HOOSAC MOUNTAIN. 99 



The .summit of ( llarksburg mountain is composed of a mass of granitoid 

 gneiss (Stamford gneiss) identical in petrographic characters with that of 

 the Hoosac tunnel (Stamford granite). This is overlain by the Clarksburg 

 quartzite (Vermont formation) on the west and south sides, and by quartzites 

 and gneisses on the east side, the contacts having been found. In this 

 quartzite Mr. Walcott has found the remains of trilobites, making it Lower 

 Cambrian, and we shall now endeavor to show that this is represented by 

 the gneiss found on the east side of the mountain. 



Near the old signal station on Clarksburg mountain the quartzite is 

 represented at the immediate contact by a blue quartz pebble conglomerate, 

 quite micaceous, the pebbles composed of aggregate quartz. Some distance 

 above the contact the quartzite contains beds of a quartz schist of consid- 

 erable thickness. The quartzite and conglomerate are found within 2 or 3 

 feet of strata of each other, the quartzite striking on the average about north 

 33° west, and dipping 25° southwest. The granitoid gneiss in part has 

 little structure, hut in several places this feature is well marked by the mica 

 planes, which are in general parallel both in strike and dip to those of the 

 quartzite, so that in so far as we can accept as stratification such structural 

 planes in the gneiss, the two rocks are parallel. From this place, on the 

 northwest edge of the mountain, the line of contact, curving gently, runs to 

 the southeast brow of the mountain above North Adams, where it turns 

 and strikes northeast. The contact here between the two rocks is very 

 close, and the structure of the granitoid gneiss obscure. The rock is massive. 

 The quartzite strikes north 30° east, dips 40° southeast. The line of contact 

 across the mountain can be traced in a general way, hut no outcrops near 

 together have been found. 



The whole south slope of the mountain down to the valley is covered 

 with the quartzite and the interbanded quartz schist. The southwest dip is 

 well marked above Williamstown, while on the North Adams side it is south- 

 east. This mountain is a large quartzite dome, doubtless with many minor 

 crumples. This quartzite is found as low down as the river hank opposite 

 the cemetery in North Adams. It is last seen in contact with the granitoid 

 gneiss at the place mentioned above, hut it is thence eroded away to the 

 north for a. distance of 2i miles, in which drift covers the valley and lower 

 slopes of the mountain, the granitoid gneiss occupying the crest. 



