he observer has to rely wholly upon the
fossils which he may meet with.

In spite of the above limitations and fallacies, there can be
no doubt as to the enormous value of palaeontology in enabling us
to work out the historical succession of the sedimentary rocks.
It may even be said that in any case where there should appear
to be a clear and decisive discordance between the physical and
the palaeontological evidence as to the age of a given series
of beds, it is the former that is to be distrusted rather than
the latter. The records of geological science contain not a few
cases in which apparently clear physical evidence of superposition
has been demonstrated to have been wrongly interpreted; but the
evidence of palaeontology, when in any way sufficient, has rarely
been upset by subsequent investigations. Should we find strata
containing plants of the Coal-measures apparently resting upon
other strata with Ammonites and Belemnites, we may be sure that
the physical evidence is delusive; and though the above is an
extreme case, the presumption in all such instances is rather that
the physical succession has been misunderstood or misconstrued,
than that there has been a subversion of the recognised succession
of life-forms.

We have seen, then, that as the collective result of observations
made upon the superposition of rocks in different localities,
from their mineral characters, and from their included fossils,
geologists have been able to divide the entire stratified series into
a number of different divisions or formations, each characterised
by a _general_ uniformity of mineral composition, and by a special
and peculiar _assemblage_ of organic forms. Each of these primary
groups is in turn divided into a series of smaller divisions,
characterised and distinguished in the same way. It is not pretended
for a moment that all these primary rock-groups can anywhere be seen
surmounting one another regularly.[8] There is no region upon the
earth where all the stratified formations can be seen together;
and, even when most of them occur in the same country, they can
nowhere be seen all succeeding each other in their regular and
uninterrupted succession. The reason of this is obvious. There
are many places--to take a single example--where one may see the
the Silurian rocks, the Devonian, and the Carboniferous rocks
succeeding one another regularly, and in their proper order. This
is because the particular region where th