BIGSBY PALAEOZOIC ROCKS OF NEW YORK. 359 



A. rhomboidea ; Posidonia (?) alata ; Orthonota curta ; Acroculia angulata ; 

 Platyostoma, n. s. ; Orthoceras annulatum ; O. virgulatum ; Comulites 

 serpularius ; Beyrichia lata ; Calymene Blumenbachii ; Homalonotus del- 

 phinocephalus ; Ceraurus (?) insignis ; Phacops limulurus. 



Of these 28 recurrents, 2 1 reappear but once, and mostly in the 

 Niagara period ; 1 twice ; and the rest more frequently. 



The curious little crustacean Beyrichia lata is in the Clinton, 

 Waterlime, and Pentamerus-limestone groups. 



The Niagara Group. 



Mineral Character. — This group commences as a dark shale or 

 slate, and passes vertically into a dark-blue or black limestone, by 

 a gradual increase of calcareous matter. When the limestone is in 

 small quantity, it is in the form of hemispherical concretions, made 

 up of successive concentric coats. It is further remarkable for nume- 

 rous cavities lined with pink calc-spar, sulphate of strontian, selenite, 

 fluor-spar, blende, or iron-pyrites. They are from a very small size 

 to the diameter of two or three feet, and were originally occupied 

 by fossils afterwards removed (Hall, Pal. ii. 3). On the whole, the 

 limestone is as thick as, or thicker than the shale, which, though very 

 thin in the west and far-west, accompanies the limestone everywhere. 



This argillaceous limestone (not always so) is the chief or central 

 mass of Upper Silurian Limestone of Middle North America. Many 

 hundred miles west of the State of New York We find the Niagara 

 group just as it is here, mineralogically and palseontologically. At 

 the western end of the Manitouline Islands, in Lake Huron (halfway 

 west), the Niagara Limestone is in great force, but it has become 

 white, hard, and siliceous. Indeed, it may be truly said that in the 

 west this group becomes massive limestone, and contains part of the 

 lead-ores of Iowa, Illinois, and Wisconsin (Hall, Pal. ii. 107). 



Transition.— In New York, the terminal or upper limestone of the 

 Clinton group is succeeded by soft argillo-calcareous shale, 80 to 100 

 feet thick (Hall, Pal. ii. 3), and this gradually, both as to fossil 

 and mineral character. In the lower parts of this shale, small ill- 

 formed fossils, not Clintonian, abound. At Verona (easterly) the 

 Niagara Limestone and Shale, much thinned, may be advantageously 

 seen, resting on the Clinton group ; sections exhibiting all this are 

 innumerable. 



In Wisconsin and other western regions the assimilation between 

 the Clinton and Niagara groups is so perfect that their fossils are 

 commingled, and they are themselves not to be distinguished. 



Place. — The Niagara is one of the great "constant" groups, and 

 always participates in their vast extensions. 



In New York it stretches westerly across the State from Roudout 

 on the Hudson River, and on the River Niagara passes into Canada. 

 In its long course it lies between the Clinton and Onondaga- Salt 

 groups, — visible as a belt which, at first narrow, gradually widens, but 

 never in New York exceeding a few miles in breadth. 



The Niagara group in its two portions, massive and shaly, is best 

 seen in New York at Rochester, Lockport, and Niagara ; but, although 



