90 PALEONTOLOGY 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 JeflFerson. This rock, 

 however, is conspicuous in Rensselaer and Washington counties, and still 

 farther north in Vermont ; but it appears to reach its fullest developraer^t 

 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. 



