108 PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jan. 9, 



Lemming, Marmot, Musk-sheep, Elk, and Eeindeer, on the other. 

 It may be objected that negative evidence has no place here, and 

 that the absence of these animals from the river- deposits does not 

 prove their non-existence in the district. When, however, we con- 

 sider the vast stores of remains belonging to all the great classes of 

 mammalia living at the time in Europe, and indicating the presence 

 of twenty-four out of the fifty-three British Pleistocene species, the ab- 

 sence of the whole northern Postglacial group, and of the four charac- 

 teristicspecies of the forest-bed, is fairly entitled to weight in the de- 

 termination of their relative age. The absence of the Eeindeer is pe- 

 cuHarly valuable, from its great abundance in nearly all the Postglacial 

 deposits. Erom these premisses one conclusion inevitably follows : 

 — that the Lower Brick-earths of Crayford, Erith, Ilford, and Grays 

 Thurrock, as affording remains of mammals in part peculiar to the 

 forest-bed of Norfolk and the Pliocene deposits of Erance and Italy, in 

 part to the Postglacial deposits of England, Erance, and Germany, 

 occupy a middle point in time between the two, being more modern 

 than the former and more ancient than the latter. 



7. The Lower Brick-earths not of late PostpUocene age. At 

 this point in the argument we must pause to consider the view of 

 the great authority on river-deposits, Mr. Prestwich. He stated, in 

 1864*, that the Lower Brick-earths at Ilford, because they occupy a 

 low level, are of late Postpliocene age, — an inference that is dia- 

 metrically opposed by the mammalian evidence, and which if proved 

 to be true would overthrow the Palaeontological value of the labours 

 of all the Tertiary mammalogists. These seem to me insuperable 

 obstacles to its adoption. If all the superficial deposits in the valley 

 of the Thames have been left by its waters at different levels above 

 the present stream, those levels will give the relative antiquity of the 

 beds of loam, sand, or gravel, provided that the land has remained 

 stationary. But if the land was elevated in one place and depressed 

 in another, as we are bound to admit was the case throughout the 

 Pleistocene period, then the evidence of level is of no importance, 

 and the low-level deposits may in some cases be of the same 

 antiquity as those at higher levels ; or, if we suppose a valley with 

 a river flowing through it to be depressed beneath the surface of the 

 sea, the higher marine deposit may even be younger than the lower 

 fluviatile one. A notable instance of this is to be found in the 

 Boulder-clay overlying the mammaliferous bed on the Norfolk shore. 

 Unless, therefore, in any particular case, there be no oscillations of 

 level, and unless there be no interference by the sea with the 

 cutting- down action of the river, relative height is no standard of 

 age. No proof of either of these conditions necessary to the truth 

 of Mr. Prestwich's theory is to be found in the lower part of the 

 Thames Yalley. On the contrary, Prseglacial Britain was depressed 

 to a depth of 2000 feet in Scotland, according to Sir. Charles Lyellf. 

 Thence the Glacial sea gradually shallowed until the line of the 

 Thames Yalley roughly marked its southern boundary. That this 

 valley should not have shared in some degree in this depression is to 

 * 0^. cit. t Antiquity of Man, 



