OF ORGANIC NATURE. 



39 



than the upper — there is no doubt about that ; but 

 what does this tell us about the age of 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 what- 

 ever 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 

 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, there 

 fore, is older than A. No doubt, as a whole, it 



Fig. 5. 



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 any events which 

 took place while a was being deposited ? It looks all 



