Geology - the Study of the Earth’s History and Development
Definition of Gelogy
Geology is the study of the earth’s history and development, as recorded in the rocks, and of the agencies which have produced that development.
From this definition it appears that the central problem in geology is the deciphering of the earth’s history, and that the historical standpoint is the dominant one. Geology deals with the earth as a cosmical unit, and is a great synthesis of all those sciences which throw light upon the structure of the globe and which may be used in interpreting its records. Astronomy, physics, chemistry, mineralogy, physical geography, zoology, and botany are all drawn upon for this purpose. The goal of our inquiries is the history of the earth as a whole and not of a single continent merely. We should endeavor to gain a true insight into those great processes of development which control the whole visible universe, and which exhibit in the most impressive way the great principles of order and of uniformly acting laws. It is, however, necessary to make a selection from the immense body of ascertained facts, and it is clearly advantageous that, so far as possible, we should make use of our own country for this purpose. It must always be remembered that the instances chosen from familiar scenes are but illustrations and examples of world-wide processes and structures. To find active examples of some important phenomena, we must travel to far-distant lands, but even of these we shall find the unmistakable traces in our own continent, as having been at work here at one time or another in the past.
Geology is one of the most modern of the sciences. In the works of certain’ classical and mediaeval writers we find, it is true, some descriptions of geological phenomena, and sound inferences were sometimes drawn from the facts. But no attempt was made to gather an extensive series of observations, or to construct a harmonious system of facts and inferences, and no one imagined that a connected history of the earth was within the bounds of human attainment. Before such a history could be written, it was necessary that the other physical and natural sciences should have reached a considerable degree of perfection; for geology, as we have seen, is a synthesis of these sciences. It was only in the latter part of the eighteenth century, that these other branches of knowledge had so far been perfected that they could offer to the geologist a firm foundation upon which to build the structure of his own science. The early workers in geology hardly attempted more than to ascertain the materials of which the earth is composed and the way in which those materials are put together. In carrying out this apparently simple task, it soon became evident that the present condition of the earth is the outcome of a long series of past changes, which must be understood if we would comprehend the earth’s structure as it is now. The past must be studied in the light of the present, and the present in the light of the past, for each supplements and helps to explain the other.
Such a conclusion is repugnant to our instinctive feeling, that the earth is stable and well-nigh unchangeable, a feeling which finds expression in Bryant’s familiar line: “The hills rock-ribbed and ancient as the sun.” This very natural belief is due to the exceeding slowness with which the surface features of the globe are modified, so that in the brief span of human life the modifications are almost imperceptible. Generations of men live and die in the same spot, while the natural features of hill and rock, valley and plain, seem to remain exactly as they were. When, however, attention was at last directed to these changes, it was found that they were unceasing, and were especially noticeable in lands which, like the countries around the Mediterranean, had been occupied for many centuries by civilized men. Occasionally, too, great tempests or earthquakes or volcanic outbursts produced changes which could not but strike the most careless observer.
When once the fact was established that the solid globe was subject to change, men looked first to the more obvious and violent natural forces as the agents of this change. To the occasional destructive fury of the hurricane, the earthquake, and the volcano was attributed far greater importance than to the ceaseless but inconspicuous work of the rain and the river. Another reason why sudden and violent catastrophes were regarded as the only important factors of change was the very general belief that the earth was only a few thousand years old. If all the modifications which the earth’s surface has demonstrably undergone were effected within such a comparatively brief period, then they must have been accomplished suddenly and violently and, in great part, by agencies of which we have had no experience. Thus, all sorts of fantastic causes, such as collisions with comets’ tails, were conjured up to account for the facts, and speculation ran riot.
The purely arbitrary character of these speculations and fancies rendered them unsatisfactory to thinking men. The progress of astronomy had gradually familiarized their minds with the idea of the almost infinite distances which separate the heavenly bodies, and with the conceptions of order and the uniform operation of law. These conceptions made the supposed cataclysms and convulsions of the earth’s history seem unnatural and improbable, and led geologists to inquire whether simpler explanations could not be devised. This, in turn, led to the careful study of those modifications of the earth’s surface which are still in progress. Gradually the conviction grew, that the agencies which are still at work upon and within the globe are the same as those which brought about the manifold changes of the past, and that the earth’s history is one of vast and unimaginable length. Scholars made little progress in deciphering the Egyptian hieroglyphics until the discovery of the Rosetta Stone, with its bilingual inscriptions, furnished the key. So the geologists found that one key to the past were to be found in the study of the forces which may be observed in actual operation at the present time.
Another advance was made while the disputes regarding the nature of geological forces and the length of geological time were still in progress. In 1799 William Smith, an English engineer, announced the discovery that the order of succession of the rocks might be determined by means of the fossils, or remains of animals and plants, which they contain. This discovery was made possible by the advances in zoology, by means of which the different species could be accurately discriminated. Since the earliest recorded times, the animals and plants of the earth have been subject to continual change, and the degrees of change give us a standard of chronology, in accordance with which the various groups of rocks may be arranged in their order of succession. The archaeologist makes a similar use of the coins, inscriptions, and objects of art, which he finds among the ruins of buried cities. These enable him to determine the races of men who built those cities and the dates at which they flourished, and to fix their place in the general history of civilization.
