96 ROCHESTER ACADEMY OF SCIENCE. [June 23, 
Microscopically the sections are seen to be composed of plagioclase, pyroxene, olivine, 
magnetite, iron, pyrrhotite (troilite) with various inclusions and alteration products, The 
general structure is granitoid like the gabbros and more coarse'y crystalline diabases. 
At my request a careful study of the minerals was made by Drs. A. C, Lane and H. 
B. Patton of the Michigan Geological Survey, and their results are given below. 
Feldspar.—Plagioclase. 
This is mostly in irregular grains which are but slightly idiomorphic. They tend to 
assume elongated lath-shaped forms that have the twinniag stripes well developed, 
although rarely they are almost or entirely wanting. The twinning is usually in more 
than one direction. The plagioclase is taken by Dr. Lane for anorthite, who states that 
it shows the albite and pericline twinning with traces of the development of o10. The 
extinctions are often over 45°, being in the symetrical forms 54°-58°. In one section 
no extinctions were observed indicatinz any feldspar more basic than labradorite. The 
cleavage planes run parallel and perpendicular to the twinning bands. In the plagioclase 
three kinds of inclusions occur which frequently render it cloudy. 
Ist, Very numerous grains, generally rounded or elongated, but sometimes very 
irregular in shape. They vary in size from 0.05 m. m. downwards, averaging about 
o.orm.m. Asa rule the grains gradually diminish in size the farther they are from the 
edge of the feldspar. The index of refraction is high—the double refraction also being 
greater for the inclusions than for the inclosing plagioclase. The grains are colorless, 
varying to light brown in the larger forms. Sometimes they are arranged without apparent 
order in the feldspar, but generally they are in parallel lines, either parallel to the twin- 
ning planes or else lying in two directions oblique to each other, apparently parallel to 
the faces o10 and oo1. ‘These grains are considered to be augite, 
2nd, Very small inclusions, that are often so minute as to present a dust like 
appearance that are considered to be gas cavities. 
3rd. Black opaque inclusions of probable magnetite. These are not abundant. 
Pyroxene. 
The pyroxene is considered by De. Patton and myself to be the diallage variety, 
but by Dr. Lane to be the augitic variety. 
This mineral is abundant and shows a high index of refraction. It also has the 
high double refraction and oblique extinction of monoclinic pyroxene, with the fine striation 
of diallage. The color is light brown, but it has frequently been stained yellow. The 
pyroxene is almost uniaxial, (4-2V<to), and has a large dispersion P<V. The striation 
runs parallel to oor, and makes a small angle with the plane of extinction (0°, 13°, 16°, 
14°, 12°, 15°). The striated sections give a disturbed axial image, apparently having 
one axis out of the line of sight. Pleochroism weak, yellowish parallel to the striation, 
piakish perpendicular to it, while the greater refraction also lies in this last direction. 
The pyroxene shows both prismatic, and more rarely, pinacoidal cleavage. Rarely we have 
a combination of this basal striation (001), probably due to twinning, with the common 
twinning after the orthopinacoid (100). Then we have, if a is the trace of the twinning 
line, b!, and b? the traces of the bases, x! and x? the extinctions corresponding to c : 
<a:b! <a:b® <pipre axe 
+12°.6 —I5°.1 +35°.4 —34°.1 
Zonet 23 30°. Zia 
20°.4 B20 0 3 
While theoretically on oro we shculd have : 
+15° 49° —15° 49° +2b45° = —ab45” 
