ot at the
same time have removed some thousands of feet of rock from many hundreds of
square miles of lofty snow-collecting plateaus, and yet have left moraines,
and blocks, and even glacial striae, undisturbed and uneffaced on the
slopes and in the valleys of these same mountains.

The theory of geological climates set forth in this volume, while founded
on Mr. Croll's researches, differs from all that have yet been made public,
in clearly tracing out the comparative influence of geographical and
astronomical revolutions, showing that, while the former have been the
chief, if not the exclusive, causes of the long-continued mild climates of
the Arctic regions, the concurrence of the latter has been essential to the
production of glacial epochs in the temperate zones, as well as of those
local glaciations in low latitudes, of which there is such an abundance of
evidence.

The next question discussed is that of geological time as bearing on the
development of the organic world. The periods of time usually demanded by
geologists have been very great, and it was often assumed that there was no
occasion to limit them. But the theory of development demands far more; for
the earliest fossiliferous rocks {538} prove the existence of many and
varied forms of life which require unrecorded ages for their
development--ages probably far longer than those which have elapsed from
that period to the present day. The physicists, however, deny that any such
indefinitely long periods are available. The sun is ever losing heat far
more rapidly than it can be renewed from any known or conceivable source.
The earth is a cooling body, and must once have been too hot to support
life; while the friction of the tides is checking the earth's rotation, and
this cannot have gone on indefinitely without making our day much longer
than it is. A limit is therefore placed to the age of the habitable earth,
and it has been thought that the time so allowed is not sufficient for the
long processes of geological change and organic development. It is
therefore important to inquire whether these processes are either of them
so excessively slow as has been supposed, and I devote a chapter to the
inquiry.

Geologists have measured with some accuracy the maximum thickness of all
the known sedimentary rocks. The rate of denudation has also been recently
measured by a method which, if not precise, at all events gives results of
the right order of magnitude and