26 GREEN MOUNTAINS IN MASSACHUSETTS. 



We have done but little work towards the study of this old core. A 

 valuable clew was found by Prof. Emerson in what he considers to be a 

 threefold division of the pre-Cambrian in southern Berkshire, where, 

 according' to his observations, chondroditic limestone separates a coarse, 

 blue quartz gneiss — possibly the Stamford granitoid — from a still older 

 gneiss. 



The massive granitoid gneiss which forms the core oil Hoosac and 

 Clarksburg mountains is in places separated from the overlying Cambrian 

 quartzite gneiss series by beds of coarse, light-colored gneisses, which have 

 interbedded layers of finer grain and darker color from the greater propor- 

 tion of biotite. These "transitional coarse gneisses" between the granitoid 

 and white gneisses are probably, to a great extent at least, Cambrian. 

 They are detrital, containing pebbles in places, as in the tunnel. Their 

 coarse feldspar is identical' with that of the granitoid gneiss, except that in 

 this transitional zone it is white, while in the granitoid it is reddish. While 

 the granitoid gneiss is preeminently a massive rock, this "transitional" zone 

 is bedded and contains micaceous layers. On the east side of the granitoid 

 area on the surface of Hoosac mountain it occupies the place of the quartz- 

 ite-white-gneiss-conglomerate and is overlaid conformably (as seen at the 

 contact) by the albite-schists. The granitoid gneiss was probably much 

 disintegrated at the time of the Cambrian transgression, and in the differ- 

 ent conditions of character of disintegrated material and of breaching and 

 sedimentation lies, perhaps, to a considerable extent, the explanation of 

 the fact that this horizon is here quartzite and there gneiss, and presents 

 itself under a great variety of aspects, due to alternating layers with vary- 

 ing proportions of quartz, feldspar, and mica. But in the field it is often 

 very difficult to distinguish, in the absence of true pebbles and of alter- 

 nating sediments, between the redeposited detritus of disintegration, which 

 has been subjected to the action of chemical and dynamic metamorphism, 

 on the one hand, and beds which, simulating these, have been produced 

 by the action of these same metamorphic agencies directly upon the older 

 gneisses, granites, or basic eruptives. 



I imagine that the Cambrian transgression found an Archean elevation 

 forming the western border of an Archean dry region. To the west of this 



