50 GEOLOGY OF THE NARRAGANSETT BASIN. 



the Narragansett Basin. The first stage of deposition in this field, when 

 the formation of the Carboniferous section began, is marked by the occur- 

 rence of fragmental beds, mainly arkoses. Where the succession is well 

 displayed, these arkoses are followed by conglomerates of no great magni- 

 tude. Succeeding these with tolerable uniformity comes a section of seA^eral 

 thousand feet where the deposits are prevailingly rather fine grained. In 

 the upper half of the section there is a return to conglomerate-making 

 conditions. This return appears to have been made rather suddenly. 



At first sight it seems reasonable to suppose that the succession of 

 events, as indicated in a general way by the section, gave (1) a pei'iod in 

 which the shore line was everywhere near to the margin of the present 

 basin, permitting the formation of arkose; (2) a period when a continued 

 depression kept the shore line always at a distance, resulting in the fine- 

 grained beds; and (3) a reelevation, which, by pushing the shores toward 

 the center of the field, led to the ready importation of coarse debris. It is 

 evident that there are many circumstances which serve to qualify the 

 interpretation which can be made as to this and other cycles of deposition. 

 Manifestly the intensity of erosion, as well as of the transportation of detritus 

 worn from the land, depends in large measure upon the ratio of the rainfall 

 at different times. As this ratio doubtless varies in different periods, or 

 even in different parts of the same period, the effect may be to produce 

 great alterations in the character of sediments brought to any particular field. 

 Moreover, along the coast line, when it is in a static condition, there is an 

 obvious tendency to produce a shelf, which, advancing from the shores of 

 the basin, may afford a slope over which, in time, coarse sediments may be 

 transported to a distance from the shore to which they could not at first 

 have attained. On these and other accounts it does not seem profitable 

 to attempt any conclusions based upon the succession of beds in this area. 



ARKOSE DEPOSITS OF THE BASIN. 



As the interpretation of much of the history of the Narragansett Basin 

 depends on the view that is taken of the arkose dejwsits which abundantly 

 occur at various points around its margin, and are, indeed, a characteristic 

 feature of that rim, it will be necessary to consider the significance of these 

 accumulations. 



Geologists who have had to deal with arkose deposits have generally 

 accepted the conclusion that they indicate the accumulation of detrital 



