278 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



or fissured zones, as lines of less resistance, and it is also 

 clear that along such zones erosion would be more rapid 

 than in the unjointed masses. At many points on the bare 

 granite slopes of the Sierra surrounding the Yosemite, the 

 course of small streams can be plainly seen to be deter- 

 mined by the joint planes. With the larger streams erosion 

 has often gone so far as to obscure the relation of the joint 

 planes to the course of the streams; yet if we plot the joints 

 on the ridges enclosing the canyons, a parallelism to the 

 general trend of the canyons may be frequently noted. 

 Not only is the direction of the stream often determined by the 

 direction of the joint planes, but the character of the enclos- 

 ing walls, where these are solid rock, is also in a consider- 

 able measure determined by these planes. Thus, if the 

 joints are vertical (See Plate XXXII), the walls will be ver- 

 tical, and if diagonal, inclined surfaces will result, as for 

 example, in the ravine of the Bridal Veil Creek above the 

 falls, and at the Three Brothers. The joint planes here 

 described should not be confused with the curved partings 

 due to weathering, as seen on most granite domes. 



In the course of long ages, the streams have cut deep 

 canyons. Portions of the canyons where no strong joint 

 planes intersect the rocks will then have the ordinary shape 

 of river or glacial valleys, according as they are above or 

 below the former limit of glacial ice ; but where strong sets 

 of vertical joints exist the river canyons will be bordered 

 by cliffs. This was the case at Yosemite Valley; but 

 before full evidence of this simple mode of the formation of 

 the valley is presented it will be advantageous to take note 

 of the views of previous observers. 



2. Views of Previous Observers. 



A valley so interesting and so much visited could hardly 

 fail to have invited suggestions as to its origin. Professor 

 J. D. Whitney, writing in 1865,^ supposed the valley to 

 have been the result of the dropping down of what is now 

 its floor. At the same time he recognized that the valley 



1 Geology of California, Vol. I, 1865, p. 422. 



