PYROXENE ANDESITK. 363 



mass of thin section 107 are similarly decomposed and colored with red oxide of iron, as 

 in the corresponding variety of andesite from Trail Hill. The same is true of thin sec- 

 tion 108, the excess of red oxide rendering the slide nearly opaque. The augite micro- 

 lites in the groundmass are very abundant and are traceable directly to the larger 

 crystals; they are in stout prisms or irregular grains and in most every case have one 

 or more magnetite grains attached. The pyroxene in these rocks, like that in the 

 andesite of Richmond Mountain consists of pleochroic hypersthene and nonpleochroic 

 augite, with the same characteristic differences throughout. 



The hornblende is much less abundant than the pyroxene and qpcurs only in 

 larger phenocrysts, with poorly denned outline, being frequently rounded and also 

 irregular, as though corroded. The cross sections are six and occasionally eight sided, 

 and show the prism and piuacoids. They are surrounded by a heavy black border, 

 the substance of which sometimes penetrates nearly to the center of the crystal. A 

 zonal arrangement of the minute .magnetite particles is seen in some individuals, thin 

 section 107. The hornblende is brown, with strong pleochroism : c = dark reddish 

 brown, b = brown, a = light brown, c>b>a. Inclusions are few, except grains of 

 magnetite, beside which there are a few prisms of apatite having a sharp hexagonal 

 cross section. 



Biotite phenocrysts are present in small amount, always with rounded outlines 

 and crowded with magnetite grains. Magnetite and apatite occur as in the Richmond 

 Mountain andesite. Quartz, though quite noticeable in macroscopic grains in the 

 hand specimens as an accessory mineral, is not found in the thin sections studied, 

 except one small particle, 0-25 nun in diameter, which carries both glass and fluid inclu- 

 sions (107). 



The groundmass is composed of feldspar and augite niicrolites, with much 

 minute magnetite associated with the augite, crowded together in a colorless glass 

 base, the whole showing a distinct flow- structure. The proportion of augite and 

 feldspar is about equal, but the size of the microlites is not so uniform as in the 

 Richmond Mountain andesite, and numerous crystals, from 0-05 to O-l""" long, arc 

 scattered through the mass, giving it a much less homogeneous texture. The funda- 

 mental structure, however, is felt-like, which completes the correspondence between 

 the two pyroxene-andesites of the district, which are indeed but 15 miles apart. 

 They represent, however, a rock of very wide occurrence in the West, judging by the 

 collection of the Exploration of the Fortieth Parallel, which, with a constant micro- 

 scopic habit of groundmass and of phenocrysts, varies only in macroscopic habit; 

 that is, in compactness, structure, and color, and in the relative size or abundance of 

 the phenocrysts, and in the absence or presence of hornblende and biotite, an excess of 

 which is generally accompanied by a modification of the groundmass, resulting in 

 difficulty deterimnable forms intermediate between pyroxeue-audesite, hornblende- 

 andesite, and hornblende-mica-andesite. From the foregoing description it is evident 



