158 QUICKSILVER DEPOSITS OF THE PACIFIC SLOPE. 



At Sulphur Bank the Ijasalt bears peculiar and very interesting rela- 

 tions to volcanic glasses. Field examinations showed that an area of 

 obsidian south of Borax Lake passed over into a rock which bore every 

 appearance of being an ordinary basalt. This glass is a dark-gray ma- 

 terial, transparent in masses a quarter of an inch or more in thickness and 

 without any peculiarly high luster. It occurs some miles from any ande- 

 site area and presents a very different appearance from the andesitic glass 

 described above. On analysis, however, it proved to contain above 75 per 

 cent, of silica. This seemed at first to preclude its reference to a basaltic 

 eruption. It was afterwards found that Xo. 48i, a fine-grained basalt from 

 one of the craters at Sulphur Bank, normal in structure, but without oli- 

 vine, of a specific gravity of 2.82, contained 57.03 per cent of silica, a very 

 high content for a basalt; while No. 202, occurring at the outskirts of the 

 obsidian area, is a somewhat glassy basalt, density 2. GO, carrying olivine, 

 augite, and hypersthene, with feldspar microlites, and showing the charac- 

 teristic structure of this rock, contains no less than G6. 87 per cent, of silica. 

 Even No. 150,/, which is macroscopically a pure glass, with a .specific grav- 

 ity of only 2. 33, and which carries 75.22 per cent, silica, proves under the 

 microscope to contain numerous small grains of pyroxene and long micro- 

 lites of plagioclase entirely similar to those in the basalts. Neither free quartz 

 nor orthoclastic feldspar has been detected in the slides of this obsidian. 

 The abundance of lath-like plagioclases in this glass distinguishes it micro- 

 scopically from the andesitic obsidian. In which the feldspars are mostly, if 

 not wholly, irregular grains or developed crvstals of primary consolidation. 



The microscope thus supports the conclusion, reached from field ob- 

 servation before either microscopical examinations or chemical determina- 

 tions had been made, that the obsidian near Borax Lake is a portion of a 

 basaltic eruption. Tlie transition has been again tested in the field- since 

 the laboratory work was completed. 



In the following table the anah^sis of the obsidian from the area imme- 

 diately soutli of Borax Lake is given under I. For comparison a second 

 basalt (No. 85, Clear Lake) was analvzed, and its composition is given 

 under II. This latter is from the basalt blutfs south of Burns Valley, at 

 no great distance from the obsidian. It is a dense, gray rock, remarkably 



