﻿GEOLOGY OF THOUSAND ISLANDS REGION 37 



thus becomes a conspicuous constituent. The rock changes from 

 deep red through lighter shades to nearly white. It varies also 

 much in texture, from throughly solid looking, crystalline appear- 

 ance to varieties which weather to a sugary, granular aspect. 



As usual in the Laurentian, inclusions albound, and as usual 

 the bulk of these are of amphibolite. Quartzite inclusions 

 also occur, but infrequently, limestone inclusions never. The 

 amphibolite inclusions are found everywhere but always most 

 abundantly near the margins, where they abound. In fact a 

 sharp boundary line between the granite gneiss and the adjacent 

 Grenville rocks can not be drawn. In passing from granite to 

 sediments the inclusions show steady increase in number until 

 they come to constitute 50^ of the rock, beyond which we find 

 sediments cut by granite dikes rather, than granite holding inclu- 

 sions of sediments. This reduces boundary mapping to a matter 

 of estimating equality or inequality in amount of the two rocks, 

 or in drawing a boundary where no real one exists. An attempt, 

 however, has been made to indicate, by convention, on the maps, 

 the actual state of things found in the field. 



The granite dikes usually represent the extreme acid state of 

 the rock. The main mass averages less acid, chiefly because of 

 the inclusions and of the attack of the granite upon them. In 

 its preliminary stages this usually takes the form of an injection 

 of the granite in thin sheets along the foliation planes of the 

 amphibolite, the so called " lit-par-lit," or leaf type of injection, 

 producing a banded rock of alternations of igneous and sedi- 

 mentary material. Then, here and there, the granite breaks out 

 from the foliation planes and spreads through the rock adjacent, 

 forcing its grains apart by the injection of a thin film of granite 

 between. This process becomes more and more pronounced, 

 until much of the rock is broken up into a granular mosaic of 

 particles cemented together by granite films, producing what 

 may be called the mosaic type of injection, as distinguished from 

 the leaf type. A fine example of injection of this type is shown 

 in plates 4 and 5. The injected rock is not amphibolite, but is green 

 schist, a closely related rock, and the type of injection is identical. 

 As a further stage, in both types of injection, the sharp bound- 

 aries become blurred, and this shading of the two rocks into one 

 another becomes more and more prominent until finally rocks 

 result which seem unquestionably to be due to the complete 

 digestion of the amphibolite by the granite, gray gneisses of 

 distinctly intermediate composition. As would be expected 



