inued
period of high excentricity necessarily brought on the glacial epoch in the
manner already described in our last chapter. A depression seems to have
occurred during the glacial period itself in North America as in Britain,
but this may have been due partly to the weight of the ice and partly to a
rise of the ocean {96} level caused by the earth's centre of gravity being
shifted towards the north.

We thus see that the last glacial epoch was the climax of a great process
of continental development which had been going on throughout long
geological ages; and that it was the direct consequence of the north
temperate and polar land having attained a great extension and a
considerable altitude just at the time when a phase of very high
excentricity was coming on. Throughout earlier Tertiary and Secondary times
an equally high excentricity often occurred, but it never produced a
glacial epoch, because the north temperate and polar areas had less high
land, and were more freely open to the influx of warm oceanic currents. But
wherever great plateaux with lofty mountains occurred in the temperate zone
a considerable _local_ glaciation might be produced, which would be
specially intense during periods of high excentricity; and it is to such
causes we must impute the indications of ice-action in the vicinity of the
Alps during the Tertiary period. The Permian glaciation appears to have
been more extensive, and it is quite possible that at this remote epoch a
sufficient mass of high land existed in our area and northwards towards the
pole, to have brought on a true glacial period comparable with that which
has so recently passed away.

_Estimate of the comparative effects of Geographical and Astronomical
Causes in producing Changes of Climate._--It appears then, that while
geographical and physical causes alone, by their influence on ocean
currents, have been the main agents in producing the mild climates which
for such long periods prevailed in the Arctic regions, the concurrence of
astronomical causes--high excentricity with winter in _aphelion_--was
necessary to the production of the great glacial epoch. If we reject this
latter agency, we shall be obliged to imagine a concurrence of geographical
changes at a very recent period of which we have no evidence. We must
suppose, for example, that a large part of the British Isles--Scotland,
Ireland, and Wales at all events--were simultaneously elevated so as to
bring extensiv