ption, the ranges

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of the Earth are built essentially of sedimentary rocks: that is
of rocks which have been accumulated at some remote past time
beneath the surface of the ocean. A volcanic core there may
sometimes be--probably an attendant or consequence of the
uplifting--or a core of plutonic igneous rocks which has arisen
under the same compressive forces which have bowed and arched the
strata from their original horizontal position. It is not
uncommon to meet among unobservant people those who regard all
mountain ranges as volcanic in origin. Volcanoes, however, do not
build mountain ranges. They break out as more or less isolated
cones or hills. Compare the map of the Auvergne with that of
Switzerland; the volcanoes of South Italy with the Apennines.
Such great ranges as those which border with triple walls the
west coast of North America are in no sense volcanic: nor are the
Pyrenees, the Caucasus, or the Himalaya. Volcanic materials are
poured out from the summits of the Andes, but the range itself is
built up of folded sediments on the same architecture as the
other great ranges of the Earth.

Before attempting an explanation of the origin of the mountains
we must first become more closely acquainted with the phenomena
attending mountain elevation.

At the present day great accumulations of sediment are taking
place along the margins of the continents where the rivers reach
the ocean. Thus, the Gulf of Mexico receiving the sediment of the
Mississippi and Rio Grande;

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the northeast coast of South America receiving the sediments of
the Amazons; the east coast of Asia receiving the detritus of the
Chinese rivers; are instances of such areas of deposition. Year
by year, century by century, the accumulation progresses, and as
it grows the floor of the sea sinks under the load. Of the
yielding of the crust under the burthen of the sediments we are
assured; for otherwise the many miles of vertically piled strata
which are uplifted to our view in the mountains, never could have
been deposited in the coastal seas of the past. The flexure and
sinking of the crust are undeniable realities.

Such vast subsiding areas are known as geosynclines. From the
accumulated sediments of the geosynclines the mountain ranges of
the past have in every case originated; and the mountains of the
future will assuredly arise and lofty ranges will stand where now
the ocean waters close over the collecting sediments. Eve