CHAP. IX. CHRONOLOGY OF FLUYIATILE DEPOSITS. 159 



period, on the borders of two distinct zoological provinces, 

 one lying to the north, the other to the south, in which case 

 many species belonging to each fauna endowed with migra 

 tory habits, like the living musk-buffalo or the Bengal tiger, 

 may have been ready to take advantage of any, even the 

 slightest, change in their favour to invade the neighbouring 

 province, whether in the summer or winter months, or 

 permanently for a series of years, or centuries. The Elephas 

 antiquus and its associated Rhinoceros leptorhinus may 

 have preceded the mammoth and tichorhine rhinoceros in the 

 valley of the Thames, or both may have alternately prevailed 

 in the same area in the post-pliocene period. 



In attempting to settle the chronology of fluviatile deposits, 

 it is almost equally difficult to avail ourselves of the evidence 

 of organic remains and of the superposition of the strata, 

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

 juxta-position, 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 fluminalis, 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, 

 fluctuations in the cold and heat of the climate all these 

 changes seem to have given rise to that complexity in the 

 fluviatile deposits of the Thames, which accounts for the small 

 progress we have hitherto made in determining their order of 

 succession, and that of the imbedded groups of quadrupeds. 

 It may happen, as at Brentford and Ilford, that sand-pits in 

 two adjoining fields may each contain distinct species of 

 elephant and rhinoceros ; and they may occur at the same 



