234 PROCEEDINGS OF THE AMERICAN ACADEMY. 



in the granite-porphyry, and is thought to l)eIong to the aegirite- 

 hedenbergite Une of pyroxenes. Magnetite or ihnenite is quite abun- 

 dant in the form of inclusions in the hornblende. Its appearance 

 suggests that it is secondary after some highly ferruginous mineral 

 other than the pyroxene. Several grains of the dark red mineral, be- 

 lieved to be aenigmatite, have been noted enclosed in the hornblende, 

 and the magnetite may be a replacement of this mineral. Fluorite in 

 the form of included grains is often very abundant in the pyroxene, and 

 it is also present, but somewhat less abundantly, in the hornblende. 



Porphyritic Phase of the Fine-Granite. — The tendency of the fine- 

 granite to become porphyritic where it probably grades into the 

 typical granite-porphyry, and also probably for a very short distance 

 where it is in original igneous contact with the slate, have been re- 

 ferred to. (The latter contacts are not satisfactorily exposed and the 

 writer is not altogether certain on this point.) This type is best 

 exposed for study in several outcrops in the Pine-Tree Brook Reserva- 

 tion and at one point just north of the entrance to Scaumaug Notch. 

 The rock while not so profusely porphyritic as the granite-porphyry 

 is still strongly porph\Titic, showing abundant rectangular feldspars 

 and rounded quartz grains. The groundmass is finer than the average 

 grain of the typical fine-granite but is distinctly coarser than the 

 groundmass of the granite-porphyry described beyond. The ground- 

 mass contains rather abundant "specky" hornblende. Microscopi- 

 cally the groundmass is essentially of the same texture as the 

 fine-granite although it often shows, in the inclusion of the 

 groundmass feldspar and quartz by the hornblende, an approach to 

 the characteristic structures found in the typical granite-porphyry 

 of the Blue Hills. 



Chemical characters. — For chemical analysis a specimen of the 

 fine-granite from an old quarry just south of Ruggles Creek, Quincy, 

 was selected. Though stained slightly brown, the rock in thin-section 

 seemed to be the freshest of any that could be obtained. The values 

 are the average of duplicate analyses except that ferrous-iron and 

 alkalies are the average of three and four determinations each. 



