wnxTAMs] GEOLOGICAL FAUNAS AND THEIR NOMENCLATURE. 21 
section, while another exposure of the same formation may be broken 
up into several fossiliferous and barren zones; and still another 
exposure of the same stratigraphical formation may be barren of 
fossils from bottom to top. 
In order to define such zones it becomes necessaiy to note and 
record their place in the vertical section of strata making the forma- 
tion. This is indicated most conveniently by measuring distance 
from bottom or top of the formation. This stratigraphical position 
of the fossiliferous zone in the section of the geological formation is 
its horizon. 
A fossiliferous zone may occupy the same horizon, a higher horizon, 
or a lower horizon in two exposures of the same formation, according 
as its position relative to the top or bottom limits of the formation is 
the same, higher, or lower. 
A fossiliferous zone may increase in thickness on following it in 
one direction, and decrease in the opposite direction, in proportion as 
the thickness of strata through Avhich the fossils prevail increases or 
decreases in the section. 
A fossiliferous zone may appear gradually on following the strata 
upward, or it may appear abruptly, being sharply contrasted with a 
subjacent nonfossiliferous zone. It is often the case that the central 
portion of a fossiliferous zone is richer in kinds of fossils than are 
its lower or upper portions. Species which are proportionately dom- 
inant at the first appearance of the fauna may disappear when the 
full expression of the fauna is seen, but reappear as the species 
become rare in the upper strata of the zone. 
Thus, for instance, Leiorhynchus is apt to occur on the borders of 
a fossiliferous zone, and is less frequently met with in the. center of a 
richly fossiliferous zone; Lingula and Discina are more frequently 
found in sparsely fossiliferous zones than in association with many 
other species or genera. 
When it is necessary to speak of a portion of a zone, be it fossilifer- 
ous or not, the terms bed or band or stratum are used. 
In this connection it is important to note that in ordinary sedi- 
mentary rocks, limestone (or the calcareous element of the sediments) 
is reasonable evidence of fossils, although present in a pulverized 
condition; and for purposes of discrimination between fossiliferous 
and nonfossiliferous zones, limestone should be classified among the 
fossiliferous zones although the forms of its fossils are obliterated. 
In like manner a coal bed is a mass of fossil plant remains. 
The kinds of strata in which the forms of fossils are in general best 
preserved are those ranging between coarse sandstone and pure 
limestone. In the former the roughness of the original conditions 
under which the formations were made was ill adapted for marine 
organisms, while the pure limestones were formed under conditions 
favorable for such organisms ; but, on account of the absence of sands 
