LOWER CRETACEOUS DEPOSITS CALIFORNIA AND OREGON

CRETACEOUS 6EDIHENTABY DIVISIONS 89
robust, thick-shelled species of Aucella, comparable to, if not specifically
identical with, contemporary boreal and Russian faunas. These are
accompanied by a number of cephalopoda, some of which are boreal,
although some are not. Species of AuceUa range through the lower
group of sediments in the district south of the delta, some appearing even
as late as Albian time (uppermost Horsetown).
In the North Cottonwood district no species of ^ ucella have been found
in contemporary beds, or in any strata below tbe Aptian, where a few
specimens have been collected. In this district the fauna of the Paskenta
group (Valanginian), although not large, consists of cepbalopod species,
only a few of which occur in the southern district, and with them are
species not found south of the delta. A few diagnostic species>are common
to both districts, including both cephalopoda and pelecypods.
The faunas of the Horsetown group in the two districts show a greater
number of common or analogous species, although at tbe north cephalopoda
occur in greater number and variety, many of which are not found at
the south. In the North Cottonwood district is found a succession of
ammonoid faunas ranging from lower Hauterivian to middle Albian; only
a few of these are found south of the delta, as is true of other classes.
* Nearly all the ammonoid stocks in the Horsetown group are of southern,
or subtropical, aspect and have their nearest analogues in southwest Asia
(Himalayas, Caucasus, Cuteh, and Australia); few of them have been
found farther north along the Pacific Coast, although our knowledge is
yet incomplete. Only a few of the many cephalopods in the Horsetown
are known to bave near analogues in Argentina or other South American
countries. In view of their number found in southwestern Asia and
Australia, this fact is surprising.
In tbe lowest part of the Horsetown group both north and south of the
delta, there are heavy beds of conglomerate and other coarse detritus, as
if at the beginning of this epoch (Hauterivian) an uplift of the mountain
areas to the west had shifted the strandlines eastward within the trough
and at the same time had stimulated tbe erosive and transporting energy
of tbe streams. It was at this juncture tbat a new cycle began in the
faunal sequence, which brought into the embayment new and unfamiliar
types and families of cephalopods, many of them of Asiatic aspect. This
disturbance at the beginning of Horsetown time seems to have been felt
along the Pacific border from Mexico to Alaska, although its effects were
differential, producing elevation in some areas more than in others. This
movement seems capable of correlation with similar phenomena described
for central Mexico and for some Andean countries far to the south. Possibly
at this epoch the land and water conditions in south Pacific regions
permitted migrations or faunal exchanges between Asiatic and American
littoral embayments.
For the later part of Cretaceous time three major events, of two phases