stanton.} FORMATIONS CHARACTERIZED BY THEIR FAUNA. 17 



region the paleontologic lines are not so sharply drawn as they are east 

 of the Front range, though the (anna! as a whole in the lower part of the 

 series corresponds with that in the lower portion of the Meek and 

 Hayden section, and is very different from that of the upper. 



In the rapid reconrioissance of such large areas as were covered by 

 the earlier surveys in the West there was little time for detailed pale- 

 ontologic work, and the geologists were therefore often obliged to 

 classify and describe the formations by their most obvious lithologic 

 and structural characteristics. But such classification can be regarded 

 only as tentative, and when, as in the case now under consideration, 

 the loAver portion of a u group" or formation is found to contain a fauna 

 that is entirely distinct from that of the upper portion, while the latter 

 is very closely related to the fauna of the overlying formation, it is 

 evident that the plane of separation has not been drawn on the right 

 horizon. A decided change in marine faunas occurring at approxi- 

 mately the same horizon over a large area is evidence of either a con- 

 siderable time interval or of some great change in external physical 

 conditions, and in either case it should have weight in geological classi- 

 fication, even though the character of the sediments may be so nearly 

 the same both above and below the faunal break that no reason other 

 than the paleontologic one can be seen for their division. Such a dif- 

 ference between adjacent marine fossil faunas certainly represents a 

 more important event than the change from a sandstone to a shale or 

 from a shale to a limestone. These general facts are universally 

 admitted and numerous examples could be cited in which the dividing 

 line between formations as widely separated in age as the Cambrian / 

 and the Carboniferous or the Jurassic and the Upper Cretaceous could 

 not be determined until the fossils were studied. But when the faunas 

 both belong to the same geologic period they are more closely related 

 and the importance of such a faunal change is likely to be underesti- 

 mated by the field geologist. 



. Meek and liayden early recognized the fact that the most marked 

 faunal break in their Cretaceous section is at the top of the Niobrara 

 division, and for that reason the line between the u Lower Series " and 

 the " Upper Series" was placed there. In Prof. Meek's last published 

 work 1 he says : 



In passing from tho Niobrara group, however, into tho succeeding rocks above, in 

 which great numbers of fossils occur, not a single species, so far as known to the 

 writer, has yet been found identical with any form yet known from either of the 

 thPee divisions below. In addition to this, the upper surface of the Niobrara beds is, 

 at several places on the Missouri river, seen to have been eroded into irregularities 

 or depressions, previous to tho deposition of the succeeding Fort Pierre group, thus 

 giving additional evidence that some kind of a physical change (perhaps slight) 

 occurs between the deposition of the latest portion of the Niobrara division and the 

 first of the Fort Pierre beds. 



It is now known that a few sj>ecies do pass from the lower series to 



1 U. S. Geol. Sur. Terr., vol. ix, p. xxxn. 



Bull. 106 2 



