Page:Popular Science Monthly Volume 74.djvu/25

Rh The second great geographic principle, that of the verity of a distinctly staged cycle of erosion comparable in a way to the periods of growth in the human individual, has done more than anything else to advance and place the study of geography on a truly genetic basis. It is to Davis we are chiefly indebted for devising for us a practical working scheme.

What the base-level of erosion is to the general theory of land degradation under conditions of a normal moist climate Passarge's great deduction of the possibility of general land leveling and lowering without regard to sea-level is to land sculpture under conditions of a dry climate. Until within the last lustrum the lineaments of the great desert regions of our globe have remained without adequate or satisfactory explanation. The genesis of the grander features of the landscape on the basis of ordinary tectonics, or of normal erosion during former wet climatic periods, or of water-action under present conditions, has always met with seemingly unsurmouutable obstacles. The origin of the salient features of the desert, its peculiar mountains, its smooth plains, its strange plateaus, its streamless surface, its remarkable rock-floor, and its many other unexpected features, are only beginning to be fully appreciated in their proper relationships. To us of the moister countries they present many novelties. They make us acquainted with the vigorous workings of geologic processes to which we are as yet almost complete strangers.

In the operations of the geologic processes under conditions of an arid climate the most noteworthy effects as compared with those under normal conditions are the prevalency, the constancy and efficiency of