﻿268 Canadian Record of Science. 



spoil ding numbers having been painted on some conspicuous 

 object on the several islands, thus enabling them to be 

 readily recognized. 



A short general account of the geology of these islands 

 was given some years since by Dr. A. P. Coleman/ in 

 a paper which appeared in this magazine, and a more 

 recent paper by Dr. C. H. Smythe, Jr.,^ gives a more 

 detailed description of the diabase dykes which cut the 

 country rock. Some additional information, obtained 

 during a recent visit to the islands, is presented in the 

 present paper. 



The prevailing rock is referred to by Dr. Coleman as a 

 granite, but as it usually possesses a more or less distinct 

 foliation it might perhaps be termed a granitoid gneiss. 

 The foliation is, however, usually very indistinct and a 

 true banded arrangement of the constituents was not 

 observed in it. It is composed essentially of quartz and 

 orthoclase with but small quantities of iron magnesia 

 constituents, these consisting of biotite and hornblende. 

 As a much more massive rock which may be classed 

 as true granite occur and is quarried on several of the 

 islands, e.g., Forsyth, Juniper and Grindstone Islands, the 

 relation of this to the granitoid gneiss just referred to is a 

 matter of much interest as bearing upon the problem 

 of the origin of the Laurentian gneisses, the question 

 presenting itself as to whether the granite in question 

 cuts this granitoid gneiss and is later than it, or whether 

 the gneiss is produced from the granite by movements set 

 up in it, either in a plastic or solid state. 



The granitoid gneiss composing several of the islands, 

 as, for example, No. 18, No. 16 and Campbell Island 

 (Sagastoweka) often shows a granulated or augen gneiss 

 structure. The granulated structure is well seen in rock 



1 Some Laurentian Rocks of tlie Thousand Islands. Can. Rec. of Sci., Vol. V., 

 No. 2, 1892. 



2 A Group of Diabase Dykes among the Thousand Islands, St. Lawrence River. 

 Trans. N.Y. Acad. Sci., VoL XIII., Aug., 1894. 



