﻿26 NEW YORK STATE MUSEUM 



In New York then the Keewatin volcanics are wholly absent, 

 except for the possibility that some of the amphibolite inclusions 

 of the granite gneiss may be greenstone fragments considerably 

 metamorphosed. Otherwise the Grenville sediments are the 

 oldest recognized rocks, and they occur in patches or in belts of 

 varying size and extent, resting on, surrounded by, and all cut 

 to pieces by the granite gneiss and the yet later intrusions. 



Grenville rocks. These rocks as originally deposited con- 

 sisted of limestones, shales and sandstones, both pure and in 

 their various transitional phases. In all probability too there was 

 some intermingled volcanic material, though the presence of such 

 material has never been definitely proved for the New York 

 Grenville. The rocks have been profoundly changed in char- 

 acter since their formation, in part owing to great compressive 

 stresses which operated throughout the district, and in part 

 owing to the heat and pressure furnished by the great igneous 

 intrusions, and also to the mineralizing agents to which these 

 gave rise. These changes moreover were brought about early in 

 Precambric time and under deep-seated conditions. As found 

 today the rocks are wholly crystalline, having completely 

 recrystallized under the severe conditions to which they were 

 subjected, with loss of all traces of their original clastic textures. 

 In their stead there has been developed a cleavage, or foliation, 

 due to parallel arrangement of the mineral particles on recrystal- 

 lization. The old bedding planes of the rocks can still be made 

 out, however, in places where the composition of the original 

 rocks changed, as where limestone was succeeded by shale or 

 by sandstone, and from these old bedding planes it can be seen 

 that the development of the foliation is parallel in direction to 

 them. The original limestones have become coarse, white 

 crystalline limestone or marble, the sandstones are now hard, 

 glassy quartzites, while the shales and impure limestones and 

 sandstones have become schists and gneisses of many types, 

 while yet other varieties are contact rocks whose nature is due 

 to action of the intrusives upon adjacent sediments. The variety 

 of rocks is so great that it would be a hopeless task to attempt 

 to map them all upon any such scale as that of the maps which 

 accompany this report. One or more beds of very thick lime- 

 stone occur, such as that along the Indian river northward from 

 Theresa, or that along Butterfield lake; thick quartzites also 

 occur, especially on Grindstone and Wellesley islands; a large 

 thickness of green schists of a peculiar type is found to the south 



