ORIGIN OF THE PEGMATITES OF MAINE 305 



in magmas, it seems necessary again in accounting for the large crys- 

 tals developed in the pegmatites to postulate the presence in the 

 magma of some substance or substances not now recognizable in the 

 derived rock. That the presence of volatile constituents in a magma 

 does influence the viscosity is shown by the fact that certain obsidians 

 may be readily melted with evident fluidity and the escape of gases, 

 but that their refusion after such gases have escaped is much more 

 difficult. Prof. Iddings' has also shown from a microscopic study 

 of the obsidian of Obsidian Cliff, Yellowstone National Park, that 

 where there was more dissolved gas the conditions were more favor- 

 able for crystallization than in other parts of the magma. 



Among those constituents of magmas which might escape, leaving 

 little record of their former presence, water gas and hydrogen are prob- 

 ably the most abundant, as is plainly indicated by analyses of the 

 gases still remaining in igneous rocks^ and by studies of the gases 

 emitted from volcanic vents. ^ 



The presence of water gas in association with subordinate amounts 

 of other gases and of certain unusual substances (mineralizers) has 

 been considered by many observers to be the competent and effective 

 cause in the development of pegmatitic textures. With this opinion 

 the present writer is in general accord, though the persuasion is based 

 more largely upon the process of reasoning already outlined than 

 upon field evidence of high water content or relatively low viscosity 

 in pegmatite magmas. The field evidence gathered in the study of 

 the Maine pegmatites must be looked upon as merely suggestive; 

 anything like a complete solution of the problem will in all probability 

 wait upon synthetic laboratory experiments upon the interaction 

 between gases and rock-forming silicates. 



The small weight of the gaseous and liquid constituents of most 

 igneous rocks as compared with the total weight of the rock might 

 lead one to question their competence to affect notably the viscosity 



1 J. P. Iddings, Seventh Ann. Rept. U.S. Geol. Survey, 283-87 (1888); idem, 

 Igneous Rocks, 185 (1909). 



2 See R. T. Chamberlin, "The Gases in Rocks," Publications o) the Carnegie 

 Institution 0} Washington No. 106 (1908). This includes a summary of earUer 

 investigations. 



3 For a review of the hterature on volcanic gases, see Clarke, "The Data of Geo- 

 chemistry," Bull. U.S. Geol. Survey No. 330, chap, viii (1908). 



