GEOLOGICAL FEATURES OF THE TRUCKEE REGION 663 



Area Described 



The area described iucludes a belt lying north and south of the Triiekee 

 river and extending from the main east front of the Sierra Nevada just west 

 of Verdi, Nevada, to the Truckee-Carson divide near Hazen. The chief topo- 

 graphic divisions, passing from west to east, are: (1) the Sierran fault front; 

 (2) the Verdi-Long Valley depression at its base; (3) the line of elevation 

 represented by the Carson range on the south and Peavine mountain and 

 others to the north; (4) the depression of Spanish Springs valley, Truckee 

 meadows, Washoe valley, etc.; (5) the Virginia range; (G) the depression of 

 Pyramid lake, the lower Truckee valley, etc.; (7) the mountains lying between 

 tlie Truckee and Humboldt regions. (For topography, see Sierraville. Keno. 

 Carson, and Wadsworth sheets of the United States Geological Survey. ) 



Bedrock Complex 



The P>edrock complex consists chiefly of granitic rocks, largely granites (in 

 part granodiorites). with residual masses of more or less metamorphosed sedi- 

 mentary and igneous rocks into which they were intruded. The chief sedi- 

 mentary types are, perhaps, quartzites and mica-schists, including muscovite- 

 schists. A petrographically interesting meta-dacite is found at intervals for 

 at least 12 or 15 miles north of Reno. Limestone and slate are occasionally 

 found, and to the south, near Mound House, gypsum. 



The Bedrock complex is found in the Sierran front, in Peavine mountain, 

 and in a number of the small ranges to the north and northeast of Reno, and 

 in the Carson range to the south. Near Steamboat springs both sediments 

 and intrusions have been mapped and described by Becker.* 



With one or two exceptions, the Bedrock complex is exposed only west of 

 the Virginia range. This range is made up of a great thickness of Tertiary 

 igneous material which has so deeply buried the older rocks that they have 

 not yet been exposed by the Truckee river, which has cut a canyon directly 

 across the range over 2,000 feet deep. The pre-Tertiaries, however, rise to the 

 surface in this range some miles south, in the vicinity of Virginia City. The 

 Marble bluff at the south end of Pyramid lake is of limestone, presumably 

 Triassic. 



Judging from a comparison between the Bedrock complex here and that in 

 the Sierran province immediately west, the Humboldt region to the east, and 

 certain areas to the south, it is probable that the schists are Mesozoic. possibly 

 in part Paleozoic, and the granitic intrusives post-Jurassic. There is abso- 

 lutely no evidence suggesting the Archean age of the rocks of the Peavine 

 Mountain area, although like several areas since i)roved Mesozoic, it was so 

 marked by the Fortieth Parallel Survey and their mapping was recently copied 

 on the map of Bulletin 208 of the United States Geological Survey. Until 

 some evidence or suggestiA'e line of reasoning indicates its presence, it is mis- 

 leading to map any areas of northwestern Nevada as Archean. f 



The chief folding of the Bedrock complex undoubtedly took place at the time 

 of the extensive post-Jurassic erogenic disturbances that affected so strongly 

 the regions just east and west of the Truckee country. The granites have 



* IT. S. Geological Survey. Monograph xlii. 



t See also Bull. Geol. Soc. Am., vol. 1.5, pp. .317, .33.5, 340. 



