XI PHENOMENA OF ORGANIC NATURE 343 
any given bed in Loch Lomond, as compared with 
that of any given bed in the Lake of Killarney ? 
It is, indeed, obvious that if any two sets of 
deposits are separated and discontinuous, there is 
absolutely no means whatever given you by the 
nature of the deposit of saying whether one is 
much younger or older than the other; but you 
may say, as many have said and think, that the 
case is very much altered if the beds which we 
are comparing are continuous. Suppose two beds 
A 
Fig.s. 
of mud hardened into rock,—A and B—are seen 
in section. (Fig. 5.) 
Well, you say, it is admitted that the lower- 
most bed is always the older. Very well; B, 
therefore, is older than A. No doubt, as a whole, 
it is so; or if any parts of the two beds which are 
in the same vertical line are compared, it is so. 
But suppose you take what seems a very natural 
step further, and say that the part a of the bed A 
is younger than the part 0 of the bed B. Is this 
‘sound reasoning? If you find any record of 
changes taking place at 6, did they occur before 
