Williams.] GEOLOGICAL FAUNAS AND THETR NOMENCLATURE. 23 
whole region occupied by the sediment-receiving sea; but the place 
in the geological section of the more eastern part of the area marked 
by the presence of the fauna would represent a different period or 
moment of time from the place in the more western section containing 
the same fauna. The difference in time could easily represent half 
the period of the existence of the fauna in the province. 
The fauna in such a case may be supposed to slowly adjust itself 
to its evironment by migration instead of by modification, keeping the 
center of its distribution within the limits of the favorable conditions 
of depth, pressure, salinity, etc. Instead of accepting an unfavorable 
environment which has invaded its original habitation, it keeps its 
relation to the favorable conditions by changing its place of habita- 
tion, and thus by slow migration maintains uniform conditions of 
environment. 
If, now, we adopt the term equivalency to express the fact that the 
faunas are alike, and continuity to mean that the stratigraphical hori- 
zon of a zone or formation is the same, the conclusion which has been 
reached may be expressed by saying that fauna! equivalency does not 
necessarily conform to format ional continuity, except for areas thai are" 
narrow in relation to the extent of the distribution of the fauna. 
This same principle of transgression of a fossiliferous zone to a 
lower or higher horizon in a formation on passing from place to 
place, applies as well to the limestone beds as to the other lithological 
characteristics of a formation. On account of the transgression it 
will be evident that formational continuity can not be interpret '< d info 
exact time equivalency, except for very limited geographical areas, the 
limits of which must be determined also upon other evidence. Not only 
may the same fossiliferous zone occupy different horizons in separate 
outcrops of the same formation, but the same formation whose strati- 
graphical continuity can be clearly traced is presumably of diverse 
age at the extremes of its geographical distribution rather than of the 
same age. Thus area, locality, distance apart, are geographical terms 
for which zone, horizon, and thickness vertically in a section are the 
corresponding geological terms. 
Systematic position in a geological section is, like geographical 
position on a map, a means of locating the place in which a formation 
is situated, and has no necessary connection with the tirru at which 
the original formation of the sedimentary deposit was made. 
Age, contemporaneity, equivalency, and correlation are terms of a 
different order, and rest for their discrimination upon the evidence of 
fossils whose preserved forms testify of the time when particular 
species of organisms lived, and thus become a distinct indication of 
time relations. 
Particular fossil species are not confined to single fossiliferous zones, 
but may recur again and again in successive zones,, irregularly sepa- 
rated by barren or nearly barren zones. This fact is itself an evidence 
