Prof. James Hall — On the Silurians of the United States. 511 



catenulatus, Favosites Niagarensis, etc., and Brachiopoda of several 

 genera. Two or three species of Trilobites also occur in the same 

 rock. 



The green shale with iron pyrites has a thickness of thirty or forty 

 feet, and, so far as known, is destitute of fossils. The proportion and 

 amount of iron pyrites is very great, and near the base of this bed 

 there is often a distinct layer of iron pyrites, which has, to some 

 extent, been mined for economic uses. This feature is of interest 

 mainly in connexion with some suggestions I propose to make re- 

 garding the characteristics of this part of the formation in its 

 western extension. 



Without occupying the time of the Association with details of 

 observations and sections at numerous points, I propose to present a 

 corresponding section at a point about sixty or seventy miles west of 

 the Schoharie valley, where the preceding section may be verified. 



We have still the same general features of country, and the 

 greater geological divisions are well marked. 



i Oriskany Sandstone. 

 Lower Helderberg Group < Shaly and Lower Pentamerus Limestone. 

 ( Tentaciilite Limestone. 

 "Water-lime Formation. 

 Red and Grey Marls of the Onondaga Salt Group. 



Niagara Group Coralline Limestone. 



Clinton Group Green Shales and Beds of Eed Haematite. 



Medina Sandstone Red and Grey Sandstone. 



At a point one hundred miles further west we have the following 

 section : 



Oriskany Sandstone. 



Upper Silurian "Water-lime Group. 



Onondaga Salt Group. 

 ( Niagara Group. 



Middle Silurian < Clinton Group. 



( Medina Sandstone. 

 Lower Silurian Hudson River Group. 



At this point every vestige of the Lower Helderberg Group has 

 disappeared, and the Water-lime and Onondaga Salt Group have 

 become developed to a thickness of more than one thousand feet. 

 The Niagara Group in its divisions of Shale and Limestone, and 

 with its characteristic fossils, is a well-marked feature. The Clinton 

 Group, with its abundant iron ores, has reached its maximum thick- 

 ness to the east of this point, and in the neighbourhood of the 

 second section. 



Still following these formations to the westward, we obtain every- 

 where for several hundred miles essentially the same order of 

 succession, with the absence of the Oriskany Sandstone. This is 

 shown upon the geological map, and the formation may be traced 

 through Western New York, Ohio, Canada, the Islands of Lake 

 Huron, Wisconsin, Illinois, Iowa, and Missouri. Nowhere in this 

 direction till we reach the Mississippi in the southern part of 

 Missouri and Illinois, do we find the least evidence of the limestones 

 or of the fossUs of the Lower Helderberg Group. 



Now, returning again to our point of departure in the Schoharie 



