No. 136.] 



61 



blend in one mass ; so that they are now regarded only a« local 

 varieties of a single rock. They are known in Northeastern 

 Pennsylvania, but are not very distinct there. Westward, how- 

 ever, they spread over a vast extent, along Northern Ohio, where 

 they are seen at Sandusky ; through Northern Indiana and Illi- 

 nois, where they are quarried near Chicago ; and across Wiscon- 

 sin to the Mississippi, where they may be seen at and above 

 Davenport. They also emerge from beneath newer rocks on the 

 Ohio, where they form the Falls at Louisville, and are known in 

 that country as the upper part of the Cliff limestone (the lower 

 part being the western extension of the Niagara limestone ; all 

 the rocks which intervene in New- York, the Onondaga-salt group, 

 the Lower Helderberg group, the Oriskany sandstone, and the 

 succeeding grits, being almost or entirely wanting in that 

 region). 



The fossils of theBe limestones are very well marked, generally 

 peculiar to them, and abundant. A large shell, Meganteris 

 ovoides, is shown in figure 26 : it is from the lower part of this 

 rock, or the grey " Onondaga limestone." Fig. 27 shows some 

 fossils of its upper portion, the " Corniferous limestone." No. 1 

 is a common trilobite, easily recognized by the "notched " edge 

 of the head and the forked tail. No. 2 is a common coiled cham- 

 bered shell. No. 7 is a fragment of a fish-bone : such are not 

 uncommon, being more abundant at the West. The fishes from 

 which they came were allied to the remarkable forms of which 

 Hugh Miller wrote in his " Footprints of the Creator."* 



In the Upper Helderberg group, we have the last or highest 

 formation of limestones of any considerable extent or thickness 

 in the State. All the southern counties, lying above or south of 

 the line of outcrop of the Onondaga and Corniferous limestones 

 as before described, are nearly destitute of this useful mineral ; 

 being formed of vast piles of slaty, shaly, and sandy strata, 

 several thousand feet in thickness, whose surfaces extend from 

 a few miles south of the Erie Canal to and beyond the Pennsyl- 

 vania line. 



These rocks give rise to peculiarities in the topographical 



* These limestones contain several trilobites, and many corals both of the columnar and 

 branching forms ; and a large number of the one-celled conical genera, shaped like horns of 

 cattle; each of which was made by a single animal tenant, living in the larger cup-shaped 

 end. 



