b CORRELATION OF GEOLOGICAL FAUNAS. [bull. 210. 
composition of the fauna of a formation must be critically examined, 
the actual association of species in each bed of rock must be analyzed, 
and the succession of species traced step by step through continuous 
sections. 
My first experiments in this field of investigation were with the 
faunas exhibited in the rocks in the neighborhood of Ithaca, N. Y. 
In these rocks, which were classified as Portage and Chemung, a 
number of zones filled with separate faunules a were discovered, some 
of which were entirely different from others in the series, but the 
order of their succession was readily distinguished in each of the 
rock sections for miles about. This integrity of the faunules in geo- 
graphical distribution, over at least the few miles of area at first 
explored, together with the sharp differences in the composition of 
successive faunules, suggested a clue to the solution of the larger 
problems involved. 
When, again, on comparison of two sections running through the 
same portion of the geological column it was found that a forma- 
tion which was clearly defined in one section was missing in the other, 
it was customary (in the absence of evidence of unconformity) to 
explain the absence of the missing member in the second section by 
the supposition that it had gradully thinned out until it disappeared. 
Its place in the second column was recognized, but the thickness of 
its sediments was reduced to nothing or to an inappreciable amount. 
Correlation of diverse formations being made on this basis, the gen- 
eral geological column was constructed of a single series of superim- 
posed formations, diversity of fossil contents standing for difference 
of formations. Each formation was thus forced to take some par- 
ticular place in a single geological column. 
As knowledge of the faunas increased, the failure to establish the 
exact identity of a newly discovered fauna with any of the faunas 
of the standard column already described led to the intercalation of 
the formation containing it between the standard formations whose 
faunas most closely resembled it. That there might be living at 
the same time two entirely distinct faunas whose records were buried 
and preserved within a few miles of each other was a possibility that 
was not then seriously contemplated. I refer to marine faunas, for 
the distinction between marine, fresh-water, and land conditions was 
clearly recognized; but almost never were faunas from diverse envi- 
« The term "faunule 11 is here and in the following pages used to distinguish an aggregate of fos- 
sils associated in a single stratum or zone from the total aggregate of species (the fauna) dis- 
tributed through a greater or less thickness of strata, each faunule containing a considerable 
proportion of the same species, but not always in the same combination or proportionate abun- 
dance. The association in the faunule is supposed to be an expression of the temporary adjust- 
ment to environment and to each other of the living species— an adjustment determined by the 
relative vigor of each species; whereas the fauna is an aggregate of species determined by sev- 
eral quite divergent conditions and factors, the fauna living on so long as these conditions and 
factors remained sufficiently intact to permit it to preserve its general characteristics and the 
dominant species to maintain their relative place in the fauna, though for a time suffering mor.e 
or less variation of composition, due to local and temporary conditions. (See page 131.) 
