Page:Popular Science Monthly Volume 74.djvu/23

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UR notions of the genesis of desert landscapes have lately undergone complete revision. In land-sculpturing under conditions of aridity we are led to recognize some entirely new phases of geologic operations. The principles deduced are not alone applicable to countries with excessively dry climates, but likewise to all lands of the earth.

In the moister regions of the globe, or those parts with which the majority of us are most familiar, moving water is so universally regarded as the chief agent of denudation that other erosive means are seldom more than barely considered. In the arid districts there is a reversal of the relative efficiencies of the erosion processes. Water-action is of quite secondary consequence. Wind-scour, or deflation, is not only the most vigorous, but often almost the sole, erosive power.

The tremendous efficiency of wind as an erosive agent has been lately brought to general notice mainly through the results of Passarge's investigations in the South African deserts. His principal deduction is far-reaching in its scope and significance, and seems destined to stand in geology as one of the grand generalizations of the new century. In our own country it opens up vast and fertile fields of geologic inquiry.

The desert regions of earth have given to modern geography its most suggestive and fundamental concepts. This is a fact that is all but forgotten by most of us who are accustomed daily to apply these basic principles in the more familiar moist tracts in which we live. Yet the definite cycle of evolution which land-forms pass through, the base-level to which all erosion tends, and a general plains-leveling, without regard to sea-level, that goes on in dry countries, are deductions of the desert.

True desert conditions prevail over a much larger proportion of our earth than most of us appreciate. Southwestern United States, the greater part of central Mexico, western South America, South Africa, northern Africa, southwestern, central and east-central Asia, eastern Europe and central Australia, all present vast areas of territory which have rain-fall insufficient to raise ordinary grain crops without artificial watering. The desert, however, is not the repulsive land that its name whenever mentioned suggests to the layman. Most of it is not