The
present outlines of its coast, its mountains and valleys, its rivers
and lakes, have mostly arisen since that time. Even the more ancient
parts of the continent have been profoundly modified through the
incessant work of rain and rivers and of the waves, tending to wear
down the land surfaces, of volcanic outbursts building them up, and of
the more mysterious agencies which raise or depress vast stretches of
mountain chains or even the whole area of a continent, and which tend
on the whole so far as we can see, to restore or increase the relief
of the continents, as the action of the surface waters tends to bring
them down to or beneath the sea level.
_Alternate Overflow and Emergence of Continents._ In a broad way these
agencies of elevation and of erosion have caused in their age-long
struggle an alternation of periods of overflow and periods of
continental emergence during geologic time. During the periods of
overflow, great portions of the low-lying parts of the continents were
submerged, and formed extensive but comparatively shallow seas. The
mountains through long continued erosion were reduced to gentle and
uniform slopes of comparatively slight elevation. Their materials were
brought down by rivers to the sea-coast, and distributed as
sedimentary formations over the shallow interior seas or along the
margins of the continents. But this load of sediments, transferred
from the dry land to the ocean margins and shallow seas, disturbed the
balance of weight (isostasy) which normally keeps the continental
platforms above the level of the ocean basins (which as shown by
gravity measurement are underlain by materials of higher specific
gravity than the continents).
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