Through The Eyes Of The Geologist

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Title:     Through The Eyes Of The Geologist
Author: John Burroughs [More Titles by Burroughs]

I

How habitually we go about over the surface of the earth, delving it or cultivating it or leveling it, without thinking that it has not always been as we now find it, that the mountains were not always mountains, nor the valleys always valleys, nor the plains always plains, nor the sand always sand, nor the clay always clay. Our experience goes but a little way in such matters. Such a thought takes us from human time to God's time, from the horizon of place and years to the horizon of geologic ages. We go about our little affairs in the world, sowing and reaping and building and journeying, like children playing through the halls of their ancestors, without pausing to ask how these things all came about. We do not reflect upon the age of our fields any more than we do upon the size of the globe under our feet: when we become curious about such matters and look upon the mountains as either old or young, or as the subjects of birth, growth, and decay, then we are unconscious geologists. It is to our interest in such things that geology appeals and it is this interest that it stimulates and guides.

What an astonishing revelation, for instance, that the soil was born of the rocks, and is still born of the rocks; that every particle of it was once locked up in the primitive granite and was unlocked by the slow action of the rain and the dews and the snows; that the rocky ribs of the earth were clothed with this fertile soil out of which we came and to which we return by their own decay; that the pulling-down of the inorganic meant the building-up of the organic; that the death of the crystal meant the birth of the cell, and indirectly of you and me and of all that lives upon the earth.

Had there been no soil, had the rocks not decayed, there had been no you and me. Such considerations have long made me feel a keen interest in geology, and especially of late years have stimulated my desire to try to see the earth as the geologist sees it. I have always had a good opinion of the ground underfoot, out of which we all come, and to which we all return; and the story the geologists tell us about it is calculated to enhance greatly that good opinion.

I think that if I could be persuaded, as my fathers were, that the world was made in six days, by the fiat of a supernatural power, I should soon lose my interest in it. Such an account of it takes it out of the realm of human interest, because it takes it out of the realm of natural causation, and places it in the realm of the arbitrary, and non-natural. But to know that it was not made at all, in the mechanical sense, but that it grew--that it is an evolution as much as the life upon the surface, that it has an almost infinite past, that it has been developing and ripening for millions upon millions of years, a veritable apple upon the great sidereal tree, ameliorating from cycle to cycle, mellowing, coloring, sweetening--why, such a revelation adds immensely to our interest in it.

As with nearly everything else, the wonder of the world grows the more we grasp its history. The wonder of life grows the more we consider the chaos of fire and death out of which it came; the wonder of man grows the more we peer into the abyss of geologic time and of low bestial life out of which he came.

Not a tree, not a shrub, not a flower, not a green thing growing, not an insect of an hour, but has a background of a vast aeon of geologic and astronomic time, out of which the forces that shaped it have emerged, and over which the powers of chaos and darkness have failed to prevail.

The modern geologist affords us one of the best illustrations of the uses of the scientific imagination that we can turn to. The scientific imagination seems to be about the latest phase of the evolution of the human mind. This power of interpretation of concrete facts, this Miltonic flight into time and space, into the heavens above, and into the bowels of the earth beneath, and bodying forth a veritable history, a warring of the powers of light and darkness, with the triumph of the angels of light and life, makes Milton's picture seem hollow and unreal. The creative and poetic imagination has undoubtedly already reached its high-water mark. We shall probably never see the great imaginative works of the past surpassed or even equaled. But in the world of scientific discovery and interpretation, we see the imagination working in new fields and under new conditions, and achieving triumphs that mark a new epoch in the history of the race. Nature, which once terrified man and made a coward of him, now inspires him and fills him with love and enthusiasm.

The geologist is the interpreter of the records of the rocks. From a bit of strata here, and a bit there, he re-creates the earth as it was in successive geologic periods, as Cuvier reconstructed his extinct animals from fragments of their bones; and the same interpretative power of the imagination is called into play in both cases, only the palaeontologist has a much narrower field to work in, and the background of his re-creations must be supplied by the geologist.

Everything connected with the history of the earth is on such a vast scale--such a scale of time, such a scale of power, such a scale of movement--that in trying to measure it by our human standards and experience we are like the proverbial child with his cup on the seashore. Looked at from our point of view, the great geological processes often seem engaged in world-destruction rather than in world-building. Those oft-repeated invasions of the continents by the ocean, which have gone on from Archaean times, and during which vast areas which had been dry land for ages were engulfed, seem like world-wide catastrophes. And no doubt they were such to myriads of plants and animals of those times. But this is the way the continents grew. All the forces of the invading waters were engaged in making more land.

