tural 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 Figure 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 Figure 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 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.

Now you must not suppose that I put this before you for the purpose of
raising a paradoxical difficulty; the fact is, that the great mass of
deposits have taken place in sea-bottoms which are gradually sinking,
and have been formed under the very conditions I am here supposing.

Do not run away with the notion that this subverts the principle I
laid down at first. The error lies in extending a principle which is
perfectly applicable to deposits in the same vertical line to deposits
which are not in that relation to one another.