MUSEUM OF COMPARATIVE ZOOLOGY. 175 
lens. Grains of dull reddish or white feldspar and of blue quartz are 
evenly distributed through the rock. The quartz grains predominate, 
having their longer axes parallel to the schistosity, and often forming 
the cores of the lenticular bands of quartz. These quartz graius aver- 
age the size of small shot, while the feldspars vary from the size of a 
large pea to that of the smallest shot. 
The feldspars are often distinctly angular in shape. Even with the 
unaided eye it is seen that they have a rim of clear glassy feldspar sur- 
rounding the inner dull red or white core, the latter sometimes preserv- 
ing its boundary, made by straight lines or sharp angles, the former 
having a ragged edge which merges imperceptibly into the cement. In 
other cases the clear rim, instead of representing only a small propor- 
tion of the diameter of the feldspar, occupies half or more of the whole 
grain, and little tongues of the clear substance then ramify into and 
across the red core. The shape of these feldspars and their occurrence 
with the larger feldspar pebbles of the coarse conglomerate make their 
detrital character evident. The same is true of the grains of blue 
quartz. 
Small prisms of tourmaline, octahedra of magnetite, and rare grains 
of apatite, occur as accessories. 
In the slides these elements are easily recognized. The mica is 
entirely a greenish yellow muscovite in thick plates, which in the 
thicker slides exhibits a pleochroism varying from yellowish green to 
colorless, is free from inclusions excepting a rare grain of magnetite 
and of titanite (1), and has the large axial angle of muscovite. The clas- 
tic grains of quartz are recognized by their large size, and by the fact 
that in polarized light they are seen to have been strained, this effect 
increasing until some grains pass into a peripheral cataclastic mosaic of 
quartz grains produced from the original grain by crushing, between 
which flakes of muscovite make their appearance. Quartz also occurs 
abundantly in the meshes of the mica, in aggregates of interlocking 
grains, which sometimes enclose muscovite, and hence must have formed 
by chemical action i situ. 
Here and there little areas of clear feldspar occur, evidently the little 
glassy crystals of the hand specimen. They often have a lenticular form, 
' flattened parallel to the schistosity of the rock, and the inclusions which 
they commonly contain have their longer axes parallel to this direction, 
and are distributed in rough parallelism to the same direction. The 
inclusions consist of little flakes of muscovite and round or elongated 
grains of quartz and of magnetite. One of these feldspars is represented 
' 
