24 CORRELATION OF GEOLOGICAL FAUNAS. [bitt.t,. 210. 
of migration ; since a recurrence of the same fossils in successive 
zones can be rationally interpreted only on the supposition that dur- 
ing the sedimentation of the barren strata the successors of the lower 
fossils and the ancestors of those that followed must have lived in 
some other locality. 
The successive zones thus become evidence of successive occupation 
of the locality at which the stratigraphic section was made, and of an 
oscillation in the movements of the shifting faunas. In order to ascer- 
tain whether the shiftings are in one direction, or back and forth, the 
successive zones must be examined and the fossils compared. The 
paleontologist is therefore obliged to examine every foot of the section 
exposed, and wherever fossils can be discovered examination must be 
made and record of the facts be preserved. . 
When a fossiliferous stratum is discovered on ascending a strati- 
graphical section, the paleontological observer stops and samples 
the stratum. The fossils thus gathered constitute a faunule. The 
fannule may be found to extend upward for several inches, or possi- 
bly several feet, without apparent change. But the collector should 
observe carefully to discover the least sign of change in the fossil 
content of the faunule. 
In recording the contents of the faunule, care is needed to observe 
the 'proportionate abundance of the species. If collections are made 
with this idea in mind the species may stand in the collection in the 
same relation to one another as in the natural faunule. In addition to 
the collection, notes should be taken of the abundant and common 
species — the rarer forms will be discovered as such during the study 
of the collection in the laboratory. 
Each fossiliferous zone should be examined, and particular attention 
should be given to any intercalated bands of rock not like the pre- 
vailing rock of the section, which may bear faunules of a different 
fauna from the one prevailing in the general fossiliferous zone of the 
region. It has been ascertained that these slight temporary incur- 
sions of a fauna, which may be conspicuous not many miles distant, 
are valuable guides to the direction of the migration, and they are often 
forerunners of a fauna belonging normally at a higher horizon in the 
formations. 
The faunule is a sample of the faunal contents of a fossiliferous 
zone, and, as a sample, care should be taken to keep together in their 
true relations all the species of the individual faunule, so as to permit 
no doubt as to the natural association of the species when the collec- 
tions come to be more minutely studied in the laboratory. The posi- 
tion of the faunule in relation to other faunules in the local section 
should be observed and recorded with precision, note being taken 
of its relative position in the fossiliferous zone, as well as its posi- 
tion in the formation as officially mapped and described in Survey 
reports of the region. 
