22 GEOLOGY OF THE NARRAGANSETT BASIN. 



movements. If the strain which produced the folding was of equal value in 

 all parts of the basin, there would be no reason why the resulting arches of 

 the strata should not have been of uniform declivity on either side. I can 

 best account for the facts in this case by supposing that, while the contraction 

 which brought about the mountain building may have acted in all parts of 

 the field, a large part of the stress was carried through the lateral girders of 

 indurated massive rocks on either side of the basin until it could be applied 

 to the newly formed, distinctly layered, and therefore less resistant materials 

 contained in the old Narragansett Basin. 



The horizontal value of the movement which was taken up in the fold- 

 ings of this field can not as yet be accurately computed. The attitudes of 

 the beds, however, indicate that it amounted to 2 miles or more. The con- 

 ditions of this action may be considered as such that the surface of the cen- 

 tral part of the disturbed area may not have been moved except down- 

 wardly, while the longitude and latitude of the points on the surface on 

 either side which were affected by the foldings were evidently changed in 

 an increasing measure as we depart from the central axis. 



The depth of the distinctively stratified rocks in the basin at the time 

 the mountain-building work was done can not well be reckoned at less 

 than that of the existing section, but as this region has been subjected to 

 an amount of erosion competent to bring the anticlines and synclines to 

 about the same level, it ma)* well have been near double that amount. 

 Therefore we may assume the depth of the section of massive rocks which 

 conveyed the thrust from an extreme area on either side to the deposits of 

 the trough as not less than 2 miles. 



It is not yet clear whether the mountain-building action which has 

 affected this basin was altogether accomplished after the latest-formed beds 

 which it now contains were accumulated. The factthat all the sections of the 

 Carboniferous series were accumulated in shallow water, or but little above 

 its level, requires us to suppose that the trough was the seat of a nearly 

 continuous depression. It is to be expected that the downsinking would 

 have been accompanied by some measure of compressive movement. So 

 far, however, the field has afforded no evidence of such mountain-building 

 work done during the subsidence of the trough. On the contrary, the 

 observations are most reconcilable on the supposition that the whole of 

 the strictly orogenic action took place after the work of depression was 



