40 



THE PAST CONDITION 



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 lowermost 

 bed is always the older. Yery well ; B, therefore, is 

 older than A. No doubt, as a whole, it is so ; or if 



Fig. 5. 



any parts of the two beds which are in the same verti- 

 cal 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 be- 

 fore any events which took place while a was being 

 deposited ? It looks all very plain sailing, indeed, to 

 say that they did ; and yet there is no proof of any- 

 thing of the kind. As the former Director of this In- 

 stitution, Sir II. De la Beche, long ago showed, this 

 reasoning may involve an entire fallacy. It is extremely 

 possible that a may have been deposited ages before b. 

 It is very easy to understand how that can be. To 

 return to Fig. 4 ; when A and B were deposited, they 

 were substantially contemporaneous; A being simply 

 the finer deposit, and B the coarser of the same detritus 

 or waste of land. Now suppose that the sea-bottom 

 goes down (as shown in Fig. 4), so that the first deposit 



