92 GREEN MOUNTAINS IN MASSACHUSETTS. 



First. In the west part of the, area, between Dry brook and the curve 

 north (about 2 miles from north to south and 1 mile wide), there is a quite 

 steady strike about north 50° west and moderate northerly dip; a perfect 

 nionoelinal structure. 



Second. In the belt east of this, 1 mile or more wide (on the map the 

 central area of flat summits), the gneisses are greatly curved and twisted. 



Third. In the licit extending from the previous one to the border of 

 the schists the normal north to south strike occurs with predominating east- 

 ern dips, as in the schists! 



This east and west strike and monoclinal north dip was a matter diffi- 

 cult of explanation, as there appeared to be a great series of gneisses and 

 epiartzites, thousands of feet in thickness, underlying the series of the north- 

 ern part of Hoosac mountain. It was not until the white gneiss-conglom- 

 erate and schist tongue had been traced around the core of the granitoid 

 gneiss, and it had become evident that there was an underturn of these 

 rocks, and that they were really geologically above the granitoid gneiss, as 

 in their normal position in the region of the tunnel, that it was possible to 

 explain the monoclinal dip of the gneisses further south. It is now believed 

 that this is due to a series of east-west transverse crinkles, pushed under 

 and collapsed from the south, so that there is a constant duplication of 

 strata in an apparent conformable series. One proof of this theory is the 

 fact that we find the actual connection between two adjacent layers of the 

 monoclinal series in several cases on the west bi'ow of the mountain. 



In one case a band of the gneiss having the schist both north and south 

 of it was traced continuously along the strike for a half mile. It gradually 

 turned to a northerly direction, the schist closely following, and then came 

 to an end, the gneiss terminating in a small crumpled outcrop and the schist 

 each side circling around and joining. The zone nearer the schist on the 

 east, with general north and south strike and easterly dip, must represent a 

 large series of similar north and south folds overturned to the west, and 

 the areas of extremely crumpled gneiss between the two represent the 

 turning pointwhere the east and west folds are twisted around to the north 

 and south direction. 



In the following details the reader should refer to the map (PI. i), on 



