138 PREHISTORIC EUROPE. 



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 em- 

 bedded group 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 the 

 fossil remains in both cases may occur at the same depth from 

 the surface, yet may be specially referable to different parts of 

 the Pleistocene Epoch, separated by thousands of years." We 

 cannot therefore infer from the occurrence of the horns of a 

 reindeer and the remains of a hippopotamus, in juxtaposition 

 in a Pleistocene deposit, that these animals have lived under 

 similar climatic conditions. It must not be supposed, however, 

 that such intimate commingling of strongly-contrasted species 

 is the rule. Not infrequently we find remains of several 

 northern animals lying associated in the same strata to the 

 entire exclusion of any of the southern forms ; and in like 

 manner the latter often appear quite unaccompanied by any 

 trace of the northern species. Thus at Gray's Thurrock in 

 Essex, the old Pleistocene alluvia of the Thames have yielded 

 Elephas antiquus, Rhinoceros leptorhinus, Hippopotamus major, 

 horse, bear, ox, stag, etc.,. but not a trace of any northern species. 

 In the same beds occur Cyrena fluminalis, Unio littoralis, and 

 Paludina (Hydrobia) marginata, which is no longer a British 

 shell, but still lives in the south of France. 



But although it is unsafe to rely exclusively upon super- 

 position as a test of the relative antiquity of fluviatile accumu- 

 lations, yet as a general rule it still holds true that the beds 

 which occupy the lower portion of any thick series will be, in 

 the main, the oldest ; while, on the other hand, those at the top 

 will commonly be the youngest. Again, in the case of those 

 river-deposits that cloak the slopes of a valley, we may feel sure 

 that those at the highest levels will be the oldest, and that the 

 younger accumulations will occupy the. lower levels ; but the 

 latter will frequently overlap upon the former, and the two will 