The geologist is bold; he is made so by the facts and processes with which he deals; his daring affirmations are inspired by a study of the features of the earth about him; his time is not our time, his horizons are not our horizons; he escapes from our human experiences and standards into the vast out-of-doors of the geologic forces and geologic ages. The text he deciphers is written large, written across the face of the continent, written in mountain-chains and ocean depths, and in the piled strata of the globe. We untrained observers cannot spell out these texts, because they are written large; our vision is adjusted to smaller print; we are like the school-boy who finds on the map the name of a town or a river, but does not see the name of the state or the continent printed across it. If the geologist did not tell us, how should we ever suspect that probably where we now stand two or more miles of strata have been worn away by the winds and rains; that the soil of our garden, our farm, represents the ashes of mountains burned up in the slow fires of the geologic ages.

Geology first gives us an adequate conception of time. The limitations which shut our fathers into the narrow close of six thousand years are taken down by this great science and we are turned out into the open of unnumbered millions of years. Upon the background of geologic time our chronological time shows no more than a speck upon the sky. The whole of human history is but a mere fraction of a degree of this mighty arc. The Christian era would make but a few seconds of the vast cycle of the earth's history. Geologic time! The words seem to ring down through the rocky strata of the earth's crust; they reverberate under the mountains, and make them rise and fall like the waves of the sea; they open up vistas through which we behold the continents and the oceans changing places, and the climates of the globe shifting like clouds in the sky; whole races and tribes of animal forms disappear and new ones come upon the scene. Such a past! the imagination can barely skirt the edge of it. As the pool in the field is to the sea that wraps the earth, so is the time of our histories to the cycle of ages in which the geologist reckons the events of the earth's history.

Through the eyes of the geologist one may look upon his native hills and see them as they were incalculable ages ago, and as they probably will be incalculable ages ahead; those hills, so unchanging during his lifetime, and during a thousand lifetimes, he may see as flitting as the cloud shadows upon the landscape. Out of the dark abyss of geologic time there come stalking the ghosts of lost mountains and lost hills and valleys and plains, or lost rivers and lakes, yea, of lost continents; we see a procession of the phantoms of strange and monstrous beasts, many of them colossal in size and fearful in form, and among the minor forms of this fearful troop of spectres we see the ones that carried safely forward, through the vicissitudes of those ages, the precious impulse that was to eventuate in the human race.

Only the geologist knows the part played by erosion in shaping the earth's surface as we see it. He sees, I repeat, the phantoms of vanished hills and mountains all about us. He sees their shadow forms wherever he looks. He follows out the lines of the flexed or folded strata where they come to the surface, and thus sketches in the air the elevation that has disappeared. In some places he finds that the valleys have become hills and the hills have become valleys, or that the anticlines and synclines, as he calls them, have changed places--as a result of the unequal hardness of the rocks. Over all the older parts of the country the original features have been so changed by erosion that, could they be suddenly restored, one would be lost on his home farm. The rocks have melted into soil, as the snow-banks in spring melt into water. The rocks that remain are like fragments of snow or ice that have so far withstood the weather. Geologists tell us that the great Appalachian chain has been in the course of the ages reduced almost to a base level or peneplain, and then reelevated and its hills and mountains carved out anew.

We change the surface of the earth a little with our engineering, drain a marsh, level a hill, sweep away a forest, or bore a mountain, but what are these compared with the changes that have gone on there before our race was heard of? In my native mountains, the Catskills, all those peaceful pastoral valleys, with their farms and homesteads, lie two or three thousand feet below the original surface of the land. Could the land be restored again to its first condition in Devonian times, probably the fields where I hoed corn and potatoes as a boy would be buried one or two miles beneath the rocks.

