Oil the Movement of Glaciers. 173 



change of dip and of pai-allelism the author attributed to the for- 

 ward movement and plasticity of the mass, together with the greater 

 amount of friction below, where the ends of the layers were in con- 

 tact with the ground, and the constant deprivation of support ante- 

 riorly and below, from the continued melting of the ice at these parts, 

 which would give the layers a constant tendency to fall forwards. 



The author then proceeded to shew that fissures or crevices in 

 glaciers could not be produced in consequence of the unequal expan- 

 sion of the ice itself, nor in consequence of the expaubion of the air 

 contained within its pores ; but that in every ease crevices were pro- 

 duced in consequence of the movement of the glacier over the in- 

 clined plane on which it rested. 



The author next passed to the second division of his subject, the 

 Movement of Glaciers, and first commented on the Dilatation Theory. 

 He endeavoured to prove that none of the phenomena observed in 

 glaciers could be accounted for by that theory ; that a glacier was 

 not retarded in its movement though riddled with crevices ; that 

 tlie supposed dilatation did not alter the form of the walls of the; e- 

 crevices ; that it did not close them at their upper extremity nor 

 widen them out below ; that it did not give rise to any convexity of 

 the surface of the glacier ; that the icy mass did not require to touch, 

 the rocky walls of the valley through which it passed ; that it could 

 move onwards for miles quite unsupported on its margins ; that du- 

 ring a whole summer, whilst its movement was greatest, it neveir 

 dilated even the hvi feet requisite to fill up the spaces interveiiino- 

 between its margin and the rocky walls of the valley ; that it ad- 

 vanced during the heat of the day, and during winter, when it is al- 

 lowed no dilatation can take place ; that it was unlikely water could 

 percolate during the course of one day through a solid mass of ice,, 

 more than 100 feet thick, especially M'hen that ice was colder thau 

 the fi-eczing point of water ; that pools of water (in the Polar Re- 

 gions) remained unfrozen for whole weeks during the summer, whilst 

 their progressive motion was greatest. For these and other reasons,. 

 the author arrived at the conclusion, " that glaciers do not advanco^ 

 in consequence of a process of dilatation of their icy mass." 



The author next enquired into the proofs of the truth of the- 

 sliding theory, and stated, that he had satisfied himself that every 

 phenomenon known to occur in glaciers could be explained by it. 

 He brought forwai'd, as explanatory circumstances, the descent of 



