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 debris, 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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deposition and crustal folding, the heat of radioactivity has been a determining factor.  We recognise in the movements of the sediments not only an influence localising and accelerating crustal movements, but one which, in subservience to the primal distribution of land and water, has determined some of the greatest geographical features of the globe.