Figure 2 represents the Jura at the present time, when the later upheavals have lifted the Jurassic strata to a sharper inclination with the Cretaceous deposits, now raised and forming the lower slope of the mountain, at the base of which is the Lake of Neufchatel.
Although this change of inclination is hardly perceptible, as one looks up against the face of the Jura range, there is a transverse cut across it which seems intended to give us a diagram of its internal structure. Behind the city of Neufchatel rises the mountain of Chaumont, so called from its bald head, for neither tree nor shrub grows on its summit. Straight through this mountain, from its northern to its southern side, there is a natural road, formed by a split in the mountain from top to bottom. In this transverse cut, which forms one of the most romantic and picturesque gorges leading into the heart of the Jura range, you get a profile view of the change in the inclination of the strata, and can easily distinguish the point of juncture between the two sets of deposits. But even after this dislocation of strata had been perceived, it was not known that it indicated the commencement of a new epoch, and it is here that my own share in the work, such as it is, belongs. Accustomed as a boy to ramble about in the beautiful gorges and valleys of the Jura, and in riper years, as my interest in science increased, to study its formation with closer attention, this difference in the inclination of the slope had not escaped my observation. I was, however, still more attracted by the fossils it contained than by its geological character: and, indeed, there is no better locality for the study of extinct forms of life than the Jura. In all its breaks and ravines, wherever the inner surface of the rock is exposed, it is full of organic remains; and to take a handful of soil from the road-side is often to gather a handful of shells. It is actually built of the remains of animals, and there are no coral reefs in existing seas presenting a better opportunity for study to the naturalist than the coral reefs of the Jura. Being already tolerably familiar with the fossils of the Jura, it occurred to me to compare those of the upper and lower slope; and to my surprise I found that they were everywhere different, and that those of the lower slope were invariably Cretaceous in character, while those of the upper slope were Jurassic. In the course of this investigation I discovered three periods in the Cretaceous and four in the Jurassic epoch, all characterized by different fossils. This led to a more thorough investigation of the different sets of strata, resulting in the establishment by D'Orbigny of a still greater number of periods, marked by the successive deposits of the Jurassic and Cretaceous seas, all of which contained different organic remains. The attention of geologists being once turned in this direction, the other epochs were studied with the same view, and all were found to be susceptible of division into a greater or less number of such periods.
I have dwelt at greater length on the Jurassic and Cretaceous divisions, because I believe that we have in the relation of these two epochs, as well as in that of the Cretaceous epoch with the Tertiary immediately following it, facts which are very important in their bearing on certain questions, now loudly discussed, not only by scientific men, but by all who are interested in the mode of origin of animals. Certainly, in the inland seas of the Cretaceous and subsequent Tertiary times, where we can trace in the same sheet of water not only the different series of deposits belonging to two successive epochs in immediate juxtaposition, but those belonging to all the periods included within these epochs, with the organic remains contained in each,—there, if anywhere, we should be able to trace the transition-types by which one set of animals is said to have been developed out of the preceding. We hear a great deal of the interruption in geological deposits, of long intervals, the record of which has vanished, and which may contain those intermediate links for which we vainly seek. But here there is no such gap in the evidence. In the very same sheets of water, covering limited areas, we have the successive series of deposits containing the remains of animals which continue perfectly unchanged during long intervals, and then, with a more or less violent shifting of the surface,[4] traceable by the consequent discordance of the strata, is introduced an entirely new set of animals, differing as much from those immediately preceding them as do those of the present period from the old Creation, (our predecessors, but not our ancestors,) traced by Cuvier in the Tertiary deposits underlying those of our own geological age. I subjoin here a tabular view giving the Epochs in their relation to the Ages, and indicating, at least approximately, the number of Periods contained in each Epoch.
It will be noticed by those who have any knowledge of geological divisions, that in this diagram I consider the Carboniferous epoch as forming a part of the Secondary age. Some geologists have been inclined, from the marked and peculiar character of its vegetation, to set it apart as forming in itself a distinct geological age, while others have united it with the Palæozoic age. For many years I myself adopted the latter of these two views, and associated the Carboniferous epoch with the Palæozoic age. But it is the misfortune of progress that one is forced not only to unlearn a great deal, but, if one has been in the habit of communicating his ideas to others, to destroy much of his own work. I now find myself in this predicament; and after teaching my students for years that the Carboniferous epoch belongs to the Palæozoic or Primary age, I am convinced—and this conviction grows upon me constantly as I free myself from old prepossessions and bias on the subject—that with the Carboniferous epoch we have the opening of the Secondary age in the history of the world. A more intimate acquaintance with organic remains has shown me that there is a closer relation between the character of the animal and vegetable world of the Carboniferous epoch, as compared with that of the Permian and Triassic epochs, than between that of the Carboniferous epoch and any preceding one. Neither do I see any reason for separating it from the others as a distinct age. The plants as well as the animals of the two subsequent epochs seem to me to show, on the contrary, the same pervading character, indicating that the Carboniferous epoch makes an integral part of that great division which I have characterized as the Secondary age.
