112 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



is fairly well armored against the attacks of erosion, which can make but compara- 

 tively slow progress. 



In a truly arid region, where there are extremes of heat and cold, rock 

 disintegration at the surface is much more rapid. Streams are rare, transient, 

 and relatively unimportant, and stream erosion is slight compared with that of 

 moister regions. Yet erosion is active, so that in the Great Basin region even 

 moderately steep slopes are stripped of debris and consist of hard, unweathered 

 rock. The lack of vegetation renders the whole surface equally susceptible of 

 attack by frosts, thaws, rains, and snows, and the disintegrated material creeps by 

 the nearest way, in the form of a sheet, into the depressions. Thus the fronts of 

 many of the Basin ranges are bordered by a continuous apron of debris sloping 

 down into the center of the valley, an enormous mass of waste which is relatively 

 slightly increased by the alluvial fans at the mouths of the gulches (PI. XV, ). 



In desert regions the more nearly equable distribution of the eroding agents 

 causes the differences in hardness of the attacked rocks to be far more prominent in 

 determining the lines of relief. In proportion as the aridity increases the topo- 

 graphic forms show more and more faithfully the resistance of the rocks. If the 

 rocks are folded and faulted the ridges will follow the lines of strike and of faulting. 

 In a country of igneous rocks a new element is introduced, but here also erosion 

 tends to preserve the original lines of structure. In intervals of moister climate 

 streams will cut gorges, a tendency which is probably antagonized in succeeding 

 arid periods. 



It is proper to insist here that these distinctions apply to truly arid climates, 

 and are more applicable as the aridity increases. Semiarid regions, where violent 

 rains are not infrequent, have a different topography. The abundant waters of the 

 storms flow down the slopes in rushing torrents, which cut their beds all the more 

 deeply because the rock is naked and disintegrated as the result of the intervening 

 periods of aridity. A rugged, well-dissected topography may sometimes result, 

 often wrongly described as typical of arid regions. 



PRECIPITATION" IX REGION NEAR TONOPAII. 



Although violent rains sometimes occur at Tonopah, especially in the spring, 

 they are rare, and the region can not be classed as semiarid; it approaches more 

 nearly true aridity, and the channeling by torrents is not so important as the 

 universal downward working of disintegrated material. 



No records of precipitation have been kept in Tonopah, for the town is only 

 a few years old. The observations made by the United Sates Weather Bureau 

 in Sodaville, 60 miles farther northwest, are as follows: 



