﻿GEOLOGY OF THOUSAND ISLANDS REGION II 



What relation the two bear to one another could not be definitely 

 ascertained. Either the augen gneiss is a crushed border phase of 

 the other, that representing an uncrushed core, or else it is a 

 separate and older rock. It is a fairly basic rock, varying much in 

 this respect, seems at times to owe its character to partial as- 

 similation of amphibolite, and so far as seen, its exposed contacts 

 are all with Grenville rocks, which it cuts. If the two intrusives 

 belong together the mass reaches considerable size and is to be 

 classed as a small bathylith. If the augen gneiss is distinct from 

 the other the latter is only a stock. 



In case the augen gneiss is distinct the question naturally arises 

 whether it may not be merely a porphyritic phase of the Lauren- 

 tian granite gneiss. A decisive answer to this question can not be 

 given owing to lack of contacts between the two classes of rock. 

 But such evidence as there is seems decidedly against such a cor- 

 relation. The rock is a more basic one than the general run of the 

 granite gneiss, and is not so severely crushed, or granulated. The 

 weight of the evidence is decidedly in favor of the view that it 

 is a gneissoid, border phase of the syenite. 



Syenite southwest of Theresa. Up the creek valley above 

 Theresa are exposures aggregating about a square mile in extent of 

 a gray to gray green rock which is a syenite. It may have con- 

 siderably greater extent underneath the sandstone which adjoins 

 it on each side. It is by no means so mashed as the granite gneiss 

 and seems clearly a younger rock, but since it is not found in as- 

 sociation with any of the other younger igneous rocks its age rela- 

 tions to them are not ascertainable. 



There is a single outcrop of a coarse, unmashed eruptive which 

 is to be classed as a gabbro, close to the upper bridge at Theresa 

 on the west bank of the river. It may have considerable extent 

 under the adjacent sandstone but with the most generous possible 

 allowance for such extension the mass would still have to be rated 

 as a stock of no great size. 



Picton granite. 1 The most extensive and important of these 

 younger Precambric intrusives is the coarse red granite which out- 

 crops widely on Grindstone, Wellesley and some of the smaller 



iThe most considerable outcrops of this rock within the State are on 

 Grindstone island, but the name of Grindstone granite would perhaps be 

 misleading, and Grindstone Island granite is too long a name. The smaller 

 Picton island is however the seat of the chief quarries at the present time 

 and the name would be wholly appropriate except for the fact that the 

 island appears on the maps as Robbins island. It is universally called 

 Picton island by residents, many of whom have no knowledge of any such 

 name as Robbins island. 



