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.
It is no more than a step to show that bound up with the
radioactive energy are most of the earthquake and volcanic
phenomena of the earth. The association of earthquakes with the
great geosynclines is well known. The work of De Montessus showed
that over 94 per cent. of all recorded shocks lie in the
geosynclinal belts. There can be no doubt that these
manifestations of instability are the results of the local
weakness and flexure which originated in the accumulation of
energy denuded from the continents. Similarly we may view in
volcanoes phenomena referable to the same fundamental cause. The
volcano was, in fact, long regarded as more intimately connected
with earthquakes than it, probably, actually is; the association
being regarded in a causative light, whereas the connexion is
more that of possessing a common origin. The girdle of volcanoes
around the Pacific and the earthquake belt coincide. Again, the
ancient and modern volcanoes and earthquakes of Europe are
associated with the geosyncline of the greater Mediterranean, the
Tethys of Mesozoic times. There is no difficulty in understanding
in a
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general way the nature of the association. The earthquake is the
manifestation of rupture and slip, and, as Suess has shown, the
epicentres shift along that fault line where the crust has
yielded.[1] The volcano marks the spot where the zone of fusion
is brought so high in the fractured crust that the melted
materials are poured out upon the surface.
In a recent work on the subject of earthquakes Professor Hobbs
writes: "One of the most interesting of the generalisations which
De Montessus has reached as a result of his protracted studies,
is that the earthquake districts on the land correspond almost
exactly to those belts upon the globe which were the almost
continuous ocean basins of the long Secondary era of geological
history. Within these belts the sedimentary formations of the
crust were laid down in the greatest thickness, and the
formations follow each other in relatively complete succession.
For almost or quite the whole of this long era it is therefore
clear that the ocean covered these zones. About them the
formations are found interrupted, and the lacuna indicate that
the sea invaded the area only to recede from it, and again at
some later period to transgress upon it. For a long time,
therefore, these earthquake belts were the sea basins—the
geosynclines. They became later the rising mountains of the
Tertiary period, and mountains they
[1] Suess, _The Face of the Earth_, vol. ii., chap. ii.
143
are today. The earthquake belts are hence those portions of the
earth's crust which in recent times have suffered the greatest
movements in a vertical direction—they are the most mobile
portions of the earth's crust."[1] Whether the movements
attending mountain elevation and denudation are a connected and
integral part of those wide geographical changes which result in
submergence and elevation of large continental areas, is an
obscure and complex question. We seem, indeed, according to the
views of some authorities, hardly in a position to affirm with
certainty that such widespread movements of the land have
actually occurred, and that the phenomena are not the outcome of
fluctuations of oceanic level; that our observations go no
further than the recognition of positive and negative movements
of the strand. However this may be, the greater part of
mechanical denudation during geological time has been done 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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