Hall.] 252 ^^^y- 



aspect than those of the higher beds in the eastern counties of the 

 State. 



Besides the contemporaneity of the sedimentary formations holding 

 dissimilar species in distant localities, we have to regard the gradual 

 lithological changes affecting the character of the fauna in the same 

 continuous deposition. There can be no longer any question that the 

 higher arenaceous and argillaceous formations of New York and the 

 adjacent portions of Pennsylvania, when traced in a southwesterly 

 direction, become intercalated with calcareous bands, gradually giving 

 out and becoming replaced in a great degree by calcareous or argillo- 

 calcareous deposits in which some of the same species of fossils con- 

 tinue, while there is an accession of other forms adapted to the 

 changed conditions of life.* In the extreme southwestern extension 

 of the Palaeozoic series the interval between the Upper Helderberg 

 group and the coal measures, which in the north is occupied by the 

 Hamilton, Portage, Chemung, and Catskill formations, which consti- 

 tute so marked a feature in New York, Pennsylvania, and Ohio, is 

 there filled almost exclusively by calcareous accumulations, but we 

 can scarcely suppose that the lapse of time required for the deposition 

 of the northern sedimentary formations remained unrecorded by a 

 fauna of some kind while the interval is filled by an accumulation of 

 a thousand or more feet of calcareous formation. 



It here becomes a matter of great interest to decide what shall 

 constitute the fauna Devonian, and what may be regarded as the 

 fauna Carboniferous. Looking at the great number of Productus 

 (for although I have used the term Productella as indicating certain 

 distinctions, the fossils are in all essential respects Productus) in the 

 central and western portions of the State, they alone would give a 

 Carboniferous aspect to the fauna. But when we find Spirifera 

 disjuncta, and other fossils of acknowledged Devonian age, we 

 instinctively allow less than the due importance to the Carboniferous 

 evidence. Nevertheless we are forced to admit, even within the 

 State of New York, a gradual diminution of the Devonian types, and 

 an augmentation of the Carboniferous types, in the same beds as we go 

 westward. And finally, we have every reason to believe that in those 



* I have already shown a similar condition existing at the period of the coal 

 measures, where some calcareous bands of a few feet in thickness in Ohio, Penn- 

 sylvania, and Virginia, become expanded, so that together with the associated 

 calcareous shales they embrace almost the entire formation towards the Rocky 

 Mountains and Mexico in the far western and southwestern regions of the United 

 States. 



