392 QUICKSILVER DEPOSITS OF THE PACIFIC SLOPE. 



sandstones not considerably altered contain cinnabar at New Idria. In 

 San Lnis Obispo Connty the ore is found in unaltered sandstones believed 

 to be Miocene; at Sulphur Bank rich ore was found in modern lake beds; 

 the andesite of Clear Lake contained deposits; so, also, does the andesite 

 near Calistoga ; and the same association occurs at Mt. Diablo; traces of 

 cinnabar occur in the basalt of Steamboat, and a large part of the best ore 

 of Sulphur Bank was found in this recent lava. On the ^Etna property 

 ore is also found in basalt. 



Alteration of the rocks. — Thc Tocks adjoluiug Ore deposits have in many cases 

 been greatly moditied. Metamorphic rocks often appear to have been con- 

 verted into or replaced by more or less dolomitic carbonates by the action 

 of solutions. This is inferable both from field and laboratory observations; 

 for, while limestone is extremely rare in the Coast Ranges as a whole, large 

 masses of more or less impure carbonates appear in the mines. This is 

 especially noticeable at New Almaden, where also the concretionary struct- 

 ure of a part of the material shows the foreign origin of the mineral. The 

 metamorphic rocks converted into carbonates are usually stained brown by 

 ferric oxide and ferrous carbonate and retain the general habitus of the 

 associated metamorphic rocks which have not undergone this change; but 

 it is often difficult to determine the exact character of the original material. 

 The metamorphic rocks where they are unaffected by carbonate solutions 

 vary so capriciously that the innnediate association of pseudodiabase with 

 carbonated rock does not prove that the latter is an altered form of the 

 former. Under the microscope it is found, too, that the alteration by car- 

 bonate solutions so quickly obliterates the character of the rock modified 

 that satisfactory transitions are comparatively rare. Both serpentine and 

 the granular metamorphic rocks seem to be subject to this conversion. 



siiicification. — In tlic tlurd chapter I have referred to a widespread partial 

 silicification of the Coast Ranges, which seems to have formed the latest 

 phase of the Post-Neocomian metamorphism. In connection with the ore 

 deposition there has also been localized silicification, sometimes on a large 

 scale. The products of the two periods of silicification are ordinarily very 

 distinct, though doubtless there are cases in the neighborhood of mines in 

 which it might be impossible to refer the silica with certainty to one or the 



