722 A TREATISE ON METAMORPHISM. 



interprets those pegmatites which by their mode of occurrence and associa- 

 tion strongly indicate an igneous character as the products of the residual 

 and therefore most acid portion of a granite magma highly charged with 

 water and other mineralizing agents. Such a siliceous material, in a state 

 intermediate between fusion and solution, has been injected into fissures and 

 there crystallized into very coarse-grained aggregates, not necessarily 

 through any great slowness of this process, but rather in virtue of the aid to 

 crystallization afforded by the abundance of mineralizers present." 



While Williams and Brogger strongly emphasize the igneous side of 

 pegmatization, they both agree that some of the pegmatites which show a 

 comb structure are essentially the result of aqueous processes. 6 



Standing at the other extreme is Hunt, who maintains a strictly 

 aqueous origin for pegmatites/ In an intermediate position, ascribing an 

 aqueo-igneous origin to pegmatites, are Elie de Beaumont, Scheerer, Leh- 

 mann, Credner, Reyer, and Crosby and Fuller. Scheerer in 1847, according 

 to Hunt, "Conceives the congealing granitic rocks to have been impreg- 

 nated with 'a juice,' which was nothing else than a highly heated aqueous 

 solution of certain mineral matters. This, under great pressure, oozed out, 

 penetrating even the stratified rocks in contact with the g-ranite, filling 

 cavities and fissures in the latter, and depositing therein crystals of quartz 

 and of hornblende, the arrangement of which shows them to have been of 

 successive growth." d Reyer, following Scheerer, regards the pegmatites as 

 "Exsudate der erstarrenden Granitmasse." 6 But of all the authors writing 

 upon pegmatites Lehmann/ followed by Crosby/ makes the closest approx- 

 imation to the view which I hold in reference to pegmatites. Lehmann 

 conceives that as a result of crystallization of the parent mass and the 

 concentration of the water in the residual uncrystallized part, a gelatinous 

 magma rich in silica is formed. ' ' Between such a gelatinous magma and a 

 saturated aqueous solution a large number of consecutive intermediate 

 stages can be imagined.'" 1 However, Lehmann insists that no part of the 



« Williams, G. H., The general relations of the granitic rocks in the middle Atlantic Piedmont 

 Plateau: Fifteenth Ann. Rept. U. S. Geol. Survey, 1895, p. 684. 



i Williams, cit., p. 679. 



«Hunt, T. Sterry, Chemical and geological essays: Osgood & Co., Boston, 1875, pp. 191-219. 



tfHunt, cit., p. 189. 



« Reyer, E., Theoretische Geologie: Schweizerbart'sche Verlagshandlung, Stuttgart, 1888, p. 101. 



/Lehmann, Johannes, Untersuchungen uber die Entstehung der altkrystallinischen Schiefer- 

 gesteine: Bonn, 1884, pp. 24-58. 



<3 Crosby, W. O., and Fuller, M. L., Origin of pegmatite: Tech. Quar. vol. 9, 1896, pp. 326-356. 



''Crosby, W. O., and Fuller, M. L., citing Lehmann, cit., p. 345. 



