126 PROCEEDINGS OF THE AMERICAN ACADEMY. 



no adequate petrographical description ; but work is now in progress 

 which it is hoped may soon make good this deficiency. 



Pegmatitic developments, so common in many granites, were almost 

 unknown in the Quincy granite until some few years ago a mass of 

 pegmatite was discovered in one of the quarries and later two other 

 pegmatites were exposed in another quarry. These pegmatites possess 

 so many features of unusual interest, structurally and mineralogically, 

 both in themselves and in their relations to the enclosing granite, that 

 it has been thought well to publish a detailed description of them ; and 

 with this task the present paper is concerned. 



Part I. The Enclosing Granite. 

 Before taking up the detailed description of the pegmatites it will be 

 appropriate to give a very brief summary of the important characters 

 of the enclosing granite. In texture it is equi-granular, hypauto- 

 morphic to xenomorphic and of medium to coarse grain. Mineralogi- 

 cally it consists of albite-microcline perthite, quartz, riebeckite with 

 related alkali -hornblendes as yet not exactly determined, and aegirite, 

 with zircon, ilmenite or magnetite, and fluorite as the chief accessories. 

 The characteristics of the various minerals including their textural 

 relations to each other, are quite similar to those of the main part of 

 the pegmatites and it will be unnecessary to go into a detailed descrip- 

 tion of them here. It may be remarked that the rock shows evidence 

 of having suffered from movements, the minerals being sometimes con- 

 siderably disturbed or even crushed and recrystallized. The granite 

 enclosing the Ballon and Fallon quarry pegmatites is practically 

 identical with that in the adjoining Hard wick quarry of which Dr. 

 H. S. Washington has given us a chemical analysis ^ which is here 

 reproduced. 



100.91 



3 Am. Jour. Sci., 6, 181 (1898). 



