304 EDSON S. BASTIN 



such infrequent occurrence as ordinarily to escape detection entirely. 

 In pegmatities where they are present their paucity or abundance seem 

 to have small influence on the textures developed. Those inclined 

 to attribute large influence in the development of pegmatitic textures 

 to the presence of rare constituents usually contend that a more 

 careful study will show that their scarcity is apparent rather than 

 real. Such an assumption is not in accord with the field obser- 

 vations of the writer in Maine and other parts of New England, and 

 appears unwarranted. 



Influence of gaseous constituents. — If neither the dominant nor the 

 rare minerals of the pegmatites have been controlling factors in the 

 development of typical pegmatitic textures, it appears necessary to 

 seek an explanation in the presence in the magmas of certain con- 

 stituents which have subsequently escaped or at least are not readily 

 recognized in the resultant rock. The fact that large crystals cannot 

 be obtained at atmospheric pressures from simple dry melts of the 

 commoner rock-forming minerals suggests at once that the crystalli- 

 zation of these minerals in nature took place either under widely 

 different physical conditions (such as high pressure) or in the pres- 

 ence of certain substances which are scarce or absent in the rocks as 

 now exposed. It has already been shown from field evidence (p. 302) 

 that in many instances differences in pressure or other external con- 

 ditions at the time of crystallization cannot reasonably be appealed 

 to, to explain the textural variations observed. In such cases an appeal 

 to the escaped constituents of the magma appears unavoidable. The 

 same conclusion appears necessary when we consider the extreme 

 viscosity exhibited (under atmospheric pressures) by silica, orthoclase, 

 and albite near their melting temperatures. The various forms of 

 silica which have been artificially produced have all crystallized from 

 a melt so viscous as to be virtually a glass. ' In the case of orthoclase 

 the viscosity of its melt is so great that all attempts to crystallize the 

 mineral from it have been unsuccessful. Since increase in pressure 

 per se can hardly be appealed to as increasing molecular mobility^ 



1 See Day and Shepherd, "The Lime-Sihca Series of Minerals," Amer. Jour. 

 Sci., XXII, 271-73 (1906). Also Day and Allen, "The Isomorphism and Thermal 

 Properties of the Feldspars," Publications 0/ the Carnegie Institution 0} Washington 

 No. 31, 28-29, 45-55 (1905). 



2 See Harker, The Natural History of Igneous Rocks, 163-64 (1909). 



