116 COEEELATION OF GEOLOGICAL FAUNAS. [bull. 210. 
of the Genesee Valley in a long stretch of about 500 feet of sediments 
above the fossiliferous Ithaca zone in the hills south of Ithaca. 
The final return shifting of the faunas westward is seen in the occu- 
pation of eastern New York by the red sediments of the Catskill 
formation. This incursion of the red sediments took place before the 
complete extinction of the Tropidoleptus carinatus fauna, and it was, 
probably, in great measure the cause of the extinction of that fauna. 
The species which lived on shifted westward, and in the eastern coun- 
ties of Pennsylvania and adjoining borders of New York we find them 
represented and mixed with the typical Spirit) r disjunctus faunas, 
which occasionally came in, intercalated between red layers of the 
Catskill. As this set of sediments is followed farther westward, the 
red sediments also pushed farther and farther westward, until they 
reached the position of Olean and corresponding positions in Pennsyl- 
vania. But during the Catskill occupation of eastern New York and 
Pennsylvania, the Spirifer disjunctus fauna, pre vailed over most of 
the western half of these Stales, in a thousand or more feet of sedi- 
ments, from which the red sediments of the Catskill are almost 
entirely, and for the more western sections entirely, absent. 
Willi each shifting of the sediments or faunas it is not simply a single 
kind of sediment that changes its position, but all of the sediments 
change t heir geographical position of accumulation; and the sequence 
of faunas (represented in any particular section cut through them) 
presents contrasts which have led to much confusion in making the 
correlations. There is, throughout the region, a gradual succession 
of faunas and species constituting the faunas. The species are modi- 
fied, chiefly, at the periods when the shifting took place. The shift- 
ing does not result, in most cases, in the extinction of the fauna, as is 
clearly indicated by the recurrence of the species in the successive 
stages. 
From an analysis of the faunas living in the New York province 
during Devonian time, we are led to believe that along with the oscil- 
lat ion of the depth of the bottom below the surface of the ocean there 
occurred shifting of the faunas as corporate wholes. The changes 
were gradual, but, with the change of condition of the bottom, the 
species of the whole fauna moved together in the direction their favor- 
able conditions of environment was taking. Coincident with such 
forced migration there was modification of some of the species, noticed 
most distinctly at first in change in the dominance of individuals, and 
followed by modification of those which maintained their strength 
and vigor, and a selection of those varieties best adapted to endure 
the new conditions. 
