GLACIAL CAflONS. 355 



In such consideration let the ice be assumed to occupy a 

 previously-formed gorge of the typical V form of water-cut 

 canons. 



The weight of the ice varies directly with its thickness, and 

 accordingly increases progressively from sides to center of the 

 gorge. The tendency of this factor is hence to continually 

 deepen the cafion and to perpetuate the V form. 



Three of the four factors into which down-stream impulse 

 may be resolved are of unequal value in different portions of the 

 width of the glacier, and from such inequality the differential 

 flow of ice-streams results ; for from sides to center the weight 

 increases uniformly, the available energy increases increasingly, 

 and the friction probably increases less rapidly than the thickness ; 

 whence the impulse at the center must ever remain predominant. 

 But if the ice-stream be conceived to consist of a parallel series 

 of longitudinal vertical laminae (for in the present discussion the 

 vertical variation of flow is immaterial), it is evident that those 

 at the edges will be retarded by the valley-sides, that the medio- 

 lateral laminae will be equally retarded and accelerated by their 

 unequally flowing neighbors, and that the central lamina will be 

 retarded by the more slowly moving ice on either side ; and if 

 the mutual interaction of the various laminae be considered, that 

 the platted ordinates of flow will form a curved figure, and 

 not a triangle homologous with the cross-section of the gorge 

 (fig. 2). Such indeed is the case of differential ice-flow, as 

 empirically established by Forbes, Agassiz, Tyndall, and other 

 observers ; though in the V gorge the curve would unquestionably 

 be less flattened than in the U gorges within which the measured 

 glaciers lie. On the whole, the disposition of the second factor 

 must be to most energetically attack the valley-bottom, but at the 

 same time to develop concavity of the valley-sides. 



Summarizing, it appears that the general tendency of the inten- 

 sity element is preeminently to deepen the canon and slightly 

 to transform the V to a U profile. 



