298 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



and along the Hudson River at Rondout and at Schoharie and elsewhere the lowermost beds 

 of tliis group rest directly upon the Waterlime beds, which we regard as the uppermost member 

 of the Onondaga salt group, indicated as a separate formation by reason of its economical 

 importance and likewise characterized by certain peculiar fossils, while the marls of the Salt 

 group are usually nonfossiliferous. 



The lowest member of the Lower Helderberg series is a thin-bedded, often thinly laminated 

 dark-blue limestone, which, from the abundance of its tentaculites, has been termed the Ten- 

 taculite limestone. Its color, texture, and composition contrast strongly with the rock below. 



The second member of this group is a thin mass of limestone, consisting almost entirely 

 of the coral Stromatopora, and constitutes a very persistent member of the group; to this 

 succeeds a limestone charged with great numbers of the broken shells of Pentamerus galeatus 

 and known as the Pentamerus limestone. This graduates above into a shaly formation, which 

 was designated in the New York reports as the Delthyris shaly limestone, from the abundance 

 of this genus of fossils. It is the most fossUiferous member of the group. * * * This shaly 

 limestone, ia physical character and composition, corresponds nearly with the shaly member of 

 the Niagara group and contaias numerous similar or representative forms. 



To this succeeds a compact crinoidal limestone, and above this is a mass of bluish-gray 

 limestone, charged with Brachiopoda, among which a Pentamerus similar to P. galeatus is so 

 abundant that the rock has been termed Upper Pentamerus limestone. 



A comparison of the species shows that the fossils of the Lower Helderberg rocks are 

 analogous to those of the Niagara group and contain among them certain species which we 

 regard as representative forms of the SUurian species in Europe; and we can not do otherwise 

 than retain this series as a member of the Silurian system. 



In 1874 Hall ^"^ published a detailed description of the distribution of the " Lower 

 Helderberg" and its relations to the Niagara and the intervening "Onondaga salt 

 group," with a map of the eastern United States illustrating the occurrence of the 

 three terranes. 



Schuchert ''"^ quotes Hall's description and gives an account of the represen- 

 tatives of the Helderbergian in several States and Canada, together with an extended 

 discussion of the faunas, from which he concludes : 



With these facts there is presented a great paleontological break between the SUuric and 

 Devonic at the top of the Cayugan group. The succumbing of the normal marine faunas of 

 the Niagaran group is undoubtedly associated with the red gypsiferous and sahferous sediments 

 of the Cayugan group. If the latter had a normal marine fauna instead of one of pecuUar 

 Crustacea, the continuity of life from the Niagaran to the Helderbergian would be probably 

 complete. However, in most areas outside of "New York and Ohio, there is a great hiatus 

 between the Niagaran and the Helderbergian, which tends to make a clear and easily discover- 

 able line for field geologists in separating the Siluric from the Lower Devonic. 



From the foregoing summary of the Helderbergian fauna it is evident that most of the 

 characteristic Siluric genera of trUobites, brachiopods, and crinoids are there absent. This 

 might be expected, for, as has been seen in the previous chapter, about 2 per cent of the Helder- 

 bergian fauna are derived from the Siluric. On the other hand, in some of the trilobites, Bryozoa, 

 and pelecypods, many of the gastropods, but more particularly in the diversified brachiopods, 

 are met organic groups which in their culmination are characteristic of the Devonic. It can not 

 be denied that the Helderbergian fauna has a Siluric facies, yet these types either have greater 

 differentiation in species or the forms attain a larger size. The fact that 9 per cent of the 

 Helderbergian fauna pass into a generally accepted Devonic horizon, the Oriskany, outweighs 

 the evidence of a Siluric facies and specific derivatives. The writer therefore concludes that 

 the Helderbergian has a fauna unlike the Siluric, but one in harmony with the Devonic and its 

 position near the base of that system. 



