As far as present
evidence goes we seem to be free to hold that the ice-ages which
have at long intervals invaded the chronicle of the earth were
due to rises of the land. Upheaval is the one constant and
clearly recognisable feature associated with, or preceding,
ice-ages. We saw this in the case of the Cambrian, Permian,
Eocene, and Pleistocene periods of cold, and may add that there
are traces of a rise of mountains before the glaciation of which
we find traces in the middle of the Archaean Era. There are
problems still to be solved in connection with each of these very
important ages, but in the rise of the land and consequent
thinning of the atmosphere we seem to have a general clue to
their occurrence. Apart from these special periods of cold,
however, we have seen that there has been, in recent geological
times, a progressive cooling of the earth, which we have not
explained. Winter seems now to be a permanent feature of the
earth's life, and polar caps are another recent, and apparently
permanent, acquisition. I find no plausible reason assigned for
this.
The suggestion that the disk of the sun is appreciably smaller
since Tertiary days is absurd; and the idea that the earth has
only recently ceased to allow its internal heat to leak through
the crust is hardly more plausible.
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