Page:Popular Science Monthly Volume 82.djvu/475

Rh appears to be due to the recency and enthusiasm with which by the scientific world the great law of the base-level of erosion had been received and to the vast dynamic possibilities which it had opened up.

Scant indeed is the attention given in the text-books on earth-history to the geologic effects of wind-action. A good and concise summary of our prevailing notions on the subject a decade ago is given by Udden. The great significance and value of the newer generalization lies not alone in the recognition of the geologic potency of wind-power as an agency of erosion, or as a means of forming such vast continental deposits as the loess, but of its tremendous efficiency as a general or regional denuding force. In far-reaching importance it compares favorably with the enunciation of the glacial theory of the last century.

It has long been the custom not only to treat the subject of general land-sculpturing independently of climatic considerations, but as if the molding of all landscape features was controlled by the same laws. The fertility of suggestion arising from the conception of a definite cycle of development through which all land-forms must pass has tended to exaggerate the evolutionary aspects of the theme at the expense of the genetic means by which the physiographic changes have taken place. Even the latest and most authoritative treatise on physical geography has premised the same derivation of physiognomy for the glacial Alps and the arid-high plateaux of western America, for the forest-clad Appalachians and the barren South African veldt, for the jungle-matted eastern Andes and the desert Australian interior. Ordinary stream-corrasion is made to account for all. Rain is regarded as the universal and sole graving-tool of land-sculpturing.

A full comprehension of the pregnant idea that wind-action under the favorable physical conditions imposed by arid climate is a general erosional agent may be said to date from the year 1904—the time of the appearance of Passarge's brief but quite remarkable essay on "Die Inselberglandschaften im tropischen Afrika." In various parts of the world during the decade previous the conception had in one way or another begun to assume form. The Trans-Caspian region had already furnished some facts bearing upon the new generalization. The vast deserts of the Dark Continent had supplied others. Our American arid lands had brought forth a host of still different suggestions. Indeed, as a definite working hypothesis the general scheme appears to have been first successfully formulated and applied in the great dry region of our own Southwest.

Whether first definitely outlined by American on the Girghiz steppes, by German on the South African plateau, or by Yankee on the Mexican tableland, it is certain that, as McGee astutely observes, the satisfactory disposal of the rock-waste of the desert by prodigious wind exportation furnishes the missing link to a rational explanation of all the long puzzling phenomena presented by arid regions throughout the world.