Page:Popular Science Monthly Volume 54.djvu/489

Rh either with or without folding of the strata. These initial wrinkles pave rise to our first mountains, and the continuation of these conditions at the present time is as surely nourishing mountain growth as at any time in the past. In this way the fluctuations of the ocean's



level, above referred to, alone are to be explained, and such form but temporary rises and falls in the history of a continent.

The rate at which an ocean bed is raised to form a mountain range is, no doubt, a variable one; always slow, often interrupted, but seldom or never violent. During this time the strata usually undergo crushing and folding; stretching takes place, and displacements of the rocks, or faulting, are not uncommon. As an example of the wrinkling that the strata may suffer under these conditions, the reader is referred to the beautiful symmetrical fold shown on the side of a mountain in the Appalachians (Fig. 7). Similar folding is the rule, but often immense areas are raised to great heights above the •ocean without disturbing the horizontal position of the beds (see Fig.-8). Coincident with the emergence of the rocks from beneath the