containing large boulders, which clearly were shaped and brought
together by glacial action. These are found in some instances far
south of the region occupied by the glaciers during the last ice
epoch. They occur in rocks of the Cambrian or Silurian age in eastern
Tennessee and western North Carolina; they are also found in India
beyond the limits to which glaciers have attained in modern times.

In closing this inadequate account of glacial action, a story which
for its complete telling would require many volumes, it is well for
the reader to consider once again how slight are the changes of
climate which may alternately withdraw large parts of the land from
the uses of life, and again quickly restore the fields to the service
of plants and animals. He may well imagine that these changes, by
driving living creatures to and fro, profoundly affect the history of
their development. This matter will be dealt with in the volume
concerning the history of organic beings.

When the ice went off from the northern part of this continent, the
surface of the country, which had been borne down by the weight of the
glacier, still remained depressed to a considerable depth below the
level of the sea, the depression varying from somewhere about one
hundred feet in southern New England to a thousand feet or more in
high latitudes. Over this region, which lay beneath the level of the
sea, the glacier, when it became thin enough to float, was doubtless
broken up into icebergs, in the manner which we now behold along the
coast of Greenland. Where the shore was swept by a strong current,
these bergs doubtless drifted away; but along the most of the coast
line they appear to have lain thickly grouped next the shores,
gradually delivering their loads of stones and finer _débris_ to the
bottom. These masses of floating ice in many cases seem to have