The architecture of the Earth’s crust, and more particularly the earth-movements registered in this architecture, provide the subject matter of Tectonics. Relevant data are furnished through study of the sedimentary rocks, most of which originated as sand, mud, or calcareous matter, deposited in a succession of approximately horizontal layers, known as beds, or strata. Exposure by erosion shows that, over wide extents of our continents, approximate horizontality of bedding has been maintained from remote antiquity. There has been elevation or depression, but comparatively little tilting or bending. Such conditions hold in the persistent platforms, represented in much of Russia and the interior of North America. On the other hand, in the folded mountains, as, for instance, the Alps and the Appalachians, the bedding has been affected by subsequent disturbances, which have produced bends, or folds, and also fractures with displacement, especially of the overriding type known as thrusting.

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