SPIRIFER.^ OF THE CHEMUNG GROUP. 257 



the Carboniferous system; so that the limestones of this age are supposed to rest 

 directly ujjon the Silurian rocks, or with a thin intervening Devonian limestone. 

 But we can scarcely suppose that the lapse of time required for the northern 

 sedimentary formations remained unrecorded by fauna of some kind, while the 

 interval is filled by an accumulation of a thousand or more feet of calcareous 

 deposition. 



It here becomes a matter of grave interest to decide what shall constitute the 

 fauna Devonian, and what may be regarded as t\xQ fauna Carboniferous. Looking 

 ct 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, thcj^ alone would give a Carbo- 

 niferous aspect to the fauna. But when we find Spirifera disjuncta, and other fos- 

 sils 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 type, 

 and an augmentation of the carboniferous type, in the same beds as we go westward. 

 And finally, mc have every reason to believe that in those sedimentary formations, 

 between the Hamilton group and the Coal measures in the east, and between the 

 same group and the Burlington (Carboniferous) limestone in the west, the Devo- 

 nian aspect of the fauna on the one hand, and its Carboniferous aspect on the 

 other, are due to geographical and physical conditions, and not to diflerence in 

 age or chronological sequence of the beds containing the fossils. 



This view of the case, which is consonant with the facts observed, will account 

 for the coming in of forms which we term Carboniferous, as we pui'sue our inves- ^ 

 tigations to the westward. 



The same opinions seem gradually to be obtaining ground in Great Britain, but 

 the idea is not new with me. It is now about fifteen years since I expressed simi- 

 lar opinions in a review and comparison of the Palaeozoic groups and systems of 

 Europe and America. 



In some of the concluding paragraphs of the eighteenth chapiter of Foster & 

 Whitney's Report on the Lake Superior Land District, after having made a com- 

 parison of the species cited as common to the Silui-ian and Devonian and to the 

 latter and the Carboniferous system, I have said : 



" The arenaceous and argillaceous deposits, which we trace uninterruptedly over 

 so wiiie an area, and which present to us such gryduul and almost imperceptible 

 changes in the fauna when studied continuously, would, if broken up and isolated 

 so (Iiat they could not be traced consecutively, pi'esent the same phases which are 

 exhil)it('d by the systems in Europe to which they are related. From all these 

 facts there seems to be but one conclusion, and that is, in the British Islands par- 

 ticnlarl)-, either there are I'cmarkable exceptions to the general law in the con- 

 tinuation of species from one to another, or there is no foundation for a dis- 

 tinction between the Devonian and Carboniferous systems." 



[ Paleontology IV.] 33 



