e on the
mountain ranges. It is, in short, indisputable that the orogenic
movements which uplift the hills have been at the basis of
geological history. To them the great accumulations of sediments
which now form so large a part of continental land are mainly
due. There can be no doubt of the fact that these movements have
swayed the entire history, both inorganic and organic, of the
world in which we live.

[1] Hobbs, _Earthquakes_, p. 58.

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To sum the contents of this essay in the most general terms, we
find that in the conception of denudation as producing the
convection and accumulation of radiothermal energy the surface
features of the globe receive a new significance. The heat of the
earth is not internal only, but rather a heat-source exists at
the surface, which, as we have seen, cannot prevail to the same
degree within; and when the conditions become favourable for the
aggregation of the energy, the crust, heated both from beneath
and from above, assumes properties more akin to those of its
earlier stages of development, the secular heat-loss being
restored in the radioactive supplies. These causes of local
mobility have been in operation, shifting somewhat from place to
place, and defined geographically by the continental masses
undergoing denudation, since the earliest times.

145

ALPINE STRUCTURE

AN intelligent observer of the geological changes progressing in
southern Europe in Eocene times would have seen little to inspire
him with a premonition of the events then developing. The
Nummulitic limestones were being laid down in that enlarged
Mediterranean which at this period, save for a few islands,
covered most of south Europe. Of these stratified remains, as
well as of the great beds of Cretaceous, Jurassic, Triassic, and
Permian sediments beneath, our hypothetical observer would
probably have been regardless; just as today we observe, with an
indifference born of our transitoriness, the deposits rapidly
gathering wherever river discharge is distributing the sediments
over the sea-floor, or the lime-secreting organisms are actively
at work. And yet it took but a few millions of years to uplift
the deposits of the ancient Tethys; pile high its sediments in
fold upon fold in the Alps, the Carpathians, and the Himalayas;
and--exposing them to the rigours of denudation at altitudes where
glaciation, landslip, and torrent prevail--inaugurate a new epoch
of sedimentation and upheaval.

146