148 DE. J. MOEISON ICE AXD ITS WOEK. 



Eemaining immelted, and more and more snow continuing to fall, 

 there would be a constantly -increasing accumulation were it not for 

 the existence of glaciers or rivers of ice which drain the snow-fields 

 in the manner which I am about to describe. As the snow in- 

 creases in depth, the particles underneath are pressed down by the 

 weight of the mass above, the separate crystals become agglutinated 

 together, and the air is squeezed out. This process continues as the 

 pressure becomes greater and greater, till at length the heap of 

 snow becomes a mass of blue compact crystalline ice. As more 

 and more snow falls, the pressure from above continues to increase, 

 and, as ice though solid is to a certain extent plastic, the eft'ect of 

 this pressure is to squeeze it out round the edges of the snowcap, 

 where by the force of gravitation it is directed down into the 

 valleys, where it forms glaciers or rivers of ice. 



These glaciers, under the influence of gra\T.tation, move slowly 

 down the valleys. The rate of motion is very slow, not more than 

 one or two feet in twenty -four hours, and it varies according to the 

 slope of the valley down which the flow takes place. A glacier 

 often extends for some considerable distance below the line of 

 perpetual snow before it melts, and this distance varies with the 

 size of the glacier, for the larger the mass of ice the greater is the 

 temperature required to melt it. A glacier may extend thousands 

 of feet below the snow-line. 



The glacier throughout its course is crossed by frequent fissures 

 called crevasses. These crevasses begin as cracks in the ice, where, 

 from the nature of its bed, it is exposed to any strain or tension. 

 These cracks gradually widen till they may become wide yawning 

 chasms extending to the very bottom of the ice. The extremity or 

 foot of a glacier varies in position according to the snowfall and 

 the temperature of the air — a greater snowfall or a lower tempera- 

 ture sendiug the ice lower down the valley. The rocks and 

 precipices which surround a glacier are always splitting up under 

 the influence of frost, and quantities of rocky fragments and debris 

 are continually rolling down and accumulating on the surface of 

 the ice. This moraine stuff, as it is called, forms long mounds or 

 bands of debris fringing the sides of the glacier, in which fragments 

 of rock of all sizes, from mere sand and grit up to blocks many tons 

 in weight, are found confusedly mixed together. A considerable 

 part of this moraine matter slips down into the crevasses, and may 

 descend to the veiy bottom of the ice and move with it along 

 the rocky floor. These fragments of rock get fixed and frozen 

 into the icy mass, and are pressed against the underlying rocks 

 with all the weight of the ice above. Pushed along with irre- 

 sistible force, these stones tear fi'om the rocks over which they 

 pass other fragments which also in like manner become frozen into 

 the ice. All these rock-fragments, being firmly held in the grasp 

 of the ice, grind and scratch the rocky pavement over which they 

 are driven, while the grit and sand which result from this grinding 

 process smooth and polish the bed over which the glacier passes. 

 This accumulation of mud, sand, and stones, which a glacier pushes 



