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In this volume is presented the procession of more important events, physical and vital, that the earth is known or believed to have gone through. It is a history of the earth, and more especially of North America, during the long geological ages, as read in the various Mnds of rocks and in the organisms of the past, the fossils, which together make up its outermost shell.
Historical Geology, sometimes also called Stratigraphical Geology, brings together all that has been made known in the other departments of Geology and Paleontology. In arranging this knowledge, we elucidate the history of the earth from the earliest time of which geologic records are known down to the time of the present. As yet, however, our knowledge is detailed only for western Europe and North America, though considerable scattering information has been gathered from all lands.
As our knowledge of Geology, and especially of Historical Geology, increases, the new facts brought to light suggest new ways of explaicing and correlating those which we have already learned. In consequence, the older views and terminology are constantly changing, and must continue to do so for a long time to come. All knowledge that is increasing is evolving toward the truth, and what is acceptable now may be rejected with the later discovery of new facts and new interpretations.
The known geological record is at best but an imperfect chronicle of the history of the earth, and will always remain so. It abounds in breaks, marking omissions of record, some of which have been caused by non-deposition of strata, others by destruction of record through weathering away of formations, or by obscuring and altering of record through metamorphism or rock change, as explained in the first part of this treatise.
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In our reading of the geologic record through the process of “ trial and error,” we are guided by the law of continuity or uniformity in the operations of nature: through the study of the forces at work to-day, and of their results, we learn to decipher the geologic history of our mundane sphere; and the time will come when geologists will be far more able than they are now to picture to themselves and describe to others the nature of the detailed relief of the lands, the extent and character of the oceans, and the essential life of any given time. Not even all the grander features of this history are yet known, and of the detail but little, so that many generations of workers still have a grand field of scientific endeavor before them. It is through the gathering of all of the detail that will be discerned not only the grander features in the evolution of the earth, but as well its everyday method of work and the incidental results of its physical and organic operations.
Dana has well said: “ In the study of Geology, there is often an expectation to find strongly drawn lines between the eras and periods, or the corresponding subdivisions of the rocks; but geological history is like human history in this respect. Time is one in its course, and all progress one in plan.”
The oceans periodically spread over the lands and the floods are as often withdrawn. Great and grand ranges of mountains have been raised many times near the borders of the continents, only to be broken up little by little and spread out as thick or thin sheets of sediments over the bottoms of the adjacent water basins. “ Rocks fall to dust, and mountains melt away.” Such a shifting of the materials of the high places of the lands into the lower ones or into the seas and oceans displaces an amount of water equivalent to the mass of sediments unloaded into the seas, and this tends to make the oceans overflow the lands. The great majority of these overfloodings of the oceans, however, are due to crustal movements, — depressions and elevations. The roots of former mountains remain forever where they once proudly reared their crests, but their one time grandeur c’an be revealed only by restoring the extent of the folds and calculating the quantity of sediments carried into the nearby areas and there laid down as formations. Why these mountains came to be, the nature of their internal rock structure, and the forces that broke them down and transported their débris to lower levels have been explained in the first part of this text-book. We are now to see the procession of these things.
The lands are slowly but continually undergoing change. Not only are the surfaces of the lands changing, but even their outlines, [ p. 3 ] and yet_there has been no general interchange in position, at a given time or m the comse of the ages, between the continents and the oceans. It is true, however, that the latter have often spread temporarily over the continents as more or less wide shallow seas, and also that vast areas of dry land have gone permanently into the abysses. The oceanic areas are the more permanent features of the earth’s surface, and their basins have grown steadily larger at the expense of the continents. Originally the continents were vastly larger and trended east and west on either side of a mediterranean (Tethys) ; now they strike north and south due to vast founderings of lands into the depths of the Atlantic and Indian oceans. Australasia is but the remains of a greater Australia and southeastern China (Sino-AustraJia).
The geography of the earth has gone through many changes, and even during each geologic period there have been alterations of a profound kind. Great mountain ranges have been folded up in one period and eroded away in the following one; many of them have been bodily elevated more than once and as often removed into the adjacent seas. As the dynamic processes are slow in action, this observation alone indicates that the divisions of geologic time are of long duration, and that the age of the earth is very great. To restore these ancient phases of geography (= Paleogeography) is one of the most difficult of geological problems. Shore-lines are rarely indicated in the sediments, and yet the marine accumulations remain where they were originally deposited. Prom their nature and their organic inclusions can be deciphered the geologic and biologic phenomena of a given time, the topographic relief of the land, and whether the climate was moist or dry, warm, cool, or cold.
The chronological order is the underlying principle in all history, and in Historical Geology the orderly sequence of time is determined by the geologist through a probable succession of rock formations. The final chronological order is ascertained (1) through the actual local order in the superposed sequence of stratified rocks, and in theirJ overlap from place to place; (2) through the degree of evolution attained by the fossils contained in the strata; (3) through the unconformities or breaks in the sequence of rock formations; and (4) through the determined order in which the igneous rocks intersect or cut one another and the stratified formations.
Just as the surface of the earth is in a continual state of slow change due to internal alterations and gravity, so in consequerce must be the atmosphere, since its gases have come out of the hot interior. Even the water is born of the interior earth. When the lands are [ p. 4 ] high and the air drier than usual, then the climates are most trying upon the mountains and especially to the plants and animals living upon their flanks. Then also igneous activity is greatest, and deepseated rock materials are being injected into the lithosphere, lavas and volcanic ashes are spread over it, and new water vapor and gases are added to the atmosphere. We are living in a time when such actions and interactions, physical and organic, are quickened and intensified, and due consideration must be given to this condition when we attempt to explain the phenomena of past geologic ages by the law of continuity or uniformity. We now know of many such crescendoes or accelerations in the ph3rsical and organic evolution of the earth. On the other hand, through vast periods of geologic time there is more or less crustal and atmospheric stability, with changes only of a mild sort. Eventually, however, the stability gives way to maximal crustal instability.
This crustal instability results in changing environments for the plants and animals, in greatest degree among the organics of the lands, and least so in the oceanic realms. Accordingly, organic change is slowest among the organisms of the oceans, and much quickened in the life of the lands. In the later half of the earth’s history, much evidence of the life of the past is preserved to us in the rock sediments, and these fossils are our main dependence in classifying and correlating the stratified rocks of a given time from place to place. During the earlier half of geologic time, the rock record is almost devoid of fossils, but by Cambrian time a fullness of marine life is at hand. Out of the marine realm came migrants upon the lands, first through choice in the fresh waters, and then through necessity in the evanescent waters and finally upon the dry lands. In the seas and oceans the organic evolution continued monotonously through the ages, but on the lands the plants and animals were in constant conflict with their variable habitats. Through ceaseless trial of effort and the weeding out of the less fit, there arose ever more perfected and higher organisms, with greater and greater mentality, to terminate finally in man. Life, once started out of water and carbon dioxide, has gone on ceaselessly, striving through force of circumstances toward better adapted mechanisms endowed with higher and higher thinking powers.