The Catskills are residual mountains, or what Agassiz calls "denudation mountains." When we look at them with the eye of the geologist we see the great plateau of tableland of Devonian times out of which they were carved by the slow action of the sub-aerial forces. They are like the little ridges and mounds of soil that remain of your garden-patch after the waters of a cloudburst have swept over it. They are immeasurably old, but they do not look it, except to the eye of the geologist. There is nothing decrepit in their appearance, nothing broken, or angular, or gaunt, or rawboned. Their long, easy, flowing lines, their broad, smooth backs, their deep, wide, gently sloping valleys, all help to give them a look of repose and serenity, as if the fret and fever of life were long since passed with them. Compared with the newer mountains of uplift in the West, they are like cattle lying down and ruminating in the field beside alert wild steers with rigid limbs and tossing horns. They sleep and dream with bowed heads upon the landscape. Their great flanks and backs are covered with a deep soil that nourishes a very even growth of beech, birch, and maple forests. Though so old, their tranquillity never seems to have been disturbed; no storm-and-stress period has left its mark upon them. Their strata all lie horizontal just as they were laid down in the old seas, and nothing but the slow gentle passage of the hand of time shows in their contours. Mountains of peace and repose, hills and valleys with the flowing lines of youth, coming down to us from the fore- world of Palaeozoic time, yet only rounded and mellowed by the aeons they have passed through. Old, oh, so old, but young with verdure and limpid streams, and the pastoral spirit of to-day!

To the geologist most mountains are short-lived. When he finds great sturdy ranges, like the Alps, the Andes, the Himalayas, he knows they are young,--mere boys. When they get old, they will be cut down, and their pride and glory gone. A few more of these geologic years and they will be reduced to a peneplain,--only their stumps left. This seems to hold truer of mountains that are wrinkles in the earth's crust--squeezed up and crumpled stratified rock, such as most of the great mountain-systems are--than of mountains of erosion like the Catskills, or of upheaval like the Adirondacks. The crushed and folded and dislocated strata are laid open to the weather as the horizontal strata, and as the upheaved masses of Archaean rock are not. Moreover, strata of unequal hardness are exposed, and this condition favors rapid erosion.

In imagination the geologist is present at the birth of whole mountain-ranges. He sees them gestating in the womb of their mother, the sea. Where our great Appalachian range now stands, he sees, in the great interior sea of Palaeozoic time, what he calls a "geosyncline," a vast trough, or cradle, being slowly filled with sediment brought down by the rivers from the adjoining shores. These sediments accumulate to the enormous depth of twenty-five thousand feet, and harden into rock. Then in the course of time they are squeezed together and forced up by the contraction of the earth's crust, and thus the Appalachians are born. When Mother Earth takes a new hitch in her belt, her rocky garment takes on new wrinkles. Just why the earth's crust should wrinkle along lines of rock of such enormous thickness is not a little puzzling. But we are told it is because this heavy mass of sediment presses the sea-bottom down till the rocks are fused by the internal heat of the earth and thus a line of weakness is established. In any case the earth's forces act as a whole, and the earth's crust at the thickest points is so comparatively thin--probably not much more than a heavy sheet of cardboard over a six-inch globe--that these forces seem to go their own way regardless of such minor differences.

The Alps and the Himalayas, much younger than our Appalachians, were also begotten and nursed in the cradle of a vast geosyncline in the Tertiary seas. We speak of the birth of a mountain-range in terms of a common human occurrence, or as if it were an event that might be witnessed, measurable in human years or days, whereas it is an event measurable only in geologic periods, and geologic periods are marked off only on the dial-face of eternity. The old Hebrew writer gave but a faint image of it when he said that with the Lord a thousand years are as one day; it is hardly one hour of the slow beat of that clock whose hours mark the periods of the earth's development.

The whole long period during which the race of man has been rushing about, tickling and scratching and gashing the surface of the globe, would make but a small fraction of one of the days that make up the periods with which the geologist deals. And the span of human life, how it dwindles to a point in the face of the records of the rocks! Doubtless the birth of some of the mountain-systems of the globe is still going on, and we suspect it not; an elevation of one foot in a century would lift up the Sierra or the Rocky Mountains in a comparatively short geologic period.

II

It was the geologist that emboldened Tennyson to sing,--

"The hills are shadows and they flow
From form to form and nothing stands,
They melt like mists, the solid lands,
Like clouds they shape themselves and go."

But some hills flow much faster than others. Hills made up of the latest or newest formations seem to take to themselves wings the fastest.

The Archaean hills and mountains, how slowly they melt away! In the Adirondacks, in northern New England, in the Highlands of the Hudson, they still hold their heads high and have something of the vigor of their prime.

The most enduring rocks are the oldest; and the most perishable are, as a rule, the youngest. It takes time to season and harden the rocks, as it does men. Then the earlier rocks seem to have had better stuff in them. They are nearer the paternal granite; and the primordial seas that mothered them were, no doubt, richer in the various mineral solutions that knitted and compacted the sedimentary deposits. The Cretaceous formations melt away almost like snow. I fancy that the ocean now, compared with the earlier condition when it must have been so saturated with mineral elements, is like thrice-skimmed milk.

