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 this occurs was always
submerged beneath the sea while these formations were being
deposited. There are, however, many more localities in which
one would find the Carboniferous rocks resting unconformably upon
the Silurians without the intervention of any strata which could
be referred to the Devonian period. This might arise from one of
two causes: 1. The Silurians might have been elevated above the
sea immediately after their deposition, so as to form dry land
during the whole of the Devonian period, in which case, of course,
no strata of the latter age could possibly be deposited in that
area.
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