86 GREEN MOUNTAINS IN MASSACHUSETTS. 



their base to the base of the schist they correspond to a thickness of 450 

 feet but on the theory of duplication to only half that aim unit, having the 

 fine-grained banded character of this western area of gneisses. These cliffs 

 strike along east with the same strike and dip. The profile of Hoosac 

 mountain seen from a distance shows plainly the step-like series of ter- 

 races, sloping gently northward, which correspond to these beds of gneiss. 

 (See PI. v, Profile xt.) Following this band of white gneiss east, at about 

 one-half mile from the point of the mountain the strike has turned to north 

 75° east. One-quarter mile farther there are again cliffs of this rock strik- 

 ing nearly east and west and dipping north; the schist overlies here again. 

 Beyond this point it is no longer possible to separate this band of gneiss 

 from that band nearest the granitoid gneiss; they merge together, after the 

 band of schist has thinned out, in the great area of contorted white gneiss 

 in the southern part of the field. 



NORTHERN AND EASTERN SCHIST AREA. 



It will be seen that the whole northern third of the region, and a broad 

 strip along the east, is occupied by the albite schist, with commonly an 

 easterly dip and north to south strike. It will be noticed that there are 

 changes in the dip to the north; (in the line of the axis of the mountain the 

 dip is north, but there is in general great uniformity, as there is in the case 

 of this rock in the tunnel. Of course this steady dip does not mean a true 

 monocline, but rather a series of folds overthrown to the west and eroded. 

 No attempt has been made in the field to unravel the more minute details 

 of this structure; this was done only in important places, where the relations 

 of the other rocks require it. It is also- possible that troughs of the over- 

 lying Rowe schist occur in this northern area, but the facts have not been 

 definitely ascertained. The quartz lenses and layers, so abundant in the 

 schist, are found to lie always parallel to the bedding at contacts with other 

 nocks of the series, where the alternation of material shows which is the 

 plane of stratification, and hence these lenses can be provisionally accepted 

 as indications of stratification elsewhere, when, as is often the case, the rock 

 lias ;i marked transverse cleavage. 1 In the vicinity of Spruce hill the schist 



1 On the Greylock side cleavage lamination and stratification in the schists have heen carefully 

 distinguished by Mr. Dale. 



