24 STRATIGRAPHICAL GEOLOGY 



we have to rely entirely on community of fossils in comparing the 

 rocks of the one with those of the other ; and if the rocks so 

 compared were deposited originally within the limits of the same 

 life-province, the groups which contain similar assemblages of 

 species may safely be regarded as contemporaneous. 



To take an example, no one would hesitate to correlate the 

 members of the Ordovician and Silurian Systems in North Wales 

 and the Lake District by means of the fossils they contain, because 

 there is every reason to suppose that the two districts formed part 

 of the same province during those periods. Similarly the members 

 of the Lower Cretaceous Series in the Isle of Wight and in the 

 Weald may be correlated with some degree of exactitude ; but 

 the same group of beds in Lincolnshire contains such a different 

 set of fossils that it obviously belonged to a different life-province, 

 and it is impossible to indicate the exact equivalents of the several 

 members of the southern series. 



Again, when we come to compare districts at a still greater 

 distance from one another as, for instance, Wales and Bohemia, 

 Sussex and the South of France, or England generally with 

 North America we find that there is certainly a general resem- 

 blance in the vertical succession of faunas which have flourished 

 in the respective areas, i.e. the rock-series in each country can 

 be divided into groups, which are characterised by the presence 

 of the same genera ; but the species are different, and it would 

 be a mistake to conclude that portions of formations containing 

 assemblages of the same genera in countries so far apart were 

 contemporaneous in the same sense as the members of the Silurian 

 Series in Wales and Cumberland are contemporaneous. 



A group of species originating in one locality and spreading 

 along the coasts of a continent would take a long time to travel 

 from Europe to America even if there were continuous coast lines. 

 Sometimes their progress would be arrested for a time, until 

 physical or geographical changes allowed them to advance ; at 

 other times they would make their way by accommodating them- 

 selves to varying conditions and by throwing out collateral varieties. 

 The date of their first appearance in one part of the world would 

 be very much in advance of their arrival in the other, so that the 

 fact of two distant formations containing fossils of closely allied 

 species can be no proof of their being really contemporaneous. 



It is certain, however, that in every part of the world the 

 geological sequence discloses the same general succession of life- 

 forms. The progress of development may have been retarded here 

 and accelerated there, but the successive fossil assemblages are 

 similar to one another, and the order in which they occur is the 



