3i6 ■ EDSON S. BASTIN 



be no reasonable doubt that these gem- and cavity-bearing portions 

 rather than the adjacent graphic portions were the last parts of the 

 pegmatite to crystallize.' 



In considering the significance of the graphic intergrowths found 

 in pegmatite it is necessary to consider not only the intergrowths of 

 feldspar and quartz but also the almost equally regular intergrowths 

 of muscovite and quartz, garnet and quartz, black tourmaline and 

 quartz, etc. Since muscovite, tourmaline and garnet are less abun- 

 dant in the pegmatites than feldspar, their intergrowths with quartz 

 are also less abundant and usually of smaller size. Such intergrowths 

 occur, however, scattered irregularly through practically all of the 

 coarser pegmatite masses. If we adopt the usual conception of the 

 eutectic as the residue of uniform composition and minimum freezing- 

 point which is the last portion to crystallize, it is manifestly impossible 

 to regard each of these intergrowths as representing a eutectic mix- 

 ture, unless indeed several portions of the pegmatite magma are 

 regarded as crystallizing more or less independently of the remainder 

 of the mass. 



Mineralogical provinces. — Most of the known pegmatites of Maine 

 which are rich in sodium and lithium minerals, that is, the gem-bear- 

 ing pegmatites, are restricted to a zone about twenty-five miles long 

 and eight to nine miles in width extending in a northwesterly direction 

 from Auburn in Androscoggin County to Greenwood in Oxford 

 County. A second and much smaller area includes the Newry and 

 Black Mountain localities in the northern part of Oxford County 

 and differs from the larger area in that the gem minerals occur 

 imbedded in the solid pegmatite and not in pockets. Within both 

 areas the lithium-bearing phases form only a small proportion of the 

 pegmatite present, most of which has the normal composition. The 

 presence locally of certain masses of unusual composition is to be 

 attributed either to a very minute excess of sodium and lithium 



I In the tourmaline-bearing pegmatites of California, according to Mr. W. T. 

 Schaller (oral communication), the zones characterized by cavities and by the presence 

 of the gems and other rare minerals, which were almost certainly the last portions 

 to crystallize, grade laterally without sharp break into graphic granite which borders 

 one wall of these pegmatite masses. Occasional stringers of pegmatite bearing lithium 

 minerals branch off from the main gem-bearing layer and cut the bordering graphic 

 granite. 



