212 ^ag <krm0ns, (Essags, anir HUfaictos. [x. 



that a vast period (even in the geological sense) of 

 time, and physical changes of almost unprecedented 

 extent, separate the two. 



But if it be a demonstrable fact that strata con- 

 taining more than 60 or 70 per cent, of species of 

 Mollusca in common, and comparatively close together, 

 may yet be separated by an amount of geological time 

 sufficient to allow of some of the greatest physical 

 changes the world has seen, what becomes of that 

 sort of contemporaneity the sole evidence of which 

 is a similarity of facies, or the identity of half a dozen 

 species, or of a good many genera \ 



And yet there is no better evidence for the contem- 

 poraneity assumed by all who adopt the hypotheses 

 of universal faunae and florae, of a universally uniform 

 climate, and of a sensible cooling of the globe during 

 geological time. 



There seems, then, no escape from the admission that 

 neither physical geology, nor palaeontology, possesses 

 any method by which the absolute synchronism of two 

 strata can be demonstrated. All that geology can 

 prove is local order of succession. It is mathematically 

 certain that, in any given vertical linear section of an 

 undisturbed series of sedimentary deposits, the bed 

 which lies lowest is the oldest. In any other vertical 

 linear section of the same series, of course, corresponding 

 beds will occur in a similar order ; but, however great 

 may be the probability, no man can say with absolute 

 certainty that the beds in the two sections were syn- 

 chronously deposited. For areas of moderate extent, 

 it is doubtless true that no practical evil is likely to 

 result from assuming the corresponding beds to be 

 synchronous or strictly contemporaneous; and there 

 are multitudes of accessory circumstances which may 

 fully justify the assumption of such synchrony. But 



