ANDESITE OF MOUNT SCHURZ. 317 



and is parted into plates parallel to the plane of contact and also into 

 columns perpendicular to the same. The megascopical habit varies consid- 

 erably in different parts of the bod3^. Fifty feet from the northwestern lower 

 contact the rock is full of prominent phenocrysts of feldspar, with smaller 

 biotites and hornblendes (1585). At 75 feet it has less prominent feldspars 

 and more noticeable hornblende and mica (1586). In the central part of 

 the body it is still lighter colored, and compact, with the same variability of 

 phenocrysts (1587, 1588). In the cavities of some of the porous forms there 

 are good crystals of tridymite (1585, 1589). 



At the higher parts of the body the rock is darker colored again 

 (1590, 1591). Among the fragments lying at its northwestern base are 

 large masses and columns of various modifications of the rock. One 

 column, 8 feet long, is a dark-colored, finely vesicular variety (1592), 

 without prominent phenocrysts. Another variety is still more vesicular, 

 with light -blue coating to the vesicles. It is basaltic in appearance, with 

 noticeable feldspars (1596). Others are dark and basaltic looking, but more 

 compact, with prominent feldspars (1594, 1595). They all belong to the 

 large andesitic body. 



The light-colored rock which constitutes the main mass of this body 

 has a groundmass that is microcrystalline and microlitic. It is verj r much 

 like that of the rock forming the summit of Mount Stevenson. It contains 

 much tridymite in clusters of minute crystals and in groups of twinned 

 plates. The abundant small phenocrysts are labradorite, brown hornblende 

 with magnetite and pyroxene border, brown biotite with magnetite border, 

 and some augite. There is a small amount of serpentinized mineral, which 

 was probably hypersthene. In the rock from within 75 to 50 feet of the 

 margin of the body the groundmass is microlitic and possibly glassy. The 

 phenocrysts of hornblende and biotite have only a very narrow magnetite 

 border in the rock at 75 feet from the contact plane, and none at all in that 

 at 50 feet from the plane. 



At the bottom contact the character of the rock is somewhat changed.' 

 The groundmass is hyalopilitic, consisting of brown glass filled with micro- 

 lites. Phenocrysts are fewer, and are' labradorite with beautiful glass inclu- 

 sions, hornblende almost completely paramorphosed to pyroxene and 

 magnetite, brown biotite with narrow border of magnetite and pyroxene, 

 many small phenocrysts of augite, and many others that are serpentine 



