BOUNDARY OF THE B ASIK 1 3 I 



North Attieboro mher. — In the Nortli Attleboro inlier (see PL XXIX, in Part 

 III), actual contacts of the Carboniferous with the subjacent Cambrian and 

 granitite are not exposed^ but the evidence points to the unconformable 

 relation of thi:; Carboniferous and Cambrian strata. 



Namasket granitite area — Homblende-granititc, similar to the rock of the 

 northern border of the basin, appears in a low outcrop about 60 feet above 

 the sea in the village of Namasket, in the town of Middleboro. The expo- 

 sure by the roadside is upward of 200 feet in length, and is at a distance of 

 8^ miles from the nearest granitite outcrops to the southeastward. The 

 nearest visible stratified rocks are 2 miles south, and nothing is known 

 regarding the contact. It is not at all improbable, as above stated, that the 

 exposure of granitite at this point marks an anticlinal structure in the Car- 

 boniferous by which a tongue of the granitite protrudes into the basin from 

 the eastern border, as does the long, broad area from Hingham westward to 

 Sheldonville, separating the Narragansett from the Norfolk County Basin. 



SUMMARY. 



The boundary in this northern part of the field appears to be mainly 

 one of downfolded beds where the line extends east and west. Down- 

 faulting is suggested only where the line runs north and south. Corre- 

 lated with this evidence is the presence of arkose along the east-west 

 borders, and its almost complete absence along north-south lines in the 

 upper part of the basin. Thebo faults are parts of a series which appear on 

 the east-west boundary lines as cross faults. 



The type of fault crossing the boundary of the basin is repeated a 

 number of times along the northern border. These cross faults along the 

 northern margin do not clearly arise out of the physical conditions engen- 

 dered at the contact of two terranes so unequally acted upon by stress, but 

 rather they are regional dislocations arising in the pre-Carboniferous ter- 

 rane, their existence being clearly brought out by the rectangular notches 

 which they introduce into the boundary line. Outcrops are wanting to 

 show how far these faults affect the beds in the basin above the lowermost 

 strata. Even if the faults were limited to the granitite and the beds imme- 

 diately at the base, we should expect to find a local deflection of the strike, 

 in the form of a flexure, in the direction of the offset at some distance 

 beyond the margin of the fault plane. 



