his cliff of
ice was formed by a sheet which lay on the bottom of the sea. On the
New Jersey coast the ice wall left the sea and entered on the body of
the continent. We will now suppose that the explorer, animated with
the valiant scientific spirit which leads the men of our day to seek
the poles, undertook a land journey along the ice front across the
continent. From the New Jersey coast the traveller would have passed
through central Pennsylvania, where, although there probably detached
outlying glaciers lying to the southward as far as central Virginia,
the main front extended westward into the Ohio Valley. In southern
Ohio a tongue of the ice projected southwardly until it crossed the
Ohio River, where Cincinnati now lies, extending a few miles to the
southward of the stream. Thence it deflected northwardly, crossing the
Mississippi, and again the Missouri, with a tongue or lobe which went
far southward in that State. Then again turning to the northwest, it
followed in general the northern part of the Missouri basin until it
came to within sight of the Rocky Mountains. There the ice front of
the main glacier followed the trend of the mountains at some distance
from their face for an unknown extent to the northward. In the
Cordilleras, as far south as southern Colorado, and probably in the
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