GNEISS AND LIMESTONE CONTACT PHENOMENA 273 



Among the specimens collected some were very rich in calcite 

 with only sparing amounts of dolomite crystals, the latter being 

 easily recognizable from their higher indices of refraction {0:0 = 

 1.683 — 1-684; the calcite always was found to have cod = 1.660). 



Other specimens collected contain pale brown biotite, having 

 /Sd = 7d = i-599 — i.6oo, and quartz, as rounded grains. 



A specimen from the wider East Lee Valley, where the Coles 

 Brook limestone occurs as a larger mass, represents a dolomite-rock 

 containing rounded grains of quarts and microcline, and minute 

 crystals of pyrite and scales of brownish, nearly uniaxial mica with 

 |Sd = 7d = 1.599=^0.002. 



It may be mentioned that these characters are the same as 

 those in the most common types of the Stockbridge limestone in 

 the Housatonic Valley. Analyses^ of the Stockbridge indicate 

 that there are all variations between dolomite-rocks and calcite- 

 rocks represented. We collected specimens of this limestone from 

 Glendale quarry. It is a dolomite-rock, with much fine scaly 

 brownish mica (/?d = Td = 1.584='= 0.002) and brown tourmaline 

 (a)D = 1.639=1= o.ooi). 



BANDED GNEISS NEAR THE LIMESTONE AND THE VARIATION 

 OF ITS MINERAL COMPOSITION 



The gneiss, near the vertical, or almost vertical, layers of 

 limestone, always shows a banded structure parallel to the strike 

 of the limestone. At the immediate contacts it is clinopyroxene 

 gneiss. Farther away there are various bands, some containing 

 hornblende, and others with almandite, biotite or magnetite as 

 the principal mafi.c minerals, and among them are bands of the 

 clinopyroxene gneiss also. 



The individual bands may be homogeneous or banded in detail 

 (Fig. 3), due to the unequal distribution of the minerals. The 

 breadth of homogeneous bands varies from less than one meter 

 to a hundred meters. Among the pyroxene-bearing bands, however, 

 none was found thicker than some ten meters. 



A common feature of all the varieties of gneiss, of widely 

 different mineral composition, is the presence of very albitic 

 plagioclase (mostly about 90 per cent Ab) and epidote whos6 



' B. K. Emerson, U.S. Geol. Sure, Bull, i^g (1899), pp. 87 and 99. 



