FOSSILS OF THE OIL-BEARING FORMATIONS OF 
SOUTHERN CALIFORNIA. 
By Ralph Arnold. 
INTRODUCTION. 
Many of the oil-bearing rocks of southern California contain the 
fossil remains of living organisms such as plants, foraminifera, sea 
urchins or echinoderms, marine and fresh-water mollusks, crabs, fish, 
and some of the higher vertebrates. The marine mollusks, owing to 
their peculiar composition and habitat, are relatively much more lia¬ 
ble to be preserved in the rocks than the individuals of most of the 
other groups, and, as a consequence, they are the most abundant of 
the organic remains, and therefore the most useful for purposes of 
correlation, not only in this region, but in all regions abounding in 
marine sediments. 
A fossil may be the more or less altered remains of the plant or ani¬ 
mal; it may be the empty mold from which all of the original material 
of the organism has been removed; or it may be the cast of the original 
form in hardened silt, sand, or some crystallized mineral which has 
replaced the organism. 
It is well known that associations of species (fauna or flora) are 
governed by two important factors—one of time and the other of 
environment. The fossil fauna of any formation, unless it be an unusu¬ 
ally thick one requiring a very long time for its deposition, is generally 
fairly constant throughout for rocks of similar lithologic character, 
owing to the fact that faunas living at approximately the same time, 
under similar conditions, and in the same general region or biologic 
province are closely related or nearly identical. Conversely the 
faunas of the different kinds of rocks in the same formation and even 
of the same kind of rock in two different formations are always more or 
less distinct. As the different marine sediments reflect the varying 
conditions of their deposition, so the faunas reflect their environ¬ 
ment; fossils are usually found in a matrix whose deposition was gov¬ 
erned by the conditions forming the environment of the fossils. Thus 
a deep-water fauna is usually found in shale or limestone, a shallow- 
water or littoral fauna in sandstone or gravel, and so on. Coarse 
conglomerates, owing to the disturbed conditions surrounding their 
deposition, rarely contain recognizable fossils. 
219 
