54 Loughlin and Hechinger — Unconformity 



Proof of unconformity. — The presence in the Dighton con- 

 glomerate of pebbles of Sterling granite gneiss and of the 

 arkose is absolute proof that an unconformity exists between 

 the Dighton and the "Coal Measures." The unconformity 

 represents a time interval long enough to account for the in- 

 trusion of an extensive granite batholith, an essentially con- 

 temporaneous period of intense folding and dynamic metamor- 

 phism,"* and a subsequent period sufficient for erosion to un- 

 cover a portion of the newly intruded granite. 



The writers were unable to find any structural indication of 

 an angular unconformity in the exposures examined by them, 

 but this is not surprising, as outcrops are separated by exten- 

 sive drift-covered areas, and it is possible that all traces of the 

 unconformable contacts are covered. Furthermore, the fact 

 that both the older and newer formations have been severely 

 compressed and folded may obscure or obliterate any uncon- 

 formable contacts. Only a beginning, however, of the attempt 

 to locate the contact has been made, and it is hoped that some 

 of those within reach of the field may have the time to make a 

 thorough search, and by correlation of beds through their 

 pebble compositions may at least succeed in mapping the 

 approximate position of the contact. 



Age of Dighton conglomerate. — Nofossilsf have been found 

 in the Dighton conglomerate, but it is now possible to assign 

 to it a definite age on structural grounds. It is separated by 

 an unconformity from the "Coal Measures," and in the writers' 

 opinion must be post-Pennsylvanian ; on the other hand, it is 

 itself intensely folded and compressed, in marked contrast to 

 the red Triassic beds to the west in Connecticut and Massa- 

 chusetts, which have suffered no conspicuous amount of folding 

 or compression. It therefore appears to be pre-Triassic, 

 although an absolute correlation on structural grounds between 

 two such isolated areas is of course impossible. The only age 

 between Pennsylvanian and Triassic is the Permian, and to 

 this age, in the writers 1 opinion, the Dighton conglomerate 

 should therefore be assigned. It is interesting to note that 

 Woodworth^ suspected the existence of the unconformity 

 because of the contrasting climatic and physiographic condi- 

 tions which must have prevailed during the deposition of the 

 relatively fine-textured " Coal Measures " with their coal beds, 



* Loughlin, G. F., this Journal (4), vol. xxix. pp. 454-455, May, 1910. 

 Lahee, F. H., Relations of the degree of nietamorpliisni to geological struc- 

 ture and to acid igneous intrusion in the Narragansett Basin, R. I., ibid., 

 xxxiii, pp. 466-467, 1912. 



f Since the writing of this paragraph, David White has orally informed 

 the writer that he has found obscure fossil plant fragments in the Dighton 

 conglomerate which are suggestive of Permian age. 



$Op. cit., pp. 186-187. 



