276 CONTEMPORANEITY OP STRATA AND THE 



unkno-wn to us, or, it may be, have been subsequently destroyed by 

 denudation. 



We may arrive at some solution of this question by considering 

 what "we must believe to have occurred at the close of any great 

 geological period — say the Cretaceous period. If we reject, as we 

 must do, the belief that the close of the Cretaceous period was 

 marked by a sudden and universal destruction and extinction of the 

 Cretaceous forms of life, there is only one other view that we can 

 accept. We know, from unmistakable physical evidence, that the 

 close of the Cretaceous period in Europe was accompanied, or rather 

 caiised, by an upheaval of the Cretaceous area, and the obliteration 

 of the Cretaceous sea, which must, at that time, have extended from 

 southern Britain at least as far as the Crimea eastwards. As a 

 matter of course, this upheaval was effected, not suddenly, but with 

 extreme slowness, and it must have resulted in bringing about 

 changes most seriously affecting the animals which swarmed in the 

 Cretaceous ocean. At the commencement of the upheaval, as the 

 sea gradually began to shallow, the marine animals would find their 

 conditions of life changed ; and as the upheaval went on, the state of 

 things would become gradually worse, till finally, the area was 

 converted into dry land. Some of the Cretaceous forms of life would, 

 from the very beginning, be probably unable to accommodate them- 

 selves to the new regime, and these would die out. Some few would 

 undergo no changes, but would simply migrate to a more favorable 

 area. Many, lastly, would migrate, and in the process of migration, 

 by reason of coming into contact with strange neighbours and untried 

 conditions, would become gradually modified, till they might assume 

 a form in which they would be regarded as distinct varieties or even 

 distinct species. The ultimate result of the whole process would be 

 the transference of many characteristic Cretaceous species to some 

 sea more or less removed in point of distance from their original 

 home. Not only so, but many of the transferred species might have 

 undergone such modifications in transitu that they would now no 

 longer be specifically identical with the forms of the chalk, but would 

 be regarded as merely allied or representative species, though truly 

 the lineal descendants of the Cretaceous animals. 



It is perfectly clear that the process of rock deposition which was 

 going on in Europe towards the close of the Cretaceous period was 



