20 
PALAEONTOLOGY OF NEW-YORK. 
counties, it is much thinner*. Continuing in the same direction through 
Canada West, there is very evidently a gradual and constant attenuation, 
which takes place in a great measure from the disappearance of the coarser 
materials of the group. 
The Sparry limestone of Prof. Eaton, which forms so striking a feature 
in the eastern and northeastern extension of this group, is almost entirely 
absent where the strata are exposed on the two sides of the Mohawk 
valley, and in the counties of Lewis, Oswego and Jefferson. This rock, 
however, is conspicuous in Rensselaer and Washington counties, and still 
farther north in Vermont; but it appears to reach its fullest development 
only in Canada, where it presents some very interesting and remarkable 
features. It occurs more or less mingled with, or interstratified among the 
slates of this part of the group, not only in continuous heavy masses, but 
in a kind of conglomeratic or brecciated condition. These beds of limestone 
are not uniform in character : some of them, weathering to a dingy 
brownish color, are found to be magnesian limestones; while others are 
destitute of magnesia, and of the same character as ordinary limestones, 
though usnally non-fossiliferous. It is these magnesian and brecciated 
limestones that have been proved to form the serpentines of Northern 
Vermont and the eastern townships of Canada. 
The Hudson-river group, though spreading far to the westward, never¬ 
theless maintains its greatest thickness in the direction of the Appalachian 
chain. In this direction have accumulated the immense amount of its 
coarser materials ; and we may conceive of that range as indicating the 
pre-existence of a long coast line from which these materials were 
abraded, forming a submarine belt of sediments in some degree parallel 
with the outline of an ancient continent on the east. The force of the 
current, which was sufficient to bring in this vast quantity of sedimentary 
matter, extended westward with diminishing force, precipitating the finer 
mud so slowly as to permit the incipient growth of coral reefs along an 
equal extent of the ocean bed. Thus from the St. Lawrence on the. north, 
* The usual estimates of the thickness of this group in Central and Northwestern New-York are 
from 800 to 1000 feet. 
