f snow many thousand feet thick,
corresponding, except in its greater bulk, to the accumulations by which
the present glaciers are caused, were stretched over an extensive level
surface? The moisture from the upper superficial layers would permeate
the larger mass as it now does the smaller one, trickling down into its
lower portions, while the pressure from above would render the bottom
hard and compact, changing it gradually into ice. If this should take
place under climatic conditions which would keep the whole as a mass in
a frozen state, the pressure from above would force out the lower ice in
every direction beyond its original circumscription, thus enlarging the
area covered by it, while the whole would subside in its bulk. Let us
for a moment assume that such an accumulation of snow takes place around
the northern and southern poles, stretching thence over the northern and
southern hemispheres to latitude forty, and that this field of snow
acquires a thickness of from twelve to fifteen thousand feet. Such a
mass would subside upon itself in consequence of its own weight; it
would be transformed into ice with greater or less rapidity and
completeness, according to the latitude determining the surrounding
climatic influences and the amount of moisture falling upon it as rain
or dew, the alternations of temperature being of course more frequent
and greater along its outer limit. In proportion as, with the rising of
the temperature, these alternations became more general, a packing of
the mass would begin, corresponding to that observed in the glacial
valleys of Switzerland, though here the action would not be intensified
by lateral pressure; an internal movement of the whole mass would be
initiated, and the result could be no other than a uniform advance in a
southerly direction from the Arctic toward the more temperate latitudes
in Europe, Asia, and North America, and from the Antarctic toward South
America, the Cape of Good Hope, and Van Diemen's Land. But we need not
build up a theoretical case in order to form an approximate idea of the
great ice-sheet stretching over the northern part of this continent
during the glacial period. It would seem that man was intended to
decipher the past history of his home, for some remnants or traces of
all its great events are left as a key to the whole. Greenland and the
Arctic regions hold all that remains of the glacial period in North
America. Their shrunken ice-fields, form