THICKNESS OF SHALES OF AQITIDNECK ISLAND. 353 



half a mile south of Lawtons Valley, and a series of exposures containing 

 sandstone extends (4) from the valley east of Coggeshall Point for at least 

 half a mile northward. Sandstone layers are also exposed (5) along- the 

 middle third of the coast both on the east and on the west side of the 

 island, and a bed is also located (6) in the lower part of the Glen valley. 

 Sandstone occurs rather frequently in the shale series (7) for a mile south 

 of Coggeshall Point, and both sandstone and small-pebbled conglomerate 

 layers occur (8) along the eastern shore from McCurrvs Point to the expo- 

 sures east of Butts Hill. 



THICKNESS OF THE SHALE SECTION EAST OF THE PORTSMOUTH 

 SYNCLIjVE. 



The geological structure of the shale area over the northern third of 

 Aquidneck Island is evidently that of a syncline, the base of the syncline 

 passing beneath Butts Hill and extending a little west of south toward the 

 west of Quaker Hill. The dip on the eastern shore of the island varies 

 between 20° and 40° W., and at the mine northeast of Butts Hill it is 

 about 20° W. According to this the coal seam opened at the eastern 

 coal mine could hardly be more than 500 or 570 feet above the shore 

 exposures along the northern third of Aquidneck, while the coarse con- 

 glomerate south of Butts Hill, and elsewhere at the same level in the 

 syncline, can hardly be more than 500 to 650 feet higher. According to 

 this the shale section exposed on the east side of the syncline probably 

 averages somewhere between 1,000 and 1,220 feet, but may be a little less 

 than 1,000 feet in thickness, and might possibly exceed 1,300 feet. 



PROBABLE THICKNESS OF THE SHALE SECTION WEST OF THE 

 PORTSMOUTH SYXCLIXE. 



Along the shore north of Coggeshall Point the dip appears to be low 

 to the east, usually not exceeding 20°. Farther east the dips are steeper to 

 the east, varying most frequently between 45° and 60° E. According 

 to this the coarse sandstone layer on the brow of the hill east of Cogges- 

 hall Point lies about 1,300 to 1,500 feet beneath the level of the coarse 

 conglomerate; the shore exposures lie between 1,300 and 1,500 feet lower, 

 and the western mine exposures occur apparently at a still lower level, per- 

 haps 400 to 500 feet beneath. According to these estimates the western 

 mon xxxiii 23 



