1846.] CREVASSES DO NOT MATERIALLY ASSIST PROGRESSION. 147 



Were the inequality of the central and lateral movement of 

 the glacier mass to be attributed to longitudinal fissures or 

 discontinuities, by means of which broad stripes of ice slide 

 past each other, we should have to demonstrate the existence 

 of such fissures, which could not be always close unless either 

 (1) The surfaces were mathematically adapted to slide over one 

 another, or (2) The ice possessed sufficient plasticity to mould 

 the surfaces to one another's asperities, in which case the 

 plasticity would alone be sufficient without the discontinuity to 

 explain the motion of the ice. These longitudinal fissures, 

 cutting the common transverse fissures perpendicularly, would 

 divide the glacier even where most level into trapezia, and no 

 transverse crevasse could be straight-edged, but must be jagged 

 like a saw, or cut en echelon.* Such a phenomenon never occurs 

 unless where a glacier is moving torrentially, or with great dis- 

 turbance and down a steep. There such longitudinal fissures 

 may occasionally be seen, but they form the exception and not 

 the rule. It has been demonstrated by an elaborate proof in 

 5, that the only trace of longitudinal discontinuity in the nor- 

 mal condition of the glacier is to be found in the veined struc- 

 ture, which, being caused by a partial discontinuity at a vast 

 number of points, admits of an insensible deformation of the 

 glacial mass without sudden or complete rents, or slips, or the 

 formation of zigzag crevasses. 



The existence of the great transverse crevasses, which, even 

 in glaciers not moving torrentially, divide the surface of a glacier 

 by rents perhaps 2000 feet long,f have been thought by some 

 to be comparable to beams of an elastic material, supported at 

 the two ends, and bending under their own weight forward, in 

 the middle. Were this the case, it would scarcely modify the 

 plastic theory as I have propounded it ; because in order that 

 such a bar of ice should conform to the known movements of 

 the glacier, opposite the Montanvert for instance, the centre 



his guides) the name of seracs, a term derived, if I recollect rightly, from a pro- 

 vincial word applied to masses of curd chopped into square pieces in the dairies of 

 the Alpine chalets. ] 



* [See Plate II., fig. 1.] f Travels, p. 171, 2d ed. 



