Rotation. 



Tilting of the Sierra. — The conclusion that an originally vertical system of 

 fissures must have been formed in this portion of the" Sierra leads to a means 

 of judging whether it has been tilted towards the Pacific ocean. The tre- 

 mendous couple which led to the vertical faults and the horizontal partings 

 tended to produce such a movement. Had it effected a sensible rotation, 

 however, the originally vertical fissures would now dip away from the coast; 

 but my observations do not reveal such a dip. In a great number of cases 

 I compared these fissures with a plumb-line without being able to detect any 

 difference in direction even where they were exposed for a vertical distance 

 of hundreds of feet. In other cases, indeed, the fissures are slightly inclined ; 

 but I observed no regularity in the direction of these divergences ; they 

 seemed to me due to local irregularities, in the resistance of the rock. 



Were such a tilting of the fissures observed, it might have taken place 

 during the disturbances of which the Assuring itself was an early feature, or 

 at any later period. I gathered some independent evidence that since the 

 beginning of the glaciation of the region no such movement has occurred. 

 Lake Tahoe appears to have been in existence since the earliest glaciation, 

 since which time it has cut down its outlet about 300 feet ; but the highest 

 beaches which I have found are at sensibly the same elevation at the north- 

 ern and southern ends of the lake and at its western side. The lake is now 

 about 22 miles long and 12 miles wide, and its earlier dimensions were ol 

 course still larger. It thus forms an excellent self-recording levelling instru- 

 ment. The highest terraces of the lake are nearly obliterated, but there 

 are well marked intermediate benches which seem to be at uniform elevations 

 above the present surface. It is possible that more exact measurements, 

 such as I propose making, will reveal a difference of a few feet ; but as the 

 whole range is only about five times the width of the present lake, it seems 

 to me impossible that the tilting should be considerable. 



Erosion of the Gravels. — The hypothesis of the tilting of the range has 

 been put forward by Professor Joseph Le Conte to account for the now well- 

 known fact, recorded by Professor Whitney, that the modern rivers of the 

 western slope of the range have eroded far below the level of the Pliocene 

 streams in the beds of which the hydraulic gravels are found. It is therefore 

 desirable for me to show that such erosion does not necessarily imply a 

 movement inconsistent with the results drawn from my observations.* 



* Professor Le Conte states " that the Sierra Nevada is a great crust block 300 miles long and 50 

 miles wide heaved and slipped on theeasiern side, forming tht-re a sreatfaultof 15,000 to 2(»,0U() feet 

 vertical displacement, and that this took place at the end of the Tertiary accompanied by floods of 

 lava. The evidence of this is found in the relation of the new to the old river beds. The rivers 

 displaced from their old beds by the lava have since that time cut far deeper than before, although 

 cutting far less time " (Amer Journ Sei., 8d Ser., vol. XXXVIII, 1889, p. '-^61). Un a previous occa- 

 sion he recognized the fissuring of the high Sierra and its connection with volcanic phenomeiia, 

 which Hgain he ascribed to the elevation of the range (Amer. Jouni. Sci., 3d Ser., vol. XIX, 1880, p. 

 190). In the same paper he says that he has observed many eruptive dikes in all the trranite region 

 above the lava flow, and these he regards as probably the roots of the flow (page 188). How the 

 range could be tilted as a single block at the same time that it> central portion was intersected by 

 numerous fissures reaching down to volcanic foci does not seem to be explained. 



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