PYROXENE ANDESITE. $61 



microlites, the reddish tint of the other varieties (Nos. 85, 86, 87, 88, 90, 91) 

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

 first mentioned variety the number of augite microlites exceeds that of the feldspar. 

 In the lighter colored fissile forms (Nos. 85, 86) the feldspar is in excess and the glass 

 base is not so abundant. In the vesicular andesite the composition of the groundmass 

 is not homogeneous throughout, for besides the amygdules of tridymite are light 

 colored spots where the augite, magnetite and globulites are almost wholly wanting 

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

 section 91, which is nricrocrystalline, with grains and lath-shaped microlites of feldspar 

 cemented together with quartz. An exceptional red variety is found in which the 

 colorless glass base is so thickly crowded with red oxide of iron as only to be detected 

 in the thinnest possible section (No. 92). 



(&.) The pyroxene-andesite of Cliff Hills is identical with that of Richmond 

 Mountain; it shows the same modifications 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 sections, 104 and 109, are slightly modified varieties. Under 

 the microscope the typical andesite has a gray groundmass of glass with microlites of 

 feldspar and augite and an abundance of magnetite. It bears phenocrysts of feldspar, 

 augite, hypersthene, and black bordered hornblende. 



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, oo Pdb, coP', GO 'P, 2 PS. The 

 largest phenocrysts are developed more equally 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 

 brachypinacoid, which correspond to anorthite, as does also the high 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 present among the 

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

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

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

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

 referred to, and also in another in the same thin section, Fig. C, PI. III. Where the 

 inner zone has a sharp crystallographic outline, while the outer 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 part in the zonal structure!, and having the same angle of extinction throughout 



