Page:Popular Science Monthly Volume 82.djvu/113

Rh shale, and sandstone, which eventually reached a thickness of 5,000–10,000 feet.

This condition did not hold without end, for eventually strong compressive forces, engendered in the underlying body of the earth, squeezed the superficial rocks into folds, and thus bulged the surface high above sea level in the region so affected. By the prompt attack of streams, winds, glaciers, and the other agencies which are incessantly sculpturing the surface of the earth, these elevated districts were, even while rising, carved into rugged mountains and deep valleys, so that the original folds were greatly disfigured even before the compressive forces ceased to operate.

It is a fact generally recognized among geologists, that in terms of geologic time such episodes of compression and folding are short-lived. They are soon followed by much longer periods during which the internal forces of the earth are quiescent, but in which the erosive agencies have free play. If any land remains indefinitely above sea level, and is not disturbed by movements from below, the mountains and hills will eventually be worn away and there will be left only a broad almost featureless plain. It is believed that China, in consequence of such a period of quiescence, was reduced to a lowland from which almost all of the preexisting mountains had been removed. In this condition it probably remained for more than one geologic period, and the western part may even have been submerged beneath the sea which at that time