60 Loughlin and Heehinger — Unconformity 



latter case there will also be absolute proof that the Dedham 

 and Milford granites are pre-Pennsylvanian and quite dis- 

 tinct from the late or post-Pennsylvanian Sterling granite- 

 gneiss ; and both of the tentative hypotheses suggested in this 

 paper will become untenable.* A little additional evidence, 

 which, though scant, is more conclusive, will be given in the 

 following section. 



The Norfolk County Basin. 



The structure of the Norfolk County Basin is obscure and 

 complicated, and the Basin as a whole has never received ade- 

 quate study. The statigraphy of the Basin, in consequence, is 

 only known in a general and probably imperfect way. The 

 part of the Basin west of Canton Junction, or the transverse 

 fault shown in fig. 1, appears, so far as could be learned from 

 reconnaissance work, to be of monoclinal structure, with north- 

 westerly dip. The southeast border is marked by an unconform- 

 able contact with granite of the Dedham type, and the north- 

 west border by a fault, along which the sedimentary strata 

 dip down against an area of Dedham granite. The strata 

 include the Pondville conglomerate, overlain by a thick series 

 of red and occasional green beds which are continuous with the 

 Wamsutta formation of the Narragansett Basin. , The red 

 beds pass upward into medium to coarse conglomerates in 

 which are intercalated occasional red and green slate beds. 

 The main series of red beds aud the overlying conglomerate 

 are identical in essential composition, and the chief difference 

 between them appears to be a difference in the amount 

 of tine red matrix which determines the degree of redness. 

 Where the fine red matrix is inconspicuous in either formation, 

 the beds have a green or greenish-gray color. Dedham 

 granite pebbles are numerous in both formations, and in the 



*As a third possible set of conditions, it may be supposed that although 

 no fossils of critical stratigraphic value may be discovered in the Pondville 

 and Wamsutta formations, some structural evidence may be found which 

 will prove the Dedham and Milford granites to be pre-Pennsylvanian and 

 therefore distinctly older than the Sterling granite-gneiss. In this case, the 

 presence of Dedham aod Milford granite pebbles in the Pondville and 

 Wamsutta formations will afford no definite clue to the age of these strata ; 

 for the fact remains that the " Coal Measures" contain no pebbles of the 

 granites in question, but do contain pebbles and grains derived from 

 formations which are certainly older than the granites and which, 

 according to this view, are supposed to have once formed a cover over the 

 granites. If the Pondville and Wamsutta formations are older than the 

 "Coal Measures," it is necessary to explain why the younger formation 

 was derived only from rocks of pre-granite age, whereas the two older 

 formations were derived largely from the granites which could have been 

 exposed only after a large part of the pre-granite rocks covering them had 

 been removed. The cause can hardly be attributed to difference in chemical 

 conditions of weathering, as the " Coal Measures" contain an abundance of 

 feldspathic material. 



