HISTORY FROM THE ROCKS 193 



that there was transition from one to the next. The lines of 

 division became more arbitrary and more difficult to fix, and 

 it became the custom to divide the rock succession into steps 

 which, although founded to some extent on the broad characters 

 of the Formations, took note more particularly of the inter- 

 pretation of the conditions under which they were formed. Thus 

 some of the old Formation names were retained, like Carboni- 

 ferous, Old Red Sandstone, Cretaceous. But in other cases it was 

 necessary to select other types of names, the most convenient 

 being chosen from the places where the rocks of the division 

 could be most easily and typically studied. Thus we have such 

 names as Cambrian, Devonian, Jurassic, all derived from place- 

 names. 



When correlation of succession over wider areas was attempted 

 it was found that no Formation, however well marked its 

 character, was world-wide in distribution, but that its character 

 gradually changed from point to point. Thus the Old Red 

 Sandstone Formation of Scotland, with its conspicuous colour 

 and its sandy composition, its fossil fishes and giant Crustacea, 

 is of the same age as marine slates and limestones in Devonshire, 

 and it would not be appropriate to extend the Formational 

 name to cover different rock types in Devonshire. It would be 

 less objectionable to use the term Devonian for the Scottish 

 sandstones, but even this can only be regarded as a temporary 

 expedient. As geological history grows out of the stage of the 

 history of local areas into the general history of the earth-crust, 

 the necessity for a wider nomenclature becomes pressing, and 

 attempts, some of them successful, are being made in this 

 direction, but the difficulties are at least as great as those found 

 in any attempt to divide the human history of the world into 

 convenient divisions applicable to the whole world. 



When fossils have been collected from the whole of the rock 

 succession, and compared with one another, it is found that the 

 older ones differ very widely from those living at the present 

 day, but that throughout the succession there is a gradual 

 approach, accelerated or retarded it may be at different times 

 and in different places, up to the faunas and floras of the present. 

 The oldest fossiliferous rocks contain only invertebrate remains ; 



VOL. VI. 13 



