4Q2 QUICKSILVER DEPOSITS OF THE PACIFIC SLOPE. 



mineral springs, and deposits of sulphui- and cinnabar form its most note- 

 worthy features. 



Metamorphic rocks of the Neocomian series nnderlie the whole country 

 so far as known, though the existence of granite pebbles in (he stream 

 which drains the lake suggests that this rock is exposed at no great distance. 

 Upon a part of the metamorphic area about Lower Lake the Chico-Tc^jon 

 occurs. The latter series is comparatively little disturbed and not meta- 

 morphosed. 



The earliest eruptions in the district seem to have been that of Chalk 

 Mountain, on the north fork of Cache Creek, and some of the rock near 

 Thurston Lake. This lava was a dense pyroxene-andesite and the eruption 

 seems to have occurred about the beginning of the Pliocene. Soon, and 

 pei'haps immediately afterward, a large body of fresh water formed, whicli I 

 have called Cache Lake. It lay mostly to the east of Clear Lake and con- 

 tinued in existence up to the end of the Pliocene. At tliis period fresh erup- 

 tions of andesites took place. They are the asperites of j\It. Konocti (or 

 Uncle Sam) and the neighborhood. A part of the lava flowed over a portion 

 of the bed of Cache Lake, and the orography was so modified as to shift the 

 position of the water to the new Clear Lake, which overlaps part of the 

 more ancient bed. The cliange must have been somewhat gradual, for the 

 same mollusks wliich lived in the earlier body of water also flourished in 

 the new one and tlie forms are lacustrine. 



Tlie asperltic andesites of Mt. Konocti are interesting because they 

 contain pyroxene and mica, but no hornblende, wliich is unusual, and be-- 

 cause they pass over into acid glasses. The asperite is often almost wholly 

 crystalline, though it has been subjected to substantially the same physical 

 conditions as the glass, and the latter has remained vitreous because of its 

 divergent chemical composition. The mountain nearly coincides in form 

 with the tlieoretical shape of a volcanic cone and its highest point is 2,936 

 feet above the lake at high water. The lake is 1,310 feet above sea-level. 



Much later than the andesite came basalt eruptions, which extended to 

 modern times. A part of this rock also is glassy. All the springs which 

 now issue at a high temperature are probably due to the basalt eruptions, 

 and the borax, sulphur, and cinnabar are referable to the same source. 



