e
there results the precession of the equinoxes. Thus the multiplication
of effects is obvious. Several of the differentiations due to the
gradual cooling of the Earth have been already noticed--as the formation
of a crust, the solidification of sublimed elements, the precipitation
of water, &c.,--and we here again refer to them merely to point out that
they are simultaneous effects of the one cause, diminishing heat. Let us
now, however, observe the multiplied changes afterwards arising from the
continuance of this one cause. The cooling of the Earth involves its
contraction. Hence the solid crust first formed is presently too large
for the shrinking nucleus; and as it cannot support itself, inevitably
follows the nucleus. But a spheroidal envelope cannot sink down into
contact with a smaller internal spheroid, without disruption: it must
run into wrinkles as the rind of an apple does when the bulk of its
interior decreases from evaporation. As the cooling progresses and the
envelope thickens, the ridges consequent on these contractions will
become greater, rising ultimately into hills and mountains; and the
later systems of mountains thus produced will not only be higher, as we
find them to be, but will be longer, as we also find them to be. Thus,
leaving out of view other modifying forces, we see what immense
heterogeneity of surface has arisen from the one cause, loss of heat--a
heterogeneity which the telescope shows us to be paralleled on the face
of Mars, and which in the moon too, where aqueous and atmospheric
agencies have been absent, it reveals under a somewhat different form.
But we have yet to notice another kind of heterogeneity of surface
similarly and simultaneously caused. While the Earth's crust was still
thin, the ridges produced by its contraction must not only have been
small, but the spaces between these ridges must have rested with great
evenness upon the subjacent liquid spheroid; and the water in those
arctic and antarctic regions in which it first condensed, must have been
evenly distributed. But as fast as the crust thickened and gained
corresponding strength, the lines of fracture from time to time caused
in it, must have occurred at greater distances apart; the intermediate
surfaces must have followed the contracting nucleus with less
uniformity; and there must have resulted larger areas of land and water.
If any one, after wrapping up an orange in tissue paper, and observing
not only how sma