168 



GEOLOGY OF THE NARRAGANSETT BASIN. 



Fossils. — North of the station black shales contain impressions of Sphe- 

 nophyllum schlotheimii, both alone and with Asterophyllites equisetiformis. 

 Pecopteris, smaller than P. unita, also occurs here. In the mudstones on 

 the bluff calamites are abundant. 



In the rocky points and islets along the shore near Silver Spring the 

 dips steepen to 45°, rising to an anticline arching over the upper narrow 

 end of Narragansett Bay — the southward continuation of the fold which is 

 more clearly indicated by the outcrops on the sides of the Seekonk River. 

 The southward extension of this anticline is not readily traceable. The 

 sandstones and conglomerates reappear in the ledge at the present mouth 



of the Pawtuxet River, and 

 again at Rocky Point, on the 

 west side of the bay. 



It is evident from a diag- 

 nosis of the dips in Provi- 

 dence and East Providence 

 that the strata in this section, 

 from the western boundary 

 eastward into the middle of 

 the basin, behave very much 

 in the manner of the layers 

 under horizontal pressure in 

 the clay models experimented upon by Mr. Bailey Willis. 1 The general 

 structure of a cross section from back of Providence southeastward to 

 Riverside is represented in the accompanying diagram, fig. 23. It will be 

 noticed that there is a belt of very highly tilted strata next the resisting 

 pre-Carboniferous terrane, in which the effects of great pressure are mani- 

 fest also by the degree of metamorphism ; thence eastward lies a belt of 

 little-disturbed strata without any marked metamorphism or even slaty 

 cleavage ; there come in then subordinate anticlines with a slight amount of 

 rock crushing. These two anticlinal axes correspond to what Mr. Willis 

 terms consequent and subsequent anticlines, respectively. 



It is a noticeable feature of many of the folds in this part of the basin 

 that the inclination of the strata increases to a maximum near the axis. 



FlQ. 23.— Theoretical section of folded structure on western margin of the 

 Narragansett BaBin. a, Providence anticlinal belt ; 6, East Providence 

 flat eyncline; c, Poraham Rock anticline. (See PI. LXXXII, figs, f-i, 

 Thirteenth Ann. Kept. U. S. Geol. Survey, Part II.) 



1 Tbe mechanics of Appalachian structure: Thirteenth Ann. Rept. IT. S. Geol. Survey, Part II, 

 1893, pp. 211-289. 



