THE GREAT ICE AGE 363 



ing ice, on the contrary, being carried by constant currents and 

 over comparatively flat surfaces, must striate and grind more 

 regularly over large areas, and with less reference to local 

 inequalities of surface. 



9. The direction of the striae and grooves produced by 

 glaciers depends on the direction of valleys. That of floating 

 ice, on the contrary, depends upon the direction of marine 

 currents, which is not determined by the outline of the surface, 

 but is influenced by the large and wide depressions of the sea 

 bottom. 



10. When subsidence of the land is in progress, floating 

 ice may carry boulders from lower to higher levels. Glaciers 

 cannot do this under any circumstances, though in their pro- 

 gress they may leave blocks perched on the tops of peaks and 

 ridges. 



I believe that in all these points of difference the boulder 

 clay and drift on the lower lands of Canada and other parts of 

 North America, correspond rather with the action of floating 

 ice than of land ice; though certainly with glaciers on such land 

 as existed at the different stages of the submergence, and these 

 glaciers drifting stones and earthy matter in different directions 

 from higher land toward the sea. More especially is this the 

 case in the character of the striated surfaces, the bedded dis- 

 tribution of the deposits, the transport of material up the 

 natural slope, the presence of marine shells, and the mechanical 

 and chemical characters of the boulder clay. In short, those 

 who regard the Canadian boulder clay as a glacier deposit, can 

 only do so by overlooking essential points of difference between 

 it and modern accumulations of this kind. 



I would wish it here to be distinctly understood, that I do 

 not doubt that at the time of the greatest Pleistocene submerg- 

 ence of Eastern America, at which time I believe the greater 

 part of the boulder clay was formed, and the more important 

 striation effected, the higher hills then standing as islands would 