The geologist is not stinted for time. He deals with big figures. It is refreshing to see him dealing out his years so liberally. Do you want a million or two to account for this or that? You shall have it for the asking. He has an enormous balance in the bank of Time, and he draws upon it to suit his purpose. In human history a thousand years is a long time. Ten thousand years wipe out human history completely. Ten thousand more, and we are probably among the rude cave-men or river-drift men. One hundred thousand, and we are--where? Probably among the simian ancestors of man. A million years, and we are probably in Eocene or Miocene times, among the huge and often grotesque mammals, and our ancestor, a little creature, probably of the marsupial kind, is skulking about and hiding from the great carnivorous beasts that would devour him.

"Little man, least of all,
Among the legs of his guardians tall,
Walked about with puzzled look.
Him by the hand dear Nature took,
Dearest Nature, strong and kind,
Whispered, 'Darling, never mind!
To-morrow they will wear another face,
The founder thou; these are thy race!'"

I fancy Emerson would be surprised and probably displeased at the use I have made of his lines. I remember once hearing him say that his teacher in such matters as I am here touching upon was Agassiz, and not Darwin. Yet did he not write that audacious line about "the worm striving to be man"? And Nature certainly took his "little man" by the hand and led him forward, and on the morrow the rest of the animal creation "wore another face."

III

In my geological studies I have had a good deal of trouble with the sedimentary rocks, trying to trace their genealogy and getting them properly fathered and mothered. I do not think the geologists fully appreciate what a difficult problem the origin of these rocks presents to the lay mind. They bulk so large, while the mass of original crystalline rocks from which they are supposed to have been derived is so small in comparison. In the case of our own continent we have, to begin with, about two million of square miles of Archaean rocks in detached lines and masses, rising here and there above the primordial ocean; a large triangular mass in Canada, and two broken lines of smaller masses running south from it on each side of the continent, inclosing a vast interior sea between them. To end with, we have the finished continent of eight million or more square miles, of an average height of two thousand feet above the sea, built up or developed from and around these granite centres very much as the body is built up and around the bones, and of such prodigious weight that some of our later geologists seek to account for the continental submarine shelf that surrounds the continent on the theory that the land has slowly crept out into the sea under the pressure of its own weight. And all this,--to say nothing of the vast amount of rock, in some places a mile or two in thickness, that has been eroded from the land surfaces of the globe in later geological time, and now lies buried in the seas and lakes,--we are told, is the contribution of those detached portions of Archaean rock that first rose above the primordial seas. It is a greater miracle than that of the loaves and the fishes. We have vastly more to end with than we had to begin with. The more the rocks have been destroyed, the more they have increased; the more the waters have devoured them, the more they have multiplied and waxed strong.

Either the geologists have greatly underestimated the amount of Archaean rock above the waters at the start, or else there are factors in the problem that have not been taken into the account. Lyell seems to have appreciated the difficulties of the problem, and, to account for the forty thousand feet of sediment deposited in Palaeozoic times in the region of the Appalachians, he presupposes a neighboring continent to the east, probably formed of Laurentian rocks, where now rolls the Atlantic. But if such a continent once existed, would not some vestige of it still remain? The fact that no trace of it as been found, it seems to me, invalidates Lyell's theory.

Archaean time in geologic history answers to pre-historic time in human history; all is dark and uncertain, though we are probably safe in assuming that there was more strife and turmoil among the earth-building forces than there has ever been since. The body of unstratified rock within the limits of North America may have been much greater than is supposed, but it seems to me impossible that it could have been anything like as massive as the continent now is. If this had been the case there would have been no great interior sea, and no wide sea-margins in which the sediments of the stratified rocks could have been deposited. More than four fifths of the continent is of secondary origin and shows that vast geologic eras went to the making of it.

It is equally hard to believe that the primary or igneous rocks, where they did appear, were sufficiently elevated to have furnished through erosion the all but incalculable amount of material that went to the making of our vast land areas. But the geologists give me the impression that this is what we are to believe.

