THE GRANITIC ROCKS OF THE SIERRA NEVADA 155 



some of the other granites. No complete analysis has been 

 made of this granite, but there are given above two partial 

 analyses which indicate some variation in chemical composition 

 if they are in reality both from the same magma. 



No. 2558 is from north Cathedral Rock in Yosemite Valley, 

 and No. 2051 is from a dike in the bed of the Middle Tuolumne 

 River, about 4.5 kilometers northeast of Bald Mountain. Both 

 analyses were made by Dr. H.N. Stokes. 



SODA-GRANITE AND APLITE 



Granitic rocks rich in soda are not abundant in the Sierra 

 Nevada. The largest mass known lies east of Cathay Valley in 

 Mariposa county. This area is clearly later than the diabase that 

 is found to the west. Along the contact in and southeast of 

 Cathay Valley a contact-breccia has been formed which is a mile 

 or more in width. This is composed of fragments of the dia- 

 base cemented by the soda-granite. On the northeast the aplite 

 area is in contact with the slates of the Mariposa formation. 

 These slates are flinty near the contact on Agua Fria Creek, 

 where also they are of a peculiar light gray color. Microscopic 

 examination does not show any very marked metamorphism. 

 Near the contact, however, the granitic rock is richer in lime 

 (analysis 399, S. N.), which it may have absorbed (?) from 

 the slates. The rock may also be regarded as a basic contact 

 facies due to differentiation. In either case the above facts susf- 

 gest that the granite is later in age than the Jurassic Mariposa 

 slates. No. 399 was collected on Agua Fria Creek near the 

 Mariposa slates, 5.2 km southwest of Mariposa. It is composed 

 of micropegmatite, quartz, oligoclase, biotite, ilmenite, and epi- 

 dote. Orthoclase is probably present although not determined in 

 the thin section. Some of the epidote is wedged in between the 

 other constituents all of which are fresh. This epidote is prob- 

 ably primary. 



No. 413 is from the interior of the area above described, 6.5 

 km west of Mariposa. This was estimated to be a fair average 

 sample of the rock. It is composed largely of albite and micro- 



