﻿GEOLOGY OF THOUSAND ISLANDS REGION IOI 



of all the types of Grenville rocks of the district does not give 

 the impression of a group of extra well foliated rocks. This is 

 largely due to the comparative scarcity of micas, and of amphi- 

 boles of slender habit, in the series and the abundance of 

 pyroxenes and of stout amphiboles. This again is a result of 

 the prominently anamorphic character of the metamorphism. 



The foliation of the Grenville rocks is parallel to the bedding. 

 In the schist series rapid alternations of materials of somewhat 

 varying composition is a feature, producing a very well banded 

 structure, sometimes so fine as to somewhat mimic a coarse 

 foliation. 



Foliation of the granite gneiss. It has been shown that the 

 Laurentian granite is characterized by frequent inclusions of 

 older rocks, chiefly of amphibolitic types, and that there is also 

 present much intermediate material, resulting from the soaking 

 of the amphibolite with granitic substance, or from its actual 

 digestion by the granite. The rock itself contains normally 

 some mica or hornblende, and hence, through the greater por- 

 tion of the mass these minerals are present in varying quantity, 

 and the rock is susceptible of foliation development under the 

 proper conditions. That such conditions have obtained is clearly 

 shown, a foliation cleavage of varying prominence appearing 

 nearly everywhere, though it becomes very obscure in those 

 relatively small portions of the mass which consist solely of 

 quartz and feldspar. The general rock is thus foliated but with 

 foliation of the crude type which proclaims the rock a gneiss, 

 rather than a schist. 



The foliation structure of the granite gneiss conforms everyzvhere 

 in dip and strike to that of the adjacent Grenville rocks. While 

 this by no means excludes the possibility that the Grenville rocks 

 may have been compressed and foliated prior to the intrusion of 

 the granite, it does demonstrate that both sets of rocks have 

 undergone compression in common, subsequent to this intrusion. 

 It is quite possible that much of this compression was a result 

 of the actual intrusion, and that the granite gneiss actually 

 solidified with a foliated structure. This is not at all uncommon 

 in great bathylithic intrusions, which, in order to make a place 

 for themselves, must endeavor to shoulder aside the rocks previ- 

 ously occupying the space. This "shouldering pressure exerted 

 on the adjacent rocks under bathylithic, or deep seated, condi- 

 tions, that is with a thick cover of overlying rocks, tends to 

 give the rocks thus compressed a foliation which parallels the 



