PLEISTOCENE RIVER-DEPOSITS. 139 



even in many cases be inextricably commingled. Nevertheless, 

 the general rule will still obtain, the high-level beds will in the 

 main belong to the oldest stage of the series. Now as it would 

 appear that remains of musk-sheep and reindeer, mammoth, 

 woolly rhinoceros, hysena, lion, elephant, hippopotamus, bison, 

 and other animals belonging to the northern, temperate, and 

 southern groups, occur at all levels in the Pleistocene river- 

 deposits, it seems only reasonable to conclude that these groups 

 must have occupied the ground alternately throughout the 

 whole of the Pleistocene Period. 



The general glance which we have taken at the more salient 

 features of the evidence presented by our Pleistocene river- 

 deposits, makes clear, as it seems to me, the following points : — 



1. They are the products of fluviatile action, and were 

 formed during the excavation of the valleys in which they lie. 



2. They were laid down under varying conditions, some of 

 the deposits indicating quiet and orderly accumulation, others 

 bespeaking tumultuous torrential waters and vast inundations. 

 But the wider spread of torrential gravels and flood-loams does 

 not necessarily imply that a cold climate predominated during 

 Pleistocene times. 



3. Their fossil organic remains point to alternating climatic 

 conditions — to periods more or less prolonged when the cold of 

 winter was severe, and the land was occupied by northern and 

 arctic forms, and to warmer periods (enduring, perhaps, for as 

 long a time as the colder ones), when the winters were extremely 

 mild and genial, so that laurels and fig-trees grew on the banks 

 of the Seine, while an abundant mammalian fauna occupied the 

 land, the hippopotamus being enabled to live as far north as the 

 latitude of Yorkshire. 



4. The depth and width attained by many of the valleys 

 which were excavated during the Pleistocene Period, and the 

 time required for great continental changes of climate, such as 

 are implied by the presence of the old mammalia, are proofs of 

 the long duration of the Pleistocene Period, and the remote 

 antiquity of its commencement. 



