1(3 GEOLOGY OF THE NARRAGANSE'JLT BASIN. 



have been orographically deepened by the suosidence caused by sedimenta- 

 tion. It will be seen that this view has a certain superficial resemblance to 

 the hypothesis commonly known as James Hall's, in which mountains are 

 explained by supposing', first, the accumulation of a thick series of beds; 

 second, a subsidence of the crust, due to the deposits, bringing about a 

 folding of the beds; third, a massive uplifting of the foundation on which 

 these foldings rest, so that the ridges come to stand on a lofty pedestal. I 

 would not have it supposed that there is any real similarity between these 

 hypotheses. The essential difficulty of the hypothesis which endeavors to 

 explain the formation of mountains first by subsidence and then by eleva- 

 tion, is that there is no sufficient means indicated whereby the reelevating 

 process can be brought about. There is also much reason to question 

 whether the downsinking movement could develop the arches of the strata. 

 In the view which I am advocating, the conceptions are much more simple 

 and rest upon more patent facts. The steps of action which are postulated 

 are as follows: First, the excavation in ancient and compact rocks, in their 

 nature good transmitters of thrusts, of a trough or basin such as is likely 

 to be formed in the estuarine section of a considerable river; second, 

 the filling in of this basin by sediments accumulated during a downward 

 oscillation of the area in which the basin lies; third, the development of 

 compression strains, such as are involved in rock folding, the relief being 

 afforded by the folding of these stratified deposits. 



If this hypothesis as to the origin of the Narragansett foldings were 

 correct, we should expect to find the maximum of disturbance in the extreme 

 margins of the basin, the central features of the area remaining less dis- 

 turbed. As will be seen from the chapters on the deposits of the basin, 

 this is essentially what we find. Wherever the area is of sufficient width 

 to afford a field for the development of the structure in a clear way, we 

 observe that the indications of lateral stressing are very clear in the belts of 

 country next the contact with the crystalline rocks, while in the central 

 portion of the field the beds exhibit lessened stress. Thus, as will .be seen 

 from fig. 1 (p. 19) and the sections across the basin where the distance is not 

 far from 20 miles, we find the marginal portion of the stratified rocks exceed- 

 ingly flexed, the resulting dislocation attaining about the highest order of 

 complexity, while the intermediate field, including much more than one-half 

 the whole length of the indicated line, is less marked by the stressing forces. 



