356 THE JOURNAL OF GEOLOGY. 



Of the factors peculiar to friction, that of indeterminate value 

 doubtless suffers increasing relative diminution as the depth of 

 ice increases, and its platted ordinates (expressed in terms of the 

 valley-profile) will hence form a curve of materially less depth 

 than the triangle formed by the tangents to its extremities (fig. 

 2). The disposition of the factor is accordingly to widen the 

 gorge and develop the U profile. 



Fig 2. 



With the less lateral velocity common to ice-streams will go 

 reduced lateral friction, and hence erosion, in a ratio correspond- 

 ing to the velocity curve ; and for a second reason, there- 

 fore, will concavity of the valley-sides be engendered and 

 developed ; though the concurrent disposition will be to deepen 

 the gorge. 



Whenever concavity of the valley-sides obtains, the contact 

 surface of the vertical prism will become variable. If, now, fric- 

 tion vary approximately with the pressure of the incumbent ice, 

 the consequent erosion will diminish with the increasing slope 

 toward the edges of the glacier ; when the disposition will be to 

 deepen the gorge and restore the V form ; but if the friction vary 

 more nearly with the contact-area, it will increase with the slope, 

 and the resulting erosion will tend to widen the gorge and, in 

 another manner, to restore the V profile. Whichever tendency 

 obtains will, however, be secondary and ever subordinate to that 

 of the principal factors of friction. (Subglacial water will at 

 once reduce friction and promote transportation directly and 

 corrasion indirectly ; also it will tend, ceteris paribus, to form a 

 continuous film between ice and rock reaching upward to 0.92 

 of the thickness of the glacier, or, if the glacial surface be highly 

 convex, perhaps quite to its margins. On the whole, then, its 



