Page:Popular Science Monthly Volume 74.djvu/31

Rh down the land. The intermont plains of the arid region of western America have beveled rock-substructures, but their surfaces are 5,000 to 8,000 feet above sea-level. The beveled character of the strata is almost conclusive evidence that these plains are not areas of extensive aggradation. In the absence of adequate water-action their origin must rest mainly in long-continued and effective deflation.

Critical examination of the substructure of the plains, in favorable places, shows clearly that the rock-floor itself is a plain. Although not always apparent at first glance this rock-floor is commonly only thinly mantled by wash débris and soil. Hence the rock-floor and the present plains-surface are not very different in their detailed relief characters.

A phenomenal feature of the desert plains is the plateau-plain. Mesas they are called in southwestern United States and Mexico. These mesas, as their Spanish name signifies, are extensive, fiat-topped, table-like areas rising abruptly from the general plain to heights of from one or two hundred feet to a thousand feet or more. The great Mesa de Maya, in northeastern New Mexico, is 3,500 feet above the next lower plain.

The surface of the plateau-plain is usually found to be composed of some hard rock layer, as in the case of the vast Llano Estacado, or "walled plains," or staked plains as it is called by the Texans; or is made up of an extensive lava flow, as, for example, the Mesa de Maya, the Ocate mesa and the majority of the plains of this kind. The surface beneath the lava flows of the mesas is itself a plain worn out on