Page:Popular Science Monthly Volume 13.djvu/661



NEW YORK —The rocks which compose New York Island and underlie the adjacent country on the north and east are chiefly gneiss and mica-schist, with heavy intercalated beds of coarse-grained, dolomitic marble and thinner layers of serpentine. These are all distinctly stratified, and have once been sedimentary beds deposited horizontally—sandstones, shales, and limestones—but now, upheaved and set on edge, are by metamorphism converted into compact crystalline strata with the obliteration of all fossils—if fossils they contained. The age of these rocks has not yet been accurately determined, although they have usually been supposed to be Lower Silurian, and a continuation of those which contain the marble-beds of Western Massachusetts and Vermont. There are some reasons, however, why they should be regarded as still older. That they do not form the southern prolongation of the marble belt of Vermont is indicated by the facts that both the marble-beds and the rocks associated with them are so unlike in the two localities that they can hardly be parts of the same formation. In Vermont, the marbles occur in what is essentially a single belt, are fine-grained, usually banded and mottled, are nearly pure carbonates of lime, and the rocks immediately associated with them are gray siliceous limestones, quartzites, and slates. In Westchester County, and on New York Island, on the contrary, the marbles are very coarsely crystalline dolomites (double carbonates of lime and magnesia), which occur in a number of parallel belts, are generally of uniform white or whitish color, and have no rocks associated with them that can represent the quartzites and argillites of Vermont. On the whole, the group of strata which forms New