Olti GEOLOGY 



tropical. The to-tincl-fro movement of the climatic zones seems 

 to have sorted out the mixed assemblage, or to have forced special 

 types into special adaptations, or both, so that to-day most species 

 are confined to definite climatic zones. Adaptation to climatic 

 zones and restriction to them seem therefore to have been favored 

 by the climatic fluctuations of the glacial period. 



Relics of glacial migrations. Significant evidence of the north- 

 erly and southerly migrations of the glacial period is found in the 

 existing 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 was of the arctic type. As the ice retired to the nort h, 

 the arctic life of the surrounding lowlands moved northward after 

 it, and the temperate life came on to take its place. In the moun- 

 tains, however, the arctic life still found congenial conditions, by 

 ascending to higher and higher altitudes as the warmer climates 

 advanced. It was thus cut off from the retreating arctic life of the 

 lowlands, and at length isolated. In the high mountains, such life 

 still finds suitable conditions. In some of the higher parts of the 

 northern Appalachians, plants, insects, and small mammals whose 

 kin now live in the arctic zone, remain to this day. The same point 

 is still more strikingly illustrated in the Alps. 



Life of the Interglacial Stages 



By far the larger part of the fossils whose exact relations to the 

 ice invasions can be fixed, are found in the interglacial beds. These, 

 therefore, possess the highest order of value. 



The Toronto beds. The most instructive interglacial beds 

 carefully studied in America are those on the Don River and in the 

 Scarboro cliffs, near Toronto. 1 The fossil-bearing beds are under- 

 lain by a sheet of bowlder clay the age of which is not determined, 

 but it is the equivalent of one of the older drift sheets. Its upper 

 surface was eroded before the fossiliferous interglacial beds of st rati- 

 fied sand and clay, with a maximum thickness of more than I"><) 



1 Coleman, Interglacial Fossils from the Don Valley, Toronto, AMI. <;<><>l., 

 Vol. XII, 1894, pp. 86-95, with references to earlier literature; also jjlucial 

 and Interglacial Beds Near Toronto, Jour. Geol., Vol. IX, 1901, pp. 285 -H>- 



