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 assamed 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 statest—a difficulty well stated in a report of the Ameri- 
can committee of the International Congress of Geologists? 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. 
+ Cf. also “Science” for 26th Jan. 1900, p. 140. 
t Rep. Geol. Congress, 1888, A. pp. 102, 144; B. pp. 144, 153, 154, 156. 
