112 CALIFORNIA ACADEMY OF SCIENCES. 



of quartz-porpliyry is met with in the mines, and is inter- 

 sected by the Baltimore, Caledonia and Knickerbocker 

 shafts. ^^ The bottom of the Forman shaft is also in this 

 rock. The last occurrence is referred to by Messrs. Hague 

 and Iddings as '*a small body," though of its size no one 

 can possibly know anything. They explain these occur- 

 rences on the same intrusive theory adopted to account for 

 the hornblende andesite in the Forman shaft. The same 

 objections stated above, in regard to the applicability of this 

 theory to the hornblende andesite, apply also to this case, 

 but with still greater force; for it seems certain that ande- 

 sites could not float in melted quartz porphyry. The 

 quartzose rock, it is true, must be viscous when melted and 

 might therefore carry up small fragments of andesite or even 

 heavier substances, but that it could lift and support a mass 

 of rock specifically heavier than itself and over 2000 feet in 

 thickness I believe quite impossible. 



Eock inclusions in quartz loorphyry — In the hope of obtain- 

 ing evidence as to the succession of the quartzose rock and 

 the andesites which should appear to every one unequivocal, 

 an earnest search was made last summer for included frag- 

 ments, near Basalt Hill. In the augite andesite nothing 

 could be found. This is perhaps not strange since this 

 rock, particularly in this neighborhood, was evidently of very 

 great fluidity. Lighter rocks would have floated upon it and 

 would have been the first portions of the mass to be re- 

 moved by erosion. Heavier rocks would have sunk to the 

 bottom. In the quartz porphyry, inclusions of metamorphic 

 rocks and of granite (entirely similar to that of the adjoining 

 area, to that of Steamboat Sprin'gs and of the Sierra Nevada), 

 were in some localities tolerably abundant. There was 

 nothing like andesite to be found, which seems strange, if 

 the quartz porphyry broke through the andesite carrying 

 with it fragments of the other rocks through which it burst. 

 This evidence, however, is only negative. 



Note ^^— See Atlas, sheet VI. 