Chamberlin and Salisbury, in their recent college geology, teach that each new formation implies the destruction of an equivalent amount of older rock--every system being entirely built up out of the older one beneath it. Lyell and Dana teach the same thing. If this were true, could there have been any continental growth at all? Could a city grow by the process of pulling down the old buildings for material to build the new? If the geology is correct, I fail to see how there would be any more land surface to-day then there was in Archaean times. Each new formation would only have replaced the old from which it came. The Silurian would only have made good the waste of the Cambrian, and the Devonian made good the waste of the Silurian, and so on to the top of the series, and in the end we should still have been at the foot of the stairs. That vast interior sea that in Archaean times stretched from the rudimentary Appalachian Mountains to the rudimentary Rocky Mountains, and which is now the heart of the continent, would still have been a part of the primordial ocean. But instead of that, this sea is filled and piled up with sedimentary rocks thousands of feet thick, that have given birth on their surfaces to thousands of square miles of as fertile soil as the earth holds.

That the original crystalline rocks played the major part in the genealogy of the subsequent stratified rocks, it would be folly to deny. But it seems to me that chemical and cosmic processes, working through the air and the water, have contributed more than they have been credited with.

It looks as if in all cases when the soil is carried to the seabottom as sediment, and again, during the course of ages, consolidated into rocks, the rocks thus formed have exceeded in bulk the rocks that gave them birth. Something analogous to vital growth takes place. It seems as if the original granite centres set the world-building forces at work. They served as nuclei around which the materials gathered. These rocks bred other rocks, and these still others, and yet others, till the framework of the land was fairly established. They were like the pioneer settlers who plant homes here and there in the wilderness, and then in due time all the land is peopled.

The granite is the Adam rock, and through a long line of descent the major part of all the other rocks directly or indirectly may be traced. Thus the granite begot the Algonquin, the Algonquin begot the Cambrian, the Cambrian begot the Silurian, the Silurian begot the Devonian, and so on up through the Carboniferous, the Permian, the Mesozoic rocks, the Tertiary rocks, to the latest Quaternary deposit.

But the curious thing about it all is the enormous progeny from so small a beginning; the rocks seem really to have grown and multiplied like organic beings; the seed of the granite seems to have fertilized the whole world of waters, and in due time they brought forth this huge family of stratified rocks. There stands the Archaean Adam, his head and chest in Canada, his two unequal legs running, one down the Pacific coast, and one down the Atlantic Coast, and from his loins, we are told, all the progeny of rocks and soils that make up the continent have sprung, one generation succeeding another in regular order. His latest offspring is in the South and Southwest, and in the interior. These are the new countries, geologically speaking, as well as humanly speaking.

The great interior sea, epicontinental, the geologists call it, seems to have been fermenting and laboring for untold aeons in building up these parts of the continent. In the older Eastern States we find the sons and grandsons of the old Adam granite; but in the South and West we find his offspring of the twentieth or twenty-fifth generation, and so unlike their forebears; the Permian rocks, for instance, and the Cretaceous rocks, are soft and unenduring, for the most part. The later slates, too, are degenerates, and much of the sandstones have the hearts of prodigals. In the Bad Lands of Arizona I could have cut my way into some of the Eocene formations with my pocket-knife. Apparently the farther away we get from the parent granite, the more easily is the rock eroded. Nearly all the wonderful and beautiful sculpturing of the rocks in the West and Southwest is in rocks of comparatively recent date.

Can we say that all the organic matter of our time is from preexisting organic matter? one organism torn down to build up another? that the beginning of the series was as great as the end? There may have been as much matter in a state of vital organization in Carboniferous or in Cretaceous times as in our own, but there is certainly more now than in early Palaeozoic times. Yet every grain of this matter has existed somewhere in some form for all time. Or we might ask if all the wealth of our day is from preexisting wealth--one fortune pulled down to build up another,--too often the case, it is true,--thus passing the accumulated wealth along from one generation to another. On the contrary, has there not been a steady gain of that we call wealth through the ingenuity and the industry of man directed towards the latent wealth of the earth? In a parallel manner has there been a gain in the bulk of the secondary rocks through the action of the world-building forces directed to the sea, the air, and the preexisting rocks. Had there been no gain, the fact would suggest the ill luck of a man investing his capital in business and turning it over and over, and having no more money at the end than he had in the beginning.

