OP CREATION. 57 



of species common in beds of an older formation, and 

 the presence in that group of other species which are 

 analogous, would lead the naturalist to conclude either 

 that a great change had taken place suddenly in the 

 depth or relative position of the sea bottom receiving 

 deposits, or else that a period had elapsed between 

 the deposit of the lower or older beds and those which 

 overlie them, and that this period was longer or 

 shorter according to the amount of difference in the 

 species examined. And this brings us to the sub- 

 ject referred to at the commencement of the present 

 chapter, viz. the existence of a break in' the continuity 

 of strata observed in the case of the rocks of the 

 second period in Scotland and Devonshire. 



With regard to these localities, however, the evi- 

 dence requires a yet more detailed statement. In 

 North Britain, the beds resting on and wrapping round 

 the gneiss, the mica schist, and other old rocks, con- 

 sist, for the most part, of coarse conglomerate or pud- 

 ding-stone, evidently made up of the broken fragments 

 of the old granitic rocks, rolled and tossed about for 

 ages in a troubled sea, the hardest stones being round- 

 ed into bullet-shaped pebbles by their long and inces- 

 sant attrition against one another. These coarse, 

 gravelly masses are not, however, universal ; and on 

 the north-eastern coast and in the Orkneys they are 

 often replaced by more regular strata of hard, dark- 

 coloured, bituminous schists, abounding with the fossil 

 remains of fishes. 



On the frontier of Wales, a deposit, in many re- 

 spects very similar to the conglomerate of Scotland, 

 and expanded to an equal and enormous thickness, is 

 found to cover up, by regular gradation, the newest 



D 5 



