing, provided the motion is +slowly effected, while at the same time it is as brittle as ice to a +sudden blow. We see, however, a similar instance of contrasted +properties in the confection known as molasses candy, a stick of which +may be indefinitely bent if the flexure is slowly made, but will fly +to pieces like glass if sharply struck. Ice differs from the sugary +substance in many ways; especially we should note that while it may be +squeezed into any form, it can not be drawn out, but fractures on the +application of a very slight tension. The conditions of its movement +we will inquire into further on, when we have seen more of its action. + +Entering on the lower part of its course, that where it flows into the +region below the snow line, the ice stream is now confined between the +walls of the valley, a channel which in most cases has been shaped +before the ice time, by a mountain torrent, or perhaps by a slower +flowing river. In this part of its course the likeness of a glacial +stream to one of fluid water is manifest. We see that it twists with +the turn of the gorge, widens where the confining walls are far apart, +and narrows where the space is constricted. Although the surface is +here and there broken by fractures, it is evident that the movement of +the frozen current, though slow, is tolerably free. By placing stakes +in a row across the axis of a glacier, and observing their movement +from day to day, or even from hour to hour if a good theodolite is +used for the purpose, we note that the movement of the stream is +fastest in the middle parts, as in the case of a river, and that it +slows toward either shore, though it often happens, as in a stream of +molten water, that the speediest part of the current is near one side. +Further observations have indicated that the movement is most rapid on +the surface and least at the bottom, in which the stream is also +riverlike. It is evident, in a word, that though the ice is not fluid +in strict sense, the bits of which it is made up move in substantially +the manner of fluids--that is, they freely slip over each other. We +will now turn our attention to some important features of a detailed +sort which glaciers exhibit. + +If we visit a glacier during the part of the year when the winter +snows are upon it, it may appear to have a very uninterrupted surface. +But as the summer heat advances, the mask of the winter coating goes +away, and we may then see the structure of the ice. First of all we +note in all valley glaciers such as we are observing that the stream +is overlaid by a quantity of rocky waste, the greater part of which +has come down with the avalanches in the manner before described, +though a small part may have been worn from the bed over which the ice +flows. In many glaciers, particularly as we approach their +termination, this sheet of earth and rock materials often covers the +ice so completely that the novice in such regions finds it difficult +to believe that the ice is under his feet. If the explorer is minded +to take the rough scramble, he can often walk for miles on these +masses of stone without seeing, much less setting foot on any frozen +water. In some of the Alaskan glaciers this coating may bear a forest +growth. In general, this material, which is called moraine, is +distributed in bands parallel to the sides of the glaciers, and the +strips may amount to a half dozen or more. Those on the sides of the +ice have evidently been derived from the precipices which they have +passed. Those in the middle have arisen from the union of the moraines +formed in two or more tributary valleys. + +[Illustration: Fig. 12.--Map of glaciers and moraines near Mont Blanc.] + +Where the avalanches fall most plentifully, the stones lie buried with +the snow, and only melt out when the stream attains the region where +the annual waste of its surface exceeds the snowfall. In this section +we can see how the progressive melting gradually brings the rocky +_debris_ into plain view. Here and there we will find a boulder +perched on a pedestal of ice, which indicates a recent down-wearing of +the field. A frequent sound in these regions arises from the tumble of +the stones from their pedestals or the slipping of the masses from the +sharp ridge which is formed by the protection given to the ice through +the thick coating of detritus on its surface. These movements of the +moraines often distribute their waste over the glacier, so that in its +lower part we can no longer trace the contributions from the several +valleys, the whole area being covered by the _debris_. At the end of +the ice stream, where its forward motion is finally overcome by the +warmth which it encounters, it leaves in a rude heap, extending often +like a wall across the valley, all the coarse fragments which it +conveys. This accumulation, composed of all the lateral moraines which +have gathered on the ice by the fall of avalanches, is called the +terminal moraine. As the ice stream itself shrinks, a portion of the +detritus next the boundary wall is apt to be left clinging against +those slopes. It is from the presence of these heaps in valleys now +abandoned by glaciers that we obtain some information as to the former +greater extent of glacial action. + +The next most noticeable feature is the crevasse. These fractures +often exist in very great numbers, and constitute a formidable barrier +in the explorer's way. The greater part of these ruptures below the +_serac_ zone run from the sides of the stream toward the centre +without attaining that region. These are commonly pointed up stream; +their formation is due to the fact that, owing to the swifter motion +in the central parts of the stream, the ice in that section draws away +from the material which is moving more slowly next the shore. As +before noted, these ice fractures when drawn out naturally form +fissures at right angles to the direction of the strain. In the middle +portions of the ice other fissures form, though more rarely, which +appear to depend on local strains brought about through the +irregularity of the surface over which the ice is flowing. + +If the observer is fortunate, he may in his journey over the glacier +have a chance to see and hear what goes on when crevasses are formed. +First he will hear a deep, booming sound beneath his feet, which +merges into a more splintering note as the crevice, which begins at +the bottom or in the distance, comes upward or toward him. When the +sound is over, he may not be able to see a trace of the fracture, +which at first is very narrow. But if the break intersect any of the +numerous shallow pools which in a warm summer's day are apt to cover a +large part of the surface, he may note a line of bubbles rushing up +through the water, marking the escape of the air from the glacier, +some remnant of that which is imprisoned in the original snow. Even +where this indication is wanting, he can sometimes trace the crevice +by the hissing sound of the air streams where they issue from the ice. +If he will take time to note what goes on, he can usually in an hour +or two behold the first invisible crack widen until it may be half an +inch across. He may see how the surface water hastens down the +opening, a little river system being developed on the surface of the +ice as the streams make their way to one or more points of descent. In +doing this work they excavate a shaft which often becomes many feet in +diameter, down which their waters thunder to the base of the glacier. +This well-like opening is called a _moulin_, or mill, a name which, as +we shall see, is well deserved from the work which falling waters +accomplish. Although the institution of the _moulin_ shaft depends +upon the formation of a crevice, it often happens that as the ice +moves farther on its journey its walls are again thrust together, +soldered in the manner peculiar to ice, so that no trace of the +rupture remains except the shaft which it permitted to form. Like +everything else in the glacier, the _moulin_ slowly moves down the +slope, and remains open as long as it is the seat of descending waters +produced by the summer melting. When it ceases to be kept open from +the summer, its walls are squeezed together in the fashion that the +crevices are closed. + +Forming here and there, and generally in considerable numbers, the +crevices of a glacier entrap a good deal of the morainal _debris_, +which falls through them to the bottom of the glacier. Smaller bits +are washed into the _moulin_, by the streams arising from the melting +ice, which is brought about by the warm sun of the summer, and +particularly by the warm rains of that season. On those glaciers +where, owing to the irregularity of the bottom over which the ice +flows, these fractures are very numerous, it may happen that all the +detritus brought upon the surface of the glacier by avalanches finds +its way to the floor of the ice. + +Although it is difficult to learn what is going on at the under +surface of the glacier, it is possible directly and indirectly to +ascertain much concerning the peculiar and important work which is +there done. The intrepid explorer may work his way in through the +lateral fissures, and even with care safely descend some of the +fissures which penetrate the central parts of a shallow ice stream. +There, it may be at the depth of a hundred feet or more, he will find +a quantity of stones, some of which may be in size like to a small +house held in the body of the ice, but with one side resting upon the +bed rock. He may be so fortunate as to see the stone actually in +process of cutting a groove in the bed rock as it is urged forward by +the motion of the glacier. The cutting is not altogether in the fixed +material, for the boulder itself is also worn and scored in the work. +Smaller pebbles are caught in the space between the erratic and the +motionless rock and ground to bits. If in his explorations the student +finds his way to the part of the floor on which the waters of a +_moulin_ fall, he may have a chance to observe how the stones set in +motion serve to cut the bed rock, forming elongated potholes much as +in the case of ordinary waterfalls, or at the base of those shafts +which afford the beginnings of limestone caverns. + +The best way to penetrate beneath the glacier is through the arch of +the stream which always flows from the terminal face of the ice river. +Even in winter time every large glacier discharges at its end a +considerable brook, the waters of which have been melted from the ice +in small part by the outflow of the earth's heat; mainly, however, by +the warmth produced in the friction of the ice on itself and on its +bottom--in other words, by the conversion of that energy of position, +of which we have often to speak, into heat. In the summer time this +subglacial stream is swollen by the surface waters descending through +the crevices and the _moulins_ which come from them, so that the +outflow often forms a considerable river, and thus excavates in the +ice a large or at least a long cavern, the base of which is the bed +rock. In the autumn, when the superficial melting ceases, this gallery +can often be penetrated for a considerable distance, and affords an +excellent way to the secrets of the under ice. The observer may here +see quantities of the rock material held in the grip of the ice, and +forced to a rude journey over the bare foundation stones. Now and then +he may find the glacial mass in large measure made up of stones, the +admixture extending many feet above the bottom of the cavern, perhaps +to the very top of the arch. He may perchance find that these stones +are crushing each other where they are in contact. The result will be +brought about by the difference in the rate of advance of the ice, +which moves the faster the higher it is above the surface over which +it drags, and thus forces the stones on one level over those below. +Where the waters of the subglacial stream have swept the bed rock +clean of _debris_ its surface is scored, grooved, and here and there +polished in a manner which is accomplished only by ice action, though +some likeness to it is afforded where stones have been swept over for +ages by blowing sand. Here and there, often in a way which interrupts +the cavern journey, the shrunken stream, unable to carry forward the +_debris_, deposits the material in the chamber, sometimes filling the +arch so completely that the waters are forced to make a detour. This +action is particularly interesting, for the reason that in regions +whence glaciers have disappeared the deposits formed in the old ice +arches often afford singularly perfect moulds of those caverns which +were produced by the ancient subglacial streams. These moulds are +termed _eskers_. + +If the observer be attentive, he will note the fact that the waters +emerging from beneath the considerable glacier are very much charged +with mud. If he will take a glass of the water at the point of escape, +he will often find, on permitting it to settle, that the sediment +amounts to as much as one twentieth of the volume. While the greater +part of this detritus will descend to the bottom of the vessel in the +course of a day, a portion of it does not thus fall. He may also note +that this mud is not of the yellowish hue which he is accustomed to +behold in the materials laid down by ordinary rivers, but has a +whitish colour. Further study will reveal the fact that the difference +is due to the lack of oxidation in the case of the glacial detritus. +River muds forming slowly and during long-continued exposure to the +action of the air have their contained iron much oxidized, which gives +them a part of their darkened appearance. Moreover, they are somewhat +coloured with decayed vegetable matter. The waste from beneath the +glacier has been quickly separated from the bed rock, all the faces of +the grains are freshly fractured, and there is no admixture of organic +matter. The faces of the particles thus reflect light in substantially +the same way as powdered glass or pulverized ice, and consequently +appear white. + +A little observation will show the student that this very muddy +character of waters emerging from beneath the glacier is essentially +peculiar to such streams as we have described. Ascending any of the +principal valleys of Switzerland, he may note that some of the streams +flow waters which carry little sediment even in times when they are +much swollen, while others at all seasons have the whitish colour. A +little further exploration, or the use of a good map, will show him +that the pellucid streams receive no contributions of glacial water, +while those which look as if they were charged with milk come, in part +at least, from the ice arches. From some studies which the writer has +made in Swiss valleys, it appears that the amount of erosion +accomplished on equal areas of similar rock by the descent of the +waters in the form of a glacier or in that of ordinary torrents +differs greatly. Moving in the form of ice, or in the state of +ice-confined streams, the mass of water applies very many times as +much of its energy of position to grinding and bearing away the rocks +as is accomplished where the water descends in its fluid state. + +The effect of the intense ice action above noted is rapidly to wear +away the rocks of the valley in which the glacier is situated. This +work is done not only in a larger measure but in a different way from +that accomplished by torrents. In the case of the latter, the stream +bed is embarrassed by the rubbish which comes into it; only here and +there can it attack the bed rock by forcing the stones over its +surface. Only in a few days of heavy rain each year is its work at all +effective; the greater part of the energy of position of its waters is +expended in the endless twistings and turnings of its stream, which +result only in the development of heat which flies away into the +atmosphere. In the ice stream, owing to its slow movement and to the +detritus which it forces along the bottom, a vastly greater part of +the energy which impels it down the slope is applied to rock cutting. +None of the boulders, even if they are yards in diameter, obstruct its +motion; small and great alike are to it good instruments wherewith to +attack the bed rocks. The fragments are never left to waste by +atmospheric decay, but are to a very great extent used up in +mechanical work, while the most of the detritus which comes to a +torrent is left in a coarse state when it is delivered to the stream; +the larger part of that which the glacier transports is worn out in +its journey. To a great extent it is used up in attacking the bed +rock. In most cases the _debris_ in the terminal moraine is evidently +but a small part of what entered the ice during its journey from the +uplands; the greater part has been worn out in the rude experiences to +which it has been subjected. + +It is evident that even in the regions now most extensively occupied +by glaciers the drainage systems have been shaped by the movement of +ordinary streams--in other words, ice action is almost everywhere, +even in the regions about the poles, an incidental feature in the work +of water, coming in only to modify the topography, which is mainly +moulded by the action of fluid water. When, owing to climatal changes, +a valley such as those of the Alps is occupied by a glacial stream, +the new current proceeds at once, according to its evident needs, to +modify the shape of its channel. An ordinary torrent, because of the +swiftness of its motion, which may, in general, be estimated at from +three to five miles an hour, can convey away the precipitation over a +very narrow bed. Therefore its channel is usually not a hundredth part +as wide as the gorge or valley in which it lies. But when the +discharge takes place by a glacier, the speed of which rarely exceeds +four or five feet a day, the ice stream because of its slow motion has +to fill the trough from side to side, it has to be some thousand times +as deep and wide as the torrent. The result is that as soon as the +glacial condition arises in a country the ice streams proceed to +change the old V-shaped torrent beds into those which have a broad +U-like form. The practised eye can in a way judge how long a valley +has been subjected to glacial action by the extent to which it has +been widened by this process. + +In the valleys of Switzerland and other mountain districts which have +been attentively studied it is evident that glacial action has played +a considerable part in determining their forms. But the work has been +limited to that part of the basin in which the ice is abundantly +provided with cutting tools in the stone which have found their way to +the base of the stream. In the region of the _neve_, where the +contributions of rocky matter to the surface of the deposit made from +the few bare cliffs which rise above the sheet of snow is small, the +snow-ice does no cutting of any consequence. Where it passes over the +steep at the head of the deep valley into which it drains, and is +riven into the _seracs_, such stony matter as it may have gathered is +allowed to fall to the bottom, and so comes into a position where it +may do effective work. From this _serac_ section downward the now +distinct ice river, being in general below the snow line, has +everywhere cliffs, on either side from which the contributions of rock +material are abundant. Hence this part of the glacier, though it is +the wasting portion of its length, does all the cutting work of any +consequence which is performed. It is there that the underrunning +streams become charged with sediment, which, as we have noted, they +bear in surprising quantities, and it is therefore in this section of +the valley that the impress of the ice work is the strongest. Its +effect is not only to widen the valley and deepen it, but also to +advance the deep section farther up the stream and its tributaries. +The step in the stream beds which we find at the _seracs_ appears to +mark the point in the course of the glacier where, owing to the +falling of stones to its base, as well as to its swifter movements and +the firmer state of the ice, it does effective wearing. + +There are many other features connected with glaciers which richly +repay the study of those who have a mind to explore in the manner of +the physicist interested in ice actions the difficult problems which +they afford; but as these matters are not important from the point of +view of this work, no mention of them will here be made. We will now +turn our attention to that other group of glaciers commonly termed +continental, which now exist about either pole, and which at various +times in the earth's history have extended far toward the equator, +mantling over vast extents of land and shallow sea. The difference +between the ice streams of the mountains and those which we term +continental depends solely on the areas of the fields and the depth of +the accumulation. In an ordinary Alpine region the _neve_ districts, +where the snow gathers, are relatively small. Owing to the rather +steep slopes, the frozen water is rapidly discharged into the lower +valleys, where it melts away. Both in the _neve_ and in the distinct +glacier of the lower grounds there are, particularly in the latter, +projecting peaks, from which quantities of stone are brought down by +avalanches or in ordinary rock falls, so that the ice is abundantly +supplied with cutting tools, which work from its surface down to its +depths. + +As the glacial accumulation grows in depth there are fewer peaks +emerging from it, and the streams which it feeds rise the higher until +they mantle over the divides between the valleys. Thus by +imperceptible stages valley glaciers pass to the larger form, usually +but incorrectly termed continental. We can, indeed, in going from the +mountains in the tropics to the poles, note every step in this +transition, until in Greenland we attain the greatest ice mass in the +world, unless that about the southern pole be more extensive. In the +Greenland glacier the ice sheet covers a vast extent of what is +probably a mountain country, which is certainly of this nature in the +southern part of the island, where alone we find portions of the earth +not completely covered by the deep envelope. Thanks to the labours of +certain hardy explorers, among whom Nansen deserves the foremost +place, we now know something as to the conditions of this vast ice +field, for it has been crossed from shore to shore. The results of +these studies are most interesting, for they afford us a clew as to +the conditions which prevail over a large part of the earth during the +Glacial period from which the planet is just escaping, and in the +earlier ages when glaciation was likewise extensive. We shall +therefore consider in a somewhat detailed way the features which the +Greenland glacier presents. + +Starting from the eastern shore of that land, if we may thus term a +region which presents itself mainly in the form of ice, we find next +the shore a coast line not completely covered with ice and snow, but +here and there exhibiting peaks which indicate that if the frozen +mantle were removed the country would appear deeply intersected with +fiords in the manner exhibited in the regions to the south of +Greenland or the Scandinavian peninsula. The ice comes down to the +sea through the valleys, often facing the ocean for great distances +with its frozen cliffs. Entering on this seaward portion of the +glacier, the observer finds that for some distance from the coast line +the ice is more or less rifted with crevices, the formation of which +is doubtless due to irregularities of the rock bottom over which it +moves. These ruptures are so frequent that for some miles back it is +very difficult to find a safe way. Finally, however, a point is +attained where these breaks rather suddenly disappear, and thence +inward the ice rises at the rate of upward slope of a few feet to the +mile in a broad, nearly smooth incline. In the central portion of the +region for a considerable part of the territory the ice has very +little slope. Thence it declines toward the other shore, exhibiting +the same features as were found on the eastern versant until near the +coast, when again the surface is beset with crevices which continue to +the margin of the sea. + +Although the explorations of the central field of Greenland are as yet +incomplete, several of these excursions into or across the interior +have been made, and the identity of the observations is such that we +can safely assume the whole region to be of one type. We can +furthermore run no risk in assuming that what we find in Greenland, at +least so far as the unbroken nature of the central ice field is +concerned, is what must exist in every land where the glacial envelope +becomes very deep. In Greenland it seems likely that the depth of the +ice is on the average more than half a mile, and in the central part +of the realm the sheet may well have a much greater profundity; it may +be nearly a mile deep. The most striking feature--that of a vast +unbroken expanse, bordered by a region where the ice is ruptured--is +traceable wherever very extensive and presumably deep deposits of ice +have been examined. As we shall see hereafter, these features teach us +much as to the conditions of glacial action--a matter which we shall +have to examine after we have completed our general survey as to the +changes which occur during glacial periods. + +In the present state of that wonderful complex of actions which we +term climate, glaciers are everywhere, so far as our observations +enable us to judge, generally in process of decrease. In Switzerland, +although the ancients even in Roman days were in contact with the ice, +they were so unobservant that they did not even remark that the ice +was in motion. Only during the last two centuries have we any +observations of a historic sort which are of value to the geologist. +Fortunately, however, the signs written on the rock tell the story, +except for its measurement in terms of years, as clearly as any +records could give it. From this testimony of the rocks we perceive +that in the geological yesterday, though it may have been some tens of +thousands of years ago, the Swiss glaciers, vastly thickened, and with +their horizontal area immensely expanded, stretched over the Alpine +country, so that only here and there did any of the sharper peaks rise +above the surface. These vast glaciers, almost continually united on +their margins, extended so far that every portion of what is now the +Swiss Republic was covered by them. Their front lay on the southern +lowlands of Germany, on the Jura district of France; on the south, it +stretched across the valley of the Po as far as near Milan. We know +this old ice front by the accumulations of rock _debris_ which were +brought to it from the interior of the mountain realm. We can +recognise the peculiar kinds of stone, and with perfect certainty +trace them to the bed rock whence they were riven. Moreover, we can +follow back through the same evidence the stages of retreat of the +glaciers, until they lost their broad continental character and +assumed something like their present valley form. Up the valley of any +of the great rivers, as, for instance, that of the Rhone above the +lake of Geneva, we note successive terminal moraines which clearly +indicate stages in the retreat of the ice when for a time it ceased to +go backward, or even made a slight temporary readvance. It is easily +seen that on such occasions the stones carried to the ice front would +be accumulated in a heap, while during the time when day by day the +glacier was retreating the rock waste would be left broadcast over the +valley. + +As we go up from the course of the glacial streams we note that the +successive moraines have their materials in a progressively less +decayed state. Far away from the heap now forming, and in proportion +to the distance, the stones have in a measure rotted, and the heaps +which they compose are often covered with soil and occupied by +forests. Within a few miles of the ice front the stones still have a +fresh aspect. When we arrive within, say, half a mile of the moraine +now building, we come to the part of the glacial retreat of which we +have some written or traditional account. This is in general to the +effect that the wasting of the glaciers is going on in this century as +it went on in the past. Occasionally periods of heavy snow would +refresh the ice streams, so that for a little time they pushed their +fronts farther down the valley. The writer has seen during one of +these temporary advances the interesting spectacle of ice destroying +and overturning the soil of a small field which had been planted in +grain. + +It should be noted that these temporary advances of the ice are not +due to the snowfall of the winter or winters immediately preceding the +forward movement. So slow is the journey of the ice from the _neve_ +field to the end of a long glacier that it may require centuries for +the store accumulated in the uplands to affect the terminal portion of +the stream. We know that the bodies of the unhappy men who have been +lost in the crevices of the glacier are borne forward at a uniform and +tolerably computable rate until they emerge at the front, where the +ice melts away. In at least one case the remains have appeared after +many years in the _debris_ which is contributed to the moraine. On +account of this slow feeding of the glacial stream, we naturally may +expect to find, as we do, in fact, that a great snowfall of many +years ago, and likewise a period when the winter's contribution has +been slight, would influence the position of the terminal point of the +ice stream at different times, according to its length. If the length +of the flow be five miles, it may require twenty or thirty years for +the effect to be evident; while if the stream be ten miles long, the +influence may not be noted in less than threescore years. Thus it +comes about that at the present time in the same glacial district some +streams may be advancing while others are receding, though, on the +whole, the ice is generally in process of shrinkage. If the present +rate of retreat should be maintained, it seems certain that at the end +of three centuries the Swiss glaciers as a whole will not have +anything like their present area, and many of the smaller streams will +entirely disappear. + +Following the method of the illustrious Louis Agassiz, who first +attentively traced the evidence which shows the geologically recent +great extension of glaciers by studying the evidence of the action in +fields they no longer occupy, geologists have now inspected a large +part of the land areas with a view to finding the proofs of such ice +work. So far as these indications are concerned, the indications which +they have had to trace are generally of a very unmistakable character. +Rarely, indeed, does a skilled student of such phenomena have to +search in any region for more than a day before he obtains indubitable +evidence which will enable him to determine whether or not the field +has recently been occupied by an enduring ice sheet--one which +survives the summer season and therefore deserves the name of glacier. +The indications which he has to consider consist in the direction and +manner in which the surface materials have been carried, the physical +conditions of these materials, the shape of the surface of the +underlying rock as regards its general contour, and the presence or +absence of scratches and groovings on its surface. As these records of +ice action are of first importance in dealing with this problem, and +as they afford excellent subjects for the study of those who dwell in +glaciated regions, we shall note them in some detail. + +The geologist recognises several ways in which materials may be +transported on the surface of the earth. They may be cast forth by +volcanoes, making their journey by being shot through the air, or by +flowing in lava streams; it is always easy at a glance, save in very +rare instances, to determine whether fragments have thus been +conveyed. Again, the detritus may be moved by the wind; this action is +limited; it only affects dust, sand, and very small pebbles, and is +easily discriminated. The carriage may be effected by river or marine +currents; here, again, the size of the fragments moved is small, and +the order of their arrangement distinctly traceable. The fragments may +be conveyed by ice rafts; here, too, the observer can usually limit +the probabilities he has to consider by ascertaining, as he can +generally do, whether the region which he is observing has been below +a sea or lake. In a word, the before-mentioned agents of +transportation are of somewhat exceptional influence, and in most +cases can, as explanations of rock transportation, be readily +excluded. When, therefore, the geologist finds a country abundantly +covered with sand, pebbles, and boulders arranged in an irregular way, +he has generally only to inquire whether the material has been carried +by rivers or by glaciers. This discrimination can be quickly and +critically effected. In the first place, he notes that rivers only in +their torrent sections can carry large fragments of rock, and that in +all cases the fragments move down hill. Further, that where deposits +are formed, they have more or less the form of alluvial deposits. If +now the observations show that the rock waste occupying the surface of +any region has been carried up hill and down, across the valleys, +particularly if there are here and there traces of frontal moraines, +the geologist is entitled to suppose--he may, indeed, be sure--that +the carriage has been effected by a glacial sheet. + +Important corroborative evidence of ice action is generally to be +found by inspecting the bed rock below the detritus, which indicates +glacial action. Even if it be somewhat decayed, as is apt to be the +case where the ice sheet long since passed away, the bed rock is +likely to have a warped surface; it is cast into ridges and furrows of +a broad, flowing aspect, such as liquid water never produces, which, +indeed, can only be created by an ice sheet moving over the surface, +cutting its bed in proportion to the hardness of the material. +Furthermore, if the bed rock have a firm texture, and be not too much +decayed, we almost always find upon it grooves or scratches, channels +carved by the stones embedded in the body of the ice, and drawn by its +motion over the fixed material. Thus the proof of glacial extension in +the last ice epoch is made so clear that accurate maps can be prepared +showing the realm of its action. This task is as yet incomplete, +although it is already far advanced. + +While the study of glaciers began in Europe, inquiries concerning +their ancient extension have been carried further and with more +accuracy in North America than in any other part of the world. We may +therefore well begin our description of the limits of the ice sheets +with this continent. Imagining a seafarer to have approached America +by the North Atlantic, as did the Scandinavians, and that his voyage +came perhaps a hundred thousand years or more before that of Leif +Ericsson, he would have found an ice front long before he attained the +present shores of the land. This front may have extended from south of +Greenland, off the shores of the present Grand Banks of Newfoundland, +thence and westward to central or southern New Jersey. This cliff of +ice was formed by a sheet which lay on the bottom of the sea. On the +New Jersey coast the ice wall left the sea and entered on the body of +the continent. We will now suppose that the explorer, animated with +the valiant scientific spirit which leads the men of our day to seek +the poles, undertook a land journey along the ice front across the +continent. From the New Jersey coast the traveller would have passed +through central Pennsylvania, where, although there probably detached +outlying glaciers lying to the southward as far as central Virginia, +the main front extended westward into the Ohio Valley. In southern +Ohio a tongue of the ice projected southwardly until it crossed the +Ohio River, where Cincinnati now lies, extending a few miles to the +southward of the stream. Thence it deflected northwardly, crossing the +Mississippi, and again the Missouri, with a tongue or lobe which went +far southward in that State. Then again turning to the northwest, it +followed in general the northern part of the Missouri basin until it +came to within sight of the Rocky Mountains. There the ice front of +the main glacier followed the trend of the mountains at some distance +from their face for an unknown extent to the northward. In the +Cordilleras, as far south as southern Colorado, and probably in the +Sierra Nevada to south of San Francisco, the mountain centres +developed local glaciers, which in some places were of very great +size, perhaps exceeding any of those which now exist in Switzerland. +It will thus be seen that nearly one half of the present land area of +North America was beneath a glacial covering, though, as before noted, +the region about the Gulf of Mexico may have swayed upward when the +northern portion of the land was borne down by the vast load of ice +which rested upon it. Notwithstanding this possible addition to the +land, our imaginary explorer would have found the portion of the +continent fit for the occupancy of life not more than half as great as +it is at present. + +In the Eurasian continent there was no such continuous ice sheet as in +North America, but the glaciers developed from a number of different +centres, each moving out upon the lowlands, or, if its position was +southern, being limited to a particular mountain field. One of these +centres included Scandinavia, northern Germany, Great Britain about as +far south as London, and a large part of Ireland, the ice covering the +intermediate seas and extending to the westward, so that the passage +of the North Atlantic was greatly restricted between this ice front +and that of North America. Another centre, before noted, was formed in +the Alps; yet another, of considerable area, in the Pyrenees; other +less studied fields existed in the Apennines, in the Caucasus, the +Ural, and the other mountains of northern Asia. Curiously enough, +however, the great region of plains in Siberia does not appear to have +been occupied by a continuous ice sheet, though the similar region in +North America was deeply embedded in a glacier. Coincident with this +development of ice in the eastern part of the continent, the ice +streams of the Himalayan Mountains, some of which are among the +greatest of our upland glaciers, appear to have undergone but a +moderate extension. Many other of the Eurasian highlands were probably +ice-bound during the last Glacial period, but our knowledge concerning +these local fields is as yet imperfect. + +In the southern hemisphere the lands are of less extent and, on the +whole, less studied than in the northern realm. Here and there where +glaciers exist, as in New Zealand and in the southern part of South +America, observant travellers have noticed that these ice fields have +recently shrunk away. Whether the time of greatest extension and of +retreat coincided with that of the ice sheets in the north is not yet +determined; the problem, indeed, is one of some difficulty, and may +long remain undecided. It seems, however, probable that the glaciers +of the southern hemisphere, like those in the north, are in process of +retreat. If this be true, then their time of greatest extension was +probably the same as that of the ice sheets about the southern pole. +From certain imperfect reports which we have concerning evidences of +glaciation in Central America and in the Andean district in the +northern part of South America, it seems possible that at one time the +upland ice along the Cordilleran chain existed from point to point +along that system of elevations, so that the widest interval between +the fields of permanent snow with their attendant glaciers did not +much exceed a thousand miles. + +Observing the present gradual retreat of those ice remnants which +remain mere shreds and patches of the ancient fields, it seems at +first sight likely that the extension and recession of the great +glaciers took place with exceeding slowness. Measured in terms of +human life, in the manner in which we gauge matters of man's history, +this process was doubtless slow. There are reasons, however, to +believe that the coming and going were, in a geological sense, swift; +they may have, indeed, been for a part of the time of startling +rapidity. Going back to the time of geological yesterday, before the +ice began its development in the northern hemisphere, all the evidence +we can find appears to indicate a temperate climate extending far +toward the north pole. The Miocene deposits found within twelve +degrees, or a little more than seven hundred miles, of the north pole, +and fairly within the realm of lowest temperature which now exists on +the earth, show by the plant remains which they contain that the +conditions permitted the growth of forests, the plants having a +tolerably close resemblance to those which now freely develop in the +southern portion of the Mississippi Valley. Among them there are +species which had the habit of retaining their broad, rather soft +leaves throughout the winter season. The climate appears, in a word, +to have been one where the mean annual temperature must have been +thirty degrees or more higher than the present average of that realm. +Although such conditions near the sea level are not inconsistent with +the supposition that glaciers existed in the higher mountains of the +north, they clearly deny the possibility of the realm being occupied +by continental glaciers. + +Although the Pliocene deposits formed in high latitudes have to a +great extent been swept away by the subsequent glacial wearing, they +indicate by their fossils a climatal change in the direction of +greater cold. We trace this change, though obscurely, in a +progressive manner to a point where the records are interrupted, and +the next interpretable indication we have is that the ice sheet had +extended to somewhere near the limits which we have noted. We are then +driven to seek what we can concerning the sojourn of the ice on the +land by the amount of wearing which it has inflicted upon the areas +which it occupied. This evidence has a certain, though, as we shall +see, a limited value. + +When the students of glacial action first began the great task of +interpreting these records, they were led to suppose that the amount +of rock cutting which was done by the ice was very great. Observing +what goes on, in the manner we have noted, beneath a valley glacier +such as those of Switzerland, they saw that the ice work went on +rapidly, and concluded that if the ice remained long at work in a +region it must do a vast deal of erosion. They were right in a part of +their premises, but, as we shall see, probably in another part wrong. +Looking carefully over the field where the ice has operated, we note +that, though at first sight the area appears to have lost all trace of +its preglacial river topography, this aspect is due mainly to the +irregular way in which the glacial waste is laid down. Close study +shows us that we may generally trace the old stream valleys down to +those which were no larger than brooks. It is true that these channels +are generally and in many places almost altogether filled in with +rubbish, but a close study of the question has convinced the writer, +and this against a previous view, that the amount of erosion in New +England and Canada, where the work was probably as great as anywhere, +has not on the average exceeded a hundred feet, and probably was much +less than that amount. + +Even in the region north of Lake Ontario, over which the ice was deep +and remained for a long time, the amount of erosion is singularly +small. Thus north of Kingston the little valleys in the limestone +rocks which were cut by the preglacial streams, though somewhat +encumbered with drift, remain almost as distinct as they are on +similar strata in central Kentucky, well south of the field which the +ice occupied. In fact, the ice sheet appears to have done the greatest +part of its work and to have affected the surface most in the belt of +country a few hundred miles in width around the edges of the sheet. It +was to be expected that in a continental glacier, as in those of +mountain valleys, the most of the _debris_ should be accumulated about +the margin where the materials dropped from the ice. But why the +cutting action should be greatest in that marginal field is not at +first sight clear. To explain this and other features as best we may, +we shall now consider the probable history of the great ice march in +advance and retreat, and then take up the conditions which brought +about its development and its disappearance. + +Ice is in many ways the most remarkable substance with which the +physicist has to deal, and among its eminent peculiarities is that it +expands in freezing, while the rule is that substances contract in +passing from the fluid to the solid state. On this account frozen +water acts in a unique manner when subjected to pressure. For each +additional atmosphere of pressure--a weight amounting to about fifteen +pounds to the square inch--the temperature at which the ice will melt +is lowered to the amount of sixteen thousandths of a degree +centigrade. If we take a piece of ice at the temperature of freezing +and put upon it a sufficient weight, we inevitably bring about a small +amount of melting. Where we can examine the mass under favourable +conditions, we can see the fluid gather along the lines of the +crystals or other bits of which the ice is composed. We readily note +this action by bringing two pieces of ice together with a slight +pressure; when the pressure is removed, they will adhere. The adhesion +is brought about not by any stickiness of the materials, for the +substance has no such property. It is accomplished by melting along +the line of contact, which forms a film of water, that at once +refreezes when the pressure is withdrawn. When a firm snowball is +made by even pressing snow, innumerable similar adhesions grow up in +the manner described. The fact is that, given ice at the temperature +at which it ordinarily forms, pressure upon it will necessarily +develop melting. + +The consequences of pressure melting as above described are in +glaciers extremely complicated. Because the ice is built into the +glacier at a temperature considerably below the freezing point, it +requires a great thickness of the mass before the superincumbent +weight is sufficient to bring about melting in its lower parts. If we +knew the height at which a thermometer would have stood in the surface +ice of the ancient glacier which covered the northern part of North +America, we could with some accuracy compute how thick it must have +been before the effect of pressure alone would have brought about +melting; but even then we should have to reckon the temperature +derived from the grinding of the ice over the floor and the crushing +of rocks there effected, as well as the heat which is constantly +though slowly coming forth from the earth's interior. The result is +that we can only say that at some depth, probably less than a mile, +the slowly accumulating ice would acquire such a temperature that, +subjected to the weight above it, the material next the bottom would +become molten, or at least converted into a sludgelike state, in which +it could not rub against the bottom, or move stones in the manner of +ordinary glaciers. + +As fast as the ice assumed this liquid or softened state, it would be +squeezed out toward the region where, because of the thinning of the +glacier, it would enter a field where pressure melting did not occur. +It would then resume the solid state, and thence journey to the margin +of the ice in the ordinary manner. We thus can imagine how such a +glacier as occupied the northern part of this continent could have +moved from the central parts toward its periphery, as we can not do if +we assume that the glacier everywhere lay upon the bed rock. There is +no slope from Lake Erie to the Ohio River at Cincinnati. Knowing that +the ice moved down this line, there are but two methods of accounting +for its motion: either the slope of the upper surface to the northward +was so steep that the mass would have been thus urged down, the upper +parts dragging the bottom along with them, or the ice sheet for the +greater part of its extent rested upon pressure-molten water, or +sludge ice, which was easily squeezed out toward the front. The first +supposition appears inadmissible, for the reason that the ice would +have to be many miles deep at Hudson Bay in order that its upper +surface should have slope enough to overcome the rigidity of the +material and bring about the movement. We know that any such depth is +not supposable. + +The recent studies in Greenland supply us with strong corroborative +evidence for the support of the view which is here urged. The wide +central field of that area, where the ice has an exceeding slight +declivity, and is unruptured by crevices, can not be explained except +on the supposition that it rests on pressure-molten water. The thinner +section next the shore, where the glacier is broken up by those +irregular movements which its wrestle with the bottom inevitably +induces, shows that there it is in contact with the bed rock, for it +behaves exactly as do the valley glaciers of like thickness. + +The view above suggested as to the condition of continental glaciers +enables us to explain not only their movements, but the relatively +slight amount of wearing which they brought about on the lands they +occupied. Beginning to develop in mountain regions, or near the poles +on the lowlands, these sheets, as soon as they attained the thickness +where the ice at their bottom became molten, would rapidly advance for +great distances until they attained districts where the melting +exceeded the supply of frozen material. In this excursion only the +marginal portion of the glacier would do erosive work. This would +evidently be continued for the greatest amount of time near the front +or outer rim of the ice field, for there, we may presume, that for +the longest time the cutting rim would rest upon the bed rock of the +country. As the ice receded, this rim would fall back; thus in the +retreat as in the advance the whole of the field would be subjected to +a certain amount of erosion. On this supposition we should expect to +find that the front of a continental glacier, fed with pressure-molten +water from all its interior district, which became converted into ice, +would attain much warmer regions than the valley streams, where all +the flow