Nothing is in the sedimentary rock that was not at one time in the original granite, or in the primordial seas, or in the primordial atmosphere, or in the heavens above, or in the interior of the earth beneath. We must sweep the heavens, strain the seas, and leach the air, to obtain all this material. Evidently the growth of these rocks has been mainly a chemical process--a chemical organization of preexisting material, as much so as the growth of a plant or a tree or an animal. The color and texture and volume of each formation differ so radically from those of the one immediately before it as to suggest something more than a mere mechanical derivation of one from the other. New factors, new sources, are implied. "The farther we recede from the present time," says Lyell, "and the higher the antiquity of the formations which we examine, the greater are the changes which the sedimentary deposits have undergone." Above all have chemical processes produced changes. This constant passage of the mineral elements of the rocks through the cycle of erosion, sedimentation, and reinduration has exposed them to the action of the air, the light, the sea, and has thus undoubtedly brought about a steady growth in their volume and a constant change in their color and texture. Marl and clay and green sand and salt and gypsum and shale, all have their genesis, all came down to us in some way or in some degree, from the aboriginal crystalline rocks; but what transformations and transmutations they have undergone! They have passed through Nature's laboratory and taken on new forms and characteristics.

"All sediments deposited in the sea," says my geology, "undergo more or less chemical change," and many chemical changes involve notable changes in volume of the mineral matter concerned. It has been estimated that the conversion of granite rock into soil increases its volume eighty-eight per cent, largely as the result of hydration, or the taking up of water in the chemical union. The processes of oxidation and carbonation are also expansive processes. Whether any of this gain in volume is lost in the process of sedimentation and reconsolidation, I do not know. Probably all the elements that water takes from the rocks by solution, it returns to them when the disintegrated parts, in the form of sediment in the sea, is again converted into strata. It is in this cycle of rock disintegration and rock re-formation that the processes of life go on. Without the decay of the rock there could be no life on the land. Water and air are always the go-betweens of the organic and inorganic. After the rains have depleted the rocks of their soluble parts and carried them to the sea, they come back and aid vegetable life to unlock and appropriate other soluble parts, and thus build up the vegetable and, indirectly, the animal world.

That the growth of the continents owes much to the denudation of the sea-bottom, brought about by the tides and the ocean-currents, which were probably much more powerful in early than in late geologic times, and to submarine mineral springs and volcanic eruptions of ashes and mud, admits of little doubt. That it owes much to extra-terrestrial sources--to meteorites and meteoric dust--also admits of little doubt.

It seems reasonable that earlier in the history of the evolution of our solar system there should have been much more meteoric matter drifting through the interplanetary spaces than during the later ages, and that a large amount of this matter should have found its way to the earth, in the form either of solids or of gases. Probably much more material has been contributed by volcanic eruptions than there is any evidence of apparent. The amount of mineral matter held in solution by the primordial seas must have been enormous. The amount of rock laid down in Palaeozoic times is estimated at fifty thousand feet, and of this thirteen thousand feet were limestone; while the amount laid down in Mesozoic times, for aught we know a period quite as long, amounts to eight thousand feet, indicating, it seems to me, that the deposition of sediment went on much more rapidly in early geologic times. We are nearer the beginning of things. All chemical processes in the earth's crust were probably more rapid. Doubtless the rainfall was more, but the land areas must have been less. The greater amount of carbon dioxide in the air during Palaeozoic times would have favored more rapid carbonation. When granite is dissolved by weathering, carbon unites with the potash, the soda, the lime, the magnesia, and the iron, and turns them into carbonates and swells their bulk. The one thing that is passed along from formation to formation unchanged is the quartz sand. Quartz is tough, and the sand we find to-day is practically the same that was dissolved out of the first crystalline rocks.

Take out of the soil and out of the rocks all that they owe to the air,--the oxygen and the carbon,--and how would they dwindle! The limestone rocks would practically disappear.

Probably not less that one fourth of all the sedimentary rocks are limestone, which is of animal origin. How much of the lime of which these rocks were built was leached out of the land-areas, and how much was held in solution by the original sea-water, is of course a question. But all the carbon they hold came out of the air. The waters of the primordial ocean were probably highly charged with mineral matter, with various chlorides and sulphates and carbonates, such as the sulphate of soda, the sulphate of lime, the sulphate of magnesia, the chloride of sodium, and the like. The chloride of sodium, or salt, remains, while most of the other compounds have been precipitated through the agency of minute forms of life, and now form parts of the soil and of the stratified rocks beneath it.

If the original granite is the father of the rocks, the sea is the mother. In her womb they were gestated and formed. Had not this seesaw of land and ocean taken place, there could have been no continental growth. Every time the land took a bath in the sea, it came up enriched and augmented. Each new layer of rocky strata taken on showed a marked change in color and texture. It was a kind of evolution from that which preceded it. Whether the land always went down, or whether the sea at times came up, by reason of some disturbance of the ocean floors in its abysmal depths, we have no means of knowing. In any case, most of the land has taken a sea bath many times, not all taking the plunge at the same time, but different parts going down in successive geologic ages. The original granite upheavals in British America, and in New York and New England, seem never to have taken this plunge, except an area about Lake Superior which geologists say has gone down four or five times. The Laurentian and Adirondack ranges have never been in pickle in the sea since they first saw the light. In most other parts of the continent, the seesaw between the sea and the land has gone on steadily from the first, and has been the chief means of the upbuilding of the land.

