3o8 CALIFORNIA ACADEMY OF SCIENCES [Proc. 3D Ser. 



10. Geological Formations about the Valley. 



Except the Pleistocene deposits, the rocks immediately 

 surrounding Yosemite Valley are entirely granolite.^ These 

 mostly contain quartz and are ordinarily called granite. 

 The dark cliffs by the Big Oak Flat road are composed of 

 amphibole-gabbro, and the large talus slope, over which the 

 road passes, is formed chiefly of this rock. This gabbro 

 mass is intersected by very numerous, nearly vertical and 

 horizontal joints, and the great size of the talus slope is 

 largely due to the ease with which frost, heat, and water, 

 acting along these joints, have dislodged the fragments. 

 The gabbro is intruded by a number of granitic dikes 

 which, by reason of their light color, are quite notice- 

 able on the cliffs in contrast with the somber tones of the 

 gabbro. Most of these dikes have followed a horizontal 

 joint system. 



The quartz-granolites or granitic rocks are of several 

 types which, for the purposes of this paper, may be desig- 

 nated as follows: El Capitan granite, granodiorite, Bridal 

 Veil granite, and quartz-monzonite. The El Capitan gran- 

 ite contains dark mica in abundance to the exclusion of 

 other ferro-magnesian minerals. El Capitan and the cliffs 

 enclosing the west end of the valley are largely made up of 

 this rock, which is probably the oldest quartz-granolite of this 

 part of the Sierra Nevada. Forming a considerable mass 

 in the drainage area of the Yosemite Creek and at Glacier 

 Point is a dioritic rock containing quartz and amphibole 

 which, in the Gold Belt folios, has been called granodio- 

 rite. This rock is found in sharp contact with the El Capi- 

 tan granite and as it contains fragments of that rock it must 

 be of later age. In the area drained by Bridal Veil Creek 

 is a fine-grained white granitic rock which occasionally shows 

 an orbicular structure. This rock forms part of the north 

 face of the north Cathedral Rock, as well as most of the cliffs 

 at the Bridal Veil Falls, but there are mixed with it patches 



1 Granolite is a general term for all granular igneous rocks, as for example, granite, 

 syenite, and gabbro. 



