Sierra Nevada to south of San Francisco, the mountain centres
developed local glaciers, which in some places were of very great
size, perhaps exceeding any of those which now exist in Switzerland.
It will thus be seen that nearly one half of the present land area of
North America was beneath a glacial covering, though, as before noted,
the region about the Gulf of Mexico may have swayed upward when the
northern portion of the land was borne down by the vast load of ice
which rested upon it. Notwithstanding this possible addition to the
land, our imaginary explorer would have found the portion of the
continent fit for the occupancy of life not more than half as great as
it is at present.

In the Eurasian continent there was no such continuous ice sheet as in
North America, but the glaciers developed from a number of different
centres, each moving out upon the lowlands, or, if its position was
southern, being limited to a particular mountain field. One of these
centres included Scandinavia, northern Germany, Great Britain about as
far south as London, and a large part of Ireland, the ice covering the
intermediate seas and extending to the westward, so that the passage
of the North Atlantic was greatly restricted between this ice front
and that of North America. Another centre, before noted, was formed in
the Alps; yet another, of considerable area, in the Pyrenees; other
less studied fields existed in the Apennines, in the Caucasus, the
Ural, and the other mountains of northern Asia. Curiously enough,
however, the great region of plains in Siberia does not appear to have
been occupied by a continuous ice sheet, though the similar region in
North America was deeply embedded in a glacier. Coincident with this
development of ice in the eastern part of the continent, the ice
streams of the Himalayan Mountains, some of which are among the
greatest of our upland glaciers, appear to have undergone but a