UNDER the general title of "Geology" are usually included at least two distinct branches of inquiry, allied to one another in the closest manner, and yet so distinct as to be largely capable of separate study. Geology,* in its strict sense, is the science which is concerned with the investigation of the materials which compose the earth, the methods in which those materials have been arranged, and the causes and modes of origin of these arrangements. In this limited aspect, Geology is nothing more than the Physical Geography of the past, just as Physical Geography is the Geology of to-day; and though it has to call in the aid of Physics, Astronomy, Mineralogy, Chemistry, and other allies more remote, it is in itself a perfectly distinct and individual study. One has, however, only to cross the threshold of Geology to discover that the field and scope of the science cannot be thus rigidly limited to purely physical problems. The study of the physical development of the earth throughout past ages brings us at once in contact with the forms of animal and vegetable life which peopled its surface in bygone epochs, and it is found impossible adequately to com * Gr. gẽ, the earth; logos, a discourse. * prehend the former, unless we possess some knowledge of the latter. However great its physical advances may be, Geology remains imperfect till it is wedded with Palæontology, a study which essentially belongs to the vast complex of the Biological Sciences, but at the same time has its strictly geological side. Dealing, as it does, wholly with the consideration of such living beings as do not belong exclusively to the present order of things, Palæontology is, in reality, a branch of Natural History, and may be regarded as substantially the Zoology and Botany of the past. It is the ancient life-history of the earth, as revealed to us by the labours of paleontologists, with which we have mainly to do here; but before entering upon this, there are some general questions, affecting Geology and Paleontology alike, which may be very briefly discussed. The working geologist, dealing in the main with purely physical problems, has for his object to determine the material structure of the earth, and to investigate, as far as may be, the long chain of causes of which that structure is the ultimate result. No wider or more extended field of inquiry could be found; but philosophical geology is not content with this. At all the confines of his science, the transcendental geologist finds himself confronted with some of the most stupendous problems which have ever engaged the restless intellect of humanity. The origin and primæval constitution of the terrestrial globe, the laws of geologic action through long ages of vicissitude and development, the origin of life, the nature and source of the myriad complexities of living beings, the advent of man, possibly even the future history of the earth, are amongst the questions with which the geologist has to grapple in his higher capacity. These are problems which have occupied the attention of philosophers in every age of the world, and in periods long antecedent to the existence of a science of geology. The mere existence of cosmogonies in the religion of almost every nation, both ancient and modern, is a sufficient proof of the eager desire of the human mind to know something of the origin of the earth on which we tread. Every human being who has gazed on the vast panorama of the universe, though it may have been but with the eyes of a child, has felt the longing to solve, however imperfectly, "the riddle of the painful earth," and has, .consciously or unconsciously, elaborated some sort of a theory as to the why and wherefore of what he sees. Apart from the profound and perhaps inscrutable problems which lie at the bottom of human existence, men have in all ages invented * Gr. palaios, ancient; onta, beings; logos, discourse. theories to explain the common phenomena of the material universe; and most of these theories, however varied in their details, turn out on examination to have a common root, and to be based on the same elements. Modern geology has its own theories on the same subject, and it will be well to glance for a moment at the principles underlying the old and the new views. It has been maintained, as a metaphysical hypothesis, that there exists in the mind of man an inherent principle, in virtue of which he believes and expects that what has been, will be; and that the course of nature will be a continuous and uninterrupted one. So far, however, from any such belief existing as a necessary consequence of the constitution of the human mind, the real fact seems to be that the contrary belief has been almost universally prevalent. In all old religions, and in the philosophical systems of almost all ancient nations, the order of the universe has been regarded as distinctly unstable, mutable, and temporary. A beginning and an end have always been assumed, and the course of terrestrial events between these two indefinite points has been regarded as liable to constant interruption by revolutions and catastrophes of different kinds, in many cases emanating from supernatural sources. Few of the more ancient theological creeds, and still fewer of the ancient philosophies, attained body and shape without containing, in some form or another, the belief in the existence of periodical convulsions, and of alternating cycles of destruction and repair. That geology, in its early infancy, should have become imbued with the spirit of this belief, is no more than might have been expected; and hence arose the at one time powerful and generally-accepted doctrine of "Catastrophism." That the succession of phenomena upon the globe, whereby the earth's crust had assumed the configuration and composition which we find it to possess, had been a discontinuous and broken succession, was the almost inevitable conclusion of the older geologists. Everywhere in their study of the