MUSEUM OF COMPARATIVE ZOOLOGY. 177 
spar, as can be seen in the figure, is irregular, the quartz grains and 
muscovite of the cement encroaching upon it. This is noticeable in the 
upper left hand corner, where a flat grain of quartz intrudes parallel to 
the cleavage. It cannot be determined from this oblique section to what 
variety of feldspar the grain belongs. The twinning excludes orthoclase ; 
it may be microcline. 
Figure 2 represents a section of another red clastic feldspar. This 
also polarizes as one crystal, and, as the section happens to be normal to 
an optic axis, it is easy to see by the position of the axial bar that 
the cloudy centre and clear rim are in like crystallographic position. 
No twinning is visible. The cleavages are not visible in the figure, 
although present in the section. The principal cleavage is parallel to 
the long dimension, and the second cleavage parallel to the narrow side 
of the grain; here again the outlines are determined by cleavage cracks, 
or by the corresponding crystallographic planes, unlike the albitic feld- 
spars. ‘The substance of the inner cloudy core is filled with little flakes 
of kaolin or muscovite arranged parallel to the second cleavage, fluid 
cavities, and opascite specks. Here and there is a large, brilliantly 
polarizing flake of muscovite. The boundary against the clear portion 
is generally quite sharp, and parallel to a cleavage line. The two large 
black cracks crossing the specimen obliquely seem to be secondary 
weathering cracks filled with limonitic products. In the clear rim the 
larger mica plates are seen here and there, arranged parallel to the edge 
of the crystal and to the general schistosity of the rock. The ragged 
form of the outer edge is very marked in comparison to the even inner 
boundary. The precise nature of this feldspar cannot be determined 
from the section, 
In the case of two other clastic feldspars (not figured) the following 
method was employed. The rock was sawn through so as to cut the 
-feldspar in two ; from one side a thin section was made, while from the 
other a thin slice containing a section of the feldspar was sawn, the rock 
surrounding the feldspar cut away with a penknife, and the specific 
gravity of the fragment thus obtained determined, while a part of it 
was crushed on a glass slide, and the cleavage sections thus developed 
studied under the microscope. 
The first had a Sp. Gr. of 2.585, and among the crushed material 
cleavage pieces with the microcline double twinning are seen. In the 
thin section the crystal has an elongate shape, but the general outline is 
much less regular than in the previous cases (see Figs. 1 and 2). The 
central portion has the same opaque clouded appearance, owing to fluid 
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