took place in the state of ice, and, furthermore, that the +speed of the going on the margin would be much more rapid than in the +Alpine streams. These suppositions are well borne out by the study of +existing continental ice sheets, which move with singular rapidity at +their fronts, and by the ancient glaciers, which evidently extended +into rather warm fields. Thus, when the ice front lay at the site of +Cincinnati, at six hundred feet above the sea, there were no glaciers +in the mountains of North Carolina, though those rise more than five +thousand feet higher in the air, and are less than two hundred miles +farther south. It is therefore evident that the continental glacier at +this time pushed southward into a comparatively warm country in a way +that no stream moving in the manner of a valley glacier could possibly +have done. + +The continental glaciers manage in many cases to convey detritus from +a great distance. Thus, when the ice sheet advanced southwardly from +the regions north of the Great Lakes, they conveyed quantities of the +_debris_ from that section as far south as the Ohio River. In part +this rubbish was dragged forward by the ice as the sheet advanced; in +part it was urged onward by the streams of liquid water formed by the +ordinary process of ice melting. Such subglacial rivers appear to have +been formed along the margins of all the great glaciers. We can +sometimes trace their course by the excavation which they have made, +but more commonly by the long ridges of stratified sand and gravel +which were packed into the caverns excavated by these subglacial +rivers, which are known to glacialists as _eskers_, or as serpent +kames. In many cases we can trace where these streams flowed up stream +in the old river valleys until they discharged over their head waters. +Thus in the valley of the Genesee, which now flows from Pennsylvania, +where it heads against the tributaries of the Ohio and Susquehanna, to +Lake Ontario, there was during the Glacial epoch a considerable river +which discharged its waters into those of the Ohio and the Susquehanna +over the falls at the head of its course. + +[Illustration: _Front of Muir Glacier, showing ice entering the sea; +also small icebergs._] + +The effect of widespread glacial action on a country such as North +America appears to have been, in the first place, to disturb the +attitude of the land by bearing down portions of its surface, a +process which led to the uprising of other parts which lay beyond the +realm of the ice. Within the field of glaciation, so far as the ice +rested bodily on the surface, the rocks were rapidly worn away. A +great deal of the _debris_ was ground to fine powder, and went far +with the waters of the under-running streams. A large part was +entangled in the ice, and moved forward toward the front of the +glacier, where it was either dropped at the margin or, during the +recession of the glacier, was laid upon the surface as the ice melted +away. The result of this erosion and transportation has been to change +the conditions of the surface both as regards soil and drainage. As +the reader has doubtless perceived, ordinary soil is, outside of the +river valleys, derived from the rock beneath where it lies. In +glaciated districts the material is commonly brought from a +considerable distance, often from miles away. These ice-made soils are +rarely very fertile, but they commonly have a great endurance for +tillage, and this for the reason that the earth is refreshed by the +decay of the pebbles which they contain. Moreover, while the tillable +earth of other regions usually has a limited depth, verging downward +into the semisoil or subsoil which represent the little changed bed +rocks, glacial deposits can generally be ploughed as deeply as may +prove desirable. + +The drainage of a country recently affected by glaciers is always +imperfect. Owing to the irregular erosion of the bed rocks, and to the +yet more irregular deposition of the detritus, there are very numerous +lakes which are only slowly filled up or by erosion provided with +drainage channels. Though several thousand years have passed by since +the ice disappeared from North America, the greater part of the area +of these fresh-water basins remains, the greater number of them, +mostly those of small size, have become closed. + +Where an ice stream descends into the sea or into a large lake, the +depth of which is about as great as the ice is thick, the relative +lightness of the ice tends to make it float, and it shortly breaks off +from the parent mass, forming an iceberg. Where, as is generally the +case in those glaciers which enter the ocean, a current sweeps by the +place where the berg is formed, it may enter upon a journey which may +carry the mass thousands of miles from its origin. The bergs separated +from the Greenland glaciers, and from those about the south pole, are +often of very great size; sometimes, indeed, they are some thousand +feet in thickness, and have a length of several miles. It often +happens that these bergs are formed of ice, which contains in its +lower part a large amount of rock _debris_. As the submerged portion +of the glacier melts in the sea water, these stones are gradually +dropped to the bottom, so that the cargo of one berg may be strewed +along a line many hundred miles in length. It occasionally happens +that the ice mass melts more slowly in those parts which are in the +air than in its under-water portions. It thus becomes top-heavy and +overturns, in which case such stony matter as remains attains a +position where it may be conveyed for a greater distance than if the +glacier were not capsized. It is likely, indeed, that now and then +fragments of rock from Greenland are dropped on the ocean floor in the +part of the Atlantic which is traversed by steamers between our +Atlantic ports and Great Britain. + +Except for the risks which they bring to navigators, icebergs have no +considerable importance. It is true they somewhat affect the +temperature of sea and air, and they also serve to convey fragments of +stone far out to sea in a way that no other agent can effect; but, on +the whole, their influence on the conditions of the earth is +inconsiderable. + +Icebergs in certain cases afford interesting indices as to the motion +of oceanic currents, which, though moving swiftly at a depth below the +surface, do not manifest themselves on the plain of the sea. Thus in +the region about Greenland, particularly in Davis Strait, bergs have +been seen forcing their way southward at considerable speed through +ordinary surface ice, which was either at rest or moving in the +opposite direction. The train of these bergs, which moves upward from +the south polar continent, west of Patagonia, indicates also in a very +emphatic way the existence of a very strong northward-setting current +in that part of the ocean. + + * * * * * + +We have now to consider the causes which could bring about such great +extensions of the ice sheet as occurred in the last Glacial period. +Here again we are upon the confines of geological knowledge, and in a +field where there are no well-cleared ways for the understanding. In +facing this problem, we should first note that those who are of the +opinion that a Glacial period means a very cold climate in the regions +where the ice attained its extension are probably in error. Natural as +it may seem to look for exceeding cold as the cause of glaciation, the +facts show us that we can not hold this view. In Siberia and in the +parts of North America bordering on the Arctic Sea the average cold is +so intense that the ground is permanently frozen--as it is, for +instance, in the Klondike district--to the depth of hundreds of feet, +only the surface thawing out during the warm summers. All this region +is cold enough for glaciers, but there is not sufficient snowfall to +maintain them. On the other hand, in Greenland, and in a less though +conspicuous degree in Scandinavia, where the waters of the North +Atlantic somewhat diminish the rigour of the cold, and at the same +time bring about a more abundant snowfall, the two actions being +intimately related, we have very extensive glaciers. Such facts, which +could be very much extended, make it clear that the climate of glacial +periods must have been characterized by a great snowfall, and not by +the most intense cold. + +It is evident that what would be necessary again to envelop the boreal +parts of North America with a glacial sheet would not be a +considerable decrease of heat, but an increase in the winter's +contribution of frozen water. Even if the heat released by this +snowfall elevated the average temperature of the winter, as it +doubtless would in a considerable measure, it would not melt off the +snow. That snowfall tends to warm the air by setting free the heat +which was engaged in keeping the water in a state of vapour is +familiarly shown by the warming which attends an ordinary snowstorm. +Even if the fall begin with a temperature of about 0 deg. Fahr., the air +is pretty sure to rise to near the freezing point. + +It is evident that no great change of temperature is required in order +to bring about a very considerable increase in the amount of snowfall. +In the ordinary succession of seasons we often note the occurrence of +winters during which the precipitation of snow is much above the +average, though it can not be explained by a considerable climatal +change. We have to account for these departures from the normal +weather by supposing that the atmospheric currents bring in more than +the usual amount of moisture from the sea during the period when great +falls of snow occur. In fact, in explaining variations in the humidity +of the land, whether those of a constant nature or those that are to +be termed accidental, we have always to look to those features which +determine the importation of vapour from the great field of the ocean +where it enters the air. We should furthermore note that these +peculiarities of climate are dependent upon rather slight geographic +accidents. Thus the snowfall of northern Europe, which serves to +maintain the glaciation of that region, and, curiously enough, in some +measure its general warmth, depends upon the movement of the Gulf +Stream from the tropics to high latitudes. If by any geographical +change, such as would occur if Central America were lowered so as to +make a free passage for its waters to the westward, the glaciers of +Greenland and of Scandinavia would disappear, and at the same time the +temperature of those would be greatly lowered. Thus the most evident +cause of glaciation must be sought in those alterations of the land +which affect the movement of the oceanic currents. + +Applying this principle to the northern hemisphere, we can in a way +imagine a change which would probably bring about a return of such an +ice period as that from which the boreal realm is now escaping. Let us +suppose that the region of not very high land about Bering Strait +should sink down so as to afford the Kuro Siwo, or North Pacific +equivalent of our Gulf Stream, an opportunity to enter the Arctic Sea +with something like the freedom with which the North Atlantic current +is allowed to penetrate to high latitudes. It seems likely that this +Pacific current, which in volume and warmth is comparable to that of +the Atlantic, would so far elevate the temperature of the arctic +waters that their wide field would be the seat of a great evaporation. +Noting once again the fact that the Greenland glaciers, as well as +those of Norway, are supplied from seas warmed by the Gulf Stream, we +should expect the result of this change would be to develop similar +ice fields on all the lands near that ocean. + +Applying the data gathered by Dr. Croll for the Gulf Stream, it seems +likely that the average annual temperature induced in the Arctic Sea +by the free entrance of the Japan current would be between 20 deg. and 30 deg. +Fahr. This would convert this wide realm of waters into a field of +great evaporation, vastly increasing the annual precipitation. It +seems also certain that the greater part of this precipitation would +be in the form of snow. It appears to the writer that this cause alone +may be sufficient to account for the last Glacial period in the +northern hemisphere. As to the probability that the region about +Bering Strait may have been lowered in the manner required by this +view, it may be said that recent studies on the region about Mount St. +Elias show that during or just after the ice epoch the shores in that +portion of Alaska were at least four thousand feet lower than at +present. As this is but a little way from the land which we should +have to suppose to be lowered in order to admit the Japan current, we +could fairly conclude that the required change occurred. As for the +cause of the land movement, geologists are still in doubt. They know, +however, that the attitudes of the land are exceedingly unstable, and +that the shores rarely for any considerable time maintain their +position. It is probable that these swayings of the earth's surface +are due to ever-changing combinations of the weight in different parts +of the crust and the strains arising from the contraction of its inner +parts. + +In the larger operations of Nature the effects which we behold, +however simple, are rarely the products of a single cause. In fact, +there are few actions so limited that they can fairly be referred to +one influence. It is therefore proper to state that there are many +other actions besides those above noted which probably enter into +those complicated equations which determine the climatal conditions of +the earth. To have these would carry us into difficult and speculative +inquiries. + +As before remarked, all the regions which have been subjected to +glaciation are still each year brought temporarily into the glacial +state. This fact serves to show us that the changes necessary to +produce great ice sheets are not necessarily of a startling nature, +however great the consequences may be. Assuming, then, that relatively +slight alterations of climate may cause the ice sheet to come and go, +we may say that all the influences which have been suggested by the +students of glaciation, and various other slighter causes which can +not be here noted, may have co-operated to produce the peculiar +result. In this equation geographic change has affected the course of +the ocean currents, and has probably been the most influential, or at +least the commonest, cause to which we must attribute the extension of +ice sheets. Next, alterations of the solar heat may be looked to as a +change-bringing action; unfortunately, however, we have no direct +evidence that this is an efficient cause. Thirdly, the variations in +the eccentricity of the earth's orbit, combined with the precession of +the equinoxes and the rotation of the apsides, may be regarded as +operative. The last of all, changes in the constitution of the +atmosphere, have to be taken into account. To these must be added, as +before remarked, many less important actions which influence this +marvellously delicate machine, the work of which is expressed in the +phenomena assembled under the name of climate. + +Evidence is slowly accumulating which serves to show that glacial +periods of greater or less importance have been of frequent occurrence +at all stages in the history of the earth of which we have a distinct +record. As these accidents write their history upon the ground alone, +and in a way impermanently, it is difficult to trace the ice times of +ancient geological periods. The scratches on the bed rocks, and the +accumulations of detritus formed as the ice disappeared, have alike +been worn away by the agents of decay. Nevertheless, we can trace here +and there in the older strata accumulations of pebbly matter often +containing large boulders, which clearly were shaped and brought +together by glacial action. These are found in some instances far +south of the region occupied by the glaciers during the last ice +epoch. They occur in rocks of the Cambrian or Silurian age in eastern +Tennessee and western North Carolina; they are also found in India +beyond the limits to which glaciers have attained in modern times. + +In closing this inadequate account of glacial action, a story which +for its complete telling would require many volumes, it is well for +the reader to consider once again how slight are the changes of +climate which may alternately withdraw large parts of the land from +the uses of life, and again quickly restore the fields to the service +of plants and animals. He may well imagine that these changes, by +driving living creatures to and fro, profoundly affect the history of +their development. This matter will be dealt with in the volume +concerning the history of organic beings. + +When the ice went off from the northern part of this continent, the +surface of the country, which had been borne down by the weight of the +glacier, still remained depressed to a considerable depth below the +level of the sea, the depression varying from somewhere about one +hundred feet in southern New England to a thousand feet or more in +high latitudes. Over this region, which lay beneath the level of the +sea, the glacier, when it became thin enough to float, was doubtless +broken up into icebergs, in the manner which we now behold along the +coast of Greenland. Where the shore was swept by a strong current, +these bergs doubtless drifted away; but along the most of the coast +line they appear to have lain thickly grouped next the shores, +gradually delivering their loads of stones and finer _debris_ to the +bottom. These masses of floating ice in many cases seem to have +prevented the sea waves from attaining the shore, and thus hindered +the formation of those beaches which in their present elevated +condition enable us to interpret the old position of the sea along +coast lines which have been recently elevated. Here and there, +however, from New Jersey to Greenland, we find bits of these ancient +shores which clearly tell the story of that down-sinking of the land +beneath the burden of the ice which is such an instructive feature in +the history of that period. + + + + + CHAPTER VII. + + THE WORK OF UNDERGROUND WATER. + + +We have already noted two means by which water finds its way +underground. The simplest and largest method by which this action is +effected is by building in the fluid as the grains of the rock are +laid down on the floors of seas or lakes. The water thus imprisoned is +firmly inclosed in the interstices of the stone, it in time takes up +into its mass a certain amount of the mineral materials which are +contained in the deep-buried rocks. The other portion of the ground +water--that with which we are now to be specially concerned--arises +from the rain which descends into the crevices of the earth; it is +therefore peculiar to the lands. For convenience we shall term the +original embedded fluid _rock water_, and that which originates from +the rain _crevice water_, the two forming the mass of the earth water. + +The crevice water of the earth, although forming at no time more than +a very small fraction of the hidden fluid, is an exceedingly potent +geological agent, doing work which, though unseen, yet affords the +very foundations on which rest the life alike of land and sea. When +this water enters the earth, though it is purified of all mineral +materials, it has already begun to acquire a share of a gaseous +substance, carbonic acid, or, as chemists now term it, carbon dioxide, +which enables the fluid to begin its role of marvellous activities. In +its descent as rain, probably even before it was gathered in drops in +the cloud realm, the water absorbs a certain portion of this gas from +the atmosphere. Entering the realm of the soil, where the decaying +organic matter plentifully gives forth carbon dioxide, a further store +of the gas is acquired. At the ordinary pressure of the air, water may +take in many times its bulk of the gas. + +The immediate effect of carbonic acid when it is absorbed by water is +greatly to increase the capacity which that fluid has for taking +mineral matters into solution. When charged with this gas, in the +measure in which it may be in the soil, water is able to dissolve +about fifty times as much limestone as it can in its perfectly pure +form take up. A familiar instance of this peculiar capacity which the +gas gives may often be seen where the water from a soda-water fountain +drips upon the marble slab beneath. In a few years this slab will be +considerably corroded, though pure water would in the same time have +had no effect upon it. + +The first and by far the most important effect of crevice water is +exercised upon the soil, which is at once the product of this action, +and the laboratory where the larger part of the work is done. +Penetrating between the grains of the detrital covering, held in large +quantities in the coating, and continually in slow motion, the +gas-charged water takes a host of substances into solution, and brings +them into a condition where they may react upon each other in the +chemical manner. These materials are constantly being offered to the +roots of plants and brought in contact with the underlying rock which +has not passed into the state of soil. The changes induced in this +stony matter lead to its breaking up, or at least to its softening to +the point where the roots can penetrate it and complete its +destruction. Thus it comes about that the water which to a great +extent divides the rocks into the state of soil, which is continually +wearing away the material on the surface, or leaching it out through +the springs, is also at work in restoring the layer from beneath. + +The greater part of the water which enters the soil does not +penetrate to any great depth in the underlying rocks, but finds its +way to the surface after no long journey in the form of small springs. +Generally those superficial springs do not emerge through distinct +channels, but move, though slowly, in a massive way down the slopes +until they enter a water course. Along the banks of any river, however +small, or along the shores of the sea, a pit a few inches deep just +above the level of the water will be quickly filled by a flow from +this sheet which underlies the earth. At a distance from the stream +this sheet spring is in contact with the bed rocks, and may be many +feet below the surface, but it comes to the level of the river or the +sea near their margins. Here and there the shape of the bed rocks, +being like converging house roofs, causes the superficial springs to +form small pipelike channels for the escape of their gathered waters, +and the flow emerges at a definite point. Almost all these sources of +considerable flow are due to the action of the water on the underlying +rock, where we shall now follow that portion of the crevice water +which penetrates deeply into the earth. + +Almost all rocks, however firm they may appear to be, are divided by +crevices which extend from the soil level it may be to the depths of +thousands of feet. These rents are in part due to the strains of +mountain-building, which tend to disrupt the firmest stone, leaving +open fractures. They are also formed in other ways, as by the +imperfectly understood agencies which produce joint planes. It often +happens that where rocks are highly tilted water finds its way +downward between the layers, which are imperfectly soldered together, +or a bed of coarse material, such as sandstone or conglomerate, may +afford an easy way by which the water may descend for miles beneath +the surface. Passing through rocks which are not readily soluble, the +water, already to a great extent supplied with mineral matter by its +journey through the soil, may not do much excavating work, and even +after a long time may only slightly enlarge the spaces in which it +may be stored or the channels by which it discharges to the surface. +Hence it comes about that in many countries, even where the waters +penetrate deeply, they do not afford large springs. It is otherwise +where the crevice waters enter limestones composed of materials which +are readily dissolved. In such places we find the rain so readily +entering the underlying rock that no part of the fall goes at once to +the brooks, but all has a long underground journey. + +In any limestone district where the beds of the material are thick and +tolerably pure--as, for instance, in the cavern district of southern +Kentucky--the traveller who enters the region notes at once that the +usual small streams which in every region of considerable rainfall he +is accustomed to see intersecting the surface of the country are +entirely absent. In their place he notes everywhere pitlike +depressions of bowl-shaped form, the sink holes to which we have +already adverted. Through the openings in the bottom of these the rain +waters descend into the depths of the earth. Although the most of +these depressions have but small openings in their bottom, now and +then one occurs with a vertical shaft sufficiently large to permit the +explorer to descend into it, though he needs to be lowered down in the +manner of a miner who is entering a shaft. In fact, the journey is +nearly always one of some hazard; it should not be undertaken save +with many precautions to insure safety. + +When one is lowered away through an open sink hole, though the descent +may at first be somewhat tortuous, the explorer soon finds himself +swinging freely in the air, it may be at a point some hundred feet +above the base of the bottle-shaped shaft or dome into which he has +entered. Commonly the neck of the bottle is formed where the water has +worked its way through a rather sandy limestone, a rock which was not +readily dissolved by the water. In the pure and therefore easily cut +limestone layers the cavity rapidly expands until the light of the +lantern may not disclose its walls. Farther down there is apt to be a +shelf composed of another impure limestone, which extends off near the +middle of the shaft. If the explorer can land upon this shelf, he is +sure to find that from this imperfect floor the cavern extends off in +one or more horizontal galleries, which he may follow for a great +distance until he comes to the point where there is again a well-like +opening through the hard layer, with another dome-shaped base beneath. +Returning to the main shaft, the explorer may continue his descent +until he attains the base of this vertical section of the cave, where +he is likely to find himself delivered in a pool of water of no great +depth, the bottom of which is occupied by a quantity of small, hard +stones of a flinty nature, which have evidently come from the upper +parts of the cavern. The close observer will have noted that here and +there in the limestone there are flinty bits, such as those which he +finds in the pool. From the bottom of the dome a determined inquirer +can often make his way along the galleries which lead from that level, +though it may be after a journey of miles to the point where he +emerges from the cavern on the banks of an open-air river. + +Although a journey by way of the sink holes through a cavern system is +to be commended for the reason that it is the course of the caverning +waters, it is, on the whole, best to approach the cave through their +exits along the banks of a stream or through the chance openings which +are here and there made by the falling in of their roofs. One +advantage of this cavity of entrance is that we can thus approach the +cavern in times of heavy rain when the processes which lead to their +construction are in full activity. Coming in this way to one of the +domes formed beneath a sink hole, we may observe in rainy weather that +the water falling down the deep shaft strikes the bottom with great +force; in many of the Kentucky caves it falls from a greater height +than Niagara. At such times the stones in the basin at the bottom of +the shaft are vigorously whirled about, and in their motion they cut +the rocks in the bottom of the basin--in fact, this cavity is a great +pot hole, like those at the base of open-air cascades. It is now easy +to interpret the general principles which determine the architecture +of the cavern realm. + +When it first enters the earth all the work which the water does in +the initial steps of cavern formation is effected by solution. As the +crevice enlarges and deepens, the stream acquires velocity, and begins +to use the bits of hard rock in boring. It works downward in this way +by the mixed mechanical and chemical action until it encounters a hard +layer. Then the water creeps horizontally through the soft stratum, +doing most of its work by solution, until it finds a crevice in the +floor through which it can excavate farther in the downward direction; +so it goes on in the manner of steps until it burrows channels to the +open stream. In time the vertical fall under the sink hole will cut +through the hard layer, when the water, abandoning the first line of +exit, will develop another at a lower level, and so in time it comes +about that there may be several stories of the cave, the lowest being +the last to be excavated. Of the total work thus done, only a small +part is accomplished by the falling of the water, acting through the +boring action of its tools, the bits of stone before mentioned; the +principal part of the task is done by the solvent action of the +carbonated waters on the limestone. In the system of caverns known as +the Mammoth Cave, in Kentucky, the writer has estimated that at least +nine tenths of the stone was removed in the state of solution. + +When first excavated, the chambers of a limestone cavern have little +beauty to attract the eye. The curves of the walls are sometimes +graceful, but the aspect of the chambers, though in a measure grand, +is never charming. When, however, the waters have ceased to carve the +openings, when they have been drained away by the formation of +channels on a lower level, there commonly sets in a process known as +stalactitization, which transforms the scene into one of singular +beauty. We have already noted the fact that everywhere in ordinary +rocks there are crevices through which water, moving under the +pressure of the fluid which is above, may find its way slowly +downward. In the limestone roofs of caverns, particularly in those of +the upper story, this ooze of water passes through myriads of unseen +fissures at a rate so slow that it often evaporates in the dry air +without dropping to the floor. When it comes out of the rocks the +water is charged with various salts of lime; when it evaporates it +leaves the material behind on the roof. Where the outflow is so slight +that the fluid does not gather into drops, it forms an incrustation of +limy matter, which often gathers in beautiful flowerlike forms, or +perhaps in the shape of a sheet of alabaster. Where drops are formed, +a small, pendent cone grows downward from the ceiling, over which the +water flows, and on which it evaporates. This cone grows slowly +downward until it may attain the floor of the chamber, which has a +height of thirty feet or more. If all the water does not evaporate, +that which trickles off the apex of the cone, striking on the floor, +is splashed out into a thin sheet, so that it evaporates in a speedy +manner, lays down its limestone, and thus builds another and ruder +cone, which grows upward toward that which is pendent above it. +Finally, they grow together, enlarged by the process which constructed +them, until a mighty column may be formed, sculptured as if by the +hands of a fantastic architect. + +[Illustration: Fig. 13.--Stalactites and stalagmites on roof and floor +of a cavern. The arrows show the direction of the moving water.] + +All the while that subterranean streams are cutting the caverns +downward the open-air rivers into which they discharge are deepening +their beds, and thereby preparing for the construction of yet lower +stories of caves. These open-air streams commonly flow in steep-sided, +narrow valleys, which themselves were caves until the galleries became +so wide that they could no longer support the roof. Thus we often find +that for a certain distance the roof over a large stream has fallen +in, so that the water flows in the open air. Then it will plunge +under an arch and course, it may be, for some miles, before it again +arrives at a place where the roof has disappeared, or perhaps attains +a field occupied by rocks of another character, in which caverns were +not formed. At places these old river caverns are abandoned by the +streams, which find other courses. They form natural tunnels, which +are not infrequently of considerable length. One such in southwestern +Virginia has been made useful for a railway passing from one valley to +another, thus sparing the expense of a costly excavation. Where the +remnant of the arch is small, it is commonly known as a natural +bridge, of which that in Rockbridge County, in Virginia, is a very +noble example. Arches of this sort are not uncommon in many cavern +countries; five such exist in Carter County, Kentucky, a district in +the eastern part of that State which abounds in caverns, though none +of them are of conspicuous height or beauty.[7] + +[Footnote 7: It is reported that one of these natural bridges of Carter +County has recently fallen down. This is the natural end of these +features. As before remarked, they are but the remnants of much more +extensive roofs which the processes of decay have brought to ruin.] + +At this stage of his studies on cavern work the student will readily +conceive that, as the surface of the country overlying the cave is +incessantly wearing down, the upper stories of the system are +continually disappearing, while new ones are forming at the present +drainage level of the country. In fact, the attentive eye can in such +a district find here and there evidences of this progressive +destruction. Not only do the caves wear out from above, but their +roofs are constantly falling to their floors, a process which is +greatly aided by the growth of stalactites. Forming in the crevices or +joints between the stones, these rock growths sometimes prize off +great blocks. In other cases the weight of the pendent stalactite +drags the ill-supported masses of the roof to the floor. In this way a +gallery originally a hundred feet below the surface may work its way +upward to the light of day. The entrance by which the Mammoth Cave is +approached appears to have been formed in this manner, and at several +points in that system of caverns the effect of this action may be +distinctly observed. + +We must now go a step further on the way of subterranean water, and +trace its action in the depths below the plane of ordinary caves, +which, as we have noted, do not extend below the level of the main +streams of the cavern district. The first group of facts to be +attended to is that exhibited by artesian wells. These occur where +rocks have been folded down into a basinlike form. It often happens +that in such a basin the rocks of which it is composed are some of +them porous, and others impervious to water, and that the porous +layers outcrop on the high margins of the depression and have +water-tight layers over them. These conditions can be well represented +by supposing that we have two saucers, one within the other, with an +intervening layer of sand which is full of water. If now we bore an +opening in the bottom of the uppermost saucer, we readily conceive +that the water will flow up through it. In Nature we often find these +basins with the equivalent of the sandy layer in the model just +described rising hundreds of feet above the valley, so that the +artesian well, so named from the village of Artois, near Paris, where +the first opening of this nature was made, may yield a stream which +will mount upward, especially where piped, to a great height. At many +places in the world it is possible by such wells to obtain a large +supply of tolerably pure water, but in general it is found to contain +too large a supply of dissolved mineral matter or sulphuretted gases +to be satisfactory for domestic purposes. It may be well to note the +fact that the greater part of the so-called artesian wells, or borings +which deliver water to a height above the surface, are not true +artesian sources, in that they do not send up the water by the action +of gravitation, but under the influence of gaseous pressure. + +Where, as in the case of upturned porous beds, the crevice water +penetrates far below the earth's surface or the open-air streams which +drain the water away, the fluid acquires a considerable increase of +temperature, on the average about one degree Fahrenheit for each +eighty feet of descent. It may, indeed, become so heated that if it +were at the earth's surface it would not only burst into steam with a +vast explosive energy, but would actually shine in the manner of +heated solids. As the temperature of water rises, and as the pressure +on it increases, it acquires a solvent power, and takes in rocky +matter in a measure unapproached at the earth's surface. At the depth +of ten miles water beginning as inert rain would acquire the +properties which we are accustomed to associate with strong acids. +Passing downward through fissures or porous strata in the manner +indicated in the diagram, the water would take up, by virtue of its +heat and the gases it contained, a share of many mineral substances +which we commonly regard as insoluble. Gold and even platinum--the +latter a material which resists all acids at ordinary +temperatures--enters into the solution. If now the water thus charged +with mineral stores finds in the depths a shorter way to the surface +than that which it descended, which may well happen by way of a deep +rift in the rocks, it will in its ascent reverse the process which it +followed on going down. It will deposit the several minerals in the +order of their solubilities--that is, the last to be taken in will be +the first to be crystallized on the walls of the fissure through which +the upflow is taking place. The result will be the formation of a vein +belonging to the variety known as fissure veins. + +[Illustration: Fig. 14.--Diagram of vein. The different shadings show +the variations in the nature of the deposits.] + +A vein deposit such as we are considering may, though rarely, be +composed of a single mineral. Most commonly we find the deposit +arranged in a banded form in the manner indicated in the figure (see +diagram 14). Sometimes one material will abound in the lower portions +of the fissure and another in its higher parts, a feature which is +accounted for by the progressive cooling and relinquishment of +pressure to which the water is subjected on its way to the surface. +With each decrement of those properties some particular substance goes +out of the fluid, which may in the end emerge in the form of a warm or +hot spring, the water of which contains but little mineral matter. +Where, however, the temperature is high, some part of the deposit, +even a little gold, may be laid down just about the spring in the +deposits known as sinter, which are often formed at such places. + +In many cases the ore deposits are formed not only in the main channel +of the fissure, but in all the crevices on either side of that way. In +this manner, much as in the case of the growth of stalactitic matter +between the blocks of stone in the roofs of a cavern, large fragments +of rock, known as "horses," are often pushed out into the body of the +vein. In some instances the growth of the vein appears to enlarge the +fissure or place of the deposit as the accumulation goes on, the +process being analogous to that by which a growing root widens the +crevice into which it has penetrated. In other instances the fissure +formed by the force has remained wide open, or at most has been but +partly filled by the action of the water. + +It not infrequently happens that the ascending waters of hot springs +entering limestones have excavated extensive caves far below the +surface of the earth, these caverns being afterward in part filled by +the ores of various metals. We can readily imagine that the water at +one temperature would excavate the cavern, and long afterward, when at +a lower heat, they might proceed to fill it in. At a yet later stage, +when the surface of the country had worn down many thousands of feet +below the original level, the mineral stores of the caverns may be +brought near the surface of the earth. Some of the most important +metalliferous deposits of the Cordilleras are found in this group of +hot-water caverns. These caverns are essentially like those produced +by cold water, with the exception of the temperature of the fluid +which does the work and the opposite direction of the flow. + +In following crevice water which is free to obey the impulses of +gravitation far down into the earth, we enter on a realm where the +rock or construction water, that which was built into the stone at +the time of its formation, is plentiful. Where these two groups of +waters come in contact an admixture occurs, a certain portion of the +rock water joining that in the crevices. Near the surface of the +ground we commonly find that all the construction water has been +washed out by this action. Yet if the rocks be compact, or if they +have layers of a soft and clayey nature, we may find the construction +water, even in very old deposits, remaining near the surface of the +ground. Thus in the ancient Silurian beds of the Ohio Valley a boring +carried a hundred feet below the level of the main rivers commonly +discovers water which is clearly that laid down in the crevices of the +material at the time when the rocks were formed in the sea. In all +cases this water contains a certain amount of gases derived from the +decomposition of various substances, but principally from the +alteration of iron pyrite, which affords sulphuretted hydrogen. Thus +the water is forced to the surface with considerable energy, and the +well is often named artesian, though it flows by gas pressure on the +principle of the soda-water fountain, and not by gravity, as in the +case of true artesian wells. + +The passage between the work done by the deeply penetrating surface +water and that due to the fluid intimately blended with the rock built +into the mass at the time of its formation is obscure. We are, +however, quite sure that at great depths beneath the earth the +construction water acts alone not only in making veins, but in +bringing about many other momentous changes. At a great depth this +water becomes intensely heated, and therefore tends to move in any +direction where a chance fissure or other accident may lessen the +pressure. Creeping through the rocks, and moving from zones of one +temperature to another, these waters bring about in the fine +interstices chemical changes which lead to great alterations in the +constitution of the rock material. It is probably in part to these +slow driftings of rock water that beds originally made up of small, +shapeless fragments, such as compose clay slates, sandstones, and +limestones, may in time be altered into crystalline rocks, where there +is no longer a trace of the original bits, all the matter having been +taken to pieces by the process of dissolving, and reformed in the +regular crystalline order. In many cases we may note how a crystal +after being made has been in part dissolved away and replaced by +another mineral. In fact, many of our rocks appear to have been again +and again made over by the slow-drifting waters, each particular state +in their construction being due to some peculiarity of temperature or +of mineral contents which the fluid held. These metamorphic phenomena, +though important, are obscure, and their elucidation demands some +knowledge of petrographic science, that branch of geology which +considers the principles of rock formation. They will therefore not be +further considered in this work. + + + VOLCANOES. + +Of old it was believed that volcanoes represented the outpouring of +fluid rock which came forth from the central realm of the earth, a +region which was supposed still to retain the liquid state through +which the whole mass of our earth has doubtless passed. Recent +studies, however, have brought about a change in the views of +geologists which is represented by the fact that we shall treat +volcanic phenomena in connection with the history of rock water. + +In endeavouring to understand the phenomena of volcanoes it is very +desirable that the student should understand what goes on in a normal +eruption. The writer may, therefore, be warranted in describing some +observations which he had an opportunity to make at an eruption of +Vesuvius in 1883, when it was possible to behold far more than can +ordinarily be discerned in such outbreaks--in fact, the opportunity of +a like nature has probably not been enjoyed by any other person +interested in volcanic action. In the winter of 1882-'83 Vesuvius was +subjected to a succession of slight outbreaks. At the time of the +observations about to be noted the crater had been reduced to a cup +about three hundred feet in diameter and about a hundred feet deep. +The vertical shaft at the bottom, through which the outbursts were +taking place, was about a hundred feet across. Taking advantage of a +heavy gale from the northwest, it was practicable, notwithstanding the +explosions, to climb to the edge of the crater wall. Looking down into +the throat of the volcano, although the pit was full of whirling +vapours and the heat was so great that the protection of a mask was +necessary, it was possible to see something of what was going on at +the moment of an explosion. + +The pipe of the volcano was full of white-hot lava. Even in a day of +sunshine, which was only partly obscured by the vapours which hung +about the opening, the heat of the lava made it very brilliant. This +mass of fluid rock was in continuous motion, swaying violently up and +down the tube. From four to six times a minute, at the moment of its +upswaying, it would burst as by the explosion of a gigantic bubble. +The upper portion of the mass was blown upward in fragments, the +discharge being like that of shot from a fowling piece; the fragments, +varying in size from small, shotlike bits to masses larger than a +man's head, were shot up sometimes to the height of fifteen hundred +feet above the point of ejection. The wind, blowing at the rate of +about forty miles an hour, drove the falling bits of rock to the +leeward, so that there was no considerable danger to be apprehended +from them. Some seconds after the explosion they could be heard +rattling down on the farther slope of the cone. Observations on the +interval between the discharge and the fall of the fragments made it +easy to compute the height to which they were thrown. + +At the moment when the lava in the pipe opened for the passage of the +vapour which created the explosion the movement, though performed in +a fraction of a second, was clearly visible. At first the vapour was +colourless; a few score feet up it began to assume a faint, bluish +hue; yet higher, when it was more expanded, the tint changed to that +of steam, which soon became of the ordinary aspect, and gathered in +swift-revolving clouds. The watery nature of the vapour was perfectly +evident by its odour. Though commingled with sulphurous-acid gas, it +still had the characteristic smell of steam. For a half hour it was +possible to watch the successive explosions, and even to make rough +sketches of the scene. Occasionally the explosions would come in quick +succession, so that the lava was blown out of the tube; again, the +pool would merely sway up and down in a manner which could be +explained only by supposing that great bubbles of vapour were working +their way upward toward the point where they could burst. Each of +these bubbles probably filled a large part of the diameter of the +pipe. In general, the phenomena recalled the escape of the jet from a +geyser, or, to take a familiar instance, that of steam from the pipe +of a high-pressure engine. When the heat is great, steam may often be +seen at the mouth of the pipe with the same transparent appearance +which was observed in the throat of the crater. In the cold air of the +mountain the vapour was rapidly condensed, giving a rainbow hue in the +clouds when they were viewed at the right angle. The observations were +interrupted by the fact that the wind so far died away that large +balls of the ejected lava began to fall on the windward side of the +cone. These fragments, though cooled and blackened on their outside by +their considerable journey up and down through the air, were still so +soft that they splashed when they struck the surface of cinders. + +Watching the cone from a distance, one could note that from time to +time the explosions, increasing in frequency, finally attained a point +where the action appeared to be continuous. The transition was +comparable to that which we may observe in a locomotive which, when it +first gets under way, gives forth occasional jets of steam, but, +slowly gaining speed, finally pours forth what to eye and ear alike +seem to be a continuous outrush. All the evidence that we have +concerning volcanic outbreaks corroborates that just cited, and is to +the effect that the essence of the action consists in the outbreak of +water vapour at a high temperature, and therefore endowed with very +great expansive force. Along with this steam there are many other +gases, which always appear to be but a very small part of the whole +escape of a vaporous nature--in fact, the volcanic steam, so far as +its chemical composition has been ascertained, has the composition +which we should expect to find in rock water which had been forced out +from the rock by the tensions that high temperature creates. + +Because of its conspicuous nature, the lava which flows from most +volcanoes, or is blown out from them in the form of finely divided +ash, is commonly regarded as the primary feature in a volcanic +outbreak. Such is not really the case. Volcanic explosions may occur +with very little output of fluid rock, and that which comes forth may +consist altogether of the finely divided bits of rock to which we give +the name of ash. In fact, in all very powerful explosions we may +expect to find no lava flow, but great quantities of this finely +divided rock, which when it started from the depths of the earth was +in a fluid state, but was blown to pieces by the contained vapour as +it approached the surface. + +If the student is so fortunate as to behold a flood of lava coming +forth from the flanks of a volcano, he will observe that even at the +very points of issue, where the material is white-hot and appears to +be as fluid as water, the whole surface gives forth steam. On a still +day, viewed from a distance, the path of a lava flow is marked by a +dense cloud of this vapour which comes forth from it. Even after the +lava has cooled so that it is safe to walk upon it, every crevice +continues to pour forth steam. Years after the flowing has ceased, and +when the rock surface has become cool enough for the growth of +certain plants upon it, these crevices still yield steam. It is +evident, in a word, that a considerable part of a lava mass, even +after it escapes from the volcanic pipes, is water which is intimately +commingled with the rock, probably lying between the very finest +grains of the heated substance. Yet this lava which has come forth +from the volcano has only a portion of the water which it originally +contained; a large, perhaps the greater part, has gone forth in the +explosive way through the crater. It is reasonably believed that the +fluidity of lava is in considerable measure due to the water which it +contains, and which serves to give the mass the consistence of paste, +the partial fluidity of flour and rock grains being alike brought +about in the same manner. + +So much of the phenomena of volcanoes as has been above noted is +intended to show the large part which interstitial water plays in +volcanic action. We shall now turn our attention again to the state of +the deeply buried rock water, to see how far we may be able by it to +account for these strange explosive actions. When sediments are laid +down on the sea floor the materials consist of small, irregularly +shaped fragments, which lie tumbled together in the manner of a mass +of bricks which have been shot out of a cart. Water is buried in the +plentiful interspaces between these bits of stone; as before remarked, +the amount of this construction water varies. In general, it is at +first not far from one tenth part of the materials. Besides the fluid +contained in the distinct spaces, there is a share which is held as +combined water in the intimate structure of the crystals, if such +there be in the mass. When this water is built into the stone it has +the ordinary temperature of the sea bottom. As the depositing actions +continue to work, other beds are formed on the top of that which we +are considering, and in time the layer may be buried to the depth of +many thousand feet. There are reasons to believe that on the floors of +the oceans this burial of beds containing water may have brought great +quantities of fluid to the depth of twenty miles or more below the +outer surface of the rocks. + +[Illustration: Fig. 15.