68 WARREN N. THAYER 



rivals that of our own Grand Canyon, which was produced by the 

 same causes and at the same time. 



It is along the courses of these westward-flowing streams that 

 the dentate character of the sierras is seen to the best advantage. 

 Their longitudinal tributaries separate chains or mountain masses, 

 the western slopes of which are marked by precipitous fault scarps. 

 The eastern slopes of these masses being more gentle, the profile is 

 clearly imitative of the teeth of a saw. 1 



Many of the longitudinal stream valleys, before being tapped by 

 the westward streams, were intermontane basins. On the sides 

 of these valleys are now to be found undeformed, stratified beds, 

 which were for a long time unaccounted for. They are now 

 regarded as loosely cemented debris which had been carried down 

 the slopes and deposited in the basins, which, when filled with 

 water, formed lagunas, and constituted "settling tanks," as Hill has 

 termed them, 2 where the detritus was roughly classified according 

 to size and deposited in layers. They are of course of a past, but 

 quite recent, geologic age. 



The general structure of this province may be described as a 

 series of folds, varying from a close, even recumbent, type, to 

 broad swells, passing out northward into monoclines, over the 

 erosion-beveled edges of which are spread flow upon flow of erup- 

 tive lavas. Le Conte, twenty years ago, 3 proposed the theory that 

 these Sierras were formed by intense folding movements in Early 

 Tertiary time, followed by profound drops at the sides of faults 

 parallel to the folds; that along these lines of less resistance, suc- 

 cessive and prolonged volcanic eruptions took place, and that subse- 

 quent active erosion accentuated the escarped form which affects 

 most of the slopes. Later studies pursued by prominent Mexican 

 geologists have confirmed this theory. 4 



In the canyon of the Aros there have been counted as many as 

 nineteen separate lava flows, one above the other, with intervening 

 beds of tuff and local conglomerate derived from the disintegration 



1 Ordonez, op. cit. ' Hill, Trans. A.I.M.E., XXXII, 163. 



3 Le Conte, "The Origin of Mountain Ranges," Jour. Geol., September-October, 

 1893. 



4 Ordonez and Aguilera, Inst. Geol. Mex., IV. 



