350 GEOLOGY OF THE NAERAGANSETT BASIN. 



PRUDENCE ISLAND. 



In the case of the coaly shales on the western side of Prudence Island, 

 it is equally probable that these shales indicate the beginning of the great 

 Aquidneck shale series immediately overlying them. On the western side 

 of Prudence Island the Aquidneck shales proper are of a dark-blue color, 

 like those on Conanicut and Aquidneck islands. The few exposures above 

 the sandstone series a mile north of Prudence Park suggest that sandstone 

 layers are, however, a more common element in the shale series here than on 

 Conanicut. The same observation also applies to the exposures south and 

 southeast of Potters Hotel (which is located on the highest point of the 

 island), where sandstones and even some thin conglomerate layers occur. 

 Color banding is not seen on the western side of Prudence Island, but on the 

 eastern side, about two-thirds of a mile north of the light-house on Sand Point, 

 color-banded shales like those on Conanicut are well exposed (PI. XXII). 

 South of the light-house the shales are sericitic, and not so fissile, or at least 

 usually not partially separated along the cleavage into thin plates, as they are 

 elsewhere, yet it is evident that these shales of eastern Prudence Island could 

 also be readily split parallel to the cleavage produced by the abundant pres- 

 ence of this micaceous mineral. An unusual featin^e is the frequent presence 

 south of the light-house of sandy courses, from 8 to 12 and even 20 inches 

 in thickness, some of which merge into thin conglomerate beds. One or two 

 of the pebbly beds attain a thickness of 2 or 3 feet and constitute a genuine 

 conglomerate. The color of the shale on this eastern side of Prudence Island 

 is more inclined toward the greenish hue of the shales at Eastons Point 

 than tow^ard the dark blue so common on Conanicut Island. A few more 

 carbonaceous shale layers occur intercalated among the greenish beds 



The eastern Prudence Island exposures south of the light-house there- 

 fore present more frequent variations from the dark-blue color and fissile 

 structure of the Aquidneck shales, and show more frequent sandstone and 

 conglomeratic layers, than do these shales on southern Conanicut or southern 

 Aquidneck. Sandstones and thin conglomerate beds are fairly frequent, 

 however, on Aquidneck or Rhode Island, directly east of the Prudence 

 Island exposures just described; also in the area from Coggeshall Point to 

 McCurrys Point and thence northward to within half a mile of Butts Hill. 

 These facts suggest an increase in the amount of sandstone, and even the 



