﻿GEOLOGY OF THOUSAND ISLANDS REGION 45 



of the dike numbered I on the map. Two have been found on 

 Wellesley island, the wider of which measures 30 feet. Seven have 

 been found on the mainland of the Alexandria sheet, in rather 

 widely scattered distribution, and in general much narrower. None 

 have been observed on the Theresa sheet. Smyth has described them 

 as abundant on the Canadian mainland and islands in the vicinity 

 of Gananoque, hence in the near vicinity of Grindstone island, 

 which would seem to have been the chief center of activity. For 

 petrographic details the reader is referred to his account which, 

 though based on Canadian material, also describes these accurately. 1 



The dikes trend in various directions, from northwest around 

 through north to northeast. Smyth states that those seen around 

 Gananoque trend chiefly to the north, and were all cutting granite. 

 It is to be noted that all those trending northeast, in our district 

 here, are cutting Grenville rocks with general northeast strike, while 

 all the dikes cutting the igneous rocks trend north or northwest. 

 This is also true of two of the dikes cutting the Grenville, but in 

 both cases the Grenville is in comparatively small bulk, and entirely 

 inclosed by igneous rocks. The dike directions are therefore appar- 

 ently determined by preexisting structures in the rocks, by the strike 

 in the Grenville, and by a joint set in the igneous rocks. Small 

 masses of Grenville rocks did not suffice to change the direction of 

 dikes passing across them, the igneous rocks here being the deter- 

 mining factor. 



Though they give no evidence of having been severely deformed, 

 yet the rock of the larger dikes does show evidences of considerable 

 pressure. Many of the feldspar crystals are distinctly bent, and both 

 the feldspar and augite of the rock shows evidence of strain by their 

 undulatory extinction. In this respect they contrast with the diabases 

 of the eastern Adirondacks, which show no such strain effects. 

 The eastern dikes also have chiefly east-west trends, differ somewhat 

 in mineralogy, and are more numerous and widespread; and are 

 also separated from this area by a wide region in which such dikes 

 are absent. We seem here therefore to be dealing with a wholly 

 different center of igneous activity, and a much less extensive one 

 than that farther east. 



Owing to their size and comparative freshness these dikes have a 

 potential value in the region as a comparatively accessible source of 

 good road metal. 



Contact rocks. The contact effects of the igneous rocks upon the 

 Grenville sediments, and vice versa, may be grouped under three 

 1 N. Y. Acad. Sci. Trans. 13 -.20^-14. 



