304 THE CANADIAN NATURALIST. [Jan. 



be similar, but greatly intensified. Then every addition to this 

 icy accumulation would depress still farther the temperature of 

 the continent, and extend the area of perennial snow. The great 

 northern ice plateau would thus increase in height and superficial 

 extent until prevented by some reactionary cause. Meteorological 

 considerations all show us that the interior of such a plateau 

 would be intensely cold, — so cold as to prevent the simultaneous 

 mo vins; f the continental glaciers in one determined direction. 

 Hence, the erosive effects which we witness are due to glacial 

 motion along the southern and seaward edge of the glacier, where 

 the snow is softened by the sun, or sea-breezes, and a slope 

 supplied by the glacial front itself. 



Depression of the Sea during the Glacial Period ; by 

 Colonel Charles Whittlesey. — The level of the ocean is 

 maintained by the evaporated water being returned through 

 rivers, etc. If part of this vapor, instead of returning, accumu- 

 late on the land as permanent snow and ice, the result will be a 

 depression of the sea level, proportionate to the extent of the ice- 

 fields. A decrease of one degree annually in the earth's 

 temperature would lower the snow line 300 feet, extend the area 

 of ice and snow, and diminish evaporation ; while additions 

 would be constantly made to the thickness of the ice beds. Now, 

 as one-fifth of the earth must have been covered by ice-fields 

 during the glacial period, and the extent of the ocean at the time 

 is known with considerable certainty, by knowing the thickness, 

 and, consequently, the mass of the beds of ice, we can easily 

 determine the decrease in the water of the sea. Ice etchings are 

 observed on rocks in British America and New England, at heights 

 varying from 1,500 to 5,300 feet above the present sea level. Ad- 

 mitting an average of 2,000 feet, and an expansion of one-tenth in 

 freezing, we have a sufficient amount of congealed water to cover 

 the above area to a depth of 1,800 feet. As nearly the entire 

 remaining surface of the earth was covered with water, the surface 

 would sink about one-fifth of the above, or 360 feet. The weight 

 of such a mass of ice would probably be sufficient to cause a 

 sinking of the land on which it rested, while that adjacent to it 

 would be elevated; just as we see Greenland settling down, and 

 Newfoundland rising, at the present day. These facts should be 

 kept in mind in studying fresh water and marine terraces, and 

 drift-beds. From the absence of these elevations on the Rocky 

 Mountains above a height of 2,000 feet, that part of the continent 



