Geol.— Vol. I.] TURNER— ORIGIN OF Y OS E MITE VALLEY. 279 



had been occupied by a glacier. At a later date he denied ^ 

 that there was any evidence of the former presence of gla- 

 cial ice in the Yosemite, and stated more fully'' his theory 

 of its formation as follows: — 



We conceive that during the process of upheaval of the Sierra, or, possibly 

 at some time after that had taken place, there was at the Yosemite a subsi- 

 dence of a limited area, marked by lines of "fault " or fissures crossing each 

 other somewhat nearly at right angles. In otlier and more simple language, 

 the bottom of the valley sank down to an unknown depth, owing to its sup- 

 port being withdrawn from underneath during some of those convulsive 

 movements which must have attended the upheaval of so extensive and ele- 

 vated a chain, no matter how slow we may imagine the process to have been. 

 Subsidence over extensive areas of portions of the earth's crust is not at 

 all a new idea in geology, and there is nothing in this peculiar application of 

 it which need excite surprise. It is the great amount of vertical displace- 

 ment for the small area implicated which makes this a peculiar case; but it 

 would not be easy to give any good reason why such an exceptional result 

 should not be brought about amid the complicated play of forces which the 

 elevation of a great mountain chain must set in motion. 



By the adoption of the subsidence theory for the formation of the Yosemite, 

 we are able to get over one difficulty which appears insurmountable with any 

 other. This is, the very small amount of debris^ at the base of the cliffs, and 

 even, at a few points, its entire absence. 



This theory of Whitney's was later adopted by Kneeland,* 

 Russell,'^ and Reyer'^ and very recently (1896) it was again 

 recognized by J. W. Gregory, in a volume entitled the 

 " Great Rift Valley". The valley referred to by Gregory 

 lies in British East Africa, west of Mount Killimanjaro. 

 By rift valley is here meant a depression formed by the 

 dropping down of faulted blocks, and not a yawning apart 

 of the earth's crust along a fracture. Such a depression 

 would be called 2i '■'■ graben'" by Suess. As previously 

 stated, Mr. W. Lindgren^ regards the basin occupied by Lake 

 Tahoe as formed in this way. The Yosemite is referred to 

 by Gregory as being a rift valley in this sense. 



iThe Yosemite Guide Book, 1874, p. 117. 



'^Ibid., pages iig-120. 



3 The largest talus slopes known to the writer in the Sierra Nevada are those of the 

 north side of Yosemite Valley, west of Kl Capitan, and of the south side east of Cathe- 

 dral Rocks and west of Bridal Veil Halls, and the lack of talus at other points is no more 

 striking than in most glacial canyons. 



^Kneeland, Proc. Bost. Soc. Nat. Hist., Vol. XV, 1873, pp. 36-47. 



6 Russell, Kighth Annual Report, U. S. Geol. Surv., Part I, i88g, p. 351. 



SReyer, Neues Jahrbuch, 1886, Beilage Band IV, Heft 2, pp. 291-326. 



'Jour. Geol., Vol. IV, 1896, p. 895. 



