RELATION OF RELIEF TO ROCK RESISTANCE. 113 



Precipitation, in inches, at Sodarille, Nev. 



1898 4.72 



1899 2.30 



1902 1.68 



1903 2.16 



Average for these four years (others not completely observed), 2.71 inches. 



DEPENDENCE AT TONOPAH OF TOPOGRAPHIC RELIEF UPON ROCK 



RESISTANCE. 



After the great amount of erosion which the Tonopah district has undergone, 

 the relief is to-day determined in a very remarkable way by the character of 

 the rocks. The relief here is not like that resulting from the work of stable 

 and .strong streams, concentrating and almost monopolizing the erosion, pushing 

 back their systematic valleys from one rock formation into another and constantly 

 broadening their domains. It is rather like that produced by the warm breath 

 of the sun on a mass of ice and snow, where the softer material fades into the 

 air and the harder skeleton of ice protrudes above the surface. 



The most prominent topographic features in the Tonopah district are the 

 denuded volcanic necks such as Butler, Brougher, Golden, Siebert, and Ararat 

 mountains, and Mount Oddie where the hard lava column has resisted erosion, 

 while the surrounding softer material has been worn down. The map shows how 

 closely the contours conform to the irregularities of the intrusion and to how 

 great a degree the difference of resistance has controlled even minor features of the 

 topography. Around the margins of the white (Oddie) rhyolite intrusions very 

 well-marked and closely set division planes parallel to the contact (platy structure) 

 render these border zones often more easily attacked. The outlying rhyolite 

 dikes also show this markedly, so that (as around Mount Oddie) such dikes, when 

 relatively narrow^ave been easily degraded to the level of the intruded rocks. 



The smaller eminences are also almost always due to a harder intrusive rock, as, 

 for example, Heller Butte. The intrusive glassy Tonopah rhyolite-dacite in the 

 northern portion of the area mapped is evidently harder than the intruded later 

 andesite and occupies in general higher ground. Study of the map shows how 

 outlying bodies of this rhyolite-dacite are frequently responsible for hills and 

 ridges, while depressions have formed along the strips of later andesite flanked on 

 the sides by the rhyolite-dacite. 



Mizpah Hill and Gold Hill an -. fault blocks whose relative relief is due to the 



greater resistance of the silicified earlier andesite. of which they are made up, as 



compared with surrounding rocks. On the east of Mizpah Hill, where the adjacent 



rocks are the soft lake beds, the scarp is fairly well developed; on the west side the 



16843 No. 4205 8 



