510 Prof. James Hall — On the Silurians of the United States. 



The geographical distribution of the Middle and Upper Silurian in 

 the United States and Canada is very clearly determined and ex- 

 tremely different in the two divisions. Each one of these divisions 

 has been the subject of long-continued study and careful investiga- 

 tion. Each one of them has furnished materials for a volume of the 

 Palseontology of the State of New York, and it may be said with 

 confidence that there is scarcely a single species common to the two 

 formations. Kepresentative species do occur in the two divisions ; 

 and wherever the physical conditions have been similar during the 

 two epochs, the species occurring in those beds bear a close similarity 

 to each other. But the entire assemblage of fossils is so different 

 that from these alone (leaving out of consideration the relative position 

 of the formations), no good Palaeontologist would recognize them as 

 the fauna of a single era in geological time. 



In a paper read before the American Association for the Advance- 

 ment of Science in 1870, Mr. A. H. Worthen, the State Geologist of 

 Illinois, proposes to dispense with the subdivisions Niagara and 

 Lower Helderberg Groups, and gives his reasons for regarding them 

 as one and the same, and that the nomenclature of the science is thus 

 unnecessarily burdened with two designations for the same forma- 

 tion. He bases his conclusions upon observations made in the 

 Schoharie valley, where, he asserts, the Lower Helderberg Group 

 rests directly upon the Sandstones of Lower Silurian age, the 

 Hudson Eiver Group. 



Having, during the last thirty years, been familiar with the Scho- 

 harie valley and its tributary valleys, and having made sections of 

 the strata at more than twenty different points, I am compelled to 

 say that I have nowhere witnessed the phenomenon asserted by 

 Mr. Worthen. Everywhere, and unmistakably too, the lowest mem- 

 ber of the Lower Helderberg Group is separated from the sandstone 

 of Lower Silurian age by two or three distinct and well-marked 

 members of the series. In order to show the true relation of the 

 rocks of the Schoharie valley, I present the following section, which " 

 may be verified in numerous localities in the Schoharie and Kobel's 

 Kill valleys. 



iOriskany Sandstone. 

 Upper Pentamerus Limestone. 

 Shaly Limestone. 

 Lower Pentamerus Limestone. 

 ' Tentaculite Limestone. 

 "Water-lime Formation. 

 Coralline Limestone. 

 Green Shales with Iron Pyrites. 

 Lower Silurian Sandstones of the Hudson Eiver Group. 



Here it will be distinctly observed that the lower member of the 

 Lower Helderberg Group is separated from the sandstones of the 

 Lower Silurian system by an impure Magnesian Limestone, which, 

 though usually destitute of fossils, contains in some localities great 

 abundance of that remarkable Crustacean the Eurypterus, and more 

 rarely its allied form Pterygotus. The Coralline Limestone, as its 

 name indicates, contains numerous corals, among which are Halysites 



