﻿GEOLOGY OF THOUSAND ISLANDS REGION 3 1 



mind it has seemed to the writer as though the weight of the evi- 

 dence were in favor of the view that at least some of the gray and 

 blue limestone was representative of the white in less metamor- 

 phosed condition, and if some, then likely all. 



Nowhere else in northern New York has the writer met with 

 Grenville limestone of this fine grained, darker colored type. A 

 comparison is at once suggested with the district in Ontario re- 

 cently described by Adams who has shown that a similar, though 

 better marked change comes over the Canadian Grenville limestone 

 when followed westward, a local development of bluish limestone 

 in thin bands within the coarser white limestones. 1 The evidence 

 seems to indicate that we have here in New York the first glimpses 

 of a similar tendency. 



The consideration of the contact effects which the various igneous 

 rocks have had upon the limestones is deferred until the igneous 

 rocks themselves have been described. 



Quartzites. There are two belts of ponderous quartzites in the 

 region, one on Wellesley and Grindstone islands, and the other in 

 the district east of Redwood (Alexandria sheet). In both cases 

 the quartzite is interbanded with various schists and amphibolites, 

 in highly folded condition, so that the number of quartzite beds is 

 uncertain, and whether there is more than one massive, thick quart- 

 zite can not be positively stated. There is certainly a considerable 

 number of thinner bands. Unless our interpretation of the struc- 

 ture is wholly at fault, these two belts represent lines of outcrop 

 of the same geologic horizon, and form the .youngest rocks of the 

 series exposed in the district. In addition to this main horizon 

 there are also frequent quartzite bands found in the general schist 

 series, and thin bands even occur at times with the limestones. 

 The more prominent of such bands are indicated upon the maps. 



The ponderous quartzites are the most resistant rocks of Pre- 

 cambric age in the region, and since they are interbedded with 

 schists which are far weaker, the districts where they outcrop are 

 quite rugged topographically, as Smyth pointed out 10 years ago. 

 The quartzite radges tower abruptly above the narrow valleys eaten 

 cut along the schists. 



Since the rock is an altered sandstone, recrystallized under heat 

 and pressure, and since sandstones often range in composition from 

 a high degree of purity to those which are quite impure, either 

 shaly, or calcareous, it is but natural to find much variation in the 

 rock from place to place. The thick bands are chiefly constituted 



1 Adams, F. D. Jour. Geol. 16 -.623-24. 



