1899.] Mr. T. H. Ko]\a,Tid— Mica-hearing Pegmatites. 87 



laro-e profits under such remarkable circumstances affords strong pre- 

 sumptive evidence of the value of the depo.sits and of the success whicli 

 should be expected, to follov^ a more scientific working of the many fine 

 pegmatite sheets hitherto untouched. 



There is probably no other group of rocks whose origin has been 

 the subject of more varied discussion than the pegmatites. De Saus« 

 sure received the support of Credner, Klockmann, Dana, Huntington, 

 Kerr and Sterry Hunt^ in likening them to metalliferous veins as the 

 result of the successive deposition of mineral matter from solution in 

 fissures, but recent researches support the earlier view of Oharpentier 

 (1823) who regarded the pegmatites as injections of granitic material 

 which, originating in the still fluid granite, deep down, was pressed into 

 the cracks of the already solidified granite and rocks above " after- 

 births," as it were, of the same granitic formation in the district in 

 which they occur. 



Even before Charpentier's time, however, similar views were pub- 

 lished by the old Cornish Greologists, Carne, Davy and others, who dis- 

 tinguished between what they called " contemporaneous veins " which 

 are I'elated genetically to the granite which they accompany and often 

 traverse, and the "true veins " filled with valuable ores and formed at 

 a distinctly subsequent period by the chemical infilling of fissures. 



It is now generally conceded that pegmatites have resulted from 

 the consolidation of injected fluid magmas, often directly traceable to 

 some large granitic mass. This view that they are merely contempora- 

 neous injections of the residual granite magma has been advocated by 

 De la Beche, Bronn, Fournet, Durocher, Angelot, Naumann, Lehmann, 

 Brogger, Reyer, Williams, Crosby and Fuller. 



Recently evidence has accumulated to show that these residual por- 

 tions of the granitic magmas, instead of being in a state of simple igneous 

 fusion, contain much larger proportions of water than the average 

 magma, and are consequently fluid at a very much- lower temperature. 

 Most, perhaps all, igneous magmas contain water, and, as in the process 

 of crystallization anhydrous minerals are separated, the water becomes 

 concentrated in the residuary mother-liquor which can thus remain 

 fluid at a much lower temperature. The injection of this aquo-igneous 

 melt into the neighbouring rocks, or into fissures in the granite just 

 solidified from the same magma, gives rise to the pegmatite veins. 

 With this view it is easy to explain the coarse grain which is so charac- 

 teristic of even the thinnest veins of pegmatite. The size of a crystal 

 is directly dependent on the freedom of molecular translation within 

 the molten magma (or solution) multiplied by the time during which 

 molecular segregation is permitted. In a magma which becomes viscous 



