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THE POPULAR SCIENCE MONTHLY 



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 prin- 

 cipally ancient crystallines and Paleozoic limestones which are followed 

 by enormous thicknesses of soft sandstones and shales that frequently 



Sierra. Oscuro, New Mexico. Rises out of plain like a volcanic isle out of the sea ; 

 height 4,000 feet ; distance 15 miles. It is composed of hard rock ; 

 the plain of soft sandstones. 



attain a vertical measurement of 10,000 feet or more. Tertiary fault- 

 ing 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 sur- 

 face 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 indica- 

 tion of former peneplanation, the lowest level to which water can wear 



