302 INDEX TO THE STRATTGKAPHY OF NORTH AMERICA. 



In lithological character this rock varies to a considerable degree in its range through the 

 district, being at the eastern extremity a fine-graiaed, compact limestone, scarcely presenting 

 any crystalline grains. Its color varies from a light grayish bliae to dark blue or black, and 

 it is sometimes even of a light-gray or drab color. It contains numerous nodules of homstone 

 and the strata are sometimes separated by irregular layers of the same. In other localities 

 these layers of hornstone increase in number and thickness to the almost entire exclusion of 

 calcareous matter, and they then present a very harsh outline. * * * 



At the eastern end of the district the hornstone is intermingled and interstratified with the 

 calcareous strata, the whole very dark colored. 



The rock for the greater part seems to have been composed of finely levigated calcareous 

 mud, probably derived from the destruction of corals at distinct points, while at the same 

 time siliceous matter often formed no inferior part in its production. The characters which 

 distinguish the last rock, viz, the presence of corals and Crinoidea, are rare in this and form 

 no essential feature. Fossils are generally few and for the most part consist of shells. Some 

 portions, it is true, though of small extent, appear as if they may have resulted from the 

 destruction of corals. 



A later account of the Onondaga is given by Grabau.^^^ 



The Erian of the New York State Survey, which comprises the Marcellus and 

 Hamilton shales, has been the object of extensive investigations. HalP^^" thus 

 described the Marcellus: 



This rock admits of two divisions. The lower is very black, slaty, and bituminous and 

 contains iron pyrites in great profusion; some portions are calcareous, and it is always marked 

 by one or more courses of concretions or septaria, which are often very large. This division 

 terminates upward by a thin band of limestone, above which the shale is more fissile, and 

 gradually passes from black to an olive or dark-slate color. 



In general characters the lower part resembles the Utica slate and is not distinguishable 

 from the Genesee slate in its general aspect; it is, therefore, more properly a slate than a shale, 

 if the distinction is to be continued. For practical purposes there is little advantage in separating 

 the upper division of this shale from the Hamilton group. The line of separation is nowhere 

 well marked, the change in lithological character being gradual, while some of the fossils con- 

 tinue from one to the other. 



The finely levigated mud composing this rock indicates a period of great tranquillity in 

 the waters, moved probably only by currents sufficient to transport the materials over the 

 wide extent we find them. The nature and condition of the fossils also indicate a quiescent 

 period, for their forms are among the most delicate and their parts are usually preserved in 

 the greatest perfection. In some instances, however, from their great numbers, they are 

 packed closely together and fracture on the separation of the laminae. 



In many places this rock contains so much l^itumen as to give out flame when thrown into 

 a fire of hot coals. 



Under the title " Hamilton group " Hall ^^^'^ describes the middle Devonian: 



This group consists of several members which may be considered distinct, but which, 

 when viewed in connection, present so many features in common that they are aU recognized 

 as the products of one period, and thus constitute one great group. In the Fourth district, 

 the only changes recognized in lithological products are from shaly to calcareous, with occa- 

 sional thin beds of limestone, and more rarely of sandy shale. 



The group, as a whole, presents an immense development of duU olive or bluish-gray 

 calcareous shales, which, on weathering, assume a light-gray or ashen tint; some portions 

 become brownish on exposure, but these are of small thickness in this district. At a few points 

 the shale becomes darker or black and exhibits a tendency to slaty structure, but as a general 