rocks they met with apparently impassable gaps, and breaches of continuity that could not be bridged over. Everywhere they found themselves conducted abruptly from one system of deposits to others totally different in mineral character or in stratigraphical position. Everywhere they discovered that well-marked and. easily recognisable groups of animals and plants were succeeded, without the intermediation of any obvious lapse of time, by other assemblages of organic beings of a different character. Everywhere they found evidence that the earth's crust had undergone changes of such magnitude as to render it seemingly irrational to suppose that they could have been produced by any process now in existence. If we add to the above the prevalent belief of the time as to the comparative brevity of the period which had elapsed since the birth of the globe, we can readily understand the general acceptance of some form of catastrophism amongst the earlier geologists. As regards its general sense and substance, the doctrine of catastrophism held that the history of the earth, since first it emerged from the primitive chaos, had been one of periods of repose, alternating with catastrophes and cataclysms of a more or less violent character. The periods of tranquillity were supposed to have been long and protracted; and during each of them it was thought that one of the great geological "formations" was deposited. In each of these periods, therefore, the condition of the earth was supposed to be much the same as it is now-sediment was quietly accumulated at the bottom of the sea, and animals and plants flourished uninterruptedly in successive generations. Each period of tranquillity, however, was believed to have been, sooner or later, put an end to by a sudden and awful convulsion of nature, ushering in a brief and paroxysmal period, in which the great physical forces were unchained and permitted to spring into a portentous activity. The forces of subterranean fire, with their concomitant phenomena of earthquake and volcano, were chiefly relied upon as the efficient causes of these periods of spasm and revolution. Enormous elevations of portions of the earth's crust were thus believed to be produced, accompanied by corresponding and equally gigantic depressions of other portions. In this way new ranges of mountains were produced, and previously existing ranges levelled with the ground, seas were converted into dry land, and continents buried beneath the ocean-catastrophe following catastrophe, till the earth was rendered uninhabitable, and its races of animals and plants were extinguished, never to reappear in the same form. Finally, it was believed that this feverish activity ultimately died out, and that the ancient peace once more came to reign upon the earth. As the abnormal throes and convulsions began to be relieved, the dry land and sea once more resumed their relations of stability, the conditions of life were once more established, and new races of animals and plants sprang into existence, to last until the supervention of another fever-fit. Such is the past history of the globe, as sketched for us, in alternating scenes of fruitful peace and revolutionary destruction, by the earlier geologists. As before said, we cannot wonder at the former general acceptance of Catastrophistic doctrines. Even in the light of our present widely-increased knowledge, the series of geological monuments remains a broken and imperfect one; nor can we ever hope to fill up completely the numerous gaps with which the geological record is defaced. Catastrophism was the natural method of accounting for these gaps, and, as we shall see, it possesses a basis of truth. At present, however, catastrophism may be said to be nearly extinct, and its place is taken by the modern doctrine of "Continuity" or "Uniformity"-a doctrine with which the name of Lyell must ever remain imperishably associated. The fundamental thesis of the doctrine of Uniformity is, that, in spite of all apparent violations of continuity, the sequence of geological phenomena has in reality been a regular and uninterrupted one; and that the vast changes which can be shown to have passed over the earth in former periods have been the result of the slow and ceaseless working of the ordinary physical forces-acting with no greater intensity than they do now, but acting through enormously prolonged periods. The essential element in the theory of Continuity is to be found in the allotment of indefinite time for the accomplishment of the known series of geological changes. It is obviously the case, namely, that there are two possible explanations of all phenomena which lie so far concealed in "the dark backward and abysm of time," that we can have no direct knowledge of the manner in which they were produced. We may, on the one hand, suppose them to be the result of some very powerful cause, acting through a short period of time. That is Catastrophism. Or, we may suppose them to be caused by a much weaker force operating through a proportionately prolonged period. This is the view of the Uniformitarians. It is a question of energy versus time; and it is time which is the true element of the case. An earthquake may remove a mountain in the course of a few seconds; but the dropping of the gentle rain will do the same, if we extend its operations over a millennium. And this is true of all agencies which are now at work, or ever have been at work, upon our planet. The Catastrophists, believing that the globe is but, as it were, the birth of yesterday, were driven of necessity to the conclusion that its history had been checkered by the intermittent action of paroxysmal and almost inconceivably potent forces. The Uniformitarians, on the other hand, maintaining the "adequacy of existing causes," and denying that the known physical forces ever acted in past time with greater intensity than they do at present, are, equally of necessity, driven to the conclusion that |