462 THE DEPTHS OF THE SEA. [CHAP, 1X, 
We found some of the glacial shelis 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 Fieroe. 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 able 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 paleontological 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 
ease, involving beds of no great depth or import- 
ance, but it is a case in which two beds correspond 
paleeontologically, and yet we know 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. 
