220 
OIL DISTRICTS OF SOUTHERN CALIFORNIA. 
It follows, then, that similar faunas from two or more different 
outcrops indicate in a general way the contemporaneity and simi¬ 
larity of conditions of deposition of the beds; that is, they indicate 
the same horizon or formation. The converse of this statement, 
however, is not always true, for it often happens, for example, that 
the shale fauna of one formation bears a stronger affinity to the shale 
fauna of a second formation either above or below it than to the fauna 
of contemporaneous sandstone or conglomerate within its owp for¬ 
mation. 
Some faunas are characterized by the predominance of one or 
more families or broader groups; some only by the occurrence or 
abundance of certain species. Analogous to the formations which 
contain them, certain faunas are distinctly isolated from others of 
the same region. A formation having an isolated, unique, or so- 
called characteristic fauna is the most easily identified and corre¬ 
lated, as the determination of only a few of its species often suffices 
to locate it definitely. The correlation of those rocks whose fauna 
is not so distinctly separated from the fauna of stratigraphic ally 
adjacent formations is much more difficult and generally requires 
some special training along paleontologic lines. Fortunately most 
of the formations in southern California contain fairly characteristic 
faunas, so that with a sufficient amount of identifiable material from 
any given stratum its correlation, or the determination of its relative 
position in the stratigraphic column, may be readily approximated. 
It is a well-known fact that the only way to determine the relative 
position or age of any bed in a series where the rocks of the different 
formations are so much alike and the structure so complex as in the 
southern California oil districts is by a determination of itsr fossil 
contents. Ordinarily lists of fossils are absolutely meaningless except 
to the few who have devoted at least a little time to the study of 
paleontology. But when these lists are supplemented by the figures 
of the species named it is an easy matter for anyone to find out for 
what each name really stands. The following plates are given to 
add meaning to the lists of fossils included in the text. The species 
figured do not represent all of those found in the oil-bearing forma¬ 
tions of southern California, but rather those which are commonly 
found in or are characteristic of the principal formations there devel¬ 
oped. It is thought that by means of these figures anyone will be 
able to locate approximately any bed in the geologic column if he 
can obtain from it a few recognizable fossils. 
The fossils are arranged on the plates in the order of the occurrence 
of the rocks containing them, beginning with the oldest, the Eocene. 
