I04 Mr. H. H. Howorth on 



The chief substantial objection against the theory that 

 it moves entirely en masse is, that the motion of its differ- 

 ent parts is not uniform, but, on the contrary, there is a 

 differential motion by which its centre moves faster than its 

 sides, and its surface layers faster than the bottom, and this 

 motion is continuous from day to day, and is not made by 

 fits and starts; so that, granting that a certain movement en 

 masse takes place, this can only explain one portion of the 

 problem, the greater portion of it remains unexplained. A 

 similar objection applies to the modifications of the sliding 

 theory propounded by Mr. Mallet and M. Martins. 



The first of these was published in a paper read before 

 the Geological Society of Dublin in 1838 by Mr. Robert 

 Mallet. He affirms that the primiim mobile which causes 

 the movements of glaciers is hydrostatic pressure acting 

 between them and the rocky bed on which they rest, 

 and thus at intervals lifting them up and floating them, or, 

 as it were, transferring them upon liquid rollers from a * 

 higher to a lower level. He goes on to argue that the bed 

 on which a glacier rests is always warmer than the glacier 

 itself, whence the bottom of the glacier is always melting^ 

 thus accounting for the torrents which underlie it. This 

 sub-glacier melting, he urges, goes on irrespective of season 

 or climate. In summer the stream is also fed by the 

 melted snow and ice of the surface, the water from which 

 finds its way below by the many fissures. These waters 

 find a ready vent in summer, and, according to Mr. Mallet, 

 the glacier would not move at all in that season but for 

 certain disturbing causes. But to give his own words : 

 " When winter has covered its whole expanse many feet 

 deep in snow, and when the embouchure of the sub- 

 glacial streams is also gelid, and partially, or sometimes 

 wholly stopped, the waters rising and pent up beneath the 

 bed of the glacier, lift its mass more or less from off the base on 

 which it rests, and with more or less regularity according to 