--Flow of lava invading a forest. A tree in the +distance is not completely burned, showing that the molten rock had +lost much of its original heat.] + +The effect of deep burial is to increase the heat of strata. This +result is accomplished in two different ways. The direct effect +arising from the imposition of weight, that derived from the mass of +stratified material, is, as we know, to bring about a down-sinking of +the earth's crust. In the measure of this falling, heat is engendered +precisely as it is by the falling of a trip-hammer on the anvil, with +which action, as is well known, we may heat an iron bar to a high +temperature. It is true that this down-sinking of the surface under +weight is in part due to the compression of the rocks, and in part to +the slipping away of the soft underpinning of more or less fluid rock. +Yet further it is in some measure brought about by the wrinkling of +the crust. But all these actions result in the conversion of energy of +position into heat, and so far serve to raise the temperature of the +rocks which are concerned in the movements. By far the largest source +of heat, however, is that which comes forth from the earth's interior, +and which was stored there in the olden day when the matter forming +the earth gathered into the mass of our sphere. This, which we may +term the original heat, is constantly flowing forth into space, but +makes its way slowly, because of the non-conductive, or, as we may +phrase it, the "blanketing" effect of the outer rock. The effect of +the strata is the same as that exercised by the non-conductive +coatings which are put on steam boilers. A more familiar comparison +may be had from the blankets used for bedclothing. If on top of the +first blanket we put a second, we keep warmer because the temperature +of the lower one is elevated by the heat from our body which is held +in. In the crust of the earth each layer of rock resists the outflow +of heat, and each addition lifts the temperature of all the layers +below. + +When water-bearing strata have been buried to the depth of ten miles, +the temperature of the mass may be expected to rise to somewhere +between seven hundred and a thousand degrees Fahrenheit. If the depth +attained should be fifty miles, it is likely that the temperature will +be five times as great. At such a heat the water which the rocks +contain tends in a very vigorous way to expand and pass into the state +of vapour. This it can not readily do, because of its close +imprisonment; we may say, however, that the tendency toward explosion +is almost as great as that of ignited gunpowder. Such powder, if held +in small spaces in a mass of cast steel, could be fired without +rending the metal. The gases would be retained in a highly compressed, +possibly in a fluid form. If now it happens that any of the strain in +the rocks such as lead to the production of faults produce fissures +leading from the surface into this zone of heated water, the tendency +of the rocks containing the fluid, impelled by its expansion, will be +to move with great energy toward the point of relief or lessened +pressure which the crevice affords. Where rocks are in any way +softened, pressure alone will force them into a cavity, as is shown by +the fact that beds of tolerably hard clay stones in deep coal mines +may be forced into the spaces by the pressure of the rocks which +overlie them--in fact, the expense of cutting out these in-creeping +rocks is in some British mines a serious item in the cost of the +product. + +The expansion of the water contained in the deep-lying heated rocks +probably is by far the most efficient agent in urging them toward the +plane of escape which the fissure affords. When the motion begins it +pervades all parts of the rock at once, so that an actual flow is +induced. So far as the movement is due to the superincumbent weight, +the tendency is at once to increase the temperature of the moving +mass. The result is that it may be urged into the fissure perhaps even +hotter than when it started from the original bed place. In proportion +as the rocky matter wins its way toward the surface, the pressure upon +it diminishes, and the contained vapours are freer to expand. Taking +on the vaporous form, the bubbles gather to each other, and when they +appear at the throat of the volcano they may, if the explosions be +infrequent, assume the character above noted in the little eruption of +Vesuvius. Where, however, the lava ascends rapidly through the +channel, it often attains the open air with so much vapour in it, and +this intimately mingled with the mass, that the explosion rends the +materials into an impalpably fine powder, which may float in the air +for months before it falls to the earth. With a less violent movement +the vapour bubbles expand in the lava, but do not rend it apart, thus +forming the porous, spongy rock known as pumice. With a yet slower +ascent a large part of the steam may go away, so that we may have a +flow of lava welling forth from the vent, still giving forth steam, +but with a vapour whose tension is so lowered that the matter is not +blown apart, though it may boil violently for a time after it escapes +into the air. + +Although the foregoing relatively simple explanation of volcanic +action can not be said as yet to be generally accepted by geologists, +the reasons are sufficient which lead us to believe that it accounts +for the main features which we observe in this class of explosions--in +other words, it is a good working hypothesis. We shall now proceed in +the manner which should be followed in all natural inquiry to see if +the facts shown in the distribution of volcanoes in space and time +confirm or deny the view. + +The most noteworthy feature in the distribution of volcanoes is that, +at the present time at least, all active vents are limited to the sea +floors or to the shore lands within the narrow range of three hundred +miles from the coast. Wherever we find a coast line destitute of +volcanoes, as is the case with the eastern coast of North and South +America, it appears that the shore has recently been carried into the +land for a considerable distance--in other words, old coast lines are +normally volcanic; that is, here and there have vents of this nature. +Thus the North Atlantic, the coasts of which appear to have gone +inland for a great distance in geologically recent times, is +non-volcanic; while the Pacific coast, which for a long time has +remained in its present position, has a singularly continuous line of +craters near the shore extending from Alaska to Tierra del Fuego. So +uninterrupted is this line of volcanoes that if they were all in +eruption it would very likely be possible to journey down the coast +without ever being out of sight of the columns of vapour which they +would send forth. On the floor of the sea volcanic peaks appear to be +very widely distributed; only a few of them--those which attain the +surface of the water--are really known, but soundings show long lines +of elevations which doubtless represent cones distributed along fault +lines, none of the peaks of sufficient height to break the surface of +the sea. It is likely, indeed, that for one marine volcano which +appears as an island there are scores which do not attain the surface. +Volcanic islands exist and generally abound in the ocean and greater +seas; every now and then we observe a new one forming as a small +island, which is apt to be washed away by the sea shortly after the +eruption ceases, the disappearance being speedy, for the reason that +the volcanic ashes of which these cones are composed drift away like +snow before the movement of the waves. + +If the waters of the ocean and seas were drained away so that we could +inspect the portion of the earth's surface which they cover as readily +as we do the dry lands, the most conspicuous feature would be the +innumerable volcanic eminences which lie hidden in these watery +realms. Wherever the observer passed from the centres of the present +lands he would note within the limits of those fields only mountains, +much modified by river action; hills which the rivers had left in +scarfing away the strata; and dales which had been carved out by the +flowing waters. Near the shore lines of the vanished seas he would +begin to find mountains, hills, and vales occasionally commingled with +volcanic peaks, those structures built from the materials ejected from +the vents. Passing the coast line to the seaward, the hills and dales +would quickly disappear, and before long the mountains would vanish +from his way, and he would gradually enter on a region of vast rolling +plains beset by volcanic peaks, generally accumulated in long ranges, +somewhat after the manner of mountains, but differing from those +elevations not only in origin but in aspect, the volcanic set of peaks +being altogether made up of conical, cup-topped elevations. + +A little consideration will show us that the fact of volcanoes being +in the limit to the sea floors and to a narrow fringe of shore next +certain ocean borders is reconcilable with the view as to their +formation which we have adopted. We have already noted the fact that +the continents are old, which implies that the parts of the earth +which they occupy have long been the seats of tolerably continuous +erosion. Now and then they have swung down partly beneath the sea, and +during their submersion they received a share of sediments. But, on +the whole, all parts of the lands except strips next the coast may be +reckoned as having been subjected to an excess of wearing action far +exceeding the depositional work. Therefore, as we readily see, +underneath such land areas there has been no blanketing process going +on which has served to increase the heat in the deep underlying rocks. +On the contrary, it would be easy to show, and the reader may see it +himself, that the progressive cooling of the earth has probably +brought about a lowering of the temperature in all the section from +the surface to very great depths, so that not only is the rock water +unaffected by increase of heat, but may be actually losing +temperature. In other words, the conditions which we assume bring +about volcanic action do not exist beneath the old land. + +Beneath the seas, except in their very greatest depths, and perhaps +even there, the process of forming strata is continually going on. +Next the shores, sometimes for a hundred or two miles away to seaward, +the principal contribution may be the sediment worn from the lands by +the waves and the rivers. Farther away it is to a large extent made up +of the remains of animals and plants, which when dying give their +skeletons to form the strata. Much of the materials laid down--perhaps +in all more than half--consist of volcanic dust, ashes, and pumice, +which drifts very long times before it finds its way to the bottom. We +have as yet no data of a precise kind for determining the average rate +of accumulation of sediments upon the sea floor, but from what is +known of the wearing of the lands, and the amount of volcanic waste +which finds its way to the seas, it is probably not less than about a +foot in ten thousand years; it is most likely, indeed, much to exceed +this amount. From data afforded by the eruptions in Java and in other +fields where the quantity of volcanic dust contributed to the seas can +be estimated, the writer is disposed to believe that the average rate +of sedimentation on the sea floors is twice as great as the estimate +above given. + +Accumulating at the average rate of one foot in ten thousand years, it +would require a million years to produce a hundred feet of sediments; +a hundred million to form ten thousand feet, and five hundred million +to create the thickness of about ten miles of bed. At the rate of two +feet in ten thousand years, the thickness accumulated would be about +twenty miles. When we come to consider the duration of the earth's +geologic history, we shall find reasons for believing that the +formation of sediment may have continued for as much as five hundred +million years. + +The foregoing inquiries concerning the origin of volcanoes show that +at the present time they are clearly connected with some process which +goes on beneath the sea. An extension of the inquiry indicates that +this relation has existed in earlier geological times; for, although +the living volcanoes are limited to places within three hundred miles +of the sea, we find lava flows, ashes, and other volcanic +accumulations far in the interior of the continents, though the energy +which brought them forth to the earth's surface has ceased to operate +in those parts of the land. In these cases of continental volcanoes it +generally, if not always, appears that the cessation of the activity +attended the removal of the shore line of the ocean or the +disappearance of great inland seas. Thus the volcanoes of the +Yellowstone district may have owed their activity to the immense +deposits of sediment which were formed in the vast fresh-water lakes +which during the later Cretaceous and early Tertiary times stretched +along the eastern face of the Rocky Mountains, forming a Mediterranean +Sea in North America comparable to that which borders southern Europe. +It thus appears that the arrangement of volcanoes with reference to +sea basins has held for a considerable period in the past. Still +further, when we look backward through the successive formations of +the earth's crust we find here and there evidences in old lava flows, +in volcanic ashes, and sometimes in the ruins of ancient cones which +have been buried in the strata, that igneous activity such as is now +displayed in our volcanoes has been, since the earliest days of which +we have any record, a characteristic feature of the earth. There is no +reason to suppose that this action has in the past been any greater or +any less than in modern days. All these facts point to the conclusion +that volcanic action is due to the escape of rock water which has been +heated to high temperatures, and which drives along with it as it +journeys toward a crevice the rock in which it has been confined. + +We will now notice some other explanations of volcanic action which +have obtained a certain credence. First, we may note the view that +these ejections from craters are forced out from a supposed liquid +interior of the earth. One of the difficulties of this view is that we +do not know that the earth's central parts are fluid--in fact, many +considerations indicate that such is not the case. Next, we observe +that we not infrequently find two craters, each containing fluid lava, +with the fluid standing at differences of height of several thousand +feet, although the cones are situated very near each other. If these +lavas came from a common internal reservoir, the principles which +control the action of fluids would cause the lavas to be at the same +elevation. Moreover, this view does not provide any explanation of the +fact that volcanoes are in some way connected with actions which go on +on the floors of great water basins. There is every reason to believe +that the fractures in the rocks under the land are as numerous and +deep-going as those beneath the sea. If it were a mere question of +access to a fluid interior, volcanoes should be equally distributed on +land and sea floors. Last of all, this explanation in no wise accounts +for the intermixture of water with the fluid rock. We can not well +believe that water could have formed a part of the deeper earth in the +old days of original igneous fusion. In that time the water must have +been all above the earth in the vaporous state. + +Another supposition somewhat akin to that mentioned is that the water +of the seas finds its way down through crevices beneath the floors of +the ocean, and, there coming in contact with an internal molten mass, +is converted into steam, which, along with the fluid rock, escapes +from the volcanic vent. In addition to the objections urged to the +preceding view, we may say concerning this that the lava, if it came +forth under these circumstances, would emerge by the short way, that +by which the water went down, and not by the longer road, by which it +may be discharged ten thousand feet or more above the level of the +sea. + +The foregoing general account of volcanic action should properly be +followed by some account of what takes place in characteristic +eruptions. This history of these matters is so ample that it would +require the space of a great encyclopaedia to contain them. We shall +therefore be able to make only certain selections which may serve to +illustrate the more important facts. + +By far the best-known volcanic cone is that of Vesuvius, which has +been subjected to tolerably complete record for about twenty-four +hundred years. About 500 B.C. the Greeks, who were ever on the search +for places where they might advantageously plant colonies, settled on +the island of Ischia, which forms the western of what is now termed +the Bay of Naples. This island was well placed for tillage as well as +for commerce, but the enterprising colonists were again and again +disturbed by violent outbreaks of one or more volcanoes which lie in +the interior of this island; at one time it appears that the people +were driven away by these explosions. + +In these pre-Christian days Vesuvius, then known as Monte Somma, was +not known to be a volcano, it never having shown any trace of +eruption. It appeared as a regularly shaped mountain, somewhat over +two thousand feet high, with a central depression about three miles in +diameter at the top, and perhaps two miles over at the bottom, which +was plainlike in form, with some lakes of bitter water in the centre. +The most we know of this central cavity is connected with the +insurrection of the slaves led by Spartacus, the army of the revolters +having camped for a time on the plain encircled by the crater walls. +The outer slopes of the mountain afforded then a remarkably fertile +soil; some traces, indeed, of the fertility have withstood the modern +eruptions which have desolated its flanks. This wonderful Bay of +Naples became the seat of the fairest Roman culture, as well as of a +very extended commerce. Toward the close of the first century of our +era the region was perhaps richer, more beautifully cultivated, and +the seat of a more elaborate luxury than any part of the shore line of +Europe at the present day. At the foot of the mountain, on the eastern +border of the bay, the city of Pompeii, with a population of about +fifty thousand souls, was a considerable port, with an extensive +commerce, particularly with Egypt. The charming town was also a place +of great resort for rich Egyptians who cared to dwell in Europe. On +the flanks of the mountain there was at least one large town, +Herculaneum, which appears to have been an association of rich men's +residences. On the eastern side of the bay, at a point now known as +Baiae, the Roman Government had a naval station, which in the year 79 +was under the command of the celebrated Pliny, a most voluminous +though unscientific writer on matters of natural history. With him in +that year there was his nephew, commonly known as the younger Pliny, +then a student of eighteen years, but afterward himself an author. +These facts are stated in some detail, for they are all involved in +the great tragedy which we are now to describe. + +For many years there had been no eruption about the Bay of Naples. The +volcanoes on Ischia had been still for a century or more, and the +various circular openings on the mainland had been so far quiet that +they were not recognised as volcanoes. Even the inquisitive Pliny, +with his great learning, was so little of a geologist that he did not +know the signs which indicate the seat of volcanic action, though they +are among the most conspicuous features which can meet the eye. The +Greeks would doubtless have recognised the meaning of these physical +signs. In the year 63 the shores of the Bay of Naples were subjected +to a distinctive earthquake. Others less severe followed in subsequent +years. In an early morning in the year 79, a servant aroused the elder +Pliny at Baiae with the news that there was a wonderful cloud rising +from Monte Somma. The younger Pliny states that in form it was like a +pine tree, the common species in Italy having a long trunk with a +crown of foliage on its summit, shaped like an umbrella. This crown of +the column grew until it spread over the whole landscape, darkening +the field of view. Shortly after, a despatch boat brought a message to +the admiral, who at once set forth for the seat of the disturbance. He +invited his nephew to accompany him, but the prudent young man relates +in his letters to Tacitus, from whom we know the little concerning the +eruption which has come down to us, that he preferred to do some +reading which he had to attend to. His uncle, however, went straight +forward, intending to land at some point on the shore at the foot of +the cone. He found the sea, however, so high that a landing was +impossible; moreover, the fall of rock fragments menaced the ship. He +therefore cruised along the shore for some distance, landing at a +station probably near the present village of Castellamare. At this +point the fall of ashes and pumice was very great, but the sturdy old +Roman had his dinner and slept after it. There is testimony that he +snored loudly, and was aroused only when his servants began to fear +that the fall of ashes and stones would block the way out of his +bedchamber. When he came forth with his attendants, their heads +protected by planks resting on pillows, he set out toward Pompeii, +which was probably the place where he sought to land. After going some +distance, the brave man fell dead, probably from heart disease; it is +said that he was at the time exceedingly asthmatic. No sooner were his +servants satisfied that the life had passed from his body than they +fled. The remains were recovered after the eruption had ceased. The +younger Pliny further relates that after his uncle left, the cloud +from the mountain became so dense that in midday the darkness was that +of midnight, and the earthquake shocks were so violent that wagons +brought to the courtyard of the dwelling to bear the members of the +household away were rolled this way and that by the quakings of the +earth. + +Save for the above-mentioned few and unimportant details concerning +the eruption, we have no other contemporaneous account. We have, +indeed, no more extended story until Dion Cassius, writing long after +the event, tells us that Herculaneum and Pompeii were overwhelmed; but +he mixes his story with fantastic legends concerning the appearance of +gods and demons, as is his fashion in his so-called history. Of all +the Roman writers, he is perhaps the most untrustworthy. Fortunately, +however, we have in the deposits of ashes which were thrown out at the +time of this great eruption some basis for interpreting the events +which took place. It is evident that for many hours the Vesuvian +crater, which had been dormant for at least five hundred years, blew +out with exceeding fury. It poured forth no lava streams; the energy +of the uprushing vapours was too great for that. The molten rock in +their path was blown into fine bits, and all the hard material cast +forth as free dust. In the course of the eruption, which probably did +not endure more than two days, possibly not more than twenty-four +hours, ash enough was poured forth to form a thick layer which spread +far over the neighbouring area of land and sea floor. It covered the +cities of Herculaneum and Pompeii to a depth of more than twenty feet, +and over a circle having a diameter of twenty miles the average +thickness may have been something like this amount. So deep was it +that, although almost all the people of these towns survived, it did +not seem to them worth while to undertake to excavate their dwelling +places. At Pompeii the covering did not overtop the higher of the low +houses. An amount of labour which may be estimated at not over one +thirtieth of the value, or at least the cost which had been incurred +in building the city, would have restored it to a perfectly +inhabitable state. The fact that it was utterly abandoned probably +indicates a certain superstitious view in connection with the +eruption. + +The fact that the people had time to flee from Herculaneum and +Pompeii, bearing with them their more valuable effects, is proved by +the excavations at these places which have been made in modern times. +The larger part of Pompeii and a considerable portion of Herculaneum +have been thus explored; only rarely have human remains been found. +Here and there, particularly in the cellars, the labourers engaged in +the work of disinterring the cities note that their picks enter a +cavity; examining the space, they find they have discovered the +remains of a human skeleton. It has recently been learned that by +pouring soft plaster of Paris into these openings a mould may be +obtained which gives in a surprisingly perfect manner the original +form of the body. The explanation of this mould is as follows: Along +with the fall of cinders in an eruption there is always a great +descent of rain, arising from the condensation of the steam which +pours forth from the volcano. This water, mingling with the ashes, +forms a pasty mud, which often flows in vast streams, and is +sometimes known as mud lava. This material has the qualities of +cement--that is, it shortly "sets" in a manner comparable to plaster +of Paris or ordinary mortar. During the eruption of 79 this mud +penetrated all the low places in Pompeii, covering the bodies of the +people, who were suffocated by the fumes of the volcanic emanations. +We know that these people were not drowned by the inundation; their +attitudes show that they were dead before the flowing matter +penetrated to where they lay. + +It happened that Pompeii lay beyond the influence of the subsequent +great eruptions of Vesuvius, so that it afterward received only slight +ash showers. Herculaneum, on the other hand, has century by century +been more and more deeply buried until at the present time it is +covered by many sheets of lava. This is particularly to be regretted, +for the reason that, while Pompeii was a seaport town of no great +wealth or culture, Herculaneum was the residence place of the gentry, +people who possessed libraries, the records of which can be in many +cases deciphered, and from which we might hope to obtain some of the +lost treasures of antiquity. The papyrus rolls on which the books of +that day were written, though charred by heat and time, are still +interpretable. + +After the great explosion of 79, Vesuvius sank again into repose. It +was not until 1056 that vigorous eruptions again began. From time to +time slight explosions occurred, none of which yielded lava flows; it +was not until the date last mentioned that this accompaniment of the +eruption began to appear. In 1636, after a repose of nearly a century +and a half, there came a very great outbreak, which desolated a wide +extent of country on the northwestern side of the cone. At this stage +in the history of the crater the volcanic flow began to attain the +sea. Washing over the edge of the old original crater of Monte Somma, +and thus lowering its elevation, these streams devastated, during the +eruption just mentioned and in various other outbreaks, a wide field +of cultivated land, overwhelming many villages. The last considerable +eruption which yielded large quantities of lava was that of 1872, +which sent its tide for a distance of about six miles. + +Since 1636 the eruptions of Vesuvius have steadily increased in +frequency, and, on the whole, diminished in violence. In the early +years of its history the great outbreaks were usually separated by +intervals of a century or more, and were of such energy that the lava +was mostly blown to dust, forming clouds so vast that on two occasions +at least they caused a midnight darkness at Constantinople, nearly +twelve hundred miles away. This is as if a volcano at Chicago should +completely hide the sun in the city of Boston. In the present state of +Vesuvius, the cone may be said to be in slight, almost continuous +eruption. The old central valley which existed before the eruption of +79, and continued to be distinct for long after that time, has been +filled up by a smaller cone, bearing a relatively tiny crater of vent, +the original wall being visible only on the eastern and northern parts +of its circuit, and here only with much diminished height. On the +western face the slope from the base of the mountain to the summit of +the new cone is almost continuous, though the trained eye can trace +the outline of Monte Somma--its position in a kind of bench, which is +traceable on that side of the long slope leading from the summit of +the new cone to the sea. The fact that the lavas of Vesuvius have +broken out on the southwestern side, while the old wall of the cone +has remained unbroken on the eastern versant, has a curious +explanation. The prevailing wind of Naples is from the southwest, +being the strong counter trades which belong in that latitude. In the +old days when the Monte Somma cone was constructed these winds caused +the larger part of the ashes to fall on the leeward side of the cone, +thus forming a thicker and higher wall around that part of the crater. + +From the nature of the recent eruptions of Vesuvius it appears likely +that the mountain is about to enter on a second period of inaction. +The pipes leading through the new cone are small, and the mass of this +elevation constitutes a great plug, closing the old crater mouth. To +give vent to a large discharge of steam, the whole of this great mass, +having a depth of nearly two thousand feet, would have to be blown +away. It seems most likely that when the occasion for such a discharge +comes, the vapours of the eruption will seek a vent through some other +of the many volcanic openings which lie to the westward of this great +cone. The history of these lesser volcanoes points to the conclusion +that when the path by way of Vesuvius is obstructed they may give +relief to the steam which is forcing its course to the surface. Two or +three times since the eruption of Pliny, during periods when Vesuvius +had long been quiet, outbreaks have taken place on Ischia or in the +Phlaegraen Fields, a region dotted with small craters which lies to the +west of Naples. The last of these occurred in 1552, and led to the +formation of the beautiful little cone known as Monte Nuovo. This +eruption took place near the town of Puzzuoli, a place which was then +the seat of a university, the people of which have left us records of +the accident. + +[Illustration: Fig. 16.--Diagrammatic sections through Mount Vesuvius, +showing changes in the form of the cone. (From Phillips.)] + +The outbreak which formed Monte Nuovo was slight but very +characteristic. It occurred in and beside a circular pool known as the +Lucrine Lake, itself an ancient crater. At the beginning of the +disturbance the ground opened in ragged cavities, from which mud and +ashes and great fragments of hard rock were hurled high in the air, +some of the stones ascending to a height of several thousand feet. +With slight intermissions this outbreak continued for some days, +resulting in the formation of a hill about five hundred feet high, +with a crater in its top, the bottom of which lay near the level of +the sea. Although this volcanic elevation, being made altogether of +loose fragments, is rapidly wearing down, while the crater is filling +up, it remains a beautiful object in the landscape, and is also +noteworthy for the fact that it is the only structure of this nature +which we know from its beginning. In the Phlaegraen Field there are a +number of other craters of small size, with very low cones about them. +These appear to have been the product of brief, slight eruptions. That +known as the Solfatara, though not in eruption during the historic +period, is interesting for the fact that from the crevices of the +rocks about it there comes forth a continued efflux of carbonic-acid +gas. This substance probably arises from the effect of heat contained +in old lavas which are in contact with limestone in the deep +under-earth. We know such limestones are covered by the lavas of +Vesuvius, for the reason that numerous blocks of the rock are thrown +out during eruptions, and are often found embedded in the lava +streams. It is an interesting fact that these craters of the +Phlaegraen Field, lying between the seats of vigorous eruption on +Ischia and at Vesuvius, have never been in vigorous eruption. Their +slight outbreaks seem to indicate that they have no permanent +connection with the sources whence those stronger vents obtain their +supply of heated steam. + +The facts disclosed by the study of the Vesuvian system of volcanoes +afford the geologist a basis for many interesting conclusions. + +In the first place, he notes that the greater part of the cones, all +those of small size, are made up of finely divided rock, which may +have been more or less cemented by the processes of change which +go on within it. It is thus clear that the lava flows are +unessential--indeed, we may say accidental--contributions to the mass. +In the case of Vesuvius they certainly do not amount to as much as one +tenth of the elevation due to the volcanic action. The share of the +lava in Vesuvius is probably greater than the average, for during the +last six centuries this vent has been remarkably lavigerous.[8] +Observation on the volcanoes of other districts show that the Vesuvian +group is in this regard not peculiar. Of nearly two hundred cones +which the writer has examined, not more than one tenth disclose +distinct lavas. + +[Footnote 8: I venture to use this word in place of the phrase +"lava-yielding" for the reason that the term is needed in the +description of volcanoes.] + +An inspection of the old inner wall of Monte Somma in that portion +where it is best preserved, on the north side of the Atria del +Cavallo, or Horse Gulch--so called for the reason that those who +ascended Vesuvius were accustomed to leave their saddle animals +there--we perceive that the body of the old cone is to a considerable +extent interlaced with dikes or fissures which have been filled with +molten lava that has cooled in its place. It is evident that during +the throes of an eruption, when the lava stands high in the crater, +these rents are frequently formed, to be filled by the fluid rock. In +fact, lava discharges, though they may afterward course for long +distances in the open air, generally break their way underground +through the cindery cone, and first are disclosed at the distance of a +mile or more from the inner walls of the crater. Their path is +probably formed by riftings in the compacted ashes, such as we trace +on the steep sides of the Atria del Cavallo, as before noted. For the +further history of these fissures, we shall have to refer to facts +which are better exhibited in the cone of AEtna. + +The amount of rock matter which has been thrown forth from the +volcanoes about the Bay of Naples is very great. Only a portion of it +remains in the region around these cones; by far the greater part has +been washed or blown away. After each considerable eruption a wide +field is coated with ashes, so that the tilled grounds appear as if +entirely sterilized; but in a short time the matter in good part +disappears, a portion of it decays and is leached away, and the most +of the remainder washes into the sea. Only the showers, which +accumulate a deep layer, are apt to be retained on the surface of the +country. A great deal of this powdered rock drifts away in the wind, +sometimes in great quantities, as in those cases where it darkened the +sky more than a thousand miles from the cone. Moreover, the water of +the steam which brought about the discharges and the other gases which +accompanied the vapour have left no traces of their presence, except +in the deep channels which the rain of the condensing steam have +formed on the hillsides. Nevertheless, after all these subtractions +are made, the quantity of volcanic matter remaining on the surface +about the Bay of Naples would, if evenly distributed, form a layer +several hundred feet in thickness--perhaps, indeed, a thousand feet in +depth--over the territory in which the vents occur. All this matter +has been taken in relatively recent times from the depths of the +earth. The surprising fact is that no considerable and, indeed, no +permanent subsidence of the surface has attended this excavation. We +can not believe that this withdrawal of material from the under-earth +has resulted in the formation of open underground spaces. We know full +well that any such, if it were of considerable size, would quickly be +crushed in by the weight of the overlying rocks. We have, indeed, to +suppose that these steam-impelled lavas, which are driven toward the +vent whence they are to go forth in the state of dust or fluid, come +underground from distances away, probably from beneath the floors of +the sea to the westward. + +Although the shores of the Bay of Naples have remained in general with +unchanged elevation for about two thousand years, they have here and +there been subjected to slight oscillations which are most likely +connected with the movement of volcanic matter toward the vents where +it is to find escape. The most interesting evidence of this nature is +afforded by the studies which have been made on the ruins of the +Temple of Serapis at Puzzuoli. This edifice was constructed in +pre-Christian times for the worship of the Egyptian god Serapis, whose +intervention was sought by sick people. The fact that this divinity of +the Nile found a residence in this region shows how intimate was the +relation between Rome and Egypt in this ancient day. The Serapeium was +built on the edge of the sea, just above its level. When in modern +days it began to be studied, its floor was about on its original +level, but the few standing columns of the edifice afford indubitable +evidence that this part of the shore has been lowered to the amount of +twenty feet or more and then re-elevated. The subsidence is proved by +the fact that the upper part of the columns which were not protected +by the _debris_ accumulated about them have been bored by certain +shellfish, known as _Lithodomi_, which have the habit of excavating +shelters in soft stone, such as these marble columns afford. At +present the floor on which the ruin stands appears to be gradually +sinking, though the rate of movement is very slow. + +Another evidence that the ejections may travel for a great distance +underground on their way to the vent is afforded by the fact that +Vesuvius and AEtna, though near three hundred miles apart, appear to +exchange activities--that is, their periods of outbreak are not +simultaneous. Although these elements of the chronology of the two +cones may be accidental, taken with similar facts derived from other +fields, they appear to indicate that vents, though far separated from +each other, may, so to speak, be fed from a common subterranean +source. It is a singular fact in this connection that the volcano of +Stromboli, though situated between these two cones, is in a state of +almost incessant activity. This probably indicates that the last-named +vent derives its vapours from another level in the earth than the +greater cones. In this regard volcanoes probably behave like springs, +of which, indeed, they may be regarded as a group. The reader is +doubtless aware that hot and cold springs often escape very near +together, the difference in the temperature being due to the depth +from which their waters come forth. + +As the accidents of volcanic explosion are of a nature to be very +damaging to man, as well as to the lower orders of Nature, it is fit +that we should note in general the effect of the Neapolitan eruptions +on the history of civilization in that region. As stated above, the +first Greek settlements in this vicinity--those on the island of +Ischia--were much disturbed by volcanic outbreaks, yet the island +became the seat of a permanent and prosperous colony. The great +eruption of 79 probably cost many hundred lives, and led to the +abandonment of two considerable cities, which, however, could at small +cost have been recovered to use. Since that day various eruptions have +temporarily desolated portions of the territory, but only in very +small fields have the ravages been irremediable. Where the ground was +covered with dust, it has in most places been again tillable, and so +rapid is the decay of the lavas that in a century after their flow has +ceased vines can in most cases be planted on their surfaces. The city +of Naples, which lies amid the vents, though not immediately in +contact with any of them, has steadfastly grown and prospered from the +pre-Christian times. It is doubtful if any lives have ever been lost +in the city in consequence of an eruption, and no great inconvenience +has been experienced from them. Now and then, after a great ash +shower, the volcanic dust has to be removed, but the labour is less +serious than that imposed on many northern cities by a snowstorm. +Through all these convulsions the tillage of the district has been +maintained. It has ever been the seat of as rich and profitable a +husbandry as is afforded by any part of Italy. In fact, the ash +showers, as they import fine divided rock very rich in substances +necessary for the growth of plants, have in a measure served to +maintain the fertility of the soil, and by this action have in some +degree compensated for the injury which they occasionally inflict. +Comparing the ravages of the eruptions with those inflicted by war, +unnecessary disease, or even bad politics, and we see that these +natural accidents have been most merciful to man. Many a tyrant has +caused more suffering and death than has been inflicted by these rude +operations of Nature. + +From the point of view of the naturalist, AEtna is vastly more +interesting than Vesuvius. The bulk of the cone is more than twenty +times as great as that of the Neapolitan volcano, and the magnitude of +its explosions, as well as the range of phenomena which they exhibit, +incomparably greater. It happens, however, that while human history of +the recorded kind has been intimately bound up with the tiny Vesuvian +cone, partly because the relatively slight nature of its disturbances +permitted men to dwell beside it, the larger AEtna has expelled culture +from the field near its vent, and has done the greater part of its +work in the vast solitude which it has created.