Geol.— Vol. IT.] ANDERSON— CRETACEOUS DEPOSITS. 23 



series as a whole is not altogether simple, and that in the 

 closing epochs disturbances were both more numerous and 

 more general. Similarly, when the series is made the sub- 

 ject of faunal study, a diversity that is still more significant 

 is soon recognized. The faunal differences that are ordi- 

 narily seen have led to the distinctions hitherto made, and 

 to the divisions of the series settled upon by the earlier 

 writers; but these differences are real and not merely 

 apparent. It is evident to one coming from the fossiliferous 

 beds upon the eastern border of the valley, where gastro- 

 pods and bivalves largely predominate, to the beds of the 

 Cottonwood, where cephalopods are so common, that one 

 has reached an entirely different faunal horizon. So, also, 

 when one proceeds to the more basal portions of the series 

 in the foot-hills of the Coast Ranges, one finds again a 

 complete faunal change. The cephalopods of the last hori- 

 zon gave place to a fauna composed almost entirely of one 

 or two species of Aiicella. These facts led to the recogni- 

 tion of the three horizons commonly known as the Chico, 

 Horsetown, and Knoxville, which, in spite of the connect- 

 ing elements uniting them, have not yet been, and ought 

 not to be, abandoned. Indeed, it is not improbable that 

 upon further study additional reasons will be found for still 

 further enforcing the distinctions, and even, as it now 

 appears, of subdividing some of the principal divisions that 

 are at present accepted as paleontological units. 



Both Diller and Stanton have been convinced of the 

 transitional character of the fauna from one level to another 

 in different parts of the series. New forms appear succes- 

 sively and continue for unequal periods and disappear at 

 different stages of the overlying series. Some forms are 

 of short duration and some are very much more persistent. 

 Many lists of species taken from different localities and 

 representing different horizons have been published, which 

 apparently show this; and undoubtedly within certain limits 

 there is a more or less gradual change, and for some pur- 

 poses these facts may well deserve attention. Yet the 



