44 
PENNSYLVANIA ANORTHOSITES 
pyroxene, amphibole, and biotite. The rock exhibits in. some 
places a distinctly foliated texture. 
In thin slices the texture is holocrystalline, inequigranular. The 
feldspars are orthoclase, microperthite, microcline, albite, oligo- 
clase, and andesine, all of which are partially altered to kaolin, 
sericite, albite, and zoisite. Quartz is present in variable amounts. 
The pyroxene is augite, or, less commonly, diopside; it may be 
altered to hornblende, chlorite, or epidote. Biotite occurs spar¬ 
ingly, and is often entirely absent. Accessory constituents are 
apatite, zircon, ilmenite, magnetite, and, more rarely, garnet, py- 
rite and rutile. The percentages of the mineral constituents vary 
within wide range. The average proportions are approximately: 
quartz, 15 per cent, feldspar, 55 per cent, mafic constituents, 30 
per cent. 
Chemical analysis of the quartz-monzonite is given below: 
Si02 
64.64 
Quartz 
8.70 
AhOa 
15.92 
Orthoclase 
36.14 
FesQa 
1.14 
Albite 
37.20 
FeO 
4.65 
Anorthite 
5.56 
MgO 
.23 
Diopside 
4.40 
CaO 
2.12 
Hypersthene 
5.15 
NasO 
4.38 
Magnetite 
1.62 
K 2 O 
6.06 
Ilmenite 
.76 
H 2 O— 
.04 
Pyrite 
.12 
H 2 O + 
.43 
H 2 O+ 
.47 
Ti02 
.42 
MnO 
.03 
Zr02 
trace 
BaO 
.10 
P 2 O 5 
trace 
S 
.06 
100.25 
MnO 
.03 
BaO 
.10 
100.22 
In the quantitative system of classification the symbol of this rock be¬ 
comes I (II) • (4)5 • (1)2:3; and its name grano-pulaskose. 
I 
In every theory on the origin of the anorthosite certain pre¬ 
vailing characteristics of the rock must be taken into account. 
These may be enumerated briefly. Anorthosite occurs charac- , 
teristically as a deep-seated rock which is notably coarse, or 
coarse- to medium-grained. Protoclastic texture and granula¬ 
tion are common phenomena. The rock has no effusive equivalent. 
Whether or not it has intrusive (dike) equivalents is still an open 
35 Quartz-monzonite, from south of Ludwig Corner, near Phoenixville, Pa., R. C. 
Wells, analyst. 
