138 Review of the Controversy Regarding the Motion of Glaciers. 



fixed in its old position, to but a little below where it was before. If B 

 has not already passed into the fluid state in consequence of heat derived 

 from the sun, the additional supply which it will receive from the 

 solidifying of A will melt it. The nionjent that B becomes fluid it 

 will descend until it reaches A. B, then, is solidified a little below its 

 former position. The game process of reasoning is in like manner ap- 

 plicable to every molecule of a glacier. Each molecule of a glacier, 

 consequently, descends step by step as it melts and solidifies,, and 

 hence, the glacier, consolidated as a mass, is in constant motion down- 

 ward." 



Mr. James Geikie, F.R.S., in his recent work, "The Great Ice 

 Age," thus speaks of Mr. Croll's hypothesis, after sketching its prin- 

 cipal features : 



"Heat is thus the great lever which forces the hard masses of com- 

 pacted snow and ice from higher to lower levels, and relieves the 

 mountains of their loads of frozen water. If it were not for that pe- 

 culiar property of ice, which enables it to behave in many respects 

 like a viscous or semi-fluid body, all the waters of the earth, the myriad 

 rivers, and lakes, and seas, would gradually be lifted up by the 

 heat of the sun and carried on the wings of the wind to the mountains, 

 there to accumulate in vast and constantly growing masses, until ocean 

 and all its feeders had been exhausted. But the heat of the sun, which, 

 falling upon clay, sand, or solid rock, merely raises the temperature, 

 without changing the condition of these masses, pulses through the 

 great piles of ice that cumber the higher elevations of Alpine coun- 

 tries. The temperature of the ice itself can not rise, but every atom 

 of its bulk is set in motion, and slowly and gradually the solid heaps 

 creep down hill-slope and valley, their progress being accelerated or 

 retarded according to the degree of heat acting upon and passing 

 through them. It is thus that during day the downward movement 

 of the ice is less sluggish than at night, and for the same reason a gla- 

 cier in summer moves more quickly on its way than in winter, when 

 its motion is exceedingly slow, sometimes not reaching to half the 

 summer rate. 



"The motion of ice, then, being due to the transmission of heat, 

 which, by momentarily converting into water each molecule of the 

 frozen mass, in succession allows it to descend by gravitation, it is quite 

 evident that a body of ice will move down any slope, however gentle 

 the inclination may be. Its motion is precisely that of running water, 

 and hence we need not be surprised to find it stealing slowly down in- 

 clinations which are so slight as to appear to the eye like level plains. 



" But it may be asked, how it happens that a body built up of atoms 



