THE 



POPULAR SCIENCE 

 MONTHLY. 



OCTOBER, 1878. 



THE GEOLOGICAL HISTORY OF NEW YORK 

 ISLAND AND HARBOR 



By J. S. NEWBEEKY. 



I NEW YORK in Ancient Geological Times. 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 lime- 

 stones 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 band- 

 ed and mottled, are nearly pure carbonates of lime, and the rocks im- 

 mediately 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 cr} r stalline 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 argil- 

 lites of Vermont. On the whole, the group of strata which forms New 



VOL. XIII. 41 



