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 



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 b of the bed B. Is this 

 sound reasoning ? If you find any record of 

 changes taking place at b, did they occur before 



51 



