Inmestone Belts of Westchester County, New York. 23 
f 
(1843), and that of Percival on the Geology of Connecticut 
(1842). Mather mentions the principal kinds of rocks, various 
localities for those that are of special or economical interest, 
and some stratigraphical facts. Many: localities of limestone 
are given, and I have thence derived much aid in the study of 
the region. On his map the positions of several of the areas 
In presenting my observations I will speak first, of the 
rocks; secondly, of the general distribution of the limestone 
areas or belts; thirdly, of the special positions and stratigraphy 
f the limestone areas; and /ourthly, of the relations of the 
rocks to one another and to the Green Mountain system. 
1. Tue Rocks. 
The rocks are (1) the ordinary metamorphic rocks of the 
County, not caleareous; (2) Calcareous rocks or limestones; 
(8) Serpentine and other hydrous materials; (4) Augitic and 
Hornblendie rocks not included above. 
(1.) Ordinary metamorphic rocks of the County, not Calcareous. 
Of these metamorphic rocks, the prevalent kinds are mica- 
ceous gneiss, mica schist, ordinary gneiss, and hard felds- 
pathic and granitoid gneiss. Besides these, and subordinate to 
them, there are hornblendie varieties of mica schist and mica- 
ceous gneiss, and hornblende schist. Granulyte is also found, 
especially in the northeastern part of the County, and in some 
places metamorphic granite. 
_In the mica schist and gneiss, both of the two common 
kinds of mica, the black (biotite) and the light-colored (musco- 
vite) are usually present together ;* but the black greatly pre- 
dominates. True muscovite gneiss or muscovite mica-schist is 
* The black may be in part lepidomelane, a point not investigated, as it requires 
a series of chemical analyses. 