[9] + +[Footnote 9: In part the excellent record of Vesuvius is due to the fact +that since the early Christian centuries the priests of St. Januarius, +the patron of Naples, have been accustomed to carry his relics in +procession whenever an eruption began. The cessation of the outbreak has +been written down to the credit of the saint, and thus we are provided +with a long story of the successive outbreaks.] + +AEtna has been in frequent eruption for a very much longer time than +Vesuvius. In the odes of Pindar, in the sixth century before Christ, +we find records of eruptions. It is said also that the philosopher +Empedocles sought fame and death by casting himself into the fiery +crater. There has thus in the case of this mountain been no such long +period of repose as occurred in Vesuvius. Though our records of the +outbreaks are exceedingly imperfect, they serve to show that the vent +has maintained its activity much more continuously than is ordinarily +the case with volcanoes. AEtna is characteristically a lava-yielding +cone; though the amount of dust put forth is large, the ratio of the +fluid rock which flows away from the crater is very much greater than +at Vesuvius. Nearly half the cone, indeed, may be composed of this +material. Our space does not permit anything like a consecutive story +of the AEtnean eruptions since the dawn of history, or even a full +account of its majestic cone; we can only note certain features of a +particularly instructive nature which have been remarked by the many +able men who have studied this structure and the effects of its +outbreak. + +The most important feature exhibited by AEtna is the vast size of its +cone. At its apex its height, though variable from the frequent +destruction and rebuilding of the crater walls, may be reckoned as +about eleven thousand feet. The base on which the volcanic material +lies is probably less than a thousand feet above the sea, so that the +maximum thickness of the heap of volcanic ejections is probably about +two miles. The average depth of this coating is probably about five +thousand feet, and, as the cone has an average diameter of about +thirty miles, we may conclude that the cone now contains about a +thousand cubic miles of volcanic materials. Great as is this mass, +it is only a small part of the ejected material which has gone forth +from the vent. All the matter which in its vaporous state went forth +with the eruption, the other gases and vapours thus discharged, have +disappeared. So, too, a large part of the ash and much of the lava has +been swept away by the streams which drain the region, and which in +times of eruption are greatly swollen by the accompanying torrential +rains. The writer has estimated that if all the emanations from the +volcano--solid, fluid, and gaseous--could be heaped on the cone, they +would form a mass of between two and three thousand cubic miles in +contents. Yet notwithstanding this enormous outputting of earthy +matter, the earth on which the AEtnean cone has been constructed has +not only failed to sink down, but has been in process of continuous, +slow uprising, which has lifted the surface more than a thousand feet +above the level which it had at the time when volcanic action began in +this field. Here, even more clearly than in the case of Vesuvius, we +see that the materials driven forth from the crater are derived not +from just beneath its foundation, but from a distance, from realms +which in the case of this insular volcano are beneath the sea floors. +It is certain that here the migration of rock matter, impelled by the +expansion of its contained water toward the vent, has so far exceeded +that which has been discharged through the crater that an uprising of +the surface such as we have observed has been brought about. + +[Illustration: _Mount AEtna, seen from near Catania. The imperfect +cones on the sky line to the left are those of small secondary +eruptions._] + +There are certain peculiarities of Mount AEtna which are due in part to +its great size and in part to the climatal conditions of the region in +which it lies. The upper part of the mountain in winter is deeply +snow-clad; the frozen water often, indeed, forms great drifts in the +gorges near the summit. Here it has occasionally happened that a layer +of ashes has deeply buried the mass, so that it has been preserved for +years, becoming gradually more inclosed by the subsequent eruptions. +At one point where this compact snow--which has, indeed, taken on the +form of ice--has been revealed to view, it has been quarried and +conveyed to the towns upon the seacoast. It is likely that there are +many such masses of ice inclosed between the ash layers in the upper +part of the mountain, where, owing to the height, the climate is very +cold. This curious fact shows how perfect a non-conductor the ash beds +of a volcano are to protect the frozen water from the heat of the +rocks about the crater. + +The furious rains which beset the mountain in times of great eruptions +excavate deep channels on its sides. The lava outbreaks which attend +almost every eruption, and which descend from the base of the cinder +cone at the height of from five to eight thousand feet above the sea, +naturally find their way into these channels, where they course in the +manner of rivers until the lower and less valleyed section of the cone +is reached. + +Such a lava flow naturally begins to freeze on the surface, the lava +at first becoming viscid, much in the manner of cream on the surface +of milk. Urged along by the more fluid lava underneath, this viscid +coating takes a ropy or corrugated form. As the freezing goes deeper, +a firm stone roof may be formed across the gorge, which, when the +current of lava ceases to flow from the crater, permits the lower part +of the stream to drain away, leaving a long cavern or scries of caves +extending far up the cone. The nature of this action is exactly +comparable to that which we may observe when on a frosty morning after +rain we may find the empty channels which were occupied by rills of +water roofed over with ice; the ice roofs are temporary, while those +of lava may endure for ages. Some of these lava-stream caves have been +disclosed, in the manner of ordinary caverns, by the falling of their +roofs; but the greater part are naturally hidden beneath the +ever-increasing materials of the cone. + +The lava-stream caves of AEtna are not only interesting because of +their peculiarities of form, which we shall not undertake to describe, +but also for the reason that they help us to account for a very +peculiar feature in the history of the great cone. On the slopes of +the volcano, below the upper cindery portion, there are several +hundred lesser cones, varying from a few score to seven hundred feet +in height. Each of these has its appropriate crater, and has evidently +been the seat of one or more eruptions. As the greater part of these +cones are ancient, many of them being almost effaced by the rain or +buried beneath the ejections which have surrounded their bases since +the time they were formed, we are led to believe that many thousands +of them have been formed during the history of the volcano. The +history of these subsidiary cones appears to be connected with the +lava caves noted above. These caverns, owing to the irregularities of +their form, contain water. They are, in fact, natural cisterns, where +the abundant rainfall of the mountain finds here and there storage. +When, during the throes of an eruption, dikes such as we know often to +penetrate the mountain, are riven outward from the crater through the +mass of the cone, and filled with lava, the heated rock must often +come in contact with these masses of buried water. The result of this +would inevitably be the local generation of steam at a high +temperature, which would force its way out in a brief but vigorous +eruption, such as has been observed to take place when these +peripheral volcanoes are formed. Sometimes it has happened that after +the explosion the lava has found its way in a stream from the fissure +thus opened. That this explanation is sufficient is in a measure shown +by observations on certain effects of lava flows from Vesuvius. The +writer was informed by a very judicious observer, a resident of +Naples, who had interested himself in the phenomena of that volcano, +that the lava streams when they penetrated a cistern, such as they +often encounter in passing over villages or farmsteads, vaporized the +water, and gave rise, through the action of the steam, to small +temporary cones, which, though generally washed away by the further +flow of the liquid rock, are essentially like those which we find on +AEtna. Such subsidiary, or, as they are sometimes called, parasitic +cones, are known about other volcanoes, but nowhere are they so +characteristic as on the flanks of that wonderful volcano. + +A very conspicuous feature in the AEtnean cone consists of a great +valley known as the Val del Bove, or Bull Hollow, which extends from +the base of the modern and ever-changeable cinder cone down the flanks +of the older structure to near its base. This valley has steep sides, +in places a thousand or more feet high, and has evidently been formed +by the down-settling of portions of the cone which were left without +support by the withdrawal from beneath them of materials cast forth in +a time of explosion. In an eruption this remarkable valley was the +seat of a vast water flood, the fluid being cast forth from the crater +at the beginning of the explosion. In the mouths of this and other +volcanoes, after a long period of repose, great quantities of water, +gathering from rains or condensed from the steam which slowly escapes +from these openings, often pours like a flood down the sides of the +mountains. In the great eruption of Galongoon, in Java, such a mass of +water, cast forth by a terrific explosion, mingled with ashes, so that +the mass formed a thick mud, was shot forth with such energy that it +ravaged an area nearly eighty miles in diameter, destroying the +forests and their wild inhabitants, as well as the people who dwelt +within the range of the amazing disaster. So powerfully was this water +driven from the crater that the districts immediately at the base of +the cone were in a manner overshot by the vast stream, and escaped +with relatively little injury. + +When it comes forth from the base of the cinder cone, or from one of +the small peripheral craters, the lava stream usually appears to be +white hot, and to flow with almost the ease of water. It does not +really have that measure of fluidity; its condition is rather that of +thin paste; but the great weight of the material--near two and a half +times that of water--causes the movement down the slope to be speedy. +The central portion of the lava stream long retains its high +temperature; but the surface, cooling, is first converted into a tough +sheet, which, though it may bend, can hardly be said to flow. Further +hardening converts these outlying portions of the current into hard, +glassy stone, which is broken into fragments in a way resembling the +ice on the surface of a river. It thus comes about that the advancing +front of the lava stream becomes covered, and its motion hindered by +the frozen rock, until the rate of ongoing may not exceed a few feet +an hour, and the appearance is that of a heap of stone slowly rolling +down a slope. Now and then a crevice is formed, through which a thin +stream of liquid lava pours forth, but the material, having already +parted with much of its heat, rapidly cools, and in turn becomes +covered with the coating of frozen fragments. In this state of the +stream the lava flow stands on all sides high above the slope which it +is traversing; it is, in fact, walled in by its own solidified parts, +though it is urged forward by the contribution which continues to flow +in the under arches. In this state of the movement trifling accidents, +or even human interference, may direct the current this way or that. + +Some of the most interesting chapters in the history of AEtna relate to +the efforts of the people to turn these slow-moving streams so that +their torrents might flow into wilderness places rather than over the +fields and towns. In the great flow of 1669, which menaced the city of +Catania, a large place on the seashore to the southeast of the cone, a +public-spirited citizen, Senor Papallardo, protecting himself and his +servants with clothing made of hides, and with large shields, set +forth armed with great hooks with the purpose of diverting the course +of the lava mass. He succeeded in pulling away the stones on the +flank of the stream, so that a flow of the molten rock was turned in +another direction. The expedient would probably have been successful +if he had been allowed to continue his labours; but the inhabitants of +a neighbouring village, which was threatened by the off-shooting +current which Papallardo had created, took up arms and drove him and +his retainers away. The flow continued until it reached Catania. The +people made haste to build the city walls on the side of danger higher +than it was before, but the tide mounted over its summit. + +Although the lavas which come forth from the volcano evidently have a +high temperature, their capacity for melting other rocks is relatively +small. They scour these rocks, because of their weight, even more +energetically than do powerful torrents of water, but they are +relatively ineffective in melting stone. On AEtna and elsewhere we may +often observe lavas which have flowed through forests. When the tide +of molten rock has passed by, the trees may be found charred but not +entirely burned away; even stems a few inches in diameter retain +strength enough to uphold considerable fringes and clots of the lava +which has clung to them. These facts bear out the conclusion that the +fluidity of the heated stone depends in considerable measure on the +water which is contained, either in its fluid or vaporous state, +between the particles of the material. + +If we consider the Italian volcanoes as a whole, we find that they lie +in a long, discontinuous line extending from the northern part of the +valley of the Po, within sight of the Alps, to AEtna, and in +subterranean cones perhaps to the northern coast of Africa. At the +northern end of the line we have a beautiful group of extinct +volcanoes, known as the Eugean Mountains. Thence southward to southern +Tuscany craters are wanting, but there is evidence of fissures in the +earth which give forth thermal waters. From southern Tuscany southward +through Rome to Naples there are many extinct craters, none of which +have been active in the historic period. From Naples southward the +cones of this system, about a dozen in number, are on islands or close +to the margin of the sea. It is a noteworthy fact that the greater +part of these shore or insular vents have been active since the dawn +of history; several of them frequently and furiously so, while none of +those occupying an inland position have been the seat of explosions. +This is a striking instance going to show the relation of these +processes to conditions which are brought about on the sea bottom. + +AEtna is, as we have noticed, a much more powerful volcano than +Vesuvius. Its outbreaks are more vigorous, its emanations vastly +greater in volume, and the mass of its constructions many times as +great as those accumulated in any other European cone. There are, +however, a number of volcanoes in the world which in certain features +surpass AEtna as much as that crater does Vesuvius. Of these we shall +consider but two--Skaptar Jokul, of Iceland, remarkable for the volume +of its lava flow, and Krakatoa, an island volcano between Java and +Sumatra, which was the seat of the greatest explosion of which we have +any record. + +The whole of Iceland may be regarded as a volcanic mass composed +mainly of lavas and ashes which have been thrown up by a group of +volcanoes lying near the northern end of the long igneous axis which +extends through the centre of the Atlantic. The island has been the +seat of numerous eruptions; in fact, since its settlement by the +Northmen in 1070 its sturdy inhabitants have been almost as much +distressed by the calamities which have come from the internal heat as +they have been by the enduring external cold. They have, indeed, been +between frost and fire. The greatest recorded eruption of Iceland +occurred in 1783, when the volcano of Skaptar, near the southern +border of the island, poured forth, first, a vast discharge of dust +and ashes, and afterward in the languid state of eruption inundated a +series of valleys with the greatest lava flow of which we have any +written record. The dust poured forth into the upper air, being finely +divided and in enormous quantity, floated in the air for months, +giving a dusky hue to the skies of Europe, which led the common people +and many of the learned to fear that the wrath of God was upon them, +and that the day of judgment was at hand. Even the poet Cowper, a man +of high culture and education, shared in this unreasonable view. + +The lava flow in this eruption filled one of the considerable valleys +of the island, drying up the river, and inundating the plains on +either side. Estimates which have been made as to the volume of this +flow appear to indicate that it may have amounted to more than the +bulk of the Mont Blanc. + +This great eruption, by the direct effect of the calamity, and by the +famine due to the ravaging of the fields and the frightening of the +fish from the shores which it induced, destroyed nearly one fifth of +the Icelandic people. It is, in fact, to be remembered as one of the +three or four most calamitous eruptions of which we have any account, +and, from the point of view of lava flow, the greatest in history. + +Just a hundred years after the great Skaptar eruption, which darkened +the skies of Europe, the island of Krakatoa, an isle formed by a small +volcano in the straits of Java, was the seat of a vapour explosion +which from its intensity is not only unparalleled, but almost +unapproached in all accounts of such disturbances. Krakatoa had long +been recognised as a volcanic isle; it is doubtful, however, if it had +ever been seen in eruption during the three centuries or more since +European ships began to sail by it until the month of May of the year +above mentioned. Then an outbreak of what may be called ordinary +violence took place, which after a few days so far ceased that +observers landed and took account of the changes which the convulsion +had brought about. For about three months there were no further signs +of activity, but on the 29th of August a succession of vast explosions +took place, which blew away a great part of the island, forming in its +place a submarine crater two or three miles in diameter, creating +world-wide disturbances of sea and air. The sounds of the outbreak +were heard at a distance of sixteen hundred miles away. The waves of +the air attendant on the explosion ran round the earth at least once, +as was distinctly indicated by the self-recording barometers; it is +possible, indeed, that, crossing each other in their east and west +courses, these atmospheric tides twice girdled the sphere. In effect, +the air over the crater was heaved up to the height of some tens of +thousands of feet, and thence rolled off in great circular waves, such +as may be observed in a pan of milk when a sharp blow pushes the +bottom upward. + +The violent stroke delivered to the waters of the sea created a vast +wave, which in the region where it originated rolled upon the shores +with a surf wall fifty or more feet high. In a few minutes about +thirty thousand people were overwhelmed. The wave rolled on beyond its +destructive limits much in the manner of the tide; its influence was +felt in a sharp rise and fall of the waters as far as the Pacific +coast of North America, and was indicated by the tide gauges in the +Atlantic as far north as the coast of Europe. + +Owing to the violence of the eruption, Krakatoa poured forth no lava, +but the dust and ashes which ascended into the air--or, in +other words, the finely divided lava which escaped into the +atmosphere--probably amounted in bulk to more than twenty cubic miles. +The coarser part of this material, including much pumice, fell upon +the seas in the vicinity, where, owing to its lightness, it was free +to drift in the marine currents far and wide throughout the oceanic +realm. The finer particles, thrown high into the air, perhaps to the +height of nearly a hundred thousand feet--certainly to the elevation +of more than half this amount--drifted far and wide in the +atmosphere, so that for years the air of all regions was clouded by +it, the sunrise and sunset having a peculiar red glow, which the dust +particles produce by the light which they reflect. In this period, at +all times when the day was clear, the sun appeared to be surrounded by +a dusky halo. In time the greater part of this dust was drawn down by +gravity, some portion of it probably falling on every square foot of +the earth. Since the disappearance of the characteristic phenomena +which it produced in the atmosphere, European observers have noted the +existence of faint clouds lying in the upper part of the air at the +height of a hundred miles or more above the surface. These clouds, +which were at first distinctly visible in the earliest stage of dawn +and in the latest period of the sunset glow, seemed to be in rapid +motion to the eastward, and to be mounting higher above the earth. It +has been not unreasonably supposed that these shining clouds represent +portions of the finest dust from Krakatoa, which has been thrown so +far above the earth's attraction that it is separating itself from the +sphere. If this view be correct, it seems likely that we may look to +great volcanic explosions as a source whence the dustlike particles +which people the celestial spaces may have come. They may, in a word, +be due to volcanic explosions occurring on this and other celestial +spheres. + +The question suggested above as to the possibility of volcanic +ejections throwing matter from the earth beyond the control of its +gravitative energy is one of great scientific interest. Computations +(not altogether trustworthy) show that a body leaving the earth's +surface under the conditions of a cannon ball fired vertically upward +would have to possess a velocity at the start of at least seven miles +a second in order to go free into space. It would at first sight seem +that we should be able to reckon whether volcanoes can propel earth +matter upward with this speed. In fact, however, sufficient data are +not obtainable; we only know in a general way that the column of +vapour rises to the height of thirty or forty thousand feet, and this +in eruptions of no great magnitude. In an accident such as that at +Krakatoa, even if an observer were near enough to see clearly what was +going on, the chance of his surviving the disturbance would be small. +Moreover, the ascending vapours, owing to their expansion of the steam +in the column, begin to fly out sideways on its periphery, so that the +upper part of the central section in the discharge is not visible from +the earth. + +It is in the central section of the uprushing mass, if anywhere, that +the dust might attain the height necessary to put it beyond the +earth's attraction, bringing it fairly into the realm of the solar +system, or to the position where its own motion and the attraction of +the other spheres would give it an independent orbital movement about +the sun, or perhaps about the earth. We can only say that observations +on the height of volcanic ejections are extremely desirable; they can +probably only be made from a balloon. An ascension thus made beyond +the cloud disk which the eruption produces might bring the observer +where he could discern enough to determine the matter. Although the +movements of the rocky particles could not be observed, the colour +which they would give to the heavens might tell the story which we +wish to know. There is evidence that large masses of stone hurled up +by volcanic eruption have fallen seven miles from the base of the +cone. Assuming that the masses went straight upward at the beginning +of their ascent, and that they were afterward borne outwardly by the +expansion of the column, computations which have a general but no +absolute value appear to indicate that the masses attained a height of +from thirty to fifty miles, and had an initial velocity which, if +doubled, might have carried them into space. + +Last of all, we shall note the conditions which attend the eruptions +of submarine volcanoes. Such explosions have been observed in but a +few instances, and only in those cases where there is reason to +believe that the crater at the time of its explosion had attained to +within a few hundred feet of the sea level. In these cases the +ejections, never as yet observed in the state of lava, but in the +condition of dust and pumice, have occasionally formed a low island, +which has shortly been washed away by the waves. Knowing as we do that +volcanoes abound on the sea floor, the question why we do not oftener +see their explosions disturbing the surface of the waters is very +interesting, but not as yet clearly explicable. It is possible, +however, that a volcanic discharge taking place at the depth of +several thousand feet below the surface of the water would not be able +to blow the fluid aside so as to open a pipe to the surface, but would +expend its energy in a hidden manner near the ocean floor. The vapours +would have to expand gradually, as they do in passing up through the +rock pipe of a volcano, and in their slow upward passage might be +absorbed by the water. The solid materials thrown forth would in this +case necessarily fall close about the vent, and create a very steep +cone, such, indeed, as we find indicated by the soundings off certain +volcanic islands which appear only recently to have overtopped the +level of the waters. + +As will be seen, though inadequately from the diagrams of Vesuvius, +volcanic cones have a regularity and symmetry of form far exceeding +that afforded by the outlines of any other of the earth's features. +Where, as is generally the case, the shape of the cone is determined +by the distribution of the falling cinders or divided lava which +constitutes the mass of most cones, the slope is in general that known +as a catenary curve--i.e., the line formed by a chain hanging between +two points at some distance from the vertical. It is interesting to +note that this graceful outline is a reflection or consequence of the +curve described by the uprushing vapour. The expansion in the +ascending column causes it to enlarge at a somewhat steadfast rate, +while the speed of the ascent is ever diminishing. Precisely the same +action can be seen in the like rush of steam and other gases and +vapours from the cannon's mouth; only in the case of the gun, even of +the greatest size, we can not trace the movement for more than a few +hundred feet. In this column of ejection the outward movement from the +centre carries the bits of lava outwardly from the centre of the +shaft, so that when they lose their ascending velocity they are drawn +downward upon the flanks of the cone, the amount falling upon each +part of that surface being in a general way proportional to the +thickness of the vaporous mass from which they descend. The result is, +that the thickest part of the ash heap is formed on the upper part of +the crater, from which point the deposit fades away in depth in every +direction. In a certain measure the concentration toward the centre of +the cone is brought about by the draught of air which moves in toward +the ascending column. + +Although, in general, ejections of volcanic matter take place through +cones, that being the inevitable form produced by the escaping steam, +very extensive outpourings of lava, ejections which in mass probably +far exceed those thrown forth through ordinary craters, are +occasionally poured out through fissures in the earth's crust. Thus in +Oregon, Idaho, and Washington, in eastern Europe, in southern India, +and at some other points, vast flows, which apparently took place from +fissures, have inundated great realms with lava ejections. The +conditions which appear to bring about these fissure eruptions of lava +are not yet well understood. A provisional and very probable account +of the action can be had in the hypothesis which will now be set +forth. + +Where any region has been for a long time the seat of volcanic action, +it is probable that a large amount of rock in a more or less fluid +condition exists beneath its surface. Although the outrushing steam +ejects much of this molten material, there are reasons to suppose that +a yet greater part lies dormant in the underground spaces. Thus in the +case of AEtna we have seen that, though some thousands of miles of +rock matter have come forth, the base of the cone has been uplifted, +probably by the moving to that region of more or less fluid rock. If +now a region thus underlaid by what we may call incipient lavas is +subjected to the peculiar compressive actions which lead to +mountain-building, we should naturally expect that such soft material +would be poured forth, possibly in vast quantities through fault +fissures, which are so readily formed in all kinds of rock when +subject to irregular and powerful strains, such as are necessarily +brought about when rocks are moved in mountain-making. The great +eruptions which formed the volcanic table-lands on the west coast of +North America appear to have owed the extrusion of their materials to +mountain-building actions. This seems to have been the case also in +some of those smaller areas where fissure flows occur in Europe. It is +likely that this action will explain the greater part of these massive +eruptions. + +It need not be supposed that the rock beneath these countries, which +when forced out became lava, was necessarily in the state of perfect +fluidity before it was forced through the fissures. Situated at great +depth in the earth, it was under a pressure so great that its +particles may have been so brought together that the material was +essentially solid, though free to move under the great strains which +affected it, and acquiring temperature along with the fluidity which +heat induces as it was forced along by the mountain-building pressure. +As an illustration of how materials may become highly heated when +forced to move particle on particle, it may be well to cite the case +in which the iron stringpiece on top of a wooden dam near Holyoke, +Mass., was affected when the barrier went away in a flood. The iron +stringer, being very well put together, was, it is said, drawn out by +the strain until it became sensibly reddened by the motion of its +particles, and finally fell hissing into the waters below. A like +heating is observable when metal is drawn out in making wire. Thus a +mass of imperfectly fluid rock might in a forced journey of a few +miles acquire a decided increase of temperature. + +Although the most striking volcanic action--all such phenomena, +indeed, as commonly receives the name--is exhibited finally on the +earth's surface, a great deal of work which belongs in the same group +of geological actions is altogether confined to the deep-lying rock, +and leads to the formation of dikes which penetrate the strata, but do +not rise to the open air. We have already noted the fact that dikes +abound in the deeper parts of volcanic cones, though the fissures into +which they find their way are seldom riven up to the surface. In the +same way beneath the ground in non-volcanic countries we may discover +at a great depth in the older, much-changed rock a vast number of +these crevices, varying from a few inches to a hundred feet or more in +width, which have been filled with lavas, the rock once molten having +afterward cooled. In most cases these dikes are disclosed to us +through the down-wearing of the earth that has removed the beds into +which the dikes did not penetrate, thus disclosing the realm in which +the disturbances took place. + +Where, as is occasionally the case in deep mines, or on some bare +rocky cliff of great height, we can trace a dike in its upward course +through a long distance, we find that we can never distinctly discover +the lower point of its extension. No one has ever seen in a clear way +the point of origin of such an injection. We can, however, often +follow it upward to the place where there was no longer a rift into +which it could enter. In its upward path the molten matter appears +generally to have followed some previously existing fracture, a joint +plane or a fault, which generally runs through the rocks on those +planes. We can observe evidence that the material was in the state of +igneous fluidity by the fact that it has baked the country rocks on +either side of the fissure, the amount of baking being in proportion +to the width of the dike, and thus to the amount of heat which it +could give forth. A dike six inches in diameter will sometimes barely +sear its walls, while one a hundred feet in width will often alter the +strata for a great distance on either side. In some instances, as in +the coal beds near Richmond, Va., dikes occasionally cut through beds +of bituminous coal. In these cases we find that the coal has been +converted into coke for many feet either side of a considerable +injection. The fact that the dike material was molten is still further +shown by the occurrence in it of fragments which it has taken up from +the walls, and which may have been partly melted, and in most cases +have clearly been much heated. + +Where dikes extend up through stratified beds which are separated from +each other by distinct layers, along which the rock is not firmly +bound together, it now and then happens, as noted by Mr. G.K. Gilbert, +of the United States Geological Survey, that the lava has forced its +way horizontally between these layers, gradually uplifting the +overlying mass, which it did not break through, into a dome-shaped +elevation. These side flows from dikes are termed laccolites, a word +which signifies the pool-like nature of the stony mass which they form +between the strata. + +In many regions, where the earth has worn down so as to reveal the +zone of dikes which was formed at a great depth, the surface of the +country is fairly laced with these intrusions. Thus on Cape Ann, a +rocky isle on the east coast of Massachusetts, having an area of about +twenty square miles, the writer, with the assistance of his colleague, +Prof. R.S. Tarr, found about four hundred distinct dikes exhibited on +the shore line where the rocks had been swept bare by the waves. If +the census of these intrusions could have been extended over the whole +island, it would probably have appeared that the total number exceeded +five thousand. In other regions square miles can be found where the +dikes intercepted by the surface occupy an aggregate area greater than +that of the rocks into which they have been intruded. + +Now and then, but rarely, the student of dikes finds one where the +bordering walls, in place of having the clean-cut appearance which +they usually exhibit, has its sides greatly worn away and much melted, +as if by the long-continued passage of the igneous fluid through the +crevice. Such dikes are usually very wide, and are probably the paths +through which lavas found their way to the surface of the earth, +pouring forth in a volcanic eruption. In some cases we can trace their +relation to ancient volcanic cones which have worn down in all their +part which were made up of incoherent materials, so that there remains +only the central pipe, which has been preserved from decay by the +coherent character of the lava which filled it. + +The hypothesis that dikes are driven upward into strata by the +pressure of the beds which overlie materials hot and soft enough to be +put in motion when a fissure enters them, and that their movement +upward through the crevice is accounted for by this pressure, makes +certain features of these intrusions comprehensible. Seeing that very +long, slender dikes are found penetrating the rock, which could not +have had a high temperature, it becomes difficult to understand how +the lava could have maintained its fluidity; but on the supposition +that it was impelled forward by a strong pressure, and that the energy +thus transmitted through it was converted into heat, we discover a +means whereby it could have been retained in the liquid condition, +even when forced for long distances through very narrow channels. +Moreover, this explanation accounts for the fact which has long +remained unexplained that dikes, except those formed about volcanic +craters, rarely, if ever, rise to the surface. + +The materials contained in dikes differ exceedingly in their chemical +and mineral character. These variations are due to the differences in +Nature of the deposits whence they come, and also in a measure to +exchanges which take place between their own substance and that of the +rocks between which they are deposited. This process often has +importance of an economic kind, for it not infrequently leads to the +formation of metalliferous veins or other aggregations of ores, either +in the dike itself or in the country rock. The way in which this is +brought about may be easily understood by a familiar example. If flesh +be placed in water which has the same temperature, no exchange of +materials will take place; but if the water be heated, a circulation +will be set up, which in time will bring a large part of the soluble +matter into the surrounding water. This movement is primarily +dependent on differences of temperature, and consequently differences +in the quantity of soluble substances which the water seeks to take +up. When a dike is injected into cooler rocks, such a slow circulation +is induced. The water contained in the interstices of the stone +becomes charged with mineral materials, if such exist in positions +where it can obtain possession of them, and as cooling goes on, these +dissolved materials are deposited in the manner of veins. These veins +are generally laid down on the planes of contact between the two kinds +of stone, but they may be formed in any other cavities which exist in +the neighbourhood. The formation of such veins is often aided by the +considerable shrinkage of the lava in the dike, which, when it cools, +tends to lose about fifteen per cent of its volume, and is thus likely +to leave a crevice next the boundary walls. Ores thus formed afford +some of the commonest and often the richest mineral deposits. At +Leadville, in Colorado, the great silver-bearing lodes probably were +produced in this manner, wherein lavas, either those of dikes or those +which flowed in the open air, have come in contact with limestones. +The mineral materials originally in the once molten rock or in the +limy beds was, we believe, laid down on ancient sea floors in the +remains of organic forms, which for their particular uses took the +materials from the old sea water. The vein-making action has served to +assemble these scattered bits of metal into the aggregation which +constitutes a workable deposit. In time, as the rocks wear down, the +materials of the veins are again taken into solution and returned to +the sea, thence perhaps to tread again the cycle of change. + +In certain dikes, and sometimes also, perhaps, in lavas known as +basalts, which have flowed on the surface, the rock when cooling, from +the shrinkage which then occurs, has broken in a very regular way, +forming hexagonal columns which are more or less divided on their +length by joints. When worn away by the agencies of decay, especially +where the material forms steep cliffs, a highly artificial effect is +produced, which is often compared, where cut at right angles to the +columns, to pavements, or, where the division is parallel to the +columns, to the pipes of an organ. + +What we know of dikes inclines us to the opinion that as a whole they +represent movements of softened rock where the motion-compelling agent +is not mainly the expansion of the contained water which gives rise to +volcanic ejection, but rather in large part due to the weight of +superincumbent strata setting in motion materials which were somewhat +softened, and which tended to creep, as do the clays in deep coal +mines. It is evident, however; it is, moreover, quite natural, that +dike work is somewhat mingled with that produced by the volcanic +forces; but while the line between the two actions is not sharp, the +discrimination is important, and occurs with a distinctness rather +unusual on the boundary line between two adjacent fields of phenomena. + + * * * * * + +We have now to consider the general effects of the earth's interior +heat so far as that body of temperature tends to drive materials from +the depths of the earth to the surface. This group of influences is +one of the most important which operates on our sphere; as we shall +shortly see, without such action the earth would in time become an +unfit theatre for the development of organic life. To perceive the +effect of these movements, we must first note that in the great +rock-constructing realm of the seas organic life is constantly +extracting from the water substances, such as lime, potash, soda, and +a host of other substances necessary for the maintenance of +high-grade organisms, depositing these materials in the growing +strata. Into these beds, which are buried as fast as they form, goes +not only these earthy materials, but a great store of the sea water as +well. The result would be in course of time a complete withdrawal into +the depths of the earth of those substances which play a necessary +part in organic development. The earth would become more or less +completely waterless on its surface, and the rocks exposed to view +would be composed mainly of silica, the material which to a great +extent resists solution, and therefore avoids the dissolving which +overtakes most other kinds of rocks. Here comes in the machinery of +the hot springs, the dikes, and the volcanoes. These agents, operating +under the influence of the internal heat of the earth, are constantly +engaged in bearing the earthy matter, particularly its precious more +solvent parts, back to the surface. The hot springs and volcanoes work +swiftly and directly, and return the water, the carbon dioxide, and a +host of other vaporizable and soluble and fusible substances to the +realm of solar activity, to the living surface zone of the earth. The +dikes operate less immediately, but in the end to the same effect. +They lift their materials miles above the level where they were +originally laid, probably from a zone which is rarely if ever exposed +to view, placing them near the surface, where the erosive agents can +readily find access to them. + +Of the three agents which serve to export earth materials from its +depths, volcanoes are doubtless the most important. They send forth +the greater part of the water which is expelled from the rocks. +Various computations which the writer has made indicate that an +ordinary volcano, such as AEtna, in times of most intense explosion, +may send forth in the form of steam one fourth of a cubic mile or +more of water during each day of its discharge, and in a single great +eruption may pour forth several times this quantity. In its history +AEtna has probably returned to the atmosphere some hundred cubic miles +of water which but for the process would have remained permanently +locked up in its rock prison. + +The ejection of rock material, though probably on the average less in +quantity than the water which escapes, is also of noteworthy +importance. The volcanoes of Java and the adjacent isles have, during +the last hundred and twenty years, delivered to the seas more earth +material than has been carried into those basins by the great rivers. +If we could take account of all the volcanic ejections which have +occurred in this time, we should doubtless find that the sum of the +materials thus cast forth into the oceans was several times as great +as that which was delivered from the lands by all the superficial +agents which wear them away. Moreover, while the material from the +land, except the small part which is in a state of complete solution, +all falls close to the shore, the volcanic waste, because of its fine +division or because of the blebs of air which its masses contain, may +float for many years before it finds its way to the bottom, it may be +at the antipodes of the point at which it came from the earth. While +thus journeying through the sea the rock matter from the volcanoes is +apt to become dissolved in water; it is, indeed, doubtful if any +considerable part of that which enters the ocean goes by gravitation +to its floor. The greater portion probably enters the state of +solution and makes its way thence through the bodies of plants and +animals again into the ponderable state. + +If an observer could view the earth from the surface of the moon, he +would probably each day behold one of these storms which the volcanoes +send forth. In the fortnight of darkness, even with the naked eye, it +would probably be possible to discern at any time several eruptions, +some of which would indicate that the earth's surface was ravaged by +great catastrophes. The nearer view of these actions shows us that +although locally and in small measure they are harmful to the life of +the earth, they are in a large way beneficent. + + + + + CHAPTER VIII. + + THE SOIL. + + +The frequent mention which it has been necessary to make of soil +phenomena in the preceding chapters shows how intimately this feature +in the structure of the earth is blended with all the elements of its +physical history. It is now necessary for us to take up the phenomena +of soils in a consecutive manner. + +The study of any considerable river basin enables us to trace the more +important steps which lead to the destructure and renovation of the +earth's detrital coating. In such an interpretation we note that +everywhere the rocks which were built on the sea bottom, and more or +less made over in the great laboratory of the earth's interior, are at +the surface, when exposed to the conditions of the atmosphere, in +process of being taken to pieces and returned to the sea. This action +goes on everywhere; every drop of rain helps it. It is aided by frost, +or even by the changes of expansion and contraction which occur in the +rocks from variations of heat. The result is that, except where the +slopes are steep, the surface is quickly covered with a layer of +fragments, all of which are in the process of decay, and ready to +afford some food to plants. Even where the rock appears bare, it is +generally covered with lichens, which, adhering to it, obtain a share +of nutriment from the decayed material which they help to hold on the +slope. When they have retained a thin sheet of the _debris_, mosses +and small flowering plants help the work of retaining the detritus. +Soon the strong-rooted bushes and trees win a foothold, and by sending +their rootlets, which are at first small but rapidly enlarge, into the +crevices, they hasten the disruption of the stones. + +If the construction of soil goes on upon a steep cliff, the quantity +retained on the slope may be small, but at the base we find a talus, +composed of the fragments not held by the vegetation, which gradually +increases as the cliff wears down, until the original precipice may be +quite obliterated beneath a soil slope. At first this process is +rapid; it becomes gradually slower and slower as the talus mounts up +the cliff and as the cliff loses its steepness, until finally a gentle +slope takes the place of the steep. + +From the highest points in any river valley to the sea level the +broken-up rock, which we term soil, is in process of continuous +motion. Everywhere the rain water, flowing over the surface or soaking +through the porous mass, is conveying portions of the material which +is taken into solution in a speedy manner to the sea. Everywhere the +expansion of the soil in freezing, or the movements imposed on it by +the growth of roots, by the overturning of trees, or by the +innumerable borings and burrowings which animals make in the mass, is +through the action of gravitation slowly working down the slope. Every +little disturbance of the grains or fragments of the soil which lifts +them up causes them when they fall to descend a little way farther +toward the sea level. Working toward the streams, the materials of the +soil are in time delivered to those flowing waters, and by them urged +speedily, though in most cases interruptedly, toward the ocean. + +There is another element in the movement of the soils which, though +less appreciable, is still of great importance. The agents of decay +which produce and remove the detritus, the chemical changes of the bed +rock, and the mechanical action which roots apply to them, along with +the solutional processes, are constantly lowering the surface of the +mass. In this way we can often prove that a soil continuously +existing has worked downward through many thousand feet of strata. In +this process of downgoing the country on which the layer rests may +have greatly changed its form, but the deposit, under favourable +conditions, may continue to retain some trace of the materials which +it derived from beds which have long since disappeared, their position +having been far up in the spaces now occupied by the air. Where the +slopes are steep and streams abound, we rarely find detritus which +belonged in rock more than a hundred feet above the present surface of +the soil. Where, however, as on those isolated table-lands or buttes +which abound in certain portions of the Mississippi Valley, as well as +in many other countries, we find a patch of soil lying on a nearly +level surface, which for geologic ages has not felt the effect of +streams, we may discover, commingled in the _debris_, the harder +wreckage derived from the decay of a thousand feet or more of vanished +strata. + +When we consider the effect of organic life on the processes which go +on in the soil, we first note the large fact that the development of +all land vegetation depends upon the existence of this detritus--in a +word, on the slow movement of the decaying rocky matter from the point +where it is disrupted to its field of rest in the depths of the sea. +The plants take their food from the portion of this rocky waste which +is brought into solution by the waters which penetrate the mass. On +the plants the animals feed, and so this vast assemblage of organisms +is maintained. Not only does the land life maintain itself on the +soil, and give much to the sea, but it serves in various ways to +protect this detrital coating from too rapid destruction, and to +improve its quality. To see the nature of this work we should visit a +region where primeval forests still lie upon the slopes of a hilly +region. In the body of such a wood we find next the surface a coating +of decayed vegetable matter, made up of the falling leaves, bark, +branches, and trunks which are constantly descending to the earth. +Ordinarily, this layer is a foot or more in thickness; at the top it +is almost altogether composed of vegetable matter; at the bottom it +verges into the true soil. An important effect of this decayed +vegetation is to restrain the movement of the surface water. Even in +the heaviest rains, provided the mass be not frozen, the water is +taken into it and delivered in the manner of springs to the larger +streams. We can better note the measure of this effect by observing +the difference in the ground covered by this primeval forest and that +which we find near by which has been converted into tilled fields. +With the same degree of rapidity in the flow, the distinct stream +channels on the tilled ground are likely to be from twenty to a +hundred times in length what they are on the forest bed. The result is +that while the brook which drains the forested area maintains a +tolerably constant flow of clean water, the other from the tilled +ground courses only in times of heavy rain, and then is heavily +charged with mud. In the virgin conditions of the soil the downwear is +very slow; in its artificial state this wearing goes on so rapidly +that the sloping fields are likely to be worn to below the soil level +in a few score years. + +Not only does the natural coating of vegetation, such as our forests +impose upon the country, protect the soil from washing away, but the +roots of the larger plants are continually at work in various ways to +increase the fertility and depth of the stratum. In the form of +slender fibrils these underground branches enter the joints and bed +planes of the rock, and there growing they disrupt the materials, +giving them a larger surface on which decay may operate. These bits, +at first of considerable size, are in turn broken up by the same +action. Where the underlying rocks afford nutritious materials, the +branches of our tap-rooted trees sometimes find their way ten feet or +more below the base of the true soil. Not only do they thus break up +the stones, but the nutrition which they obtain in the depths is +brought up and deposited in the parts above the ground, as well as in +the roots which lie in the true soil, so that when the tree dies it +becomes available for other plants. Thus in the forest condition of a +country the amount of rock material contributed to the deposit in +general so far exceeds that which is taken away to the rivers by the +underground water as to insure the deepening of the soil bed to the +point where only the strongest roots--those belonging to our +tap-rooted trees--can penetrate through it to the bed rocks. + +Almost all forests are from time to time visited by winds which uproot +the trees. When they are thus rent from the earth, the underground +branches often form a disk containing a thick tangle of stones and +earth, and having a diameter of ten or fifteen feet. The writer has +frequently observed a hundred cubic feet of soil matter, some of it +taken from the depth of a yard or more, thus uplifted into the air. In +the path of a hurricane or tornado we may sometimes find thousands of +acres which have been subjected to this rude overturning--a natural +ploughing. As the roots rot away, the _debris_ which they held falls +outside of the pit, thus forming a little hillock along the side of +the cavity. After a time the thrusting action of other roots and the +slow motion of the soil down the slope restore the surface from its +hillocky character to its original smoothness; but in many cases the +naturalist who has learned to discern with his feet may note these +irregularities long after it has been recovered with the forest. + +Great as is the effect of plants on the soil, that influence is almost +equalled by the action of the animals which have the habit of entering +the earth, finding there a temporary abiding place. The number of +these ground forms is surprisingly great. It includes, indeed, a host +of creatures which are efficient agents in enriching the earth. The +species of earthworms, some of which occupy forested districts as well +as the fields, have the habit of passing the soil material through +their bodies, extracting from the mass such nutriment as it may +contain. In this manner the particles of mineral matter become +pulverized, and in a measure affected by chemical changes in the +bodies of the creatures, and are thus better fitted to afford plant +food. Sometimes the amount of the earth which the creatures take in in +moving through their burrows and void upon the surface is sufficient +to form annually a layer on the surface of the ground having a depth +of one twentieth of an inch or more. It thus may well happen that the +soil to the depth of two or three feet is completely overturned in the +course of a few hundred years. As the particles which the creatures +devour are rather small, the tendency is to accumulate the finer +portions of the soil near the surface of the earth, where by solution +they may contribute to the needs of the lowly plants. It is probably +due to the action of these creatures that small relics of ancient men, +such as stone tools, are commonly found buried at a considerable depth +beneath the earth, and rarely appear upon the surface except where it +has been subjected to deep ploughing or to the action of running +streams. + +Along with the earthworms, the ants labour to overturn the soil; +frequently they are the more effective