160 Mr. R. Mountford Deeley's Theory 



also of their former existence in areas where they are now 

 absent. Such evidence may be seen scattered over almost 

 the whole of Northern and Central Europe. 



We have so far only considered the initial motion of a 

 glacier resulting from its weight and elasticity. As soon as 

 the strains became proportional to the stresses (we shall be 

 sufficiently accurate if we regard the problem as a kinematical 

 one), all movements ceased; and, without the existence of some 

 further reason for motion, the ice would remain motionless in 

 its bed. Generally speaking the strains produced by weight 

 alone would not be sufficient to cause more than local rupture 

 in such a hard material. The distortion so far undergone 

 may be considered as one of elastic shear. 



Displace a glacier by an equal bulk of firm clay. The clay, 

 being heavier than the ice and nothing like so capable of re- 

 sisting shear, ought, on the fracture theory, to move; but in 

 reality it would remain in the valley for a lengthened period 

 without showing any signs of continuous motion. Now, if 

 the weight of the clay is not sufficient to produce shear and 

 give rise to motion, why should gravity act so powerfully 

 upon each particle of ice, a much harder and more cohesive 

 material, and cause the whole to move like a sluggish river ? 

 Fracture cannot occur until an amount of general movement 

 has taken place sufficient to throw strains upon some portions 

 greater than the ice at such points can withstand without 

 rupture. What we have, therefore, to explain is how this 

 general movement comes about : fracture and regelation are 

 mere secondary phenomena arising from glacier motion. 



Now we have seen how a glacier takes an initial set, or 

 elastic shear, by reason of its weight and elasticity. Every 

 portion of the mass is supporting a certain proportion of the 

 load; and every internal particle of ice which at any time 

 thaws and ceases to be an element of strength gives rise to a 

 further set or movement. On the other hand, when the 

 water in a cavity freezes, it does not begin to afford any 

 support until a strain is thrown upon it by the differential 

 motion of surrounding particles. Gravity may therefore give rise 

 to a slow but continuous change of form in an elastic substance 

 in the interior of which liquefaction and re solidification are con- 

 stantly going on. Changes of bulk (I do not mean change of 

 form or reduction of mass by thawing) do not take place to 

 any extent in a glacier as a whole ; what we have to consider 

 is shear, a sliding of particle over particle, producing change 

 of form or distortion, whether internal or external. In an 

 experiment with the steel plate it was proved that by destroy- 

 ing at numerous points the support afforded by the metal, 



