MUSEUM OF COMPARATIVE ZOOLOGY. 177 



spar, as can be seen in the figure, is irregular, the quartz grains and 

 muscovite of tiie 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 tlie 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. Tlie cleavages are not visible in the figui-e, 

 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 cr^'stallographic 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 opascitc 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 ai'e 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 

 feldspiir 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 tlie specific 

 gravity of the fragment thus ol^tained 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 c;ises (see Figs. 1 and 2). Tlie 

 central portion has the same opaque clouded appearance, owing to fluid 



VOL XVI. — NO. 10. 12 



