358 THE JOURNAL OF GEOLOGY. 



of erosion. Now ordinary valleys, whether occupied by streams 

 or glaciers, are of progressively diminishing declivity from source 

 to terminus ; ordinary glacial valleys exhibit successive zones of 

 active erosion, feeble erosion, slight deposition, and abundant 

 deposition in passing from their upper reaches to the broader 

 valleys into which they embouch or upon the plains with which 

 they merge ; and in such cases the down-stream impulse must 

 wane to practically nothing at the extremities of the glaciers, and 

 must hence greatly fail of the weight. It follows that at some 

 point (or at diverse points) in every extended glacier-course 

 the components weight and impulse are equal at the centre of 

 the glacier. 



Since glacier ice but slightly approaches perfect fluidity and 

 the flow of the center is greatly retarded by the sides, the ratio 

 of impulse to weight (and with it the effectiveness) continually 

 and largely increases from center to sides : if the central effec- 

 tiveness be just zero, that at the sides will nevertheless remain 

 important; if it be minus centrally, it may still be considerable 

 laterally ; and however great may be its value at the center, it must 

 have far greater value at the sides. The disposition, then, will ever 

 be to protect the bottom and equally to attack the sides of the 

 valley ; and since the down-stream impulse of the several parallel 

 laminae forms a curve when platted, so will the disposition also 

 be to form concave valley-sides. 



Of the unknown factor in the effectiveness term, the first 

 component (rock-crushing) can be but trivial in the ordinary 

 case, while the second (pressure-liquefaction) exercises antagon- 

 istic influences. It may, accordingly, be safely neglected. 



Collectively, the tendencies of the third element of glacial 

 erosion are (i) to effectually protect the valley-bottom through- 

 out a considerable portion of the glacier course, (2) to develop 

 the U form of canon, and (3) to materially increase the relative 

 width of the gorge. 



The fifth feature of glacial canons is explained by the opera- 

 tion of this element, and in turn establishes the importance of 

 the element. 



