CONTEMPORANEITY OF STRATA. 25 



than would any particular historical epoch be delayed until the 

 survivors of the preceding one had died out. Period is an 

 arbitrary time-division. The Chalk or the ' London Clay ' 

 formations mark definite stratigraphical divisions. We may 

 speak of the period of the London Clay, or we may speak of 

 the Tertiary period. It merely refers to the ' time when ' 

 either were in course of construction. The occurrence of 

 Triassic forms in the Jurassic series, of Oolitic forms in the 

 Cretaceous series, and of Cretaceous forms in the Eocene, in 

 no way lessens the independence of each series, although it 

 may sometimes render it difficult to say where one series ceases 

 and the other commences. The land and littoral faunas are 

 necessarily more liable to change than a deep-sea fauna, 

 because an island or part of a continent may be submerged, 

 and all on it destroyed, while the fauna of the adjacent oceans 

 would survive ; and as we cannot suppose the elevation of 

 entire ocean-beds at the same time, the maritime fauna of one 

 period must be in part almost necessarily transmitted to the 

 next." 



In accordance, therefore, with the principles here laid down, 

 we may conclude that it is not correct to say that we " are 

 living in the Cretaceous period," in any other sense than one 

 might say that we are living in the Silurian period, with this 

 difference, that the Cretaceous period is much nearer to us in 

 point of time than the Silurian, and that we can therefore trace 

 a relationship between certain .Cretaceous types and certain 

 living forms that we can not hope to establish in the case of 

 Silurian fossils. 



It is to be observed, lastly, that certain classes of animals 

 are always likely to flourish in places and times in which 

 favourable conditions are present, wholly irrespective of any 

 genetic connection between successive faunae. Thus, the con- 

 ditions present in the deep Atlantic are such as favour the 

 existence of numerous Foraminifera, Sponges, Echinoderms, 

 &c. Similar conditions existed in the seas in which the Chalk 

 was deposited ; and we need not, therefore, be surprised at the 

 predominance of similar organisms in the Cretaceous period. 

 In the same way, there are portions of the Carboniferous 

 Limestone fairly comparable to the Chalk in mineral characters 

 (making due allowance for difference of age), and containing 

 forms of life which may be regarded as representative of the 

 Cretaceous fauna such as Foraminifera, smooth Terebratulce, 

 Crinoids, and Sea-urchins. The conditions, however, present 

 in the deep Atlantic are not exactly similar to those under which 

 the Chalk was deposited, for there are certain great classes, 



