Gbol— Vol. II.] ANDERSOM— CRETACEOUS DEPOSITS. 25 



if but a few of its localities are taken independently. It is 

 found in this way that there are recognizable elements 

 apparent in each fauna, which may be safely depended 

 upon, and that while there is more or less of a transitional 

 character in the fauna of a given level, yet it does not 

 depart from the main type to any considerable extent until 

 the time arrives for an almost complete change. The 

 Horsetown fauna, for example, consists of a large number 

 of cephalopod forms, which is as great if not greater than 

 the whole number of other mollusks combined. This can 

 not be claimed for the Chico upon the eastern side of the 

 valley, where the whole number of cephalopods known is 

 not greater than one-eighth of the number of other mol- 

 lusks, and even in the strata immediately overlying the 

 Horsetown upon the west, which have been hitherto re- 

 ferred to the Chico, the proportion of cephalopods known 

 is not more than one-third that of the others. The rapid 

 increase in the number of gastropod and bivalve species 

 in the Chico is, however, the noteworthy fact; while at the 

 same time, the number of cephalopods as rapidly dimin- 

 ishes, except, perhaps, in more favored localities. 



In the Great Valley basin of California the transition of 

 faunas is more gradual than it has been in any other basin 

 of the Pacific border; and for that reason the faunas repre- 

 sentative of the different horizons are not so easily distin- 

 guished. For purposes of correlation, therefore, it is safer 

 to select for study, if possible, localities lying outside of the 

 boundaries of the Great Valley, in which these distinctions 

 can be more readily made. And for the Chico epoch this 

 is both possible and especially desirable. The faunas of 

 the Chico are therefore represented in the following lists, 

 massed from a number of the more significant localities, as 

 will be shown later. Each division of the Chico, the Upper 

 and the Lower, is represented by four^ such localities, 

 the lists being for the most part compiled, in a somewhat 

 revised form, from others already published. For the 

 Upper Chico the localities selected are in the Sacramento 



