know at least that there was
a great uprise of land in Europe and North America in the Pliocene and
Pleistocene and may leave the precise determination of the point to a
later age. At the same time other local causes are not excluded. There
may have been a large extension of the area of atmospheric depression
which we have in the region of Greenland to-day.

When we turn to the question of chronology we have the same acute
difference of opinion as we have found in regard to all questions of
geological time. It used to be urged, on astronomical grounds, that the
Ice-Age began about 240,000 years ago, and ended about 60,000 years
ago, but the astronomical theory is, as I said, generally abandoned.
Geologists, on the other hand, find it difficult to give even
approximate figures. Reviewing the various methods of calculation,
Professor Chamberlin concludes that the time of the first spread of the
ice-sheet is quite unknown, the second and greatest extension of the
glaciation may have been between 300,000 and a million years ago, and
the last ice-extension from 20,000 to 60,000 years ago; but he himself
attaches "very little value" to the figures. The chief ice-age was some
hundreds of thousands of years ago, that is all we can say with any
confidence.

In dismissing the question of climate, however, we should note that a
very serious problem remains unsolved. 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.