140 GEOLOGY OF THE NARRAGANSETT BASIN. 



SUPRABASAL CONGLOMERATES. 



The conglomerates which occur near the base along the margin of the 

 basin are not precisely basal in the sense in which that word finds its most 

 exact use. At a few points conglomerates actually rest upon the basement 

 floor of granite, but they more usually overlie beds of arkose. The reason 

 for this order of deposits lies in the fact, before noted, that the granitic land 

 area was, in the beginning of Carboniferous erosion and deposition in this 

 field, so decayed at surface that pebble making did not go on until the layer 

 of disintegrated granite was stripped off. It is in full accord with this chem- 

 ical preparation of the sediments that we find the first conglomerates prevail- 

 ingly quartzose and composed of quartzite — rocks which do not yield to 

 atmospheric decay so readily as the granite. On account of this sequence 

 in the deposition — first arkose, and then conglomerate — the term swpra- 

 basal conglomerate expresses more exactly than basal conglomerate the 

 nature of the inferior pebble beds in the Narragansett Basin. Of these 

 suprabasal conglomerates there are a few noteworthy exposiires believed 

 to be approximately on the same horizon. The beds are often composed 

 of quartz pebbles, the remnants of veins in the granitic border, the latter 

 rock occurring less abundantly than in the higher conglomerates. These 

 beds form the closest analogy to the typical Millstone grit which occurs in 

 the basin, and the following locality, being typical, has been chosen for the 

 type, although the relations to the arkose and the basement rocks are better 

 shown elsewhere. 



Miners River conglomerate. — In the valley of Millers River, in Cumberland, 

 Rhode Island, there is a broad exposure of conglomerate beds underlying the 

 red Wamsutta series. In this gray series there are three or four thick beds 

 of conglomerate with small quartz and quartzite pebbles. The best section 

 is exposed on the farm of Mr. James A. Miller. The thickness is here 

 unusually great, being as much as 300 to 400 feet. 



south Attieboro exposure. — Between South Attleboro and Lanesville, in the 

 triangular area between the southern end of the great horseshoe fold of the 

 Wamsutta and the Pawtucket area of these rocks, is an exposure of hard 

 quartzose conglomerate, with quartz veins. These beds evidently underlie 

 the Wamsutta, which once arched over them. They are probably con- 

 tinuous with the Millers River outcrops on the northwest, though that 



