Vol. 54.] ON THE GLICIAL GEOLOGY OF SPITSBEKGEN. 223 



some distance from the edge of the ice. J^one of the river-channels 

 that we saw either in or on the ice contained much debris. 



In this connexion we may mention that a few drumliu-like mounds 

 were met with, but not in such a position as to throw any further 

 light on their formation. 



(8) Differential Flow in Glaciers_, and the Striation of 

 Intraglacial Material. 



Differential flow occurs in glaciers on a small scale, owing to the 

 movement along shearing-planes ; and one consequence of this 

 appears to be a striation of the materials carried in the glacier. 

 Prof. Hussell ^ has recently pointed out that ice charged with debris 

 must be more rigid than pure ice otherwise of the same character. 

 "When, therefore, a block composed of alternate layers of clean and 

 debris-charged ice is subjected to pressure, it will yield more readily 

 along the pure layers, and thus at once cause a differential flow. 

 The pure layers are not absolutely free from debris, but usually 

 contain some scattered pebbles, which will be dragged forward across 

 the face of the debris-charged layer, causing mutual abrasion of 

 the rock-fragments. Only by some such process can we account 

 for the striation and rounding of the intraglacial material in some 

 Spitsbergen glaciers.^ 



(9) The Evidence for Interglacial Periods. 



The advance and retreat of the Spitsbergen glaciers are very 

 irregular, and appear to be often due to quite local climatic 

 changes. Ground from which ice has receded soon becomes covered 

 with vegetation and inhabited by reindeer and foxes. The latter 

 animals make caches of dead reindeer-meat as a supply for the 

 winter, for we found heaps of bones which can have been collected 

 only by foxes. The remains of plants and bones may thus readily 

 become interstratified between beds of glacial drift by a further 

 local advance of the ice. 



We found moreover, in several places, a growth of plants on mud- 

 hills covering sheets of ' fossil ice.' Owing to the protection of the 

 overlying debris, the ice was melting very slowly ; but as it 

 dwindles in mass, numerous slips take place in the mud-hillsy 

 leadiDg to the burial of the plants in a very admirable preserving 

 medium. A future examination of these beds would probably show 

 a remarkable interstratification of true glacial deposits with clays 

 containing fossil plants and reindeer-bones. 



^ J. 0. Russell, ' The Influence of Debris on the Flow of Glaciers,' Joura, 

 Geol. vol. iii (1895) p. 823. 



^ The striation of boulders carried intra glacially has been suggested by- 

 Kendall, Glac. Mag. vol. ii (1894) p. 45. But the angularity of most of the 

 Swiss intraglacial debris is a further indication of the simpler character of the 

 movements in the ice there. The fact has been used by Chamberlin, Bull. 

 Geol. Soc. Amer. vol. v (1894) p. 85, as an argument against the uplift of 

 material in glaciers. 



