188 H. S. WILLIAMS — SHIFTING OF FAUNAS 



that mutation of homceotopic faunas chiefly takes place at times of rapid 

 shifting. An example is the case of the Hamilton fauna, which presents 

 very slight change either in its proportionate composition or in the form 

 of its species or in introduction of new species for some thousand feet of 

 section in Cayuga lake, according to Doctor Cleland's analysis of the 

 facts ; whereas a little later, at the time of the introduction of the Ithaca 

 fauna, the shifting is from the east to west, and a large number of the 

 species at Ithaca present mutational relations to the Hamilton species 

 which are less marked a hundred miles eastward, where they may have 

 come by direct descent from Hamilton species of the underlying rocks. 

 The same fact is observed in Warren and Crawford counties, Pennsyl- 

 vania ; at the upper range of the Chemung fauna mutation was taking 

 place as the Waverly stage is reached in species which preserved their 

 characteristics so long as the equilibrium of the Chemung fauna was 

 intact. 



6. DOMINANT AND CHARACTERISTIC SPECIES 



Dominant and characteristic species, so called, must be used with 

 caution in making correlations, for with the shifting of the faunas they 

 preserve their integrit}^ with great persistence, and afterwards will recur 

 long after the fauna to which they belong has by shifting been generally 

 modified and decimated. Examples of this are the recurrence of Tropi- 

 doleptus and half a dozen other dominant Hamilton species after the 

 Chemung fauna is evolved at the cliffs along Chemung river and at 

 Owego and in Tioga county. New York; also the appearance of Spirifer 

 disjunctus and associate species in Warren county, Pennsylvania, over 

 100 feet above where Syringotheris has appeared in abundance, fully de- 

 veloped and in large size. 



7. STRATIGRAPHIC SUCCESSION NOT CERTAIN EVIDENCE OF CHRONOLOGIC 

 SUCCESSION OF FA UNAS 



The succession of one fauna above another is not certain evidence 

 that the upper fauna is really younger than the lower. This must be 

 regarded as true when we observe the shifting of one fauna locally over 

 another, the two coexisting in the same basin in rocks at a lower level. 

 Thus the Portage and Ithaca faunas are in succession in the Ithaca sec- 

 tion, but in the Genesee section the Portage occupies the whole interval 

 from the Genesee to the Chemung. 



8. CHRONOLOGIC VALUE OF FIRST APPEARANCE OF A NEW FAUNA 



It is probable that the first appearance, on passing upward in a geo- 

 logical section, of new types of fossils is a more accurate and more 



