is 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. 2. The Devonian might have been deposited upon the Silurian,
and then the whole might have been elevated above the sea, and
subjected to an amount of denudation sufficient to remove the
Devonian strata entirely. In this case, when the land was again
submerged, the Carboniferous rocks, or any younger formation,
might be deposited directly upon Silurian strata. From one or
other of these causes, then, or from subsequent disturbances
and denudations, it happens that we can rarely find many of the
primary formations following one another consecutively and in
their regular order.

[Footnote 8: As we have every reason to believe that dry land
and sea have existed, at any rate from the commencement of the
Laurentian period to the present day, it is quite obvious that
no one of the great formations can ever, under any circumstances,
have extended over the entire globe. In other words, no one of
the formations can ever have had a greater geographical extent
than that of the seas of the period in which the formation was
deposited. Nor is there any reason for thinking that the proportion
of dry land to ocean has ever been materially different to what
it is at present, however greatly the areas of sea and land may
have changed as regards their place. It follows from the above,
that there is no sufficient basis for the view that the crust of
the earth is composed of a succession of concentric layers, like
the coats of an onion, each layer representing one formation.]

In no case, however, do we ever find the Devonian resting upon
the Carboniferous, or the Silurian rocks reposing on the Devonian.
We have therefore, by a comparison of many different areas, an
established order of succession of the stratified formations, as
shown in the subjoined ideal section of the crust of the earth
(fig. 17).

The main subdivisions of the stratified ro