380 PALEONTOLOGY OF. NEW- YORK. 



with other Silurian strata, and containing a fauna eminently silurian in 

 character. 



The Waterlime group is in many places not clearly separable from the 

 upper part of the Onondaga-salt group, but is entirely above the Gypsum 

 beds. The latter group, in its lower portions, is almost entirely argillaceous 

 or calcareo-argillaceous in composition, forming a shale or marl ; but in 

 its central and upper portions it contains alternating bands of magnesian 

 limestone, which is variably argillaceous and siliceous; and finally the 

 magnesian character becomes developed, and forms a group of strata from 

 which the hydraulic cement is everywhere obtained. Hence the name 

 "Waterlime group" has been given, from the common term applied to 

 distinguish hydraulic cement from ordinary lime*. 



The Tentaculite limestone was originally united with the rock below, 

 imder the term "Waterlime group" ; but since the former rock is a thinly 

 bedded blue or black limestone, abounding in certain organic remains, 

 and reaching only from the Hudson river valley to the central part of the 

 State ; while the other rock, characterized by its gray or drab-colored 

 surface and darker interior color, and almost destitute of fossils, extends 

 throughout the State and lies below the first, it was thought proper to 

 separate the two formations. The Eurypterus, though known at present 

 but at a few points, has thus far, with a single exception, been found 

 only in the lower strata. Several years since, I received from my friend 

 Ledyard Lincklaen, esquire, of Cazenovia, a specimen of grayish blue 

 limestone, containing the carapace of a small Eurypterus, together with 

 the little Spirifer plicatus so common in the Tentaculite limestone. The 

 species is quite distinct, in the form of the part preserved, from any other 

 known to me. The specimen is from the central part of the State, and 

 about as far west as the known extension of the Tentaculite limestone. 



• At the thne Dr. Dekat described the Eurypterut, very little was known of the sequence of onr foi^ 

 inations, and even the lithological characters were not always clearly defined, and of this rock he wrote as 

 follows : " It is said to be clay slate by Dr. Mitchell; graywacke slate, calciferous sandrock, transition 

 sandrock, etc. by others." Subsequent authors have several times cited tliis fossil as from the Slate or 

 Graywacke formation, while in truth it is in the midst of the most extensive Calcareous formation of the 

 I'alseozoic strata in the Eastern United States. 



