1887.] NATURAL SCIENCES OF PHILADELPHIA. 321 



of progressive faunas, requires for its acceptance facts of a very 

 different character from any that geology has thus far brought for- 

 ward. It is true that geologists may have erred in locating or defin- 

 ing their time-periods (or formations), and, doubtless, more accur- 

 ate research will alter, or completely obliterate, much that has been 

 done in the way of chronological classification, but to maintain, as 

 some geologists would lead us to believe, that the Cretaceous and 

 Tertiary formations (or pei'iods) no longer exist as such by virtue 

 of the discovery of a series of intermediate connecting beds ; or that, 

 for a somewhat similar reason, the Oligocene formation has given 

 up its right to existence independently of the Eocene, is main- 

 taining a position which no geologist who has examined into the 

 premises will be willing to concede. We may as well admit that 

 the various periods recognized in human history do not exist because 

 they can be continuously traced without break of any kind ; and 

 similarly, centuries and years have no existence because they are 

 continuously consecutive. The geologist should bear in mind that 

 the time-periods (et conseq. the corresiDOnding formations) are arti- 

 ficial constructions based upon equivalent phenomena for the entire 

 world, and serve only for the purpose of a universal classification. 

 The finding of an intermediate formation (period) in no way oblit- 

 erates the formations between which it may have been found, pro- 

 vided the limits of these formations have been satisfactorily deter- 

 mined ; for example, the discovery of a connecting series of deposits 

 between the true Cretaceous and Tertiary, uniting them by way of 

 continuous sedimentation, does not render the entire series Cretaceo- 

 Tertiary, but the connecting beds are alone entitled to this designa- 

 tion. The Cretaceous and Tertiary sections of the series remain as truly 

 Cretaceous and Tertiary in a general scheme of classification as they 

 were before the intermediate beds were discovered. Manifestly, in 

 a complete geological scale, whose near realization is by no means im- 

 , possible, all the various formations now recognized will be found 

 united to one another through the intermedium of connecting beds, 

 but certainly no geologist will go so far as to urge that, for this rea- 

 son, a classification of time, based upon some artificial basis of chro- 

 nology, will no longer be necessary or desirable. 



It has also recently been objected that a classification based upon 

 percentages of survivals is an absurdity, and wholly illogical. Why 

 illogical, it might be asked ? Surely, if there has been a general, 

 and neai-ly equivalent, faunal succession or development for the 



