and the Adjacent Sedimentary Formations. 25 



Field study of the giant conglomerate provest 'hat bowlders 

 and pebbles of Quiney granite-porphyry are confined to those 

 outcrops close to the south base of the Blue Hills range. 

 Several outcrops of coarse conglomerate were studied along, 

 and near, the curving road (Pecunit St.) which borders the 

 Neponset River swamp, a mile and a half southwest of Great 

 Blue Hill, and here, too, pebbles of the Quiney granite series 

 were found to be limited to the northernmost exposures, giving 

 way southward to an increasing number of the epidotic granite, 

 felsite, quartzite, and slate pebbles. The giant character of 

 the conglomerate, however, prevailed even where pebbles of 

 the Quiney series were inconspicuous or absent.. Thin pebbly 

 beds in the red and gray sandstones and shales were found in 

 several places, and in these too the absence of Quiney granite 

 material and abundance of pebbles derived from the biotite 

 granite series is the rule without exception. 



Interpretation of Structure. 



Significance of the Giant Conglomerate.— There can, there- 

 fore, be little doubt that the strictly local giant conglomerate 

 is the basal member of the Norfolk Basin sediments along the 

 Blue Hills. 



The presence in the conglomerate of bowlders and pebbles 

 of only the granite-porphyry phase implies that only the upper 

 part of the Quiney granite mass could have been exposed at 

 the time. Again, the gradation upward from the giant con- 

 glomerate into the grit, sandstone, and shale, of different com- 

 position is evidence that the Quiney granite mass was protected 

 from erosion during the deposition of all but the basal member 

 of the Norfolk Basin sediments. Whether it was submerged 

 below sea-level, or buried beneath continental deposits, is a 

 question beyond the scope of the present paper. The consider- 

 able thickness, 450-500 feet, of so extremely coarse a conglom- 

 erate, and the absence in it of sharply angular bowlders suggest 

 a deposit similar in origin to an alluvial cone, or an accumula- 

 tion at the base of a wave-beaten cliff as the original form of 

 the giant conglomerate. Either of these forms, as well as the 

 submergence of the Quiney granite mass during sedimentation, 

 would accord with Prof. Crosby's conclusion that the sedi- 

 ments were depositedin a graben.* The bulk of the Norfolk 

 Basin sediments were derived from a land composed of the 

 biotite-granite series, the older felsite, and the Cambrian sedi- 

 ments. 



"Whether or not identical conditions then existed along the 

 north boundary of the Quiney granite mass is an unanswerable 



*W. O. Crosby, op. cit.,pp. 500-502. 



