102 



GEOLOGY OF THE NARRAGANSETT BASIN. 



without other means of correlation than these physical criteria, as to which 

 horizon formed the axis of the synclinal structure. Where beds have been 

 stripped off from areas several miles in width, as between the Narragansett 



and Norfolk County basins, 



\ ,.. _. there is difficulty in making 



\ \ ,.-'' ,.--' ""--.. "\ /' correlations. Owingtodenu- 



\ " ,/ \ \ / / dation, the uppermost beds 



/' .---—-. \ \ \ /' /' of a formation, as originally 



\ \ /' / \ \ \ \ / / / deposited, may be entirely 



lost. Thus there is doubt 

 as to the correlation of the 

 upper conglomerates near 

 NeAvport with those in the 

 Taunton syncline. The Newport conglomerates may be in the position of 

 the left-hand syncline in fig. 3, while those near Taunton correspond to the 

 upper conglomerate in the right-hand syncline. 



GLACIATION. 



The chief difficulty arising from glaciation is the coating of drift 

 which is left upon the rocks of a country. In the Carboniferous field of 

 southern New England the embarrassment from this source is particularly 

 great, for the reason that the area received a thick coating of the deposits 

 of the retreating ice sheet. So effectually are the bed rocks concealed in 

 parts of the Carboniferous basin, that outcrops may not be seen oftener 

 than from 3 to 5 miles in any direction. The tracing of highly inclined 

 strata along their strike can not be safely undertaken in these areas, 

 and the mapping of the formation must be done with reference to the 

 broadest possible groups, or perhaps be limited to the indication of the 

 presence of undiscriminated members of a large rock series. The distri- 

 bution of fragments of rock along known lines of glacial carriage is some- 

 times a help in fixing the boundaries of the underlying bed rocks. 1 In this 

 field the transportation of bowlders was from north to south. Where the 

 rocks trend in bands from east to west, the line along which each differ- 

 ent kind of rock appears in the drift is approximately the boundary line 



' See Fence-wall geology, by A. F. Foerste : Am. Geo]., Vol. IV, 1889, pp. 367-371. 



