enudation can have no tendency to diminish the
thickness of any entire deposit.[39] Unless, therefore, a formation is
completely destroyed by denudation in every part of the world (a thing very
improbable), we may have in existing rocks a not very inadequate
representation of the _mean thickness_ of all that have been formed, and
even of the _maximum_ thickness of the larger portion. This will be the
more likely because it is almost certain that many rocks contemporaneously
formed are counted by geologists as distinct formations, whenever they
differ in lithological character or in organic remains. But we know that
limestones, sandstones, and shales, are always forming at the same time;
{110} while a great difference in organic remains may arise from
comparatively slight changes of geographical features, or from difference
in the depth or purity of the water in which the animals lived.[40]

_How to Estimate the Average Rate of Deposition of the Sedimentary
Rocks._--But if we take the estimate of Professor Haughton (177,200 feet),
which, as we have seen, is probably excessive, for the maximum thickness of
the sedimentary rocks of the globe of all known geological ages, can we
arrive at any estimate of the rate at which they were formed? Dr. Croll has
attempted to make such an estimate, but he has taken for his basis the
_mean_ thickness of the rocks, which we have no means whatever of arriving
at, and which he guesses, allowing for denudation, to be equal to the
_maximum_ thickness as measured by geologists. The land-area of the globe
is, according to Dr. Croll, 57,000,000[41] square miles, and he gives the
coast-line as 116,000 miles. This, however, is, for our purpose, rather too
much, as it allows for bays, inlets, and the smaller islands. An
approximate measurement on a globe shows that 100,000 miles will be nearer
the mark, and this has the advantage of being an easily remembered even
number. The distance from the coast, to which shore-deposits usually
extend, may be reckoned at about 100 or 150 miles, but by far the larger
portion of the matter brought down from the land will be deposited
comparatively close to the shore; that is, within twenty or thirty miles.
If we suppose the portion deposited beyond thirty miles to be added to the
deposits within that distance, and the whole reduced to a uniform thickness
in a direction at right angles to the coast, we should probably include all
areas where deposits of the maxim