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.  Every mountain range upon the Earth enforces the certainty of this prediction.

The mountain-forming movement takes place after a certain great depth of sediment is collected.  It is most intense where the thickness of deposit is greatest.  We see this when we examine the structure of our existing mountain ranges.  At either side where the sediments thin out, the disturbance dies away, till we find the comparatively shallow and undisturbed level sediments which clothe the continental surface.