IX GEOLOGICAL CONTEMPORANEITY 285 



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 contem- 

 poraneity the sole evidence of which is a simi- 

 larity 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 

 contemporaneity assumed by all who adopt the 

 hypothesis 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 admis- 

 sion that neither physical geology, nor palseonto- 

 logy, possesses any method by which the absolute 

 synchronism of two strata can be demonstrated. 

 All that geology can prove is local order of succes- 

 sion. 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 many other vertical 

 linear sections of the same series, of course, cor- 

 responding 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 synchronously 

 deposited. For areas of moderate extent, it is 

 doubtless true that no practical evil is likely to 

 result from assuming the corresponding beds to 



