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 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 first deposit is carried no farther than 
a, forming the bed A, and the coarse no farther 
than b, forming the bed B', the result will be the 
formation of two continuous beds, one of fine 
sediment (A <A’) over-lapping another of coarse 
sediment (B B'). Now suppose the whole sea- 
pottom is raised up, and a section exposed about 
the point A‘; 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 B1; for A was deposited long before B4 