To the slow and oft-repeated labor-throes of the sea we owe the continents. But the sea devours her children. Large areas, probably continental in extent, have gone down and have not yet come up, if they ever will. The great Mississippi Valley was under water and above water time after time during the Palaeozoic period. The last great invasion of the land by the sea, and probably the greatest of all, seems to have been in Cretaceous times, at the end of the Mesozoic period. There were many minor invasions during Tertiary times, but none on so large a scale as this Cretaceous invasion. At this time a large part of North and South America, and of Europe, and parts of Asia and Australia went under the ocean. It was as if the earth had exhaled her breath and let her abdomen fall. The sea united the Gulf of Mexico with the Arctic Ocean, and covered the Prairie and the Gulf States and came up over New Jersey to the foot of the Archaean Highlands. This great marine inundation probably took place several million years ago. It was this visitation of the sea that added the vast chalk beds to England and France. In parts of this country limestone beds five or six thousand feet thick were laid down, as well as extensive chalk beds. The earth seems to have taken another hitch in her girdle during this era. As the land went down, the mountains came up. Most of the great Western mountain-chains were formed during this movement, and the mountains of Mexico were pushed up. The Alps were still under the sea, but the Sierra and the Alleghanies were again lifted.

It is very interesting to me to know that in Colorado charred wood, and even charcoal, have been found in Cretaceous deposits. The fact seems to give a human touch to that long-gone time. It was, of course, long ages before the evolution of man, as man, had taken place, yet such is the power of association, that those charred sticks instantly call him to mind, as if we had come upon the place of his last campfire. At any rate, it is something to know that man, when he did come, did not have to discover or invent fire, but that this element, which has played such a large part in his development and civilization, was here before him, waiting, like so many other things in nature, to be his servant and friend. As Vulcan was everywhere rampant during this age, throwing out enough lava in India alone to put a lava blanket four or five feet thick over the whole surface of the globe, it was probably this fire that charred the wood. It would be interesting to know if these enormous lava-flows always followed the subsidence of some part of the earth's crust. In Cretaceous times both the subsidence and the lava- flows seem to have been worldwide.

IV

We seem to think that the earth has sown all her wild oats, that her riotous youth is far behind her, and that she is now passing into a serene old age. Had we lived during any of the great periods of the past, we might have had the same impression, so tranquil, for the most part, has been the earth's history, so slow and rhythmical have been the beats of the great clock of time. We see this in the homogeneity of the stratified rocks, layer upon layer for thousands of feet as uniform in texture and quality as the goods a modern factory turns out, every yard of it like every other yard. No hitch or break anywhere. The bedding-planes of many kinds of rock occur at as regular intervals as if they had been determined by some kind of machinery. Here, on the formation where I live, there are alternate layers of slate and sandstone, three or four inches thick, for thousands of feet in extent; they succeed each other as regularly as the bricks and mortar in a brick wall, and are quite as homogeneous. What does this mean but that for an incalculable period the processes of erosion and deposition went on as tranquilly as a summer day? There was no strike among the workmen, and no change in the plan of the building, or in the material.

The Silurian limestone, the old red sandstone, the Hamilton flag, the Oneida conglomerate, where I have known them, are as homogeneous as a snowbank, or as the ice on a mountain lake; grain upon grain, all from the same source in each case, and sifted and sorted by the same agents, and the finished product as uniform in color and quality as the output of some great mill.

Then, after a vast interval, there comes a break: something like an end and a new beginning, as if one day of creation were finished and a new one begun. The different formations lie unconformably upon each other, which means revolution of some sort. There has been a strike or a riot in the great mill, or it has lain idle for a long period, and when it has resumed, a different product is the result. Something happened between each two layers. What?

Though in remote geological ages the earth-building and earth-shaping forces were undoubtedly more active than they are now, and periods of deformation and upheaval were more frequent, yet had we lived in any of those periods we should probably have found the course of nature, certainly when measured by human generations, as even and tranquil as we find it to-day. The great movements are so slow and gentle, for the most part, that we should not have been aware of them had we been on the spot. Once in a million or a half-million years there may have been terrific earthquakes and volcanic eruptions, such as seem to have taken place in Tertiary time, and at the end of the Palaeozoic period. Yet the vast stretches of time between were evidently times of tranquillity.

