﻿GEOLOGY OF THOUSAND ISLANDS REGION IIO, 



Smyth. 1 The numerous dikes, chiefly of granite, which every- 

 where cut the Grenville give every facility for determining their 

 presence. They are in great number but for the most part of 

 very trifling displacement. Similar faulting locally in the 

 Paleozoic rocks suggests that this faulting is of Paleozoic date, 

 but the much greater number of faults noted in the older rocks 

 indicates some Precambric faulting at least, and of this there is 

 direct evidence in some instances. The hand specimen shown in 

 plate 5, lower figure, presents an adequate illustration. The rock is a 

 well banded, acid Grenville gneiss,. consisting chiefly of feldspar and 

 quartz and seems certainly a sediment, a metamorphosed shaly sand- 

 stone. The bands vary in color from a light reddish to a black- 

 ish red, and are very plain, though without sufficient contrast to 

 photograph. clearly. They are parallel to the bedding and seem 

 certainly to represent original lamination in the rock. Shearing 

 has occurred, with development of fracture cleavage, principally 

 at a high angle with the bedding, but with secondary fractures 

 which rudely follow it, and along many of the former minute 

 slips of the rock have taken place. These old cracks are now 

 solidly welded up with secondary minerals, black in color, except 

 for an occasional, shining pyrite crystal, and it is this secondary 

 filling which furnishes the evidence for the date of the deforma- 

 tion and gives the chief interest to the rock. Pyroxene, horn- 

 blende and black mica (biotite), stated in order of abundance, 

 are the minerals composing the filling, their grain somewhat 

 coarser than that of the rock. They are of the same types as the 

 minerals of the Grenville green schists. They argue for fairly 

 deep seated conditions at the time of the deformation. The 

 fractures show that the rock was above the zone of flow, but the 

 minerals, the pyroxene especially, indicate anamorphic condi- 

 tions and point to deformation in the lower part of the zone of 

 fracture. Such faulting seems not only of Precambric date, but 

 to have preceded the greater part of the long, Precambric erosion 

 interval. Its date is made quite certain by the numerous dikes 

 of Picton granite which cut the schists, the granite being younger 

 than the filling of the shear zones. 



There are also frequent shear zones in the Precambric rocks, 

 zones of no great breadth but of considerable linear extent, along 

 which the rock is shattered into quite small blocks by a multitude 

 of close spaced joints, and along which some faulting has cer- 

 tainly taken place, small slips along many planes. No such shear 



1 N. Y. State Geol. 19th An. Rep't, pi. 15. 



