kness of 72,600 feet; while Professor Haughton,
from a survey of the whole world, estimates the _maximum_ thickness of the
known stratified rocks at 177,200 feet. Now these _maximum_ thicknesses of
each deposit will have been produced only where the conditions were
exceptionally favourable, either in deep water near the mouths of great
rivers, or in inland seas, or in places to which the drainage of extensive
countries was conveyed by ocean currents; and this great thickness will
necessarily be accompanied by a corresponding thinness, or complete absence
of deposit, elsewhere. How far the series of rocks found in any extensive
area, as Europe or North America, represents the whole series of deposits
which have been made there we cannot tell; but there is no reason to think
that it is a very inadequate representation of their _maximum_ thickness,
though it undoubtedly is of their _extent_ and _bulk_. When we see in how
many distinct localities patches of the same formation occur, it seems
improbable that the whole of the deposits formed during any one period
should have been destroyed, even in such an area as Europe, while it is
still more improbable that they should be so destroyed over the whole
world; and {109} if any considerable portion of them is left, that portion
may give a fair idea of their average, or even of their maximum, thickness.
In his admirable paper on "The Mean Thickness of the Sedimentary
Rocks,"[38] Dr. James Croll has dwelt on the extent of denudation in
diminishing the mean thickness of the rocks that have been formed,
remarking, "Whatever the present mean thickness of all the sedimentary
rocks of our globe may be, it must be small in comparison to the mean
thickness of all the sedimentary rocks which have been formed. This is
obvious from the fact that the sedimentary rocks of one age are partly
formed from the destruction of the sedimentary rocks of former ages. From
the Laurentian age down to the present day the stratified rocks have been
undergoing constant denudation." This is perfectly true, and yet the mean
thickness of that portion of the sedimentary rocks which remains may not be
very different from that of the entire mass, because denudation acts only
on those rocks which are exposed on the surface of a country, and most
largely on those that are upheaved; while, except in the rare case of an
extensive formation being _quite horizontal_, and wholly exposed to the sea
or to the atmosphere, d