QUARRY MATERIALS OF NEW YORK 69 



Section 4 



FIELD OCCURRENCE OF THE GRANITES, GNEISSES, 



TRAPS, ETC. 



THE ST LAWRENCE RIVER GRANITES 



Granite and granitic gneiss are exposed on several of the larger 

 islands in the St Lawrence river, particularly in the stretch from 

 Clayton to Alexandria Bay and over a considerable area on the 

 adjacent mainland. They are outlying representatives of the 

 Adirondack crystallines, though separated from the main area of 

 the latter by an interval in which the surface formations consist 

 mainly of undisturbed Paleozoic sediments. -These rocks un- 

 doubtedly covered the whole region at one time, but have been 

 eroded away here and there so as to expose the underlying Pre- 

 cambric basement. In contrast with the Adirondacks, the Pre- 

 cambric area along the St Lawrence presents very little relief, for 

 the most part being less than 100 feet above the river and much 

 of it is quite flat. Suitable quarry sites are therefore not so com- 

 mon in this section as in the interior highland where rocks of 

 similar or identical character occur, but the region is favored by 

 the facilities for water transportation which give access to the im- 

 portant markets on the St Lawrence and Great Lakes at very low 

 rates. 



The most valuable quarry material in this section is the red 

 granite of Grindstone, Picton and Wellesley islands, a product with 

 which the name Thousand Island granite is popularly associated. 

 It has had a fairly large sale for building and monumental purposes, 

 taking rank with the best of the red granites from American 

 quarries. In general it is a bright red, coarsely textured rock, but 

 medium-grained and fine-grained varities also occur. It has a 

 thoroughly massive appearance, and the grain is very uniform so 

 far as relates to the product of a single quarry. 



The present exposures of this granite have been traced on the 

 geological maps prepared by Cushing and others for the report 

 on the " Geology of the Thousand Islands Region." 1 The granite 

 extends from the central part of Wellesley island, where it is in 

 contact with the older granitic gneiss series, to the western limits 

 of that island, and reappears on Grindstone, of which it constitutes 



1 N. Y. State Mus. Bui. 145, 1910. The red granite lies mainly within 

 the Grindstone quadrangle. 



