18 
The Ancient Fauna of Essex. 
meeting here with the Rhinoceros leptorhimis and meyarhinus. 
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 
Roding, 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 
Reading 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 ; 
