MICROSCOPICAL DESCRIPTION. 117 



In specimen 149 (161, 162) the feldspathic ellipsoids are drawn out 

 into irregular, flattened, disk-like forms. The rock shows a tendency to 

 cleave along the face of the augite bands when the breakage is parallel 

 to the longer axes of the disks. 



Under the microscope both feldspar and augite are much finer grained 

 than in the preceding. The latter especially appears' in smaller and 

 more rounded grains, which lie distributed in a linear direction in the 

 granular feldspathic matrix. The diallage-like structure, due to the 

 presence of twinned lamellae parallel to the orthopinacoid, appears some- 

 times in sections cut transverse to the vertical axis. Indications of oro- 

 graphic pressure appear in one instance in the breaking of an augite 

 grain, calcite being deposited in the crack, and, further, in the appear- 

 ance of fractures extending across adjoining grains of pyroxene. The 

 granular feldspar grains often meet each other in sharp, straight lines. 

 In one case two grains of microcline, one of which is bordered by a small 

 amount of fresh plagioclase feldspar, are separated by a narrow band of 

 the latter, which in polarized light is seen to be twinned, the part on 

 either side of the twinning plane behig very nearly, but not quite, in 

 optical orientation with its adjacent microcline grain. 



In specimen 138 (144)., taken from the dike intersecting the pyroxenite 

 and quartzite, shown on page 101, the feldspathic lumps and interstitial 

 bands are both of a uniformly fine grained granular texture. 



In thin section the feldspathic lens-shaped masses are shown to consist 

 of rather fine but relatively uniform grains of microcline, with a mod- 

 erate amount of quartz and the usual portion of titanite. Occasionally 

 the microcline carries nodular quartz inclusions, but they are few. 



In the interstitial zones the constituents are somewhat finer, and the 

 grain lie with their longer axes extended in the same direction. The 

 granular feldspar has the characteristic structure of microcline, while 

 plagioclase is almost altogether wanting. However, these granules carry 

 a considerable amount of nodular quartz. 



The augite appears in small grains disseminated in the granular feld- 

 spar matrix. This specimen ditfers from the preceding in the presence 

 of a considerable amount of hornblende associated with the augite; some- 

 times connected with it zonally. Some of the hornblende presents idio- 

 morphic outlines with well developed prism faces meeting at angles of 

 56° or 124.° 



STREAKED SYENITE-GNEISS. 



Specimen 140, associated with the last in the exposure described on 

 page 101, shows a nearly complete flattening of the ellipsoids, giving a 

 well developed gneissoid banding. Its identity with the leopard rock, 

 however, is plainly evident from the anastomosing of the augite streaks 



