450 J. D. Dana—Geological relations of the 
Art. XLIX.—Geological relations of the Limestone Belts of 
Westchester County, New York; by JAMES D. Dana. (With 
Plates VIII and [X.) 
[Continued from page 375.] 
3. The Limestones and the conformably associated rocks of West- 
chester County and New York Island are Lower Silurian in 
age—the Cambrian or Primordial being here included. 
No evidence with regard to the age of the Westchester 
County and New York Island limestones, and the conformably 
associated rocks (gneisses of various kinds, mica schist, horn- 
blende schist, ete.), can be wholly satisfactory that is not based 
on fossils. But the fossils may exist at points outside of 
the region if only they are within the same system of conform- 
able strata or formations. This kind of evidence as to the age 
of these rocks is afforded in three ways: 
_ First: by the relations which exist between the limestone _ 
areas and schists of this county and those of Western New 
Engiand and Eastern New York to the north. 
Secondly: by the special relations between the areas of north- 
western Westchester County, south of the Putnam County 
Archean, and those of Dutchess County, north of it. 
Thirdly: by the relations of both the Westchester and 
Dutchess County rocks to those west of the Hudson in Southern 
New York and Northern New Jersey. : 
_ The age to which the facts from these different sources point 
is the Lower Silurian. The Cambrian or Primordial era is 
here included with the Lower Silurian because in the geology 
of the region there is no possibility of separating them ; more- 
over, no stratigraphic or paleontological reason for the separa- 
tion is afforded by the geology of North America, and little 
0H that of Great Britain where the separation was first 
e 
1. Relations to the limestone areas and associated schists of the 
regions to the north.—In order that the facts under this head 
may be appreciated, I have brought together in one map 
(Plate VIII) the southern portion of the Green Mountain region, 
from the northern boundary of Connecticut to New York 
Island. The northern portion of the map was published with 
my paper on Dutchess County ;* the rest is the Westchester 
County map (Plate V) reduced to the same scale, or that of 
ten inches to the mile. The limestone areas of the Connecti- 
cut portion, east of Dutchess County, N. Y., are mainly from 
* This Journal, III, xvii, 375, May, 1879. 
