Page:Popular Science Monthly Volume 74.djvu/30

26 most rapid degradation. The much stronger contrasts of belts of soft and hard rocks observable in the arid lands than in the moist lands appear to be due to this very fact. Then, too, wind-scour and not water-action must be reckoned with as the main erosional means, another fact that has not been usually taken into account.

Without exception the mountain rocks of the western deserts are very hard and resist the attacks of erosive action to an eminent degree. The plains rocks being mainly non-resistant rocks, as we have seen, there thus appears a notable alternation of hard and soft rock-belts. In the geologic succession the rocks composing the mountains are principally ancient crystallines and Paleozoic limestones which are followed by enormous thicknesses of soft sandstones and shales that frequently

attain a vertical measurement of 10,000 feet or more. Tertiary faulting on a grand scale has brought the soft strata to the same level as the resistant beds. In the general and profound wearing down of the surface of the country towards sea-level, marked contrasts of relief are produced between the various rock-belts. In the moist countries the ipolated residual eminences of general erosion, known to geographers by the special name of monadnocks, after the New England mountain regarded as the type, are of rare occurrence. In the desert region the majority of the mountains are of this character.

One of the most peculiar of the many odd features of the desert is the beveled rock structure of the plains-surface. In the moist regions a high-lying plain with the substructure beveled is taken as an indication of former peneplanation, the lowest level to which water can wear