and the Adjacent Sedimentary Formations. 21 



A minor phase of the series in West Quincy is a basic alka- 

 line porphyry, containing microperthite as phenocrysts and 

 angite as a constituent of the ground mass. The groundmass, 

 where noted along faults or shear zones, is altered more or less 

 to epidote. This is evidence that where the composition of the 

 rock was favorable, i. e., where lime-alumina silicates were 

 present, alteration to epidote took place during the period of 

 compression. The absence, therefore, of such alteration pro- 

 ducts as epidote and chlorite in the Quincy granite seems due 

 rather to the absence of lime and magnesia in the rock than, 

 as might at first be supposed, to any wide difference in age. 



Alkali Felsite. — The Quincy granite series is cut, on, and 

 west of, Pine Hill in West Quincy, by an irregular mass of 

 felsite, which may be confused megascopically with the felsites 

 above mentioned ; but the microscope proves it to be of dis- 

 tinctly alkalic character, and closely related to the Quincy 

 granite in mineral composition. Intrusive bodies of the same 

 type of felsite are found at a few other places in the Blue 

 Hills, and an extrusive body lies between Fine Hill and the 

 ponds to the south. 



Evidence in the Boston Basin. 



The Boston Basin sediments, assigned to the Carboniferous 

 Age, include a thick mass of Roxbury conglomerate conform- 

 ably overlain by the Cambridge slate. They are bounded on 

 the north by a fault scarp, which extends west-southwest from 

 Lynn to Waltham or farther, and separates them from the 

 hills of granite, diorite, and felsite ; on the south by another 

 fault, of nearly east-west trend, which separates them from 

 the Quincy granite. The sediments narrow westward, and end 

 in a narrow band which curves southward through Needham 

 and Dover. The whole series is thrown into a series of nearly 

 east-west folds, the erosion of which has left alternating bands 

 of conglomerate and slate. 



The conglomerate with its intercalated sandy and slaty beds, 

 consists chiefly of felsite, granite, and quartzite fragments, 

 with a minor amount of basic igneous rocks and Cambrian 

 slate. The felsite pebbles are, in most cases, readily identified 

 megascopically as derived from the older felsite masses. A 

 few, which resemble the younger alkaline felsite megascop- 

 ically, have been proved in thin-section to belong to the older 

 felsite. The granite pebbles also have been proved megascop- 

 ically and microscopically to belong to the biotite-granite series, 

 though an occasional pebble resembles closely the much 

 weathered surfaces of Quincy granite. Slaty intercalated beds 

 are, so far as seen, of prevailing reddish or purplish color. 



