470 WARREN AND POWERS DIAMOND HILL-CUMBERLAND DISTRICT 



uncovered until after Carboniferous times; at least no fragments of it 

 have been identified in the conglomerates of the Xarragansett series. 



The accompanying sections, AA and BB, will serve to convey the au- 

 thors^ conception of the structure relations of the riebeckite and asso- 

 ciated rocks. 



The mineral composition of the riebeckite-n?girite rocks here as well as 

 their mode of occurrence are closely analogous to those of the Quincy- 

 Blue Hille batholith 20 miles to the north, as well as to a small interme- 

 diate mass at Eattlesnake Hill, Sharon, Massachusetts,^^ and there can 

 be no doubt that the two masses- possess a similar origin and history and 

 are of the same age of intrusion. The Quincy mass is later than the 

 Middle Cambrian and earlier than the Norfolk Basin conglomerates of 

 Carboniferous age. If the three occurrences are of the same age, we 

 have proof now that they are later and cut the biotite granite series, just 

 as the similar alkaline rocks to the north of Boston cut the biotite granite 

 (Saugus) of that region. The age of the alkaline granites is apparently 

 the same as that of the Sterling granite of southern Rhode Island, the 

 younger series of granites of western Massachusetts, and the alkaline 

 granites of northeastern Massachusetts. As shown by Loughlin,* the 

 Sterling granite cuts the Kingstown series of the Narragansett Basin, 

 which is the equivalent of the Pawtucket formation on the north, and 

 pebbles of the granite are found in the Dighton conglomerate. The dias- 

 trophism which accompanied the intrusions of these various batholiths 

 took place in Middle Pennsylvanian time, as is shown both in Xova Scotia 

 and New Brunswick (above the Mispec, Riversdale-Union and Coal Meas- 

 ures) and in the Narragansett Basin. 



*i In the fan of 1912 Dr. F. H. Lahee called the writer's attention to the occurrence 

 at Rattlesnake Hill. Sharon. Massachusetts, about 10 miles southwest of the Blue Hills 

 and an equal distance northwest of Diamond IIill-("umberland area, of a hitherto un- 

 known small stock of riebeckite granite. ' This occurrence has since been studied by 

 Messrs. W. L. Whitehead and K. C. Foster under the direction of Doctor Lahee and the 

 writer, and the results were incorporated in thesis for the bachelor's degree in geology. 



Their report may be summarized as follows : A coarse-grained riebeckite granite iden- 

 tical with the Quincy granite in general appearance and in mineral composition, except 

 that no a?girite is present, occurs on Ilattlesnake Hill. I'his is surrounded on all sides 

 by a fine-grained granite of the same composition, the whole forming a roughly circular 

 mass about one mile in diameter. Contacts are not exi)osed, but the mass is surrounded 

 on all sides by a biotite granite of the type found in Weymouth and Dedliam. The coarse 

 and tine granite appear to l)lend rapidly into one another. Occasional pegmatitic spots 

 are found with very coarse riebeckite crystals. 



There can be no doubt that in this mass we have another upward extension (cupola) 

 of the alkaline granite magma more largely developed to the north in Quincy and the 

 Blue Hills and to the south near Diamond Hill, Rhode Island. The riebeckite granite 

 of Sharon Is almost identical, so far as can be told without an analysis, with the fine 

 granite of eastern and southern Quincy and northern Weymouth, and there appears to 

 be no doubt but that the relations of the coarse and fine granite In the two localities are 

 Identical. — C. IT. Warre.v. 



♦ Am. Jour. Scl., vol. 29, 1910, pp. 447-457, and vol. 32, 1911, pp. 17-32. 



