PLEISTOCENE RIVER-DEPOSITS. 137 



that transported the sediments, and spread them out where we 

 now see them, behaved in precisely the same manner as any 

 other river at the present day. Gravel was laid down here, 

 sand there, and mud in some other place ; then, owing to 

 changes in the direction or velocity of the current, these de- 

 posits were disturbed, broken up, wholly or partially, and their 

 materials distributed over another part of the river's bed. After 

 a considerable accumulation of such deposits had taken place — 

 many feet or even yards in depth, — the river might again 

 gradually undermine and re-arrange them. The gravel would 

 be pushed along and come to rest farther away, and so would 

 it be with the sand and silt. Any animal remains, such as 

 bones or teeth, which these older deposits may have contained 

 would in like manner be rolled along and embedded in another 

 position. Thus in a series of fluviatile strata like the Pleistocene 

 gravels and sands, it is often quite impossible to tell whether 

 the animal remains that lie side by side in the same stratum 

 belong to species that were exactly contemporaneous, in the 

 sense of occupying the same country at the same time. Sir 

 Charles Lyell has some remarks upon this subject which are so 

 apposite that I cannot do better than quote them in full. He 

 says: 1 "In attempting to settle the chronology of fluviatile 

 sediments, it is almost equally difficult to avail ourselves of the 

 evidence of organic remains, or of the superposition of the 

 strata, for we may find two old river-beds on the same level in 

 juxtaposition, one of them perhaps many thousands of years 

 posterior in date to the other. I have seen an example of this 

 at Ilford, where the Thames, or a tributary stream, has at some 

 former period cut through sands containing Cyrena JluwAnalis, 

 and again filled up the channel with argillaceous matter, 

 evidently derived from the waste of the Tertiary London-clay. 

 Such shiftings of the site of the main channel of the river, the 

 frequent removal of gravel and sand previously deposited, and 

 the throwing down of new alluvium, the flooding of tributaries, 

 the rising and sinking of the land, fluctuation in the cold and 



1 Antiquity of Man, 4th edition, p. 206. 



