338 GEOLOGY OF THE I^TAERAGAKSETT BASIK 



larity of the Dutch Island rocks and the Kingstown sandstone series may 

 be regarded as conclusive of their close geological relationsliip. 



The Dutch Island exposures must therefore be added to the sections 

 exposed on the mainland in order to form an estimate of the total thickness 

 of th Kingstown series in the southwestern part of the Narragansett Bay 

 region. This is discussed more fully in the third paragraph following. 



Lithoiogy of the Dutch Island series. — III gcuoral, tlio Strata of Dutcli Islaud may 

 be described as consisting chiefly of sandstones with subsidiary conglom- 

 erate layerS; underlain on the western side of the island by a series in 

 which' black, often very carbonaceous, shales predominate. Considering 

 the black color of some of the Bonnet shales, the Carbonaceous character 

 of the fern-bearing shales on Dutch Island is not unexpected. The con- 

 glomerate layers evidently contain larger pebbles than most of the conglom- 

 erates found high up in the series on the western side of the bay, but it is 

 evident that their length is largely due to stretching. The pebbles are also 

 less qiiartzitic than lower down. But it is the considerable abundance of 

 sandstones in the Dutch Island section that suggests relationship with the 

 Kingstown series. 



Beaver Head section. — At Beavcr Hcad, couglomcrate is exposed only on the 

 western border of the hill, near low-water mark. The main mass of the 

 hill is evidently composed of black shales, with intercalated subsidiary 

 thin sandstone beds and some arkose. The shales are decidedly carbona- 

 ceous. The Beaver Head section is evidently an introduction to the Aquid- 

 neck shale series as exposed farther eastward on the island, and seems to 

 have its likeness in the carbonaceous shales at the base of the shale series 

 on the western side of Prudence Island. 



Total thickness of the Kingstown series, including the conglonaerate at Beaver Head. Taking 40^ 



as an average of the eastward dips on Dutch Island, the thickness of the 

 Dutch Island section is about 1,050 feet; this, added to the unknown inter- 

 val of 975 to 1,200 feet occupied by the western passage (see bottom of pre- 

 ceding page) and the estimated thickness of 9,100 feet for the series as 

 exposed on the western .shores of the bay from the Bonnet to Hazzard's 

 quarry, Indian Comer, and southward (see pp. 333 and 334), would give 

 a thickness of from 11,125 to 11,350 feet for the whole series of Kingstown 

 sandstones, including Dutch Island. An additional thickness of 225 feet 

 would probably include the conglomerate layers exposed at low tide on the 

 western margin of Beaver Head, so that 11,500 feet would express, in round 



