﻿GEOLOGY OF THOUSAND ISLANDS REGION 47 



ing the color as beyond question original. The red color which so 

 many feldspars possess is usually ascribed to ferric oxid, though in 

 general without any definite proof in the matter. In such case the 

 loss of color might be ascribed to simple reduction of the iron, but 

 what reducing agent the limestone might furnish is a difficult prob- 

 lem and greenish, rather than white, feldspar would likely result. 

 Analyses of both white and red granites are given on a later page, 

 where the matter will be somewhat further discussed. The chemical 

 differences between the two rocks are but slight, and we are in 

 doubt whether in any recognizable respect they are due to influence 

 of the limestone. The field relations are, however, perfectly clear, 

 and susceptible of no other explanation. 



Mixed rocks. Rocks which seem definitely of intermediate com- 

 position between the intrusive and a sediment, to be due to the 

 intimate penetration and final digestion of the latter by the former, 

 and which show all stages in the process, occur as the result of action 

 of granite upon amphibolite and upon quartzite. In the former the 

 action is chiefly seen in the case of the amphibolite inclusions which 

 so abound in the granite gneiss, and which are found in all stages of 

 being first penetrated by films of the granite and later slowly 

 absorbed by it. The process has already been described ; so has the 

 gradation of granite into quartzite which is found in some localities 

 and which seems only explainable on the assumption of production 

 of a border zone of true mixed rock between the two. 



Contact rocks.. These, as here understood, result from the injec- 

 tion into the sediment of fluids from the igneous rock which contain 

 only certain of its constituents instead of all, and which may, and 

 often do, differ very materially in composition from the rock itself. 

 The injection is apt to be more or less local, here much, there little, 

 or none at all; the injected fluid may differ in composition at 

 different points along the border of the igneous mass; the bordering 

 rocks themselves differ from place to place, and finally the various 

 igneous masses are quite sure to differ among themselves in the 

 character of their mineralizing fluids. Since we have here three 

 separate granite bathyliths, to say nothing of the syenites and smaller 

 granite masses, and Grenville rocks of great variety of composition, 

 the opportunity for contact action ot diverse sorts is exceedingly 

 good. 



Green schists in Alexandria. Reference to the geologic map of 

 the Alexandria quadrangle will show, to the south and southwest 

 of Alexandria Bay, three northeast-southwest ridges of Grenville 

 schists. These are cut out on the north by the granite of the 



