THE PLEISTOCENE OR GLACIAL PERIOD. 



but as caves are rather transient features, subject to early oblitera- 

 tion, they and their contents are rarely preserved in the record of the 

 more ancient periods. Those which were formed so late as the Pleis- 

 tocene period, however, have frequently endured, and have become 

 the receptacles of valuable relics. The cave earth and the stalag- 

 mite that accumulated on the bottoms of the caves enveloped and 

 retained animal relics more often than most superficial deposits, for 

 the obvious reason that the caves were not only frequented by many 

 predaceous animals, but were the depository of the inedible relics of 

 the prey these animals dragged into their retreats. So long as the 

 bottoms of the caves were occupied by cave earth only, this was liable 

 to be dug over by the fossorial forms of the cave-frequenting animals, 

 and the relics of different stages mixed. When, however, the earth 

 was periodically covered by a floor of stalagmite, mixture was restricted 

 to the intervening stages, and the inter-stalagmite relics recorded the 

 order of occupancy with measurable fidelity. Cave deposits are chiefly 

 limited to non-glacial regions, and to those glaciated regions where 

 erosion did not cut them away or the deep drift bury them beyond 

 reach. Fissures, as well as sinks and caverns, occasionally preserved 

 the relics of animals that fell or were washed into them from above. 

 In these cases, the order of burial is usually subject to some doubt 

 owing to irregularities in the mode of filling; but in some cases the 

 succession is fairly certain. In such cases, however, the known order 

 of the life succession is usually more depended upon to determine the 

 age of the several portions of the deposits, than is the order of the 

 deposits to fix the age of the life. 



Existing alpine remnants of the migrations. — Significant evidence 

 of the northerly and southerly migrations of the glacial period is 

 found recorded in the present life of the higher mountains within 

 or near the borders of the once glaciated areas. It is obvious that 

 at the time the ice stood in the vicinity of these mountains, the only 

 life which could occupy them, if any at all, was of the arctic type. 

 As the ice retired to the north, the arctic life of the surrounding low- 

 lands moved northward after it, and the temperate life came on to 

 take its place. Upon the mountain sides and summits, however, the 

 arctic life still found congenial conditions; but it was compelled to 

 ascend to higher and higher altitudes as the warmer climates advanced. 

 It was thus soon cut off from the retreating arctic life of the lowlands, 



