CHAP, vi.] MID PLEISTOCENE CAVERNS. 143 



has remained stationary at one level above the sea. 

 The valley of the lower Thames was probably excavated 

 in the Pleiocene age, and is proved by the large sheets 

 of boulder clay and the marine shingle in Essex and 

 Hertfordshire to have been submerged after the end of 

 the early Pleistocene age. It was re-elevated while the 

 late Pleistocene deposits were being formed in the area 

 between Oxford and the mouth of the Thames. Here, 

 then, as in the case of the submarine forest-bed of Nor- 

 folk, we cannot consider that the age is settled by the 

 level. The lower brick-earths seem to me to be isolated 

 patches of a series of fluviatile deposits, of which the 

 higher and more exposed portions have been destroyed 

 by the rain, rivers, snow, ice, and sea, and other agents 

 ever at work in re-modelling the surface of the earth. 



Mid Pleistocene Caverns. 



It is a very singular and striking fact, that although 

 caverns must have existed in all ages of the earth's 

 history, and have been used for shelter by the animals, 

 there are none older than the mid Pleistocene times. 

 There is every reason to believe that they were haunted 

 by the Eocene, Meiocene, and Pleiocene beasts of prey, 

 and that the anoplotheres and palseotheres, the deino- 

 theres and mastodons, the deer and the antelopes, were 

 either dragged in by the carnivores, or swept in by the 

 flow of water, after the same manner as the remains of 

 the successive groups of animals have been introduced 

 which have inhabited Europe from the Pleistocene age 

 down to the present day. Why then do we not meet 

 with ossiferous caverns of those times ? The simple 

 answer is to be found in the realisation of the enormous 



