These alternations now appear to me to form the problem in geology that it is of most importance to solve, or rather to define and circumscribe within due limits; for, in order to resolve it satisfactorily, it would be necessary to discover the cause of these events,—an undertaking which presents a difficulty of quite a different kind.

I repeat it, we see pretty clearly what is going on at the surface of the continents in their present state; we have formed a tolerable conception of the uniform progress and regular succession of the primitive formations, but the study of the secondary formations has been little more than merely commenced. That wonderful series of unknown zoophytes and marine mollusca, succeeded by reptiles and fresh-water fishes equally unknown; and these again replaced, in their turn, by other zoophytes and mollusca, more nearly related to those of the present day; those land animals, and those equally unknown fresh water mollusca and other animals which next occupied the surface, to be again displaced but by mollusca and other animals similar to those of our present seas; the relations of these diversified beings to the plants the remains of which accompany theirs, the connection of these two kingdoms with the mineral strata in which they are deposited; the greater or less uniformity existing between these different orders of beings in the different basins;—these are phenomena which appear to me imperiously to demand the attention of philosophers.

Rendered interesting by the variety of the products of the partial or general revolutions of this epoch, and by the abundance of the various species that figure alternately on the stage, this study is divested of the dryness of that of the primordial formations, and does not, like it, almost necessarily launch into hypotheses. The facts are so direct, so curious, and so evident, that they are sufficient, so to speak, to satisfy the most ardent imagination; and the conclusions to which they lead from time to time, however scrupulous the observer may be, having nothing vague in them, are equally free of any thing arbitrary. In fine, it is in those events that approach nearer to our own times, that we may hope to find some traces of more ancient events, and of their causes; if, indeed, after so many fruitless attempts as have been already made, one may be permitted to flatter himself with such a hope.

These ideas have haunted, I may almost say have tormented me, during my researches among fossil bones, the results of which I have lately presented to the public; researches which embrace but a very small part of those phenomena of the age preceding the last general revolution of the globe, and which are yet intimately connected with all the others. It was almost impossible that the desire should not arise of investigating the general mass of these phenomena, at least as they occur in a limited space around us. My excellent friend, M. Brongniart, in whose mind other studies excited the same desire, had the complaisance to associate me with himself in the task; and it is thus that we have laid the first foundations of our labours upon the environs of Paris. But this work, while it still bears my name, has become almost entirely that of my friend, from the infinite attention which he has bestowed, since the first conception of our plan, and since our journeys, upon the profound investigation of the objects, and the perfecting and arranging of the whole. I have placed it, with M. Brongniart’s consent, in the second part of my “Recherches,” in that in which I treat of the fossil bones of our neighbourhood. Although apparently relating only to a rather limited extent of country, it affords numerous results, which are applicable to geology in general, and, in this point of view, it may be considered as intimately connected with the present discourse; at the same time, that it is, without a doubt, one of the best ornaments of my work[244].

In it there is presented the history of the most recent changes that have taken place in a particular basin, and it descends so far as the Chalk formation, the extent of which over the globe is vastly more considerable than that of the materials of the basin of Paris. The chalk, which has been considered so modern, is thus found to be advanced in antiquity among the ages of the great period preceding the last catastrophe. It forms a sort of limit between the most recent formations, those to which the name of Tertiary may be reserved, and the formations which are named Secondary, which have been deposited before the Chalk, but after the Primitive and Transition formations.