of the two agents. The common +species, though they make no permanent hillocks, have been observed by +the writer to lay upon the surface each year as much as a quarter of +an inch of sand and other fine materials which they have brought up +from a considerable depth. In many regions, particularly in those +occupied by glacial drift, and pebbly alluvium along the rivers, the +effect of this action, like that of earthworms, is to bring to the +surface the finer materials, leaving the coarser pebbles in the +depths. In this way they have changed the superficial character of the +soil over great areas; we may say, indeed, over a large part of the +earth, and this in a way which fits it better to serve the needs of +the wild plants as well as the uses of the farmer. + +Many thousand species of insects, particularly the larger beetles, +have the habit of passing their larval state in the under earth. Here +they generally excavate burrows, and thus in a way delve the soil. As +many of them die before reaching maturity, their store of organic +matter is contributed to the mass, and serves to nourish the plants. +If the student will carefully examine a section of the earth either in +its natural or in its tilled state, he will be surprised to find how +numerous the grubs are. They may often be found to the number of a +score or more of each cubic foot of material. Many of the species +which develop underground come from eggs which have carefully been +encased in organic matter before their deposition in the earth. Thus +some of the carrion beetles are in the habit of laying their eggs in +the bodies of dead birds or field mice, which they then bury to the +depth of some inches in the earth. In this way nearly all the small +birds and mammals of our woods disappear from view in a few hours +after they are dead. Other species make balls from the dung of cattle +in which they lay their eggs, afterward rolling the little spheres, it +may be for hundreds of feet, to the chambers in the soil which they +have previously prepared. In this way a great deal of animal matter is +introduced into the earth, and contributes to its fertility. + +Many of our small mammals have the habit of making their dwelling +places in the soil. Some of them, such as the moles, normally abide in +the subterranean realm for all their lives. Others use the excavations +as places of retreat. In any case, these excavations serve to move the +particles of the soil about, and the materials which the animals drag +into the earth, as well as the excrement of the creatures, act to +enrich it. This habit of taking food underground is not limited to the +mammals; it is common with the ants, and even the earthworms, as noted +by Charles Darwin in his wonderful essay on these creatures, are +accustomed to drag into their burrows bits of grass and the slender +leaves of pines. It is not known what purpose they attain by these +actions, but it is sufficiently common somewhat to affect the +conditions of the soil. + +The result of these complicated works done by animals and plants on +the soil is that the material to a considerable depth are constantly +being supplied with organic matter, which, along with the mineral +material, constitutes that part of the earth which can support +vegetation. Experiment will readily show that neither crushed rock nor +pure vegetable mould will of itself serve to maintain any but the +lowliest vegetation. It requires that the two materials be mixed in +order that the earth may yield food for ordinary plants, particularly +for those which are of use to man, as crops. On this account all the +processes above noted whereby the waste of plant and animal life is +carried below the surface are of the utmost importance in the creation +and preservation of the soil. It has been found, indeed, in almost all +cases, necessary for the farmer to maintain the fertility of his +fields to plough-in quantities of such organic waste. By so doing he +imitates the work which is effected in virgin soil by natural action. +As the process is costly in time and material, it is often neglected +or imperfectly done, with the result that the fields rapidly diminish +in fertility. + +The way in which the buried organic matter acts upon the soil is not +yet thoroughly understood. In part it accomplishes the results by the +materials which on its decay it contributes to the soil in a state in +which they may readily be dissolved and taken up by the roots into +their sap; in part, however, it is believed that they better the +conditions by affording dwelling places for a host of lowly species, +such as the forms which are known as bacteria. The organisms probably +aid in the decomposition of the mineral matter, and in the conversion +of nitrogen, which abounds in the air or the soil, into nitrates of +potash and soda--substances which have a very great value as +fertilizers. Some effect is produced by the decay of the foreign +matter brought into the soil, which as it passes away leaves channels +through which the soil water can more readily pass. + +By far the most general and important effect arising from the decay +of organic matter in the earth is to be found in the carbon dioxide +which is formed as the oxygen of the air combines with the carbon +which all organic material contains. As before noted, water thus +charged has its capacity for taking other substances into solution +vastly increased, and on this solvent action depends in large part the +decay of the bed rocks and the solution of materials which are to be +appropriated by the plants. + +Having now sketched the general conditions which lead to the formation +of soils, we must take account of certain important variations in +their conditions due to differences in the ways in which they are +formed and preserved. These matters are not only of interest to the +geologist, but are of the utmost importance to the life of mankind, as +well as all the lower creatures which dwell upon the lands. First, we +should note that soils are divisible into three great groups, which, +though not sharply parted from each other, are sufficiently peculiar +for the purposes of classification. Where the earth material has been +derived from the rocks which nearly or immediately underlie it, we +have a group of soils which may be entitled those of immediate +derivation--that is, derived from rocks near by, or from beds which +once overlaid the level and have since been decayed away. Next, we +have alluvial soils, those composed of materials which have been +transported by streams, commonly from a great distance, and laid down +on their flood plains. Third, the soils the mineral matters of which +have been brought into their position by the action of glaciers; these +in a way resemble those formed by rivers, but the materials are +generally imperfectly sorted, coarse and fine being mingled together. +Last of all, we have the soils due to the accumulation of blown dust +or blown sand, which, unlike the others, occupy but a small part of +the land surface. It would be possible, indeed, to make yet another +division, including those areas which when emerging from the sea were +covered with fine, uncemented detritus ready at once to serve the +purposes of a soil. Only here and there, and but seldom, do we find +soils of this nature. + +It is characteristic of soils belonging to the group to which we have +given the title of immediate derivation that they have accumulated +slowly, that they move very gradually down the slopes on which they +lie, and that in all cases they represent, with a part of their mass +at least, levels of rock which have disappeared from the region which +they occupied. The additions made to their mass are from below, and +that mass is constantly shrinking, generally at a pretty rapid rate, +by the mineral matter which is dissolved and goes away with the spring +water. They also are characteristically thin on steep slopes, +thickening toward the base of the incline, where the diminished grade +permits the soil to move slowly, and therefore to accumulate. + +In alluvial soils we find accumulations which are characterized by +growth on their upper surfaces, and by the distant transportation of +the materials of which they are composed. In these deposits the +outleaching removes vast amounts of the materials, but so long as the +floods from time to time visit their surfaces the growth of the +deposits is continued. This growth rarely takes place from the waste +of the bed rocks on which the alluvium lies. It is characteristic of +alluvial soils that they are generally made up of _debris_ derived +from fields where the materials have undergone the change which we +have noted in the last paragraph; therefore these latter deposits have +throughout the character which renders the mineral materials easily +dissolved. Moreover, the mass as it is constructed is commonly mingled +with a great deal of organic waste, which serves to promote its +fertility. On these accounts alluvial grounds, though they vary +considerably in fertility, commonly afford the most fruitful fields of +any region. They have, moreover, the signal advantage that they often +may be refreshed by allowing the flood waters to visit them, an +action which but for the interference of man commonly takes place once +each year. Thus in the valley of the Nile there are fields which have +been giving rich grain harvests probably for more than four thousand +years, without any other effective fertilizing than that derived from +the mud of the great river. + +The group of glaciated soils differs in many ways from either of those +mentioned. In it we find the mineral matter to have been broken up, +transported, and accumulated without the influence of those conditions +which ordinarily serve to mix rock _debris_ with organic matter during +the process by which it is broken into bits. When vegetation came to +preoccupy the fields made desolate by glacial action, it found in most +places more than sufficient material to form soils, but the greater +part of the matter was in the condition of pebbles of very hard rock +and sand grains, fragments of silex. Fortunately, the broken-up state +of this material, by exposing a great surface of the rocky matter to +decay, has enabled the plants to convert a portion of the mass into +earth fit for the uses of their roots. But as the time which has +elapsed since the disappearance of the glaciers is much less than that +occupied in the formation of ordinary soil, this decay has in most +cases not yet gone very far, so that in a cubic foot of glaciated +waste the amount of material available for plants is often only a +fraction of that held in the soils of immediate derivation. + +In the greater portion of the fields occupied by glacial waste the +processes which lead to the introduction of organic matter into the +earth have not gone far enough to set in effective work the great +laboratory which has to operate in order to give fertile soil. The +pebbles hinder the penetration of the roots as well as the movement of +insects and other animals. There has not been time enough for the +overturning of trees to bring about a certain admixture of vegetable +matter with the soil--in a word, the process of soil-making, though +the first condition, that of broken-up rock, has been accomplished, +is as yet very incomplete. It needs, indeed, care in the introduction +of organic matter for its completion. + +It is characteristic of glacial soils that they are indefinitely deep. +This often is a disadvantageous feature, for the reason that the soil +water may pass so far down into the earth that the roots are often +deprived of the moisture which they need, and which in ordinary soils +is retained near the surface by the hard underlayer. On the other +hand, where the glacial waste is made up of pebbles formed from rocks +of varied chemical composition, which contain a considerable share of +lime, potash, soda, and other substances which are required by plants, +the very large surface which they expose to decay provides the soil +with a continuous enrichment. In a cubic foot of pebbly glacial earth +we often find that the mass offers several hundred times as much +surface to the action of decay as is afforded by the underlying solid +bed rock from which a soil of immediate derivation has to win its +mineral supply. Where the pebbly glacial waste is provided with a +mixture of vegetable matter, the process of decay commonly goes +forward with considerable rapidity. If the supply of such matter is +large, such as may be produced by ploughing in barnyard manure or +green crops, the nutritive value of the earth may be brought to a very +high point. + +It is a familiar experience in regions where glacial soils exist that +the earth beneath the swamps when drained is found to be +extraordinarily well suited for farming purposes. On inspecting the +pebbles from such places, we observe that they are remarkably decayed. +Where the masses contain large quantities of feldspar, as is the case +in the greater part of our granitic and other crystalline rocks, this +material in its decomposition is converted into kaolin or feldspar +clay, and gives the stones a peculiar white appearance, which marks +the decomposition, and indicates the process by which a great variety +of valuable soil ingredients are brought into a state where they may +be available for plants. + +In certain parts of the glacial areas, particularly in the region near +the margin of the ice sheet, where the glacier remained in one +position for a considerable time, we find extensive deposits of +silicious sand, formed of the materials which settled from the +under-ice stream, near where they escaped from the glacial cavern. +These kames and sand plains, because of the silicious nature of their +materials and the very porous nature of the soil which they afford, +are commonly sterile, or at most render a profit to the tiller by dint +of exceeding care. Thus in Massachusetts, although the first settlers +seized upon these grounds, and planted their villages upon them +because the forests there were scanty and the ground free from +encumbering boulders, were soon driven to betake themselves to those +areas where the drift was less silicious, and where the pebbles +afforded a share of clay. Very extensive fields of this sandy nature +in southeastern New England have never been brought under tillage. +Thus on the island of Martha's Vineyard there is a connected area +containing about thirty thousand acres which lies in a very favourable +position for tillage, but has been found substantially worthless for +such use. The farmers have found it more advantageous to clear away +the boulders from the coarser drift in order to win soil which would +give them fair returns. + +Those areas which are occupied by soil materials which have been +brought into their position by the action of the wind may, as regards +their character, be divided into two very distinct groups--the dunes +and loess deposits. In the former group, where, as we have noted (see +page 123), the coarse sea sands or those from the shores of lakes are +driven forward as a marching hillock, the grains of the material are +almost always silicious. The fragments in the motion are not taken up +into the air, but are blown along the surface. Such dune accumulations +afford an earth which is even more sterile than that of the glacial +sand plains, where there is generally a certain admixture of pebbles +from rocks which by their decomposition may afford some elements of +fertility. Fortunately for the interests of man, these wind-borne +sands occupy but a small area; in North America, in the aggregate, +there probably are not more than one thousand square miles of such +deposits. + +Where the rock material drifted by the winds is so fine that it may +rise into the air in the form of dust, the accumulations made of it +generally afford a fertile soil, and this for the reason that they are +composed of various kinds of rock, and not, as in the case of dunes, +of nearly pure silica. In some very rare cases, where the seashore is +bordered by coral reefs, as it is in parts of southern Florida, and +the strand is made up of limestone bits derived from the hard parts +which the polyps secrete, small dunes are made of limy material. +Owing, however, in part to the relatively heavy nature of this +substance, as well as to the rapid manner in which its grains become +cemented together, such limestone dunes never attain great size nor +travel any distance from their point of origin. + +As before noted, dust accumulations form the soil in extended areas +which lie to the leeward of great deserts. Thus a considerable part of +western China and much of the United States to the west of the +Mississippi is covered by these wind-blown earths. Wherever the +rainfall is considerable these loess deposits have proved to have a +high agricultural value. + +Where a region has an earth which has recently passed from beneath the +sea or a great lake, the surface is commonly covered by incoherent +detritus which has escaped consolidation into hard rock by the fact +that it has not been buried and thus brought into the laboratory of +the earth's crust. When such a region becomes dry land, the materials +are immediately ready to enter into the state of soil. They commonly +contain a good deal of waste derived from the organic life which +dwelt upon the sea bottom and was embedded in the strata as they were +formed. Where these accumulations are made in a lake, the land +vegetation at once possesses the field, even a single year being +sufficient for it to effect its establishment. Where the lands emerge +from the sea, it requires a few years for the salt water to drain away +so that the earth can be fit for the uses of plants. In a general way +these sea-bottom soils resemble those formed in the alluvial plains. +They are, however, commonly more sandy, and their substances less +penetrated by that decay which goes on very freely in the atmosphere +because of the abundant supply of oxygen, and but slowly on the sea +floor. Moreover, the marine deposits are generally made up in large +part of silicious sand, a material which is produced in large +quantities by the disruption of the rocks along the sea coast. The +largest single field of these ocean-bottom soils of North America is +found in the lowland region of the southern United States, a wide belt +of country extending along the coast from the Rio Grande to New York. +Although the streams have channelled shallow valleys in the beds of +this region, the larger part of its surface still has the peculiar +features of form and composition which were impressed upon it when it +lay below the surface of the sea. + +Local variations in the character of the soil covering are exceedingly +numerous, and these differences of condition profoundly affect the +estate of man. We shall therefore consider some of the more important +of these conditions, with special reference to their origin. + +The most important and distinctly marked variation in the fertility of +soils is that which is produced by differences in the rainfall. No +parts of the earth are entirely lacking in rain, but over considerable +areas the precipitation does not exceed half a foot a year. In such +realms the soil is sterile, and the natural coating of vegetation +limited to those plants which can subsist on dew or which can take on +an occasional growth at such times as moisture may come upon them. +With a slight increase in precipitation, the soil rapidly increases in +productivity, so that we may say that where as much as about ten +inches of water enters the earth during the summer half of the year, +it becomes in a considerable measure fit for agriculture. Observations +indicate that the conditions of fertility are not satisfied where the +rainfall is just sufficient to fill the pores of the soil; there must +be enough water entering the earth to bring about a certain amount of +outflow in the form of springs. The reason of this need becomes +apparent when we study the evident features of those soils which, +though from season to season charged with water, do not yield springs, +but send the moisture away through the atmosphere. Wherever these +conditions occur we observe that the soil in dry seasons becomes +coated with a deposit of mineral matter, which, because of its taste, +has received the name of alkali. The origin of this coating is as +follows: The pores of the soil, charged from year to year with +sufficient water to fill them, become stored with a fluid which +contains a very large amount of dissolved mineral matter--too much, +indeed, to permit the roots of plants, save a few species which have +become accustomed to the conditions, to do their appointed work. In +fact, this water is much like that of the sea, which the roots of only +a few of our higher plants can tolerate. When the dry season comes on, +the heat of the sun evaporates the water at the surface, leaving +behind a coating composed of the substances which the water contains. +The soil below acts in the manner of a lamp-wick to draw up fluid as +rapidly as the heat burns it away. When the soil water is as far as +possible exhausted, the alkali coating may represent a considerable +part of the soluble matter of the soil, and in the next rainy season +it may return in whole or in part to the under-earth, again to be +drawn in the manner before described to the upper level. It is +therefore only when a considerable share of the ground water goes +forth to the streams in each year that the alkaline materials are in +quantity kept down to the point where the roots of our crop-giving +plants can make due use of the soil. Where, in an arid region, the +ground can be watered from the enduring streams or from artificial +reservoirs, the main advantage arising from the process is commonly +found in the control which it gives the farmer in the amount of the +soil water. He can add to the rainfall sufficient to take away the +excess of mineral matter. When such soils are first brought under +tillage it is necessary to use a large amount of water from the +canals, in order to wash away the old store of alkali. After that a +comparatively small contribution will often keep the soil in excellent +condition for agriculture. It has been found, however, in the +irrigated lands beside the Nile that where too much saving is +practised in the irrigation, the alkaline coating will appear where it +has been unknown before, and with it an unfitness of the earth to bear +crops. + +Although the crust of mineral matters formed in the manner above +described is characteristic of arid countries, and in general peculiar +to them, a similar deposit may under peculiar conditions be formed in +regions of great rainfall. Thus on the eastern coast of New England, +where the tidal marshes have here and there been diked from the sea +and brought under tillage, the dissolved mineral matters of the soil, +which are excessive in quantity, are drawn to the surface, forming a +coating essentially like that which is so common in arid regions. The +writer has observed this crust on such diked lands, having a thickness +of an eighth of an inch. In fact, this alkali coating represents +merely the extreme operation of a process which is going on in all +soils, and which contributes much to their fertility. When rain falls +and passes downward into the earth, it conveys the soluble matter to a +depth below the surface, often to beyond the point where our ordinary +crop plants, such as the small grains, can have access to it, and +this for the reason that their roots do not penetrate deeply. When dry +weather comes and evaporation takes place from the surface, the fluid +is drawn up to the upper soil layer, and there, in process of +evaporation, deposits the dissolved materials which it contains. Thus +the mineral matter which is fit for plant food is constantly set in +motion, and in its movement passes the rootlets of the plants. It is +probably on this account--at least in part--that very wet weather is +almost as unfavourable to the farmer as exceedingly dry, the normal +alternation in the conditions being, as is well known, best suited to +his needs. + +So long as the earth is subjected to conditions in which the rainfall +may bring about a variable amount of water in the superficial detrital +layer, we find normal fruitful soils, though in their more arid +conditions they may be fit for but few species of plants. When, by +increasing aridity, we pass to conditions where there is no tolerably +permanent store of water in the _debris_, the material ceases to have +the qualities of a soil, and becomes mere rock waste. At the other +extreme of the scale we pass to conditions where the water is +steadfastly maintained in the interstices of the detritus, and there +again the characteristic of the soil and its fitness for the uses of +land vegetation likewise disappear. In a word, true soil conditions +demand the presence of moisture, but that in insufficient quantities, +to keep the pores of the earth continually filled; where they are thus +filled, we have the condition of swamps. Between these extremes the +level at which the water stands in the soil in average seasons is +continually varying. In rainy weather it may rise quite to the +surface; in a dry season it may sink far down. As this water rises and +falls, it not only moves, as before noted, the soluble mineral +materials, but it draws the air into and expels it from the earth with +each movement. This atmospheric circulation of the soil, as has been +proved by experiment, is of great importance in maintaining its +fertility; the successive charges of air supply the needs of the +microscopic underground creatures which play a large part in enriching +the soil, and the direct effect of the oxygen in promoting decay is +likewise considerable. A part of the work which is accomplished by +overturning the earth in tillage consists in this introduction of the +air into the pores of the soil, where it serves to advance the actions +which bring mineral matters into solution. + +[Illustration: _Mountain gorge, Himalayas, India. Note the difference +in the slope of the eroded rocks and the effect of erosion upon them; +also the talus slopes at the base of the cliffs which the torrent is +cutting away. On the left of the foreground there is a little bench +showing a recent higher line of the water._] + +In the original conditions of any country which is the seat of +considerable rainfall, and where the river system is not so far +developed as to provide channels for the ready exit of the waters, we +commonly find very extensive swamps; these conditions of bad drainage +almost invariably exist where a region has recently been elevated +above the level of the sea, and still retains the form of an irregular +rolling plain common to sea floors, and also in regions where the work +done by glaciers has confused the drainage which the antecedent +streams may have developed. In an old, well-elaborated river system +swamps are commonly absent, or, if they occur, are due to local +accidents of an unimportant nature. + +For our purpose swamps may be divided into three groups--climbing +bogs, lake bogs, and marine marshes. The first two of these groups +depend on the movements of the rain water over the land; the third on +the action of the tides. Beginning our account with the first and most +exceptional of these groups, we note the following features in their +interesting history: + +Wherever in a humid region, on a gentle slope--say with an inclination +not exceeding ten feet to the mile--the soil is possessed by any +species of plants whose stems grow closely together, so that from +their decayed parts a spongelike mass is produced, we have the +conditions which favour the development of climbing bogs. Beginning +usually in the shores of a pool, these plants, necessarily of a +water-loving species, retain so much moisture in the spongy mass +which they form that they gradually extend up the slope. Thus +extending the margin of their field, and at the same time thickening +the deposit which they form, these plants may build a climbing bog +over the surface until steeps are attained where the inclination is so +great that the necessary amount of water can not be held in the spongy +mass, or where, even if so held, the whole coating will in time slip +down in the manner of an avalanche. + +The greater part of the climbing bogs of the world are limited to the +moist and cool regions of high latitudes, where species of moss +belonging to the genus _Sphagnum_ plentifully flourish. These plants +can only grow where they are continuously supplied with a bath of +water about their roots. They develop in lake bogs as far south as +Mexico, but in the climbing form they are hardly traceable south of +New England, and are nowhere extensively developed within the limits +of the United States. In more northern parts of this continent, and in +northwestern Europe, particularly in the moist climate of Ireland, +climbing bogs occupy great areas, and hold up their lakes of +interstitially contained water over the slopes of hills, where the +surface rises at the rate of thirty feet or more to the mile. So long +as the deposit of decayed vegetable matter which has accumulated in +this manner is thin, therefore everywhere penetrated by the fibrous +roots of the moss, it may continue to cling to its sloping bed; but +when it attains a considerable thickness, and the roots in the lower +part decay, the pulpy mass, water-laden in some time of heavy rain, +break away in a vast torrent of thick, black mud, which may inundate +the lower lands, causing widespread destruction. + +In more southern countries, other water-loving plants lead to the +formation of climbing bogs. Of these, the commonest and most effective +are the species of reeds, of which our Indian cane is a familiar +example. Brakes of this vegetation, plentifully mingled with other +species of aquatic growth, form those remarkable climbing bogs known +as the Dismal and other swamps, which numerously occur along the coast +line of the United States from southern Maryland to eastern Texas. +Climbing bogs are particularly interesting, not only from the fact +that they are eminently peculiar effects of plant growth, but because +they give us a vivid picture of those ancient morasses in which grew +the plants that formed the beds of vegetable matter now appearing in +the state of coal. Each such bed of buried swamp material was, with +rare exceptions, where the accumulation took place in lakes, gathered +in climbing bogs such as we have described. + +Lake bogs occur in all parts of the world, but in their best +development are limited to relatively high latitudes, and this for the +reason that the plants which form vegetable matter grow most +luxuriantly in cool climates and in regions where the level of the +basin is subject to less variation than occurs in the alternating wet +and dry seasons which exist in nearly all tropical regions. The +fittest conditions are found in glaciated regions, where, as before +noted, small lakes are usually very abundant. On the shores of one of +these pools, of size not so great that the waves may attain a +considerable height, or in the sheltered bay of a larger lake, various +aquatic plants, especially the species of pond lilies, take root upon +the bottom, and spread their expanded leaves on the surface of the +water. These flexible-leaved and elastic-stemmed plants can endure +waves which attain no more than a foot or two of height, and by the +friction which they afford make the swash on the shore very slight. In +the quiet water, rushes take root, and still further protect the +strand, so that the very delicate vegetation of the mosses, such as +the _Sphagnum_, can fix itself on the shore. + +As soon as the _Sphagnum_ mat has begun its growth, the strength given +by its interlaced fibres enables it to extend off from the shore and +float upon the water. In this way it may rapidly enlarge, if not +broken up by the waves, so that its front advances into the lake at +the rate of several inches each year. While growing outwardly it +thickens, so that the bottom of the mass gradually works down toward +the floor of the basin. At the same time the lower part of the sheet, +decaying, contributes a shower of soft peat mud to the floor of the +lake. In this way, growing at its edge, deepening, and contributing to +an upgrowth from the bottom, a few centuries may serve entirely to +fill a deep basin with peaty accumulation. In general, however, the +surface of the bog closes over the lake before the accumulation has +completely filled the shoreward portions of the area. In these +conditions we have what is familiarly known as a quaking bog, which +can be swayed up and down by a person who quickly stoops and rises +while standing on the surface. In this state the tough and thick sheet +of growing plants is sufficient to uphold a considerable weight, but +so elastic that the underlying water can be thrown into waves. Long +before the bog has completely filled the lake with the peaty +accumulations the growth of trees is apt to take place on its surface, +which often reduces the area to the appearance of a very level wet +wood. + +[Illustration: Fig. 17.--Diagram showing beginning of peat bog: A, +lake; B, lilies and rushes; C, lake bog; D, climbing bog.] + +Climbing and lake bogs in the United States occupy a total area of +more than fifty thousand square miles. In all North America the total +area is probably more than twice as great. Similar deposits are +exceedingly common in the Eurasian continent and in southern +Patagonia. It is probable that the total amount of these fields in +different parts of the world exceeds half a million square miles. +These two groups of fresh-water swamps have an interest, for the +reason that when reduced to cultivation by drainage and by subsequent +removal of the excess of peaty matter, by burning or by natural decay, +afford very rich soil. The fairest fields of northern Europe, +particularly in Great Britain and Ireland, have been thus won to +tillage. In the first centuries of our era a large part of +England--perhaps as much as one tenth of the ground now tilled in that +country--was occupied by these lands, which retained water in such +measure as to make them unfit for tillage, the greater portion of this +area being in the condition of thin climbing bog. For many centuries +much of the energy of the people was devoted to the reclamation of +these valuable lands. This task of winning the swamp lands to +agriculture has been more completely accomplished in England than +elsewhere, but it has gone far on the continent of Europe, +particularly in Germany. In the United States, owing to the fact that +lands have been cheap, little of this work of swamp-draining has as +yet been accomplished. It is likely that the next great field of +improvement to be cultivated by the enterprising people will be found +in these excessively humid lands, from which the food-giving resources +for the support of many million people can be won. + +[Illustration: Fig. 18.--Diagram showing development of swamp: A, +remains of lake; B, surface growth; c, peat.] + +The group of marine marshes differs in many important regards from +those which are formed in fresh water. Where the tide visits any +coast line, and in sheltered positions along that shore, a number of +plants, mostly belonging to the group of grasses, species which have +become accustomed to having their roots bathed by salt water, begin +the formation of a spongy mat, which resembles that composed of +_Sphagnum_, only it is much more solid. This mat of the marine marshes +soon attains a thickness of a foot or more, the upper or growing +surface lying in a position where it is covered for two or three hours +at each visit of the tide. Growing rapidly outward from the shore, and +having a strength which enables it to resist in a tolerably effective +manner waves not more than two or three feet high, this accumulation +makes head against the sea. To a certain extent the waves undermine +the front of the sheet and break up masses of it, which they +distribute over the shallow bottom below the level at which these +plants can grow. In this deeper water, also, other marine animals and +plants are continually developing, and their remains are added to the +accumulations which are ever shallowing the water, thus permitting a +further extension of the level, higher-lying marsh. This process +continues until the growth has gone as far as the scouring action of +the tidal currents will permit. In the end the bay, originally of +wide-open water, is only such at high tide. For the greater part of +the time it appears as broad savannas, whose brilliant green gives +them the aspect of rare fertility. + +Owing to the conditions of their growth, the deposits formed in marine +marshes contain no distinct peat, the nearest approach to that +substance being the tangle of wirelike roots which covers the upper +foot or so of the accumulation. The greater part of the mass is +composed of fine silt, brought in by the streams of land water which +discharge into the basin, and by the remains of animals which dwelt +upon the bottom or between the stalks of the plants that occupy the +surface of the marshes. These interspaces afford admirable shelter to +a host of small marine forms. The result is, that the tidal marshes, +as well as the lower-lying mud flats, which have been occupied by the +mat of vegetation, afford admirable earth for tillage. Unfortunately, +however, there are two disadvantages connected with the redemption of +such lands. In the first place, it is necessary to exclude the sea +from the area, which can only be accomplished by considerable +engineering work; in the second place, the exclusion of the tide +inevitably results in the silting up of the passage by which the water +found its way to the sea. As these openings are often used for +harbours, the effect arising from their destruction is often rather +serious. Nevertheless, in some parts of the world very extensive and +most fertile tracts of land have thus been won from the sea; a large +part of Holland and shore-land districts in northern Europe are made +up of fields which were originally covered by the tide. Near the mouth +of the Rhine, indeed, the people have found these sea-bottom soils so +profitable that they have gone beyond the zone of the marshes, and +have drained considerable seas which of old were permanently covered, +even at the lowest level of the waters. + +On the coast of North America marine marshes have an extensive +development, and vary much in character. In the Bay of Fundy, where +the tides have an altitude of fifty feet or more, the energy of their +currents is such that the marsh mat rarely forms. Its place, however, +is taken by vast and ever-changing mud flats, the materials of which +are swept to and fro by the moving waters. The people of this region +have learned an art of a peculiar nature, by which they win broad +fields of excellent land from the sea. Selecting an area of the flats, +the surface of which has been brought to within a few feet of high +tide, they inclose it with a stout barrier or dike, which has openings +for the free admission of the tidal waters. Entering this basin, the +tide, moving with considerable velocity, bears in quantities of +sediment. In the basin, the motion being arrested, this sediment +falls to the bottom, and serves to raise its level. In a few months +the sheet of sediment is brought near the plane of the tidal movement, +then the gates are closed at times when the tide has attained half of +its height, so that the ground within the dike is not visited by the +sea water, and can be cultivated. + +[Illustration: Fig. 19.--Map of Ipswich marshes, Massachusetts, formed +behind a barrier beach.] + +Along the coast of New England the ordinary marine marshes attain an +extensive development in the form of broad-grassed savannas. With this +aspect, though with a considerable change in the plants which they +bear, the fringe of savannas continues southward along the coast to +northern Florida. In the region about the mouth of the Savannah River, +so named from the vast extent of the tidal marshes, these fields +attain their greatest development. In central and southern Florida, +however, where the seacoast is admirably suited for their development, +these coastal marshes of the grassy type disappear, their place being +taken by the peculiar morasses formed by the growth of the mangrove +tree. + +In the mangrove marshes the tree which gives the areas their name +covers all the field which is visited by the tide. This tree grows +with its crown supported on stiltlike roots, at a level above high +tide. From its horizontal branches there grow off roots, which reach +downward into the water, and thence to the bottom. The seeds of the +mangrove are admirably devised so as to enable the plant to obtain a +foothold on the mud flats, even where they are covered at low tide +with a depth of two or three feet of water. They are several inches in +length, and arranged with booklets at their lower ends; floating near +the bottom, they thus catch upon it, and in a few weeks' growth push +the shoot to the level of the water, thus affording a foundation for a +new plantation. In this manner, extending the old forests out into the +shallow water of the bays, and forming new colonies wherever the water +is not too deep, these plants rapidly occupy all the region which +elsewhere would appear in the form of savannas. + +[Illustration: Fig. 20.--Diagram showing mode of growth of mangroves.] + +The tidal marshes of North America, which may be in time converted to +the uses of man, probably occupy an area exceeding twenty thousand +square miles. If the work of reclaiming such lands from the sea ever +attains the advance in this country that it has done in Holland, the +area added to the dry land by engineering devices may amount to as +much as fifty thousand square miles--a territory rather greater than +the surface of Kentucky, and with a food-yielding power at least five +times as great as is afforded by that fertile State. In fact, these +conquests from the sea are hereafter to be among the great works which +will attract the energies of mankind. In the arid region of the +Cordilleras, as well as in many other countries, the soil, though +destitute of those qualities which make it fit for the uses of man, +because of the absence of water in sufficient amount, is, as regards +its structure and depth, as well as its mineral contents, admirably +suited to the needs of agriculture. The development of soils in desert +regions is in almost all cases to be accounted for by the former +existence in the realms they occupy of a much greater rainfall than +now exists. Thus in the Rocky Mountain country, when the deep soils +of the ample valleys were formed, the lakes, as we have before noted, +were no longer dead seas, as is at present so generally the case, but +poured forth great streams to the sea. Here, as elsewhere, we find +evidence that certain portions of the earth which recently had an +abundant rainfall have now become starved for the lack of that supply. +All the soils of arid regions where the trial has been made have +proved very fertile when subjected to irrigation, which can often be +accomplished by storing the waters of the brief rainy season or by +diverting those of rivers which enter the deserts from well-watered +mountain fields. In fact, the soil of these arid realms yields +peculiarly ample returns to the husbandman, because of certain +conditions due to the exceeding dryness of the air. This leads to an +absence of cloudy weather, so that from the time the seed is planted +the growth is stimulated by uninterrupted and intense sunshine. The +same dryness of the air leads, as we have seen, to a rapid evaporation +from the surface, by which, in a manner before noted, the dissolved +mineral matter is brought near the top of the soil, where it can best +serve the greater part of our crop plants. On these accounts an acre +of irrigated soil can be made to yield a far greater return than can +be obtained from land of like chemical composition in humid regions. + +In many parts of the world, particularly in the northern and western +portions of the Mississippi Valley, there are widespread areas, which, +though moderately well watered, were in their virgin state almost +without forests. In the prairie region the early settlers found the +country unwooded, except along the margins of the streams. On the +borders of the true prairies, however, they found considerable areas +of a prevailingly forested land, with here and there a tract of +prairie. There were several of these open fields south of the Ohio, +though the country there is in general forested; one of these prairie +areas, in the Green River district of Kentucky, was several thousand +square miles in extent. At first it was supposed that the absence of +trees in the open country of the Mississippi Valley was due to some +peculiarity of the soil, but experience shows that plantations +luxuriantly develop, and that the timber will spread rapidly in the +natural way. In fact, if the seeds of the trees which have been +planted since the settlement of the country were allowed to develop as +they seek to do, it would only be a few centuries before the region +would be forest-clad as far west as the rainfall would permit the +plants to develop. Probably the woods would attain to near the +hundredth meridian. + +In the opinion of the writer, the treeless character of the Western +plains is mainly to be accounted for by the habit which our Indians +had of burning the herbage of a lowly sort each year, so that the +large game might obtain better pasturage. It is a well-known fact to +all those who have had to deal with cattle on fields which are in the +natural state that fire betters the pasturage. Beginning this method +of burning in the arid regions to the west of the original forests, +the natural action of the fire has been gradually to destroy these +woods. Although the older and larger trees, on account of their thick +bark and the height of their foliage above the ground, escaped +destruction, all the smaller and younger members of the species were +constantly swept away. Thus when the old trees died they left no +succession, and the country assumed its prairie character. That the +prairies were formed in this manner seems to be proved by the +testimony which we have concerning the open area before mentioned as +having existed in western Kentucky. It is said that around the +timberless fields there was a wide fringe of old fire-scarred trees, +with no undergrowth beneath their branches, and that as they died no +kind of large vegetation took their place. When the Indians who set +these fires were driven away, as was the case in the last decade of +the last century, the country at once began to resume its timbered +condition. From the margin and from every interior point where the +trees survived, their seeds spread so that before the open land was +all subjugated to the plough it was necessary in many places to clear +away a thick growth of the young forest-building trees. + +The soils which develop on the lavas and ashes about an active volcano +afford interesting subjects for study, for the reason that they show +how far the development of the layer which supports vegetation may +depend upon the character of the rocks from which it is derived. Where +the materials ejected from a volcano lie in a rainy district, the +process of decay which converts the rock into soil is commonly very +rapid, a few years of exposure to the weather being sufficient to +bring about the formation of a fertile soil. This is due to the fact +that most lavas, as well as the so-called volcanic ashes, which are of +the same material as the lavas, only blown to pieces, are composed of +varied minerals, the most of which are readily attacked by the agents +of decay. Now and then, however, we find the materials ejected from a +particular volcano, or even the lavas and ashes of a single eruption, +in such a chemical state that soils form upon them with exceeding +slowness. + + * * * * * + +The foregoing incomplete considerations make it plain that the +soil-covering of the earth is the result of very delicate adjustments, +which determine the rate at which the broken-down rocks find their +path from their original bed places to the sea. The admirable way in +which this movement is controlled is indicated by the fact that almost +everywhere we find a soil-covering deep enough for the use of a varied +vegetation, but rarely averaging more than a dozen feet in depth. Only +here and there are the rocks bare or the earth swathed in a profound +mass of detritus. This indicates how steadfast and measured is the +march of the rock waste from the hills to the sea. Unhappily, man, +when by his needs he is forced to till the soil, is compelled to break +up this ancient and perfect order. He has to strip the living mantle +from the earth, replacing it with growth of those species which serve +his needs. Those plants which are most serviceable--which are, indeed, +indispensable in the higher civilization, the grains--require for +their cultivation that the earth be stripped bare and deeply stirred +during the rainy season, and thus subjected to the most destructive +effect of the rainfall. The result is, that in almost all grain fields +the rate of soil destruction vastly surpasses that at which the +accumulation is being made. We may say, indeed, that, except in +alluvial plains, where the soil grows by flood-made additions to its +upper surface, no field tilled in grain can without exceeding care +remain usable for a century. Even though the agriculturist returns to +the earth all the chemical substances which he takes away in his +crops, the loss of the soil by the washing away of its substance to +the stream will inevitably reduce the region to sterility. + +It is not fanciful to say that the greatest misfortune which in a +large way man has had to meet in his agriculture arises from this +peculiar stress which grain crops put upon the soil. If these grains +grew upon perennial plants, in the manner of our larger fruits, the +problem of man's relation to the soil would be much simpler than it is +at present. He might then manage to till the earth without bringing +upon it the inevitable destruction which he now inflicts. As it is, he +should recognise that his needs imperil this ancient and precious +element in the earth's structure, and he should endeavour in every +possible way to minimize the damage which he brings about. This result +he may accomplish in certain simple ways. + +First, as regards the fertility of the soil, as distinguished from the +thickness of the coating, it may be said that modern discoveries +enable us to see the ways whereby we may for an indefinite period +avoid the debasement of our great heritage, the food-giving earth. We +now know