ll are the wrinkles, but how evenly the intervening
spaces lie upon the surface of the orange, will then wrap it up in thick
cartridge-paper, and note both the greater height of the ridges and the
larger spaces throughout which the paper does not touch the orange, he
will realize the fact that, as the Earth's solid envelope grew thicker,
the areas of elevation and depression increased. In place of islands
homogeneously dispersed amid an all-embracing sea, there must have
gradually arisen heterogeneous arrangements of continent and ocean. Once
more, this double change in the extent and in the elevation of the
lands, involved yet another species of heterogeneity--that of
coast-line. A tolerably even surface raised out of the ocean must have a
simple, regular sea-margin; but a surface varied by table-lands and
intersected by mountain-chains must, when raised out of the ocean, have
an outline extremely irregular both in its leading features and in its
details. Thus, multitudinous geological and geographical results are
slowly brought about by this one cause--the contraction of the Earth.

When we pass from the agency termed igneous, to aqueous and atmospheric
agencies, we see the like ever-growing complications of effects. The
denuding actions of air and water, joined with those of changing
temperature, have, from the beginning, been modifying every exposed
surface. Oxidation, heat, wind, frost, rain, glaciers, rivers, tides,
waves, have been unceasingly producing disintegration; varying in kind
and amount according to local circumstances. Acting upon a tract of
granite, they here work scarcely an appreciable effect; there cause
exfoliations of the surface, and a resulting heap of _debris_ and
boulders; and elsewhere, after decomposing the feldspar into a white
clay, carry away this and the accompanying quartz and mica, and deposit
them in separate beds, fluviatile and marine. When the exposed land
consists of several unlike kinds of sedimentary strata, or igneous
rocks, or both, denudation produces changes proportionably more
heterogeneous. The formations being disintegrable in different degrees,
there follows an increased irregularity of surface. The areas drained by
different rivers being differently constituted, these rivers carry down
to the sea different combinations of ingredients; and so sundry new
strata of unlike compositions are formed. And here we may see very
simply illustrated, the truth, which we shall present