44 THE OKYSTAL FALLS IKON-BEAlilNG DISTRICT. 



tendency to this ai-rangement, and are usually parallel to the banding. 

 The most perfect schistosity is thus developed parallel to the micaceous 

 bands. The banding- and the schistose structure are plainly of secondary 

 origin, the result of dynamic action. 



Others of the gnessoid granites, however, when examined inider the 

 microscope, are decidedly inassive, and it is only on a large scale that the 

 banding shows distinctly. In such cases the cause of the banding could not 

 be determined, and might by some be ascribed to differentiation, though, 

 from the association of these gneissoid granites with those just described, it 

 is assumed that the banded str xcture is due to dynamic action. If this be 

 the case, however, a complete recr3^stallization has taken place, and slight 

 dynamic effects are now shown. The strike of the banding, wherever it 

 was taken, was uniform, varying from N.-S. to nearly N. A5° W., agreeing, 

 on the whole, with the trend of the Archean oval area. 



The microscope shows that the constituent minerals of the gneissoid 

 granites are the same as those which compose the granites just described. 

 These show also the same relations to one another and the same general char- 

 acters as in the granites, except where mashing has completely obliterated 

 the original texture, and hence no further description of them is necessary. 



The crushing to which the gneissoid granites have been subjected is 

 very clearly shown in the present cataclastic condition of the quartz and 

 feldspars. 



As stated above, both the gneissoid granite and the granite proper are 

 found in the border area of the Archean. In those rocks in which the con- 

 tact shows a gradual transition from the banded rock to the unbanded, the 

 micaceous bands are clearly secondary, and are the result of the crushing 

 of the original gi'anite, these lines representing macroscopic and microscopic 

 shearing planes along which the feldspar and quartz have been thoroughly 

 granulated, and sericite and some biotite produced, as was found to be the 

 case also in some of the granites. These rocks thus agree in their dynamic 

 origin with a similar but apparently more extensive and better developed 

 gneissoid border facies in the Morbihan (Brittany) granites, which have 

 been described, and whose origin has been so clearly demonstrated by 

 BaiTois.^ Numerous other similar cases have been described recently from 

 the Canadian granite massifs and from Sweden and other districts. 



'Ann. Soc. G(Sol. du Nord., 1887, p. 40. 



