PETROGRAPHY. 335 



ated structure in gueisses and schists, together with its relation to the 

 original bedded structure, where such existed, has received a good share 

 of attention, particuhirlj^ from British i)etrographers, and some very 

 interesting and instructive results have been obtained. That a massive 

 eruptive rock of the composition of gabbro may through dynamic agen- 

 cies undergo structural changes, including a paramori)hism of its py- 

 roxenic constituent, and give rise to schistose dioritic forms, was first 

 conclusively shown in America by the researches of Williams* on the 

 gabbros in the vicinity of Baltimore, Maryland, and for several years 

 there has been a growing feeling among petiologists that the schistose 

 structure in many of the so called metamorphic rocks (meaning meta- 

 morphosed sedimentary deposits) was not due to or in any way con- 

 nected with an original bedding, but that these rocks were in reality of 

 eruptive origin. That this is to a certain extent a correct supposition 

 may now be considered as settled beyond disjjute so far as it is applied 

 to certain limited areas. How general this mode of origin may have 

 been and how far it is applicable to the great group of distinctly banded 

 Archfeau schists and gneisses is as yet largely conjectural, and few 

 would care to claim it as universal. That such structures are in any 

 way indicative of bedding and consequent sedimentary origin has been 

 vigorously combated by Lawson,t who finds similar structures in rocks 

 of undoubted eruptive origin in the Kainy Lake region of Canada. 

 Lawson, however, regards the structure as due to pressure supplied 

 not by orographic movements but by expansion in the erupted mass 

 itself during the process of consolidation, a view which is somewhat at 

 variance with that held by other investigators. Teall| has shown that 

 the banded rocks of the Lizard district include granite, diorite, and 

 gabbro ; are, in short, rocks of igneous origin, and their banded structure 

 due to the deformation to which the original rock masses have been 

 subjected. Bonney,§ too, has given it as his opinion that the foliated 

 glaucophane schist of the Isle de Groix is an eruptive altered by pres- 

 sure. His conclusions from a study of this rock, together with the 

 gneisses of the district around Quimperle and the gneisses, granites, and 

 amphibolites of the Koscott" and Morlaix districts, were to the effect 

 that while both igneous and stratified rocks have undergone a certain 

 amount of pressure metamorphism, the igneous rocks being converted 

 into gneisses and schists, yet many of them evidently possessed a true 

 foliation prior to the earth movements. Callaway || has likewise con- 

 tended in favor of the igneous origin of many of tiie gneisses and schist- 

 ose rocks of the Malvern Hills; [)roof to this effect being afforded by 



* Bull. U. S. Geol, Survey, No. 28, 18ri6. 



tGueissic Fpliiition and Sclijstoso Cloavas© in Dikes, Proc. Can. Inst, of TorontOj 

 }88t), p. 1K-, 



f Geol, Mag,, Noveniltor, 1887, \^. 484, 



ij Qiiar, Jour, Geol. 8ci., Aufjnst, lii'rt?, \o\, %mi, p. 301, 



II /W(/., 1687, p. 5ir, 



