72 GREEN MOUNTAINS IN MASSACHUSETTS. 



central mass of coarse granitoid gneiss (Stamford gneiss) forming the core 

 of Hoosac mountain; that this is flanked on either side by a band of the 

 white gneiss-conglomerate (Vermont formation), the eastern band having a 

 steady east dip and conformably overlain by the albite-schist series, the 

 western band being broader, with varying dips passing by gradual transitions 

 into the coarse gneiss, and bounded on the west by a narrow band of the 

 albite-schist (Hoosac schist); the contact between the two rocks being con- 

 formable and transitional. The schist band is succeeded on the west by another 

 band of fine-grained white gneiss (Vermont) and this in turn by limestone 

 (Stockbridge), no contacts being observed. We shall speak of this anti- 

 clinal structure further, after describing the geology of the surface of the 

 mountain. 



THE REGION EMBRACING THE CENTRAL PART OF HOOSAC MOUNTAIN. 



The map shows the distribution of the formations in this area. The 

 central part, forming the crest of the mountain, is occupied by a long irreg- 

 ularly oval area of the granitoid gneiss, the long axis of which runs nearly 

 north and south parallel to the trend of the mountain, with a length of 5 

 miles and a widtli at one place of 1\ miles. This is surrounded by a zone 

 of the white gneiss series (Vermont) about one-half mile wide, which at 

 the southern end of the granitoid gneiss core expands into a broad area of 

 white gneiss-quartzite, extending down to the southern border of the map. 

 To the east, tlie great expanse of the albite-sclhsts (Hoosac schist) borders 

 the zone of the white gneiss-conglomerate, running in an almost straight line 

 along the whole eastern edge of that formation to the southern edge of the 

 sheet. It circles around this formation to the north, forming the surface 

 rock in the whole northern part of the Hoosac mountain, and sends a long 

 narrow tongue down on the west side of the white gneiss zone, which bends 

 around with this at the southern end of the granitoid gneiss area, and 

 becomes gradually thinner until it can lie only doubtfully traced by loose 

 blocks at the extreme point of the curve. 



Lying west of this tongue of Hoosac, schist we have another area of 

 fine grained white gneisses or quartzites, with a variable width, which dis- 

 appear under the drift a little north of the tunnel, and at the south join 

 tin- LiT'-at mass of white gneiss at the southern end. 



