Geological Wot.es on the Sierra We-tfoda, — Turner. 305 
The graaodiorite occurs in very large ureas in the range and is the 
rock of the Yosemite valley. According to Mr. W. Lindgren the gran- 
odiorife at Folsom on the American river has metamorphosed the Mari- 
posa slates into mica-schists; and Mr. II. W. Fairbanks* states thai the 
slates and schists near Bridgeport in Mariposa enmity (this includes the 
Mariposa slates) are cut off and metamorphosed by the granite, an ob- 
servation which the writer later verified. It therefore appears thai much 
of the granodiorite is later in age than the Mariposa slates. 
GrEANITE-PoRHPYRT. 
To the east of the granodiorite area of the Yosemite valley 
is a mass of granitoid rock, in which are developed large pink 
orthoclase feldspars. About nine miles nearly north of Mount 
Hoffman, to the north of the Tuolumne river, is a little lake 
which was visited a few years since by Mr. G. F. Becker and 
myself, and to it the name of Granite lake was given, partly 
for the sake of a name to use in notes and partly on account 
of the magnificent granite exposures about the lake. In fact 
there is nothing but granite in the vicinity, and as the loose 
material has been swept away by the glacier that formerly 
covered the spot, the rocks are beautifully cleaned off. The 
granite-porphyry occurs about the head of the lake, and the 
contact there and at other points in the neighborhood with 
the biotite-hornblende granite is everywhere sharp. Dike- 
like protrusions of the granite-porphyry were seen extending 
into the biotite-hornblende-granite, indicating the former to 
be the younger rock. 
The granite-porphyry ma\ be described as a holocrystalline rock of 
hypidiomorphic structure with a coarse ground mass of plagioclase, 
orthoclase, and sometimes microcline, with plenty of brown mica, a lit- 
tle primary hornblende, and titanite and iron oxide. In this ground 
mass are developed large orthoclase phenocrysts, which usually contain 
ajnindant inclusions of the minerals of the ground mass, both in the in- 
terior and along the edge of the phenocrysts. Plagioclase, biotite, quartz, 
titanite, and iron oxide were noted in the porphyritic orthoclases, which 
are often more than two inches in length. It is evident that these phen- 
DCrj sts were formed a ft el' some of I lie ol her const it llell Is of I lie rock, and 
.in this particular the granite-porphyrj appears to stronglj resemble the 
porphyrite with large orthoclase phenocrysts described by Mr. Cross 
(not yet published) as forming a part of the laccolites of the Kl |<" 
mountains, Colorado. Prof. A. < '. Lawson^j has lately described a pran- 
*Amkkk an Geologist, 1891, p. 211. 
f "The Geology of Carmelo Bay." Bull. Dept. Geology "i the Univer- 
sity of ( 'ali fornia, vol. i. pp. 1-59. 
