ORIGIN OF THE PEGMATITES OF MAINE 301 



certain types of apatite, etc. Increase either in sodium and littiium 

 or in fluorine is usually accompanied by an increase in phosphorus 

 content. The pegmatite characterized by these rarer minerals con- 

 stitutes only an exceedingly small part of the total volume of pegmatite 

 in any district. 



In coarseness, the pegmatites vary from little coarser than normal 

 granites to masses showing single crystals of feldspar 20 feet across. 

 Their distinguishing feature is therefore not coarseness but extreme 

 irregularity of grain. Distribution of the constituent minerals in 

 bands, such as is observed in pegmatites in some other regions, is 

 entirely absent in Maine, the minerals usually (except for the graphic 

 intergrowths) being distributed with complete irregularity. Graphic 

 intergrowths are very abundant and include those of quartz with feld- 

 spar and less frequently of quartz with muscovite, feldspar with 

 muscovite, garnet with quartz, black tourmaline with quartz, and 

 spodumene with quartz. All of these intergrowths may occur in 

 the same pegmatite mass. 



Cavities are practically absent from the finer-grained pegmatites, 

 and in most of the coarse-grained pegmatites constitute less than i 

 per cent of the total volume. The gem-bearing pegmatites, however, 

 are usually characterized by a central zone in which albite, lepidolite, 

 tourmaline, amblygonite, etc., are particularly abundant and in which 

 miarolitic cavities are also quite abundant. It is on the walls of such 

 cavities that the gem tourmalines were developed. At Mt. Mica, 

 which is the largest and most famous tourmaline locality in Maine, 

 the largest pocket found was 20X12X7 feet, but the majority do not 

 average more than a foot or two in diameter. Only a few of these 

 pockets contain gem minerals in any considerable amounts, but groups 

 of quartz crystals are developed on the walls of many others. 



Origin. — The writer does not purpose in this article to attempt 

 a discussion of the voluminous literature on pegmatites except in so 

 far as it bears closely upon those of the region under discussion. Pre- 

 vious writings and theories have been well summarized by George 

 H. Williams' and especialy by Brogger.^ 



1 George H. Williams, Fifteenth Ann. Rept. U.S. Geol. Survey, 675-84. 



2 Brogger; "Der Syenitpegmatitgange der siidnorwegischen Augit und Nephe- 

 linsyenite," Zeitschr. }. Kryst., XVI (1890). Sections on genesis translated in 

 Canadiatt Records of Science, VI (1894), 33-46, 61-71. 



