Well, you say, it is admitted that the lowermost bed is always the
older. Very well; B, therefore, is older than A. No doubt, 'as a
whole', it 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 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 Al, and the
coarse no farther than 'b', forming the bed B1, the result will be the
formation of two continuous beds, one of fine sediment (A A1)
over-lapping another of coarse sediment (B Bl). Now suppose the whole
sea-bottom is raised up, and a section exposed about the point Al; 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 Bl; for A was deposited long before
B1. In fine, if, instead of comparing immediately adjacent parts of