and the Adjacent Sedimentary Formations. 19 



are clearly older than all the igneous rocks of the region 

 and need no further mention in this paper. 



Biotite-Granite Series. — The next formation in order of age 

 is the biotite-granite series. It extends over a great part of 

 eastern Massachusetts. On the map it is shown along the 

 southern edge, separated from the Norfolk Basin by a pro- 

 found fault. It also extends around the west and north mar- 

 gins of the Boston Basin with more or less continuity. It 

 includes several phases ; alaskite (very subordinate), normal 

 granite, granodiorite, and diorite of varying basicity, which in 

 some places are gradational into one another, and in others are 

 marked by sharp contacts. Where sharp contacts exist, the 

 more acid phases are intrusive into the more basic. 



It is of prevailing subalkaline character. The more acid 

 phases, those involved in the present problem, are gray to red- 

 dish in color, fine- to rather coarse-grained, and in places por- 

 phyritic with reddish feldspar. The chief minerals are gray 

 to white plagioclase (oligoclase to andesine) microscopically 

 altered to aggregates of sericite and epidote with more or less 

 calcite, reddish microcline-microperthite, in which the potassic 

 member is dominant and, as a rule, very free from products of 

 deep-seated alteration ; quartz in gray to white glassy grains ; 

 biotite in small evenly scattered flakes, largely altered to chlor- 

 ite. Hornblende, where present, is considerably altered to 

 chlorite and epidote. The most common order of crystalliza- 

 tion is : (1) plagioclase accompanied or followed by the femic 

 minerals ; (2) microperthite and quartz. The rock is nearly 

 everywhere full of irregular slickensided fractures filled with 

 the secondary minerals, especially epidote. The epidote, as 

 well as the granite walls, is slickensided. 



Older Felsite. — Felsites, not including those in the Blue 

 Hills, are found more or less abundantly over much of the 

 same area as the biotite-granite series. They vary somewhat 

 in texture and mineral composition. Where porphyritic, the 

 chief feldspar is gray to white plagioclase, more or less altered 

 to sericite and epidote. Quartz phenocrysts are also com- 

 monly present. The predominating femic mineral is green 

 hornblende, more or less chloritized. The groundmass is even- 

 microgranular. 



Quincy Alkaline Granite Series. — The Quincy granite 

 series covers a roughly elliptical area, with an east-west major 

 axis, extending across Milton and Quincy, including the entire 

 Blue Hill range, and the northern parts of Braintree and Wey- 

 mouth. It is bounded on the north by an east-west fault con- 

 tact with the Boston Basin sediments ; on the south in Brain- 

 tree by another east-west fault, and farther west by uncon- 

 formable contact with the Norfolk Basin sediments. Certain 



