LOWER CRETACEOUS DEPOSITS CALIFORNIA AND OREGON

72 3WJWEH CRETACEOUS DEPOSITS IN CALIFORNIA AND OREGON
contrasts. The interval between them is little more than 20 miles. In
both, the stratigraphic sequence of the Shasta series is nearly complete,
each containing tbe two major groups of strata of similar thickness. In
fact the depositional sequences in the two areas seem to be parts of the
same general body of sediment. It may be supposed, however, that the
southern area contains Cretaceous strata somewhat older than the northern,
and this may be true to a limited extent. But such a fact affords no
explanation of the faunal contrasts which are evident, and which run
almost entirely throughout the Shasta series in these areas.
In the district between Thomes and Elder creeks, the fauna of the older
group, through a vertical range of 1800 feet, is dominated by AuceUa, of
which there are at least 11 distinct species, as determined by Pavlow.
With them are found various species of cephalopoda, including berriasellids,
olcostephanids, and neocomitids, specifically different from any
found in the northern district. Only a few species of ammonites are
known to be common to the two areas in the lower group. Many of the
cephalopoda found at the south are ^ponfined to this area. Among belemnoids
tbe case is somewhat different, and a number of species is common to
both districts. The contrasts in faunas are illustrated by the total absence
of AuceUa in the lower group at the north. None have yet been
found, although a few, and mostly different, species of Aucella have been
found in the upper group at the north. Reference to the lists of species
from the Paskenta group in tbe two areas will illustrate these facts.
Similar contrasts of faunas continue almost throughout tbe Horsetown
group in tbe two districts. The abundant cephalopoda in the Horsetown
group in the Cottonwood district have few representatives in this group
in Tehama County, although a few species are common to both districts.
There is a general scarcity of ammonites in the Horsetown group south of
tbe delta. The plant remains in the Paskenta group, as far as known,
show similar contrasts, in that they are abundant south of the delta and
comparatively rare to the north. On the contrary, the Horsetown group
north of the delta contains much fossil wood, some leaves, cones, and nuts
especially in the Hulen beds, but at the south these are not plentiful.
The causes of the faunal contrasts here described probably lay in the
hydrological condition of the embayment already described, namely, the
inflow of large volumes of fresh water laden with sediment from the land
areas to tbe west, seasonally cooled, and having a southward current along
the western side of the embayment; tidal currents bringing in marine
water that followed the eastern shore northward and into the areas north
of the delta; and lastly the influence of these currents upon the molluscan
life entering tbe embayment from the sea, but in part coming from opposite
directions along tbe littoral corridors of the time.
Sources of the Faunas—Earlier writers have pointed out that the faunas