FAUNA OF YORKSHIRE. 185 



sometimes shelly, which contain fragments washed out of the 

 older clay, and worn and rounded in various degrees. 



POSTGLACIAL PERIOD. To this submersion we may perhaps 

 attribute the extinction in our district of many of these " ante- 

 diluvian" or "preglacial" animals; for after the land rose again, 

 so as to admit of freshwater lakes in hollows of the boulder clay 

 and gravel beds, we find but few of the forms of life analogous 

 to tropical species. It is the Irish Elk, the Red Deer, and Fal- 

 low Deer, which most frequently lie in the old lacustrine de- 

 posits. 



This last great elevation laid dry the old sea-channels of the 

 Vale of York, the Vale of Pickering, and Holderness; exposing in 

 each the boulder clay and other aggregations of the glacial period 

 to rapid waste by shore currents and land streams. These 

 causes are still in operation, following out the design of perpe- 

 tual but regulated change on the face of nature ; nor does any 

 evidence appear that they have ever been interrupted here by 

 epochs of great violence. No sudden movement has disturbed 

 the relative level of the land and sea ; no extraordinary change 

 of climate has destroyed the races of animals and plants, or 

 greatly modified the qualities of the atmosphere. The land and 

 sea, the hills and valleys, the rains and winds, the clouds and 

 atmosphere of the present day may be regarded as unaltered in 

 main features since the retirement of the boreal ocean ; it can- 

 not therefore be surprising that remains of man and the works 

 of man should appear in the deposits of this age, though not in 

 the earliest of them, along with boaes of many animals essential 

 to the comfort of human existence. 



FAUNA OF YORKSHIRE. 



Let us now survey the successive phases of animal and vege- 

 table life which occupy the latest seras of geology, and consider 

 them in relation to the earliest historic periods. 



The preglacial fauna of Yorkshire is chiefly known to us by the 



