114 CORRELATION OF GEOLOGICAL FAUNAS. [bull 210. 
superimposed strata, composed of the same class of 'lifhological sediments) 
may contain a large number of zones, each with a faunule differing 
in particular from the others; but all the faunules from bottom, to top 
may be made up of varying combinations of a common list of species, 
i. e. , the common fauna of the formation. 
The absence or presence of the individual species in the separate 
zones of faunules is more rationally explained on the assumption of 
this temporary shifting of the species than by the hypothesis that 
either the species temporarily ceased to live or they were simply not 
recorded in the sediments. So long as the species continued to live 
there must have been some locality in which favorable conditions 
for their living were found. The conclusion is reasonable, therefore, 
that they shifted their place of habitation — in the case of faunules, 
not far enough in distance to disturb the normal equilibrium of 
species in the general fauna. 
This difference in the relative abundance of the component faunules 
of a continuous fauna leads to the conclusion that we are dealing with 
parts of the fauna at varying distances from its center, or metropolis, 
rather than with fluctuations of the composition of the whole fossil 
contents. This actual fact of (6) frequt nt difference inrelative abun- 
dance of the species of the faunules of a continuous fauna is established 
by the statistics already givi n. 
By the hypothesis proposed the shif tings are adjustments of the 
species to constantly but in general slowly shifting conditions of 
environment of the life of the species. 
It is believed that these zones of different sedimentation might be 
recognized (if we had the whole record before us) all around the 
shores of such a marine basin as we have now under investigation. 
It is supposed, second, that the difficulties arising from correlation 
of the sediments which are cut through by sections in different parts 
of such a basin are due in great measure to neglect of this fact of utter 
difference, as far as adaptation to species is concerned, in the sedi- 
ments synchronously forming. Across the central part of New York 
State the shifting of these sediments was recognized early in the 
eighties, and it is represented in the region about Ithaca and imme- 
diately eastward in the following way: 
The Hamilton formation is found underlying the whole State, reach- 
ing from eastern New York across the State and into Ontario, Canada. 
It contains a rich marine fauna, and for that reason is clearly tracea- 
ble wherever it appears. 
This formation, as an arenaceous, sometimes argillaceous, shale, 
occupied a large area of near-shore bottom of a sea which extended over 
what is now New York State. The sediments became more calcareous 
on passing south westward, and in Ohio and Indiana the calcareous 
beds increase, the limestone conditions of the Onondaga continuing 
upward after the time of occupation of the region by the Tropidolep- 
tus carinatus fauna. Taking the presence of this fauna as the basis 
