PAL^ONTOLOGICAL KECORD. 385 



the present day. It is preferable, therefore, to believe that no such 

 violent processes of destruction and re-peopling took place, but that 

 the great and 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 immigration into it would set in 

 from neighboring seas. By this time, however, the Cretaceous 

 animals must have mainly died out or must have been greatly altered 

 in 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 slowly, 

 marine species especially so, and we have here the disappearance of a 

 whole and entire fauna, 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 a denuded and eroded surface of Chalk, 

 and to fill up " pipes " and winding hollows which descend far below 

 its general surface. 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, and affording unquestionable proof that the Chalk 

 had been subjected to great denudation before the Eocene beds were 

 deposited upon its surface. In short, the Eocene strata rest " uncon- 

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

 following sequence of phenomena : — Firstly, the beds of Chalk were 

 deposited in a horizontal position at the bottom of the 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 must inevitably have suffered vast denudation. Thirdly, 

 after another wholly indefinite interval, the Chalk was again sub- 

 merged beneath the sea, in which process it would be subjected to 

 still further denudation, and an approximately level surface would 

 be formed upon it. Fourthly, strata of Eocene age were deposited 



