'24 LougMm — Relations between the Quincy Granite 



the base of the Blue Hills from north of Great Pond westward 

 to Houghton's Pond. Its dip is nearly, or quite, vertical. It 

 is composed chiefly of bowlders, up to four feet in diameter, of 

 a dark reddish granite-porphyry, with smaller rounded pebbles 

 of felsite, slates, and sandstones. The matrix is inconspicuous, 

 save in occasional sandy or shaly lenses, which have a red 

 color. 



Prof. Crosby* considered that the granite-porphyry bowlders 

 had been derived from the adjacent hills of Quincy granite- 

 porphyry ; and accordingly, that the giant conglomerate was a 

 basal conglomerate ; but Dr. Mansfieldf failed to identify any 

 pebbles of the Quincy granite series in the giant conglomerate, 

 finding only granite of the biotite type. He, therefore, favored 

 the conclusion that the giant conglomerate was not basal and 

 that the true basal member had been concealed through down- 

 faulting along the present base of the Blue Hills. 



The writer first visited the locality with Prof. Crosby in 

 1909, and later made a microscopic study of the granite-por- 

 phyry then collected. It proved undoubtedly to be of the 

 Quincy type, and was free from sericite, epidote, and chlorite. 

 Some of the bowlders in the conglomerate contained portions 

 of small quartz veins which did not pass in the matrix. The 

 same type of quartz vein was found in the adjacent granite- 

 porphyry ledges. The felsite pebbles were not studied in 

 thin-section, and may be partly or wholly derived from the 

 older felsite, but their study was not necessary, so long as the 

 character of the granite-porphyry was proved. Evidence thus 

 proves the presence of Quincy granite detritus in the exposed 

 sediments of the Norfolk Basin, and its absence in the exposed 

 sediments of the Boston Basin. 



The giant conglomerate, where measured, has an exposed 

 thickness of 450 to 500 feet. It grades abruptly upward into 

 a greenish gray sericitic grit, or fine conglomerate, in which 

 only one or two small pebbles of Quincy granite-porphyry have 

 been found. Thin lenses of red shale or fine sandstone are 

 also intercalated in the grit. The best exposure of this grada- 

 tional relation is on the north side of the eastward-flowing 

 stream, south of Bear Hill and but a few steps west of the fork 

 in the north-south road (Randolph Ave.). The grit on the 

 south side of the stream passes upward into red sandstone and 

 slate, which, to the southwest along the railroad, gives way to 

 gray sandstone and slate4 



*Op. cit., pp. 471-478. + Op. cit., pp. 221-226. 



X The outcrops of gray sandstone and shale are isolated and their strati- 

 graphic position uncertain. They may overlie the red sandstone, or may be 

 the equivalent of the grit or fine conglomerate and form the south limb of a 

 syncline. In figs. 3 and 4, the gray sandstone is shown tentatively above 

 the red sandstone. 



