486 SCIENCE PROGRESS. 



from land, the thickness decreases from east to west. It is 

 198 feet thick at Dover, 116 feet in the Isle of Wight, and 

 only 10 feet in Devon, and even this 10 feet is partly 

 sandy. Moreover, as the thickness decreases, the pro- 

 portion of insoluble residue increases : in Kent there is 

 only some 15 per cent., in the Isle of Wight 40 per cent. 

 Both these circumstances again point to the fact that land 

 still lay towards the west and the sea had deepened. 



In the Middle Chalk the amount of insoluble material 

 is still further reduced. It has fallen to two per cent, in 

 the Isle of Wight, and four per cent, at Folkestone. In the 

 Upper Chalk the proportion of residue is equally low. 



If, then, we may assume that the insoluble residue was 

 derived from land, and the calcareous material was formed 

 in the sea, we have clear evidence of a nearly continuous 

 depression of the sea floor and recession of the coast line — 

 a few small fluctuations there were, but these are unim 

 portant. And that this assumption is justified is shown by 

 the fact that the beds in which insoluble residue pre- 

 dominates thin out towards the east ; while those in which 

 the calcareous matter is most abundant thin towards the 

 west. Moreover, as the insoluble residue diminishes, the 

 heavy minerals, such as zircons, which must have been 

 derived from land, become first of all smaller in size and 

 then disappear, and the glauconite grains disappear also. 



The depth of the Middle and Upper Chalk sea must, 

 therefore, have been considerably greater than that of the 

 Upper Greensand sea ; and this view is supported by the 

 decrease in numbers of the gasteropods and the pro- 

 portionate increase of monomyarian lamellibranchs. Hume 

 also bases part of his argument on the Foraminifera, and 

 his final conclusion is that the Upper Cretaceous was laid 

 down in a sea at least as deep as the Mediterranean. 



In questions of this kind, when the argument is based 

 upon the assumption that fossil forms lived and flourished 

 under the same conditions as their recent allies, it must 

 always be borne in mind that such an assumption is open 

 to grave doubt. There is nothing to prevent an animal 

 from adapting itself to live under a new set of conditions 



