290 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



abundant and dark brown, with strong absorption. It incloses some mag- 

 netite, and in one case lias grown around augite. Idiomorphic and broken 

 crystals occur beside one another. Biotite, with the same color as hornblende, 

 is less abundant. Some crystals are idiomorphic; others are bent and 

 crumpled by the pressure from other phenocrysts prior to the solidification 

 of the mass. Small crystals of these minerals, almost microscopic, occur in 

 the glassy gronndmass. Magnetite, apatite, and zircon are present in idio- 

 morphic crystals. The groundmass is colorless glass with a relatively small 

 amount of very minute crystals. It is pumiceous, with vesicles more or less 

 spherical, or drawn out into spindle-shaped tubes. 



Hombiende-mica-andesites. — The varieties of these rocks are microcrystalline 

 andesites with multitudes of minute phenocrysts of plagioclase, feldspar, and 

 hornblende, and fewer of biotite, so that in some cases they might be 

 called mica-bearing hornblende-andesites. In thin sections the hornblendes 

 are seen to be differently colored in different rocks. In some they are green 

 and brownish green, with very slight border of magnetite grains, or none at 

 all. In others they are deep chestnut brown, without borders ; in still others, 

 strong reddish brown with pleochroism from orange to red without border, 

 or with narrow magnetite margin. Otherwise they are alike, having similar 

 forms and inclusions. They are partly idiomorphic. Inclusions are not 

 common. Some contain numerous glass inclusions. 



Biotite varies also in color and is easily confused with hornblende in 

 some of the rocks, usually having the same colors where the hornblende 

 is brown and red-brown; but where hornblende is green, biotite is brown. 

 Its chief inclusions are magnetite and apatite. 



Hypersthene and augite occur in only a few rocks, and then in very 

 small crystals. In one case hypersthene with pronounced pleochroism has 

 a dark border, as though from magmatic action. It accompanies red horn- 

 blendes. In another instance, where the hornblendes are red, the few small 

 augites are nearly colorless. 



The plagioclase is toward the labradorite part of the series, exhibiting 

 rather high extinction angles and having forms and inclusions similar to 

 those of the plagioclases of the dacites just described, except that zonal 

 structure is not so frequent, being most pronounced in the larger, nearly 

 equidimensional crystals, and nearly wanting in the smaller, narrow 



rectangular ones. 



