PALAChE.] 



Soda-Rhyolite North of Berkeley. 



69 



But while the three facies of the rock exhibit a pronounced dif- 

 ferentiation as regards their acidity, they have some strongly marked 

 features in common. In all these types iron is present only to the 

 extent of a fraction of one per cent., and is probably represented 

 mineralogically entirely by the minute granules of magnetite seen in 

 the slides. 



Magnesia is found only in traces, as was to be inferred from the 

 absence of ferro-magnesian silicates. The important chemical 

 feature, however, which the three facies have in common, is the 

 ratio of the alkalies. In each facies the soda greatly preponderates 

 over the potash. 



NOMENCLATURE. 



The latter fact, taken together with the known presence of acid 

 plagioclase in addition to the orthoclase, supplies us with all the 

 information necessary for locating the rock definitely in the classi- 

 ficatory scheme. 



But here we are met by the natural difficulty that a name 

 applied to any one of the three facies is not strictly applicable to the 

 other two. Thus the porphyritic facies approaches in its char- 

 acters a soda-rich quartz-porphyry, or q,uartz-keratophyre. The 

 spherulitic facies, considered by itself, is a soda-rhyolite, and the 

 glassy facies is often an obsidian. Yet the three facies are clearly 

 a geological unit, and in general discussions must be referred to by 

 a single name. The intermediate facies, as the most abundant form, 

 is therefore taken as representative of the essential features of the 

 formation, and the rock as a whole is designated a soda-rhyolite. 



A comparison of the analyses of the Berkeley rocks with those 

 of soda-rhyolite and quartz-keratophyre given in the table, shows 

 that the nomenclature adopted is justified by the composition of the 

 various facies. 



OTHER OCCURRENCES. 



Volcanic rocks of the acid rhyolitic type have been hitherto 

 reported from but one locality in the Coast Ranges, a dyke of 

 rhyolite near the New Almaden mine, Santa Clara County, described 

 by Dr. G. F. Becker.* This dyke he regards as certainly post- 



*Quicksilver Deposits of the Pacific Coast, Monograph XIII, U. S. G. S., 

 pp. 123 and 313. 



