STRATIGRAPHY OF SOUTHERN CALIFORNIA 125 
strata were laid down. The sediments consist for the most part 
of medium grained gray sands, such as are being deposited at 
the present time quite near shore off San Pedro. The abund- 
ance in the lower San Pedro deposits of certain species which 
live only between tides also offers evidence of deposition of the 
strata near the shore. 
The fauna of the lower San Pedro series is one of transition 
from the boreal or subboreal fauna of the late Pliocene to the 
semi-tropical fauna of the upper San Pedro series. Two causes 
account for this change: (1) The deposits being laid down in 
shallower water than that in which the Pliocene sediments were 
deposited would necessarily contain fewer of the colder, deeper 
water forms of the Pliocene; and (2) the lower temperature 
prevailing during the latter part of the Pliocene epoch was giv- 
ing place to warmer conditions, which caused the boreal species 
to migrate and brought in more of the species which commonly 
inhabit warm waters. Of the fauna of nearly 250 species found 
in the lower San Pedro series, over 17 per cent. are now found 
living only north of San Pedro. This percentage is only a little 
lower than that of the northern species found in the Pliocene, 
(the latter being 18.5 per cent.), and indicates, considering the 
fact that the lower San Pedro beds are shallower water deposits 
than those of the Pliocene, that the climatic conditions had 
changed but little during the period, intervening between the 
deposition of the Pliocene and the overlying lower San Pedro 
series. 
The evidence afforded by several species in the lower San 
Pedro series (about 4 per cent. of the fauna of that series), which 
are now found living only south of San Pedro, shows that the 
conditions were changing from cold to warm. Besides the dis- 
tinctly northern forms which either disappear or become scarce 
in the period between the Pliocene and lower San Pedro, there 
are many species now more commonly found in northern waters, 
which, though common in the Pliocene, become rarer in the 
Pleistocene ; while species showing a predilection for southern 
conditions become common in the fauna of the latter period. 
