24 GEOLOGY VACAVILLE TO CHICO CREEK. 



surround their bases, exhibit little traces of disturbance, and do not cover their sides ; from 

 which it is apparent that they were not covered by the waters by which these transported mate- 

 rials were deposited. 



We have here arrived at nearly the eastern limits of the Sacramento valley. The foot hills 

 of the Sierra Nevada are but few miles distant, and although immediately on Feather river, we 

 are still on the lower alluvial plain which borders the Sacramento river, and occupies all the 

 central portion of the valley within a mile to the north; the surface rises more than 200 feet, 

 and a series of rounded hills commence, which are so characteristic of the borders of the valley. 

 Chico creek. — No rock wa3 found in place from Hamilton, where we left Feather river, to this 

 point. The surface is somewhat undulating ; the higher grounds being destitute of trees, the 

 soil gravelly and unproductive, while the lower grounds immediately adjacent to the streams 

 which we crossed are fertile, and sustain groves of oak timber. 



The beds of the streams contain gravel and rounded stones in larger quantities than those 

 before passed, and the fragments are of larger size. They consist principally of trap, compact 

 or vesicular, some of it being scoriaceous. 



Fossils. — At Bidwell's on Chico creek we saw fragments of calcareous sandstone — brought 

 from a locality not far distant on the same stream, in the foot hills of the Sierra Nevada — whicb 

 contained great numbers of fossils very perfectly preserved. The rock in which they occur is 

 light brown in color, and quite hard and compact. The fossils are exclusively marine shells, 

 and present an interesting mingling of forms, to which attention was first called by Dr. Trask, 

 in the Proc. Cal. Acad. Sciences. The genera represented are Mactra, Tellina, Nucula, Fusus, 

 Turritella, Natica, &c, of which the species are as yet undetermined, with the exception of two — 

 a Mactra and Nueula — considered by Mr. Conrad as probably identical with two species, Nucula 

 divaricata and Mactra albaria, described by him in the Geology of the Exploring Expedition, and 

 obtained by Professor Dana from the shales and sandstones near Astoria, on the Columbia river. 

 The Astoria shales contain large numbers of fossils, many of which have been described by 

 Mr. Conrad, (op. cit.) and have been regarded by him as of Miocene age. These shales and 

 sandstones are also probably identical with the San Francisco group, which, as we have seen, 

 may for still other reasons be considered as recent as the Miocene epoch. 



Baculiles and Ammonites have never heretofore been found in any strata of more recent date 

 than the cretaceous, and there these genera have been regarded as furnishing the most certain 

 criteria by which to distinguish between cretaceous and tertiary deposits. There can be no doubt, 

 however, of their occurrence in the sandstones of Chico creek, and, from their mode of fossilization, 

 could not have been transported from older deposits, but must have lived in the sex with the 

 other associated genera. 



The species of the fossil shells found with the Ammonites and Baculites in this locality are 

 probably all new, unless the Nucula and Mactra alluded to are, as has been supposed, identical 

 with those of Astoria. They are, therefore, of little or-no value in determining the age of the 

 deposit within the limits in question. 



The genera represented are, perhaps, equally common in cretaceous and tertiary rocks, and, 

 so far as known, do not afford any forms which, though specifically new, might be regarded as 

 by their general pattern to indicate a cretaceous rather than a tertiary age, or vice versa. 



One of two conclusions is therefore inevitable: either the Nucula and Mactra, which are 

 supposed to link these strata to others known to be Miocene tertiary, are distinct from those of 

 the shales of Astoria, and the Chico creek strata are cretaceous ; or they are identical, and 

 Baculiles and Ammonites cease to be criteria in distinguishing tertiary from cretaceous rocks. 



