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 B1). 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.

It is in consequence of circumstances of this kind, and of others that I
might mention to you, that our conclusions on and interpretations of
the record are really and strictly only valid so long as we confine
ourselves to one vertical section. I do not mean to tell you that there
are no qualifying circumstances, so that, even in very considerable
areas, we may safely speak of conformably superimposed beds being older
or