144 QUIUKSlLVEIt DEPOSITS OF THE PACIFIC SLOPE. 



Granite from the Coast Ranges. — Tile speciuieiis tVoiii the Gavilaii Range, from 

 the Farallones, from Marin County, and from Horsetown, Shasta County, 

 do not differ notably from those of Steamboat Springs. A portion of the 

 Montei'cy granite is extremely coarse. Microcline was observed in slides 

 from two of the localities mentioned; muscovite occurs, but is not common ; 

 and hornblende is present in some of them, but not in large quantities. The 

 crystals of primary and secondary consolidation are less marked in tlie 

 Coast Ranges than in the Nevada granite, and in a granite from Olema, 

 Marin County, there is some granophyric structure. These differences, 

 slight as they are, are probably due rather to the accidents of collection 

 than to any persistent difference in type. 



Older porphyries of the Coast Ranges. No erUptive rOCks antedating tlie PoSt- 



Miocene upheaval have been detected in place during the present investi- 

 gation ; but in the conglomerates of the Knoxville series, at Knoxville, and 

 in a conglomerate stratum lying just above the base of the Chico, at New 

 Idria, pebbles of porphyry have been found. In appearance these sets of 

 pebbles strongly i-esemble each other, and, although there is considerable 

 variation in their mineralogical composition, they seem at present fairly ref- 

 erable to a single group. They are all quartzose and the quartzes contain 

 fluid inclusions, often in abundance. There are also patches in the quartzes 

 which resemble devitrified glass. There are in some cases remains of por- 

 phyritic hornblende-cr3-stals, naostly decomposed. A large portion of the 

 porphyritic feldspars are plagioclastic and appear to be refei'able to oligo- 

 clase and andesine. Many of the porphyritic feldspars are unstriated, but 

 none was detected showing good cleavages and certainly referable to ortlio- 

 clase. The feldspar of the groundmass is microcrystalline and cannot 

 be satisfactorily determined under the microscope. The quantity of the 

 groundmass is usually so large that the feldspar which it contains must 

 determine the classification of the rock. A j)artial analysis was made of 

 one of tlie Knoxville specimens (No. 76) with the following result: 



Per cent. 



Silica, SiO- 68.800 



Soila, Na-0 10.021 



Potnssa, K=0 0.912 



This is evidently a plagioclase rock and must be considered u por- 

 phyritic diorite. 



