PREFACE. XIX 



The fourth section of this volume furnishes evidence of 

 the wide-spread occurrence of the Cretaceous system on the 

 Pacific coast. The Coast ranges of California and Oregon 

 are, indeed, to a large extent, made up of rocks of this age, 

 usually somewhat metamorphic in their condition, and often 

 highly so; but still, on the whole, forming the richest fossili- 

 ferous formation of California, having furnished already, in 

 the course of our necessarily rapid explorations, a more or 

 less extensive series of species at over thirty localities. The 

 Cretaceous formation is also extensively developed on the 

 flanks of the Sierra Nevada, in the northern part of the 

 State, and in this position being less metamorphosed than in 

 the Coast ranges, several of the localities discovered have 

 proved to be prolific in well-preserved as well as interesting 

 and novel forms. According to Mr. Gabb, it appears that 

 the Cretaceous, so far as is yet known, is represented on the 

 Pacific coast by but a single member, — the Upper or White 

 Chalk, — and that it may be divided into two well-marked 

 groups, the lower of which corresponds to the No. 4 of Meek 

 and Ilayden, or the Fort Pierre group. This division (Div. 

 A.) of this volume, has yielded 152 species in California, out 

 of about 2G0 collected in the Cretaceous, and of these only 

 about half a dozen are common to this and the upper divi- 

 sion (Div. B.). This latter should, probably, judging from 

 its stratigraphical position, correspond with the Fox Hill 

 group, or No. 5 of Meek and Hayden ; although all the species, 

 with a single doubtful exception, are peculiar to California, 

 and that species is referred to one described from No. 4 in 

 New Jersey and Tennessee. 



