HOOSAC MOUNTAIN. 83 



uniform and peculiar character; it belongs to the fine grained porphyritic 

 gneiss already described and has a tendency to pass into micaceous quartz- 

 ite or even pure quartzite. 



The first exposure found is on the side of the mountain about 1 mile 

 north of the tunnel line, where it is within a fow feet of the albite-schist, 

 which here extends up the mountain. Both rocks are conformable, strike 

 north 30° east, dip 60° east; to the north the rock is covered with glacial 

 drift, so that it is uncertain where it finally disappears, but the two bands 

 of albite-schist come close together both east and west of it. This rock 

 shows a remarkable tendency to disintegrate. This "rotten gneiss" caused 

 great expense and loss of time in building the western part of the tunnel. 

 At the tunnel line outcrops of this rock are found on the surface at the 

 west shaft and on the mountain above for over 100 feet, when they are suc- 

 ceeded by the schist: but transitional rocks made it impossible to draw a 

 line. Toward the west edge of this gneiss band, a few hundred yards north 

 of the tunnel line, an old iron mine alongside the road is composed of a 

 massive quartzite containing masses of limonitic iron ore, the structure of 

 which is not determinable. This gneiss was also found in the tunnel at 

 several manholes, and in the creek just south of the tunnel line we find 

 several outcrops of this rock as indicated on the map, all striking about 

 north "20° east and dipping east at varying angles. Also a few hundred 

 yards south of the portal of the tunnel we have an outcrop the strike of 

 which would carry it very close to the portal. 



When we come to the sharp little hill of Profile iv" ("the Buttress") 

 we have fine exposures of this gneiss (see Plate v). It is plain, from this 

 section, that in this band of gneiss we have considerable folding. < hie 

 sharp anticlinal is plainly shown hen- with many smaller crumples. There 

 are several hundred feet of covered space between the western outcrop of 

 gneiss and easternmost of limestone, but the contact with the schist is very 

 close. The folds of this gneiss have a strong northerly pitch of as much 

 as 10° in Profile rv°. 



From Profile iv a for ljf miles to Profile vn we have only two or three 

 scattering outcrops of this rock (see PI. v). At Profile vn it is represented 

 by one outcrop of micaceous quartzite closely underlying and conformable 



