LOWER CRETACEOUS DEPOSITS CALIFORNIA AND OREGON

93 LOWEB CBETACEODB DEPOSITS IN CALIFORNIA. AND OREGON
A similar retreat of the sea is described from Texas and Arizona, and, as
suggested, also in Alaska.
It may be carrying the rights of inference too far to suggest that the
soutb Pacific land bridge may have been incomplete or only partially
effective in the earlier part of Cretaceous time and that not until Hauterivian
time were connections between Asia and America sufficiently complete
to permit migrations hither. Nevertheless, such suggestions are
inherent in the subject and arise spontaneously in tbe study of the faunas,
their origin, relations, and succession in lower, middle, and later Horsetown
deposits in the Great Valley embayments of California.
By what other route could tbe families, the genera, and even nearspecific
analogues have reached the American coast during the epochs of
their dominance in those distant regions (Caucasus, Cutch, and Australia)?
Disturbances similar to those at the beginning of Horsetown deposition
brought this epoch to a close in late Albian time, resulting in the spreading
of Chico sediments unconfonnably upon those of the Shasta and older
terrains. The biotic results of this disturbance at the close of the Shasta
period cannot be given here, but it may be added that the faunal changes
effected at this time in California and Oregon are comparable to those
already described for the beginning of the Shasta period and at the beginning
of Horsetown time; new and linfnmiTiar stocks were brought into
the embayments in late Albian time from apparently remote sources;
these facts seem most readily explainable by the supposition of widespread
earth movements at this point of geologic history.
The tectonic record of the Chico period i3 replete with incidents, as is
also the records of arrivals of new assemblages of invertebrate life.
GENERAL SUMMARY
The Cretaceous deposits of California and Oregon are probably the
most complete and representative of this period that occur on the Pacific
Coast and they contain a succession of marine invertebrate faunas unequalled
in any other known area on the Pacific border. Yet they have
been but little explored, and therefore little has been known concerning
them, either of the circumstances of their accumulation, their stratigraphical
features, or their faunal character of order. Although large
areas of Cretaceous deposits are known in other troughs in the Coast
Ranges of these States, and farther north, the thickest, most fossilifcrous,
and in many ways the most important, are those found in the Great Valley
trough of California. Within this trough the maximum thickness of the
succession (Shasta and Chico series) is not less than 50,000 feet. For
the most part the Cretaceous troughs in California and Oregon, as in
other parts of the Coast, are inherited from Jurassic time, or perhaps in