462 THE DEPTHS OF THE SEA. [CHAP. ix. 



We found some of the glacial shells of the Clyde 

 beds living on the northern outskirts of our region, 

 Tellina calcarea, for instance, was very common 

 in some of the Fjords in Fseroe. It seems evident 

 that this fauna quietly retreated northwards in the 

 face of slowly' altering circumstances. Such an 

 instance of change of fauna, which we are ahle in 

 a great degree to trace step by step, has an interest- 

 ing bearing upon the great question of the contem- 

 poraneity of beds containing generally the same 

 fauna at distant localities. We can well imagine 

 that a block of perfectly recent silt might be brought 

 from a locality on the verge of the Arctic circle, 

 imbedding precisely the same species of mollusca 

 as those contained in a block of the Clyde glacial 

 clay, and the mineral character of the matrix in 

 the two cases might correspond most closely ; apply- 

 ing the ordinary geological rule, those two blocks 

 agreeing in their palgeontological characters ought 

 to be contemporaneous, but we know that while 

 the northern silt belongs to the present period, the 

 British glacial clays are overlain by a deep series 

 of modern deposits, representing the lapse of a 

 period of time considerable even in a geological 

 sense, and containing a fauna of a very different 

 character. This is no doubt a comparatively trifling 

 case, involving beds of no great depth or import- 

 ance, but it is a case in which two beds correspond 

 palseontologically, and yet we Imow that they are 

 not contemporaneous from one of them being overlain 

 by a considerable thickness of newer strata, while the 

 other is now forming, and thus furnishes a date, a 

 rare and valuable thing in geology. 



