162 



GEOLOGY OF THE NAKRAGANSETT BASIN. 



Pawtucket shales,- 



-Still farther northward in Pawtucket the shales are well 

 exposed in the banks of the Blackstone River on Division street, and at 

 Valley Falls they are exploited in a graphite mine. 



The best natural exposure is along the eastward bend of the Black- 

 stone River between Central Falls and Valley Falls, where, on the south 

 bank of the river, a section of bluish and black carbonaceous shales, with 

 fossils, is thrown into a broken fold with minor contortions (see fig. 18). 

 The strike here is approximately east and west, and the dips are steep, 

 mainly 80^ N. The beds evidently overlie the grits and red series half a 

 mile south. The slate layers contain distorted and disjointed fragments of 

 plants. Furtlier evidence of the movement which the rock has under- 



Fi&. 18.— Folded and faulted Carboniferous shales on the Blackstone River at Pawtucket, Rhode Island. (Looking east.> 



gone is shown in minute joints, in antiparallel sets, accompanying small 

 puckerings of the slate. Along each joint plane there has been a minute 

 fault movement of the normal kind. 



The thickness of the beds included under the term Pawtucket shales,. 

 if we place here all the soft beds in the troughs so far described, can not be 

 safely stated. If the structure about Providence is due to the duplication 

 of beds by overturned folds, as indicated in the theoretical section (fig. 17, 

 p. 160), and since there are at least 3,500 feet of beds between the sandstones 

 of the westei^n or Sockanosset ridge and the base or western boundary, the 

 apparently great thickness of beds in the other valleys may be readily 



