ORIGIN OF PEGMATITES. 721 



may be called pegmatites. So far as my observation goes, many of the 

 more complex phenomena of pegmatization which can not be explained 

 by igneous injection alone are in regions in which the rocks have been 

 buried to a very considerable depth. They commonly form in the lower 

 part of the zone of fracture, in the zone of combined fracture and flowage, 

 or in the part of the zone of flowage in which continuous fractures may 

 exist temporarily The phenomena occur to a great extent in the lower 

 zone in connection with offshoots from the deep-seated batholiths — i. e., 

 injection dikes which have made their way along planes of rock weakness, 

 such as contacts or cleavage planes. Probably a strictly logical arrange- 

 ment would require that the treatment of pegmatites be deferred until the 

 relations of the zones of anamorphism and katamorphism are considered, 

 but the relations of pegmatites to deep-seated intrusives are so close that, 

 for the sake of continuity, the subject is dealt with here. 



Pegmatization has been variously explained as the result of true igneous 

 injection, of acpieo-igneous action, and of water impregnation or cementa- 

 tion. Brogger has strongly enforced the idea that many pegmatite veins 

 are true igneous injections." In support of this idea he cites the undoubted 

 frecpient association of pegmatitic veins with intrusive masses of acid rock, 

 the fact that many of the pegmatitic veins behave like other eruptives, and 

 that their structure is that of igneous rocks. 



Williams is in substantial agreement with Brogger in assigning' an 

 essentially eruptive orig'in for the greater number of the pegmatite dikes of 

 the Piedmont Plateau. 6 In favor of this view he cites (1) the likeness in 

 chemical and mineralogical composition of the granite masses to the pegma- 

 tites, although it is stated that the pegmatites are usually somewhat more 

 acid; (2) the fact that the size and abundance of pegmatites are directly 

 proportional to their nearness to eruptive granite masses; (3) that in com- 

 position the pegmatites are independent of the rocks which surround them, 

 and (4) that they do not show the drusy or symmetrical character of veins. 

 While Williams thus holds to the igneous origin of the pegmatites, he 

 thinks they show a greater activity of mineralizing agencies than the normal 

 granite. In conclusion of his discussion, he says: "The writer therefore 



"Brogger, W. C, Syenitpeginatitgiinge der jsiidnorwegischen Augit-und Nephelinsyenite: Zeitsch. 

 liirKryst, vol. 16, 1890, pp. 215-235. 



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

 Plateau: Fifteenth Ann. Kept, U. S. Geol. Survey, 1895, pp. 675-684. 

 MON XLVII — 0-1 46 



