IMPERFECTION OF PAL^ONTOLOGICAL RECORD. 63 



but that the marked break in the life of the two periods in- 

 dicates an enormous lapse of time. The Cretaceous animals, 

 in consequence of the elevation of the British area at the 

 close of the Cretaceous period, must have mostly migrated, 

 some doubtless perishing, and others probably becoming 

 modified in the process. When the British area became 

 once more submerged beneath the sea, and became again a 

 fitting home for marine life, an immigration into it would 

 set in from neighbouring seas. By this time, however, the 

 Cretaceous animals must have mostly died out, or must have 

 become greatly changed in their characters ; and the new 

 immigrants would be forms characteristic of the Lower 

 Eocene. How long the processes here described may have 

 taken, it is utterly impossible to say, even approximately. 

 Judging, however, from what we can observe at the present 

 day, the palseontological break between the Chalk and the 

 Eocene indicates a perfectly incalculable lapse of time ; for 

 all species change or die out slowly, marine species especially 

 so ; and we have here the disappearance of a large fauna 

 almost in its entirety, and its replacement by another wholly 

 distinct. 



In the second place, to come to the physical evidence, the 

 Eocene strata in Britain are seen to rest upon an eroded and 

 denuded surface of Chalk, filling up " pipes " and winding 

 hollows which descend far below the general surface of the 

 latter. Not only so, but the base of the Eocene rocks is 

 commonly composed of a bed of rolled and rounded flints, 

 derived from the Chalk, affording incontestable proof that 

 the Chalk had been greatly worn down and removed by 

 denudation before the Eocene beds were deposited upon its 

 surface. In short, the Eocene rocks repose "unconform- 

 ably " upon the Chalk, and this, as is well known, indicates 

 the following series of phenomena : Firstly, the Chalk was 

 deposited in horizontal layers at the bottom of the Cretaceous 

 sea. Secondly, at some wholly indefinite time after its 

 deposition, after it had become more or less consolidated, 

 the Chalk must have been raised by a gradual process of 

 elevation above the level of the sea, during which it would 

 inevitably suffer vast denudation. Thirdly, after another 



