46 GREEN MOUNTAINS IN MASSACHUSETTS. 



effect of great pressure in crystalline rocks. The feldspars contain little 

 dull grains of quartz, black specks of mica, and crystals of magnetite, and 

 are often crossed by little branches from the layers of mica outside. At 

 the edges the feldspars often pass very irregularly into the quartz, which 

 then forms the narrow parts of the lens of which the feldspar forms the 

 center (" Augen" structure). 



The quartz is characteristically blue, but when crushed by pressure in 

 the rock is often white or sugary in appearance, the blue cores then rep- 

 resenting the uncrushed material. In other varieties of this rock it has 

 almost the structure of a coarse granite. The quartz is deep blue, the 

 feldspar colorless and in Carlsbad twins, and the mica layers black. The 

 gneissic structure is almost wanting. 



Certain other variations occur in the structure of the gneiss. In the bed 

 of Roaring brook, Stamford, Vermont, the gneiss on the weathered surface 

 has numerous rounded elliptical masses which by the absence of quartz 

 and scarcity of mica stand out by contrast with the rock as a whole, and 

 look like pebbles. They are composed of feldspar aggregates and flakes 

 and patches of biotite. The microscope shows that these feldspars are mi- 

 crocline with some plagioclase and perhaps orthoclase; the v have the gen- 

 eral structure of the gneiss itself, without the quartz, and are probably of 

 contemporaneous origin. West of Stamford village the rock contains Carls- 

 bad twins of microcline an inch or two across, which weather out from the 

 rock, become rounded by decay, and look like pebbles. 



The microscopic characters of this rock are quite uniform; the large 

 feldspars are generally microcline, 1 with whatever crystalline boundary they 

 may have once possessed obliterated by the great mechanical changes thev 

 have undergone. The crystals are often faulted and the edges crushed; 

 little veins of secondary quartz, mixed with little grains or crystals of an 

 unstriated feldspar (albite?) traverse them along the fault lines. (See PI. 

 vn, B.) With a low power the feldspar substance appears cloudy, owing to 

 fluid cavities and little prisms of epidote in great numbers. These epidote 

 grains are sometimes arranged parallel to the twinning planes of the feld- 



1 In the "Geology of Vermont," vol. 2, p. 561, there is an analysis of the feldspar from the Stam- 

 ford granite, according to h nich it contains from 61 to liOi percent silica, 10 to 11 percent potash, and 

 2 to :u per ceni soda. 



