REVISED PRINCIPLES OF CORRELATION 473 



tion of faunas and floras is, and has always been, a continuous, though 

 probably not a uniformly continuous, process. Doubtless, also, it is un- 

 necessary to argue that the stratigraphic column, so far as it is accessible 

 and known, contains but an imperfect record of particular stages of the 

 developmental process. Granting all this, several considerations are 

 readily suggested. 



Beginning with a general proposition, I may say that as the faunal or 

 floral conception ascribed to any one system or period is but a part of a 

 continuous process of organic evolution, it is manifestly impossible to 

 decide that the first appearance of a given species or fauna in any conti- 

 nental basin marks at the same time its inception, nor that its highest 

 known occurrence in the stratigraphic column marks its extinction every- 

 where. Clearly, then, the determination of this collection of fossils as an 

 Ordovician fauna, that as a Silurian, and another as Cambrian is nothing 

 better than a more or less arbitrary age assignment of species according to 

 recorded or personal knowledge and belief regarding their vertical or 

 time ranges. 



Much confusion has arisen through unwarranted additions to faunas 

 commonly accepted as diagnostic of certain ages and periods. For in- 

 stance, prior to the discovery that the Richmond of Ohio and Indiana is 

 of the age of the Lower or Queenston division of the Medina in New 

 York, lists of characteristic Ordovician fossils always included those 

 found only in the Richmond. As previously stated, there was no other, 

 even then, valid reason for this misassociation than the now discredited 

 belief that the formations of the Richmond group are entirely older than 

 the base of the Medina — yes, even older than the Oswego sandstone — with 

 which the Silurian system, or the Upper Silurian, as it was formerly 

 designated, began in the standard New York section. 



Similarly, the large and extraordinary fishes and other fossils found in 

 the Ohio shale in Ohio and adjoining States have for 50 years been in- 

 cluded in the list of Devonian fossils. In fact, more than two-thirds of 

 the Upper Devonian fish fauna as given in current literature is made up 

 of these Ohio shale species. But I have on many occasions claimed, 

 though seemingly without much avail, that this series of mainly black 

 Ohio shales is younger than the Chemung of New York, and therefore 

 that the beds and the fossils in them are not of Devonian but of Missis- 

 sippian age. 



Whether the future proves my contention right or wrong, the fact re- 

 mains that so long as the older correlation is in reasonable question these 

 Ohio shale fossils can not properly be included in lists of characteristic 

 Devonian fossils. 



