THE " GREAT ICE AGE." 121 



of yielding to mechanical strain without rupturing. But 

 the great tension to which it is subjected takes effect in the 

 usual way, and the ice yields, not by bending and stretch- 

 ing, but by breaking." Mr. Geikie illustrates this by a 

 diagram showing the " calving" of an iceberg. 



In spite of my respect for Mr. Geikie as a geological au- 

 thority, I have no hesitation in contradicting some of the 

 physical assumptions included in the above. 



Ice has no such rigidity as here stated. It does possess 

 in a high degree "the property of yielding to mechanical 

 strain without rupturing. " We need not go far for evidence 

 of this. Everybody who has skated or seen others skating 

 on ice that is but just thick enough to "bear" must have 

 felt or seen it yield to the mechanical strain of the skater's 

 weight. Under these conditions it not only bends under 

 him, but it afterwards yields to the reaction of the water 

 below, rising and falling in visible undulations, demon- 

 strating most unequivocally a considerable degree of flexi- 

 bility. It may be said that in this case the flexibility is due 

 to the thinness of the ice ; but this argument is unsound, 

 inasmuch as the manifestation of such flexibility does not 

 depend upon absolute thickness or thinness, but upon the 

 relation of thickness to superficial extension. If a thin 

 sheet of ice can be bent to a given arc, a thick sheet may 

 be bent in the same degree, but the thicker ice demands a 

 greater radius and proportionate extension of circumference. 

 But we have direct evidence that ice of great thickness 

 actual glaciers may bend to a considerable curvature before 

 breaking. This is seen very strikingly when the uncrevassed 

 ice-sheet of a slightly inclined neve suddenly reaches a pre- 

 cipice and is thrust over it. If Mr. Geikie were right, the 

 projecting cornice thus formed should stand straight out, 

 and then, when the transverse strain* due to the weight of 

 this rigid overhang exceeded the resistance of tenacity, it 

 should break off short, exposing a face at right angles to 

 the general surface of the supported body of ice. Had 

 Mr. Geikie ever seen and carefully observed such an over- 

 hang or cornice of ice, I suspect that the above-quoted pas- 

 sage would not have been written. 



Some very fine examples of such ice-cornices are well seen 



