280 GEOLOGY 



of ice, and (2) partly to the shearing and foliation of the ice, or to 

 the causes which produce these structures. In this position, then, 

 the slipping of crystal plates on one another may be a factor in 

 glacial motion. 



The Probable Fundamental Element in Glacier Motion 

 Melting and freezing. There seems to be no escape from the 

 conclusion that the primal cause of glacier motion is one which may 

 operate even under the relatively low temperatures, the relatively 

 dry conditions, and the relatively granular textures which affect 

 the heads of glaciers. These considerations lead to the view that 

 movement there takes place by the minute individual movements 

 of the grains upon one another. While they are in the spheroidal 

 form, as in the neve, this would not seem to be difficult. They may 

 rotate and slide over each other as the weight of the neve increases. 

 After they become interlocked by further growth, rotation and 

 sliding must be more difficult. The amount of motion required of 

 an individual granule is surprisingly small. In order to account for 

 a movement of three feet per day in a glacier six miles long, the 

 mean motion of the average granule relative to its neighbor would 

 be roundly, Trrornr f its own diameter per day; in other words, it 

 should change its relations to its neighbors to the extent of its diameter 

 once in about thirty years. A change of such slowness under the 

 conditions of granular alteration can scarcely be thought incredible, 

 or even improbable, in spite of the interlocking which the granules 

 may develop. It is conceivable that there may be motion between 

 the granules comparable to that between shot in great quantities 

 in similar positions; but the movement is supposed to be effected 

 chiefly by the temporary passage of minute portions of the granules 

 into the fluid form at the points of greatest compression, the transfer 

 of the water thus produced to adjoining points, and its resnlidifi- 

 cation. The points of greatest compression are obviously those 

 whose yielding most promotes motion, and the successive yielding 

 of points which come in succession to oppose motion most (and thus 

 to receive the greatest stresses), permits continuous motion. It is 

 only necessary to assume that the gravity of the accumulated mass 

 is sufficient to produce a little temporary liquefaction at the points 



