Mineral Industries and Geology of Certain Areas - Vermont Agency ...

56
REPORT OF THE VERMONT STATE GEOLOGIST.
MgO.............................
CaO
.66 104
.............................. 2.07 2.40
Na20...............................
K20 7 4.08
.............................. 4.29
1.-I0 at 1100
4.14
.
.31
H 20 ignit ..........................23
P20 ............. ................ Trace
99.13
I. Granite. Barre, Vt. G. I. Finlay anal. 99.76
II. "Shap" Granite. England. J. B. Cohen anal. Q. J.
G. S. XLVII. 276.
An analysis of the English "Shap" granite, made for
H. Harker, is put beside it for reference.
By the aid of the micrometer eye-piece three other varieties
of the granite, as noted below, were measured for a quantitative
estimate of their mineralogical contents. It is believed that the
figures thus obtained, after calculating the percentage by weight
for each constituent, are roughly expressive of the range in
mineralogical composition throughout the granite mass.
I II III.
Microcijue .....................
47.9
Orthoclase 56.8 ...... 38.4
..............
Plagioclase
7 - 5 2.1 8.9
.................... I 5
Quartz
.3
........................
3 .7
26.0 28.5
Biotjte
18.4
.............................10.2
i\Iuscovjte
26.1
.....................
3.3 . I
Titanite
.2
........................... .6
Magnetite
2.6
............................ .
.0
Apatite ....................................5
99.8 99.6 99.8
I. Acid Granite Southern end of Millstone Hill.
II. Granite. Medium Stock. Eastern slope of Millstone
Hill.
III. Basic Segregation. Granite. Northern slope of Mill-
:stone Hill.
REPORT OF THE VERMONT STATE GEOLOGIST. 57
The most striking variations from the normal quality of the
granite throughout its mass are found in rare miarolitic pockets.
One of these, which is notable as showing three distinct types
of rock side by side over a very small space, is figured in
Plate VIII. In the center of the picture, in the upper half
of that portion of the granite defined by its contact with the
schist on the left, and the vertical crack in dark shadow, on
the right, a rudely V-shaped outline may be seen, delimiting a
space of about two square feet. At the top smoky quartz is
found. Its presence is significant, since the observation has
been made by Dr. A. A. Julien that the occurrence of smoky
quartz in the central portions of many pegmatitic veins marks
the last stages of vein filling in such cases. At the bottom, in
the point of the V, the excessively coarse-grained variety of
granitic rock, which makes up the miarolitic seam above referred
to in Messrs. Lamson and \Vells quarry, is developed.
The sides of the V carry a very fine-grained facies of the granite,
lacking in the usual biotite, but abundantly supplied with muscovite.
Between the patches of this muscovite-bearing rock on
either side, and sharply outlined against them, as showing no
gradual changing over to their substance, or into the coarsegrained
member in the lowest portion of the V, a porphyritic
facies of the granitic magma occurs.
In the hand specimen this rock is bluish by reason of the
color of its ground-mass. Phenocrysts of white feldspar are
far more abundant than those of quartz. Biotite in minute
plates, strewn everywhere through the rock, is seen to he the
most important dark constituent.
Under the microscope large porphyritic crystals of microdine
and quartz, with much less orthoclase and plagioclase, and
abundant biotite, appear in a coarse holocrystalline groundmass,
composed of quartz and feldspar, in irregular grains. The
quartz phenocrysts give at best only a rough approximation, on
basal sections, to hexagonal outlines. Microcline usually
shows but slight evidences of twinning. Its substance is glassy,
rarely kaolinized. Some orthoclase is present in rudely auto-