PYlfOXENi: ANDKSITE. 3(33 



iiiiiss of thill section 107 arc siiiiilail\' dcconiitciscd and colorcil with red oxide of iron, as 

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

 tion lOS, the excess of red oxide rendering the slide nearly opa(|ue. The augite micro- 

 lites in the groundinass are very abundant and are tracealde 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 

 aude.site of Kichmond ^Mountain consists of pleochroic hyi)ersthene and nonpleochroic 

 augite, with the same characteristic differences throughout. 



The hornblende is much less abundant thau the pyroxene and occurs only in 

 larger i>henocrysts, with poorly defined outline, being frequently rounded and also 

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

 and show the prism and pinacoids. They are surrounded by a hea\y ijlack border, 

 the .substance of which sometimes i)enetrates 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 = browu, a = light brown, c>b>ii. 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 jn-esent in small amount, always with loiinded outlines 

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

 ^Mountain andesite. (Juartz, though quite noticeable in macroscopic grains in the 

 hand specimens as an accessory mineral, is uot found in the thin sections studied, 

 except one small particle, 0-2.5 """ in diameter, which carries both glass and fluid inclu- 

 sions (107). 



The groundmass is composed of feldspar and augite microlites, 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 

 feld-spar is about equal, but the size of the microlites is uot .so uniform as in the 

 Kichmond Mountain andesite, and numerous crystals, from 0-0.5 to 0-1""" loug, are 

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

 mental structure, however, is felt-like, which comi)letes the correspondence between 

 the two pyroxene-ande-sites of the di.strict, which are indeed but 15 miles ai)art. 

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

 collection of the Exploi'ation of the Fortieth Parallel, which, with a constant micro- 

 scopic habit of grounilmass and of- pheiuujrysts, varies only in ina(-roscoi)ic lial)ii; 

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

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

 which is geiu'rally accompanied by a modification of the groundmass. resulting in 

 difficulty determinable forms intermediate between pyroxene-andcsite, hornblcnde- 

 andesite, and hornblende niica-andcsitc. l'"roni the foregoing de.scri]>tion it is evident 



