HOOSAC MOUNTAIN. 75 



covered by our Profile ix, PI. v, (which is plotted from a stadia section, 

 checked by triangulation) it will be seen that there is nearly 800 feet of the 

 albite-schist to the sunmiit of Spruce hill, where the section stops. The 

 schist preserves its gentle northerly dip throughout, with a quite uniform 

 character, often very rich in the albite crystals. 



The profile we have just described gives us the key and starting- point 

 for the geology of Hoosac mountain. As will be seen later, this profile is 

 taken at the northern end of the overturned anticlinal axis of Hoosac moun- 

 tain, tha whole axis having this gentle pitch or plunge to the north which 

 causes the dip of 15° to 20° northerly. The granitoid gneiss disappears 

 at the surface here and is found again in the center of the Hoosac tunnel 

 in the same meridian line, but 1,400 feet lower in level. Although the north- 

 erly pitch of the axis has here brought the top of the arch of the granitoid 

 gneiss far below the surface, enough of the arch remains above the tunnel 

 to allow a Length of several thousand feet of the excavation to lie in this 

 rock. (See Profile x, PI. v.) 



Now going - back to the contact of granitoid gneiss and gneiss-con- 

 glomerate at the south end of Profile ix, PI. v, and tracing the contact of 

 the two rocks westward, in a few hundred feet we come to the extreme west 

 cnst of Hoosac mountain overlooking the valley (see PI. iv). Here we find 

 the continuation of the two rocks in contact again with the same strike and 

 dip. The granitoid gneiss runs a hundred yards north and then disappears; 

 the white gneiss-conglomerate makes a sharp turn over the prong of the other 

 rock and comes in on the west side of it, on the slope of the mountain; the 

 white gneiss strikes north 40° east and dips 50° west, instead of striking north 

 67° west and dipping northeast. The manner in which one rock mantles 

 over the other can be seen very plainly; at the turn they are within 20 

 feet of each other. The successive outcrops of the white gneiss on lines 

 radiating out from this point of the turn show the same curving - around of 

 the outcrops from a northwest to a northeast strike. 



Following in the same way the top of the conglomerate towards the 

 west, we find it strikes northwest until the extreme west crest of the moun- 

 tain is reached, closely overlain by the Hoosac schist; the outcrops then 

 suddenly turn and descend the slope of the mountain obliquely in a north- 



