34 HOLLAND: MICA DEPOSITS OF INDIA. 



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 tem- 

 perature. Adopting the view expressed by Charpentier's expres- 

 sive phrase, the hydrous condition of the magma injected to form 

 these pegmatitic veins is capable of a simple explanation :— 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 pegma- 

 tite veins. 



With this view it is easy to explain also the coarse grain which is 

 so characteristic 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 on cooling, and in which the consolidation is rapidly 

 accomplished, the crystals formed are necessarily small, as they 

 always are for instance at the selvages of basic dykes, the converse 

 being the case when the magma retains its fluidity for a long period. 

 With what Reyer calls a hydatopyrogenetic (aquo-igneous) magma 

 the latter condition is possible, for there is then a small difference 

 between the temperature of the magma and of the rock into which it 

 is injected, and consequently a very slow dissipation of heat. The 

 reduction of temperature is still more retarded on account of the great 

 specific heat of the water contained in an aquo-igneous melt ; for to 

 reduce water by one degree in temperature involves the equivalent 

 rise of some three times the amount of average rock. The water 

 therefore, which becomes concentrated in the magmas that form our 



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