correlating Geological Formations. 423 



the dominant species are permanently replaced by other species, 

 the change in fauna has become apparent. This can be 

 observed, although in the meantime there may have appeared 

 faunules of entirely different composition intercalated between 

 successive stages of the prevailing fauna. 



Again, the distribution of the fauna becomes at once evi- 

 dent when we deal with the dominant species in sufficient 

 number to represent the fauna in its characteristic expression. 



The testing of this method has been carried out in detail 

 by the United States Geological Survey in the study of the 

 Devonian faunas, of which, I believe, we have now a fuller 

 set of statistics illustrating this law than of any other system in 

 the whole geological column. The statistics have been worked 

 out regarding these faunas to a degree which enables us to 

 give figures, and state definite results of correlation. 



The Hamilton formation has been carefully studied by 

 many workers, but only within a few years have the statistics 

 been brought out with sufficient precision to make them avail- 

 able for the determination of the bionic values of the several 

 species of the fauna. The fauna of the Hamilton formation 

 of New York State is called the Tropidoleptus fauna / the 

 fauna of the typical Chemung formation is the Sjnrifer dis- 

 junctus fauna ; the fauna of a zone intermediate between 

 them in central New York is the Productella sjpeciosa 'fauna, 

 and the zone is generally known as the Ithaca formation. The 

 relative position of the Hamilton, Ithaca and Chemung forma- 

 tions is well established on stratigraphic grounds ; but the cor- 

 relation of the formations is not so clearly understood. The 

 bionic method makes it possible to say that the Tropidoleptus 

 fauna lived on in the eastern New York area till after the 

 Spirifer disjunctus stage had been reached in the western part 

 of New York state. To establish these facts the statistics of 

 146 local faunules of the Hamilton formation in eastern New 

 York and Pennsylvania have been examined, gathered chiefly 

 by Prof. C. S. Prosser.* Faunules from 76 zones examined in 

 the 1224 feet of section of the Hamilton formation at Cayuga 

 Lake, made and tabulated by Dr. H. F. Cleland, and reduced 

 to 25 temporary stages of the faunal history, were studied. f 

 The Eighteen Mile Creek section worked out by Mr. Grabau % 



* Classification and distribution of the Hamilton and Chemung series of 

 central and eastern New York. 15th and 17th Ann. Repts. State Geologist, 

 N. Y., 1895 and 1900. The Devonian System of Eastern Pennsylvania and 

 New York. Bull. 120, U. S. G. S., 1894. 



f A study of the fossil faunules in the Hamilton formation of the Cayuga 

 Lake section in central New York. A Thesis for Doctor of Philosophy 

 degree (given by Yale University in 1900), by Herdman F. Cleland. 



\ The faunas of the Hamilton Group of Eighteen Mile Creek and vicinity, 

 in western New York, by Amadeus W. Grabau. 16th Ann. Kept, of State 

 Geologist, N.Y., 1898. 



