512 Frof. James Hall — On the Silurians of the United States. 



valley, we can trace the Lower Helderberg group of beds to the east 

 and south everywhere underlaid by the Water-lime formation. 

 The blue line marked upon the map indicates the geographical ex- 

 tension of the formation, which is everywhere marked by its charac- 

 teristic fossils, and everywhere resting on the Water-lime beds, 

 through all its meanderings caused by the plications of the strata 

 through south-eastern New York, Pennsylvania, and Maryland. 



The Coralline Limestone can be traced southward for fifty miles 

 along the Hudson Eiver valley, but is lost to observation near the 

 southern limits of the State, and I do not know that the same or 

 equivalent formation has been recognized in Pennsylvania or Mary- 

 land. In some parts of Maryland at least the Water-lime rests on 

 shales of the Clinton Group. 



Returning again to the point of first starting, we are not able to take 

 up a continuous line of outcrop ; but the Lower Helderberg group 

 has been determined by Sir William Logan in the neighbourhood of 

 Montreal. It is largely developed at Gaspe and other places in 

 Canada, and apparently constitutes the main feature in the Limestone 

 formation of this age, so clearly drawn out on the great map of Sir 

 W. E. Logan. 



Not being personally familiar with this area of country, I am of 

 course unable to enter into the details regarding the individual 

 members of the series ; but I believe that throughout this great ex- 

 tent the Niagara group, as known in New York and Canada West, 

 has scarcely any existence. 



Whatever Upper Silurian fossils there may be which are not 

 strictly referable to the Lower Helderberg Group are from beds dis- 

 tinctly separable from it, and lying at a lower horizon. I say this of 

 my own personal knowledge, having critically examined the collec- 

 tions of the Geological Survey of Canada made near Montreal, and 

 at Gaspe and elsewhere. 



So far, therefore, we find both the physical fact of super-position 

 and the evidence of fossils coinciding to prove the Lower Helder- 

 berg group a distinct and overlying formation to the Niagara group, 

 and separated from it by the Onondaga Salt group and Water-lime 

 formation, wherever these latter formations exist. 



It is claimed, however, that in southern Illinois, Missouri, and 

 in Tennessee the Niagara and Lower Helderberg groups are physically 

 and zoologically inseparable. Having seen these formations in 

 Missouri, I am prepared to assert and to prove that the Lower 

 Helderberg and Niagara formations are both physically and by their 

 fossil contents distinguishable the one from the other. 



With the localities in Tennessee I am not so familiar ; but from 

 the examination of large collection of fossils collected in that region, 

 and from the sections given by Prof. Safford, I believe the formations 

 to be quite distinct. It happens, however, from the thinning out 

 of the Salt Group, and Water-lime formations, that the disputed 

 groups of strata do come in contact ; and the facts which would i^rove 

 a lapse of time between the ending of the one and the beginning of 

 the other have not yet been observed, or if observed, have not 

 received that consideration to which they are entitled. 



