CORNIFEROUS PERIOD. 269 



2. COKNIFEROUS PERIOD (9). 



Epochs. — 1. Cauda-Galli, or that of the Cauda-Gralli grit (9 a) ; 

 2. Schoharie, or that of the Schoharie grit (9 b) ; 3. Upper Hel- 

 derberg, or that of the Onondaga and Corniferous limestones (9 c). 



I. Rocks: kinds and distribution. 



1. Cauda-Gtalli Epoch. 

 The term Cauda-Gralli refers to the feathery forms of an abundant 

 fossil supposed to be the impressions of a sea-weed (fig. 441). The 

 rock is a drab or brownish argillaceous sandstone, often shaly and 

 crumbling. It rests upon the Oriskany sandstone, but its position 

 is somewhat more easterly, it lying altogether east of the west limit 

 of Oneida co., N.Y., and thickening towards the Hudson. 



In the Helderberg Mountains, south of Albany, the thickness is 50 or 60 feet. 

 It extends southerly, and has been observed in Sussex co., New Jersey, and on 

 the eastern border of Pennsylvania, with its characteristic Cauda-galli fossil. 



2. Schoharie Epoch. 

 The Schoharie grit is a fine-grained calcareous sandstone, contain- 

 ing numerous fossils. The lime becomes dissolved out on exposure, 

 leaving a rusty rock full of casts of the fossils and holes left by the 

 removed shells. In New York the beds are confined to the eastern 

 part of the State. It resembles the Oriskany sandstone, but has 

 very different fossils. 



3. Upper Helderberg Epoch. 



The rocks of the Upper Helderberg epoch are limestones. They 

 spread widely over the Interior Continental basin from eastern New 

 York to the States beyond the Mississippi. 



The formation in New York is divided into the Onondaga lime- 

 stone (the lower part) and the Corniferous limestone (the upper). 

 The latter contains disseminated masses of hornstone (or imperfect 

 flint), lying in layers of the limestone between other layers that con- 

 tain little or no hornstone (just as the flint lies in the chalk-bed) ; 

 and hence the name corniferous (from the Latin cornu, horn). The 

 thickness of the whole series of strata is in some places 350 feet. 



The color of the limestone is dark grayish and occasionally black in New 

 York, and light gray, drab, and buff in the Mississippi basin. 



(a.)Interior Continental basin. — In New York the beds have a thickness seldom 

 over 20 feet for the Onondaga limestone and 50 feet for the Corniferous. The 



