278 CONTEMPORANEITY OF STRATA AND THE 



concealed since their formation by the deposition on them of other 

 newer rocks ; or they may be situated in areas which are at present 

 hidden from us by the ocean. Fourthly, there may have been times 

 in which great changes in life were actively progressing in areas in 

 which there might be little or no contemporaneous deposition of 

 rock. 



From these and similar causes, it is almost certain that we shall 

 never be able to point to a complete series of deposits linking one 

 great geological period, such as the Cretaceous, to another, such as 

 the Eocene. Still, we may well have a strong conviction that such 

 deposits must exist, or must have at one time existed, though all 

 traces of them may now be lost. Upon any theory of " evolution," 

 at any rate, it is certain that there can be no break in the great 

 series of stratified deposits, but that there must have been a complete 

 "continuity" of life and of deposition, from the Laurentian period to 

 the present day. There was and could have been no such continuity 

 in any one given area ; but it is not credible that the chain should 

 ever have been snapped at one point, and taken up again at another 

 wholly different one. The links may, indeed must, have been forged 

 in different places ; but the chain nevertheless remained t^nbroken. 

 From this point of view, there would be little impropriety in saying 

 that we are still living in the Silurian period ; but we could say so 

 in a very limited sense only. Most geologists probably would admit 

 that there must in nature have been such an actual continuity of the 

 great geological periods. Nevertheless it remains certain that we 

 can never dispense with the division of the stratified series into 

 definite rock-groups and life-periods. We can never hope to discover 

 all of the lost links of the geological chain, and the great formations 

 will ever be separated from one another by more or less pronounced 

 physical or palseontological breaks or by both combined. The utmost 

 we can at present do is to arrive at the conviction that the lines of 

 demarcation between the great formations only mark gaps in our 

 knowledge, and that there can be in nature no hiatus in the long 

 series of fossiliferous deposits. 



The theory, then, of geological "continuity" may in practice be 

 carried so far as to be useless or even injurious to the progress of 

 science. This would seem to be the case with a recent attempt by 

 Professor Wyville Thomson to show that " we are still living in the 



