OF ORGANIC NATURE 



35 



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 

 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 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 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 that 

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



FIG. 5 



fine deposit is carried no farther than a, forming the bed 

 A 1 , and the coarse no farther than b, 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 1 ; for A was deposited long before B 1 . In fine, if, 

 instead of comparing immediately adjacent parts of two 

 beds, one of which lies upon another, we compare distant 

 parts, it is quite possible that the upper may be any number 

 of years older than the under, and the under any number 

 of years younger than the upper. 



