222 QUICKSILVEIJ DEPOSITS OF THE PACIFIC SLOPE. 



tlie material of Mt. Shasta and the countiy suiTOunding it. Between tliis 

 region and Clear Lake the country is practically nnknown. At Clear Lake 

 asperites form the bulk of the andesitic eruptions. Andesitic areas also 

 extend almost uninterruptedl}' in a southwesterly direction along the Ma- 

 yacmas Range, including Jit. St. Helena, to the neighborhood of Vallejo, on 

 San Pablo Bay, which is practically the northern end of the Bay of San 

 Francisco. Most of this andesite belongs in the asperite group. Andesites 

 reappear at Mt. Diablo and to the eastward of Tres Pinos. Comparatively 

 small amounts of older dense andesites occur at Clear Lake and in the Ma- 

 yacmas Range. Rhyolite in the areas under discussion has been found only 

 at New Almaden, but basalt is widely distributed. It occurs at Steamboat 

 Springs and at Washoe and is abundant near Mt. Shasta and at Clear Lake 

 In the I'anges to the southward of Clear Lake basalt appears to be more 

 widely distributed than andesite, occurring at Knoxville, in Sonoma County 

 at tlie Mt. Pisgah quarr}-, at Mt. Diablo, and to the south of the Bay of San 

 Francisco as far at least as the Panoche Valley. The volume of the basaltic 

 eruptions is much inferior to that of the andesites. 



No eruptive rocks of the Pre-Tertiary age are known to be intercalated 

 in the Knoxville or Chico-Tijon series or to have broken through them. 

 The only earlier eruptions encountered are represented b}- pebbles in the 

 Knoxville and Chico conglomerates, and these are believed to have cut 

 the granite before the deposition of the Knoxville beds. Excepting these 

 pebbles, the earliest eruption known is pyroxene-andesite, which preceded 

 the Cache Lake period. Had this eruption antedated the Miocene, pebbles 

 of the lava would almost certainly have been found in the Chico-Tejon 

 series of Lower Lake. It may have accompanied the Post-Miocene up- 

 heaval or it may have followed this uplift after an interval. I think it 

 probable that the eruption took place at the time of the orographical change 

 which dammed back the waters of Cache Lake, probably early in the Plio- 

 cene or jiist before it. Another outbreak took place at the close of the 

 Cache Lake period after an interval long enough to permit of the deposition 

 of at least a thousand feet of fresh-water strata. This eruption, represented 

 by the asperites of Mt. Konocti, accompanied an orographical cliange which 

 shifted the waters of Cache Lake to the present Clear Lake, and the lavu 



