Lyell opposed this view with all his power. According to him we can not judge of geological causes and processes except by study of causes and processes now in operation and producing effects under our eyes. The slow operation of similar causes and processes is sufficient—given time enough—to account for all the phenomena in geological history. Thus arose the extreme opposite doctrine of uniformitarianism. Things have gone on from the beginning and throughout all time much as they are going on now. This view, of course, required illimitable time, and was of great service in enforcing this idea. But, in revulsion from the previous idea of catastrophism, it undoubtedly was pushed much too far.
Meanwhile the theory of evolution was incubating in the mind of Darwin. Even Lyell, while he established the doctrine of slow uniform changes so far as inorganic Nature was concerned, was still compelled to admit supernatural catastrophic changes in organic Nature. Species, even for Lyell, were still immutable—still there were supernatural creation of first individuals, and continuance of similar individuals by natural process of generation. On the publication of Darwin’s Origin of Species by Descent with Modification, Lyell at once embraced the new view as a completion of his principle of causes now in operation and his doctrine of uniformitarianism. In a certain superficial sense evolution is certainly confirmatory of the doctrine of uniformity of causes and processes in the past and the present, but in a deeper sense it is quite contrary in its spirit. Uniformitarians of the Lyell school look upon geology as a chronicle of events—evolutionists as a life history of the earth. The one regards the slow changes as irregular, uncertain, without progress or purpose or goal; the other as an evolution to higher and higher conditions, as a gradual movement onward toward the present condition and toward man as its goal. The recognition of this is only now approaching clearness. If geology is the history of the evolution of the earth from primal chaos until now, then the conditions have changed at every step, and absolute uniformity is impossible. Extreme uniformitarianism is therefore untenable. Catastrophism and uniformitarianism are opposite extremes which must be combined and reconciled. This reconciliation is only now being completed, and we therefore put off its discussion for the present. Suffice it to say now that geologic thought in this regard has passed through three stages—catastrophism, uniformitarianism, and evolutionism. And this latter is the final stage, because (1) it is a complete reconciliation between the other two, and (2) because it is plastic and indefinitely modifiable and progressive, while the other two are equally rigid and unchangeable by their mutual antagonism.
With these fundamental principles in mind, we proceed to touch briefly the most important advances during the century.
EVOLUTION OF EARTH FORMS.
The idea of the progressive development of the earth in its greater features throughout all geologic time by the action of forces resident in the earth itself preceded the acceptance of the evolution of organic forms. We have said that the fundamental idea of geology is that of the evolution of the earth through all time. Now, it was Dana who first studied geology wholly from this point of view. For him geology was the development of the earth as a unit. Before him, doubtless, geology was a kind of history—i. e., a chronicle of thrilling events—but Dana first made it a philosophic history. Before Dana, geology was an account of the succession of formations and their fossil contents. Dana made it an account of the evolution of earth forms and the concomitant and resulting evolution of organic forms. It is true that first and for a long time his evolutional conception was incomplete. It is true that while he attributed the evolution of earth forms to natural causes and processes, he still shrank from applying similar causes to the changes in life forms, but this was the almost necessary result of the then universal belief in the supernatural origin and the unchangeableness of organic forms. He lived to make his conception of evolution as a natural process, both of the earth and of organic forms, complete.
Ocean Basins and Continents.—If we divide geological causes and processes into two general kinds as to their origin—viz., internal, or earth-derived, and external, or sun-derived—evidently the former is the original and fundamental kind. These determine earth forms, while the other only modify them; these determine the great features, the other only the lesser features; the former rough-hews the earth features, the latter shapes them. It is the effects of these interior earth forces which are the most important to study. And among these effects the most fundamentally important of all is the formation of those greatest features—the ocean basins and continental arches. The most probable view is that they are formed by unequal radial contraction in the secular cooling of the earth. The earth was certainly at one time an incandescently hot mass, which gradually cooled and contracted to its present temperature and size. Now, if it were perfectly homogeneous both in density and in conductivity in all parts, then, cooling and contracting equally in every part, it would retain its symmetric oblate-spheroid form, though diminishing in size. But if there were any, the least, heterogeneity either in density or especially in conductivity over large areas, then the more conductive areas, contracting more rapidly toward the center radially, would form hollows or basins, and the less conductive areas would stand out as higher arches. Thus were formed the oceanic basin and the continental arches of the lithosphere. The same causes which produced would continue to increase them, and thus the ocean basins would increase in depth and the continents in height.
The hydrosphere is still to be added. In the beginning of this process doubtless the lithosphere was hot enough to maintain all the water in the form of vapor in the atmosphere. But when the surface was cool enough the water would precipitate and partly or wholly cover the earth—whether partly or wholly would depend on the amount of precipitated water and the amount of inequality which had already taken place. The amount of water, as we know, is sufficient, if the inequalities were removed, to cover the whole surface two and a half miles deep. Inasmuch as the forming of the inequalities is progressive and still going on, it seems improbable that the inequalities had become sufficiently great, at the time of precipitation, to hold the waters. If this be so, then the primeval ocean was universal and the future continents existed only as continental banks in the universal ocean.
However this may have been, there seems little doubt that the same cause which produced the inequalities continued to operate to increase them. The ocean basins, so far as these causes are concerned, must have become deeper and deeper, and the continents larger and larger. In spite of many oscillations producing changes mostly on the margins, but sometimes extending over wide areas in the interior of the continent, this, on the whole, seems to be in accordance with the known geological history of the earth. If so, then the oceanic basins have always been oceanic basins, and the places of the continents have always been substantially the same. This introduces a subject on which there has been much discussion recently—viz.:
The Permanency of Ocean Basins.—Closely associated with the Lyellian uniformitarianism was the doctrine of extreme instability of earth features, especially the forms and places of sea and land. Crust movements were irregularly oscillating to such a degree that in the course of geologic history sea and land frequently and completely changed places. Abundant evidence of this was supposed to be found in the unconformities so frequent in the stratified series. The tendency of that time was toward a belief in up-and-down movements, back-and-forth changes, without discoverable law, rather than progressive onward movement. On first thought it might seem that such lawless movement was rather in keeping with catastrophism than uniformitarianism. But not so, for the movements are supposed to be very slow. Again, it might seem on first thought that gradual progressive change—in a word, evolution—would be peculiarly in accord with uniformitarian ideas. But again not so, because this doctrine was, above all, a revulsion from the idea of supernatural purpose or design or goal contained in catastrophism. Uniformitarianism strongly inclined toward purposelessness, because of its supposed identity with naturalism. Thus for a long time, and still with many geologists, the tendency is toward a belief in irregular movements without discoverable law, toward instability of even the greatest features of the earth—viz., sea basins and continental arches. Geology for them is a chronicle, not a life history.
The contrary movement of thought may be said to have commenced with Dana. Dana studied the earth as a unit, as in some sense an organism developing by forces within itself. The history of the earth is a life history moving progressively toward its completion. The forces originating oceanic basins and continental arches still continue to deepen the former and enlarge the latter. From this point of view, oceanic basins and continental arches must have always been substantially in the same places. Oscillations there have been at all times and in all places, but they affect mainly the outlines of these great features, though sometimes affecting also the interior of continents and mid-sea bottoms, but not sufficiently to change greatly their general form, much less to interchange their places.