366 
The American Geolos[ist. 
December, 190O 
The primary minerals are: 
Quartz . Allanite 
Urthoclase Enstatite 
Andesine Bronzite 
Labradorite Diopside 
Cordierite Augite 
Apatite 
Chrysolite 
Fa va lite 
Spinel 
Zircon 
Magnetite 
Hercynite 
Staurolite 
Graphite 
The other minerals are secondary. Qtiartz and magnetite 
are often secondary. Anomite is of primary origin in the 
cordierite noryte. Cordierite, fayalite, hercynite. staurolite, 
spinel, and graphite are minerals especially common in regions 
of metamorphism. 
Quartz occurs only as a secondary mineral in the normal 
gabbros, where it is rare. Its presence in certain types is 
therefore evidence of some modifying influence, which seems 
in the cases studied to be the absorption of pre-existing rocks. 
OrtJioclasc is the characteristic mineral of the orthoclase 
gabbro. It is crowded full of inclusions, especially hematite, 
which give the rock its pectiliar reddish color. 
Andesine occtirs only in those types showing a tendency to 
become more acid than the normal gabbro. It is rare in the 
gabbro unmodified by contact. In the most acid type of the 
series (the quartz gabbro), it is the onK feldspar found. It 
occurs in subordinate amount in the cordierite noryte and in 
the orthoclase gabbro. 
Labradorite is certainly the most abundant mineral in the 
series of rocks studied ; it constitutes ihe larger part of the 
gabbros, and is lacking only in the silicoferrolyte, which con- 
tains no feldspar, and in the quartz gabbro, where it is re- 
placed by andesine. It is so nearly pure in the plagioclasyte 
as to furnish excellent material for detailed study. JNIost briefly 
expressed, the chief results obtained are : 
Chemical 
Composition. 
Refringence and 
Birefrinsenoe. 
Extinction 
angles. 
Cleavaee 
angle. 
.■\ClltP 
rJisectrix. 
SiOg 
AUO, 
FcOg 
MgO" 
CaO . 
Na.O 
K..0 
h:.o 
53-38 
29.70 
.21 
trace 
I I.QO 
4-30 
.56 
•37 
100.42 
Sp. Gr .2.701 
«g = 1.5705 
«m = 1.5660 
«p =1.5626 
N =1.566 
«g -«rn = -0046 
«m-«p =.0036 
«g-«p =.0082 
s«g -37° 
T//p57K° 
Parallel t()/>(oor) 12' 
Parallel to^' (010)26" 
Maximum equal ex- 
tinction angle in 
zone perpendicu- 
lar to .4'-' (010) -38° 
/>^Mooi)A(oio) 
=94 = 16' 
Optic angle 
2VTI =83°29' 
2VXa =83^47' 
2VLi =84 9' 
ns 
Disper- 
sion a- 
bout ?/g 
p>JJ 
