MUSEUM OF COMPARATIVE ZOOLOGY. 175 



leas. Grains of dull reddish or white feldspar and of blue quartz are 

 evenly distributed thx'ough the rock. The quartz grains pi'edominate, 

 having their longer axes parallel to the schistosity, and often forming 

 the cores of the lenticular bands of quartz. These quartz grains aver- 

 age the size of small shot, while the feldspars vazy 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 

 eutii'ely a greenish yellow muscovito 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 (?), and has the large axial angle of muscovite. The clas- 

 tic grains of quartz are i-ecognized 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 hi the meshes of the mica, in aggregates of interlocking 

 grains, which sometimes enclose muscovite, and hence must have formed 

 by chemical action in 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 



