Mr. Hopkins on the Mechanism of Glacial Motion. IV. SSI 



is that in which the greatest sliding motion ought to take 

 place. 



The lines exhibited on Prof. Forbes's models of plaster of 

 Paris, and which have been considered by him as represen- 

 tatives of the veined structure in glaciers, are to be referred, 

 I have no doubt, to the same cause as the linear ridges de- 

 scribed in my experiment. In fact, if it were possible to con» 

 tinue the central motion in my experiments, to the same ex- 

 tent as in the more fluid mass used in the experiments of Prof. 

 Forbes, the two sets of lines in these cases respectively would 

 assume exactly similar positions. I have shown experiment- 

 ally that the lines in my experiments were not associated with 

 any discontinuity of the mass, such as that which Prof. Forbes 

 has considered to be indicated by the corresponding lines in 

 his models. How such indication is afforded by the lines in 

 question, I do not in any degree understand. 



When the central motion was continued the fissures were 

 necessarily brought more nearly to perpendicularity with the 

 axis ; but when their obliquity to their original position, /. e. to 

 the line of maximum pressure, became considerable, they began 

 to close. (Second Letter, an. 24, p. 163.). We thus account 

 for the fact, that transverse fissures never remain open long 

 enough to acquire a position in which their convexity would 

 be turned towards the lower extremity of the glacier, a posi- 

 tion which they must necessarily assume by the more rapid 

 motion of the centre, unless there were some cause originating 

 in that motion constantly tending to obliterate them. 



When the central motion was continued long enough, the 

 fissures along the flanks became more irregular and ran into 

 each other, after which the central portion moved nearly as a 

 continuous mass, sliding past the narrow lateral portions, from 

 which it was severed on either side by the lateral fissures 

 running into each other as just described. 



Another effect of the pressure superinduced by the motion 

 of the central portion is not undeserving of notice. When 

 the mass projected beyond the lower end of the trough, it was 

 pushed outwards at the sides by the pressure acting trans- 

 versely, so that by this action and the dislocation along the 

 flank of the mass, the lateral portions turned round the ex- 

 tremities of the trough almost as completely as if the mass 

 had been fluid. This explains the expansion of glaciers in 

 expanding valleys without the hypothesis of fluidity. (Second 

 Letter, art. 23, p. 163.) 



IL When the experiment was made within an hour or two 

 after the layer of mortar had been prepared, and when the 

 mortar was comparatively soft, parallel lines of dislocation 



