Vol. 6] Reid: The Geomorphogeny of the Sierra Nevada. 93 



relatively much less amount. In the Carson area, just west of 

 Lakeview, there are many small isolated spots, the smallest hav- 

 ing a diameter of only ten yards. The largest areas are little 

 more than a veneer over the granite, so that it is evident that the 

 work of removing the roof of the granite is nearly completed. 

 The smaller areas owe their preservation in large part to the fact 

 that they are slightly sunken in the crystalline rock. 



Granodiorite. — The petrography of the intrusive plutonic is 

 in itself a complex chapter in the geology of the region. There is 

 much variation in composition, both chemical and physical. 

 Wherever a contact is exposed, the crystalline rock is intrusive 

 into the older schists and the several facies are therefore regarded 

 as merely component parts of the one great batholith, even 

 though physical connection cannot be found. Chemically, there 

 is a range from the granite of Steamboat Springs, described by 

 Becker, 3 to a rather basic granodiorite or diorite, developed in 

 certain localities along the east flank of the Sierra Nevada. In 

 general the plutonic is probably best classed as a granodiorite, 

 although some areas might well be placed with the quartz 111011- 

 zonites. Lindgren's description of the rock on the Truckee area 

 covers the main features. Hornblende in well-formed prisms, 

 often 2 cm. long, is quite common in the Carson area, particu- 

 larly near the irruptive contacts with the overlying schists. 

 Biotite, in grains up to 5 mm. in diameter, is usually in much 

 less amount than the other dark constituent. The orthoclase is 

 always present in larger grains than the plagioclase, and not 

 very much less in amount. The grain normally is coarse. The 

 quartz occurs from a small percentage up to 30%. There are 

 two spots where local variation is extreme, one a few miles north 

 of Franktown, and the other north of Carson. Here the grain 

 changes in the distance of a few inches from fine to coarse ; 

 chemically within the same distance an acid granite exists next 

 a basic diorite. In some small spots the rock is a coarse-grained 

 biotite granite; in others, nearly no quartz nor biotite are to be 

 found. Frequently the normal rock exhibits dike-like areas of 

 fine grained acid diorite or quartz diorite, with possible flow de- 



3 G. r. Becker, "The Geology of the Quicksilver Deposits of the Pacific 

 Slope," United States Geological Survey, Mon. xiii, 1888, pp. 141-3. 