Within the Periods there is a still more limited kind of geological division, founded upon the special character of local deposits. These I would call geological Formations, indicating concrete local deposits, having no cosmic character, but circumscribed within comparatively narrow areas, as distinguished from the other terms, Ages, Epochs, Periods, which have a more universal meaning, and are, as it were, cosmopolitan in their application. Let me illustrate my meaning by some formations of the present time. The accumulations along the coast of Florida are composed chiefly of coral sand, mixed of course with the remains of the animals belonging to that locality; those along the coast of the Southern States consist principally of loam, which the rivers bring down from their swamps and low, muddy grounds; those upon the shores of the Middle States are made up of clay from the disintegration of the eastern slopes of the Alleghanies; while those farther north, along our own coast, are mostly formed of sand from the New-England granites. Such deposits are the local work of one period, containing the organic remains belonging to the time and place. From the geological point of view, I would call them Formations; from the naturalist's point of view, I would call them Zoölogical Provinces.
Of course, in urging the application of these names, I do not intend to assume any dictatorship in the matter of geological nomenclature. But I do feel very strongly the confusion arising from an indiscriminate use of terms, and that, whatever names be selected as most appropriate or descriptive for these divisions, geologists should agree to use them in the same sense.
There is one other geological term, bequeathed to us by a great authority, and which cannot be changed for the better: I mean that of Geological Horizon, applied by Humboldt to the whole extent of any one geological division,—as, for instance, the Silurian horizon, including the whole extent of the Silurian epoch. It indicates one level in time, as the horizon which limits our view indicates the farthest extension of the plain on which we stand in space.
We left America at the close of the Carboniferous epoch, when the central part of the United States was already raised above the water. Let us now give a glance at Europe in those early days, and see how far her physical history has advanced. What European countries loom up for us out of the Azoic sea, corresponding in time and character to the low range of hills which first defined the northern boundary of the United States? what did the Silurian and Devonian epochs add to these earliest tracts of dry land in the Old World? and where do we find the coal basins which show us the sites of her Carboniferous forests? Since the relation between the epochs of comparative tranquillity and the successive upheavals has been so carefully traced in Europe, I will endeavor, while giving a sketch of that early European world, to point out, at the same time, the connection of the different systems of upheaval with the successive stratified deposits, without, however, entering into such details as must necessarily become technical and tedious.
In the European ocean of the Azoic epoch we find five islands of considerable size. The largest of these is at the North. Scandinavia had even then almost her present outlines; for Norway, Sweden, Finland, and Lapland, all of which are chiefly granitic in character, were among the first lands to be raised. Between Sweden and Norway, there is, however, still a large tract of land under water, forming an extensive lake or a large inland sea in the heart of the country. If the reader will take the trouble to look on any geological map of Europe, he will see an extensive patch of Silurian rock in the centre of Sweden and Norway. This represents that sheet of water gradually to be filled by the accumulation of Silurian deposits and afterwards raised by a later disturbance. There is another mass of land far to the southeast of this Scandinavian island, which we may designate as the Bohemian island, for it lies in the region now called Bohemia, though it includes, also, a part of Saxony and Moravia. The northwest corner of France, that promontory which we now call Bretagne, with a part of Normandy adjoining it, formed another island; while to the southeast of it lay the central plateau of France. Great Britain was not forgotten in this early world; for a part of the Scotch hills, some of the Welsh mountains, and a small elevation here and there in Ireland, already formed a little archipelago in that region. By a most careful analysis of the structure of the rocks in these ancient patches of land, tracing all the dislocations of strata, all the indications of any disturbance of the earth-crust whatsoever, Élie de Beaumont has detected and classified four systems of upheavals, previous to the Silurian epoch, to which he refers these islands in the Azoic sea. He has named them the systems of La Vendée, of Finistère, of Longmynd, and of Morbihan. These names have, for the present, only a local significance,—being derived, like so many of the geological names, from the places where the investigations of the phenomena were first undertaken,—but in course of time will, no doubt, apply to all the contemporaneous upheavals, wherever they may be traced, just as we now have Silurian, Devonian, Permian, and Jurassic deposits in America as well as in Europe.