y 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.

But, unfortunately, no attempt has been made in the treatises under
review to avoid this manifest source of confusion, and