MICROSCOPICAL DESCRIPTION. 113 



appear in larger grains in the more coarsely crystallized portions. In 

 the finer grained areas the constituents are quite uniformly distributed. 

 In thin section the microcline shows a pronounced development of 

 microperthitic intergrowth with a more strongly doubly refracting, fresh- 

 looking feldspar (albite ?). In some sections the albite bands show a 

 very fine transverse striation, upon which the extinction is about five 

 degrees. Sometimes small grains of q\\SiYtz appear inclosed in the albite 

 bands. The inclusions appearing in the microcline occur also in the 

 albite bands and in the quartz and correspond to those already described. 

 In some cases plates of biotite extend across the boundary between the 

 microcline and albite and are partially inclosed in each. 



The space between the microcline grains is occupied by finely granu- 

 lar microcline and a striated feldspar, with here and there larger grains 

 of green augite. The striated feldspar is fresher in appearance than the 

 microcline. The laminations are very fine, and according to a number 

 of measurements extinguish at from 4° to 5° on either side of the twin- 

 ning plane where this bisects the angle of extinction of the two lamellae, 

 thus corresponding to albite or oligoclase. In addition to biotite_ plates, 

 these plagioclase feldspars often contain large numbers of rounded or 

 nodular inclusions of quartz.* 



Specimens 134 (137) and 135 (138), taken from the same block as the 

 above, are much finer grained and contain a considerable amount of 

 quartz, showing with the feldspar a somewhat obscurely banded arrange- 

 ment. Augite is present in comparatively small amount. The feldspar 

 is pinkish, except in the coarser patches, where it is the usual gray color. 

 Grains of microcline up to the size of peas, sometimes inclosing augite, 

 occur scattered through the finer grained areas. Under the microscope 

 these specimens are seen to differ from the preceding in the greater ex- 

 tent of the interstitial granular areas, the lessened amount of augite, and 

 greater abundance of quartz. 



In addition to the microperthitic intergrowth with albite, the larger 

 microcline grains show an abundance of the nodular quartz inclusions, 

 with here and there similar forms made up of an aggregate of albite 

 grains. These nodular quartz inclusions are of considerable size, varying 

 from .035 of a millimeter to .425 of a millimeter in diameter, the larger 

 ones sometimes reaching a length of .95 of a millimeter. The largest in- 

 dividuals are often separated into a number of parts, each with slightly 

 different extinction. It is noticeable that in many of them the more 

 prominent fractures extend nearly in the same direction. In the granular 



*These correspond to the " quartz de corrosion " of tlie French authors, a term quite inappro- 

 priate, as they in no sense represent corrosive action. In correspondence with their peculiarities 

 of shape, the term " noduhir quartz" is here adopted for tliem. 



