Williams.] PRINCIPLES OF CORRELATION. 11 
DUAL NOMENCLATURE. 
It was while acting as a member of the American com initio,, which 
was engaged in preparing reports on the American systems for the 
International Congress that I became impressed with the necessity of 
a dual nomenclature. The common usage abroad, as here, was to 
name and classify geological formations only. Fossils were a means 
of their identification, but no attempt had been made to distinguish 
the limits of the life range of the fossil faunas from the formation al 
boundaries which were established on lithological and stratigraphica] 
grounds. 
The principle of distinguishing the faunal from the formational 
classification and nomenclature was thus summarized in the Compte 
Rendu of the Fourth Congress. 
Prof. H. S. Williams at the Albany meeting [1887] suggested an important 
fundamental idea, and one which may influence materially the final distribution 
of terms in stratigraphic nomenclature, viz, the adoption of a dual set of designa- 
tions— one set, that referring to the lithological character of the rock masses and 
based on geographic names, will be liable to vary as the strata change from place 
to place: and the other, based on some great and persistent life characters, shall 
refer to the faunas of those rock masses and be substantially constant over large 
areas, and perhaps over the world. It is very evident that great confusion has 
resulted in the past, among geologists, by confounding these distinctions, and 
much controversy has arisen in attempting to maintain one or the other of these 
different zonal designations. Stratigraphic work has been ignored, or at least 
neglected, by paleontologists, and the practical field geologist has been tempted, 
in some instances, to ignore, if not to deny, the assertions of the paleontologist. 
Instead of this confusion there should be introduced some new departure. The 
confusion results from a confusion of nomenclature. Faunal characters have 
been made to have the force and the usage of stratigraphic designations and have 
been extended as stratigraphic features over strata where the faunal characters 
are wanting. Again, stratigraphy, based on natural and great lithological dis- 
tinctions, having been defined in one region by its faunal associations, is extended 
over other States by one geologist so far as he finds the lithology to warrant, and 
by another so far as he finds the paleontology to warrant. 
There are, hence, two laws by which we must be governed in framing a scheme 
of nomenclature which shall allow the freest rein both to the stratigraphic geolo- 
gist and to the paleontologist. One relates to the work of the stratigrapher, who 
takes account of the great physical changes to which the earth's surface has been 
subjected, and the other refers to the work of the paleontologist, who strives to 
delineate the organic changes which the surface of the earth has witnessed. 
These changes have been supposed to be coeval and coextensive: but our investi- 
gations show they have not been so entirely. But we sometimes have the same 
fauna, or nearly the same, living under different circumstances, and. perhaps, 
also at different dates, in different parts of the world. 
So long as the geology of the United States, for instance, was known accurately 
in only one part (New York State) the faunal characters which the formations 
were found to exhibit were seen to be coincident with the si ratigraphic fcosogreal 
an extent that there was no reason to dissociate them under separate schemes; 
but since the whole area of the United States is being brought under careful 
examination, it is found that the close connection which these two classes of 
characters have in New York State is broken up and they begin to diverge grad 
