204 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Great Blue Hill, with an altitude of about 600 feet (200 meters) and 

 extends with gradually diminishing elevations, in an easterly direc- 

 tion, with a gentle bow to the south, as a series of rounded hills which 

 die out as the shores of Quincy and Hingham bays are approached. 



The Blue Hills proper, or that portion which lies between the Great 

 Blue Hill and the last prominent hills near West Quincy, now form a 

 public park, the Blue Hills ReserAation, which ranks as one of the 

 most beautiful parks in the neighborhood of any large city. Although 

 traversed by convenient roads and foot-paths, the natural wild and 

 wooded character of the hills has ])een perfectly preserved. From 

 the higher points pleasing and extensive panoramas of the surround- 

 ing countr\' may be seen on the north south and west, while to the 

 east and northeast, the ocean and its numerous bays form a distant 

 background in a view of great beauty. 



With the exception of relatively small areas of Cambrian slate and 

 a few diabase dikes, the abundant rock exposures over this area con- 

 sist entirely of an alkali-liornl^lende-aegirite-granite or closely related 

 porphyries, and it is with the petrology of these rocks that the present 

 paper is concerned. 



Previous work on the geology of the region. — In an extended memoir 

 entitled "The Blue Hill Complex," Professor W. O. Crosby ^ has 

 furnished us with a very detailed and valual)le discussion of the geology 

 of this area. His description of the rocks was essentially only a mega- 

 scopic one, inasmuch as he had at his command comparatively little 

 microscopic or chemical data, and that which he had appears to liave 

 been imperfect and to some extent misleading. Up to the present 

 time the only ]:)etrographic work which has appeared describing 

 the rocks are: — brief notes descriptive of the granite by G. N. Hawes ^, 

 IVI. E. Wadsworth,^ G. P. Merrill;* an imperfect description of the 

 granite and porphyries, by Dr. T. G. White ^; a brief description 

 of the Quincy granite accompanied by an excellent analysis by H. S. 

 Washington ^; and a rather detailed description of the granite from 

 the more important quarries of the Quincy district by T. Nelson 

 Dale.^ The last mentioned paper contains, besides the descriptions 



1 Occasioaal Papers, Boston Soc. Nat. Hist., 4, 19, (1895). 



2 Tenth Census U. S., 10, p. 18. 



3 Descriptive Cat. of American & Foreign Rocks, Boston, No. 71 (1883). 



4 Building and Ornamental Stones. Report U. S. Nat. Mus., p. 409 (1886), 



5 Notes on the petrography of the Boston Basin. Boston Soc. Nat. Hist. 

 28, No. (1897). 



6 American Journal of Science, 156, p. 181 (1898). 



7 Bull. No. 354, U. S. G. S. (1908). 



