IMPERFECTION OF PAL^ONTOLOGICAL RECORD. 33 



introduced en masse into the same waters. This theory, how- 

 ever, calls upon the stage forces of which we know nothing, 

 and is contradicted by the whole tenor of the operations which 

 we see going on around us at the present day. It is prefer- 

 able, therefore, to believe that no such violent processes of 

 destruction and re-peopling took place, but that the marked 

 break in the life of the two periods indicates 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 immi- 

 gration 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 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 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 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 com- 

 posed 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 " unconformably " upon the Chalk, and 

 this, as is well known, indicates the following series of pheno- 

 mena : 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 surfer vast denudation. Thirdly, 

 after another wholly indefinite period, the Chalk was again 

 submerged beneath the sea, in which process it would be sub- 

 jected to still further denudation, and an approximately level 

 c 



