

loughlin and Hechinger — Narragansett Basin. 45 



Art. V. — An Unconformity in the Narragansett Basin of 

 Rhode Island and Massachusetts; by G. F. Loughlin 

 and L. A. Hechinger.* 



Introduction and Statement of Conclusions. 



As the presence of Permian beds associated with the earlier 

 Carboniferous strata of eastern Massachusetts and Rhode Island 

 has been suggested at various times, it may be of interest to 

 present the evidence of a strong, though obscure unconformity 

 within this sedimentary series which accords in position with 

 the approximate division between what have been tentatively 

 regarded as Permian and Pennsylvanian rocks. The investiga- 

 tion of this unconformity is far from complete, but, as its exist- 

 ence has been demonstrated and as neither of the writers is 

 likely to have any further opportunity to visit the region, it 

 seems proper to place on record the results thus far obtained, 

 with the hope that other investigators may continue the work. 

 The existence of this unconformity was suggested by the results 

 of previous work by the senior author, f who found that the 

 Sterling granite gneiss was intrusive into the "Coal Measures" 

 along the southwestern border of the Narragansett Basin (tig. 

 1), whereas the sedimentary rocks of the Norfolk County and 

 Boston Basins were younger than all different types of granite 

 in the areas surrounding them. These contrasting results gave 

 rise to the alternative hypotheses : (1) that there are present in 

 the region two or more granite batholiths, one or more older, 

 and one younger than all the sedimentary rocks ; or (2) that 

 the granites are not necessarily of widely different ages, but 

 that there is within the sedimentary series a pronounced uncon- 

 formity, representing a period of upheaval and granite intrusion 

 and a succeeding period of erosion long enough to expose the 

 newly intruded granite. 



As a satisfactory study of the granite problem would have 

 required extensive areal mapping, whereas the existence of the 

 unconformity could be investigated within a limited area and 

 in the limited time at the writers' disposal, the latter problem 

 was undertaken. Lithologic proof of the unconformity's exist- 

 ence has been found, but field evidence as a whole is so obscure 

 that it is not yet, and may never be, possible to give more than 

 a rough approximation to the location of this important struc- # 

 tural feature. 



* Published by permission of the Director of the United States Geological 

 Survey. 



f Loughlin, G. F., this Journal, 4th ser., vol. xxix, pp. 447-457, 1910. 

 Loughlin, G. F., this Journal, 4th ser., vol. xxxii, pp. 17-32, 1911. 



