THICKNESS OF AQUIDNECK SHALES OF PRUDENCE. 351 



introduction of conglomerate, in the lower part of the Aquidneck shales on 

 going- northward. 



THICKNESS OF THE SHALE SERIES ON EACH SIDE OP THE PRUDENCE ISLAND 



SYNCLINE. 



The structure of Prudence Island is a syncline, the sides dipping 20° 

 to 25° E. on the west side (PL XXIII), and 70° to 80° W. on the east side. 

 The bottom of the syncline appears to be at Potters Hotel or to the west- 

 ward. The thickness of shales forming the western side of the syncline as 

 far as exposed may be 2,200 feet. The thickness included in the exposures 

 forming the eastern side of the syncline appears with more certainty to be 

 at least 1,750 feet. In that case the lower 450 feet exposed on the western 

 side of the syncline may not be exposed on the. eastern side of the island. 

 Since these estimates are not based on a continuous series of exposures in 

 an east-west direction, there being no exposures for almost a mile west of 

 Potters Hotel, the figures given can not have great value, although they 

 serve to give at least some notion as to the probable minimum thickness of 

 the shale formation. The sandstones and conglomerates on the western 

 side of Prudence Island north of the wharf being assigned to the Kingstown 

 series, it is evident that the Aquidneck shale series as exposed on this side 

 of the island overlies the Kingstown series. For this reason the western 

 part of the Prudence Island section is believed to furnish evidence in favor 

 of the more recent age of the Aquidneck shale as compared with the Kings- 

 town sandstones. This corroborates the evidence furnished by the Beaver 

 Head section on Conanicut. 



BRISTOL NECK. 



The greenish, dark-blue, and greenish-blue shales of Bristol Neck are 

 known to dip low eastward on the western side of the neck. If the sand- 

 stone and conglomerate beneath the shales on the west belong to the 

 Kingstown sandstones, the more recent age of the Aquidneck shales is 

 here indicated once again. A sandstone layer, becoming conglomeratic 

 near the Warren-Bristol road, occurs in the shale series about 400 feet 

 above the coarse conglomerate layer at the supposed base of the shales, 

 and thus shows the existence of occasional sandy and conglomeratic layers 

 in the Aquidneck shale series northward. Sandstone layers are not known 



