464 QUICKSILVER DEPOSITS OF THE PACIFIC SLOPE. 



Very large quantities of quicksilver have been taken from this prop- 

 erty, but it has not been worked witli system and has been insufficiently 

 prospected below the basalt. There is no reason to suppose, however, that 

 it is nearly exhausted. 



Close to Borax Lake lies a very interesting area of glassy basalts, 

 ranging all the way from a nearly normal olivinitic rock to a pure glass. 

 As is the case with the andesites across the lake, the glass is very acid and 

 contains little lime, but much alkali. 



Borax Lake is a shallow pond, without an outlet, into which springs 

 similar to those now flowing from Sulphur Bank once drained. These 

 springs came from the obsidian area, and to them the borax contents of the 

 lake is due. They issued at the point called Little Sulphur Bank, which 

 is still hot and moist and shows native sulphur. It is stated on excellent 

 authority that cinnabar in small quantities was found here. Maggots of 

 Epliifdra califoniica and of a species of Stratiomys live in the lake. 



The Knoxviue district. — The rcglou about Knoxville consists of metamor- 

 phosed and unaltered rocks of Neocomian age, through which a small basalt 

 eruption has broken, and contains a number of quicksilver mines and pros- 

 pects. I know of no other district so favorable as this for the determina- 

 tion of the age of the metamorphic rocks ^md for a study of their cliaracter, 

 excepting Mt. Diablo. Rocks occur in all stages of metamorphism, and 

 the transitions, together with the structural relations, show that even the 

 serpentine is not of eruptive origin. The metamorphosed and unaltered 

 rocks are also so related as to preclude the sup[)Osition that the former are 

 crystalline sediments. One side of an eroded anticlinal is metamorphosed, 

 while the other is unchanged and fossiliferous. Fossiliferous strata in a 

 nearlv vertical position pass over into metamorphic rocks in the direction 

 of their strike, and patches of unchanged rocks remain in metamorphosed 

 masses. The fossils of the unaltered strata are of Neocomian age and the 

 principal species are Amelia concenlrica and A. mosqneiisis. The series carry- 

 ing these shells are called the Knoxville group from the name of this locality. 



Excelleirt opportunities are here afforded for studying the passage of 

 sandstone into pseudodiabase and pseudodiorite and the alteration of tliese 

 rocks to serpentine. The direct change of slightl}- altered sandstones to 



