io8 Mr. H. H. Howorth on 



incapable of being stretched because it is covered with fine 

 slits. Again, it is seldom the crevasses intersect even when 

 most numerous, and they do not therefore separate the whole 

 mass into blocks or fragments, and when they do so, it would 

 seem that they are very shallow, causing only a surface dis- 

 location or they would fall away in avalanches. 



Were, he says, the inequality of the central and lateral 

 movement of the glacier mass to be attributed to longitudinal 

 fissures or discontinuities, by means of w^hich broad strips 

 of ice slide past each other, we should have to demonstrate 

 the existence of such fissures, which could not be always 

 close unless either (i) 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 cre- 

 vasse-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 distur- 

 bances, 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, that the only trace of longitudinal discontinuity in the 

 normal condition of the glacier is to be found in the veined 

 structure, 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 (/^/^/Z. Trans. ^ 1846, 197). 



In the course of this controversy Mr. Hopkins urged 

 that both the sliding and the viscous theories, that is 

 his own and Forbes's theories, agree in assigning gravity 

 as the primary cause of glacier motion, but in the one 



