288 GEOLOGY -OF THE NARRAGANSETT BASIN. 



origin, but they do not resemble any known rock in the Carboniferous basin. 

 Fragments of a reddish rock, often resembling jasper, occur here and there. 

 In the fifth place, the absence of granite in the form of pebbles is very 

 noticeable. 



In several of these particulars the greenish Sachuest conglomeratic rock 

 resembles the more northern exposures of the Dumpling rock on the eastern 

 shore of Conanicut, south of Jamestown. There seems to be no question 

 of the pre-Carboniferous character of the greenish rock, both shale and 

 conglomerate. The absence of all granite fragments from the green series 

 suggests that the granite is intrusive and therefore of later origin. The 

 nearest granite exposures of any size are now at Cormorant Rock, but the 

 exposure just beneath the arkose series suggests that it once reached as far as 

 Sachuest Neck. The ai'kose series probably once rested upon the green 

 series, a relation now obscured by faulting. It derived its materials, at least 

 the coarser part, from the granite, which is therefore pre-Carboniferous. The 

 grit is Carboniferous, as is shown by the interbedded coaly shale layers 

 containing fossils. The interbedding of coaly fossiliferous shales with the 

 grit seems to be characteristic of exposures along the eastern margin of the 

 Narragansett Basin, several such occurrences being known at Steep Brook, 

 Fall River, and Tiverton. The relative age of this eastern arkose as com- 

 pared with that of the Aquidneck shale series can not be determined at 

 present. 



EASTERN SHORE OF AQUIDNECK ISLAND AS FAR SOUTH AS THE 

 SECOND COVE NORTHWEST OF BLACK POINT. 



The most northern exposure of Carboniferous rock on Aquidneck 

 Island on the east shore occurs southwest of the cove, near the eastern 

 base of Butts Hill. Coaly shale and sandstone are interbedded, being 

 several times repeated southward for half a mile, as far as the wharf east 

 of Portsmouth village. North of the wharf a short distance there is evidence 

 of a low anticline, pitching southward, the dip being low on both sides. 

 The exposures at the wharf and southward belong to the western side of 

 this anticline, and therefore all dip westward. At the wharf there is sand- 

 stone on the north side, showing a cleavage dipping 60° W. Overlying 

 the sandstone on the south side is a thin layer of conglomerate dipping 

 about 10° E. But south along the shore exposures of conglomerate 

 and dark coaly shales strike N. 3° E., dip 45° W. Southward sandstone 



