assumed from the known rate of
surface-increase now, we ought to remember that we have not a tittle of
proof that the rate of increase has not at certain remote periods been
suddenly and immensely augmented. There are many facts on record which
tend to shew that the rate at which geologic changes take place in
certain localities affords no reliable data whatever to infer that at
which phenomena apparently quite parallel have occurred in other
localities. An upheaval or a subsidence of one part of a country may
rapidly effect a great change in the amount of soil or gravel
precipitated by streams, without destroying or changing their channels,
and yet the deposit may be made sufficiently gradually to allow the
burial of shells or of bones of creatures which lived and died on the
spot.

The degradation of a cliff, either suddenly or gradually, might throw a
vast quantity of fragments into a rapid stream, and cause a deposit of
gravel of considerable breadth and thickness in a comparatively short
period of time,--say a century or two.

Sir Charles Lyell has adduced examples of very rapid formation of
certain stony deposits, which should make us cautious how we assert that
such and such a thickness _must_ have required a vast number of years.
In one of them there is a thickness of 200 or 300 feet of travertine of
recent deposit, while in another, a solid mass _thirty feet thick was
deposited in about twenty years_. There are countless places in Italy
where the formation of limestone may be seen, as also in Auvergne and
other volcanic districts.[23]

From these and similar considerations it seems to me by no means
unreasonable that the four thousand years which elapsed between the
Creation and the commencement of Western European history should have
been amply sufficient for many of those geological operations whose
results are seen in what are known as the later Tertiary deposits,--the
crag, the drift, the cavern-accumulations, and the like. And, as a
corollary to this, that the great extinct Mammalia may have extended
into this period, and thus have been contemporary with man, for a
greater or less duration, according to the species; some, probably,
having been extinguished at a very early period of the era, while others
lived on to the time I have named, or even later.

But have we nothing better for this conclusion than an assumption of the
possibility, and a more or less probable conjecture? Yes; we have some
facts