Vol. 6] Beid: The Oeomorphogeuy of the Sierra Nevada. 101 



nately it is nowhere in contact with the rhyolite. In the north 

 part of Little Valley, near the head of the canon of Franktown 

 Creek, is a small area of the typical hornblende andesite, with 

 which is associated some breccia similar to that of the small 

 dike to the south. A correlation is made for this reason. A few 

 other dikes associated with cones exist. The andesitic cones are 

 interesting features of the geology. There are two well-formed 

 cones in the area, the larger directly north of Carson, and the 

 smaller a mile west of the southwest corner of Washoe Valley. 

 The andesite in both these cones is more massive than the rock 

 in the dikes, and frequently show well-developed joints. The 

 petrography of the andesites yet needs investigation. The cone 

 north of Carson (pi. 20a) is held to be such because (1) of its 

 perfect conical form and (2) the contact between the volcanic 

 and surrounding schist is marked by much breaking and distor- 

 tion of the latter rock, its intense metamorphism and vertical 

 dip facing the cone. Near the summit of the cone joints are 

 well shown and result in the formation of rough hexagonal 

 columns lying with their axis nearly horizontal about N55°W. 

 The cone west of Washoe Valley (pi. 20b) is held to be such on 

 account of its form and the existence of a few radial dikes in 

 the southwestern flank. Some of these dikes are peculiar in 

 their being composed of andesite full of angular fragments of 

 granodiorite. One dike nearest the cone has a well-developed 

 jointage (pi. 21a). Of the residuals of the andesitic flows little 

 further need be stated. They all appear to be connected either 

 with dikes or the cones. 



A second well-marked variety of andesite occurs in the area 

 just east of Glenbrook. A large part of this appears in the field 

 to be quite similar to the andesite already described, but part is 

 very different. This latter rock is microscopically identical with 

 Becker's "later hornblende andesite" at Virginia City. It is a 

 coarse-grained rock with phenocrysts of mica, hornblende and 

 feldspar in a purplish gray ground-mass. On the road east of 

 Glenbrook it has the appearance and character of dikes intruded 

 into the other andesite. Not sufficient field work was done to 

 settle this point definitely. Microscopically the two andesites 

 are quite similar to Becker's earlier and later hornblende ande- 

 sites at Virginia City. 



