and the Adjacent Sedimentary Formations. 23 



is exposed adjacent to the fault, its pink micropertbite shows 

 no marked alteration to sericite. It therefore seems probable 

 that, had the " arkose" been derived from the Quincy granite, 

 considerable micropertbite would be found in thin-section. 



This evidence also disproves the suggestion of Dr. Mans- 

 field* that the arkose was the basal member of the series, rest- 

 ing unconformably upon the Quincy granite, and that the 

 boundary fault lay some distance to the north. Further evi- 

 dence against this suggestion was found by the present writer 

 in the summer of 1909 at Blue Hill village, where a new road 

 cut had exposed the Quincy granite in fault contact with the 

 " arkose.' 1 The exposure was small, but showed the fault to 

 be reversed. Its strike crossed that of the " arkose" at a very 

 low angle, its dip was uneven but averaged about 80° south. 

 The dip of the " arkose" was very steep to the north. 



The above evidence is reasonable proof that the portion of 

 the conglomerate series now exposed in the Boston Basin was 

 not derived from the Quincy granite ; but the absence, on the 

 other hand, of any intrusions from the Quincy granite series 

 into the conglomerate series fails to indicate that the latter is 

 older than the former. Evidence thus far is wholly negative. 



Evidence in the Norfolk Basin. 



The Norfolk Basin comprises a series of sediments, extend- 

 ing in a belt one to over two and a half miles wide, from Great 

 Pond in Braintree west-southwest to "Wrentham, where it 

 joins the Narragansett Basin. Its eastern boundary is con- 

 cealed by drift, but is believed to be a north-south fault. Its 

 southern boundary is a nearly east-west fault which separates 

 it from the biotite-granite series. The northern boundary, 

 within the area considered, is the south base of the Blue Hills. 

 The sediments consist to some extent of very coarse conglom- 

 erate, grit and arkose, but mostly of reddish sandstone and 

 slate. As a whole they strike nearly east-west, and dip 70° to 

 90° south ; outcrops are few and far between, but their atti- 

 tude as a whole is suggestive of a monoclinal, or closely folded 

 synclinal structure bevelled by the south boundary fault, and 

 modified by minor faults along certain shaly beds. One or 

 two outcrops on the west side of Great Pond have a nearly 

 southward strike and westward dip, indicating a westward 

 pitching syncline. The minor faults are more or less clearly 

 exposed in the railroad cuts about three miles southwest of 

 Great Blue Hill. 



The lowest member of the series is called by Prof. Crosby 

 a " giant conglomerate." It is exposed intermittently along 



*Op. cit., p. 206. 