in various parts of the world extensive and practically +inexhaustible deposits, whence may be obtained the phosphates, +potash, soda, etc., which we take from the soil in our crops. We also +have learned ways in which the materials contained in our sewage may +be kept from the sea and restored to the fields. In fact, the recent +developments of agriculture have made it not only easy, but in most +cases profitable, to avoid this waste of materials which has reduced +so many regions to poverty. We may fairly look forward to the time, +not long distant, when the old progressive degradation in the +fertility of the soil coating will no longer occur. It is otherwise +with the mass of the soil, that body of commingled decayed rock and +vegetable matter which must possess a certain thickness in order to +serve its needs. As yet no considerable arrest has been made in the +processes which lead to the destruction of this earthy mass. In all +countries where tillage is general the rivers are flowing charged with +all they can bear away of soil material. Thus in the valley of the Po, +a region where, if the soil were forest-clad, the down-wearing of the +surface would probably be at no greater rate than one foot in five +thousand years, the river bears away the soil detritus so rapidly that +at the present time the downgoing is at the rate of one foot in eight +hundred years, and each decade sees the soil disappear from hillsides +which were once fertile, but are now reduced to bare rocks. All about +the Mediterranean the traveller notes extensive regions which were +once covered with luxuriant forests, and were afterward the seats of +prosperous agriculture, where the soil has utterly disappeared, +leaving only the bare rocks, which could not recover its natural +covering in thousands of years of the enforced fallow. + +Within the limits of the United States the degradation of the soil, +owing to the peculiar conditions of the country, is in many districts +going forward with startling rapidity. It has been the habit of our +people--a habit favoured by the wide extent of fertile and easily +acquired frontier ground--recklessly to till their farms until the +fields were exhausted, and then to abandon them for new ground. By +shallow ploughing on steep hillsides, by neglect in the beginning of +those gulches which form in such places, it is easy in the hill +country of the eastern United States to have the soil washed away +within twenty years after the protecting forests have been destroyed. +The writer has estimated that in the States south of the Ohio and +James Rivers more than eight thousand square miles of originally +fertile ground have by neglect been brought into a condition where it +will no longer bear crops of any kind, and over fifteen hundred miles +of the area have been so worn down to the subsoil or the bed rock that +it may never be profitable to win it again to agricultural uses. + +Hitherto, in our American agriculture, our people have been to a great +extent pioneers; they have been compelled to win what they could in +the cheapest possible way and with the rudest implements, and without +much regard to the future of those who were in subsequent generations +to occupy the fields which they were conquering from the wilderness +and the savages. The danger is now that this reckless tillage, in a +way justified of old, may be continued and become habitual with our +people. It is, indeed, already a fixed habit in many parts of the +country, particularly in the South, where a small farmer expects to +wear out two or three plantations in the course of his natural life. +Many of them manage to ruin from one to two hundred acres of land in +the course of half a century of uninterrupted labour. This system +deserves the reprobation of all good citizens; it would be well, +indeed, if it were possible to do so, to stamp it out by the law. The +same principle which makes it illegal for a man to burn his own +dwelling house may fairly be applied in restraining him from +destroying the land which he tills. + +There are a few simple principles which, if properly applied, may +serve to correct this misuse of our American soil. The careful tiller +should note that all soils whatever which lie on declivities having a +slope of more than one foot in thirty inevitably and rapidly waste +when subject to plough tillage. This instrument tends to smear and +consolidate the layer of earth over which its heel runs, so that at a +depth of a few inches below the surface a layer tolerably impervious +to water is formed. The result is that the porous portion of the +deposit becomes excessively charged with water in times of heavy rain, +and moves down the hillside in a rapid manner. All such steep slopes +should be left in their wooded state, or, if brought into use, should +be retained as pasture lands. + +Where, as is often the case with the farms in hilly countries, all the +fields are steeply inclined, it is an excellent precaution to leave +the upper part of the slope with a forest covering. In this condition +not only is the excessive flow of surface water diminished, but the +moisture which creeps down the slope from the wooded area tends to +keep the lower-lying fields in a better state for tillage, and +promotes the decay of the underlying rocks, and thus adds to the body +and richness of the earth. + +On those soils which must be tilled, even where they tend to wash +away, the aim should be to keep the detritus open to such a depth that +it may take in as much as possible of the rainfall, yielding the water +to the streams through the springs. This end can generally be +accomplished by deep ploughing; it can, in almost all cases, be +attained by under-drainage. The effect of allowing the water to +penetrate is not only to diminish the superficial wearing, but to +maintain the process of subsoil and bed-rock decay by which the +detrital covering is naturally renewed. Where, as in many parts of the +country, the washing away of the soil can not otherwise be arrested, +the progress of the destruction can be delayed by forming with the +skilful use of the plough ditches of slight declivity leading along +the hillsides to the natural waterways. One of the most satisfactory +marks of the improvement which is now taking place in the agriculture +of the cotton-yielding States of this country is to be found in the +rapid increase in the use of the ditch system here mentioned. This +system, combined with ploughing in the manner where the earth is with +each overturning thrown uphill, will greatly reduce the destructive +effect of rainfall on steep-lying fields. But the only effective +protection, however, is accomplished by carefully terracing the +slopes, so that the tilled ground lies in level benches. This system +is extensively followed in the thickly settled portions of Europe, but +it may be a century before it will be much used in this country. + +The duty of the soil-tiller by the earth with which he deals may be +briefly summed up: He should look upon himself as an agent necessarily +interfering with the operations which naturally form and preserve the +soil. He should see that his work brings two risks; he may impoverish +the accumulation of detrital material by taking out the plant food +more rapidly than it is prepared for use. This injurious result may be +at any time reparable by a proper use of manures. Not so, however, +with the other form of destruction, which results in the actual +removal of the soil materials. Where neglect has brought about this +disaster, it can only be repaired by leaving the area to recover +beneath the slowly formed forest coating. This process in almost all +cases requires many thousands of years for its accomplishment. The man +who has wrought such destruction has harmed the inheritance of life. + + + + + CHAPTER IX. + + THE ROCKS AND THEIR ORDER. + + +In the preceding chapters of this book the attention of the student +has been directed mainly to the operations of those natural forces +which act upon the surface of the earth. Incidentally the consequences +arising from the applications of energy to the outer part of the +planet have been attended to, but the main aim has been to set forth +the work which solar energy, operating in the form of heat, +accomplishes upon the lands. We have now to consider one of the great +results of these actions, which is exhibited in the successive strata +that make up the earth's crust. + +The most noteworthy effect arising from the action of the solar forces +on the earth and their co-operation with those which originate in our +sphere is found in the destruction of beds or other deposits of rock, +and the removal of the materials to the floors of water basins, where +they are again aggregated in strata, and gradually brought once more +into a stable condition within the earth. This work is accomplished by +water in its various states, the action being directly affected by +gravitation. In the form of steam, water which has been built into +rocks and volcanically expelled by tensions, due to the heat which it +has acquired at great depths below the surface, blows forth great +quantities of lava, which is contributed to the formation of strata, +either directly in the solid form or indirectly, after having been +dissolved in the sea. Acting as waves, water impelled by solar energy +transmitted to it by the winds beats against the shores, wearing away +great quantities of rock, which is dragged off to the neighbouring sea +bottoms, there to resume the bedded form. Moving ice in glaciers, +water again applying solar energy given to it by its elevation above +the sea, most effectively grinds away the elevated parts of the crust, +the _debris_ being delivered to the ocean. In the rain the same work +is done, and even in the wind the power of the sun serves to abrade +the high-lying rocks, making new strata of their fragments. + +As gravity enters as an element in all the movements of divided rock, +the tendency of the waste worn from the land is to gather on to the +bottoms of basins which contain water. Rarely, and only in a small +way, this process results in the accumulation of lake deposits; the +greater part of the work is done upon the sea floor. When the beds are +formed in lake basins, they may be accumulated in either of two very +diverse conditions. They may be formed in what are called dead seas, +in which case the detrital materials are commonly small in amount, for +the reason that the inflowing streams are inconsiderable; in such +basins there is normally a large share of saline materials, which are +laid down by the evaporation of the water. In ordinary lakes the +deposits which are formed are mostly due to the sediment that the +rivers import. These materials are usually fine-grained, and the sand +or pebbles which they contain are plentifully mingled with clay. Hence +lake deposits are usually of an argillaceous nature. As organic life, +such as secretes limestone, is rarely developed to any extent in lake +basins, limy beds are very rarely formed beneath those areas of water. +Where they occur, they are generally due to the fact that rivers +charged with limy matter import such quantities of the substance that +it is precipitated on the bottom. + +As lake deposits are normally formed in basins above the level of the +sea, and as the drainage channels of the basins are always cutting +down, the effect is to leave such strata at a considerable height +above the sea level, where the erosive agents may readily attack them. +In consequence of this condition, lacustrine beds are rarely found of +great antiquity; they generally disappear soon after they are formed. +Where preserved, their endurance is generally to be attributed to the +fact that the region they occupy has been lowered beneath the sea and +covered by marine strata. + +The great laboratory in which the sedimentary deposits are +accumulated, the realm in which at least ninety-nine of the hundred +parts of these materials are laid down, is the oceanic part of the +earth. On the floors of the seas and oceans we have not only the +region where the greater part of the sedimentation is effected, but +that in which the work assumes the greatest variety. The sea bottoms, +as regards the deposits formed upon them, are naturally divided into +two regions--the one in which the _debris_ from the land forms an +important part of the sediment, and the other, where the remoteness +of the shores deprives the sediment of land waste, or at least of +enough of that material in any such share as can affect the character +of the deposits. + +What we may term the littoral or shore zone of the sea occupies a belt +of prevailingly shallow water, varying in width from a few score to a +few hundred miles. Where the bottom descends steeply from the coast, +where there are no strong off-shore setting currents, and where the +region is not near the mouth of a large river which bears a great tide +of sediment to the sea, the land waste may not affect the bottom for +more than a mile or two from the shore. Where these conditions are +reversed, the _debris_ from the air-covered region may be found three +or four hundred miles from the coast line. It should also be noted +that the incessant up-and-down goings of the land result in a constant +change in the position of the coast line, and consequently in the +extension of the land sediment, in the course of a few geological +periods over a far wider field of sea bottom than that to which they +would attain if the shores remained steadfast. + +It is characteristic of the sediments deposited within the influence +of the continental detritus that they vary very much in their action, +and that this variation takes place not only horizontally along the +shores in the same stratum, but vertically, in the succession of the +beds. It also may be traced down the slope from the coast line to deep +water. Thus where all the _debris_ comes from the action of the waves, +the deposits formed from the shore outwardly will consist of coarse +materials, such as pebbles near the coast, of sand in the deeper and +remoter section, and of finer silt in the part of the deposit which is +farthest out. With each change in the level of the coast line the +position of these belts will necessarily be altered. Where a great +river enters the sea, the changes in the volume of sediment which it +from time to time sends forth, together with the alternations in the +position of its point of discharge, led to great local complexities in +the strata. Moreover, the turbid water sent forth by the stream may, +as in the case of the tide from the Amazon, be drifted for hundreds of +miles along the coast line or into the open sea. + +The most important variations which occur in the deposits of the +littoral zone are brought about by the formations of rocks more or +less composed of limestone. Everywhere the sea is, as compared with +lake waters, remarkably rich in organic life. Next the shore, partly +because the water is there shallow, but also because of its relative +warmth and the extent to which it is in motion, organic life, both +that of animals and plants, commonly develops in a very luxuriant way. +Only where the bottom is composed of drifting sands, which do not +afford a foothold for those species which need to rest upon the shore, +do we fail to find that surface thickly tenanted with varied forms. +These are arranged according to the depth of the bottom. The species +of marine plants which are attached to fixed objects are limited to +the depth within which the sunlight effectively penetrates the water; +in general, it may be said that they do not extend below a depth of +one hundred feet. The animal forms are distributed, according to their +kinds, over the floor, but few species having the capacity to endure +any great range in the pressure of the sea water. Only a few forms, +indeed, extend from low tide to the depth of a thousand feet. + +The greatest development of organic life, the realm in which the +largest number of species occur, and where their growth is most rapid, +lies within about a hundred feet of the low-tide level. Here sunlight, +warmth, and motion in the water combine to favour organic development. +It is in this region that coral reefs and other great accumulations of +limestone, formed from the skeletons of polyps and mollusks, most +abundantly occur. These deposits of a limy nature depend upon a very +delicate adjustment of the conditions which favour the growth of +certain creatures; very slight geographic changes, by inducing +movements of sand or mud, are apt to interrupt their formation, +bringing about a great and immediate alteration in the character of +the deposits. Thus it is that where geologists find considerable +fields of rock, where limestones are intercalated with sandstones and +deposits of clay, they are justified in assuming that the strata were +laid down near some ancient shore. In general, these coast deposits +become more and more limy as we go toward the tropical realms, and +this for the reason that the species which secrete large amounts of +lime are in those regions most abundant and attain the most rapid +growth. The stony polyps, the most vigorous of the limestone makers, +grow in large quantities only in the tropical realm, or near to it, +where ocean streams of great warmth may provide the creatures with the +conditions of temperature and food which they need. + +As we pass from the shore to the deeper sea, the share of land +detritus rapidly diminishes until, as before remarked, at the distance +of five hundred miles from the coast line, very little of that waste, +except that from volcanoes, attains the bottom of the sea. By far the +larger part of the contributions which go to the formation of these +deep-sea strata come from organic remains, which are continually +falling upon the sea floor. In part, this waste is derived from +creatures which dwell upon the bottom; in considerable measure, +however, it is from the dead bodies of those forms which live near the +surface of the sea, and which when dying sink slowly through the +intermediate realm to the bottom. + +Owing to the absence of sunlight, the prevailingly cold water of the +deeper seas, and the lack of vegetation in those realms, the growth of +organic forms on the deep-sea floor is relatively slow. Thus it +happens that each shell or other contribution to the sediment lies for +some time on the bottom before it is buried. While in this condition +it is apt to be devoured by some of the many species which dwell on +the bottom and subsist from the remains of animals and plants which +they find there. In all cases the fossilization of any form depends +upon the accumulation of sediment before the processes of destruction +have overtaken them, and among these processes we must give the first +place to the creatures which subsist on shells, bones, or other +substances of like nature which find their way to the ocean floor. In +the absolute darkness, the still water, and the exceeding cold of the +deeper seas, animals find difficult conditions for development. +Moreover, in this deep realm there is no native vegetation, and, in +general, but little material of this nature descends to the bottom +from the surface of the sea. The result is, the animals have to +subsist on the remains of other animals which at some step in the +succession have obtained their provender from the plants which belong +on the surface or in the shallow waters of the sea. This limitation +of the food supply causes the depths of the sea to be a realm of +continual hunger, a region where every particle of organic matter is +apt to be seized upon by some needy creature. + +In consequence of the fact that little organic matter on the deeper +sea floors escapes being devoured, the most of the material of this +nature which goes into strata enters that state in a finely divided +condition. In the group of worms alone--forms which in a great +diversity of species inhabit the sea floor--we find creatures which +are specially adapted to digesting the _debris_ which gathers on the +sea bottom. Wandering over this surface, much in the manner of our +ordinary earthworms, these creatures devour the mud, voiding the +matter from their bodies in a yet more perfectly divided form. Hence +it comes about that the limestone beds, so commonly formed beneath the +open seas, are generally composed of materials which show but few and +very imperfect fossils. Studying any series of limestone beds, we +commonly find that each layer, in greater or less degree, is made up +of rather massive materials, which evidently came to their place in +the form of a limy mud. Very often this lime has crystallized, and +thus has lost all trace of its original organic structure. + +One of the conspicuous features which may be observed in any +succession of limestone beds is the partings or divisions into layers +which occur with varied frequency. Sometimes at vertical intervals of +not more than one or two inches, again with spacings of a score of +feet, we find divisional planes, which indicate a sudden change in the +process of rock formation. The lime disappears, and in place of it we +have a thin layer of very fine detritus, which takes on the form of a +clay. Examining these partings with care, we observe that on the upper +surface on the limestone the remains of the animal which dwelt on the +ancient sea floor are remarkably well preserved, they having evidently +escaped the effect of the process which reduced their ancestors, +whose remains constitute the layer, to mud. Furthermore, we note that +the shaly layer is not only lacking in lime, but commonly contains no +trace of animals such as might have dwelt on the bottom. The fossils +it bears are usually of species which swam in the overlying water and +came to the bottom after death. Following up through the layer of +shale, we note that the ordinary bottom life gradually reappears, and +shortly becomes so plentiful that the deposit resumes the character +which it had before the interruption began. Often, however, we note +that the assemblage of species which dwelt on the given area of sea +floor has undergone a considerable change. Forms in existence in the +lower layer may be lacking in the upper, their place being taken by +new varieties. + +So far the origin of these divisional planes in marine deposits has +received little attention from geologists; they have, indeed, assumed +that each of these alterations indicates some sudden disturbance of +the life of the sea floors. They have, however, generally assumed that +the change was due to alterations in the depth of the sea or in the +run of ocean currents. It seems to the writer, however, that while +these divisions may in certain cases be due to the above-mentioned +and, indeed, to a great variety of causes, they are in general best to +be explained by the action of earthquakes. Water being an exceedingly +elastic substance, an earthquake passes through it with much greater +speed than it traverses the rocks which support the ocean floor. The +result is that, when the fluid and solid oscillate in the repeated +swingings which a shock causes, they do not move together, but rub +over each other, the independent movements having the swing of from a +few inches to a foot or two in shocks of considerable energy. + +When the sea bottom and the overlying water, vibrating under the +impulse of an earthquake shock, move past each other, the inevitable +result is the formation of muddy water; the very fine silt of the +bottom is shaken up into the fluid, which afterward descends as a +sheet to its original position. It is a well-known fact that such +muddying of water, in which species accustomed to other conditions +dwell, inevitably leads to their death by covering their breathing +organs and otherwise disturbing the delicately balanced conditions +which enable them to exist. We find, in fact, that most of the tenants +of the water, particularly the forms which dwell upon the bottom, are +provided with an array of contrivances which enable them to clear away +from their bodies such small quantities of silt as may inconvenience +them. Thus, in the case of our common clam, the breathing organs are +covered with vibratory cilia, which, acting like brooms, sweep off any +foreign matter which may come upon their surfaces. Moreover, the +creature has a long, double, spoutlike organ, which it can elevate +some distance above the bottom, through which it draws and discharges +the water from which it obtains food and air. Other forms, such as the +crinoids, or sea lilies, elevate the breathing parts on top of tall +stems of marvellous construction, which brings those vital organs at +the level, it may be, of three or four feet above the zone of mud. In +consequence of the peculiar method of growth, the crinoids often +escape the damage done by the disturbance of the bottom, and thus form +limestone beds of remarkable thickness; sometimes, indeed, we find +these layers composed mainly of crinoidal remains, which exhibit only +slight traces of partings such as we have described, being essentially +united for the depth of ten or twenty feet. Where the layers have been +mainly accumulated by shellfish, their average thickness is less than +half a foot. + +When we examine the partitions between the layers of limestone, we +commonly find that, however thin, they generally extend for an +indefinite distance in every direction. The writer has traced some of +these for miles; never, indeed, has he been able to find where they +disappeared. This fact makes it clear that the destruction which took +place at the stage where these partings were formed was widespread; so +far as it was due to earthquake shocks, we may fairly believe that in +many cases it occurred over areas which were to be measured by tens of +thousands of square miles. Indeed, from what we know of earthquake +shocks, it seems likely that the devastation may at times have +affected millions of square miles. + +Another class of accidents connected with earthquakes may also +suddenly disturb the mud on the sea bottom. When, as elsewhere noted, +a shock originates beneath the sea, the effect is suddenly to elevate +the water over the seat of the jarring and the regions thereabouts to +the height of some feet. This elevation quickly takes the shape of a +ringlike wave, which rolls off in every direction from its point of +origin. Where the sea is deep, the effect of this wave on the bottom +may be but slight; but as the undulation attains shallower water, and +in proportion to the shoaling, the front of the surge is retarded in +its advance by the friction of the bottom, while the rear part, being +in deeper water, crowds upon the advancing line. The action is +precisely that which has been described as occurring in wind-made +waves as they approach the beach; but in this last-named group of +undulations, because of the great width of the swell, the effect of +the shallowing is evident in much deeper water. It is likely that at +the depth of a thousand feet the passing of one of these vast surges +born of earthquakes may so stir the mud of the sea floor as to bring +about a widespread destruction of life, and thus give rise to many of +the partitions between strata. + +If we examine with the microscope the fine-grained silts which make up +the shaly layers between limestones, we find the materials to be +mostly of inorganic origin. It is hard to trace the origin of the +mineral matter which it contains; some of the fragments are likely to +prove of Volcanic origin; others, bits of dust from meteorites; yet +others, dust blown from the land, which may, as we know, be conveyed +for any distance across the seas. Mingled with this sediment of an +inorganic origin we almost invariably find a share of organic waste, +derived not from creatures which dwelt upon the bottom, but from those +which inhabited the higher-lying waters. If, now, we take a portion of +the limestone layer which lies above or below the shale parting, and +carefully dissolve out with acids the limy matter which it contains, +we obtain a residuum which in general character, except so far as the +particles may have been affected by the acid, is exactly like the +material which forms the claylike partition. We are thus readily led +to the conclusion that on the floors of the deeper seas there is +constantly descending, in the form of a very slow shower, a mass of +mineral detritus. Where organic life belonging to the species which +secrete hard shells or skeletons is absent, this accumulation, +proceeding with exceeding slowness, gradually accumulates layers, +which take on a shaly character. Where limestone-making animals +abound, they so increase the rate of deposition that the proportion of +the mineral material in the growing strata is very much reduced; it +may, indeed, become as small as one per cent of the mass. In this case +we may say that the deposit of limestone grew a hundred times as fast +as the intervening beds of shale. + +The foregoing considerations make it tolerably clear that the sea +floor is in receipt of two diverse classes of sediment--those of a +mineral and those of an organic origin. The mineral, or inorganic, +materials predominate along the shores. They gradually diminish in +quantity toward the open sea, where the supply is mainly dependent on +the substances thrown forth from volcanoes, on pumice in its massive +or its comminuted form--i.e., volcanic dust, states of lava in which +the material, because of the vesicles which it contains, can float for +ages before it comes to rest on the sea bottom. Variations in the +volcanic waste contributed to the sea floor may somewhat affect the +quantity of the inorganic sediments, but, as a whole, the downfalling +of these fragments is probably at a singularly uniform rate. It is +otherwise with the contributions of sediment arising from organic +forms. This varies in a surprising measure. On the coral reefs, such +as form in the mid oceans, the proportion of matter which has not come +into the accumulation through the bodies of animals and plants may be +as small as one tenth of one per cent, or less. In the deeper seas, it +is doubtful whether the rate of animal growth is such as to permit the +formation of any beds which have less than one half of their mass made +up of materials which fell through the water. + +In certain areas of the open seas the upper part of the water is dwelt +in by a host of creatures, mostly foraminifera, which extract +limestone from the water, and, on dying, send their shells to the +bottom. Thus in the North Atlantic, even where the sea floor is of +great depth beneath the surface, there is constantly accumulating a +mass of limy matter, which is forming very massive limestone strata, +somewhat resembling chalk deposits, such as abundantly occur in Great +Britain, in the neighbouring parts of Europe, in Texas, and elsewhere. +Accumulations such as this, where the supply is derived from the +surface of the water, are not affected by the accidents which divide +beds made on the bottom in the manner before described. They may, +therefore, have the singularly continuous character which we note in +the English chalk, where, for the thickness of hundreds of feet, we +may have no evident partitions, except certain divisions, which have +evidently originated long after the beds were formed. + +We have already noted the fact that, while the floors of the deeper +seas appear to lack mountainous elevations, those arising from the +folding of strata, they are plentifully scattered over with volcanic +cones. We may therefore suppose that, in general, the deposits formed +on the sea floor are to a great extent affected by the materials which +these vents cast forth. Lava streams and showers represent only a +part of the contributions from volcanoes, which finally find their way +to the bottom. In larger part, the materials thrown forth are probably +first dissolved in the water and then taken up by the organic species; +only after the death of these creatures does the waste go to the +bottom. As hosts of these creatures have no solid skeleton to +contribute to the sea floor, such mineral matter as they may obtain is +after their death at once restored to the sea. + +Not only does the contribution of organic sediment diminish in +quantity with the depth which is attained, but the deeper parts of the +ocean bed appear to be in a condition where no accumulations of this +nature are made, and this for the reason that the water dissolves the +organic matter more rapidly than it is laid down. Thus in place of +limestone, which would otherwise form, we have only a claylike +residuum, such as is obtained when we dissolve lime rocks in acids. +This process of solution, by which the limy matter deposited on the +bottom is taken back into the water, goes on everywhere, but at a rate +which increases with the depth. This increase is due in part to the +augmentation of pressure, and in part to the larger share of carbonic +dioxide which the water at great depths holds. The result is, that +explorations with the dredge seem to indicate that on certain parts of +the deeper sea floors the rocks are undergoing a process of +dissolution comparable to that which takes place in limestone caverns. +So considerable is the solvent work that a large part of the inorganic +waste appears to be taken up by the waters, so as to leave the bottom +essentially without sedimentary accumulations. The sea, in a word, +appears to be eating into rocks which it laid down before the +depression attained its present great depth. + +We should here note something of the conditions which determine the +supply of food which the marine animals obtain. First of all, we may +recur to the point that the ocean waters appear to contain something +of all the earth materials which do not readily decompose when they +are taken into the state of solution. These mineral substances, +including the metals, are obtained in part from the lands, through the +action of the rain water and the waves, but perhaps in larger share +from the volcanic matter which, in the form of floating lava, pumice, +or dust, is plentifully delivered to the sea. Except doubtfully, and +at most in a very small way, this chemical store of the sea water can +not be directly taken into the structures of animals; it can only be +immediately appropriated by the marine plants. These forms can only +develop in that superficial realm of the seas which is penetrated by +the sunlight, or say within the depth of five hundred feet, mostly +within one hundred feet of the surface, about one thirtieth of the +average, and about one fiftieth of the maximum ocean depth. On this +marine plant life, and in a small measure on the vegetable matter +derived from the land, the marine animals primarily depend for their +provender. Through the conditions which bring about the formation of +_Sargassum_ seas, those areas of the ocean where seaweeds grow afloat, +as well as by the water-logging and weighting down of other vegetable +matter, some part of the plant remains is carried to the sea floor, +even to great depths; but the main dependence of the deep-sea forms of +animals is upon other animal forms, which themselves may have obtained +their store from yet others. In fact, in any deep-sea form we might +find it necessary to trace back the food by thousands of steps before +we found the creature which had access to the vegetable matter. It is +easy to see how such conditions profoundly limit the development of +organic being in the abysm of the ocean. + +The sedentary animals, or those which are fixed to the sea bottom--a +group which includes the larger part of the marine species--have to +depend for their sustenance on the movement of the water which passes +their station. If the seas were perfectly still, none of these +creatures except the most minute could be fed; therefore the currents +of the ocean go far by their speed to determine the rate at which life +may flourish. At great depths, as we have seen, these movements are +practically limited to that which is caused by the slow movement which +the tide brings about. The amount of this motion is proportional to +the depth of the sea; in the deeper parts, it carries the water to and +fro twice each day for the distance of about two hundred and fifty +feet. In the shallower water this motion increases in proportion to +the shoaling, and in the regions near the shores the currents of the +sea which, except the massive drift from the poles, do not usually +touch the bottom, begin to have their influence. Where the water is +less than a hundred feet in depth, each wave contributes to the +movement, which attains its maximum near the shore, where every surge +sweeps the water rapidly to and fro. It is in this surge belt, where +the waves are broken, that marine animals are best provided with food, +and it is here that their growth is most rapid. If the student will +obtain a pint of water from the surf, he will find that it is clouded +by fragments of organic matter, the quantity in a pound of the fluid +often amounting to the fiftieth part of its weight. He will thus +perceive that along the shore line, though the provision of victuals +is most abundant, the store is made from the animals and plants which +are ground up in the mill. In a word, while the coast is a place of +rapid growth, it is also a region of rapid destruction; only in the +case of the coral animals, which associate their bodies with a number +of myriads in large and elaborately organized communities, do we find +animals which can make such head against the action of the waves that +they can build great deposits in their realm. + +It should be noted that a part of the advantage which is afforded to +organic life by the shore belt is due to the fact that the waters are +there subjected to a constant process of aeration by the whipping into +foam and spray which occurs where the waves overturn. + +It will be interesting to the student to note the great number of +mechanical contrivances which have been devised to give security to +animals and plants which face these difficult conditions arising from +successive violent blows of falling water. Among these may be briefly +noted those of the limpets--mollusks which dwell in a conical shell, +which faces the water with a domelike outside, and which at the moment +of the stroke is drawn down upon the rock by the strong muscle which +fastens the creature to its foundation. The barnacles, which with +their wedge-shaped prows cut the water at the moment of the stroke, +but open in the pauses between the waves, so that the creature may +with its branching arms grasp at the food which floats about it; the +nullipores, forms of seaweed which are framed of limestone and cling +firmly to the rock--afford yet other instances of protective +adaptations contrived to insure the safety of creatures which dwell in +the field of abundant food supply. + + * * * * * + +The facts above presented will show the reader that the marine +sediments are formed under conditions which permit a great variety in +the nature of the materials of which they are composed. As soon as the +deposits are built into rocks and covered by later accumulations, +their materials enter the laboratory of the under earth, where they +are subjected to progressive changes. Even before they have attained a +great depth, through the laying down of later deposits upon them, +changes begin which serve to alter their structure. The fragments of a +soluble kind begin to be dissolved, and are redeposited, so that the +mass commonly becomes much more solid, passing from the state of +detritus to that of more or less solid rock. When yet more deeply +buried, and thereby brought into a realm of greater warmth, or perhaps +when penetrated by dikes and thereby heated, these changes go yet +further. More of the material is commonly rearranged by solution and +redeposition, so that limestone may be converted into crystalline +marble, granular sandstones into firm masses, known as quartzites, and +clays into the harder form of slate. Where the changes go to the +extreme point, rocks originally distinctly bedded probably may be so +taken to pieces and made over that all traces of their stratification +may be destroyed, all fossils obliterated, and the stone transformed +into mica schist, or granite or other crystalline rock. It may be +injected into the overlying strata in the form of dikes, or it may be +blown forth into the air through volcanoes. Involved in +mountain-folding, after being more or less changed in the manner +described, the beds may become tangled together like the rumpled +leaves of a book, or even with the complexity of snarled thread. All +these changes of condition makes it difficult for the geologist to +unravel the succession of strata so that he may know the true order of +the rocks, and read from them the story of the successive geological +periods. This task, though incomplete, has by the labours of many +thousand men been so far advanced that we are now able to divide the +record into chapters, the divisions of the geologic ages, and to give +some account of the succession of events, organic and geographic, +which have occurred since life began to write its records. + + + EARTHQUAKES. + +In ordinary experience we seem to behold the greater part of the earth +which meets our eyes as fixed in its position. A better understanding +shows us that nothing in this world is immovable. In the realm of the +inorganic world the atoms and molecules even in solid bodies have to +be conceived as endowed with ceaseless though ordered motions. Even +when matter is built into the solid rock, it is doubtful whether any +grain of it ever comes really to rest. Under the strains which arise +from the contraction of the earth's interior and the chemical changes +which the rocks undergo, each bit is subject to ever-changing +thrusts, which somewhat affect its position. If we in any way could +bring a grain of sand from any stratum under a microscope, so that we +could perceive its changes of place, we should probably find that it +was endlessly swaying this way and that, with reference to an ideally +fixed point, such as the centre of the earth. But even that centre, +whether of gravity or of figure, is probably never at rest. + +Earth movements may be divided into two groups--those which arise from +the bodily shifting of matter, which conveys the particles this way or +that, or, as we say, change their place, and those which merely +produce vibration, in which the particles, after their vibratory +movement, return to their original place. For purposes of illustration +the first, or translatory motion, may be compared to that which takes +place when a bell is carried along upon a locomotive or a ship; and +the second, or vibratory movement, to what takes place when the bell +is by a blow made to ring. It is with these ringing movements, as we +may term them, that we find ourselves concerned when we undertake the +study of earthquakes. + +It is desirable that the reader should preface his study of +earthquakes by noting the great and, at the same time, variable +elasticity of rocks. In the extreme form this elasticity is very well +shown when a toy marble, which is made of a close-textured rock, such +as that from which it derives its name, is thrown upon a pavement +composed of like dense material. Experiment will show that the little +sphere can often be made to bounce to the height of twenty feet +without breaking. If, then, with the same energy the marble is thrown +upon a brick floor, the rebound will be very much diminished. It is +well to consider what happens to produce the rebound. When the sphere +strikes the floor it changes its shape, becoming shorter in the axis +at right angles to the point which was struck, and at the same instant +expanded along the equator of that axis. The flattening remains for +only a small fraction of a second; the sphere vibrates so that it +stretches along the line on which it previously shortened, and, as +this movement takes place with great swiftness, it may be said to +propel itself away from the floor. At the same time a similar movement +goes on in the rock of the floor, and, where the rate of vibration is +the same, the two kicks are coincident, and so the sphere is impelled +violently away from the point of contact. Where the marble comes in +contact with brick, in part because of the lesser elasticity of that +material, due to its rather porous structure, and partly because it +does not vibrate at the same rate as the marble, the expelling blow is +much less strong. + +All rocks whatever, even those which appear as incoherent sands, are +more or less set into vibratory motion whenever they are struck by a +blow. In the crust of the earth various accidents occur which may +produce that sudden motion which we term a blow. When we have examined +into the origin of these impulses, and the way in which they are +transmitted through the rocks, we obtain a basis for understanding +earthquake shocks. The commonest cause of the jarrings in the earth is +found in the formation of fractures, known as faults. If the reader +has ever been upon a frozen lake at a time when the weather was +growing colder, and the ice, therefore, was shrinking, he may have +noted the rending sound and the slight vibration which comes with the +formation of a crack traversing the sheet of ice. At such a time he +feels a movement which is an earthquake, and which represents the +simpler form of those tremors arising from the sudden rupture of fault +planes. If he has a mind to make the experiment, he may hang a bullet +by a thread from a small frame which rests upon the ice, and note that +as the vibration occurs the little pendulum sways to and fro, thus +indicating the oscillations of the ice. The same instrument will move +in an identical manner when affected by a quaking in the rocks. + +Where the rocks are set in vibration by a rent which is formed in +them, the phenomena are more complicated, and often on a vastly larger +scale than in the simple conditions afforded by a sheet of ice. The +rocks on either side of the rupture generally slide over each other, +and the opposing masses are rent in their friction upon one another; +the result is, not only the first jar formed by the initial fracture, +but a great many successive movements from the other breakages which +occur. Again, in the deeper parts of the crust, the fault fissures are +often at the moment of their formation filled by a violent inrush of +liquid rock. This, as it swiftly moves along, tears away masses from +the walls, and when it strikes the end of the opening delivers a blow +which may be of great violence. The nature of this stroke may be +judged by the familiar instance where the relatively slow-flowing +stream from a hydrant pipe is suddenly choked by closing the stopcock. +Unless the plumber provides a cushion of air to diminish the energy of +the blow, it is often strong enough to shake the house. Again, when +steam or other gases are by a sudden diminution of pressure enabled to +expand, they may deliver a blow which is exactly like that caused by +the explosion of gunpowder, which, even when it rushes against the +soft cushion of the air, may cause a jarring that may be felt as well +as heard to a great distance. Such movements very frequently occur in +the eruptions of volcanoes; they cause a quivering of the earth, which +may be felt for a great distance from the immediate seat of the +disturbance. + +When by any of the sudden movements which have been above described a +jar is applied to the rocks, the wave flies through the more or less +elastic mass until the energy involved in it is exhausted. This may +not be brought about until the motion has travelled for the distance +of hundreds of miles. In the great earthquake of 1755, known as the +Lisbon shock, the records make it seem probable that the movement was +felt over one eighth part of the earth's surface. Such great +disturbances probably bring about a motion of the rocks near the point +of origin, which may be expressed in oscillations having an amplitude +of one to two feet; but in the greater number of earthquakes the +maximum swing probably does not exceed the tenth of that amount. Very +sensible shaking, even such as may produce considerable damage to +buildings, are caused by shocks in which the earth vibrates with less +than an inch of swing. + +When a shock originates, the wave in the rocks due to the compression +which the blow inflicts runs at a speed varying with the elasticity of +the substance, but at the rate of about fifteen hundred feet a second. +The movements of this wave are at right angles to the seat of the +originating disturbance, so that the shock may come to the surface in +a line forming any angle between the vertical and the nearly +horizontal. Where, as in a volcanic eruption, the shock originates +with an explosion, these waves go off in circles. Where, however, as +is generally the case, the shock originates in a fault plane, which +may have a length and depth of many miles, the movement has an +elliptical form. + +If the earthquake wave ran through a uniform and highly elastic +substance, such as glass, it would move everywhere with equal speed, +and, in the case of the greater disturbances, the motion might be felt +over the whole surface of the earth. But as the motion takes place +through rocks of varying elasticity, the rate at which it journeys is +very irregular. Moving through materials of one density, and with a +rate of vibration determined by those conditions, the impulse is with +difficulty communicated to strata which naturally vibrate at another +speed. In many cases, as where a shock passing through dense +crystalline strata encounters a mass of soft sandstone, the wave, in +place of going on, is reflected back toward its point of origin. These +earthquake echoes sometimes give rise to very destructive movements. +It often happens that before the original tremors of a shock have +passed away from a point on the surface the reflex movements rush in, +making a very irregular motion, which may be compared to that of the +waves in a cross-sea. + +The foregoing account of earthquake action will serve to prepare the +reader for an understanding of those very curious and important +effects which these accidents produce in and on the earth. Below the +surface the sensible action of earthquake shocks is limited. It has +often been observed that people in mines hardly note a swaying which +may be very conspicuous to those on the surface, the reason for this +being that underground, where the rocks are firmly bound together, all +those swingings which are due to the unsupported position of such +objects as buildings, columnar rocks, trees, and the waters of the +earth, are absent. The effect of the movements which earthquakes +impress on the under earth is mainly due to the fact that in almost +every part of the crust tensions or strains of other kinds are +continually forming. These may for