GEOLOGICAL CONTEMPORANEITY 381 



whether such deposits were formed simultaneously, or 

 whether they possess any given difference of antiquity. 

 To return to an example already given : All competent 

 authorities will probably assent to the proposition that 

 physical geology does not enable us in any way to reply 

 to this question Were the British Cretaceous rocks 

 deposited at the same time as those of India, or are they a 

 million of years younger or a million of years older ? 



Is paleontology able to succeed where physical geology 

 fails ? Standard writers on paleontology, as has been seen, 

 assume that she can. They take it for granted, that 

 deposits containing similar organic remains are synchronous 

 at any rate in a broad sense ; and yet, those who will 

 study the eleventh and twelfth chapters of Sir Henry 

 De la Beche's remarkable Researches in Theoretical Geology, 

 published now nearly thirty years ago, and will carry out 

 the arguments there most luminously stated, to their 

 logical consequences, may very easily convince themselves 

 that even absolute identity of organic contents is no 

 proof of the synchrony of deposits, while absolute diversity 

 is no proof of difference of date. Sir Henry De la Beche 

 goes even further, and adduces conclusive evidence to 

 show that the different parts of one and the same stratum, 

 having a similar composition throughout, containing the 

 same organic remains, and having similar beds above and 

 below it, may yet differ to any conceivable extent in 

 age. 



Edward Forbes was in the habit of asserting that the 

 similarity of the organic contents of distant formations was 

 primd facie evidence, not of their similarity, but of their 

 difference of age ; and holding as he did the doctrine of 

 single specific centres, the conclusion was as legitimate 

 as any other ; for the two districts must have been occupied 

 by migration from one of the two, or from an intermediate 

 spot, and the chances against exact coincidence of migration 

 and of imbedding are infinite. 



In point of fact, however, whether the hypothesis of 

 single or of multiple specific centres be adopted, similarity 

 of organic contents cannot possibly afford any proof of 

 the synchrony of the deposits which contain them ; on 

 the contrary, it is demonstrably compatible with the lapse of 

 the most prodigious intervals of time, and with interposition 

 of vast changes in the organic and inorganic worlds, between 

 the epochs in which such deposits were formed. 



