at Sulphur BanJc, California. 25 



The time of the volcanic activity is uncertain but cannot be 

 earlier than the beginning of the Pliocene, for this is the date 

 of the formation of the Coast chain. It is probable that it com- 

 menced with the formation of these mountains at the I 

 of the Pliocene and may have continued to a much later time. 

 In that case it would be contemporaneous with the much 

 greater volcanic activity of the Sierra and Cascade ranges, 

 which seems to have occupied the whole of the Pliocene and 

 perhaps a portion of the Quaternary.* The eruptive rocks in 

 all this region are andesites and trachytes, somewhat strongly 

 contrasted, but no attempt has been made to determine the 

 order, if any, in which the two kinds have been erupted. 



The immediate vicinity of Sulphur Bank— One of the most 

 active centers of vulcanism during Pliocene times and of solfa- 

 taric action now, is about Sulphur Bank. This place is situa- 

 ted at the extremity of an eastward-extending bay of Clear 

 Lake, and in its immediate neighborhood are distinctly visible 

 four or five low volcanic cones with almost perfect craters, the 

 nearest two being less than a mile distant. The Bank itself is a 

 low, rounded hill, rising from the lake margin and apparently 

 the lake ward extremity of a lava-stream from one of the near- 

 est volcanoes to the east, toward and almost to which it may 

 be easily traced as a low ridge of lava-blocks. The rock 

 which forms this lava-stream, as well as that of the crater from 

 which it apparently flowed, according to Mr. W. Jackson, 

 the instructor in mineralogy and lithology, to whom we re- 

 ferred it, is an augite-andesite.f During all our earlier visits, 

 the mines were simple excavations in this hill, open to the sky, 

 and none more than 50 to 60 feet deep ; but when last visited, 

 in 1881, reg ommenced by sinking 



a shaft 260 feet deep, and running drifts at various levels. 

 The stratified rock in the vicinity, when not covered with lava 

 and concealed from view, consists of sandstones and shales in- 

 clined at high angles. 



