DIFFERENTIATION OF LAVAS FROM A UNIFORM TYPE. 61 



lava, the lava of Mount Oddie shows an increase of 4.86 per cent silica and of 1.41 

 per cent potash; and a decrease of 2.25 per cent soda, and probably 2 per cent lime. 

 The course followed by this gradual change from Brougher Mountain to Mount 

 Oddie by way of Butler Mountain, Golden Mountain, and Kushton Hill, is almost 

 circular; and while more extended knowledge is desirable, it has probably a signifi- 

 cance, for, as already explained, all these vents belonged to the same period, though 

 they were not necessarily absolutely contemporaneous. They may well have been 

 successive centers of outbreak in the order given. 



THEORY OF DIFFERENTIATION OF TONOPAH LAVAS FROM A UNIFORM TYPE. 



PSEUDOMORPHS IX RHVOLITE. 



Character of pseudomorphs. The description of the first specimen of rhyolite 

 analyzed, as seen under the microscope (column 10 in table on p. 58), is as follows: 



Specimen 376, from Belmont shaft, 50 feet down. This rock shows to the naked 

 eye small fresh crystals of orthoclase (sanidine), quartz, and biotite in a pinkish- 

 white groundmass. Abundant small, dull-white spots often have crystalline form, 

 and seem to play the part of phenocrysts. 



Under the microscope the rock is seen to be fresh. The sanidine shows 

 sometimes Carlsbad twinning; it is often broken, and may be partly resorbed by 

 the magma. The quartz is frequently in dihexahedral crystals, rounded and 

 invaded by the resorbing magma. The biotite is fresh, in small crystals, and in 

 very small amount. The groundmass is a fine microgranular aggregate of quartz 

 and feldspar. 



Some of the dull-white spots noticed in the hand specimen are without 

 crystal outlines, while others have sharp outlines. Inspection of u number of 

 longitudinal and cross sections leads to the conclusion that the forms are probably 

 those of hornblende. The material, however, is evidently pseudomorphous, for 

 it is a fine transparent aggregate of low single and double refraction, which 

 under high powers is seen to be spherulitic. It separates itself from the rest of 

 the groundmass chiefly by its greater fineness. In several cases small biotite 

 crystals were observed in this aggregate, as large as many in the rest of the 

 rock, and these were clustered together with a tendency to a diverging or radial 

 arrangement. 



The description of the second specimen of rhyolite analyzed is as follows: 



Specimen 337, from face of G. and H. Tunnel, Mount Oddie, contains larger 

 phenocrysts than usual of quartz, orthoclase, and a little biotite in a fine microgran- 

 ular groundmass of quartz and orthoclase. The feldspar is glassy and fresh sani- 

 dine. The biotite contains apatite crystals, which are clear, not smoky like those 

 of the andesites. 



