38 CORRELATION OF GEOLOGICAL FAUNAS. [bull. 210. 
district. In such a case the formation holding the two faunas would 
be identified, by their fossils, as belonging to two separate epochs; 
and the stratigrapher would take the third example as proof positive 
that the one fauna followed the other and therefore that the two epochs 
were successive and not contemporaneous. 
An example of such a case is discussed in detail beyond. The Che- 
mung formation is known to follow (to lie above) the Hamilton for- 
mation in western New York by the fact that normal faunas of the 
former are some thousand feet higher up in the section. But that the 
faunas are actually contemporaneous in a part of their existence is 
shown by the recurrence of a faunule of Hamilton species at Owego, 
N. Y., in the midst of strata containing below as well as above char- 
acteristic Chemung fossils. 
These two sets of facts — the formations and the faunas — must there- 
fore be dealt with separately. While the presence of the fossils of a 
particular fauna does stand for something in a column of sedimentary 
rocks, it docs not stand for the whole of any particular period or inter- 
val of time. It represents some portion of the life period of the fauna, 
but the limits observed in a local column between one fauna and a 
succeeding one may not be the horizons of the beginning or of the 
close of the life history of the fauna; they may be the limits of the 
formation for that section. 
There seems to be necessity of considering also a third element, 
which Mr. Bailey Willis has recently emphasized. I refer to the time 
element of geological classification. Formations are lithological and 
physical. Faunas arc biological and must be treated of as living. 
Time divisions are conceptions, and their use depends upon the 
accuracy and reliability with which they may be represented by 
visible formations or faunas. 
The primary basis of distinguishing the time relation of formations 
is stratigraphieal sequence. But the formation itself is a lithological 
aggregate, and the lithological characters by which one formation is 
distinguished from another have no regular order of stratigraphieal 
sequence, hence stratigraphieal sequence has no positive time value; 
it is only the element of sequence of time which is recorded by the 
observed facts. 
When faunas are considered separately from formations, in this 
way, we are ready to notice that faunas may have shifted geograph- 
ically, and may thus cause confusion in the classification and correla- 
tion of the formations of contiguous basins. When we consider the 
confusion which has already arisen in the classification of the geology 
of the various counties of Pennsylvania, which is probably to be 
accounted for in this way, the necessity for more light on the subject 
is apparent. The consideration of a possible shifting of faunas may 
therefore be necessary to the proper interpretation of facts which 
otherwise greatly confuse the geologist. Classification based upon 
