of Glacier Motion. 159 



If a mountain-mass could be freed from the attraction of the 

 earth, it would, owing to the elasticity of its component rocks, 

 increase in height by many feet. Every change of atmo- 

 spheric pressure produces an exceedingly small rise or fall of 

 the earth's surface. The ebb and flow of the tides has a 

 similar effect upon the land in the neighbourhood of the coast- 

 lines ; indeed, every substance not absolutely rigid (and ab- 

 solutely rigid substances do not exist) must, when subjected 

 to any stress whatever, show a corresponding strain or de- 

 formation. 



A glacier upon whose mass we will suppose gravity has 

 previously had no effect is suddenly put within the sphere of 

 the earth's attraction. It instantly undergoes a change of 

 form. In fact strains are produced in the ice each propor- 

 tional to the measure of the corresponding stress, and the ice 

 takes a small step in a downward direction, that is in the 

 direction of least resistance. If the glacier rested in a valley 

 with straight smooth sides, the ice would descend bodily and 

 continue to slide until stopped by an angle in its channel or 

 other obstruction. Even on an uneven or crooked bed a 

 certain amount of slide occurs until the bending strains are 

 proportional to the stresses. We therefore have two kinds of 

 motion — one a bodily slide in a downward direction, and 

 another descent due to the differential motion of the ice not 

 in contact with the ground, the resultant motion being pro- 

 portional to these two components. De Saussure supported a 

 sliding theory; but in a rocky uneven gorge, before continuous 

 sliding can take place, the ice must be capable of continually 

 changing its form to creep over the inequalities of its rocky 

 bed. Of course, when ice is pressed hard against a boss of 

 rock, thawing must go on at a temperature below the ordinary 

 freezing-point, a portion of the water produced recongealing 

 on liberation. A similar effect is obtained by passing a wire 

 over a block of ice and hanging a weight upon it. The wire 

 passes through the mass without splitting it into two portions. 

 That glaciers do slide is capable of absolute proof. Examine 

 the rocky floor over which a glacier has moved. It is fre- 

 quently beautifully polished, scratched, and grooved. Stones 

 of all sizes have become embedded in the ice, and as it carried 

 them along, the adjacent rocks against which they were 

 pressed underwent considerable abrasion. The polishing and 

 finer markings are effected by the clay, sand, and smaller 

 stones. In this way many of the inequalities of the ground 

 are worn away by glaciers; and the markings give us con- 

 clusive proof, not only of the bodily motion of glaciers, but 



