552 CENOZOIC ERA— AGE OF MAMMALS. 



regions, like Switzerland, the Himalayas, etc., the glaciers run in all 

 directions; but the Drift was carried over wide areas, in a general 

 direction. Such a general direction is easily accounted for by the 

 action of icebergs carried by marine currents. 2. The agent of the 

 Drift seems to have been often uninfluenced by the direction of valleys 

 and ridges even of considerable size ; thus, for instance, bowlders are 

 carried across valleys 500 or 1,000 feet deep, and lodged as high up on 

 the mountain-slope on the other side. This is perfectly consistent 

 with the action of icebergs drifting over an uneven sea-bottom, but in- 

 consistent with our usual notions of glacial action. 3. The great dis- 

 tance carried, sometimes one hundred miles or more, is precisely what 

 we might expect of icebergs, but difficult to reconcile with our usual 

 notions of glaciers. 4. Alpine glaciers will not move on a slope of less 

 than 2° or 3°, but such a slope, carried several hundred miles, would 

 produce an incredible elevation of land. A slope of 2|-° for 200 miles 

 would produce an elevation of nearly nine miles ! 



These were unanswerable objections so long as our ideas of glaciers 

 were confined to those of temperate climates ; but they all find their 

 complete answer in the phenomena of the polar ice-sheet. Greenland is 

 1,200 miles long and 400 or 500 miles wide. This whole area of over a 

 half-million of square miles is covered 3,000 to 6,000 feet deep with 

 ice.* This ice-mantle moves en masse seaward, molding itself on the 

 surface inequalities of the country, and molding that surface beneath 

 itself, producing universal glaciation, and only separating into distinct 

 glaciers at its margin. In antarctic regions, the general ice-sheet is 

 even still more extensive and thick. Now, it is to such an ice-mantle 

 that the Drift is to be ascribed, for it moves irrespective of smaller val- 

 leys, in one general direction over great areas, to great distances, and 

 over a slope of only 1° or even £°. 



Probable Condition of Things in the Eastern part of the Continent 

 during the Glacial Epoch. — The continental elevation, which com- 

 menced in the Pliocene, culminated during this time. In the northern 

 part of the continent it probably reached 2,000 to 3,000 feet above its 

 present level. The shore-line was at least as far out as the submerged 

 continental margin, and all the coast islands of this part were added to 

 the continent. The evidence of this is found in deep submarine chan- 

 nels off the mouths of all the great rivers, such as the St. Lawrence, 

 the Hudson, the Delaware, etc., cutting through the submerged conti- 

 nental border, and evidently formed by erosion during the Tertiary. 

 The axis of elevation was the Canadian Archaean highlands, and thence 

 it became less both northward and southward, but undoubtedly ex- 

 tended to the shores of the Gulf. Coincidently with this elevation, 



* Nansen, Nature, vol. xl, p. 210, 1889. 



