﻿30 NEW YORK STATE MUSEUM 



and white rock which is of common occurrence in the Grenville 

 wherever known. In the writer's experience this is far from being 

 true of the quartzose limestone which occurs in much greater force 

 here than is customary. 



Of the various Grenville rocks the limestones are much more 

 yielding under compressive stresses than are the schists and quart- 

 zites, behave more like plastic and less like brittle bodies, and hence 

 change shape more readily. As a result rocks which much resemble 

 coarse conglomerates are a frequent feature in the Grenville lime- 

 stone. Frequent dikes of granite traverse it, many of which are of 

 slender width. Under compression these are brittle under condi- 

 tions which are sufficient to cause flowage in the limestone, hence 

 the dikes fracture, the separate fragments are somewhat shifted in 

 position and limestone is squeezed in between them. The same 

 thing takes place where thin bands of quartzite or of schist are 

 present in the limestone, as is frequently the case. These frag- 

 ments of granite, quartzite or schist weather less rapidly than the 

 surrounding limestone, and hence project somewhat on weathered 

 surfaces, with considerable increase in conspicuousness, and the 

 separate fragments surrounded by calcite give an admirable mim- 

 icry of a conglomerate in appearance. 



In addition to the normal white limestone frequent patches or 

 streaks of gray or blue limestone also occur in association with it, 

 which outwardly look much more like ordinary limestone. This 

 is in line with the further fact that all the Grenville rocks seem 

 somewhat less severely metamorphosed than is the case with the 

 equivalent rocks to the eastward. Even the white marble has at 

 times a grayish or bluish cast, and does not average as coarsely 

 crystalline as the eastern Grenville limestone. On the other hand 

 limestone of these characters is commonly not so pure as is much 

 of the white limestone, and these gray or blue portions often occur 

 in such situation as to suggest that they are contact effects of 

 igneous rocks on the white limestone. In some instances certainly 

 the white limestone changes to gray adjacent to an igneous rock 

 mass of good size, and in others gray patches in white limestone 

 occur in direct contact with granite dikes, an unlikely situation if 

 they are really less metamorphosed portions of the white limestone. 

 It is also true, however, that some of the gray limestone is very 

 pure, that in some places it has no discoverable nearness to any 

 igneous rock, and that in general the contact action of the igneous 

 rocks of the district upon the limestone has been but slight, though 

 with local exceptions to this statement. With such arguments in 



