Williams.] EQUIVALENCY AS INTERPRETED BY GEOLOGISTS. 123 
which appears in the Ithaca formation contains all of these represent- 
ative species of the Hamilton formation, thus making- a faunal equiva- 
lency with known discordance as to formational equivalency. It is 
known that stratigraphically the Ithaca formation is not equivalent 
to the Hamilton formation. However, if we were to detect the species 
named in the dominant Hamilton list in a section in Indiana, the 
inference would be drawn at once that the Hamilton fauna was pres- 
ent. The truth is that the Tropidolepius fauna is present, but that the 
Hamilton formation may or may not be represented in Indiana. The 
evidence of the equivalency of the Sellersburg formation with the 
Hamilton formation in Indiana, furnished by the presence of the few 
specimens of the Tropidoleptus fauna, is not so great as the evidence 
of equivalency of the Ithaca formation with the Hamilton in New York. 
This case brings out the distinction between faunal and formational 
equivalencies. It also illustrates the importance of the recognition of 
some other basis than simple presence of species in order to certify the 
fauna to which they belong. The facts are not present for carrying 
correlations by this careful method through the whole series of forma- 
tions known to occur within the boundaries of the intercontinental 
basin, but sufficient is known to make it certain that the general faunas 
prevailing in one section of the basin during a period of time, the 
formational equivalency of which may be clearly established in 
another section, are faunally diverse in the two sets of sediments 
representing the same period of time. 
