1842.] STRUCTURE THE RESULT OF UNEQUAL PROGRESSION. 23 



inent of the glacier ; and if the laws of movement, ascertained 

 independently, shall coincide with, or confirm, the phenomena 

 of structure, we shall be better able, from the comparison of the 

 two classes of facts, to decide upon the cause of movement. 



What I have hitherto stated is matter of fact. I will state 

 very briefly what I am disposed to deduce by way of hypothesis. 



It is impossible to consider these structural bands on the 

 surface of the glacier, in combination with the fact established 

 in my former letters, that the centre of the glacier moves con- 

 siderably faster than its edges, without believing that the bands 

 are an indication of the motion, and that the motion gives rise 

 to the veined structure. These dirt-bands perfectly resemble 

 those of froth and scum which every one has seen upon the 

 surface of slowly-moving foul water ; and their figure at once 

 gives the idea of fluid motion, freest in the middle, obstructed 

 by friction towards the sides and bottom. It will be found 

 that the analogies are entirely favourable ; the glacier struggles 

 between a condition of fluidity and rigidit}^ It cannot obey 

 the law of semifluid progression (maximum velocity at the 

 centre, which is no hypothesis in the case of glaciers, but a 

 fact), without a solution of continuity perpendicular to its sides. 

 If two persons hold a sheet of paper, so as to be tense, by the 

 four corners, and one moves two adjacent corners, whilst the 

 other two remain at rest, or move less fast, the tendency will 

 be to tear the paper into shreds parallel to the motion ; in the 

 glacier, the fissures thus formed are filled with percolated water, 

 which is then frozen. It accords with this view 1. That the 

 glacier moves fastest in the centre, and that the loop of the 

 curves described coincides (by observation) with the line of 

 swiftest motion. 2. That the bands are least distinct near the 

 centre, for there the difference of velocity of two adjacent stripes 

 parallel to the length of the glacier is nearly nothing ; but near 

 the sides, where the retardation is greatest, it is a maximum. 

 3. It accords with direct observation (see my last Letter), that 

 the difference of velocity of the centre and sides is greatest near 

 the lower extremity of the glacier, and that the velocity is more 



