96 University of California Publications in Geological Sciences 





diabase, respectively ; both formations are of presumed Lower Jurassic age. Simi- 

 lar rocks have been reported in the Virginia City area to the southeast (Thompson, 

 1956), and considered Triassic in age. 



Light-gray, hornblende-biotite granodiorite representing part of the Sierran 

 batholith of Late Jurassic age is exposed in three areas. On the hillside east of 

 Fleish the sinuous contact between granodiorite and metavolcanics appears to be 

 intrusive. The nearest metasediments, about 1,500 feet west of the contact, con- 

 tain andalusite which is suggestive of contact metamorphism and hence tends to 

 support the intrusive relation. In addition, the metasediments are intruded by 

 granodiorite veins. The outcrops of granodiorite north of the river are similar to 

 the rocks at Fleish, and presumably represent a part of the same pluton. There, 

 both the Alta andesite and Coal Valley formation rest on it unconformably. 



ALTA ANDESITE 



At its type area in the Virginia City region 15 miles southwest (Gianella, 1936; 

 Thompson, 1956), Alta andesite comprises a succession of pyroxene and horn- 

 blende andesite flows, tuff-breccias, possibly some contemporaneous intrusions, and 

 a thin but prominent tuffaceous sand-shale-eonglomerate unit, the Sutro member. 

 The Alta section attains a maximum thickness of approximately 2,700 feet. Much 

 of it has been altered hydrothermally, resulting in wide areas of bleached rocks. 

 The Middle Oligocene age of the section is indicated by a rich flora that has been 

 recovered from the Sutro member (Axelrod, 1949a). 



Andesites similar in all respects to the Alta are exposed on the lower flanks of 

 Peavine Mountain north of the Truckee River, where they rest on granodiorite. 

 The section appears to be chiefly one of hornblende andesite and pyroxene andesite 

 flows, but detailed mapping may demonstrate that sills and domes are represented. 

 A thin, lenticular, andesite tuff -breccia is also present. The lavas range from dark 

 gray-black to greenish-gray, gray -brown, and light gray in color. Plagioclase and 

 hornblende phenocrysts are readily visible in most of the samples examined, though 

 they are not clearly evident in some hand specimens, and locally some of the rocks 

 contain hornblendes exceeding a centimeter in length. Parts of the section have 

 been altered hydrothermally, especially near the larger faults where the rocks are 

 commonly white to yellow, reddish and rust brown, with gossan often present on 

 the contact. Not all of the alteration seems related to faulting, however ; some of it 

 away from the large faults may have been controlled by the complex joint system 

 in the section. 



The thickness and structure of the exposed section is not readily determinable 

 because reliable attitudes are difficult to obtain in these highly jointed rocks. That 

 important structures are present is clear from the relations in the upper part of 

 Bull Ranch Creek, where a syncline is well exposed. Although no definite statement 

 can be made as to the maximum thickness of the section, it almost certainly is in 

 excess of 2,000 feet. 



A much thinner section of the Alta andesite crops out on the north front of the 

 Carson Range about 2 miles south of the area mapped, where it rests on metamor- 

 phie and granitic rocks. In general the Alta has a much more patchy distribution 

 than the Kate Peak, which unconformably overlies it. 



