IMPERFECTION OF PAL^EONTOLOGICAL RECORD. 69 



mous 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 be- 

 came 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 be- 

 come 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 ob- 

 serve at the present day, the palaeontological break between the 

 Chalk and the Eocene indicates a perfectly incalculable lapse of 

 time ; for all species change or die out slowly, marine species es- 

 pecially 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 sur- 

 face of Chalk, filling up " pipes " and winding hollows which de- 

 scend 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 incon- 

 testable proof that the Chalk had been greatly worn down and re- 

 moved by denudation before the Eocene beds were deposited upon 

 its surface. In short, the Eocene rocks repose " unconformably " 

 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 

 wholly indefinite period, the Chalk was again submerged beneath 

 the sea, in which process it would be subjected to still further denu- 

 dation, and an approximately level surface would be formed upon it. 

 Fourthly, strata of Eocene age were deposited upon the denuded 

 surface of the Chalk, filling up all the hollows and inequalities of its 

 eroded surface (fig. 15). 



In the unconformability, then, between the Chalk and the Eocene 

 rocks, we have unequivocal evidence — irrespective of anything that 

 we learn from Palaeontology — that the break between the two for- 

 mations was one of enormous length. In Britain the interval of 

 time thus indicated is not represented by any deposits ; and in 



