28 CORRELATION OF GEOLOGICAL FAUNAS. [biti.l 210. 
The geologist is liable to regard fossils, in determination of cor- 
relation, as of the same order as minerals (viz, chondrodite) or pet- 
rographical characters (limestone, sandstone), and then to associate 
them with other diagnostic characters of the formations, but a closer 
consideration of the facts will show that the quality of the fossil by 
which it becomes evidence of a particular point of geological time, and 
from which it derives its value in correlation, is biological, and is due 
to the fact that in biology incessant change is taking place. 
While a formation has a bottom and a top and thickness; which, to 
be sure, must have started and ended at particular points of time, 
those particular points of time can not be determined in the general 
history of the earth except upon evidence which changed with the 
passage of time. The validity of this statement will become apparent 
by attempting to ascertain the geological age of an igneous rock with- 
out noting its relation to some fossil-bearing rock. 
In dealing with formations, therefore, whenever fossils are brought 
in, a new bodj^ of evidence is introduced, and a number of terms not 
applicable to formations are required for the scientific discrimination 
of this evidence. 
FAUNAL AGGREGATES. 
Fossils when spoken of in aggregates are faunas or floras. Faunas 
are particularly spoken of in this paper, not to the exclusion of floras, 
but because in most respects the remarks which apply to the geolog- 
ical relations of faunas apply also to floras. The term fauna, however, 
will be used in its strict sense of an aggregate of animals. The first 
reason for making the distinction between formation and faunas is 
that the aggregation of the species which makes up a fauna is not 
determined by the formation. The generally accepted practice, which 
was formulated in Dewalque's report a for the committee on uniformity 
of nomenclature at the International Geological Congress at Berlin — 
by which the chronological divisions (era, period, epoch, and age) are 
adopted as names for the duration of time corresponding to the strati- 
graphical divisions called group, system, series, and stage — does not deal 
with faunas as such but only with the nomenclature and classification 
of geological formations. 
Professor Renevier took a step toward the recognition of fossil 
faunas, as distinct from formations, in his "Chronographe Geolo- 
gique," 6 by distinguishing separate "fades "of the same formation 
deposited at the same time with other facies. 
In 1884 Renevier defined "facies" as follows: 
"Les facies sont done en definitive les differ entes sortes de forma- 
tions, sedimentaires ou autres, qui peuvent s'etre produites simultane- 
oCompte Rendu Congres Geol. Internat., third session, Berlin, 1888, p. 322. 
?>Compte Rendu Congres Geol. Internat., sixth session, Zurich, 1894, p. 519. 
