WAMSUTTA GROUP. 



157 



low, as in the nose of a broad shallow syncline (see fig. 15). On the 

 western arm of the area the dips vary from east to west. Such marked 

 inversion of strata warrants the explanation that the beds have been com- 

 pressed into the fan structure by the marginal collapse of a more or less 

 quaquaversal anticline which formed over the Hoppin Hill inlier. It is 

 owing to this extreme folding, together with the imperfection of the expo- 

 sures by reason of glacial drift, that the region is so difficult of interpreta- 

 tion. Northward, near Arnolds Mill, the apparent structure is indicated in 

 the section, fig. 16. 



In the southern areas of red rocks in Pawtucket there is the most sat- 

 isfactory reason for believing that the broad exposures of alternating red 

 and gray rocks are due to close folding. This district, indeed, furnishes a 

 clue to the structure of the nearly vertical beds southward along the 



Flo. 1G.— GL'ulogiral section iu the Arnuliis ilills region. 



western margin of the Narragansett Basin, in the Cranston beds, and in the 

 equivalent Kingstown series, described by Dr. Foerste in another section of 

 this monograph. 



This same field, showing the red and gray Carboniferous strata folded 

 into isoclinal relations, affords strong evidence for believing that the Wam- 

 sutta series, in the main basin at least, was not folded until the deposition 

 of the Coal Measures, and that the entire thick section of sediments in the 

 basin underwent plication after the period of deposition. All the facts from 

 various points in the field support the view that there was but one period 

 of elevation^ and not two, as was formerly thought by Edward Hitchcock. 

 There are a few disturbances along the northern margin in the Wamsutta 

 area, which have been thought to indicate an upturning of the red series 

 before the deposition of the Coal Measures in that section, but to my mind 

 the evidence is not clearly demonstrative of this view. 



