ng 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 _debris_ to the
bottom. These masses of floating ice in many cases seem to have
prevented the sea waves from attaining the shore, and thus hindered
the formation of those beaches which in their present elevated
condition enable us to interpret the old position of the sea along
coast lines which have been recently elevated. Here and there,
however, from New Jersey to Greenland, we find bits of these ancient
shores which clearly tell the story of that down-sinking of the land
beneath the burden of the ice which is such an instructive feature in
the history of that period.

CHAPTER VII.

THE WORK OF UNDERGROUND WATER.

We have already noted two means by which water finds its way
underground. The simplest and largest method by which this action is
effected is by building in the fluid as the grains of the rock are
laid down on the floors of seas or lakes. The water thus imprisoned is
firmly inclosed in the interstices of the stone, it in time takes up
into its mass a certain amount of the mineral materials which are
contained in the deep-buried rocks. The other portion of the ground
water--that with which we are now to be specially concerned--arises
from the rain which descends into the crevices of the earth; it is
therefore peculiar to the lands. For convenience we shall term the
original embedded fluid _rock water_, and that which originates from
the rain _crevice water_, the two forming the mass of the earth water.