THE WORK OF SNOW AND ICE. 321 



than in summer^ and the whole thickness of the glacier should experi- 

 ence this rigidity chiefly at the ends and edges, where the relative thin- 

 ness of the ice permits the low temperature to reach its bottom. The 

 motion in these parts during the winter is, therefore, very small. 



In this view may also be found an explanation of the movement of 

 glaciers for considerable distances on upward slopes, even when the sur- 

 face as well as the base is incHned backwards. So far does this go that 

 superglacial streams sometimes run for some distance backwards, i.e. 

 toward the heads of the gla,ciers, while in other places surface-waters 

 are collect; ed into ponds and lakelets. Such a slope of the surface of 

 ice is not difficult to understand if the movement be due to thrust from 

 behind, or if it be occasioned by internal crystalline changes acting upon 

 a rigid body; but it must be regarded as very remarkable if the move- 

 ment be that of a fluid body, no matter how viscous, for the length of 

 the acclivity is sometimes several times the thickness of the ice. Crevas- 

 sing and other evidences of brittleness and rigidity find a ready eluci- 

 dation under the view that the ice is a really soUd body at all times, 

 and that its apparent fluency is clue to the momentary fluidity of small 

 portions of the mass assumed in succession as compression demands. 



In addition to the considerations already adduced, it may be urged 

 that a glacier does not flow as a stiff Hquid because its granules are not 

 habitually drawn out into elongated forms, as are cavities in lavas and 

 plastic lumps in viscous bodies. Flowage fines comparable to those in 

 lavas are unknown in glaciers. 



All this is strictly consistent with our primary thesis, that a glacier 

 is a crystalline rock of the purest and simplest type, and that it never has 

 other than the crystalline state. This strictly crystalline character is 

 incompatible with viscous liquidity. 



Other Views of Glacier Motion. ^ 



While these views of glacial motion seem to us to best accord with 

 the known facts, they are not to be regarded as established in scientific 

 opinion, or as the views most commonly held. The mode of glacial 



^ References on glacier structure and motion. — L. Agassiz, Etudes sur les Glaciers, 

 Neuchatel, 1840. Rendu, Theorie des Glaciers de la Savoie, Soc. Roy, Acad., Savoie, 

 Mem. 1840 (in English, ed. by Geo. Forbes, London, 1874). J. de Charpentier, Essai 

 sur les Glaciers et le terrain erratique du Basin du Rhone, Lausanne, 1841. F. J. Hugi, 



