THE 



AMERICAN JOURNAL OF SCIENCE 



[FOURTH SERIES.] 



Aet. XXXV. — Fossil Faunas and their use in correlating 

 Geological Formations* ; by Heney S. Williams. 



[By permission of the Director of the U. S. Geological Survey.] 



It has long been the practice of geologists to recognize fos- 

 sils as the best available means of determining the systematic 

 correlation of geological formations. And in making corre- 

 lations by fossils it is probably true that the degree of likeness, 

 expressed by numbers of identical species, is taken in general 

 as the measure of the contemporaneity of the formations com- 

 pared. As a means of gaining a rough approximation to 

 contemporaneity this principle may be safely followed. The 

 geological systems may thus be recognized in different parts of 

 the world, and from comparisons of the faunas of the Paleo- 

 zoic systems it has been estimated by the writer that a later 

 and an earlier, and in some cases a middle division of the 

 grand systems, can be distinguished throughout the known 

 world by this method of interpretation of identity of species 

 into contemporaneity. The plan of restricting uniformity of 

 correlation, in an international scheme of classification, to sys- 

 tems and divisions of the systems to be indicated by the pre- 

 fixes eo-, meso-, and neo- to the system name, is based upon this 

 calculation, f 



When, however, the attempt is made to correlate formations 

 on a firm basis, and to trace or estimate the limits of local 

 formations by means of fossils, some more accurate method 

 than mere identity of species is demanded. 



The reasons for this conclusion are various ; among which 

 is the fact that very many single species, whose range has been 



* Eead before the Connecticut Academy, Feb. 12, 1902. 

 f Journal of Geology, vol. ii, p. 157; Comptes Eendus, Congres Geol. 

 Internat., vii, cl and cli ; Comptes Eendus, viii, p. 202. 



Am. Jour. Scl— Fourth Series, Vol. XIII, No. 78.— June, 1902. 

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