THE PEGMATITES OF THE RIEBECKITE-AEGIRITE 



GRANITE OF QUINCY, MASS., U. S. A. ; THEIR 



STRUCTURE, MINER.\LS, AND ORIGIN. 



Bt Chakles H. Warhen and Charles Palache. 



Table of Contents. 



Introductory 125 



Part I. The Enclosing Granite 120 



Occurrences of Pegmatite in the Area 127 



Pegmatite-pipe of the Ballou Quarry, North Common Hill, Quincy , . 129 

 Pegmatite-pipes of the Fallon Quarry, " " " " 132 



Discussion Relating to the Chemical Composition, Sequence of Crj'staliz- 



ation, and Mode of Origin of the Pegmatite-Pipea 143 



Part II. Special notes of the minerals of the Quincy-granite pegmatites ; 



quartz; the feldspars; inclusions in the feldspars; riebeckite; crocidolite 



and the needle-like hornblende; aegirite; parisite; ilmenite; octahedrite; 



fluorite ; calcite ; wulfenite 147 



Introductory. 



The general geologic features of the area of riebeckite-aegirite rocks 

 which have an exten.sive development a few miles south of Boston in 

 the city of Quincy, and to the west of Quincy, in what is known as the 

 Blue Hill reservation, have been brought most fully to the attention of 

 geologists by Professor W. 0. Crosby in a well-known memoir. ^ Taken 

 as a whole the igneous rocks of this area form an intrusive mass of 

 rudely elliptical outline measuring some H miles in length by 2 to 3 

 in breadth. They are intru.sive into Cambrian strata and are quite 

 certainly precarboniferous. They consist essentially of granite, exten- 

 sive developments of granite-porphjTy and quartz-alkali-feldspar-por- 

 pbyry, the two last forming a cover over the granite ; nielano and 

 leucocratic ditTerentiation products ; and large masses of apo-rhyolite, 

 intrusive through the other rocks. On account of the economic im- 

 portance of the Quincy granite it has received considerable attention 

 from petrographers, and of it alone we have a somewhat full petro- 

 graphical description.* Of the other rocks of the area, we have as yet 



^ The IJlue Hill Complex, Boston Soc. Nat. Historv, Occas. Papers, Vol. 4. 



« See particularly T. X. Dale, Bull. U. S. Geol. Sur., No. 351. Dr. Dale, 

 besides giving a description of the granite, gives abo the more important 

 references to the literature of this region. 



