302 EDSON S. BASTIN 



If, as seems necessary from the field evidence, we admit a genetic 

 connection between the pegmatites and granites, it is next of impor- 

 tance to inquire what evidence is afforded by the Maine pegmatites as 

 to the physical and chemical conditions which resulted in the crys- 

 tallization, from related magmas, of rocks of such widely varying 

 character. 



Influence of external conditions. — Differences in external conditions 

 at the time of crystallization appear inadequate to explain the observed 

 textural differences. This is shown by the close associations of the 

 two types of rocks, already cited as evidence of their genetic relation- 

 ships. The field relations show that in many instances the external 

 conditions, such as the nature and temperature of the wall-rock, depth 

 at which solidification took place, etc., were similar for both types 

 of rocks. In cases, such as shown for example in Fig. i, the general 

 external conditions must have been practically the same for both rocks. 

 A similar conclusion is justified in numerous other instances where 

 granite and pegmatite grade into each other and especially in cases 

 where pegmatite forms segregation-like masses wholly inclosed in 

 granite. Conversely, the broad general similarity of the pegmatites 

 over very large areas where the external conditions were certainly not 

 constant also indicates that the causes of their peculiar textures were 

 in the main internal rather than external. It seems necessary to look 

 therefore to differences inherent in the magmas themselves for an 

 explanation of the textural variations. 



Influence of dominant constituents. — The characters shown by 

 the Maine pegmatites accord with the evidence obtained from many 

 other districts in indicating that the pegmatite magma were charac- 

 terized as a general rule by the presence of certain components in 

 larger amounts than in normal granite magmas and that these differ- 

 ences in composition were in large measure responsible for the differ- 

 ences in texture. The exact nature of such difi^erences is, however, 

 more largely a matter of inference than of direct field observation. 



In the great mass of the pegmatite, what may be termed the normal 

 pegmatite, it is exceedingly difficult, if not impracticable, to make a 

 satisfactory estimate of the relative proportions of the different min- 

 eral constituents, but as far as can be judged without measurements 

 the proportions are of the same general order as in the normal granites, 



