240 GEOLOGICAL NOMENCLATURE 



and below by great unconformities and intrusions of granitic 

 rocks. The only evidence brought against the name Devonian 

 is that of certain fossils assumed to have a definite range in 

 geological time. Were such a means of classification possible in 

 the present state of our knowledge, and we were to understand 

 that, for example, rocks must not be called Devonian above the 

 horizon of the appearance of amphibians, or Silurian above that 

 of fishes, such a classification would be quite satisfactory. 

 Paleontology is not, however, one of the mathematical or 

 exact sciences, but has its limitations even in the countries .in 

 which it has been most diligently studied ; consequently, uncer- 

 tainty exists both in Europe and America regarding the proper 

 limits of this and other formations. We have heard, for 

 example, the work of the Second Geological Survey of Pennsyl- 

 vania, a most important and thorough industrial investigation, 

 described as conducted on the plan that correlations can best be 

 made by lithological means. " Frequently one meets with 

 expressions of lack of confidence in the evidence offered by 

 fossils." And in a recent report on the Devonian and Car- 

 boniferous* " the whole subject of the value of fossil plants 

 as means of correlation " is said to be " under consideration." 



It is only necessary to read this report to realize the diffi- 

 culty met with in attempting to group these rocks by their 

 fossils in Pennsylvania, Ohio, Virginia, Illinois, Michigan and 

 other statesf a difficulty well stated in a report of the Ameri- 

 can committee of the International Congress of Geologists J as 

 follows : (1) " Shall we include the Catskill rocks (and, when no 

 marine faunas occur, up to the base of the Olean conglomerate and 

 equivalents) in the Devonian ? (2) Shall the Chemung marine 

 fauna be taken as the uppermost fauna of the Devonian ? Or 

 shall a part or the whole of the marine faunas between the 

 middle Devonian and the conglomerate which introduces the 

 Coal Measures be called Devonian ? If an arbitrary line is to 



* Bulletin of the U. S. Geol. Survey, No. 80, pp. 123, 208 and 228. 



t Cf. also " Science " for 26th Jan. 1900, p. 140. 



J Rep. Geol. Congress, 1888, A. pp. 102, 144 ; B. pp. 144, 153, 154, 156. 



