48 MAN AND THE GLACIAL PERIOD. 



true in view of Tyndall's suggestion that the contour of 

 the bottom over which the ice flows may greatly affect the 

 rate in certain places. A sudden deepening of the chan- 

 nel may affect the motion of ice in a glacier as much as it 

 does that of water in a river. 



Other observations also amply sustain the conclusions 

 of Helland. As already stated, the Danish surveying party 

 under Steenstrup, after several years' work upon the south- 

 western coast of Greenland, have ascertained that the nu- 

 merous glaciers coming down to the sea in that region and 

 furnishing the icebergs incessantly floating down Baffin's 

 Bay, move at a rate of from thirty to fifty feet per day, 

 while Lieutenants Ryder and Bloch, of the Danish Navy, 

 who spent the year 1887 in exploring the coast in the 

 vicinity of Upernavik, about latitude 73°, found that the 

 great glacier entering the fiord east of the village had a 

 velocity of ninety-nine feet per day during the month of 

 August.* 



It is easier to establish the fact of glacial motion than 

 to explain how the motion takes place, for ice seems to be 

 as brittle as glass. This, however, is true of it only when 

 compelled suddenly to change its form. When subjected 

 to slow and long-continued pressure it gradually yet readi- 

 ly yields, and takes on new forms. From this capacity of 

 ice, it has come to be regarded by some as a really viscous 

 substance, like tar or cooling lava, and upon that theory 

 Professor Forbes endeavours to explain all glacial move- 

 ment. 



The theory, however, seems to be contradicted by fa- 

 miliar facts ; for the iceman, after sawing a shallow groove 

 across a piece of ice, can then split it as easily as he would 

 a piece of sandstone or wood. On the glaciers themselves, 

 likewise, the existence of innumerable crevasses would 

 seem to contradict the plastic theory of -glacier motion ; 



* Nature, December 29, 1887. 



