18 The Ancient Fauna of Essex.
meeting here with the Rhinoceros leptorhinus and megarhinus.
Still further back in time, it seems as if there must have been a
still colder period, marked by glacial deposits on the top of the
high grounds forming the divisions between the Thames, the
Boding, and the Lea, and also on such high grounds as at
Muswell Hill. Then after this period, which must have been
one of submergence as well as of cold, we had no doubt a cold
period of emergence, when the land stood higher than at
present. That gradually passed away and was succeeded by
a somewhat warmer period, marked by a corresponding change
in the fauna of the valley, and the predominance of more
southern forms; and that must have extended up to those
Norfolk beds which have yielded a large number of southern
forms. Then we reach the Norwich Crag with a fauna
characteristic of a period of depression, and then a warmer
period, marked by the Red and Coralline Crags of Suffolk.
Lastly an Eocene Period, with its subtropical fauna and flora.
Mr. Gardner has mentioned to me that he has found what
appears to him to be evidence of a truly Arctic flora in the
Beading plant-beds ; so that just at the close of the Eocene
Period, and at the top of the Chalk, there was probably an
intervening cold land period. But there does not appear to
have been any cold period again till we reach the Post-
Tertiary deposits. These oscillations of climate have no
doubt recurred again and again during long stretches of
time, but with vast intervals between. We must not
suppose for a moment that the land has risen and fallen
like the mercury in a barometer. These processes of
elevation and subsidence have been very slow, and have
occupied inconceivably long periods of time; and when
one is asked to believe in repeated glacial periods marked
by only a few feet thickness of strata, one ought to be very
careful in receiving such doctrines. I do not believe that
any sound geologist who has carefully considered what is
involved in any great change of climate can ever entertain
such an idea. It takes a very, very long period of centuries or
thousands of years to bring about any considerable climatal
change. We can easily imagine a local change taking place;