412 PKOCEEDINGS OF THE GEOLOGICAL SOCIETY, [June 5, 



strata is clear and decisive. In many cases, as at Walton, in Essex, 

 and Fisherton, near Salisbury, tlie former underlie the latter, and 

 must therefore be older. 



In other cases they form part of the basin in which the Pre- 

 historic deposits lie, as in the case of the gravel-beds of "Windsor 

 or of London, and must therefore, from their position, be of 

 higher antiquity than the latter. Nearly every valley in Great 

 Britain (the glacier- areas, to which I shall return presently, being 

 excepted) contains beds of brick- earth or of gravel, which were 

 formed, as Mr. Prestwich * has clearly shown, befoi'e the valleys 

 were cut by the streams to their present depths ; and the dif- 

 ference between the levels of these old river-terraces has been 

 shown by that eminent observer to be a rough measure of their 

 relative antiquity, the highest being the oldest. The Prehistoric 

 deposits, on the other hand, occupy for the most part the bottom of 

 the vaUej-B, and are seldom raised much above the level of the pre- 

 sent stream. There is also a marked difference between the two in 

 the materials of which they are composed. The Prehistoric as well 

 as the present alluvia are for the most part formed of claj'^s, 

 more or less stiff ; and the gravels are composed of pebbles more or 

 less evenly sorted ; and both were formed under conditions of climate 

 not very different from the present. The Pleistocene brick-earths, 

 on the other hand, very seldom consist of stiff clays ; and the gravels 

 contain large and small pebbles and angular blocks confusedly 

 mixed together, which indicate that the conditions under which 

 they were formed were different from those which are now presented 

 by the temperate region of Europe. But the difference offered by 

 the fauna which they present is, perhaps, the most striking. 



It will be unnecessary to give the river-deposits in Great Britain 

 which have furnished the remains of the following animals, since 

 they have already been published in the Quarterly Journal of the 

 Geological Society, vol. xxv. pp. 192 et seqcj^. 



It would, of course, be unreasonable to expect that the remains 

 of all the animals inhabiting the country at the time would be pre- 

 sent in one small river-deposit, and stiU more improbable that they 

 would all be discovered in the small area which happens to be open 

 for examination. By correlating, however, the animals from many 

 localities, a fair estimate can be obtained of the whole fauna. The 

 greater liability of one animal to drowning than another must be 

 taken into consideration. For this reason the Otter, probably from its 

 aquatic habits, is extremely rare, while \the Squirrel, living in trees, 

 would run little risk of a watery grave, and has only been found in 

 one Pleistocene deposit in Great Britain. Altogether the following 

 twenty-eight species of animals have left their remains to prove that 

 they existed on the surface of the Pleistocene continent that was 

 di-ained by the rivers in the deposits of which their remains have 

 been found, 



* Philosophical Transactions, vol. clir, 



