from the Pre-Camhrian of Netv Jersey. 357 



faces of the rock at right angles to this structure are quite 

 even-grained. 



Of the visible minerals, feldspar and hornblende pre- 

 dominate, while quartz and magnetite appear very spar- 

 ingly. The feldspar is grayish-green in color, and 

 possesses good and rather highly lustrous cleavage faces. 

 The hornblende occurs in grains of various sizes, with 

 large individuals the more common. The hornblende 

 pencils, mentioned above, constitute the most striking 

 feature of the rock. The quartz is light gray and glassy. 

 A few small grains of magnetite are visible. The light 

 constituents make up about 80 per cent of the rock. 



Microscopic. — Microscopic examination shows the 

 gneiss to be composed of plagioclase, microcline-micro- 

 perthite, hornblende, microcline, and quartz, named in the 

 order of their importance ; and very subordinate amounts 

 of accessory magnetite, biotite, zircon, and apatite. Mica- 

 ceous and chloritic decomposition products occur to a 

 limited extent. The texture is hypidiomorphic, medium 

 to coarse granular. Evidences of pressure are rare, 

 though certain of the feldspars show slightly curved 

 twinning lamellae, and both the quartz and the feldspar 

 exhibit faint strain shadows. 



Feldspars. — Of the feldspars, plagioclase and microper- 

 thite each compose about one-third of the rock, while 

 microcline is much less abundant. The feldspars are 

 quite clear, except for small stringers of micaceous 

 decomposition products which have developed along the 

 cleavage cracks. 



Plagioclase occurs in large anhedral grains. The 

 maximum extinction angle perpendicular to M(OIO) is 

 9°, hence the mineral is an oligoclase having a composition 

 between Ab^gAusg and AbjoAugo- Microperthite occurs 

 in coarse, interrupted intergrowths of microcline and 

 albite, which are present in about equal amounts. The 

 microperthite individuals are much larger than those of 

 any of the other constituents. Microcline is abundant in 

 small areas. Much of it presents clear and distinctive 

 cross-hatched twinning. Orthoclase is not present in the 

 Van Nest Gap rock although it is commonly found in the 

 Byram gneisses. 



Hornblende. — This mineral is the most abundant dark 

 constituent. The grains vary in size; the larger are 

 irregular, while many of the smaller individuals show 



