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
moderate extension. Many other of the Eurasian highlands were probably
ice-bound during the last Glacial period, but our knowledge concerning
these local fields is as yet imperfect.

In the southern hemisphere the lands are of less extent and, on the
whole, less studied than in the northern realm. Here and there where
glaciers exist, as in New Zealand and in the southern part of South
America, observant travellers have noticed that these ice fields have
recently shrunk away. Whether the time of greatest extension and of
retreat coincided with that of the ice sheets in the north is not yet
determined; the problem, indeed, is one of some difficulty, and may
long remain undecided. It seems, however, probable that the glaciers
of the southern hemisphere, like those in the north, are in process of
retreat. If this be true, then their time of greatest extension was
probably the same as that of the ice sheets about the southern pole.
From certain imperfect reports which we have concerning evidences of
glaciation in Central America and in the Andean district in the
northern part of South America, it seems possible that at one time the
upland ice along the Cordilleran chain existed from point to point
along that system of elevations, so that the widest interval between
the fields of permanent snow with their attendant glaciers did not
much exceed a thousand miles.

Observing the present gradual retreat of those ice remnants which
remain mere shreds and patches of the ancient fields, it seems at
first sight likely that the extension and recession of the great
glaciers took place with exceeding slowness. Measured in terms of
human life, in the manner in which we