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.
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