CONFLICTING VIEWS 293 



thousand feet above the level of the sea. Le Conte conceived that ele- 

 vated portion of the earth's crust between the Sierra Nevada on the west 

 and the Wasatch on the east, in the course of its slow upheaval to its 

 present exalted position, to be something like a great arch, the incompe- 

 tent material of which was continually readjusting itself by breaking- 

 down into blocks that gradually shifted along planes of normal or gravity 

 faulting. The upturned edges or elevated tops of some blocks form the 

 mountain ranges ; the downthrown edges or depressed tops of others, the 

 intervening valleys. The Sierra Nevada forms one end of the great arch, 

 with its mighty scarp facing eastward, the Wasatch being its counterpart 

 on the far eastern limb, with its scarp facing westward.* 



Western geologists have in general accepted the " faulted block " as 

 the prevailing type of mountain structure in the Great basin, and the 

 expression u Basin Range structure " has passed into familiar usage as 

 a general designation of such type. 



But recently an attempt has been made to overthrow these views,f and 

 to prove that the ranges of the Great basin are not faulted blocks, but 

 were formed as follows : 



" The process of mountain building in this region has been complicated, so it is 

 to be expected that when the details shall have been more closely studied many 

 types of ranges will be found. But at present we can hardly distinguish more 

 than two— those formed chiefly by erosion and those due directly to deformation. 

 To the first class seem to belong most of the mountains of the region. To the 

 second class probably belong part of the ranges of two outlying provinces. J • • • 

 The faults actually observed in this region are comparatively few. Actually ascer- 

 tained heavy faults along the main fronts of ranges are exceedingly rare.$ . . . 

 According to this conclusion these mountains are not simple in origin and structure. 

 However, the writer would compare the typical Basin range to the less compressed 

 portions of the Appalachians and the Alps." || 



Besides the sweeping attack of Spurr just referred to, it may be noted 

 that James D. Dana considered the fault hypothesis as not yet proved. 



He says : ^f 



"The ridges of the Great Basin, made thus of upturned and plicated rocks, 

 have been assumed to be each limited by faults and to have undergone up and 

 down movements and variously tilting displacements, and thus to have become in 

 effect ' monoclinal orographic blocks' in the ' Basin System,'— each block making 

 by itself a monoclinal mountain, even when not so in its bedding (Russell, 1885). 

 In the ideal sections made to illustrate this hypothesis, the wide intervals of allu- 



* These views are explained rather fully and illustrated with several diagrams in Am. Jour. Sci., 

 3d ser., vol. 38, pp. 257-263, especially p. 259 et seq. 

 t Spurr: Bull. Geol. Soc. Am., 1901, vol. 12, pp. 217-270, pis. 20-25. 

 % Log. cit., p. 241. 

 g Idem, p. 259. 

 || Idem, p. 266. 

 U Manual of Geology, fourth edition, 1895, p. 366. 



