8 GEOLOGY OF THE NARRAGANSETT BASIN. 



The north-south extension of the Narragansett Basin, including the 

 related area of the Norfolk Basin, the axis of its greatest length, is from 

 the southern portion of Narragansett Bay to near Walpole, a distance of 

 about 50 miles. The east-west diameter, from the western part of Cum- 

 berland, Rhode Island, to the town of Scituate, Massachusetts, is about 

 30 miles. Althoug'h its outline has many irregularities, which will be 

 hereafter described, the basin has in general a rudely curved form, concave 

 on the southeastern side. The sections given in a later chapter of this 

 report show that this trough has great depth, the lowest stratified beds 

 disclosed on the margins possibly attaining in its central portions a level 

 of from 10,000 to 15,000 feet below the plane of the sea. The sections 

 also indicate that the correlative anticlines, at least those in the western 

 and central parts of the field, probably had in their original form an eleva- 

 tion comparable in amount to the depression of the great trough which 

 they inclose. In a word, the facts indicate that the mountain-building 

 work effected in this district, and altering the original reliefs, was consider- 

 ably greater, and gave rise to sharper foldings, than in the more interior 

 parts of the eastern coast of North America, where the elevations still 

 retain the mountainous character. 



An examination of the structure and attitude of the rocks in this 

 basin, as will be shown in a detailed way in the later sections of this report, 

 indicates that this region originally contained an extensively developed 

 series of pre-Cambrian rocks, the age of which is not yet determinable. 

 They may for convenience be referred to that limbo of ill-discriminated 

 formations, the upper Archean (of Dana), or Algonkian. Above and 

 probably upon the eroded surfaces of these ancient strata, known in this 

 report as the Blackstone series, there lie, apparently in detached, much 

 worn patches, considerable remnants of the Olenellus horizon, or the lower- 

 most stage of the Cambrian. On top of this formation and the granites 

 which have broken through it, which were in turn much degraded, come 

 the Carboniferous beds, strata which, owing in part to their great thickness 

 and in part to their having escaped the nearly complete destruction which 

 overtook the lower-lying beds, now occupy the greater part of the basin. 



The evidence indicates that, on the western border of the basin at 

 least, the margin of the field was determined before the beginning of Cam- 

 brian time. At the beginning of the Carboniferous, there is proof that 



