EROSION IN ARID CLIMATES. Ill 



remain (p. 45). These outer cones must have been very extensive in comparison 

 with the necks and must have covered the whole area of the quadrangle deeply. 

 There is a difference of 700 feet in elevation from the top of Butler Mountain to 

 the lowest point, near its base, where the dacite neck cuts the intruded rock, so 

 that 700 feet is less than the minimum possible thickness of the material that has 

 been removed between these two points. 



A still better measure of the amount of erosion is supplied by a study of the 

 faulting. In general, the southern half of the area has been depressed by faulting 

 below the northern half by a distance which has not been closely measured, but 

 which is certainly many hundred feet; yet this differential movement has been 

 entirely compensated by erosion, and there has been stripped from the northern 

 half a crustal layer of a thickness equal to the sum of the amount of the displace- 

 ment and the thickness of the material removed from the southern half during 

 the same period. Similarly, the individual faults, elsewhere considered, show that 

 erosion has compensated for their dislocations. 



FEATURES OF EROSION Ilf ARID CL.IMATES. 



In the arid Great Basin region the conditions governing the origin of topo- 

 graphic forms are different from and more complicated than those which exist in 

 well-watered regions, where most of the reliable physiographic conclusions have 

 been formulated. It is therefore important that in a region where the topography 

 is well mapped and the geology fairly well understood, as in Tonopah, the origin 

 of the forms should be examined. 



The writer has previously remarked that in the greater part of the arid Great 

 Basin region the effect of the scant moisture as an agency of erosion is equaled or 

 exceeded by disintegration, gravity, and wind action, with the result that in the 

 lower valleys leveling instead of dissection is brought about, and in the higher ones 

 dissection is much less marked than in moister regions." The general conclusions 

 reached by the writer concerning processes of erosion in the Great Basin region, 

 as expressed in an unpublished paper read before the Geological Society of 

 Washington in the spring of 1903, are as follows: 



Climate controls not only the speed of erosion, but its manner. In moist 

 climates the precipitated moisture gathers into permanent bodies of running water, 

 a stream system is maintained, and erosion goes on chiefly along these lines. Thus 

 even those rocks which offer no differences in weakness are thoroughly dissected, not 

 because the materials in the valleys are less resistant, but because there the eroding 

 activity is concentrated. The disintegrated rock or soil, except along these naked 

 stream beds, is cemented with moisture and bound together by vegetation, and so 



a Bull. Geol. Soc. Am., vol. 12. p. 237. 



