The Theory of Glacier motion. 107 



pressed the view that the friction of the ice against the 

 sides of the valley produces a dislocation of the glacier into 

 longitudinal stripes, and as a result the central portions 

 slide past those adjacent to them, and so on for successive 

 strips as we approach the sides, the more rapid retardation 

 near the sides being rendered mechanically possible by the 

 increased number of these longitudinal dislocations. The 

 result in such a case, it was argued by Hopkins, would be 

 that the ice would advance by echelons^ or that strips of ice 

 of a certain number of feet, or yards, or fathoms, would 

 move either suddenly or by gradual sliding, but at all events 

 so as to mark by an abrupt separation at the longitudinal 

 fissure, that the one portion of the ice had slipped past the 

 other by a distinct measurable quantity. 



In regard to this notion of a glacier being a congeries of 

 moving masses, Forbes maintains that a rugged channel 

 like that of a glacier being packed with angular solid 

 fragments would speedily be choked, and that further 

 pressure from behind would tend to wedge the fragments 

 more tightly. . . . and if the figure of the channel be irregular 

 i.e.^ have expansions and contractions, however smooth 

 its surface, and however small the sliding angle, the 

 choking of a strait or contraction by the piling of the 

 fragments will be as complete as if the lateral friction were 

 excessive. This points to the impossibility of the discharge 

 of a fragmentary solid through a gorge by long strips 

 fractured parallel to its length, and constituting parallelo- 

 pipedons of a certain breadth ; secondly, he urges that actual 

 observation proves that a glacier is not a mass of fragments 

 or parallelopipedons, as some have supposed. Most of the 

 crevasses at a small depth shrink to mere slits, and perhaps 

 disappear altogether, and the area they occupy is small 

 compared with that of the unbroken ice, and when viewed 

 as a whole, is capable of conveying strains as thrusts, its 

 cohesion is no more destroyed than a parchment sieve is 



