Page:Popular Science Monthly Volume 74.djvu/27

Rh wind-scouring, the very subordinate, local and sporadic character of water-action, and the remarkable plains-forming tendency which deflative erosion effects.

Since most of our conceptions of landscape genesis are derived from our experiences in a normal moist or wet climate, the erosion agency with which we are best acquainted is running water. In the desert regions actually and necessarily water plays but small part in erosion. With less than ten inches of annual rainfall, most of which sinks into the earth as soon as it touches it, as in the dry regions of southwestern United States and the northern part of the Mexican tableland, or less than one inch as in the Nubian and Libian deserts of north Africa, the erosive influences of water must be all but a negligible quantity.

With water-action reduced to relative impotency in the desert region, wind-scour assumes a rôle the denuding power of which has been heretofore little considered. Its action is general and constant. Its effects are probably even more vigorous than the work of water under normal climatic conditions. In the effort to reduce the land surface to a low-lying plain the belts of hard and soft rocks are brought into somewhat stronger contrast than in the case when water is the chief agency of planationplantation [sic]. The geologic structures are more sharply accentuated. The rock-floors are cleaner swept. The belts of weak rocks are faster removed. At all times the plain is more characteristically the main relief feature.

Contrary to popular belief the great desert tracts of the earth are mountainous regions. The mountain character has many novel and instructive peculiarities. Yet so dominant is the plains feature locally that the mountain ranges, bold and lofty as they often are, rise sharply from the level expanse as do volcanic isles out of the sea. So characteristic is this aspect that it is, in the South African deserts, appropriately denominated the "Inselgeberglandschaft."

In regard to the manner of their development the salient lineaments of the desert deserve much more attention than ever has been given them. They acquire new meanings when their peculiarities are considered in the light of an origin eolian in nature. Notwithstanding the fact that some of us, whose lifelong experiences have been mainly with the workings of the geologic processes in the moister parts of the globe, may find it a little difficult to fully appreciate at first the direct significance of many of the details of the relief features, a visit to the desert soon convinces us of their verity. There are at least a score of these physiographic characteristics of the dry lands that are especially striking.

The dominant feature of such desert regions as the western part of our own country and of Mexico is the interrupted plain the general surface of which is 5,000 to 7,000 feet above the sea. Out of it rise