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on a large scale in the Himalayan area till the Tertiary
upheaval, it is, in the majority of cases, literally correct to
speak of the mountains as having their generations like organic
beings, and passing through all the stages of birth, life, death
and reproduction. The Alps, the Jura, the Pyrenees, the Andes,
have been remade more than once in the course of geological time,
the _débris_ of a worn-out range being again uplifted in succeeding
ages.
Thus to dwell for a moment on one case only: that of the
Pyrenees. The Pyrenees arose as a range of older Palmozoic rocks
in Devonian times. These early mountains, however, were
sufficiently worn out and depressed by Carboniferous times to
receive the deposits of that age laid down on the up-turned edges
of the older rocks. And to Carboniferous succeeded Permian,
Triassic, Jurassic and Lower Cretaceous sediments all laid down
in conformable sequence. There was then fresh disturbance and
upheaval followed by denudation, and these mountains, in turn,
became worn out and depressed beneath the ocean so that Upper
Greensand rocks were laid down unconforrnably on all beneath. To
these now succeeded Upper Chalk, sediments of Danian age, and so
on, till Eocene times, when the tale was completed and the
existing ranges rose from the sea. Today we find the folded
Nummulitic strata of Eocene age uplifted 11,000 feet, or within
200 feet of the greatest heights of the Pyrenees. And so they
stand awaiting
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the time when once again they shall "fall into the portion of
outworn faces."[1]
Only mountains can beget mountains. Great accumulations of
sediment are a necessary condition for the localisation of
crust-flexure. The earliest mountains arose as purely igneous or
volcanic elevations, but the generations of the hills soon
originated in the collection of the _débris_, under the law of
gravity, in the hollow places. And if a foundered range is
exposed now to our view encumbered with thousands of feet of
overlying sediments we know that while the one range was sinking,
another, from which the sediments were derived, surely existed.
Through the "windows" in the deep-cut rocks of the Swiss valleys
we see the older Carboniferous Alps looking out, revisiting the
sun light, after scores of millions of years of imprisonment. We
know that just as surely as the Alps of today are founding by
their muddy torrents ranges yet to arise, so other primeval Alps
fed into the ocean the materials of these buried pre-Permian
rocks.
This succession of events only can cease when the rocks have been
sufficiently impoverished of the heat-producing substances, or
the forces of compression shall have died out in the surface
crust of the earth.
It seems impossible to escape the conclusion that in the great
development of ocean-encircling areas of
[1] See Prestwich, _Chemical and Physical Geology_, p. 302.
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