WAMSUTTA GROUP. 



145 



along - the northern border are of much importance in that they help to 

 define the stratigraphic position of the rock. It is here interbedded with 

 the red conglomerates and shales. 



Traces of this rock are seen at points eastward in the area under dis- 

 cussion. Bowlders of a similar rock bestrew the hillside where the North 

 River passes from the Carboniferous area into the region occupied by the 

 granitites. Here the sandstone is well bedded and alternates with bands of 

 pebbles and slate. This variety of sandstone along the northern border is 

 apparently much thinner than in North Attleboro. The probably volcanic 

 origin of this rock, in the form of ash, is referred to in the account of the 

 occurrences about North Attleboro. 



[Ill' Noli I II A I 1 l.Kl-.oIii ' UiEA. 



The most characteristic exposure of the Wamsutta group occurs as a 

 horseshoe-shaped area, open on the north, in the towns of Wrentham and 



Fig. 12 — Geological section northward from Robinson Hill. 



North Attleboro, Massachusetts, and Cumberland, Rhode Island. The 

 formation is a series of conglomerates, sandstones, shales, and calcareous 

 beds with associated felsites, felsite breccias, felsite agglomerates, and 

 diabases. A characteristic of the area is the very great thickness of 

 conglomerates. 



Beginning on the northeast, the formation makes its appearance about a 



mile northeast of the village of North Attleboro, in Robinson Hill (see fig. 12), 



an eminence which overlooks the valley excavated in the softer strata of the 



overlying Coal Measures extending westward from Mansfield. The section 



mon xxxiii 10 



