308 GEOLOGY OF THE NARRAGANSETT BASIN. 



ROSE ISLAND. 



The main body of Rose Island, all except tlie narrow tongue projecting 

 northward, is composed of the greenish rock already mentioned as occurring 

 on the Gull Rocks anc at the south end of Coasters Harbor Island. The 

 strike, as heretofore, is approximately E.-W., the dip 15^ to 20^ N. 

 Some of the layers are coarser and resemble sandstone, while others are 

 tinted purplish, as at Gull Island hght-house. They are considered to be 

 of pre-Carboniferous age. 



North of the green series of rocks on the eastern side of Rose Island 

 arkose appears. It seems to strike north-south and dips almost vertically. 

 Farther north, forming the lunate extension of the tongue, are coaly sand- 

 stone and shale, with strike N. 25*^ E. and very steep dip, perhaps 70^ W. 

 On the western side arkose layers occur also southward toward the contact 

 with the green shales, green shales coming in contact with the arkose as 

 though brought together by a fault.^ Reference has already been made to 

 the arkose on Bishop Rock and Coasters Harbor Island. 



CONANICUT ISLAND. 



The arkose northwest of the granite area east of Mackei'el Cove has 

 already been described. Farther eastward, also north of the gi*anite, 

 occurs a pre-Carboniferous greenish rock, here called the Dumpling rock. 



LINE OF SEPARATION BETWEEN CARBONIFEROUS AND PRE-CARBONIFEROUS 



ROCKS. 



If now the northern line of outcrop of the western part of the granite 

 and the pre-Carboniferous greenish Dumpling rock farther eastward on 

 Conanicut be connected with the northern exposures of the green shales on 

 Rose Island, the most northern exposures of green shales on Gull Rock 

 Island, and the most northern of the green shales exposed along the southern 

 end of Coasters Harbor Island, east of the arkose exposure, then it will be 

 noticed that arkose occurs immediately north of this line at Mackerel Cove, 

 Conanicut, on the southern end of the tongue of land extending ptorth from 

 Rose Island, and at the southwest corner of the tongue of land at the south- 



Jprof. T. Helson Dale suggests tliat unconformities between the Cambrian and Carboniferous are 

 quite natural. There may be an unconformity here, but the abrupt linear contact between the arkose 

 and the green shale, with the very marked variance between their dips and strikes, seems to exceed 

 that of ordinary unconformities. There is no direct evidence that the Carboniferous arkose is resting 

 upon the eroded surface of the Cambrian shale. 