It is probable that the great glacial winter of Pleistocene times came on as gradually as our own winter, or through a long period of slowly falling temperature, and as it seems to have been many hundred thousand times as long, this preceding period, or great fall, was probably equally long--so long that the whole of recorded human history would form but a small fraction of it. It may easily be, I think, that we are now living in the spring of the great cycle of geologic seasons. The great ice-sheet has withdrawn into the Far North like snowbanks that linger in our wood in late spring, where it still covers Greenland as it once covered this country. When the season of summer is reached, some hundreds of thousands of years hence, it may be that tropical life, both animal and vegetable, will again flourish on the shores of the Arctic Ocean, as it did in Tertiary times. And all this change will come about so quietly and so slowly that nobody will suspect it.

That the crust of the earth is becoming more and more stable seems a natural conclusion, but that all folding and shearing and disruption of the strata are at an end, is a conclusion we cannot reach in the face of the theory that the earth is shrinking as it cools.

The earth cools and contracts with almost infinite slowness, and the great crustal changes that take place go on, for the most part, so quietly and gently that we should not suspect them were we present on the spot, and long generations would not suspect them. Elevations have taken place across the beds of rivers without deflecting the course of the river; the process was so slow that the river sawed down through the rock as fast as it came up. Nearly all the great cosmic and terrestrial changes and revolutions are veiled from us by this immeasurable lapse of time.

Any prediction about the permanence of the land as we know it, or as the race has known it, or of our immunity from earthquakes or volcanic eruptions, or of a change of climate, or of any cosmic catastrophe, based on human experience, is vain and worthless. What is or has been in man's time is no criterion as to what will be in God's time. The periods of great upheaval and deformation in the earth's crust appear to be separated by millions of years. Away back in pre-Cambrian times, there appear to have been immense periods during which the peace and repose of the globe were as profound as in our own time. Then at the end of Palaeozoic time--how many millions of years is only conjectural--the truce of aeons was broken, and the dogs of war let loose; it was a period of revolution which resulted in the making of one of our greatest mountain-systems, the Appalachian, and in an unprecedented extinction of species. Later eras have witnessed similar revolutions. Why may they not come again? The shrinking of the cooling globe must still go on, and this shrinking must give rise to surface disturbances and dislocations, maybe in the uplift of new mountain-ranges from the sea-bottom, now undreamed of, and in volcanic eruptions as great as any in the past. Such a shrinkage and eruption made the Hawaiian Islands, probably in Tertiary times; such a shrinkage may make other islands and other continents before another period of equal time has elapsed.

Of course the periods and eras into which the geologists divide geologic time are as arbitrary as the months and seasons into which we divide our year, and they fade out into each other in much the same way; but they are really as marked as our seasonal divisions. Not in their climates--for the climate of the globe seems to have been uniformly warm from pole to pole, without climatic zones, throughout the vast stretch of Palaeozoic and Mesozoic times--but in the succession of animal and vegetable life which they show. The rocks are the cemeteries of the different forms of life that have appeared upon the globe, and here the geologist reads their succession in time, and assigns them to his geologic horizons accordingly. The same or allied forms appeared upon all parts of the earth at approximately the same time, so that he can trace his different formations around the world by the fossils they hold. Each period had its dominant forms. The Silurian was the great age of trilobites; the Devonian, the age of fishes; Mesozoic times swarm with the gigantic reptiles; and in Tertiary times the mammals are dominant. Each period and era has its root in that which preceded it. There were rude, half-defined fishes in the Silurian, and probably the beginning of amphibians in the Devonian, and some small mammalian forms in the Mesozoic time, and doubtless rude studies of the genus Homo in Tertiary times. Nature works up her higher forms Jike a human inventor from rude beginnings. Her first models barely suggest her later achievements.

In the vegetable world it has been the same; from the first simple algae in the Cambrian seas up to the forests of our own times, the gradation is easily traced. Step by step has vegetable life mounted. The great majority of the plants and animals of one period fail to pass over into the next, just as our spring flowers fail to pass over into summer, and our summer flowers into fall. But the law of evolution is at work, and life always rises on stepping-stones of its dead self to higher things.

[The end]
John Burroughs's essay: Through The Eyes Of The Geologist

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