142 EARLY MAN IN BRITAIN. [CHAP. vi. 



of the other animals, buried in the fluviatile strata below, 

 may be considered preglacial. If, however, it be referred 

 to the action of the snows and the frosts of the late 

 Pleistocene age, the strata in question, from their position 

 below, must be older than that age. The mammalia 

 then inhabiting the district are intermediate in character 

 between those of the forest-bed and those of the late 

 Pleistocene strata, and lived in that area before the cold 

 was sufficiently severe to drive away the big-nosed 

 rhinoceros, and cause the feeding grounds of the stags, 

 fallow deer, and uri, to be enjoyed by the countless herds 

 of reindeer and bisons. 



Level not an Absolute Test of Age. 



For these reasons, these river-deposits in the valley 

 of the Thames and at Clacton are taken to be older 

 than those of the late Pleistocene so widely distributed 

 through middle and southern England, and they may date 

 back to the preglacial age, as Dr. Falconer 1 inferred 

 from the study of the mammalia. They are, on the other 

 hand, assigned by Professor Prestwich 2 to a late period in 

 the Pleistocene age because they are at a low level. 

 The use of relative levels as a test of age is, however, 

 valid only under two conditions. 3 The valley must be 

 assumed to have been cut down by the stream flowing 

 along the bottom, and the fluviatile deposits to have 

 been formed at different levels as the river bottom be- 

 came lowered. It must also be assumed that the land 



1 Falconer, Quart. Jour. Geol Soc. Lond., xiv. p. 83. 



2 Prestwich, Geol Mag., i. 245. 



3 For full statement of this argument see Quart. Jour. Geol. Soc. 

 Lond., xxiii. p. 91. 



