CHAPTER VII. 



DESCRIPTIVE GEOLOGY OP SULPHUR BANK. 



[Atlas Sheet IV.] 



sedimsntary rocks of th= district. — TliG results of a geiioral stucly of tlie region 

 of Clear Lake, undertaken for the purpose of throwing hgiit upon the his- 

 tory and geological relations of Sulphur Bank, have been i)resented in the 

 preceding chapter. The area delineated in the detailed map of Sulphur 

 Bank includes few formations. The underlying rock everywhere belongs 

 to the Knoxville series (Neocomian), representing the opening of the Cre- 

 taceous period. This rock was intensel}' crushed and irregularly metamor- 

 phosed not long after its depositiort, but neither the quicksilver deposits of 

 this locality nor those of any other included in this menioii' were formed 

 at the epoch of metamorphism. All the varieties of metamorphic rocks 

 described in Chapter III occur in this small area, from almost unaltered 

 sandstones up to material so highly recrystallized as closely to resemble 

 an eruptive porphyry. Serpentine is also found in very small quantities on 

 the ridge north of Borax Lake. It appears again near the end of Sunken 

 Point, shown on Atlas Sheet III. The little spot of serpentine near Borax 

 Lake might be shown on the detailed map by a separate color, but the met- 

 amorphism is so irregular in intensity that it would be quite impossible to 

 delineate areas of pseudodiabase, pseudodiorite, and glaucophane schist. 



No en;ptive rocks are interbedded in the metamorphic series at Sulphur 

 Bank. This was established by careful observation in the field prior to the 

 microscopical examinations. The latter showed that some specimens, so far 

 as their microscopic character is concerned, might possibly be either ex- 



