TEACHYTIO EHYOLITE. 323 



a number of cases the symmetrical extinction angles tested by Michel Levy's 

 method indicate that the feldspar is labradorite as high in lime as Ab 2 An 3 . 

 In other rock sections the only symmetrical extinction angles are low, but 

 they are few, and may possibly belong to labradorite. These feldspars also 

 are quite free from inclusions. They are more easily decomposed than sani- 

 dine, and in a number of rock sections are completely altered, while the 

 sanidine is fresh. The usual alteration is to a microcrystalline aggregate 

 with low double refraction, probably kaolin. Occasionally the feldspar is 

 replaced by calcite. Biotite occurs in irregularly outlined crystals of very 

 small size. It is brown and has a very small optic angle. Augite, when 

 present, forms idiomorphic and also irregularly shaped crystals, apparently 

 fragments of larger crystals. Its colors and general character are the same 

 as those of the augite in the associated andesites. The same is true of the 

 few small fragments of green and brownish -green hornblende. Ilmenite, 

 or titaniferous magnetite, is present in comparatively large microscopic 

 crystals and grains. Its character is indicated by its alteration product 

 and its form, since it alters to a white opaque mineral crossed by lines in 

 three directions. It is similar to the occurrence of titaniferous iron oxide 

 in the rhyolite of the region. With it are associated colorless apatite and 

 zircon. There are a few yellow, almost isotropic, pseudomorphs, possibly 

 after hypersthene. 



The groundmass is in many cases brecciated, and is made up of patches 

 with different kinds of microstructure. It also contains fragments of other 

 rocks, such as andesite and the crystalline schists. Large fragments of the 

 latter are found in places, and the massive lava grades into tuff-breccia in 

 some localities. Only a small part of the rock is glassy and unaltered (683). 

 Numerous thin sections show that the rock was once glassy in many places 

 but has become more or less completely devitrified. The glassy form con- 

 sists of glass that is globulitic and brown in places, with streaks and lumps 

 that are colorless, and microlitic and trichitic, with pyroxene and magnetite, 

 quite like some rhyolitic glasses. It has eutaxitic and flow structures and 

 is spherulitic in places. Perlitie cracking is developed to some extent. 

 Small lumps with beautiful brown glass full of microlites, and others con- 

 taining augite and hypersthene, and noncrystalline pieces of pyroxene- 

 andesite in this glassy lava, suggest that its eruption was subsequent to that 

 of pyroxene-andesite, which is the case for similar trachytic lava farther 



