ORIGIN OF SEDIMENTS. 35 



ing- differ from those of normal mountains. In the last-named group, as 

 has been often remarked, the streams very generally come to occupy 

 the geological highlands — the crests of the anticlines. The reason of the 

 departure from the general rule in these antecedent basin mountains is that 

 in them the rim is of hard rock, while the central portion of the area con- 

 tains softer materials. 



It should furthermore be noted that the downsinkings which lowered 

 these valleys, and thus afforded the opportunity for deposition, were not 

 generally so extensive as to induce the formation of normal marine deposits. 

 After the beginning of the Carboniferous, indeed, it ma}- be doubted whether 

 the submergence beneath the level of the sea was, until Cretaceous times, 

 ever great enough to mantle far over the surface of the country. Apart 

 from the local downsinkings, I see no reason to believe that the shore 

 within this time has swayed downward or upward more than a few hundred 

 feet. 



So far as the examination has been carried, the seat of origin of the 

 detrital materials contained in the Narragansett Basin is tolerably well 

 explained. The granitic, trappean, schistose, and other rocks represented 

 in the conglomerates, with a single exception, may be paralleled from 

 deposits the like of which are known within a few miles of the margin of 

 the basin. The exception — a most notable one — is in the case of certain 

 quartzite pebbles, sometimes containing an abundance of ill-preserved 

 brachiopods. These quartzites are all fine grained, hardened, but not 

 greatly metamorphosed, and of a hue varying- from blue to white. The 

 age of the material, as determined by Walcott, is that of the Potsdam 

 sandstone. 



Pebbles of these quartzites plentifully occur in the upper conglomerates 

 of the Narragansett series, as is elsewhere noted in this memoir. They are, 

 however, best known from their occurrence in the drift deposits, where their . 

 presence is doubtless to be explained by the breaking up of the Carbon- 

 iferous beds in which they formerly lay. On the northern shore of Marthas 

 Vineyard they can readify be gathered to the number of many thousands, 

 and on Cape Cod they occur less plentifully as far east as Highland Light. 



As the pebbles, so far as observed, are always small, never exceed- 

 ing about a foot in diameter, and as the)- are always rounded in a sub- 

 spherical form, it seems clear that none of them have been brought into 



