NIAGARA GROUP. 151 



1. In the ascending- order, thin laminated bluish green shales, tender, and subject to disintegration. 



2. Dark blue limestone, the lower beds or beds of passage argillaceous : the beds of passage into the 



limestone are silico-argillaccous ; when recently exposed, they are of bluish green ; on exposure, 



they become gray. 

 The limestone is often distinguished by cavities lined with crystals of pearl and dog-tooth 

 spar, and hence received the name of geodijcrous limestone by the earlier writers on geology. 



1. NiAOARA SHALE. 



The color of this shale is dark liluish, which invariably wiiilens on exposure to the 

 weather, passing into laminated fragments, and finally into a stiff cloy : alternations of a 

 dry and wet surface favor this change. The whole mass is slightly calcareous, but the 

 lower parts do not furnish calcareous bands ; the middle and upper, hovi'ever, exhibit in- 

 terlaminations of impure limestone. 



2. Niagara limestone. 



This rock is dark colored and bituminous ; often strongly so. It admits of the following 

 subdivisions : 



1. From the shale upwards, beds of gray siliceous limestone, often quarried for cement, or for hydraulic 



mortars. 



2. Thin-bedded shaly limestone, alternating with scams of dark colored shale. 



3. Thick-bedded limestone above ; below, the beds are thinner, and often bent or contorted, or even 



concretionary. 



4. Bituminous limestone, cherty, thin-bedded and gray or brown : geodes abound. 



At Rochester, the limestone is crystalline, sparkling upon a dark ground, and brittle, 

 breaking with an uneven fracture : it is cjuite harsh, and apparently siliceous. 

 At Lockport, the Niagara limestone appears with some additions to its strata : 



1. Reddish gray crystalline limestone, susceptible of a fine polish : the color is due to the numerous 



broken stems of encrinftes. 



2. Concretionary and irregular-bedded limestone, with cavities containing spar in various forms. 



3. Bituminous limestone wiili cavities, generally dark colored. 



4. Gray limestone, with thin bituminous shale between its layers. 



In Oneida and Herkimer comities, this rock first appears as one of the members of the 

 New- York system ; and so different are its features where it first appears at its eastern 

 position, that it would not be recognized as the western geodiferous limestone, if it could 

 not be traced almost uninterruptedly in its western route. It takes its origin only a few 

 miles west of the Clinton group, which it accompanies all the way to Niagara falls. 



On Swift creek in Oneida county, it is a dark concretionary mass, about four or five feet 

 thick, accompanied with a dark colored slate. The concretions form segments of large 

 curves or semicircles, and may be split or separated from each other in tables a yard square 

 or more. The mass possesses the same characters at Hart's mill. Steel's creek, near 

 Hamilton College, at Vernon, and near Skanandoa. As the rock proceeds west, it be- 

 comes a purer lime, and loses in part its concretionary character. 



