TOPOGIJAPHY OF ARID EEGI0:NS. 255 



The geologist is enabled to reverse this process, with greater or less success, 

 and, from the records in the rocks, determine the prevailing climatic condi- 

 tions of bygone ages. The interi>rettUion of the Laliontan records in terms 

 of climate is at the same time the most interesting and the most difficult of 

 the problems that their study has suggested. The character of the Quater- 

 nary climate of the Great Basin has been treated in a comprehensive manner 

 by Mr. Gilbert in a monograph on Lake Bonneville, which, it is expected, 

 will soon be published. We are thus, not unwillingly, constrained to con- 

 fine our studies to the evidence afforded by the records of Lake Lahon- 

 tan. Our attention will necessarily be mainly directed to the questions of 

 humidity and of temperature. 



Among the topographic characteristics of arid regions are angular 

 mountain tops, canons with precipitous walls, and alluvial cones where 

 streams from the mountains lose their grade upon reaching the valleys. 

 The last of these features is perhaps as striking as an} of the others, and 

 is of special interest in the present discussion. Many times the bases of 

 mountains are completely encircled by a sloping pediment of unassorted 

 debris, either angular or rounded, which is the most abundant at the mouths 

 of canons. Such accumulations form alluvial slopes, and when the dehrii:! 

 occurs in more or less conical or fan-shaped piles it forms alluvial cones. 

 These deposits have been studied especially by Drew'"' and Gilbert;*" by 

 the former in the arid regions of Southern Asia, by the latter in the Gi'eat 

 Basin. As descpibed by Gilbert, "The sculpture of a mountain by rain is 

 a twofold process — on the one hand destructive, on the other constructive. 

 The upper parts are eaten away in gorges and amphitheaters until the 

 intervening remnants are reduced to sharp-edged spurs and crests, and all 

 the detritus thus produced is swept outward and downward by the flowing 

 waters and deposited beyond the mouths of the mountain gorges. A large 

 share of it remains at the foot of the mountain mass, being built into a 

 smooth sloping pediment. If the outward flow of water were equal in all 

 directions this pediment would be uniform upon all sides, but there is a 

 principle of concentration involved, whereby rill joins with rill, creek with 



"Journal Geological Society of London, Vol. XXIX, 1873, pp. 441-471. 

 *• Second Annual Rejioit U. S. Geological Survey, ji. 183 et seq. 



