Geol.— Vol. I.] TURNER— ORIGIN OF YOSEMITE VALLEY. 309 



of a dark dioritic rock which confuse the relations at some 

 points. It is designated Bridal Veil granite. Forming the 

 east wall of the valley is a coarse granolite containing scat- 

 tered prisms of amphibole. As it is found in sharp contact 

 with the basic granodiorite of Glacier Point, and as it is 

 everywhere very uniform in grain and texture, showing little 

 evidence of crushing, it is presumed to be later than most of 

 the other granites. It is genetically related to granodiorite, 

 forming the alkali rich extreme of that rock, but in this 

 paper will be called quartz-monzonite.^ 



The evenly granular texture of the quartz-monzonite 

 lends itself to the process of weathering which has been 

 called exfoliation, and it may be noted that all the domes 

 about the valley, except Sentinel Dome and Boundary Hill, 

 are composed of this rock; that is to say, North Dome, 

 Basket Dome, Half Dome, Cloud's Rest, and Starr King. 

 In addition to these types of granite, each of which forms a 

 considerable mass, there are the usual pegmatite and aplite 

 dikes cutting all of the other granolites and therefore clear ly 

 later in age. 



The Pleistocene deposits are of three kinds — the talus, 

 the ordinary alluvial deposits formed by stream action, and 

 the moraines. Of these the oldest are the moraines and the 

 youngest the talus. The alluvial deposits may be quite 

 deep, but as there is rock in place at the west edge of the 

 valley by the outlet, this would imply that the valley is a 

 rock-basin filled with detritus. There are no deep wells or 

 borings in the valley, so that any estimate of the depth of 

 the alluvium must rest, for the present, on conjecture. 



77. The yoint Systems of the Valley. 



Enough has been said in the preceding pages concerning 

 the joints that intersect the granitic rocks to make plain to 

 the reader their general nature and occurrence. It now 



1 Monzonites are granolites containing both orthoclase and plagioclase in abundance. 

 Strictly speaking the iCl Capitan granite above described is also quartz-monzonite. 



(4) November 10, 1890. 



