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



remains to examine more definitely the joint planes about 

 the Yosemite and to determine whether they have exerted a 

 controlling influence on the topographic forms. 



Vertical Joints. — The strike of the vertical joints and 

 those approximating to verticality was plotted on the geo- 

 logical map. These clearly show that the most abundant 

 vertical joints belong to a system with a northeast strike. 

 Exact measurements were not made of the finely developed 

 joints at Ribbon Falls, or those just east of the cliff at the 

 lower falls of Cascade Creek. In a general way it may be 

 said that vertical joints are more abundant at the Yosemite 

 than at any other deep canyon valley in the Sierra Nevada, 

 so far as my own observation goes. The walls on the 

 south side in particular show vertical jointing splendidly, as 

 may be seen at Sentinel Rock and Cathedral Spires. It is 

 even better shown on the south side of the spur connecting 

 Sentinel Rock with the plateau to the south. The sheeted 

 or joint structure may be seen also at other points on the 

 south wall, between Sentinel Rock spur and Cathedral 

 Rocks. At one point it is brought strikingly to the notice 

 of the observer, since along some of the more thinly sheeted 

 zones the granite has weathered out, leaving narrow, deep, 

 open spaces which are locally known as " the Fissures." 



Plate XXXII, representing the spur east of Bridal Veil 

 Creek, shows how erosion along vertical partings has pro- 

 duced the Cathedral Spires and the Cathedral Rocks. Still 

 farther west is the spur which has been referred to before 

 as forming the west wall of the valley, to the northwest of 

 Fort Munroe. Vertical joints penetrate through this spur, 

 and are clearly the cause of the vertical cliffs on the south 

 side of the Merced Canj^on west of the valley. On the 

 north side of the valley the vertical jointing is not every- 

 where evident. It is finely exhibited, however, in the ell 

 just northwest of El Capitan, where the vertical slabs of 

 granite resemble a pile of boards set on edge. Vertical 

 sheeting is also shown in the bluffs west of the top of 

 Yosemite Falls. A highly inclined set of joints may be 

 seen forming the escarpment just east of Yosemite Falls, 



