PTKOXENE-ANDESITE. 361 



microlites, the nuldish tint, i>\' tho other varieties (Nos. 85, StJ, 87, 88, 90, 01) 

 arising from the presence of a higher oxide of iron iiicrusting the magnetite. In the 

 tirst nicntion(^(l variety the number of augite microlites exceeds that of the feUlspar. 

 In the lighter colored tissih^ forms (Nos. 85, 8(5) the fclds])ar is in excess and the glass 

 base is not so abundant. In the vesi(!nlaraiid(;sitethe composition of the groundmass 

 is not homogeneons throughout, for besides the aniygdnles of tridymite are light 

 colored spots where the augite, magnetite an»l globulites are almost wholly wanting 

 (No. 88). Glass base is altogether absent from the mica-bearing groundmass of thin 

 section 91, which is microcrystalline, with gi'ains and lath-shaped microlites of ftildspar 

 cemented together with (puirtz. An exceptional red variety is found in wiiich the 

 colorless glass base is so thickly crowded with red oxide; of iron as oidy to be (b^tected 

 in the thinnest possible section (No. 9;i). 



(b.) The i)yroxene-andesite of ClifC Ilills is identical with that of Richnioud 

 Mountain; it shows the same nioditications in the field as the latter, corresponding to 

 which are the same microscopic characters. Thin section 102 is from a resinous blue- 

 black variety similar to Nos. 77, 78, 79 of Richmond; .section 107 is from a reddish 

 purple form, and corresponds to No. 90 from Trail Hill. Thin section 108 is like No. 

 92, and the remaining two sc^ctions, 104 and 109, are slightly modified varieties. Under 

 the micr()S(M)pe the typical andesite has a gniy groundmass of glass with microlites of 

 feldspar and augite and an abunilaiice of 7nagnctite. It bears phenocrysts of fcldspai-, 

 augitt;, hypersthene, and black bordered hornblen<le. 



The feldspar is triclinic without any admixture of recognizable orthoclase, the 

 individuals are all striated by multiple twinning. Their outline is mostly rectangular, 

 some with the angles truncated or rounded, indicating their form to have been prisms in 

 the direction of the brachydiagonal, having the faces OP, go P cfc, tx P', oo 'P, 2 PqE. The 

 largest plienocrysts are developed more eiiually in the direction of the three axes; the 

 feldspar microlites in the groundmass are wholly lath-shaped. The angles of extinction 

 of the porphyritical crystals reach 35°, 40'^, and 44° in the zone at right angles to the 

 bra(;hypinacoid, which correspond to anorthite, as does also the higli light they exhibit 

 between crossed nicols in very thin sections. Optically it can not be determined 

 whether other species of triclinic feldspar are at the same time i)resent among the 

 larger phenocrysts, unless the great divergence of extinction angles in the zoinilly 

 built individuals, which reaches in one instance .32° (102), be taken as evidence of 

 difterence in chemical composition between the different zones. The zonal struc- 

 ture is beautifully developed in some individuals, especially so in the crystal just 

 referreil to, and also in another in the same thin section, Fig. 0, PI. iii. Where the 

 inner zone has a sharp crystallographic outline, while the tmter one is rounded at 

 the corners, the angle of extinction for the former being 38'^ and for the latter only 

 18°, narrow strips of twinned feldspar pass through the different zones, without tak- 

 ing i>art in the zcnial structure, and having the same angle of extinction througliout. 



