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.