40 GEOLOGY OF THE NARRAGANSETT BASIN. 



ANCIENT MARGIN OF THE BASIN. 



The original extension of the Carboniferous beds the remains of 

 which are found in the Narragansett Basin can not be determined. The 

 evidence goes to show that in the process of filling the trough the margin 

 of the field in which the deposits were accumulated extended in a somewhat 

 continuous manner in every direction, this extension being in a way coin- 

 cident with the progressive subsidence of the area. There must thus have 

 been a succession of shore lines, each lying farther aAvay from what is now 

 the central portion of the field. It is probable that the arkose deposits which 

 are now found around a large part of the margin of the existing Carbon- 

 iferous area were accumulated at no great distance from what was the shore 

 line at the time the)^ were formed; but the later shore, answering in age to 

 the upper conglomerate, may have been some scores of miles beyond the 

 present limits of the Carboniferous rocks, the materials being brought in 

 over the shelf of earlier-formed deposits. The fact before adverted to, that 

 the fossil-bearing quartzite pebbles come from some unknown and possibly 

 rather remote district, indicates the validity of this hypothesis. 



Although the Carboniferous section of this basin is thick, the fact that 

 the conditions favored the formation of rapidly accumulating' conglomerates 

 of itself suggests that some portion of the section has been worn away 

 The fact that no higher-lying beds than the Carboniferous exist in this por- 

 tion of New England, although there is abundant evidence that a large 

 amount of erosion has taken place since the time of the Coal Measures, is 

 also evidence that a considerable thickness of stratified rocks must have 

 disappeared from this field. The margin of these vanished formations 

 must have been far beyond the limits of the Narragansett Basin. 



RELATIVE EROSION OF EAST AND WEST APPALACHIANS. 



It requires but a glance at the topography of the districts lying- to the 

 east and to the west of the ancient or mid-Appalachian field to show 

 the observer that there has been a great difference in their erosional history. 

 On the west we find the mountain folds on the whole well preserved as 

 regards both their anticlinal and their synclinal elements, the average pres- 

 ervation of the structural features being more perfect than that of any 

 other equally well-known great mountains, except, it may be, portions of 



