GEOLOGICAL FEATURES OF THE TRUCKEE REGION 665 



The second type gives rise to silieified zones, which weather in pi-omineut 

 reefs. The silica occurs partly as a deposit in cracks or other open spaces 

 with distinct crustiflcatiou and often drusy structures, partly as a cement 

 between the crushed fragments and particles of the rock and partly as a re- 

 placement of the rock, the original porphyritic structure often being visible in 

 the silieified product. These three modes of deposition are commonly found 

 in the same ledge. Many of these occur in low hills north of the Truckee 

 meadows. 



The Olinghouse Canyon mines are in fissured (commonly crushed and 

 sheeted) zones in audesite and carry gold without much deposited gangue ma- 

 terial. The best known deposits in these rocks are along the Comstock lode and 

 need not here be described. They belong to the third type mentioned above. 



RHYOLITES 



The rhyolites (and geologically related dacites) occur to the north and 

 northeast of Reno, and more abundantly in the Virginia range, where they 

 vai-y from vitreous, sometimes perlitic, to lithoidal. often porphyritic, and 

 occur both as lava flows and dikes. They are less abundant than the ande- 

 sites. They may also be accompanied by mineralization as in the White 

 Horse district. White Horse mountain is a stock of rhyolite, and dikes pass 

 from it out into the surrounding andesites. Good values have been found in 

 some of the dikes. Veins in rhyolite also occur to the south near Gold Hill. 



The latest of the Tertiary eruptions are basalts. They are found about 

 Steamboat springs, along the summit region of the Vii'ginia range, and abun- 

 dantly in the ranges just east of this. There is commonly evidence of some 

 deformation before the outpouring of the basalts. 



DISTRIBUTIOIV OF TYPES 



A noteworthy change takes place in the character of the predominant types 

 of Tertiary lavas as we pass from west to east across the Truclvee region. In 

 the western part the andesites occupy the field almost to the exclusion of other 

 types. In the central part, especially in the Virginia range, we get the greatest 

 complex. The andesites show the greatest mineralogical varieties, and 

 although still the dominant rock type they are accompanied by considerable 

 quantities of rhyolite and basalt. To the east of the Virginia range the ande- 

 sites practically disappear, and we have a broad area, forming the eastern edge 

 of the Truckee region, but lying chiefly in the Humboldt region, which is char- 

 acterized by a great thickness of rhyolite flows and tuffs overlain by caps of 

 basalt. 



Tertiary Sediments 



The Tertiary sediments are entirely freshwater or subaerial. The greatest 

 part has been included under the title Truckee beds. The t.vpe locality is the 

 Kawsoh mountains and along the east edge of the Virginia range, at the east- 

 ern edge of the Truckee region. Other areas are found in the summit region 

 of the Virginia mountains south of the Truckee river ; in the Truckee meadows, 

 especially flanking the Carson block and extending up the river to the Sierra 

 fault front west of Verdi ; along the west side of Spanish Springs valley ; in 