ages prove without effect until the +earth is jarred, when motions will suddenly take place which in a +moment may alter the conditions of the rocks throughout a wide field. +In a word, a great earthquake caused by the formation of an extensive +fault is likely to produce any number of slight dislocations, each of +which is in turn shock-making, sending its little wave to complicate +the great oscillation. Nor does the perturbing effect of these jarring +movements cease with the fractures which they set up and the new +strains which are in turn developed by the motions which they induce. +The alterations of the rocks which are involved in chemical changes +are favoured by such motions. It is a familiar experience that a +vessel of water, if kept in the state of repose, may have its +temperature lowered three or four degrees below the freezing point +without becoming frozen. If the side of the vessel is then tapped with +the finger, so as to send a slight quake through the mass, it will +instantly congeal. Molecular rearrangements are thus favoured by +shocks, and the consequences of those which run through the earth are, +from a chemical point of view, probably important. + +The reader may help himself to understand something of the complicated +problem of earth tensions, and the corresponding movements of the +rocks, by considering certain homely illustrations. He may observe how +the soil cracks as it shrinks in times of drought, the openings +closing when it rains. In a similar way the frozen earth breaks open, +sometimes with a shock which is often counted as an earthquake. Again, +the ashes in a sifter or the gravel on a sieve show how each shaking +may relieve certain tensions established by gravity, while they create +others which are in turn to be released by the next shock. An ordinary +dwelling house sways and strains with the alternations of temperature +and moisture to which it is subjected in the round of climatal +alterations. Now and then we note the movements in a cracking sound, +but by far the greater part of them escape observation. + +With this sketch of the mechanism of earthquake shocks we now turn to +consider their effects upon the surface of the earth. From a +geological point of view, the most important effect of earthquake +shocks is found in the movement of rock masses down steep slopes, +which is induced by the shaking. Everywhere on the land the agents of +decay and erosion tend to bring heavy masses into position where +gravitation naturally leads to their downfall, but where they may +remain long suspended, provided they are not disturbed. Thus, wherever +there are high and steep cliffs, great falls of rock are likely to +occur when the earthquake movements traverse the under earth. In more +than one instance observers, so placed that they commanded a view of +distant mountains, have noticed the downfall of precipices in the path +of the shock before the trembling affected the ground on which they +stood. In the famous earthquake of 1783, which devastated southern +Italy, the Prince of Scylla persuaded his people to take refuge in +their boats, hoping that they might thereby escape the destruction +which threatened them on the land. No sooner were the unhappy folk on +the water than the fall of neighbouring cliffs near the sea produced a +great wave, which overwhelmed the vessels. + +Where the soil lies upon steep slopes, in positions in which it has +accumulated during ages of tranquillity, a great shock is likely to +send it down into the valleys in vast landslides. Thus, in the +earthquake of 1692, the Blue Mountains of Jamaica were so violently +shaken that the soil and the forests which stood on it were +precipitated into the river beds, so that many tree-clad summits +became fields of bare rock. The effect of this action is immensely to +increase the amount of detritus which the streams convey to the sea. +After the great Jamaica shock, above noted, the rivers for a while +ceased to flow, their waters being stored in the masses of loose +material. Then for weeks they poured forth torrents of mud and the +_debris_ of vegetation--materials which had to be swept away as the +streams formed new channels. + +In all regions where earthquake movements are frequent, and the shock +of considerable violence, the trained observer notes that the surfaces +of bare rock are singularly extensive, the fact being that many of +these areas, where the slope lies at angles of from ten to thirty +degrees, which in an unshaken region would be thickly soil-covered, +are deprived of the coating by the downward movement of the waste +which the disturbances bring about. A familiar example of this action +may be had by watching the workmen engaged in sifting sand, by casting +the material on a sloping grating. The work could not be done but for +an occasional blow applied to the sifter. An arrangement for such a +jarring motion is commonly found in various ore-dressing machines, +where the object is to move fragments of matter over a sloping +surface. + +Even where the earth is so level that an earthquake shock does not +cause a sliding motion of the materials, such as above described, +other consequences of the shaking may readily be noted. As the motion +runs through the mass, provided the movement be one of considerable +violence, crevices several feet in width, and sometimes having the +length of miles, are often formed. In most cases these fissures, +opened by one pulsation of the shock, are likely to be closed by the +return movement, which occurs the instant thereafter. The consequences +of this action are often singular, and in cases constitute the most +frightful elements of a shock which the sufferer beholds. In the great +earthquake of 1811, which ravaged the section of the Mississippi +Valley between the mouth of the Ohio and Vicksburg, these crevices +were so numerously formed that the pioneers protected themselves from +the danger of being caught in their jaws by felling trees so that they +lay at right angles to the direction in which the rents extended, +building on these timbers platforms to support their temporary +dwelling places. The records of earthquakes supply many instances in +which people have been caught in these earth fissures, and in a single +case it is recorded that a man who disappeared into the cavity was in +a moment cast forth in the rush of waters which in this, as in many +other cases, spouts forth as the walls of the opening come together. + +Sometimes these rents are attended by a dislocation, which brings the +earth on one side much higher than on the other. The step thus +produced may be many miles in length, and may have a height of twenty +feet or more. It needs no argument to show that we have here the top +of a fault such as produced the shock, or it may be one of a secondary +nature, such as any earthquake is likely to bring about in the strata +which it traverses. In certain cases two faults conjoin their action, +so that a portion of the surface disappears beneath the earth, +entombing whatever may have stood on the vanished site. Thus in the +great shock known as that of Lisbon, which occurred in 1755, the stone +quay along the harbour, where many thousand people had sought refuge +from the falling buildings of the city, suddenly sank down with the +multitude, and the waters closed over it; no trace of the people or of +the structure was to be found after the shock was over. There is a +story to the effect that during the same earthquake an Arab village in +northern Africa sank down, the earth on either side closing over it, +so that no trace of the habitations remained. In both these instances +the catastrophes are best explained by the diagram. + +[Illustration: Fig. 21.--Diagram showing how a portion of the earth's +surface may be sunk by faulting. Fig. A shows the original position; +B, the position after faulting; b b' and c c' the planes of the +faults; the arrows the direction of the movement.] + +In the earthquake of 1811 the alluvial plains on either side of the +Mississippi at many points sank down so that arable land was converted +into lakes; the area of these depressions probably amounted to some +hundred square miles. The writer, on examining these sunken lands, +found that the subsidences had occurred where the old moats or +abandoned channels of the great river had been filled in with a +mixture of decaying timber and river silt. When violently shaken, this +loose-textured _debris_ naturally settled down, so that it formed a +basin occupied by a crescent-shaped lake. The same process of settling +plentifully goes on wherever the rocks are still in an uncemented +state. The result is often the production of changes which lead to the +expulsion of gases. Thus, in the Charleston earthquake of 1883, the +surface over an area of many hundred square miles was pitted with +small craters, formed by the uprush of water impelled by its contained +gases. These little water volcanoes--for such we may call +them--sometimes occur to the number of a dozen or more on each acre of +ground in the violently shaken district. They indicate one result of +the physical and chemical alterations which earthquake shocks bring +about. As earthquakes increase in violence their effect upon the soil +becomes continually greater, until in the most violent shocks all the +loose materials on the surface of the earth may be so shaken about as +to destroy even the boundaries of fields. After the famous earthquake +of Riobamba, which occurred on the west coast of South America in +1797, the people of the district in which the town of that name was +situated were forced to redivide their land, the original boundaries +having disappeared. Fortunately, shocks of this description are +exceedingly rare. They occur in only a few parts of the world. + +Certain effects of earthquakes where the shock emerges beneath the sea +have been stated in the account of volcanic eruptions (see page 299). +We may therefore note here only certain of the more general facts. +While passing through the deep seas, this wave may have a height of +not more than two or three feet and a width of some score miles. As it +rolls in upon the shore the front of the undulation is retarded by the +friction of the bottom in such a measure that its speed is diminished, +while the following part of the waves, being less checked, crowds up +toward this forward part. The result is, that the surge mounts ever +higher and higher as it draws near the shore, upon which it may roll +as a vast wave having the height of fifty feet or more and a width +quite unparalleled by any wave produced from wind action. Waves of +this description are most common in the Pacific Ocean. Although but +occasional, the damage which they may inflict is very great. As the +movement approaches the shore, vessels, however well anchored, are +dragged away to seaward by the great back lash of the wave, a +phenomenon which may be perceived even in the case of the ordinary +surf. Thus forced to seaward, the crews of the ships may find their +vessels drawn out for the distance of some miles, until they come near +the face of the advancing billow. This, as it approaches the shore, +straightens up to the wall-fronted form, and then topples upon the +land. Those vessels which are not at once crushed down by the blow are +generally hurled far inland by the rush of waters. In the great +Jamaica earthquake of 1692 a British man-of-war was borne over the +tops of certain warehouses and deposited at a distance from the shore. + +Owing to the fact that water is a highly elastic material, the shocks +transmitted to it from the bottom are sent onward with their energy +but little diminished. While the impulse is very violent, these +oscillations may prove damaging to shipping. The log-books of mariners +abound in stories of how vessels were dismasted or otherwise badly +shaken by a sudden blow received in the midst of a quiet sea. The +impression commonly conveyed to the sailors is that the craft has +struck upon a rock. The explanation is that an earthquake jar, in +traversing the water, has delivered its blow to the ship. As the speed +of this jarring movement is very much greater than that of any +ordinary wave, the blow which it may strike may be most destructive. +There seems, indeed, little reason to doubt that a portion of the +vessels which are ever disappearing in the wilderness of the ocean are +lost by the crushing effect of these quakings which pass through the +waters of the deep. + +We have already spoken of the earthquake shock as an oscillation. It +is a quality of all bodies which oscillate under the influence of a +blow, such as originates in earthquake shocks, to swing to and fro, +after the manner of the metal in a bell or a tuning fork, in a +succession of movements, each less than the preceding, until the +impulse is worn out, or rather, we should in strict sense say, +changed to other forms of energy. The result is, that even in the +slightest earthquake shock the earth moves not once to and fro, but +very many times. In a considerable shock the successive diminishing +swingings amount to dozens before they become so slight as to elude +perception. Although the first swaying is the strongest, and generally +the most destructive, the quick to-and-fro motions are apt to continue +and to complete the devastation which the first brings about. The +vibrations due to any one shock take place with great rapidity. They +may, indeed, be compared to those movements which we perceive in the +margin of a large bell when it has received a heavy blow from the +clapper. The reader has perhaps seen that for a moment the rim of the +bell vibrates with such rapidity that it has a misty look--that is, +the motions elude the sight. It is easy to see that a shaking of this +kind is particularly calculated to disrupt any bodies which stand free +in the air and are supported only at their base. + +In what we may call the natural architecture of the earth, the +pinnacles and obelisks, such as are formed in many high countries, the +effect of these shakings is destructive, and, as we have seen, even +the firmer-placed objects, such as the strong-walled cliffs and steep +slopes of earth, break down under the assaults. It is therefore no +matter of surprise that the buildings which man erects, where they are +composed of masonry, suffer greatly from these tremblings. In almost +all cases human edifices are constructed without regard to other +problems of strength than those which may be measured by their weight +and the resistance to fracture from gravitation alone. They are not +built with expectation of a quaking, but of a firm-set earth. + +The damage which earthquakes do to buildings is in most cases due to +the fact that they sway their walls out of plumb, so that they are no +longer in position to support the weight which they have to bear. The +amount of this swaying is naturally very much greater than that which +the earth itself experiences in the movement. A building of any height +with its walls unsupported by neighbouring structures may find its +roof rocked to and fro through an arc which has a length of feet, +while its base moves only through a length of inches. The reader may +see an example of this nature if he will poise a thin book or a bit of +plank a foot long on top of a small table; then jarring the table so +that it swings through a distance of say a quarter of an inch, he will +see that the columnar object swings at its top through a much greater +distance, and is pretty sure to be overturned. + +Where a building carries a load in its upper parts, such as may be +afforded by its heavy roof or the stores which it contains, the effect +of an earthquake shock such as carries the earth to and fro becomes +much more destructive than it might otherwise be. This weight lags +behind when the earth slips forward in the first movement of the +oscillation, with the effect that the walls of the building are pretty +sure to be thrust so far beyond the perpendicular that they give way +and are carried down by the weight which they bore. It has often been +remarked in earthquake shocks that tall columns, even where composed +of many blocks, survive a shock which overturns lower buildings where +thin walls support several floors, on each of which is accumulated a +considerable amount of weight. In the case of the column, the strains +are even, and the whole structure may rock to and fro without toppling +over. As the energy of the undulations diminish, it gradually regains +the quiet state without damage. In the ordinary edifice the irregular +disposition of the weight does not permit the uniform movement which +may insure safety. Thus, if the city of Washington should ever be +violently shaken, the great obelisk, notwithstanding that it is five +hundred feet high, may survive a disturbance which would wreck the +lower and more massive edifices which lie about it. + +Where, as is fortunately rarely the case, the great shock comes to +the earth in a vertical direction, the effect upon all movable objects +is in the highest measure disastrous. In such a case buildings are +crushed as if by the stroke of a giant's hand. The roofs and floors +are at one stroke thrown to the foundations, and all the parts of the +walls which are not supported by strong masonry continuous from top to +bottom are broken to pieces. In such cases it has been remarked that +the bodies of men are often thrown considerable distances. It is +asserted, indeed, that in the Riobamba shock they were cast upward to +the height of more than ninety feet. It is related that the solo +survivor of a congregation which had hastened at the outset of the +disturbance into a church was thrown by the greatest and most +destructive shock upward and through a window the base of which was at +the height of more than twenty feet from the ground. + +It is readily understood that an earthquake shock may enter a building +in any direction between the vertical and the horizontal. As the +movement exhausts itself in passing from the place of its origin, the +horizontal shocks are usually of least energy. Those which are +accurately vertical are only experienced where the edifices are placed +immediately over the point where the motion originates. It follows, +therefore, that the destructive work of earthquakes is mainly +performed in that part of the field where the motion is, as regards +its direction, between the vertical and the horizontal--a position in +which the edifice is likely to receive at once the destructive effect +arising from the sharp upward thrust of the vertical movement and the +oscillating action of that which is in a horizontal direction. Against +strains of this description, where the movements have an amplitude of +more than a few inches, no ordinary masonry edifice can be made +perfectly safe; the only tolerable security is attained where the +building is of well-framed timber, which by its elasticity permits a +good deal of motion without destructive consequences. Even such +buildings, however, those of the strongest type, may be ruined by the +greater earthquakes. Thus, in the Mississippi Valley earthquake of +1811, the log huts of the frontiersmen, which are about as strong as +any buildings can be made, were shaken to pieces by the sharp and +reiterated shocks. + +It is by no means surprising to find that the style of architecture +adopted in earthquake countries differs from that which is developed +in regions where the earth is firm-set. The people generally learn +that where their buildings must meet the trials of earthquakes they +have to be low and strong, framed in the manner of fortifications, to +withstand the assault of this enemy. We observe that Gothic +architecture, where a great weight of masonry is carried upon slender +columns and walls divided by tall windows, though it became the +dominant style in the relatively stable lands of northern Europe, +never gained a firm foothold in those regions about the Mediterranean +which are frequently visited by severe convulsions of the earth. There +the Grecian or the Romanesque styles, which are of a much more massive +type, retain their places and are the fashions to the present day. +Even this manner of building, though affording a certain security +against slight tremblings, is not safe in the greater shocks. Again +and again large areas in southern Italy have been almost swept of +their buildings by the destructive movements which occur in that +realm. The only people who have systematically adapted their +architectural methods to earthquake strains are the Japanese, who in +certain districts where such risks are to be encountered construct +their dwellings of wood, and place them upon rollers, so that they may +readily move to and fro as the shock passes beneath them. In a measure +the people of San Francisco have also provided against this danger by +avoiding dangerous weights in the upper parts of their buildings, as +well as the excessive height to which these structures are lifted in +some of our American towns. + +Earthquakes of sensible energy appear to be limited to particular +parts of the earth's crust. The regions, indeed, where within the +period of human history shocks of devastating energy have occurred do +not include more than one fifteenth part of the earth's surface. There +is a common notion that these movements are most apt to happen in +volcanic regions. It is, indeed, true that sensible shocks commonly +attend the explosions from great craters, but the records clearly show +that these movements are very rarely of destructive energy. Thus in +the regions about the base of Vesuvius and of AEtna, the two volcanoes +of which most is known, the shocks have never been productive of +extensive disaster. In fact, the reiterated slight jarrings which +attend volcanic action appear to prevent the formation of those great +and slowly accumulated strains which in their discharge produce the +most violent tremblings of the earth. The greatest and most continuous +earthquake disturbances of history--that before noted in the early +days of this century, in the Mississippi Valley, where shocks of +considerable violence continued for two years--came about in a field +very far removed from active volcanoes. So, too, the disturbances +beneath the Atlantic floor which originated the shocks that led to the +destruction of Lisbon, and many other similar though less violent +movements, are developed in a field apparently remote from living +volcanoes. Eastern New England, which has been the seat of several +considerable earthquakes, is about as far away from active vents as +any place on the habitable globe. We may therefore conclude that, +while volcanoes necessarily produce shocks resulting from the +discharge of their gases and the intrusion of lava into the dikes +which are formed about them, the greater part of the important shocks +are in no wise connected with volcanic explosions. + +With the exception of the earthquake in the Mississippi Valley, all +the great shocks of which we have a record have occurred in or near +regions where the rocks have been extensively disturbed by +mountain-building forces, and where the indications lead us to +believe that dislocations of strata, such as are competent to rive the +beds asunder, may still be in progress. This, taken in connection with +the fact that many of these shocks are attended by the formation of +fault planes, which appear on the surface, lead us to the conclusion +that earthquakes of the stronger kind are generally formed by the +riving of fissures, which may or may not be developed upward to the +surface. This view is supported by many careful observations on the +effect which certain great earthquakes have exercised on the buildings +which they have ravaged. The distinguished observer, Mr. Charles +Mallet, who visited the seat of the earthquake which, in 1854, +occurred in the province of Calabria in Italy, with great labour and +skill determined the direction in which the shock moved through some +hundreds of edifices on which it left the marks of its passage. +Platting these lines of motion, he found that they were all referred +to a vertical plane lying at the depth of some miles beneath the +surface, and extending for a great distance in a north and south +direction. This method of inquiry has been applied to other fields, +with the result that in the case of all the instances which have been +subjected to this inquiry the seat of the shock has been traced to +such a plane, which can best be accounted for by the supposition of a +fault. + +The method pursued by Mr. Mallet in his studies of the origin of +earthquakes, and by those who have continued his inquiry, may be +briefly indicated as follows: Examining disrupted buildings, it is +easy to determine those which have been wrecked by a shock that +emerged from the earth in a vertical direction. In these cases, though +tall walls may remain standing, the roofs and floors are thrown into +the cellars. With a dozen such instances the plane of what is called +the seismic vertical is established (_seismos_ is the Greek for +earthquake). Then on either side of this plane, which indicates the +line but not the depth of the disturbance, other observations may be +made which give the clew to the depth. Thus a building may be found +where the northwest corner at its upper part has been thrown off. Such +a rupture was clearly caused by an upward but oblique movement, which +in the first half of the oscillation heaved the structure upwardly +into the northwest, and then in the second half, or rebound, drew the +mass of the building away from the unsupported corner, allowing that +part of the masonry to fly off and fall to the ground. Constructing a +line at right angles to the plane of the fracture, it will be found to +intersect the plane, the position of which has been in part determined +by finding the line where it intersects the earth, or the seismic +vertical before noted. Multiplying such observations on either side of +the last-mentioned line, the attitude of the underground parts of the +plane, as well as the depth to which it attained, can be approximately +determined. + +It is worth while to consider the extent to which earthquake shocks +may affect the general quality of the people who dwell in countries +where these disturbances occur with such frequency and violence as to +influence their lives. There can be no question that wherever +earthquakes occur in such a measure as to produce widespread terror, +where, recurring from time to time, they develop in men a sense of +abiding insecurity, they become potent agents of degradation. All the +best which men do in creating a civilization rests upon a sense of +confidence that their efforts may be accumulated from year to year, +and that even after death the work of each man may remain as a +heritage to his kind. It is likely, indeed, that in certain realms, as +in southern Italy, a part of the failure of the people to advance in +culture is due to their long experience of such calamities, and the +natural expectation that they will from time to time recur. In a +similar way the Spanish settlements in Central and South America, +which lie mostly in lands that are subject to disastrous shocks, may +have been retarded by the despair, as well as the loss of property +and life, which these accidents have so frequently inflicted upon +them. It will not do, however, to attribute too much to such +terrestrial influences. By far the most important element in +determining the destiny of a people is to be found in their native +quality, that which they owe to their ancestors of distant +generations. In this connection it is well to consider the history of +the Icelandic people, where a small folk has for a thousand years been +exposed to a range and severity of trials, such as earthquakes, +volcanic explosions, and dearth of harvests may produce, and all these +in a measure that few if any other countries experience. +Notwithstanding these misfortunes, the Icelanders have developed and +maintained a civilization which in all else, except its material +results, on the average transcends that which has been won by any +other folk in modern times. If a people have the determining spirit +which leads to high living, they can successfully face calamities far +greater than those which earthquakes inflict. + +It was long supposed that the regions where earthquakes are not +noticeable by the unaided senses were exempt from all such +disturbances. The observations which seismologists have made in recent +years point to the conclusion that no part of the earth's surface is +quite exempt from movements which, though not readily perceived, can +be made visible by the use of appropriate instruments. With an +apparatus known as the horizontal pendulum it is possible to observe +vibrations which do not exceed in amplitude the hundredth part of an +inch. This mechanism consists essentially of a slender bar supported +near one end by two wires, one from above, the other from below. It +may readily be conceived that any measurable movement will cause the +longer end of the rod to sway through a considerable arc. Wherever +such a pendulum has been carefully observed in any district, it has +been found that it indicates the occurrence of slight tremors. Even +certain changes of the barometer, which alter the weight of the +atmosphere that rests upon the earth to the amount indicated by an +inch in the height of the mercury column, appears in all cases to +create such tremors. Many of these slight shocks may be due to the +effect of more violent quakings, which have run perhaps for thousands +of miles from their point of origin, and have thus been reduced in the +amplitude of their movement. Others are probably due to the slight +motion brought about through the chemical changes of the rocks, which +are continuously going on. The ease with which even small motions are +carried to a great distance may be judged by the fact that when the +ground is frozen the horizontal pendulum will indicate the jarring due +to a railway train at the distance of a mile or more from the track. + +In connection with the earth jarring, it would be well to note the +occurrence of another, though physically different, kind of movement, +which we may term earth swayings, or massive movements, which slowly +dislocate the vertical, and doubtless also the horizontal, position of +points upon its surface. It has more than once been remarked that in +mountain countries, where accurate sights have been taken, the heights +of points between the extremities of a long line appear somewhat to +vary in the course of a term of years. Thus at a place in the +Apennines, where two buildings separated by some miles of distance are +commonly intervisible over the crest of a neighbouring peak, it has +happened that a change of level of some one of the points has made it +impossible to see the one edifice from the other. Knowing as we do +that the line of the seacoast is ever-changing, uprising taking place +at some points and down-sinking at others, it seems not unlikely that +these irregular swayings are of very common occurrence. Moreover, +astronomers are beginning to remark the fact that their observatories +appear not to remain permanently in the same position--that is, they +do not have exactly the same latitude and longitude. Certain of these +changes have recently been explained by the discovery of a new and +hitherto unnoted movement of the polar axis. It is not improbable, +however, that the irregular swaying of the earth's crust, due to the +folding of strata and to the alterations in the volume of rocks which +are continually going on, may have some share in bringing about these +dislocations. + +Measured by the destruction which was wrought to the interests of man, +earthquakes deserve to be reckoned among the direst calamities of +Nature. Since the dawn of history the records show us that the +destruction of life which is to be attributed to them is to be counted +by the millions. A catalogue of the loss of life in the accidents of +this description which have occurred during the Christian era has led +the writer to suppose that probably over two million persons have +perished from these shocks in the last nineteen centuries. +Nevertheless, as compared with other agents of destruction, such as +preventable disease, war, or famine, the loss which has been inflicted +by earth movements is really trifling, and almost all of it is due to +an obstinate carelessness in the construction of buildings without +reference to the risks which are known to exist in earthquake-ridden +countries. + +Although all our exact knowledge concerning the distribution of +earthquakes is limited to the imperfect records of two or three +thousand years, it is commonly possible to measure in a general way +the liability to such accidents which may exist in any country by a +careful study of the details of its topography. In almost every large +area the process of erosion naturally leaves quantities of rock, +either in the form of detached columns or as detrital accumulations +deposited on steep slopes. These features are of relatively slow +formation, and it is often possible to determine that they have been +in their positions for a time which is to be measured by thousands of +years. Thus, on inspecting a country such as North America, where the +historic records cover but a brief time, we may on inquiry determine +which portions of its area have long been exempt from powerful shocks. +Where natural obelisks and steep taluses abound--features which would +have disappeared if the region had been moved by great shocks--we may +be sure that the field under inspection has for a great period been +exempt from powerful shaking. Judged by this standard, we may safely +say that the region occupied by the Appalachian Mountains has been +exempt from serious trouble. So, too, the section of the Cordilleras +lying to the east of what is commonly called the Great Basin, between +the Rocky Mountains and the Sierra Nevada, has also enjoyed a long +reign of peace. In glaciated countries the record is naturally less +clear than in those parts of the world which have been subjected to +long-continued, slow decay of the rocks. Nevertheless, in those fields +boulders are often found poised in position which they could not have +maintained if subjected to violent shaking. Judged by this evidence, +we may say that a large part of the northern section of this +continent, particularly the area about the Great Lakes, has been +exempt from considerable shocks since the glacier passed away. + +The shores which are subject to the visitations of the great marine +waves, caused by earthquake shocks occurring beneath the bottom of the +neighbouring ocean, are so swept by those violent inundations that +they lose many features which are often found along coasts that have +been exempted from such visitations. Thus wherever we find extensive +and delicately moulded dunes, poised stones, or slender pinnacled +rocks along a coast, we may be sure that since these features were +formed the district has not been swept by these great waves. + +[Illustration: Fig. 22.--Poised rocks indicating a long exemption from +strong earthquakes in the places where such features occur.] + +Around the northern Atlantic we almost everywhere find the glacial +waste here and there accumulated near the margin of the sea in the +complicated sculptured outlines which are assumed by kame sands and +gravels. From a study of these features just above the level of high +tide, the writer has become convinced that the North Atlantic district +has long been exempt from the assaults of other waves than those which +are produced during heavy storms. At the present time the waves +formed by earthquakes appear to be of destructive violence only on the +west coast of South America, where they roll in from a region of the +Pacific lying to the south of the equator and a few hundred miles from +the shore of the continent, which appears to be the seat of +exceedingly violent shocks. A similar field occurs in the Atlantic +between the Lesser Antilles and the Spanish peninsula, but no great +waves have come thence since the time of the Lisbon earthquake. The +basin of the Caribbean and the region about Java appear to be also +fields where these disturbances may be expected, though in each but +one wave of this nature has been recorded. Therefore we may regard +these secondary results of a submarine earthquake as seldom phenomena. + + + DURATION OF GEOLOGICAL TIME. + +Although it is beyond the power of man to conceive any such lapses of +time as have taken place in the history of this earth, it is +interesting, and in certain ways profitable, to determine as near as +possible in the measure of years the duration of the events which are +recorded in the rocks. Some astronomers, basing their conclusions on +the heat-containing power of matter, and on the rate at which energy +in this form flows from the sun, have come to the conclusion that our +planet could not have been in independent existence for more than +about twenty million years. The geologist, however, resting his +conclusions on the records which are the subject of his inquiry, comes +on many different lines to an opinion which traverses that entertained +by some distinguished astronomers. The ways in which the student of +the earth arrives at this opinion will now be set forth. + +By noting the amount of sediment carried forth to the sea by the +rivers, the geologist finds that the lands of the earth--those, at +least, which are protected by their natural envelopes of +vegetation--are wearing down at a rate which pretty certainly does +not exceed one foot in about five thousand years, or two hundred feet +in a million years. Discovering at many places on the earth's surface +deposits which originally had a thickness of five thousand feet or +more, which have been worn down to the depths of thousands of feet in +a single rather brief section of geological time, the student readily +finds himself prepared to claim that a period of from five to ten +million years has often been required for the accomplishment of but a +very small part of the changes which he knows to have occurred on this +earth. + +As the geologist follows down through the sections of the stratified +rocks, and from the remains of strata determines the erosion which has +borne away the greater part of the thick deposits which have been +exposed to erosion, he comes upon one of those breaks in the +succession, or encounters what is called an unconformity, as when +horizontal strata lie against those which are tilted. In many cases he +may observe that at this time there was a great interval unrepresented +by deposits at the place where his observations are made, yet a great +lapse of time is indicated by the fact that a large amount of erosion +took place in the interval between the two sets of beds. + +Putting together the bits of record, and assuming that the rate of +erosion accomplished by the agents which operate on the land has +always been about the same, the geologist comes to the conclusion that +the section of the rocks from the present day to the lowest strata of +the Laurentian represents in the time required for their formation not +less than a hundred million years; more likely twice that duration. To +this argument objection is made by some naturalists that the agents of +erosion may have been more active in the past than they are at +present. They suggest that the rainfall may have been much greater or +the tides higher than they now are. Granting all that can be claimed +on this score, we note the fact that the rate of erosion evidently +does not increase in anything like a proportionate way with the +amount of rainfall. Where a country is protected by its natural +coating of vegetation, the rain is delivered to the streams without +making any considerable assault upon the surface of the earth, however +large the fall may be. Moreover, the tides have little direct cutting +power; they can only remove detritus which other agents have brought +into a condition to be borne away. The direct cutting power of the +tidal movement does not seem to be much greater in the Bay of Fundy, +where the maximum height of the waves amounts to fifty feet, than on +the southern coast of Massachusetts, where the range is not more than +five. So far as the observer can judge, the climatal conditions and +the other influences which affect the wear of rocks have not greatly +varied in the past from what they are at the present day. Now and then +there have been periods of excessive erosion; again, ages in which +large fields were in the conditions of exceeding drought. It is, +however, a fair presumption that these periods in a way balance each +other, and that the average state was much like that which we find at +present. + +If after studying the erosive phenomena exhibited in the structure of +the earth the student takes up the study of the accumulations of +strata, and endeavours to determine the time required for the laying +down of the sediments, he finds similar evidence of the earth's great +antiquity. Although the process of deposition, which has given us the +rocks visible in the land masses, has been very much interrupted, the +section which is made by grouping the observations made in various +fields shows that something like a maximum thickness of a hundred and +fifty thousand feet of beds has been accumulated in that part of +geologic time during which strata were being laid down in the fields +that are subjected to our study. Although in these rocks there are +many sets of beds which were rapidly formed, the greater part of them +have been accumulated with exceeding slowness. Many fine shales, such +as those which plentifully occur in the Devonian beds of this country, +must have required a thousand years or more for the deposition of the +materials that now occupy an inch in depth. In those sections a single +foot of the rock may well represent a period of ten thousand years. In +many of the limestones the rate of accumulation could hardly have been +more speedy. The reckoning has to be rough, but the impression which +such studies make upon the mind of the unprejudiced observer is to the +effect that the thirty miles or so of sedimentary deposits could not +have been formed in less than a hundred million years. In this +reckoning it should be noted that no account is taken of those great +intervals of unrecorded time, such as elapsed between the close of the +Laurentian and the beginning of the Cambrian periods. + +There is a third way in which we may seek an interpretation of +duration from the rocks. In each successive stage of the earth's +history, in different measure in the various ages, mountains were +formed which in time, during their exposure to the conditions of the +land, were worn down to their roots and covered by deposits +accumulated during the succeeding ages. A score or more of these +successively constructed series of elevations may readily be observed. +Of old, it was believed that mountain ranges were suddenly formed, but +there is, however, ample evidence to prove that these disturbed +portions of the strata were very gradually dislocated, the rate of the +mountainous growth having been, in general, no greater in the past +than it is at the present day, when, as we know full well, the +movements are going on so slowly that they escape observation. Only +here and there, as an attendant on earthquake shocks or other related +movements of the crust, do we find any trace of the upward march which +produces these elevations. Although not a subject for exact +measurements, these features of mountain growth indicate a vast lapse +of time, during which the elevations were formed and worn away. + +Yet another and very different method by which we may obtain some +gauge of the depths of the past is to be found in the steps which have +led organic life from its lowest and earliest known forms to the +present state of advancement. Taking the changes of species which have +occurred since the beginning of the last ice epoch, we find that the +changes which have been made in the organic life have been very small; +no naturalist who has obtained a clear idea of the facts will question +the statement that they are not a thousandth part of the alterations +which have occurred since the Laurentian time. The writer is of the +opinion that they do not represent the ten thousandth part of those +vast changes. These changes are limited in the main to the +disappearance of a few forms, and to slight modifications in those +previously in existence which have survived to the present day. So far +as we can judge, no considerable step in the organic series has taken +place in this last great period of the earth's history, although it +has been a period when, as before noted, all the conditions have +combined to induce rapid modifications in both animals and plants. If, +then, we can determine the duration of this period, we may obtain a +gauge of some general value. + +Although we can not measure in any accurate way the duration of the +events which have taken place since the last Glacial period began to +wane, a study of the facts seems to show that less than a hundred +thousand years can not well be assumed for this interval. Some of the +students who have approached the subject are disposed to allow a +period of at least twice this length as necessary for the perspective +which the train of events exhibits. Reckoning on the lowest estimate, +and counting the organic changes which take place during the age as +amounting to the thousandth part of the organic changes since the +Laurentian age, we find ourselves in face once again of that +inconceivable sum which was indicated by the physical record. + +Here, again, the critics assert that there may have been periods in +the history of the earth when the changes of organic life occurred in +a far swifter manner than in this last section of the earth's history. +This supposition is inadmissible, for it rests on no kind of proof; it +is, moreover, contraindicated by the evident fact that the advance in +the organic series has been more rapid in recent time than at any +stage of the past. In a word, all the facts with which the geologist +deals are decidedly against the assumption that terrestrial changes in +the organic or the inorganic world ever proceed in a spasmodic manner. +Here and there, and from time to time, local revolutions of a violent +nature undoubtedly occur, but, so far as we may judge from the aspect +of the present or the records of the past, these accidents are +strictly local; the earth has gone forward in its changes much as it +is now advancing. Its revolutions have been those of order rather than +those of accident. + +The first duty of the naturalist is to take Nature as he finds it. He +must avoid supposing any methods of action which are not clearly +indicated in the facts that he observes. The history of his own and of +all other sciences clearly shows that danger is always incurred where +suppositions as to peculiar methods of action are introduced into the +interpretation. It required many centuries of labour before the +students of the earth came to adopt the principle of explaining the +problems with which they had to deal by the evidence that the earth +submitted to them. Wherever they trusted to their imaginations for +guidance, they fell into error. Those who endeavour to abbreviate our +conception of geologic time by supposing that in the olden days the +order of events was other than that we now behold are going counter to +the best traditions of the science. + +Although the aspect of the record of life since the beginning of the +Cambrian time indicates a period of at least a hundred million years, +it must not be supposed that this is the limit of the time required +for the development of the organic series. All the important types of +animals were already in existence in that ancient period with the +exception of the vertebrates, the remains of which have apparently now +been traced down to near the Cambrian level. In other words, at the +stage where we first find evidence of living beings the series to +which they belong had already climbed very far above the level of +lifeless matter. Few naturalists will question the statement that half +the work of organic advance had been accomplished at the beginning of +the Cambrian rocks. The writer is of the opinion that the development +which took place before that age must have required a much longer +period than has elapsed from that epoch to the present day. We thus +come to the conclusion that the measurement of duration afforded by +organic life indicates a yet more lengthened claim of events, and +demands more time than appears to be required for the formation of the +stratified rocks. + +The index of duration afforded by the organic series is probably more +trustworthy than that which is found in the sedimentary strata, and +this for the reason that the records of those strata have been +subjected to numerous and immeasurable breaks, while the development +of organic life has of necessity been perfectly continuous. The one +record can at any point be broken without interrupting the sequences; +the other does not admit of any breaches in the continuity. + + + THE MOON. + +Set over against the earth--related to, yet contrasted with it in many +ways--the moon offers a most profitable object to the student of +geology. He should often turn to it for those lessons which will be +briefly noted. + +In the beginning of their mutual history the materials of earth and +moon doubtless formed one vaporous body which had been parted from the +concentrating mass of the sun in the manner noted in the sketch of +the history of the solar system. After the earth-moon body had +gathered into a nebulous sphere, it is most likely that a ring +resembling that still existing about Saturn was formed about the +earth, which in time consolidated into the satellite. Thenceforth the +two bodies were parted, except for the gravitative attraction which +impelled them to revolve about their common centre of gravity, and +except for the light and heat they might exchange with one another. + +The first stages after the parting of the spheres of earth and moon +appear to have been essentially the same in each body. Concentrating +upon their centres, they became in time fluid by heat; further on, +they entered the rigid state--in a word, they froze--at least in their +outer parts. At this point in their existence their histories utterly +diverge; or rather, we may say, the development of the earth continued +in a vast unfolding, while that of the moon appears to have been +absolutely arrested in ways which we will now describe. + +With the naked eye we see on the moon a considerable variation in the +light of different parts of its surface; we discern that the darker +patches appear to be rudely circular, and that they run together on +their margins. Seeing this little, the ancients fancied that our +satellite had seas and lands like the earth. The first telescopes did +not dispel their fancies; even down to the early part of this century +there were astronomers who believed the moon to be habitable; indeed, +they thought to find evidence that it was the dwelling place of +intelligent beings who built cities, and who tried to signal their +intellectual kindred of this planet. When, however, strong glasses +were applied to the exploration, these pleasing fancies were rudely +dispelled. + +Seen with a telescope of the better sort, the moon reveals itself to +be in large part made up of circular depressions, each surrounded by a +ringlike wall, with nearly level but rough places between. The +largest of these walled areas is some four hundred miles in diameter; +thence they grade down to the smallest pits which the glass can +disclose, which are probably not over as many feet across. The writer, +from a careful study of these pits, has come to the conclusion that +the wider are the older and the smaller the last formed. The rude +elevations about these pits--some of which rise to the height of ten +thousand feet or more--constitute the principal topographic reliefs of +the lunar surface. Besides the pits above mentioned, there are +numerous fractures in the surface of the plains and ringlike ridges; +on the most of these the walls have separated, forming trenches not +unlike what we find in the case of some terrestrial breaks such as +have been noted about volcanoes and elsewhere. It may be that the +so-called canals of Mars are of the same nature. + +[Illustration: Fig. 23.