ORIGIN OF THE PEGMATITES OF MAINE 319 



are not primarily responsible for the pegmatitic textures. The pres- 

 ence of the rarer elements seems to have had only a minor influence 

 on the texture since in many typical pegmatites such elements appear 

 entirely absent. Theoretical considerations and the presence of 

 miarolitic cavities in certain pegmatites point to the gaseous constit- 

 uents of the pegmatite magmas, especially water vapor, as the pri- 

 mary cause of their textures. 



While certain facts, such as the pinch and swell phenomena 

 observed in many pegmatite dikes in contrast with the parallel- 

 walled character of most of the granite dikes, indicate somewhat 

 greater mobility in the pegmatite than in the granite magmas, other 

 facts, such as the sharpness of many of the contacts between pegmatite 

 and schist, the absence of absorption along any of the contacts, the 

 presence of angular schist fragments now surrounded by pegmatite, 

 the small proportion by volume which the cavities bear to the whole 

 pegmatite mass, the absence of notably greater contact metamorphic 

 effects near pegmatite than near granite contacts, and the batholithic 

 dimensions of some pegmatite bodies, all suggest that the difference 

 in average composition between the granite pegmatites and the normal 

 granites was perhaps not so great as has generally been supposed. 



In his textbook on Igneous Rocks Iddings' in discussing the peg- 

 matites says: "The amount of gases concentrated in such magmas 

 was not many times that of the gases originally distributed throughout 

 the magma from which the pegmatite was differentiated; possibly 

 not more than ten times as much." The present writer would be 

 inclined, in the case at least of the granite pegmatites of New England, 

 to limit the gaseous content of these rocks still further. 



The experiments of Messrs. Wright and Larsen on quartz from 

 pegmatites from Maine and elsewhere indicate that some at least of 

 the coarser pegmatites began to crystallize at a temperature slightly 

 above the inversion-point of quartz (about 575° C.) and completed 

 their crystallization somewhat below this temperature. It is probable 

 that many of the finer-grained pegmatites crystallized wholly above 



575° C. 



The theory that the graphic intergrowths in pegmatites represent 

 eutectic mixtures cannot be regarded as proven by the published 



I Joseph P. Iddings, Igneous Rocks (1909), I, 276. 



