Geol.— Vol. I.] TURNER-ORIGIN OF YOSEMITE VALLEY. 317 



2. That it was a river-cut canyon, but that the vertical 

 walls are due to the sapping action of ice. Johnson. 



3. That it was formed by a drop fault. Whitney, 

 Reyer, Le Conte? and Russell. 



4. That it was formed by river erosion facilitated by 

 strong jointing. Becker, Branner, and Turner. 



1. As it has not been shown that ice can dig deep 

 canyons, the first theory does not appear to be well 

 founded. 



2. Having obtained good evidence that glacial cirques 

 were made, or at least enlarged, by the sapping action of 

 the ice which they contained, Mr. Johnson felt encouraged 

 to apply the same method to the formation of deep steep- 

 sided valleys. That is to say, while recognizing that the 

 Merced canyon must have been partly excavated by a 

 river, he would suppose that this canyon had the ordinary 

 V-shape of a river canyon, and that the ice by its sapping 

 action cut into the V-slopes, eating backward until vertical 

 cliffs were formed. While sapping action as described by 

 Johnson eats into the slopes of river canyons and results in 

 a U-shape, there is no evidence, so far as my observations 

 go, that it forms high vertical cliffs, except when the rock 

 possesses a vertical structure. 



3. The faulting theory is held by some famous geolo- 

 gists. Drop faults, or longitudinal blocks dropped down 

 between two parallel fractures, seem actually to occur in 

 nature, and if one observes the steep walls of the valley as 

 given in the cross-section (Plate XXXVI-Z?) such an origin 

 seems plausible. The vertical joints described in the pre- 

 ceding pages appear, however, to satisfactorily account for 

 the steeply inchned or vertical walls. Moreover, it is diffi- 

 cult to understand how faulting on so extensive a scale can 

 have taken place, and yet end so abruptly at the east and 

 west ends of the valley, unless, indeed, we regard Tenaya 

 Canyon as part of the faulted area or graben. There is, 

 to be sure, a sheeted zone, as already described, just north 

 of Half Dome,. and some faulting may have occurred 

 along this zone ; but in general the well exposed rocks of 



