628 BULLETIN UNITED STATES GEOLOaiCAL SURVEY. 



dom lias been greater on this continent than it has been in Europe. 

 Hence, a student of the flora of these American strata, using a series of 

 European standards, would naturally refer those which he found to con- 

 tain certain vegetable forms to the Tertiary period, while the associated 

 or superimposed remains of animal life might all show them to be of 

 Cretaceous age, according to the same series of European standards. 



The fact that the physical changes which took place in Western Korh 

 America during the Mesozoic and Cenozoic periods were very gradual, 

 and without any important break, is of itself sufficient to lead us to ex- 

 pect to find tiiose animals whose existence was not necessarily affected 

 by a change from a saline to a fresh condition of the waters, would be 

 found to have propagated their respective types beyond the period 

 which those types in their culmination distinctively characterized. Thus 

 the discovery of these perpetuated types that in their culmination dis- 

 tinctively characterized the Cretaceous period does not necessarily prove 

 the Cretaceous age of those strata, because they are evidently the last 

 of their kind, and because all the other known fossil remains of the 

 group are indicative of a later period. 



Again, the plants that have been found in the Dakota group when 

 first discovered were, by European standards, referred to the Tertiary 

 period, until it was found that all the other Cretaceous groups of the 

 Western North American series belong above the one that contained 

 them. These raay serve as extreme examples of acceleration on the one 

 hand, and of retardation or prolongation on the other, of the evolutional 

 life-currents before referred to ; and as such they are exceedingly sig- 

 nificant. 



These currents have been very variable also among invertebrate ani- 

 mals, the Mollusca of marine, brackish, and fresh waters affording strik- 

 ing examples. In marine waters, these animals have, from early geolog- 

 ical times, attained a maximum of wide and various differentiation, in 

 brackish waters much less, and in fresh waters a minimum. The value of 

 their remains as indices of the geological age of the strata containing 

 them is therefore in a similar ratio, those of marine origin being of much 

 the greatest value, and those of fresh-water origin the least. Indeed, the 

 evolutional life-current among the latter, as well as among the land Mol- 

 lusks, has been so sluggish that alone they are of comparatively little 

 value to the geologist in determining the geological age of the strata in 

 which he finds their remains. For example, if it were not for the known 

 stratigraphical relations of the brackish- and fresh-water Wealden de- 

 posits of Europe, no paleontologist would be justified in referring them 

 to Mesozoic rather than Cenozoic age. Many other, but less widely 

 known, examples might also be cited from our Western North American 

 strata. 



With a few doubtful exceptions, none of the strata of the Laramie 

 groifp were deposited in open-sea waters ; and, with equally few excep- 

 tions, none have yet furnished invertebrate fossils that indicate the 