--Lunar mountains near the Gulf of Iris.] + +The most curious feature on the moon's surface are the bands of +lighter colour, which, radiating from certain of the volcanolike +pits--those of lesser size and probably of latest origin--extend in +some cases for five hundred miles or more across the surface. These +light bands have never been adequately explained. It seems most likely +that they are stains along the sides of cracks, such as are sometimes +observed about volcanoes. + +The eminent peculiarity of the moon is that it is destitute of any +kind of gaseous or aqueous envelope. That there is no distinct +atmosphere is clearly shown by the perfectly sharp and sudden way in +which the light of a star disappears when it goes behind the moon and +the clear lines of the edge of the satellite in a solar eclipse. The +same evidence shows that there is no vapour of water; moreover, a +careful search which the writer has made shows that the surface has +none of those continuous down grades which mark the work of water +flowing over the land. Nearly all of the surface consists of shallow +or deep pits, such as could not have been formed by water action. We +therefore have not only to conclude that the moon is waterless, but +that it has been in this condition ever since the part that is turned +toward us was shaped. + +As the moon, except for the slight movement termed its "libration," +always turns the same face to us, so that we see in all only about +four sevenths of its surface, it has naturally been conjectured that +the unseen side, which is probably some miles lower than that turned +toward us, might have a different character from that which we behold. +There are reasons why this is improbable. In the first place, we see +on the extreme border of the moon, when the libration turns one side +the farthest around toward the earth, the edge of a number of the +great walled pits such as are so plenty on the visible area; it is +fair to assume that these rings are completed in the invisible realm. +On this basis we can partly map about a third of the hidden side. +Furthermore, there are certain bands of light which, though appearing +on the visible side, evidently converge to some points on the other. +It is reasonable to suppose that, as all other bands radiate from +walled pits, these also start from such topographic features. In this +way certain likenesses of the hidden area to that which is visible is +established, thus making it probable that the whole surface of the +satellite has the same character. + +Clearly as the greater part of the moon is revealed to us--so clearly, +indeed, that it is possible to map any elevation of its surface that +attains the height of five hundred feet--the interpretation of its +features in the light of geology is a matter of very great +difficulty. The main points seem to be tolerably clear; they are as +follows: The surface of the moon as we see it is that which was formed +when that body, passing from the state of fluidity from heat, formed a +solid crust. The pits which we observe on its surface are the +depressions which were formed as the mass gradually ceased to boil. +The later formed of these openings are the smaller, as would be the +case in such a slowing down of a boiling process. + +As the diameter of the moon is only about one fourth of that of the +earth, its bulk is only about one sixteenth of that of its planet; +consequently, it must have cooled to the point of solidification ages +before the larger sphere attained that state. It is probable that the +same changeless face that we see looked down for millions of years on +an earth which was still a seething, fiery mass. In a word, all that +vast history which is traceable in the rocks beneath our feet--which +is in progress in the seas and lands and is to endure for an +inconceivable time to come--has been denied our satellite, for the +reason that it had no air with which to entrap the solar heat and no +water to apply the solar energy to evolutionary processes. The heat +which comes upon the moon as large a share for each equal area as it +comes upon the earth flies at once away from the airless surface, at +most giving it a temporary warmth, but instituting no geological work +unless it be a little movement from the expansion and contraction of +the rocks. During the ages in which the moon has remained thus +lifeless the earth, owing to its air and water, has applied a vast +amount of solar energy to geological work in the development and +redevelopment of its geological features and to the processes of +organic life. We thus see the fundamental importance of the volatile +envelopes of our sphere, how absolutely they have determined its +history. + +It would be interesting to consider the causes which led to the +absence of air and water on the moon, but this matter is one of the +most debatable of all that relates to that sphere; we shall therefore +have to content ourselves with the above brief statements as to the +vast and far-acting effects which have arisen from the non-existence +of those envelopes on our nearest neighbour of the heavens. + + + METHODS IN STUDYING GEOLOGY. + +So far as possible the preceding pages, by the method adopted in the +presentation of facts, will serve to show the student the ways in +which he may best undertake to trace the order of events exhibited in +the phenomena of the earth. Following the plan pursued, we shall now +consider certain special points which need to be noted by those who +would adopt the methods of the geologist. + +At the outset of his studies it may be well for the inquirer to note +the fact that familiarity with the world about him leads the man in +all cases to a certain neglect and contempt of all the familiar +presentations of Nature. We inevitably forget that those points of +light in the firmament are vast suns, and we overlook the fact that +the soil beneath our feet is not mere dirt, but a marvellous +structure, more complicated in its processes than the chemist's +laboratory, from which the sustenance of our own and all other lives +is drawn. We feel our own bodies as dear but commonplace possessions, +though we should understand them as inheritances from the +inconceivable past, which have come to us through tens of thousands of +different species and hundreds of millions of individual ancestors. We +must overlook these things in our common life. If we could take them +into account, each soul would carry the universe as an intellectual +burden. + +It is, however, well from time to time to contemplate the truth, and +to force ourselves to see that all this apparently simple and ordinary +medley of the world about us is a part of a vast procession of events, +coming forth from the darkness of the past and moving on beyond the +light of the present day. Even in his professional work the +naturalist of necessity falls into the commonplace way of regarding +the facts with which he deals. If he be an astronomer, he catalogues +the stars with little more sense of the immensities than the man who +keeps a shop takes account of his wares. Nevertheless, the real profit +of all learning is in the largeness of the understanding which it +develops in man. The periods of growth in knowledge are those in which +the mind, enriched by its store, enlarges its conception while it +escapes from commonplace ways of thought. With this brief mention of +what is by far the most important principle of guidance which the +student can follow, we will turn to the questions of method that the +student need follow in his ordinary work. + +With almost all students a difficulty is encountered which hinders +them in acquiring any large views as to the world about them. This is +due to the fact that they can not make and retain in memory clear +pictures of the things they see. They remember words rather than +things--in fact, the training in language, which is so large a part of +an education, tends ever to diminish the element of visual memory. The +first task of the student who would become a naturalist is to take his +knowledge from the thing, and to remember it by the mental picture of +the thing. In all education in Nature, whether the student is guided +by his own understanding or that of the teacher, a first and very +continuous aim should be to enforce the habit of recalling very +distinct images of all objects which it is desired to remember. To +this end the student should practise himself by looking intently upon +a landscape or any other object; then, turning away, he should try to +recall what he has beheld. After a moment the impression by the sight +should be repeated, and the study of the memory renewed. The writer +knows by his own experience that even in middle-aged people, where it +is hard to breed new habits, such deliberate training can greatly +increase the capacity of the memory for taking in and reproducing +images which are deemed of importance. Practice of this kind should +form a part of every naturalist's daily routine. After a certain time, +it need not be consciously done. The movements of thought and action +will, indeed, become as automatic as those which the trained fencer +makes with his foil. + +Along with the habit of visualizing memories, and of storing them +without the use of words, the student should undertake to enlarge his +powers of conceiving spaces and directions as they exist in the field +about him. Among savages and animals below the grade of man, this +understanding of spacial relations is very clear and strong. It +enables the primitive man to find his way through the trackless +forest, and the carrier pigeon to recover his mate and dwelling place +from the distance of hundreds of miles away. In civilized men, +however, the habit of the home and street and the disuse of the +ancient freedom has dulled, and in some instances almost destroyed, +all sense of this shape of the external world. The best training to +recover this precious capacity will now be set forth. + +The student should begin by drawing a map on a true scale, however +roughly the work may be done, of those features of the earth about him +with which he is necessarily most familiar. The task may well be begun +with his own dwelling or his schoolroom. Thence it may be extended so +as to include the plan of the neighbouring streets or fields. At +first, only directions and distances should be platted. After a time +to these indications should be added on the map lines indicating in a +general way contours or the lines formed by horizontal planes +intersecting the area subject to delineation. After attaining certain +rude skill in such work, the student may advantageously make +excursions to districts which he can see only in a hurried way. As he +goes, he should endeavour to note on a sketch map the positions of the +hills and streams and the directions of the roads. A year of holiday +practice in such work will, if the tasks occupy somewhere about a +hundred hours of his time, serve greatly to extend or reawaken what +may be called the topographic sense, and enable him to place in terms +of space the observations of Nature which he may make. + +In his more detailed work the student should select some particular +field for his inquiry. If he be specially interested in geologic +phenomena, he will best begin by noting two classes of facts--those +exhibited in the rocks as they actually appear in the state of repose +as shown in the outcrops of his neighbourhood, and those shown in the +active manifestations of geological work, the decay of the rocks and +the transportation of their waste, or, if the conditions favour, the +complicated phenomena of the seashores. + +As soon as the student begins to observe, he should begin to make a +record of his studies. To the novice in any science written, and +particularly sketched, notes are of the utmost importance. These, +whether in words or in drawings, should be made in face of the facts; +they should, indeed, be set down at the close of an observation, +though not until the observer feels that the object he is studying has +yielded to him all which it can at that time give. It is well to +remark that where a record is made at the outset of a study the +student is apt to feel that he is in some way pledged to shape all he +may see to fit that which he has first written. In his early +experience as a teacher, the writer was accustomed to have students +compare their work of observation and delineation with that done by +trained men on the same ground. It now seems to him best for the +beginner at first to avoid all such reference of his own work to that +of others. So great is the need of developing independent motive that +it is better at the outset to make many blunders than to secure +accuracy by trust in a leader. The skilful teacher can give fitting +words of caution which may help a student to find the true way, but +any reference of his undertakings to masterpieces is sure to breed a +servile habit. Therefore such comparisons are fitting only after the +habit of free work has been well formed. The student who can afford +the help of a master, or, better, the assistance of many, such as some +of our universities offer, should by all means avail himself of this +resource. More than any other science, geology, because of the +complexity of the considerations with which it has to deal, depends +upon methods of labour which are to a great extent traditional, and +which can not, indeed, be well transmitted except in the personal way. +In the distinctly limited sciences, such as mathematics, physics, or +even those which deal with organic bodies, the methods of work can be +so far set forth in printed directions that the student may to a great +extent acquire sound ways of work without the help of a teacher. + +Although there is a vast and important literature concerning geology, +the greater part of it is of a very special nature, and will convey to +the beginner no substantial information whatever. It is not until he +has become familiar with the field with which he is enabled to deal in +the actual way that he can transfer experience thus acquired to other +grounds. Therefore beyond the pleasing views which he may obtain by +reading certain general works on the science, the student should at +the outset of his inquiry limit his work as far as possible to his +field of practice, using a good text-book, such as Dana's Manual of +Geology, as a source of suggestions as to the problems which his field +may afford. + +The main aim of the student in this, as in other branches of inquiry, +is to gain practice in following out the natural series of actions. To +the primitive man the phenomenal world presents itself as a mere +phantasmagoria, a vast show in which the things seen are only related +to each other by the fact that they come at once into view. The end of +science is to divine the order of this host, and the ways in which it +is marshalled in its onward movement and the ends to which its march +appears to be directed. So far as the student observes well, and thus +gains a clear notion of separated facts, he is in a fair way to +gather the data of knowledge which may be useful; but the real value +of these discernments is not gained until the observations go +together, so as to make something with a perspective. Until the store +of separate facts is thus arranged, it is merely crude material for +thought; it is not in the true meaning science, any more than a store +of stone and mortar is architecture. When the student has developed an +appetite for the appreciation of order and sources of energy in +phenomena, he has passed his novitiate, and becomes one of that happy +body of men who not only see what is perceived by the mass of their +fellows, but are enabled to look through those chains of action which, +when comprehended, serve to rationalize and ennoble all that the +senses of man, aided by the instruments which he has devised, tell us +concerning the visible world. + + + + + + INDEX. + + AEtna, Mount, 381. + + Agriculture, + American, 346; + in England, winning swamp lands for, 335; + recent developments of, 345. + + Alaska, changes on the coast of, 96. + + Ants taking food underground, 319; + work of the, on the soil, 318. + + Apsides, revolution of the, 61, 62. + + Arabians, chemical experiments of the, 13. + + Arches, natural, in cavern districts, 258. + + Artesian wells, 258, 259. + + Arts, advance of Italian fine, 19. + + Asteroids, 53; + motions of, about their centres and about the sun, 53. + + Astronomers, the solar system and the early, 79. + + Astronomy, 31-80; + growth of, since the time of Galileo, 33, 34; + the first science, 10. + + Atmosphere, 97-206; + along the tropical belt, 102; + as a medium of communication between different regions, 99; + deprived of water, containing little heat, 105; + beginning of the science of the, 117; + counter-trade movements of the, 105; + envelope of the earth, 98; + expansion of, in a hollow wall during the passage of a storm, 114; + heat-carrying power of the, 105; + heights to which it extends, 99; + in water, 99; + movements no direct influence on the surface of the earth, 122; + movements of the, qualified by the condition which + it encounters, 118; + of mountains, 98; + of the seashore, 98; + of the earth, 98; + of the sun, 73; + snow as an evidence of, 65; + supplying needs of underground creatures, 331; + uprushes of, 101, 102; + upward strain of the, next the earth, 107; + weight and motion of the, 120, 121. + + Atmospheric circulation of the soil, 330, 331; + envelopes, 97. + + Aurora borealis, 168. + + Avalanches, 210-213; + dreaded, in the Alpine regions, 212; + great, in the Swiss Oberland, 211, 212; + rocky, 175-177. + + Axis, + imaginary changes in the earth's, 59; + of the earth's rotation, 58; + polar, inclined position of, 58; + polar, nodding movement of the axes, 54; + rotations of the planetary spheres on their axes, 56. + + + Barometer, causes of changes in the, 117, 118. + + Basalts, 309. + + Beaches, 93, 142, 144; + boulder, 142, 143; + pebbly, 142; + sand, 144. + + Beetles, work of, on the soil, 318, 319. + + Belief of the early astronomers about the solar system, 79. + + _Bergschrund_, the, 214. + + Birds and mammals contributing to the fertility of the soil, 319. + + "Blanketing," 269. + + Bogs, + climbing, 331-334; + lake, 331-333; + peat, 334, 335; + quaking, 334. + + Botany, rapid advance in, 14, 15. + + Boulders, 217, 220. + + Breakers, 135, 137, 139. + + Bridges, natural, 257, 258. + + + Canals of Mars, 67. + + Canon, newly formed river cutting a, 195. + + Cataracts, 193. + + Caves, 253-258, 261; + architecture of, 255-258; + hot-water, 261; + mammoth cave, 258; + stalactites and stalagmites on the roof and floor of, 257. + + Chasms, 140, 141. + + Chemistry, 6, 12, 14; + advance of, 12; + modern, evolving from the studies of alchemists, 13, 14. + + Chromosphere, 73. + + Civilization of the Icelanders, 384. + + Cliffs, sea-beaten, 132, 141, 142. + + Climate, + changes of, due to modifications of the ocean streams, 153; + effect of the ocean on the, 147; + of the Gulf Stream, 149, 150. + + Clouds, 159; + formation of, 162, 163; + shape of, 163; + water of, usually frozen, 207; + cloud-making, laws of, 161, 162. + + Coast, + changes on the Scandinavian, 96; + line, effect of tide on the, 145; + of Greenland, 226; + of New Jersey sinking, 95; + marine, changes in, 95. + + Cold in Siberia, 243. + + Comets, 47, 50; + collisions of, 50; + kinship of meteorites and, 48; + omens of calamity to the ancients, 50; + the great, of 1811, 49, 50. + + Cones. See under VOLCANOES. + + Conflict between religion and science, 20, 22; + between the Protestant countries and the followers of science, 20. + + Continental shelves, 125. + + Continents and oceans, 83; + changes in position of, 91; + cyclones of the, 111; + forms of, 90; + proofs that they have endured for many years, 92; + shape of, 84, 96. + + Coral reefs, 153, 353. + + Corona, realm of the, 73. + + Craters. See under VOLCANOES. + + Crevasse, a barrier to the explorer, 218. + + Crevice water, 250. + + Curds, 214. + + Currents, + coral reefs in Florida affecting the velocity of, 153; + equatorial, 150; + of the Gulf Stream, 147-149; + hot and cold, of the sea, 102; + ocean, 145; + oceanic action of trade winds on, 145; + effect on migration of, 157; + icebergs indicating, 243; + surface, history of, 172; + uprushing, near the equator, 106. + + Cyclones, 111; + cause of, 111; + of North America, 111; + secondary storms of, 112. + + + Deltas, 173, 187. + + Deposits, vein, 260, 261. + + Deserts, interior, 158. + + Dew, 159, 160; + a concomitant of cloudless skies, 160, + and vegetation, 160; + formation of, 159-161. + + Diablerets, 174. + + Diagram of a vein, 260; + showing development of swamp, 335; + how a portion of the earth's surface may be sunk by faulting, 374; + growth of mangroves, 340; + the effect of the position of the fulcrum point + in the movement of the land masses, 94. + + Diameter of our sphere at the equator, 62; + of the earth, 82. + + Dikes, 192, 293; 305-310; + abounding in volcanic cones, 305; + cutting through coal, 306; + driven upward, 307; + formation of, 305, 310; + material of, 307, 308; + representing movements of softened rock, 309; + their relation to volcanic cones, 307; + variations of the materials of, 307, 308; + waterfalls produced by, 192; + zone of, 306. + + Dismal Swamp, 95, 333. + + Distances, + general idea of, 27; + good way to study, 27, 28; + training soldiers to measure, 28. + + Doldrums, 104, 109; + doldrum of the equator, 109; + of the hurricane, 109. + + Drainage, imperfect, of a country affected by glaciers, 242. + + Dunes, 123, 124, 325, 326, 387; + moulded, 387. + + Duration of geological time, 389. + + Dust accumulations from wind, in China, 122. + + + Earth, + a flattened sphere, 82; + air envelope of the, 98; + amount of heat falling from the sun on the, 41; + antiquity of the, 391; + atmosphere of the, 98; + attracting power of the, 127; + axis of the rotation of the, 58; + composition of the atmosphere of the, 98; + crust of the, affected by weight, 93; + deviation of the path of the, varied, 61; + diameter of the, 82; + of the, affected by loss of heat, 131; + difference in altitude of the surface of the, 83; + discovery that it was globular, 31, 32; + effect of imaginary changes in the relations of sun and, 59; + effect of the interior heat of the, 309, 310; + effect of the sun on the, 60, 61; + formerly in a fluid state, 82; + imaginary view of the, from the moon, 81; + important feature of the surface of the, 83; + jarring caused by faults, 367; + surface of the, determined by heat and light from the sun, 57; + most important feature of the surface of the, 83; + motion of the, affecting the direction of trade winds, 103; + movements, 366; + natural architecture of the, 377; + no part of the, exempt from movement, 384; + parting of the moon and, 396; + path of the, around the sun, 55, 56, 59, 60; + revolving from east to west, 103; + shrinking of the, from daily escape of heat, 89; + soil-covering of the, 343; + study of the, 81-96; + swaying, 385; + tensions, problem of, 371; + tremors, caused by chemical changes in the rocks, 385; + tropical belt of the, 74; + viewed from the surface of the moon, 311, 312; + water store of the, 125. + + Earthquakes, 277, 278, 280, 356, 358, 370-384, 388-390; + accidents of, 358; + action of, 356; + agents of degradation, 383, 384; + basis of, 367; + certain limitations to, 380, 381; + Charleston, of 1883, 374, 375; + countries, architecture in, 381; + echoes, 369, 370; + damages of, 377, 390; + effect of, + on the soil, 375; + the surface of the earth, 371; + formed by riving of fissures, 382; + great, occurring where rocks have been + disturbed by mountain-building, 381, 382; + Herculaneum and Pompeii destroyed by an, 277, 280; + Italian, in 1783, 371, 372; + important, not connected with volcanic explosions, 381; + Jamaica, in 1692, 372, 376; + Lisbon, in 1755, 368, 369, 373, 374, 381; + maximum swing of, 369; + measuring the liability to, 386, 387; + mechanism of, 370, 371; + method of the study of, followed by Mr. Charles Mallet, 382, 383; + Mississippi, in 1811, 373, 374, 380, 381; + movement of the earth during, 377; + originating from a fault plane, 367, 369, 370; + originating from the seas, 358, 375; + oscillation of, 376; + poised rocks indicating a long exemption from strong, 388; + Riobamba, in 1797, 375; + shocks of, and their effect upon people, 383; + the direct calamities of Nature, 386; + waves of, 389. + + Earthworms, 317-319; + taking food underground, 319. + + Eclipses, record of ancient, 130. + + Electrical action in the formation of rain and snow, 164. + + Elevations of seas and lands, 83. + + Energy indestructible, 23. + + Envelope, lower, of the sun, 74. + + Equator, + diameter of our sphere at the, 62; + doldrum of the, 109; + updraught under the, 102; + uprushing current near the, 106. + + Equinoxes, precession of the, 61, 62. + + _Eskers_, 221. + + Expansion of air contained in a hollow wall during the + passage of the storm, 114. + + Experiment, illustrating consolidation of disseminated + materials of the sun and planets, 40. + + + Falls. See WATERFALLS. + + Fault planes, 382. + + Feldspar, 324. + + Floods, 180, 197; + rarity of, in New England, 121; + river, frequent east of Rocky Mountains, 198. + + Foehns, 121. + + Forests, salicified, 124. + + Fossilization, 354-356. + + Fulcrum point, 95. + + + Galactic plane, 45. + + Galongoon, eruption of, 294. + + Geological work of water, 168-206. + + Glacial action in the valleys of Switzerland, 224; + periods, 63, 243, 246; + in the northern hemisphere, 246; + waste, 324. + + Glaciation, + effect of, + in North America, 241; + in Central America, 234; + South America, 234. + + Glaciers, 207-249; + action of ice in forming, 230-232; + Alaskan, 216; + continental, 225, 239, 240; + discharge of, 220; + exploring, 220; + extensive, in Greenland and Scandinavia, 244; + former, of North America, 232, 234; + map of, and moraines near Mont Blanc, 217; + motions of, 213; + retreat of the, 228, 230, 235; + secrets of the under ice of, 221; + speed of a, 224; + study of, in the Swiss valleys, 222; + testimony of the rocks regarding, 228; + when covered with winter snows, 216; + valley, 216. + + Gombridge, 1830, 74. + + Gravitation, law of, 4. + + Greeks' idea of the heavens, 31; + not mechanically inventive, 22. + + Gulf Stream, current of the, 147. + + + Heat, + amount of, daily escaping from the earth, 89; + amount of, falling from the sun on the earth, 41; + belief of the ancients regarding, 42; + dominating effect on air currents of tropical, 104; + energy with which it leaves the sun, 41; + internal, + of the earth, 88, 89; + of the earth's interior, 309, 310; + sun, effect on the atmosphere of the, 100; + Prof. Newcomb's belief regarding the, of the sun, 52; + radiation of the earth's, causing winds, 101; + solar, 41; + tropical, and air currents, 104. + + Hills, sand, 123. + + Horizontal pendulum, 384. + + Horse latitudes, 104. + + "Horses," 261. + + Hurricanes, 107, 110, 317; + commencement of, 107; + doldrum of, 109; + felt near the sea, 110; + in the tropics, 110. + + Hypothesis, + nebular, 34, 35, 39, 52, 56; + working, 4, 5. + + + Ice action, + effect of intense, 222, 223; + in forming glaciers, 230, 232; + recent studies in Greenland of, 239; + depth of, in Greenland, 227; + effect of, on river channels, 196; + effect of, on stream beds, 196; + expanding when freezing, 237; + epoch, 92, 93, 246; + floating, 242; + made soils rarely fertile, 241; + mass, greatest, in Greenland, 226, 227; + moulded by pressure, 215; + streams, + continental, 225, 226; + of the mountains, 225; + of the Himalayan Mountains, 234. + + Icebergs, 242, 243; + indicating oceanic currents, 243. + + Iceland, volcanic eruptions in, 297, 298. + + Instruments, first, astronomical, 10, 11. + + Inventions, mechanical, aiding science, 22. + + Islands, 84, 272; + continental, 84; + in the deeper seas made up of volcanic ejections, 272; + volcanic, 272. + + + Jack-o'-lantern, 167. + + Jupiter, + gaseous wraps of, 97; + path of the earth affected by, 59, 60; + the largest planet of the sun, 69. + + + Kames, 325. + + Kant, Immanuel, and nebular hypothesis, 34. + + Kaolin, 324. + + Klondike district, cold in, 243, 244. + + Krakatoa, + eruption of, 298-300; + effect of, on the sea, 299; + effect of, on the sun, 300. + + + Lacolites, 306. + + Lacustrine beds, 351. + + Lagoons, salt deposits found in, 200. + + Lake basins, + formation of, 200, 201; + bogs, 331, 333, 334; + deposits, 350, 351. + + Lakes, 199-206; + effect of, on the river system, 205; + fresh-water, 145; + formed from caverns, 202; + great, changing their outlets, 205; + of extinct volcanoes, 203; + temporary features of the land, 203; + volcanic, 203. + + Lands, + great, relatively unchangeable, 96; + table, 91; + movements resulting in change of coast line, 351, 352; + shape of the seas and, 83, 84; + accounting for the changes in the attitude of the, 95; + and water, divisions of, 84; + dry, surface of, 85; + general statement as to the division of the, 83, 84; + surface, shape of the, 85; + triangular forms of great, 90. + + Latitudes, horse, troublesome to mariners, 104. + + Laplace and nebular hypothesis, 34. + + Lava, 266-268, 270, 271, 292, 293, 295, 296, 303, 304; + flow of, invading a forest, 268; + from Vesuvius, 293; + of 1669, 295, 296; + temperature of, 295, 296; + incipient, 304; + outbreaks of, 292, 303; + stream eaves, 292, 293. + + Law, natural, + Aristotle and, 3; + of gravitation, 4; + of the conservation of energy, 23. + + Leaves, radiation of, 160. + + Length of days affected by tidal action, 131. + + Level surfaces, 91. + + Life, organic, evolution of, 15, 16. + + Light, belief of the ancients regarding, 42. + + Lightning, 24, 164-168; + noise from, 166; + proceeding from the earth to the clouds, 165; + protection of buildings from, 165; + stroke, wearing-out effect of, 165. + + Limestones, 353, 357, 358, 360, 364; + formation of, 357, 360. + + Lisbon, earthquake of, 1755, 368, 369. + + Lowell, Mr. Percival, observations on Venus, 64. + + Lunar mountains near the Gulf of Iris, 397. + + + Mackerel sky, 35. + + Mallet, Mr. Charles, and the study of earthquakes, 382, 383. + + Man as an inventor of tools, 10. + + Mangroves, 340; + diagram showing mode of growth, 340; + marshes of, 339. + + Map of glaciers and moraines near Mont Blanc, 217; + of Ipswich marshes, 338. + + Mapping with contour lines, 27. + + Maps, + desirable, for the study of celestial geography, 77; + geographic sketch, 26, 27. + + Marching sands jeopardizing agriculture, 123. + + Marine animals, sustenance of, 361-363; + deposits, 325-327, 349, 356; + marshes, 336-340; + waves caused by earthquakes, 387. + + Mars, 65-67, 84, 97; + belief that it has an atmosphere, 65; + canals of, 67; + gaseous wraps of, 97; + more efficient telescopes required for the study of, 67; + nearer to the earth than other planets, 65. + + Marshes, + mangrove, 339; + map of Ipswich, 338; + marine, 336-340; + deposits found in, 336; + of North America, 337; + on the coast of New England, 339; + phenomena of, 167, 168; + tidal, good earth for tillage, 337; + tidal, of North America, 340. + + Mercury, 55, 63, 78; + nearest to the sun, 63; + time in which it completes the circle of its year, 55. + + Meteorites, 47, 48; + kinship of comets and, 48. + + Meteors, 47; + falling, 47; + composition of, 48; + flashing, 39, 40, 47; + speed of, 47; + inflamed by friction with air, 99. + + Methods in studying geology, 400. + + Milky Way, 45; + voyage along the path of the, 44, 45. + + Mineral crusts, 328, 329; + deposits, 308. + + Moon, 38, 395-400; + absence of air and water on the, 399; + attended by satellites, 57; + attraction which it exercises on the earth, 62; + curious feature of the, 397; + destitute of gaseous or aqueous envelope, 397; + diameter of the, 399; + imaginary view of the earth from the, 81; + "libration" of the, 398; + made up of circular depressions, 396, 397; + movements of the, 78; + no atmosphere in the, 97; + parting of the earth and, 396; + position of the, in relation to the earth, 62; + tidal action and the, 131; + tides of the, 126, 127; + why does the sun not act in the same manner as the, 78. + + Moraines, 216, 218, 229, 230; + map of glaciers and, near Mont Blanc, 217; + movements of the, 216-218; + terminal, 228. + + _Moulin_, 219. + + Mount AEtna, 288-310; + lava yielding, 290, 293, 294; + lava stream caves of, 292, 293; + more powerful than Vesuvius, 297; + peculiarities of, 291, 292; + size of, 289-291; + turning of the torrents of, 295. + + Mountain-building, 90-93, 304; + folding, 86, 87, 90, 365; + attributed to cooling of the earth, 88; + growth, 392; + Swiss falls, 174; + torrents, energy of, 177. + + Mountains, 85, 86, 89, 90-93; 174-178; + form and structure of, 86; + partly caused by escape of heat from the earth, 89; + sections of, 87. + + Mount Nuova, formation of, 284. + + Mount Vesuvius, 263-285, 288, 289, 293, 302, 381; + description of the eruption of, in A.D. 79, 277-280; + diagrammatic sections through, showing changes in the form + of the cone, 283; + eruption of, in 1056, 281; + in 1882-'83, 264, 266; + eruption of, in 1872, 282; + eruptions of, increased since 1636, 282; + flow of lava from, 285; + likely to enter on a period of inaction, 282, 283; + outbreak of, in 1882-'83, 264, 266. + + + Naples, prosperity of the city, 289. + + Nebular hypothesis, 34, 35, 39, 52. + + Neptune, 70. + + _Neve_, the, 214; + no ice-cutting in the region of the, 224. + + Newcomb's (Prof.) belief regarding the heat of the sun, 52. + + Niagara Falls, 191, 192, 204; + cutting back of, 204. + + North America, + changes in the form of, 91, 92; + triangular form of, 90. + + + Ocean, + average depth of the, 89; + climatal effect of the, 147; + currents, 145; + effect of, on migration, 156; + effect of, on organic life, 154; + floor, 85, 93; + hot and cold currents of the, 102; + sinking of the, 93, 94; + the laboratory of sedimentary deposits, 351; + depth of the, 89, 126. + + Oceanic circulation, effect of, on the temperature, 152. + + Oceans and continents, 83. + + Orbit, + alterations of the, and the seasons, 60, 61; + changing of the, 59-63; + shape of the, 61-63. + + Organic life, 315, 317, 321, 352, 353, 363; + action of, on the soil, 317, 321; + advantages of the shore belt to, 363; + development of in the sea, 352, 353; + effect of ocean currents on, 154; + processes of, in the soil, 315; + decay of, in the earth, 321. + + Orion, 46. + + Oscillations of the shores of the Bay of Naples, 287. + + Oxbow of a river, 182, 183. + + Oxbows and cut-off, 182. + + + Pebbles, + action of seaweeds on, 143; + action of the waves on, 142, 144. + + Photosphere, 74. + + Plains, 86; + alluvial, 91, 179, 182, 184-186, 325; + history of, 91; + sand, 325. + + Planets, 38; + attended by satellites, 57; + comparative sizes of the, 68; + experiments illustrating consolidation of disseminated + materials of the sun and, 40; + gaseous wraps of, 97; + important observations by the ancients of fixed stars + and planets, 43; + movements of, 57-61; + outer, 78; + table of relative masses of sun and, 77. + + Plant life in the Sargassum basins, 156. + + Plants and animals, + protection of, + by mechanical contrivances, 364; + and trees, work of the roots of, on the soil, 316, 317; + water-loving, 181; + forming climbing bogs, 332. + + Polar axes, nodding movement of, 54. + + Polar snow cap, 66. + + Polyps, 155, 353. + + Pools, circular, 203. + + Prairies, 340, 342. + + + Radiation of heat, 159. + + Rain, 152, 156, 164, 168, 170, 328, 330; + circuit of the, 156-168; + drops, force of, 169, 170; + spheroidal form of, 170; + electrical action in the formation of snow and, 164; + work of the, 171. + + Realm, unseen solar, 75. + + Reeds, 332. + + Religion, + conflict between science and, 20, 22; + struggle between paganism and, 21. + + Rivers and _debris_, 183; + changes in the course of, in alluvial plain, 182; + deposition of, accelerated by tree-planting, 181; + great, always clear, 205; + inundation of the Mississippi, eating away land, 182; + muds, 222; + newly formed, cutting a canon, 195; + of snow-ice, 211; + origin of a normal, 173; + oxbow of a, 182,183; + sinking of, 199; + swinging movement of, 179-181; + river-valleys, 193, 194; + diversity in the form of 188-191. + + Rocks, 145; + accidents from falling, 174; + cut away by sandstones, 188; + divided by crevices, 252; + duration of events recorded in, 389, 390, + ejection of, material, 311; + falling of, 174-176; + formation of, 262, 263; + from the present day to the strata of the Laurentian, 390; + migration of, 291; + poised, indicating a long exemption from strong earthquakes, 388; + rents in, 252, 253; + stratification of, 349, 350, 352, 365, 390; + testimony of the, in regard to glaciers, 228; + under volcanoes, 303; + variable elasticity of, 366; + vibration of, 367, 368; + rock-waste, march of the, 343; + water, 250, 267. + + Rotation of the earth affected by tides, 130; + of the planetary spheres on their axes, 56. + + + Salicified forests, 124. + + Salt deposits formed in lagoons, 200; + found in lakes, 199-200. + + Sand bars, 183; + endurance of, against the waves, 145; + hills, travelling of, 123; + marching, 123; + silicious stones cutting away rooks, 188. + + Satellites, 53, 54; + motions of, about their centres and about the sun, 53, 54. + + Saturn, 38, 53, 57, 396; + cloud bands of, 70; + gaseous wraps of, 97; + path of the earth affected by, 59, 60. + + Savages, primitive, students of Nature, 1. + + Scandinavia, changes on the coasts of, 96. + + Science, + advance of, due to mechanical inventions, 22; + astronomy beginning with, 10; + chemical, characteristics of, 14; + conflict between religion and, 20, 22; + conflict between the Roman faith and, 20; + mechanical inventions as aids to, 22, 23; + modern and ancient, 4; + natural, 5, 6; + of botany in Aristotle's time, 14; + of physiology, 15; + of zooelogy in Aristotle's time, 14; + resting practically on sight, 10. + + Scientific development, + historic outlines of, 17; + tools used in measuring and weighing, as an aid to vision, 12. + + Sea, + battering action of the, 140; + coast ever changing, 385, 386; + effect of volcanic eruptions on the, 299; + floor deposits of the, affected by volcanoes, 360, 361; + in receipt of organic and mineral matter, 359; + hot and cold currents of the, 102; + littoral zone of the, 351, 352; + puss, 142; + rich in organic life, 352, 353; + solvent action of the, 361; + strata, formation of, 354; + water, minerals in, 185; + weeds, 155, 156. + + Seas, dead, + originally living lakes, 200; + water of, buoyant, 199; + eventually the seat of salt deposits, 199-201; + general statement as to division of, 83, 84; + shape of the, 83, 84. + + Seashore, air of the, 98. + + Seasons, changing the character of the, 61, 62. + + Sense of hearing, 9,10; + of sight, 10; + of smell, 9, 10; + of taste, 9, 10; + of touch, 9, 10. + + _Seracs_, 214. + + Shocks, earthquake. See under EARTHQUAKES. + + Shore lines, variation of, 83, 84. + + Shores, cliff, 138-142. + + Sink holes, 202; + in limestone districts, 253, 254. + + Skaptar, + eruption of, 297, 298; + lava from the eruption of, 298. + + Sky, mackerel, 35. + + Snow, 207-225, 244; + as an evidence of atmosphere, 65; + blankets, early flowers beginning to blossom under, 208; + covering, difference between an annual and perennial, 210; + effect of, on plants, 208; + electrical action in the formation of rain and, 164; + flakes, formation of, 164; + red, 210; + slides, 210; + slides, phenomena of, 210, 211. + + Soil, + alluvial, 321, 322; + atmospheric circulation of, 330, 331; + conditions leading to formation of, 313, 331; + continuous motion of the, 314; + covering of the earth, 343; + decay of the, 314, 315; + degradation of the, 344-348; + means for correcting, 346-348; + destruction in grain fields greater than the accumulation, 344; + developing on lava and ashes an interesting study, 343; + development of, in desert regions, 340; + effect of animals and plants on the, 317-320; + effect of earthquakes on the, 375; + fertility of the, distinguished from the coating, 344, 345; + fertility of, affected by rain, 327; + formation of, 314-321; + glacial, characteristics of, 324; + glaciated, 323, 324; + irrigation of the, 328-330; + local variation of, 327; + mineral, 321; + of arid regions fertile when subjected to irrigation, 341; + of dust or blown sand, 321; + of immediate derivation, 321, 322; + phenomena, 313; + processes of organic life in the, 315; + variation in, 321-331; + vegetation protecting the, 316, 317; + washing away of the, 346, 347; + winning, from the sea, 337; + work of ants on the, 318; + tiller, duty of the, 348. + + Solar bodies, + general conditions of the, 63-71; + forces, action of, on the earth, 349; + system, 52, 56; + independent from the fixed stars system, 43; + original vapour of, 52, 53; + singular features of our, 68; + tide, 127. + + Spheres, + difference in magnitude of, 51; + motions of the, 50, 51; + planetary, rotation of, on their axes, 56. + + Spots, sun, 72. + + Spouting horn, 141. + + Springs, formation of small, 252. + + Stalactitization, 256. + + Stalagmites and stalactites on the roof and floor of a cavern, 257. + + Stars as dark bodies in the heavens, 47; + discovery of Fraunhofer and others on, 23, 38; + double, 39; + and tidal action, 131; + earliest study of, 10; + fixed, important observations by the ancients of planets and, 43; + not isolated suns, 38, 39; + variation in the light of, 46; + limit of, seen by the naked eye, 11; + revolution of one star about another, 46, 47; + shooting, 47; + speed of certain, 51; + study of, 31-80; + sudden flashing forth of, due to catastrophe, 46; + voyage through the, 44, 45; + star, wandering, 74. + + Stellar realm, 31-80. + + Storms, + circular, 111; + desert, 121, 122; + expansion of air contained in a hollow wall during + the passage of, 114; + great principle of, 105, 106; + in the Sahara, 121; + lightning, more frequent in summer, 167; + paths of, 115; + secondary, of cyclones, 112; + spinning, 115; + thunder, 165-167; + whirling, 106, 124; + whirling peculiarity of, 108, 109. + + Strabo, writings of, 18. + + Sun, + atmosphere of the, 73; + constitution of the, 72; + distance of the earth from the, 29; + effect from changes in the, and earth, 59; + envelope of the, 73, 74, 97; + experiments illustrating consolidation of disseminated + materials of planets and, 40; + finally, dark and cold, 42; + formation of the eight planets of the, 53; + heat leaving the, 41; + heat of the, 76; + imaginary journey from the, into space, 44; + mass of the, 76, 77; + path of the earth around the, 55; + physical condition of the, 71; + Prof. Newcomb's belief regarding the heat of the, 52; + spots, 75; + abundant at certain intervals, 72; + difficulty in revealing cause of, 75; + structure of the, a problem before the use of the telescope, 72; + table of relative masses of, and planets, 77; + three stages in the history of the, 71; + tides, 126; + why does it not act in the same manner as the moon? 78. + + Surfaces, level, 90. + + Surf belt, swayings of the, 137. + + Swamps, + diagram showing remains of, 335; + Dismal Swamp, 95, 333; + drainage of, 334, 335; + fresh-water, 334, 335; + phenomena of, 167, 168. + + + Table-lands, 91. + + Table of relative masses of sun and planets, 77. + + Telescopes, 11, 12, 45; + first results of, 72; + power of, 11; + revelations of, 45. + + Temperature, + effects of, produced by vibration, 42; + in the doldrum belt, 118; + of North America, 118; + of the Atlantic Ocean, 118. + + Tempests, rate of, 99, 100. + + Thunder, 166; + more pronounced in the mountains, 166. + + Thunderstorms, 165, 166; + distribution of, 166, 167. + + Tidal action, + recent studies of, 131, 132; + marshes of North America, 340. + + Tides, + carving channels, 129; + effecting the earth's rotation, 130; + effect of, on marine life, 130; + height of, 128, 129; + moon and sun, 126, 127; + normal run of the, 127; + production of, 131; + of the trade winds, 150; + solar, 127; + travelling of, 127, 128. + + Tillage introducing air into the pores of the soil, 331. + + Tornadoes, 112, 113, 317; + development of, 113; + effect of, on buildings, 113; + fiercest in North America, 113; + length of, 115; + resemblance of, to hurricanes, 115; + upsucking action of, 114, 115. + + Torrents, 177-179, 204. + + Trade winds. See under WINDS. + + Training in language, + diminishing visual memory, 401; + soldiers to measure distances, 28; + to measure intervals of time, 28; + for a naturalist, 25-29. + + Tunnels, natural, 257. + + + Uranus, 70. + + + Valley of Val del Bove formed from disturbances of Mount AEtna, 294. + + Valleys, + diversity in the form of river, 188-191; + river, 193. + + Vapour, 156, 157, 159, 163; + gravitative attraction of, 34, 35; + nebular theory of, 52, 53; + original, of the solar system, 52, 53. + + Vegetation, + and dew, 160; + in a measure, independent of rain, 160; + protecting the soil, 316, 317. + + Vein, diagram of a, 260. + + Venus, 64, 78; + recent observations of, by Mr. Percival Lowell, 64. + + Vesuvian system, study of the, 285. + + Vesuvius. See MOUNT VESUVIUS. + + Visualizing memories, 402, 403. + + Volcanic action, 268-276. + + Volcanic eruption of A.D. 79, 288; + important facts concerning, 276-279; + islands, 272; + lava a primary feature in, 266; + observations of, made from a balloon, 301; + peaks along the floor of the sea, 272, 273; + possibility of throwing matter beyond control of gravitative + energy, 300. + + Volcanoes, 125, 203, 263; + abounding on the sea floor, 302; + accidents from eruptions of, 288; + along the Pacific coast, 271; + ash showers of, maintaining fertility of the soil, 289; + distribution of, 271; + eruption of, 286-294, 368; + explosions from, coming from a supposed liquid interior + of the earth, 275; + exporting earth material, 310; + little water, 375; + Italian, considered collectively, 296, 297; + Neapolitan eruptions of and the history of civilization, 288; + subsidence of the earth after eruption of, 287, 291; + origin of, 263-274; + phenomena of, 263-267; + submarine, 301; + travelling of ejections from, 287, 288. + + + Waters, + crevice, 250; + of the earth, 250, 251; + cutting action of, 117, 192; + drift, from the poles, 151; + journey of, from the Arctic Circle to the tropics, 151, 152; + dynamic value of, 171; + expansion of, in rocks, 270; + geological work of, 168-206; + in air, 99; + of the clouds usually frozen, 207; + pure, no power for cutting rocks, 204; + rock, 250, 263; + sea, minerals in, 185; + store of the earth, 125; + system of, 125, 156; + tropical, 151; + velocity of the, under the equator, 150; + wearing away rocks, 178, 179; + underground, carrying mineral matter to the sea, 193; + chemical changes of, leading to changes in rock material, 262, 263; + effect of carbonic-acid gas on, 251; + operations of the, 126; + wearing away rocks, 178, 179; + work of, 250. + + Waterfalls, 189-193; + cause of, 191; + the Yosemite, 192; + Niagara, 191, 192; + numerous in the torrent district of rivers, 192; + produced by dikes, 192; + valuable to manufactures, 192, 193. + + Waterspouts, 115, 116; + atmospheric cause of, 116; + firing at, 116; + life of a, 116; + picturesqueness of, 116; + the water of fresh, 117. + + Waves, 128, 129, 132, 145; + action of friction on, 135, 136; + break of the, 136; + endurance of sand against the, 145; + force of, 133, 136, 139; + marine, caused by earthquakes, 387; + of earthquakes, 389; + peculiar features in the action of, 137; + size of, 137, 138; + stroke of the, 144; + surf, 135; + tidal height of, 132; + undulations of, 132; + wind, 132; + wind influence of, on the sea, 134, 135; + wind-made, 128. + + Ways and means of studying Nature, 9. + + Weeds of the sea, 155. + + Well, artesian, 258, 259. + + Whirling of fluids and gas, 36, 37. + + Whirlwinds in Sahara, 121. + + Will-o'-the-wisp, 167. + + Winds, 101, 110, 122, 317; + effect of sand, 122; + hurricane, 110; + illustration of how they are produced, 101; + in Martha's Vineyard, 120; + of the forests, work of the, 317; + of tornadoes, effect of, 113; + on the island of Jamaica, 119, 120; + regimen of the, 119; + variable falling away in the nighttime, 100; + trade, 102-105; 145, 146, 150; + action of, on ocean currents, 145: + affected by motion of the earth, 103; + belt, motion of the ocean in, 146; + flow and counter-flow of the, 150; + tide of the, 150; + uniform condition of the, 102; + waves, work of, 132, 134, 135. + + Witchcraft, belief of, in the early ages, 21. + + + Zooelogy, rapid advance in, 14, 15. + + + + + + +End of the Project Gutenberg EBook of Outlines of the Earth's History, by +Nathaniel Southgate Shaler + +*** END OF THIS PROJECT GUTENBERG EBOOK OUTLINES OF THE EARTH'S HISTORY *** + +***** This file should be named 18562.txt or 18562.zip ***** +This and all associated files of various formats will be found in: + http://www.gutenberg.org/1/8/5/6/18562/ + +Produced by Brendan Lane, Riikka Talonpoika, Jeroen van +Luin and the Online Distributed Proofreading Team at +http://www.pgdp.net + + +Updated editions will replace the previous one--the old editions +will be renamed. + +Creating the works from public domain print editions means that no +one owns a United States copyright in these works, so the Foundation +(and you!) can copy and distribute it in the United States without +permission and without paying copyright royalties. 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