COKRASION BY GRAVITY STREAMS. 261 



What we seek is not so much the knowledge as to the 

 ultimate composition or texture of a glacier as information 

 concerning its general character when in motion. It may 

 move by rotation of its constituent granules upon each 

 other; it may move by differential motion being set up 

 between other sets of textural units than ice granules ; 

 this does not concern us here, for if it really be a stream, 

 then flow may possibly be transferred from one set of tex- 

 tural units to another by simple readjustment of its volume 

 (see Introduction) without alteration of its general stream 

 characteristics. Its motion as a whole does interest us 

 however. 



Evidence of Sherzer. — According to this observer, 1 

 experimenters such as Main, McOonnell, Koch, Miigge and 

 others, have bent, elongated, compressed and twisted bars 

 of ordinary ice and glacier ice without visible rupture, even 

 when such ice has been kept continually below freezing 

 temperature. It would appear, also, as a result of these 

 experiments that the apparent plasticity increased with 

 the pressure. This latter is a most important and significant 

 feature of the case. Sherzer also as a result of careful 

 observation, apparently demonstrates that "glacier ice is 

 capable of showing a certain type of plasticity " (p. 130). 



The writer has observed the same glaciers which Sherzer 

 here is discussing, and his (Sherzer's) deduction as to the 

 plastic mode of motion exhibited by glaciers appears justi- 

 fiable from the evidence yielded by a study of these ice 

 masses in the field. This conclusion, it must be remem- 

 bered, was reached by Sherzer for some of the dirtiest 

 glaciers known to science. 



Chamberlin and Salisbury. — These authors 2 devote a 

 considerable space to the discussion of glacier motion. No 



1 W. H. Sherzer, Glaciers of the Canadian Selkirks and Bockies, 

 Smithsonian Institution, 1907, pp. 129 -131. 

 8 Geology, Vol. i, pp. 261 - 263. 



