Since as ordinarily developed the scheme of the geographic cycle postulates an upraised land surface exposed to stream action and other erosive influences peculiar to a humid climate, its designation “normal scheme” is perhaps as fitting as any other term possibly could be. The normal plan, however, has had to be modified to meet two special conditions. On the one hand it has been adapted to a glacial climate, and on the other hand to an arid climate. A number of writers have recently described these variations, but always with the essentials of the normal scheme conspicuously before them.
In all of these considerations stream action or water alone is regarded as the prime agency of regional leveling and lowering. That there should be need of distinctive treatment to meet the requirements of those special conditions of climate where the snows of winter do not all vanish in summer,