ing power of ice in
periods of glaciation; and last, but not least, the wholesale melting up
of sedimentary formations whenever these have sunk for any considerable
distance beneath the earth's surface:--all these agencies taken together
constitute so prodigious a sum of energies combined through
immeasureable ages in their common work of destruction, that when we
try to realise what it must amount to, we can scarcely fail to wonder,
not that the geological record is highly imperfect, but that so much of
the record has survived as we find to have been the case. And, if we add
to these erosive and solvent agencies on land the erosive and solvent
agencies of the sea, we may almost begin to wonder that anything
deserving the name of a geological record is in existence at all.

That such estimates of the destructive powers of nature are not mere
matters of speculative reasoning may be amply shown by stating one
single fact, which, like so many others where the present subject is
concerned, we owe to the generalizations of Darwin. Plutonic rocks,
being those which have emerged from subterranean heat of melting
intensity, must clearly at some time or another have lain beneath the
whole thickness of sedimentary deposits, which at that time occupied any
part of the earth's surface where we now find the Plutonic rocks exposed
to view. Or, in other words, wherever we now find Plutonic rocks at the
surface of the earth, we must conclude that all the sedimentary rocks by
which they were covered when in a molten state have since been entirely
destroyed; several vertical miles of the only kinds of rocks in which
fossils can possibly occur must in all such cases have been abolished
_in toto_. Now, in many parts of the world metamorphic rocks--which have
thus gradually risen from Plutonic depths, while miles of various other
rock-formations have been removed from their now exposed surfaces--cover
immense areas, and therefore testify by their present horizontal range,
no less than by their previously vertical depth, to the enormous scale
on which a total destruction has taken place of everything that once lay
above them. For instance, the granitic region of Parime is at least
nineteen times the size of Switzerland; a similar region south of the
Amazon is probably larger than France, Spain, Italy, and Great Britain
all put together; and, more remarkable still, over the area of the
United States and Canada, granitic rocks exceed in the pro