enient diagram showing
what occurred by pressing a number of leaves of this book so that the
sheets of paper are thrown into ridges and furrows. By this experiment
he also will see that the easiest way to account for such foldings as
we observe in mountains is by the supposition that some force residing
in the earth tends to shove the beds into a smaller space than they
originally occupied. Not only are the rocks composing the mountains
much folded, but they are often broken through after the manner of
masonry which has been subjected to earthquake shocks, or of ice which
has been strained by the expansion that affects it as it becomes
warmed before it is melted. In fact, many of our small lakes in New
England and in other countries of a long winter show in a miniature
way during times of thawing ice folds which much resemble mountain
arches.

At first geologists were disposed to attribute all the phenomena of
mountain-folding to the progressive cooling of the earth. Although
this sphere has already lost a large part of the heat with which it
was in the beginning endowed, it is still very hot in its deeper
parts, as is shown by the phenomena of volcanoes. This internal heat,
which to the present day at the depth of a hundred miles below the
surface is probably greater than that of molten iron, is constantly
flowing away into space; probably enough of it goes away on the
average each day to melt a hundred cubic miles or more of ice, or, in
more scientific phrase, the amount of heat rendered latent by melting
that volume of frozen water. J.R. Meyer, an eminent physicist,
estimated the quantity of heat so escaping each day of the year to be
sufficient to melt two hundred and forty cubic miles of ice. The
effect of this loss of heat is constantly to shrink the volume of the
earth; it has, indeed, been estimated that the sphere on this account
contracts on the average to the amount of some inches each thousand
years. For the reason that almost all this heat goes from the depths
of the earth, the cool outer portion losing no considerable part of
it, the contraction that is brought about affects the interior
portions of the sphere alone. The inner mass constantly shrinking as
it loses heat, the outer, cold part is by its weight forced to settle
down, and can only accomplish this result by wrinkling. An analogous
action may be seen where an apple or a potato becomes dried; in this
case the hard outer rind is forced to wrinkle,