wti.ltams] SHIFTING OF FAUNAS. 103 
proved their dominance in the fauna are not replaced bj' other 
species. 
Upon reading on this basis the time value of the Leiorhynchus 
globuliforme faunule of Chenango County, we are able to say, from 
the study of the faunas, that the dominance of the Tropidoleptus 
fauna is already passed, although 27 of its species are present. The 
epoch of the Productella speciosa fauna of the Ithaca formation is 
also far advanced, but the Spirifer disjunctus stage has not been 
reached in force, as only a slight representation of its species is seen. 
The dominant species are those of the Productella speciosa fauna of 
the Ithaca formation. 
So long as the majority of the species, including a majority of the 
dominant species, belong to the faunas characteristic of the Hamilton 
and Ithaca formations, the evidence is strong for its contemporaneity 
with some part of the Portage formation of the Genesee River section. 
The mingling of species of two adjacent faunas by slight and 
repeated sh if tings is well illustrated in a paper by Dr. J. M. Clarke. 
He has shown how the species of the "Portage (Ithaca) fauna" are 
mingled with the species of the "Portage (Naples) fauna," as he calls 
them, in central New York. 6 In this paper is brought out the evi- 
dence of the great difference in composition between the fauna of 
western New York in the Portage rocks and the faunas occupying the 
same horizon in central New York. The method of accounting for 
the presence of both faunas in the same section is that advocated in 
this paper. Dr. Clarke speaks of the fauna of the western extension 
of the Portage group as an "exotic fauna," and describes the faunas 
of the central and eastern sections as "indigenous." Confirmation of 
the interpretation given in the present discussion appears in the state- 
ment that the Ithaca group fauna is a modified Hamilton fauna, with 
the following : "It contains a more abundant representation of unmod- 
ified Hamilton species in the meridional section along the Chenango 
River." If we had passed the time in which the "Hamilton," i. e., 
Tropidoleptus carinatus, fauna was living in its integrity the species 
would show modification. The greater abundance of these " unmodi- 
fied " species in the eastern outcrops points to the metropolis of this 
fauna, in which the fauna itself has maintained its bionic integrity. 
Although outside, only a hundred miles westward, a new fauna, exotic 
in origin, has occupied this ground with partial replacement there of 
the indigenous species of the region. 
PRINCIPLES INVOLVED IN SHIFTING OF GEOLOGICAL FAUNAS. 
This brings us to a consideration of the fundamental principles 
involved in the shifting of faunas, announced in 1883, the outlines of 
which were further set forth in 1892 in the vice-presidential address 
"The stratigraphic and faunal relations of the Oneonta sandstones and shales, tin- [thaca and 
the Portage groups in central New York: Fifteenth Ann. Rept. State Geologist New Fork, 
189?, pp. 81-81. 
''Ibid.; see p. 53, etc., for the lists, and fi^r. .">. i> 51, for the diagram. 
