444 DILLER AND STANTON — THE SHASTA-CHICO SERIES. 



The Chico beds of this section, extending from a short distance above 

 the mouth of Hulen creek to Gas Point, have a thickness of 3,600 feet. 

 In the basal portion, as on Cold fork and Elder creek, conglomerates and 

 sandstones prevail, but with a thickness deminishing northward. They 

 contain near the bo.ttom a large number of characteristic Chico fossils, 

 and associated with these at this point, as well as on Cold fork and 

 Elder creek, have been found the characteristic Wallala form, CoralUo- 

 chama orcutti. 



At Gas Point the topmost Chico beds are unconformably overla})ped 

 by the post-Cretaceous deposits which fill the Sacramento valley. It is 

 not probable, however, that much of the section is covered, for upon the 

 eastern side of the Sacramento valle,y, where the upper portion of the 

 Chico is exposed, the fossils indicate approximately the same horizon as 

 the beds at Gas Point. No trace of the Tejon has 3^et been found in 

 California north of the fortieth parallel. 



Faunas of the Series. 

 chico fauna. 



The Chico fauna is most typically developed at Pence's ranch, Butte 

 creek, Chico creek and other localities along the eastern border of the 

 Sacramento valley, where it was first recognized by the California Geo- 

 logical Survey. A great many species were described from this region 

 by Mr W. M. Gabb, a few were described earlier by Dr Trask, and some 

 additions to the fauna have since been made by Dr C. A. White. The 

 strata exposed at these localities are comparatively thin, and on structural 

 grounds they are correlated with the upper portions of the beds referred 

 to the Chico in the sections on the west side of the Sacramento valley. 

 This correlation is supported by the fact that the few fossils, such as 

 Ammonites {Schloenbachia) chicoensis and Nucula truncata, found only in 

 the upper parts of these sections, are common at the t^^pical localities 

 above referred to, though the very small number of species makes the 

 comparison with the rich fauna of the east side of the valley less satis- 

 factory than is desirable. On the other hand, the lower 1,000 feet of the 

 Chico in the measured sections are very fossiliferous, and the fauna they 

 have yielded, though not as large as that of the typical Chico localities, 

 has many species in common with it, together with some that have not 

 been found there. Such species as Cucullsea truncata, Trigonia evansana, 

 Trigonia leana, Meehia sella, Pectimculus veatchi, Chione varians, Anchura 

 falciformis and others that are common on the east side of the valley 

 occur at the very base of the Chico. This comparatively small change 

 in the fauna indicates that the Chico was a period of rapid sedimentation 



