University of California . 



[Vol. 3. 



The amygdaloidal cavities with their secondary lining are 

 much less abundant here than the other variety, which presum- 

 ably belongs to the more superficial portions of the flows, though 

 field data in regard to this point are lacking. 



The chemical composition of a typical specimen from Ante- 

 lope, with the second type of groundmass, is shown in the 

 following 1 analysis made by the writer: 



Analysis of Rhyolite, Antelope Valley. 



Si0 2 



75.40 



A1 2 3 



13. 56 



Fe 2 3 



0.21 



*FeO 



0.61 



MgO 



0.07 



CaO 



0.38 



Na 2 



4.64 



K 2 



4.40 



H 2 at 110° 



0.44 



H 2 above 110° 



0.94 



Ti0 2 



0.04 



P 2 5 



0.09 



MnO 



trace 



Total 



100.78 



The rock is a decidedly acid rhyolite, with soda slightly pre- 

 ponderating over potash, and with iron, lime and magnesia in 

 insignificant amount. Completely crystallized, this magma 

 would form a rock essentially of the following mineralogical 

 composition : 



Albite, 40.3 

 Orthoclase, 26.7 

 Anorthite, 1.4 

 Quartz, 31.6 



In view of the large excess of silica, the absence of quartz 

 phenocrysts is remarkable. The amount of quartz in the ground 

 mass apparently falls so far short of 30 per cent, that it would 

 seem that there must be a more considerable amount of siliceous 

 glass base than is apparent, or that quartz fibres enter into the 

 composition of the spherulites. 



In the central part of Turtle Cove, overlapping the edge of 

 the rhyolite whose petrographical characters we have been 

 describing, is a flow eight or ten feet in thickness of a glassy 

 lava of a different aspect. 



*PeO probably too high. 



