156 QUICKSILVER DEPOSITS OF THE PACIFIC SLOPE. 



Pinos and New Idria, many of which are entirely similar to the northern 

 andesites. Tliat these rjcks must be in place somewhere In tlie hills is of 

 course certain, but they were not encountered. 



Rhyoiite. — Onlv one occurrence of rhyolite is known in the whole area 

 dealt with in this memoir. This is a dike at New Almaden, far from any 

 other known outcrop of eruptive rock. Its occurrence here is specially 

 significant when considered in connection with the ore deposits. The crop- 

 pings are niostlv of a tufo-like consistency, and in consequence have de- 

 composed to a considerable extent. They are light colored, and without 

 careful scruthiy might be mistaken for clastic material. Under the micro- 

 scope they are found to be composed of porphyritic grains of quartz and 

 feldspars, both striated and unstriated, with a little lirown mica in a ground- 

 mass, which is sometimes holocrystalliue (possibly as a result of devitrifi- 

 cation) and sometimes shows the pseudospherolitic structure so common in 

 rhyolites. In the latter case some glass remains in the groundmass, and 

 inclusions of glass are common in the quartzes of both varieties. To de- 

 termine the character of the predominant feldspars a portion of specimen 

 No. 47 was reduced to coarse powder as usual and separated in a Thoulet 

 solution of a specific gravity of 2.59. Eighty-one per cent, floated. This 

 consisted of feldspar with groundmass and a little adhering quartz It 

 gave a strong potash reaction. This i-hyolite is later than the Post-Miocene 

 npheaval. 



Basalt. — Basalt is very widely distributed on tlie Pacific slope Being 

 the last eruption, it is seldom covered over by material of anv kind. The 

 thin flows of this rock have also spread over a greater area than the com- 

 parati\ely viscous andesites of equal volume could ha\e done. Both 

 macroscopically and microscopically the rock is Aery monotonous in its 

 character and presents comparatively few points of interest. 



At Steamboat Springs the thin flows of olivinitic basalt are found 

 under the microscope to be entirely normal and precisely similar to the 

 basalt of the Washoe district. Similar rocks are found in the Panoche Val- 

 ley, in the San Benito Valley, near Mt. Diablo, and at many points between 

 the Bay of San Francisco and Clear Lake. Near Clear Lake they are 

 somewhat more notable, since much of the rock at this point is scoriaceons 



