i68 STUDIES FOR STUDENTS 



may be cited the coalescent gravel fans of Persia forming the "skirts 

 of the mountains," and the detrital slopes of the mountains in New 

 Mexico and Arizona. 



As a description of the character of erosion in the heart of the desert 

 mountain region of Arizona may be cited a paragraph by Ransome, 

 who, writing of the Globe copper district, says : 



With the exception of the timbered slopes of the Pinal Mountains, and a 

 few alluvial areas along the main arroyos, the surface of the region is almost 

 destitute of soil. The scanty shrubbery, and the sparse grass and herbage 

 which spring up with wonderful rapidity after the rains, are insufficient to prevent 

 such soil as may form from being quickly washed away. The humus acids, 

 which in moister climates and beneath the covering of soil aid in rock decay, 

 have in this region little opportunity to form or to attack the rocks. The latter 

 crumble or flake under the influence of sharp atmospheric changes, and these 

 fragments are rapidly carried into the valleys. The granitic masses crumble 

 into particles of quartz, flakes of mica, and angular fragments or crystals of 

 comparatively fresh feldspar. The rains acting on this disintegrated material 

 soon wash it down to the larger streams, which carry off the quartz and mica. 

 The larger fragments of feldspar often build up alluvial fans at the mouths of 

 the small ravines heading in a granitic area, and such fans are remarkable for 

 the purity and freshness of the feldspathic material which composes them, the 

 numerous cleavage faces flashing brightly in the sun. Excellent examples of 

 these fans were observed along Pinto Creek, north of Horrell's west ranch. They 

 are evidently transient phenomena, accumulating untfl an exceptionally wet 

 season causes Pinto Creek to rise and sweep them away.^ 



The rainfall varies in adjacent parts of this region and in succes- 

 sive years, the amounts ranging from ii to 20 inches per year, a 

 considerable proportion falling as rain during the sudden and violent 

 downpours which are common in July and August.^ 



Upon a climatic change causing more voluminous streams to flow 

 across the piedmont slopes or drain the interior basins, the stream 

 profile for equilibrium is altered, a large amount of coarse, unleached, 

 and incoherent waste may be quickly swept downstream, even to 

 the delta, and be deposited either upon its upper surface as partly 

 subaerial products, or swept to the front, building the delta farther 

 seaward. 



I Geology of the Globe Copper District, Arizona, Professional Paper No. 12, U. S. 

 Geological Survey, 1903, p. 21. 

 3 Op. cit., p. 20. . 



