8 Canadian Record of Science. 



deformation and alteration — considered that the foliation 

 and banding in them were evidence of original stratifica- 

 tion. And with the facilities for investigation then at 

 their command, this is not surprising. 



With increased knowledge, resulting from due con- 

 sideration of the effects produced by great dynamic 

 action, from detailed mapping in the field, and perhaps 

 more than anything else, from the application of the 

 microscope to the study of petrography, and the con- 

 sequent recognition of the exact composition and structure 

 of many of these rocks, this old idea has been completely 

 superseded. And in recent years, work by Lossen, 

 Lehmann, Daubree, Nauman, Eeusch, Schmidt, Milch, 

 Teal, Heim, G. H. Williams, and others has proved 

 beyond doubt that foliation may be produced in massive 

 plutonic rocks as a result of pressure before the rock has 

 wholly solidified ; and Lawson 1 has drawn attention to 

 the fact that " the various foliated crystalline rocks 

 usually classified as Laurentian were largely plutonic 

 rocks which have crystallized slowly, probably under an 

 extremely gradual diminution of temperature, from a 

 thickly viscid, coherent, or tough hydrothermal magma." 2 

 The foliation was explained as a result of " differential 

 pressure which, by causing a yielding or deformation, 

 induced a flow in the mass." 



From these considerations it is evident that much 

 valuable information may be obtained by adding to the 

 results of stratigraphic methods, those of a careful 

 microscopic examination of thin sections of the various 

 rocks; and that has been the aim in the following division. 



PETROGRAPHY. 



In the pages which follow, certain terms used to describe 

 structure may admit of a somewhat different meaning 

 than the one given to them here ; in order to avoid any 



1. Opera '-it. 



2. Barlow, A. E., Geol. Surv. Can., Ann. Rep't., 1S97, Vol. X., Part I, p. 51. 



