On the arid plains of the Southwest the two most striking features of the landscape are the lofty isolated desert ranges and the lower truncated hills which rise abruptly as walls above the general plains surface. The first mentioned of these relief characters has long attracted wide attention from travelers; the second, notwithstanding the fact that it is fully as impressive as the first, has scarcely received any notice at all. R. T. Hill alone seems to have given them special consideration and a name. For these plains above plains, or plains within plains, the term plateau plains appears to be a very appropriate title. At the present time particular interest attaches to the plateau plains because of the fact that they seem to furnish the most direct and convincing testimony we have of the deflative nature of general erosion under conditions of aridity. . . .