mountains. But, of course, the lower down it flows the higher the
temperature it meets, and it will sooner or later reach a point where it
will melt as fast as it advances. This stream of ice flowing down from
snow-clad mountains is called a glacier. Those we are best acquainted
with are but puny things compared with those of the polar regions,
where in one case a great river of ice sixty miles wide, flowing from an
unknown distance, some thousands of feet in depth (or height), pours out
into the sea.<20>

We at once perceive that such a mass of ice could not pour down a valley
without leaving unmistakable signs of its passage. The sides of the
mountains would be deeply scarred and smoothed. Projecting knobs
would be worn away. The surface of the valley, exposed to the enormous
grinding power of the moving ice, would be crushed, pulverized, and
dragged along with it. Pieces of stone, like that here represented,
would form part of this moving _debris,_ and as they were crowded along
they would now and then grate over another piece of stone more firmly
seated, and so their surface would be deeply scratched in the direction
of their greatest length. There is always more or less water circulating
under the Alpine glaciers, and the streams that flow from them are
always very muddy, containing, as they do, quantities of crushed rock,
sand, and clay.

Illustration of Scratched Stone---------

If, for any reason, this earthy matter was not washed out it would form
a bed of hard clay, in places packed with these striated stones. Such
beds of clay are known as "till" or bowlder clay.<21>

This is descriptive, though in a very general way, of the glaciers as
they exist to-day. Geologists have long been aware of the fact that they
have convincing proofs of the former presence of glaciers in Northern
Europe, where now the climate is mild. The mountains of Scotland and
Wales show as distinct traces of glaciers as do those of the Alps. It
is not necessary, in this hasty sketch, to enumerate the many grounds
on which this conclusion rests. It is sufficient to state that by the
united labors of many investigators in that field we are in possession
of many conclusions relating to the great glaciers of this age
which almost surpass belief; and yet they are the results of careful
deductions. The former presence of this ice sheet itself is shown in a
most conclusive manner by the bowlder clay formed underneath the
great glacier, con