104 CORRELATION OF GEOLOGICAL FAUNAS. [bull. 210. 
before Section E of the American Association for the Advancement of 
Science. 
In a paper read before the American Association in August, 1885, 
the fact of shifting of faunas was illustrated by a chart based -on the 
detailed examination of the faunules of ten sections cutting across 
the strata of the Devonian, extending from Cuyahoga County, Ohio, 
eastward to Unadilla, in Otsego County, New York. A brief report 
of the paper was published in the proceedings, and the formulated 
expression of the law was given in the following words: 
The actual order of faunas met with in a vertical section is not necessarily 
expressive of biologic sequence, but signifies the sequence of the occupants of that 
particular area. 
The change in the species. from one stratum to the next may express the shift- 
ing for miles of the actual inhabitants, and if the change, within a few feet of 
strata, is to an entirely distinct group of species, the evidence should be taken as 
pointing to a considerable shifting of conditions of the bottom. If in such case 
each fauna is kept distinct, the means of tracing the geographic distribution and 
modification are at hand. If mingled, then the collection, though made at the 
same locality, will only confuse. Two such faunas meet at Owego, Tioga County, 
in distinct strata, but in rocks which are of similar lithologic character; one is a 
remnant of a prevailing western fauna, the other is an eastern and late stage of a 
new fauna. 
It was there shown how, by the shifting of faunas and formations, 
the lower part of the Catskill formation of the Hudson River section 
was actually equivalent to the Oneonta formation of the Delaware 
County section, 1<> the Ithaca formation of the Cayuga Lake section, 
and to the Portage formation of the Genesee River section. 
From the established lad that the Catskill (a formation discrimi- 
nated on a lithological basis) did not occupy the same horizon, when 
the horizons were determined on a paleontological basis in sections 
not over 50 miles apart, it was argued that there is need of differ- 
entiating by nomenclature the vertical divisions discriminated by 
fossils from the lithological divisions called formations. 
The same subject was further elaborated in a discussion before the 
Geological Society at Boston in 1893, the immediate topic then under 
examination being the place of the Catskill formation in the geological 
time scale. In that discussion I proposed the use of dual nomencla- 
ture in geological classification, and again showed how the shifting 
of faunas from place to place necessitates their appearance at different 
horizons in separate sections, using horizon in the sense of synchrony 
in time. By this interpretation of the facts the Catskill formation 
was shown to occupy in eastern New York the actual horizon of the 
Oneonta of Delaware County, of the Ithaca formation of the Cayuga 
Lake section, and of the Portage of the Genesee River section. 
The lack of statistics for the discussion of migration of faunas was 
greatly felt in all those early studies of the subject. 
The deep interest taken in the question by numerous investigators 
has been shown by the many papers which have been published since 
