294 PROCEEDINGS OF THE AMERICAN ACADEMY. 



several on the small hill soiitli of Xorth Common Hill, Quincy, and just 

 east of the northern extension of the Pine Hill Area; and several 

 small dikes and patches of irregular outline in the coarse-granite near 

 its contact with the fine-granite in the old quarry just off Quincy Ave. 

 in Wej'mouth. In all of these occurrences it is noteworthy that the 

 pegmatitic dikes occur either at the contact with the granite-porphyry 

 (Rattle Rock and northeast of Pine Hill), or with its fine-granite 

 equivalent (Weymouth). Their origin is probably to be found in the 

 filling of fractures in the upper parts of the granite by injection of 

 fluid material from beneath. The banded texture of the larger veins 

 — coarse margins, fine centers, etc. — have been discussed elsewhere.*^ 



PART II. 

 General Discussion. 



Chemical and Mineral characters. — Although these characters have 

 been more or less fully discussed under each type it may be well to 

 briefly summarize certain important features brought out by the 

 chemical analyses. 



The entire series are characterized by relatively high iron and 

 alkalies, by exceedingly low magnesia and also by almost equally low 

 lime, except in the case of the rhombenporphyry and the rhomben- 

 porphyry xenoliths, in both of which the lime reaches about three 

 percent owing to the presence of a lime-iron rich pyroxene probably a 

 hedenbergitic augite. 



There is a considerable fluctuation (see table No. I) in the total 

 iron oxide content even between the coarse-granites (compare 1-2 

 with 3), while the rhombenporphyries show a great difference, com- 

 pared with the rest, in their high iron content. It is to be noted that 

 there is a strong sympathy between the iron oxides and high alkalies; 

 as the amount of feldspar increases the sodic-iron silicates increase 

 also. The relative amounts of K2O and Na20 are not far from equal, 

 with the exception of the rhombenporphyry xenoliths, and this ab- 

 normality may point to some later addition of soda from an enclosing 

 hot magma.* The potash on the whole slightly predominates in 

 amount. 



48 Palache & Warren, op. cit., p. 127. 