The lines along which geology has developed were nearly all laid down late in the last century, or early in the present one, but the progress of the science has led to many changes in men’s conceptions of the subject, some of which changes have been revolutionary. Geology began with the study of Western Europe, and on account of this narrowly restricted range of view, erroneous notions naturally crept into the new science. As the study has been extended to other continents, new and larger views have been gained, and doubtless, when the whole earth has been accurately examined, many of our present opinions will need revision, though we cannot hope ever to reach final certainty upon all points.
To many intelligent people this continual modification of scientific opinion, which is a necessary consequence of advancing knowledge, is a source of annoyance. This attitude of mind comes from a failure to discriminate between fact, and inference or hypothesis. Accurately observed facts may be added to, but they remain trustworthy: the changeable element is the inference which is drawn from the facts. These inferences are of very different degrees of certainty. Some such deductions which were made centuries ago remain unshaken to-day, while others of far more recent date have proved illusory. Thus, when we find a rock composed of cemented sand-grains, arranged in regular beds or layers, we infer that it was laid down under water, because of its exact resemblance to accumulations of sand which are forming under water to-day. If the sandstone be full of marine shells, we infer that it was formed under the sea, and further that the land where the rock is now found was once covered by the sea. Such inferences are practically certain, because they explain all the known facts and are in conflict with none. On the other hand, the hypotheses of Cuvier and others as to the character of the earth’s development, and the manner in which the successive assemblages of animals and plants were called into being, were abandoned long ago.
In the process of reasoning from the known to the unknown, the inferences become the more uncertain, the farther we recede from demonstrable facts. Hypotheses are assumptions which we make to explain and coordinate large numbers of facts, and so long as their true nature is understood, they are useful, indeed indispensable, means of reaching the truth. The objection is that they are too often taught as though they were established beyond dispute. A true hypothesis will prove to be in harmony with newly discovered facts, which will take their place under it simply and naturally. A false hypothesis, on the other hand, may be in accordance with all the facts known at the time when it was proposed, but the progress of discovery will bring to light facts which are inconsistent with the hypothesis, until it is plainly seen to be inadequate and misleading. Yet even a false hypothesis may serve a useful purpose, for it puts before us a definite problem, instead of a mere catalogue of uncorrelated facts. The pathway of every science is strewn with wrecks of hypotheses which have been used, worn out, and thrown aside. In all our thinking and reasoning the distinction between hypothesis and fact must be steadily held in view.
While, in its most comprehensive sense, geology consists in the application of nearly all the physical and natural sciences to the elucidation of the earth’s history, the geologist has his own special field of investigation. This he finds in the rocks, and every exposure of rocks yields him material. It might seem that, at best, his studies must be very superficial, and that he must soon eke out his scanty facts with daring guess-work. Such is happily not the case. The fact that rocks of different ages were formed in different places, and that great disturbances have so tilted immense bodies of rocks that their edges are exposed to view, enables the observer to study vast thicknesses of them, without descending below the surface of the ground. Seventy-five years ago Playfair saw and expressed this truth. “Men can see much further into the interior of the globe than they are aware of, and geologists are reproached without reason for forming theories of the earth, when all they can do is but to make a few scratches on the surface.”
The history of the earth has been recorded, for the most part, upon its successive surfaces, and it is not necessary to penetrate deep into the interior of the globe. In later chapters we shall learn how these surfaces came to be buried to great depths and yet retained the characters impressed upon them when they were superficial in position, as written pages are buried under fresh accumulations of manuscript. This fact, together with the disturbances which have made the deep-seated rocks accessible to study, renders the task of deciphering the record less hopeless than might be imagined.
The study of geology must be carried on at first hand, and cannot be adequately learned from books alone. The use of books is to serve as guides in directing the learner in what to look for and to enable him to compare distant lands with his own. The arrangement and treatment of such a complex subject as geology cannot avoid a certain artificial character, which will surely mislead the student, unless he learns to observe and reason for himself. Some parts of our country are more favorable to geological study than others, but none is entirely devoid of geological interest, and the operations of the dynamical agencies may be watched everywhere. To one who thus familiarizes himself with the structure and history of the country, every landscape will offer a renewed charm and interest. The study of the rocks will lead him step by step to the widest outlook over the history of the earth, to the contemplation of infinities of energy, space, and time, bringing him at last face to face with the mystery of the Universe. To this inscrutable mystery every line of scientific inquiry must ultimately lead, for human knowledge is like a dim taper which illumines a little space, but is surrounded by immeasurable darkness.
The very diverse lines of inquiry which together make up the science of geology must be classified and divided for the purposes of orderly treatment. The following are the principal divisions of the science.
Dynamical Geology - or the study of the natural forces such as wind, weather and water which are now at work and continue a modification of the surface of the Earth. Additionally, a study of the chemical and mechanical changes which they effect. This is the key by which we may interpret past changes.
Structural Geology - or the study of the materials of which the earth is composed and of the manner in which they are arranged. Normally such changes are associated shifts in climatic weather; together with such explanations of the modes in which the arrangement was produced, as may be inferred from the structure.
Physiographical Geology - is an examination of the topographical features of the earth and of the mode in which they were produced. Primarily, this subject is a province of physical geography, but it is a valuable adjunct to geology. The three foregoing divisions together constitute a larger division, which is called Physical Geology, and which is contrasted with;
Historical Geology - This is the study of the earth’s history and development, the changes of level between land and sea, of topography, of climate, and of the successive groups of animals and plants which have lived upon the globe. As we have seen, the historical is the dominant standpoint in geology, the main problem of which is to interpret the records of the earth’s history. The other departments are to be regarded as the means to this great end.