184 GEOLOGICAL HISTORY. 



of migration from one region to another which nature is con- 

 tinually employing. To determine the position of the local 

 centres from which, by these processes long continued, our 

 earliest British fauna and flora were derived, is a problem which 

 every naturalist will acknowledge to be difficult, but which we 

 ought not to abandon as impracticable, until we find unavailing 

 in this case the methods of research which have been so emi- 

 nently successful in tracing the geographical distribution of the 

 animals and plants now in existence. 



Species of Elephant, Rhinoceros, and Hippopotamus ; of Lion, 

 Hyaena, Bear, and Wolf; of Ox, Deer, and Horse, very similar 

 to existing races, yet for the most part really distinct, were 

 among the earliest tenants of the Yorkshire hills and plains ; 

 and not of these only, for the same conclusion has been drawn 

 from observations in many parts of England. 



One proof of this is in the fact, that in the gravel and clay 

 which spread round these hills in considerable quantities, and 

 contain masses of stone drifted from the hills, lie bones of the 

 animals named. Another proof is the occurrence in fissures and 

 caverns of the remains of many of these animals, together with 

 others, under circumstances which leave no doubt of their having 

 lived in the immediate vicinity, or actually, as in the case of 

 Kirkdale is well ascertained, in the cavern itself. 



GLACIAL PERIOD. But again subsidence occurred, so as to 

 plunge considerable portions of the dry and inhabited surface 

 beneath the sea-level, and allow of their being covered by great 

 quantities of bluish clay (boulder clay) full of fragments derived 

 from the old lands of Cumbria, the Penine chain, the northern 

 moorlands and the chalk hills, fragments procured by the waste 

 and breaking up of the surface of these districts consequent on 

 the littoral action of water, aided perhaps by the operation of 

 glaciers on the land and icebergs in the sea; for this was a 

 glacial sea, a cold ocean, as the shells which it has left among 

 the drifted masses which it deposited testify. Above the boulder 

 clay are usually extensive deposits of various gravels and sands, 



