344 THE CAUSES OF THE 



XI 



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 

 anything of the kind. As the former Director of 

 this Institution, Sir H. 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 I. 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 

 that sea-bottom goes down (as shown in Fig. 4), 

 so that the first deposit is carried no farther than 

 a, forming the bed A 1 , and the coarse no farther 

 than &, forming the bed B 1 , the result will be the 

 formation of two continuous beds, one of fine 

 sediment (A A 1 ) over-lapping another of coarse 

 sediment (B B 1 ). Now suppose the whole sea- 

 bottom is raised up, and a section exposed about 

 the point A 1 ; no doubt, at this spot, the upper 

 bed is younger than the lower. But we should 

 obviously greatly err if we concluded that the 

 mass of the upper bed at A was younger than the 

 lower bed at B ; for we have just seen that they 

 are contemporaneous deposits. Still more should 

 we be in error if we supposed the upper bed at A 

 to be younger than the continuation of the lower 

 bed at B l ; for A was deposited long before B \ 



