I 
PENNSYLVANIA ANORTHOSITES 41 
sian constituents in very minute quantity are found in a few 
specimens where they form faint narrow bands. Iron oxide stains 
are sometimes visible. 
In thin section under the microscope the texture of the rock is 
seen to be holocrystalline, medium to coarse-grained, inequigran- 
ular. From 95 to 100 per cent of the rock is composed of feld¬ 
spar which is for the most part labradorite. The only other pri¬ 
mary constituents are the accessory minerals, apatite and mag¬ 
netite, both of which occur in very small amounts as scattered 
grains, and occasionally biotite. The following minerals occur as 
secondary products: Zoisite, sericite, clinozoisite, epidote, green 
hornblende, calcite, and leucoxene. Usually in the pure type, 
zoisite and calcite are the only alteration products. Rarely, ac- 
tinolite and epidote occur in very small amount. The feldspars 
are the intermediate plagioclases, labradorite and andesine-labra- 
dorite, the former always predominating. In a specimen from 
Forrest, of which chemical analysis was made, the feldspar so 
far as determinate under the microscope proved to be andesine, 
but many of the feldspars were obscured by zoisitization. The 
chemical analysis indicates that labradorite predominates over 
andesine. The feldspar crystals vary greatly in size; crystal 
boundaries occur rarely, if ever. Zonal growth is absent. Albite 
twinning alone, or combined with pericline, or Carlsbad twinning, 
is common. Zoisite and sericite form minute aggregates along 
cleavage cracks, in irregular fractures, or within the feldspar 
crystals. Occasionally sericite occurs in larger flakes. Zoisite 
shows the characteristic indigo blue or blue gray, between crossed 
nicols. The colorless, monoclinic epidote, clinozoisite, has slightly 
higher double refraction, first order yellow and red. Epidote, 
with its characteristic pistachio-green color, is undoubtedly a 
decomposition product of the green hornblende. It is not found 
disseminated generally through the rock as is the zoisite, but is 
confined to specific areas where, in association with actinolite, 
calcite, zoisite, and magnetite, it forms dense aggregates. The 
amphibole, which gives an extinction of 17 degrees with pleochro- 
ism greenish yellow to yellowish green, is actinolite. Although 
there is no indication of its being secondary to a pyroxene, the fact 
that diopside is found abundantly in the border facies suggests 
that pyroxene is in all cases the original constituent. No titanite 
