Tnmestone Belts of Westchester County, New York. 27 
(2.) Caleareous Rocks. 
The limestone of the County is, in general, coarsely crystal- 
line, and of a white to grayish-white color, and in many places 
it is quarried for use as an architectural marble. But in the 
northwestern part of the County, in the vicinity of the Ar- 
chan, it is feebly crystalline, and in part has the gray color and 
texture of the unaltered rock. The absence of crystallization 
is so marked in the part of the northwestern belt which occurs 
on the west side of the Hudson River, at Tompkins’ Cove, that 
much of it, especially in the western beds toward the Archean, 
is like ordinary gray and unaltered limestone, so that if the rock 
is without fossils—a point yet to be made certain—the reason is 
not their obliteration by metamorphism. 
According to the few analyses that have been published, the 
rock is a magnesian limestone or dolomite. Iron replaces in 
some beds a portion of the magnesium ; and when so the blocks 
of “marble” show it by becoming rusty in color after exposure. 
Tron is not unfrequently present also in pyrite (FeS,) another 
source of rust and destruction, and less frequently in pyr- 
rhotite (Fe,S,); chalcopyrite (or copper pyrites) is of occasional 
occurrence. Scales of mica, mostly of the species muscovite, 
are often distributed through the beds, and such micaceous 
blende) is very common in bladed crystals and fibrous or 
and in the limestone of New York Island near 208th stree 
localities mentioned by Mather, and the latter from the obser- 
vations of Professor Gale. I have not succeeded in my at- 
tempts to verify the fact at either place. 
_ Chlorite in scales is distributed through some limestone beds 
in the southern part of the County, as, for example, at a locality 
a mile northeast of Central Bridge, over Harlem River, and in 
Eastern Morrisania. Graphite is sometimes sparingly present, 
and more rarely small crystals of sphene. Apatite is found only 
in an occasional minute erystal. Chondrodite has not been 
observed. 
Much of the limestone crumbles on exposure, making sandy 
hills of its outcrops; but this is true of the same rock in West- 
ern Connecticut and Massachusetts. 
e Westchester County limestone beds are much thinner 
