PETROGRAPHY. 335 



ated strnotiire in <;neis.ses and schists, tosPtlier witli its relation to the 

 original bcchled structnre, where such existed, lias received a f?ood share 

 of attention, j)articnlarly IVoni British petro^raphers, and some very 

 interesting: and instructive results have been obtained. That a massive 

 ernptive rock of the composition of ^abbro may througli dynamic ngen- 

 cies underfio structural changes, including^ a i)aramor])hism of its ])y- 

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

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

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

 there has been a growing- leeling' among- ])eti'ologi'sts that the schistose 

 structure in manj^ of the so(!alled metamorphic rocks (meaning meta- 

 morphosed sedimentary de])osits) 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 dispute so far as it is api)lied 

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

 been and how far it is applicable to the great group of distinctl}^ banded 

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

 would care to claim it as universal. That sncli 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 Rainy 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 wiiich 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 (^uimperle 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 metamorphisu), 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- 

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

 ose rocks of the Malvern Hills; proof to this effect being afforded by 



* Bull. 1 1. S. Geol. Survey, No. 28, 188(5. 



tOueissic Foliation and Schistose Cleavaf^e in Dikes, Proo. Can. lust, of Toronto, 

 1886, p. 115. 

 {Geol. Mag., Noveniher, 1887, p. 484. 

 ^ Qiiar. Jour. Geol. Sci., Angnst, 1887, Vol, XLiil. )). WJ, 

 II Ibid., 1887, p. 517. 



