326 



PRINCIPLES OF STRATIGRAPHY 



ginal portion one of the higher peaks may project through the ice 

 as a nunatack.''' 



Stratification of Ice. Since the ice of the snow fields and of 



Fig. 6i. Map of North America in Pleistocenic time, showing four centers of 

 dispersion of the ice. I., CordiUeran ; II., Keevvatin ; III., Labra- 

 doran; IV., Newfoundland. (After Wilson.) 



glaciers is the product of successive deposits of snow in more or 

 less continuous sheets, each of which at the time forms the surface 

 layer of the earth at that point, it follows that snow ice has a 



* For types of glacial deposits see Chapter XII. 



