22 Loughlin — Relations between the Quincy Granite 



These colors are also present in the matrix of the conglomerate 

 where not obscured by the abundance of pebbles or by a 



strong development of green alteration minerals. 



The Cambridge slate, which overlies the conglomerate con- 

 formably, is characterized by a bluish gray to nearly black 

 color, and by a distinct, though not prominent, banding. 



While evidence throughout the Basin is of general import- 

 ance, that of most critical value lies in the outcrops nearest the 

 Quincy granite along the south boundary. The structural 

 relations, largely concealed by an extensive covering of glacial 

 drift, have been interpreted as follows by Prof. Crosby* : A. 

 band of arkose, measuring from 450 to 500 feet across the 

 strike and dipping steeply to the north, extends eastward along 

 the boundary fault from north of great Blue Hill as far as 

 East Milton. Its terminations west and east are concealed 

 under drift, but evidence as a whole favors the conclusion 

 that its eastward end is bevelled off by the fault. Overlying 

 the arkose is a bed of purplish slate, 100 feet thick, and over 

 that a conglomerate extending through Milton Centre and 

 south of Forbes Hill in Quincy, where it is in contact with the 

 boundary fault. The dip of the sediments varies from 40° N. 

 to vertical or even slightly overturned to the north, and is 

 attributed in part to the drag effected by the faulting. The 

 conglomerate south of Forbes Hill is overlain by Cambridge 

 slate, and has a thickness of 200 to over 400 feet. The thick- 

 ness of the entire conglomerate series is far greater, and all but 

 the upper part of the series has been concealed by the fault. 

 The displacement along the fault has been estimated by Prof. 

 Crosby to be at least 2000 feet, and is believed to have grown 

 during the deposition of the conglomerate series. The writer's 

 own evidence neither confirms nor denies these conclusions. 



Prof. Crosby believed many granite pebbles in the con- 

 glomerate south of Forbes Hill to be of weathered Quincy 

 granite, but megascopic and microscopic study by tlie present 

 writer have failed to identify any of the granite pebbles as 

 belonging to the Quincy type. The so-called arkose was 

 believed both by Prof. Crosby and Dr. Mansfield to contain 

 detritus of Quincy granite, but microscopic evidence fails to 

 confirm this opinion. The rock contains no feldspar, but con- 

 sists of purplish quartz grains cemented and somewhat 

 enlarged by a recrystallized matrix of sericite and quartz. 

 One or two sericitic aggregates found were more suggestive of 

 altered felsite than of altered feldspar. The "arkose" then 

 gives only negative evidence. It is worth noting, however, that in 

 the biotite granite, even where much altered, the microperthite 

 is very free from sericite ; also that where the Quincy granite 



*Op. cit., pp. 417-485. 



