the
sun's heat. However brought about, from very ancient days to the present
time large portions of the earth's surface have occasionally had
climatal conditions which cause the rainfall to descend in the form of
snow, the snow falling in such quantities that it did not melt away in
the summer season. This condition now exists about either pole, and to a
certain extent on the high mountains, even those of tropical lands.

From time to time, owing to the variable adjustments of climate, these
periods of excessive snow have endured for ages, in which the glacial
sheet has extended in either hemisphere far towards the equator. In our
present day the earth is just escaping from the last of these wonderful
ice epochs. At a time so recent that it may be called a geological
yesterday the greater part of Europe and of North America was buried
beneath accumulations of snow, or rather of ice formed from it, the
sheets having in places the depth of a mile or more, and, according to
their strange nature, moving slowly over the surface, crushing and
grinding the rocks as they went, until the ice either reached the sea,
where it would float off as icebergs, or a place on the land where it
was far enough south to be melted away.

[Illustration: THE ICE SHEET WAS DEEP ENOUGH TO FLOW OVER THE TOP OF
MOUNT WASHINGTON.]

On the surface of North America the ice sheet, the remnant of which
still covers Greenland, expelled all life from the region of Canada and
the United States from a line a little to the east of the Rocky
Mountains, and in general north of the Ohio and the Mississippi rivers
to the sea-coast. It was deep enough to flow over the top of Mount
Washington in New Hampshire, and a primitive man (for there were such in
those days) might possibly have journeyed over all the realm without
discerning the least trace of the earth's rock surface, for even the
higher mountains were buried.

We do not yet know how many of these glacial periods there have been, or
whether they occur at the same time in both the northern and southern
hemispheres, but it is clear that they have been of frequent occurrence.
In the intervals between the ice epochs warm conditions appear to have
prevailed even up to the pole of the hemisphere, which was shortly
afterwards to experience the dreadful winter of an ice-time. Thus, at a
period which in its geological sense was not long before the last
glacial epoch, the Greenland district bore a forest much li