The following may serve as an analysis of the evidence on which the theory above stated depends. "We observe," says M. de Beaumont, "when we attentively examine nearly all mountain-chains, that the most recent rocks extend horizontally up to the foot of such chains, as we should expect would be the case if they were deposited in seas or lakes, of which these mountains have partly formed the shores; whilst the other sedimentary beds, tilted up, and more or less contorted, on the flanks of the mountains, rise in certain points even to their highest crests."[245] There are, therefore, in and adjacent to each chain, two classes of sedimentary rocks, the ancient and inclined beds, and the newer or horizontal. It is evident that the first appearance of the chain itself was an event "intermediate between the period when the beds now upraised were deposited, and the period when the strata were produced horizontally at its feet." [Illustration: Fig. 11.] Thus the chain A assumed its present position after the deposition of the strata _b_, which have undergone great movements, and before the deposition of the group _c_, in which the strata have not suffered derangement. [Illustration: Fig. 12.] If we then discover another chain B, in which we find not only the formation _b_, but the group _c_ also, disturbed and thrown on its edges, we may infer that the latter chain is of subsequent date to A; for B must have been elevated _after_ the deposition of _c_, and before that of the group _d_; whereas A had originated _before_ the strata _c_ were formed. It is then argued, that in order to ascertain whether other mountain ranges are of contemporaneous date with A and B, or are referable to _distinct_ periods, we have only to inquire whether the inclined and undisturbed sets of strata in each range correspond with or differ from those in the typical chain A and B. Now all this reasoning is perfectly correct, so long as the period of time required for the deposition of the strata _b_ and _c_ is not made identical in duration with the period of time during which the animals and plants found fossil in _b_ and _c_ may have flourished; for the latter, that is to say, the duration of certain groups of species, may have greatly exceeded, and probably did greatly exceed, the former, or the time required for the accumulation of certain local deposits, such as _b_ and _c_ (figs. 11 and 12). In order, moreover, to render the reasoning correct, due latitude must be given to the term contemporaneous; for this term must be understood to allude, not to a moment of time, but to the interval, whether brief or protracted, which elapsed between two events, namely, between the accumulation of the inclined and that of the horizontal strata.