﻿GEOLOGY OF THOUSAND ISLANDS REGION 9 



New York, been able to discover anywhere any trace of the 

 older rocks which formed the floor upon which these water-laid 

 sediments were deposited, though plainly, with such an origin, 

 they must originally have been laid down on some such floor of 

 older rocks. It follows therefore that we do not know the base 

 of this Grenville series. Neither do we know its summit, since 

 that has apparently been everywhere removed by erosion. 

 Hence we can not know its thickness, though we do know that 

 it is a very thick rock series, several thousands of feet at least. 



Since the deposition of this formation it has undergone many 

 changes. The rocks have been greatly compressed and intri- 

 cately folded and plicated. They have been invaded from be- 

 neath by huge masses of igneous rocks, which have broken up the 

 once continuous Grenville formation into separate and discon- 

 nected belts and patches, have probably engulfed and digested 

 large amounts of it, and are likely responsible for the utter dis- 

 appearance of the old floor on which the formation originally 

 rested. As a result of this mishandling the rocks have been pro- 

 foundly changed in character. They have been entirely re- 

 crystallized, with complete destruction of the textures which, as 

 sediments, they originally possessed, and with the production of 

 a foliation cleavage, or schistosity, due to a banded arrangement 

 of the minerals formed by the recrystallization. In addition a 

 quantity of contact rocks has been produced in the vicinity of, 

 and by the action of, the igneous rocks, which interact with the 

 others to produce rocks quite different from either, and with 

 opportunities for manifold variation, with variation in the 

 character of either or both sets of the original rocks. In this 

 manner many rock types have arisen, often of puzzling nature. 



The changes which have been produced in these Grenville rocks 

 are of such nature as to lead to the confident belief that they took 

 place at some considerable depth below the surface, or in other 

 words that a considerable thickness of other rocks then overlay 

 them, a rock thickness which subsequently disappeared because of 

 surface wear continued through long ages. 



Igneous intrusions 



As has been implied the Grenville sediments are the most ancient 

 rocks of "which we have definite knowledge in northern New York. 

 Subsequent to their formation they were repeatedly invaded from 

 beneath by igneous rocks in molten condition. In the immediate 

 district the bulk of this igneous rock consisted of granite, and 



