MOUNT RAINIER 



upon the ferromagnesian silicates, and four rock types 

 are represented hypersthene-andesite, pyroxene-an- 

 desite, augite-andesite, and basalt any of which 

 may carry small amounts of hornblende. A rigid 

 separation of these rock types, however, is impossible, 

 since insensible gradations connect the most acid with 

 the most basic. In the same flow hypersthene-andesite 

 may occur in one portion, while in close proximity the 

 lava is an augite-andesite. 



These lavas have groundmass textures that vary 

 from almost holo-crystalline to glassy. The felted or 

 hyalopilitic texture is the most common, and plagio- 

 clase is the principal groundmass constituent. The 

 feldspars are lath-shaped, often with castellated termi- 

 nations. In the more basic phases anhedrons of augite 

 and of olivine appear, and magnetite grains are usually 

 present. Flowage is often beautifully expressed by the 

 arrangement of the slender laths of feldspar. 



Among the phenocrysts feldspar is the most promi- 

 nent. It has the usual twinning characteristic of 

 plagioclase and belongs to the andesine-labradorite 

 series, extinction angles proving basic andesine and 

 acid labradorite to be the most common. Zonal 

 structure is characteristic, being noticeable even with- 

 out the use of polarized light. Zonal arrangement of 

 glass inclusions testifies to the vicissitudes of crystal- 

 lization, and often the core of a feldspar phenocryst 

 is seen to have suffered corrosion by the magma and 

 subsequently to have been repaired with a zone of 

 feldspar more acid in composition. 



Of the darker phenocrysts, the pyroxenes are 

 more abundant than the olivine or hornblende. Hy- 

 persthene and augite occur alone or together, and 

 are readily distinguished by their different crystallo- 

 graphic habits as well as by their optical properties. 

 The hypersthene is usually more perfectly idiomorphic 

 and occurs in long prisms, with the pinacoidal planes 

 best developed, while the augite is in stout prisms, 



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