because, losing no
water, it does not diminish in its extent, and can only accommodate
itself to the interior by a wrinkling process. In one case it is water
which escapes, in the other heat; but in both contraction of the part
which suffers the loss leads to the folding of the outside of the
spheroid.

Although this loss of heat on the part of the earth accounts in some
measure for the development of mountains, it is not of itself
sufficient to explain the phenomena, and this for the reason that
mountains appear in no case to develop on the floors of the wide sea.
The average depth of the ocean is only fifteen thousand feet, while
there are hundreds, if not thousands, of mountain crests which exceed
that height above the sea. Therefore if mountains grew on the sea
floor as they do upon the land, there should be thousands of peaks
rising above the plain of the waters, while, in fact, all of the
islands except those near the shores of continents are of volcanic
origin--that is, are lands of totally different nature.

Whenever a considerable mountain chain is formed, although the actual
folding of the beds is limited to the usually narrow field occupied by
these disturbances, the elevation takes place over a wide belt of
country on one or both sides of the range. Thus if we approach the
Rocky Mountains from the Mississippi Valley, we begin to mount up an
inclined plane from the time we pass westward from the Mississippi
River. The beds of rock as well as the surface rises gradually until
at the foot of the mountain; though the rocks are still without
foldings, they are at a height of four or five thousand feet above the
sea. It seems probable--indeed, we may say almost certain--that when
the crust is broken, as it is in mountain-building, by extensive folds
and faults, the matter which lies a few score miles below the crust
creeps in toward those fractures, and so lifts up the country on which
they lie. When we examine the forms of any of our continents, we find
that these elevated portions of the earth's crust appear to be made up
of mountains and the table-lands which fringe those elevations. There
is not, as some of our writers suppose, two different kinds of
elevation in our great lands--the continents and the mountains which
they bear--but one process of elevation by which the foldings and the
massive uplifts which constitute the table-lands are simultaneously
and by one process formed.

Looking upon continents