HOOSAC MOUNTAIN. 



45 



THE STAMFORD GNEISS. 



The basement rock is a coarse granitoid gneiss, which forms the core 

 of Hoosae mountain proper, occupying' the surface of the mountain for 

 several miles, then disappearing below the overlying rock, but cut in 

 Hoosae tunnel for nearly 5,000 feet; hence this rock figures prominently 

 on the dumps of the tunnel shafts. Another area of the same rock under- 

 lies the fossiliferous Cambrian quartzite of Clarksburg mountain, north of 

 Williamstown, continuing some miles northward into Vermont — the "Stam- 

 ford granite" of the Vermont geological report. 



Fig. 12. — Granitoid gneiss (Stamford gneiss), from dump Central shaft. Natural size. 



This is the variety with a well-marked gneissoid structure. The dark streaks are composed of the micas inclosing 

 irregularly lenticular areas of feldspar and quartz. 



In its most typical form the rock is a coai-se banded gneiss (see Fig. 12), 

 composed of long lenticular crystals of pinkish feldspar, flattened lenses of 

 blue quartz, and thin, irregular, greenish layers of a micaceous element 

 (biotite or muscovite, or both) mixed with small epidote crystals, which 

 cause in part the greenish color. We notice at once that the broad cleav- 

 ages of the feldspar often do not reflect as one surface, but as a num- 

 ber of little disconnected areas, which are often curved — a well-known 



