lb'4 SCIENCE IN SHORT CHAPTERS. 



Greenland ; and (as the structure of the southern icebergs 

 prove) everywhere within the great Antarctic ice barrier. 



What, then, must happen when the snow-line comes down, 

 or nearly down, to the sea-level ? It is evident that the out- 

 thrust glaciers, the overflow down the valleys, cannot come to 

 an end like the present Swiss and Scandinavian glaciers, by the 

 direct melting action of the sun. They may be somewhat 

 thinned from below by the heat of the earth, and that gen- 

 erated by their own friction on the rocks, but these must be 

 quite inadequate to overcome the perpetual accumulation due 

 to the snowfall upon their own surface and the vast overflow 

 from the great snow-fields above. They must go on and on, 

 ever increasing, until they meet some new condition of climate 

 or some other powerful agent of dissipation something that 

 can effectively melt them. 



This agent is very near at hand in the case of the Scandina- 

 vian valleys and those of Scotland. It is the sea. I think I 

 may safely say that the valley glaciers of these countries during 

 the great ice age must have reached the sea, and there have 

 terminated their existence, just as tho Antarctic glaciers ter- 

 minate at the present Antarctic ice- wall. 



What must happen when a glacier is thus thrust out to sea ? 

 This question is usually answered by assuming that it slides 

 along the bottom until it reaches such a depth that flotation 

 commences, and then it breaks off or " calves" as icebergs. 

 This view is strongly expressed by Mr. Geikie (p. 47) when he 

 says that, "The seaward portion of an Arctic glacier cannot 

 by any possibility be floated up without sundering its connec- 

 tion with the frozen mass behind. So long as the bulk of the 

 glacier much exceeds the depth of the sea, the ice will of 

 course rest upon the bed of the fiord or bay without being sub- 

 jected to any strain or tension. But when the glacier creeps 

 outward to greater depths, then the superior specific gravity of 

 the sea-water will tend to press the ice upward. That ice, 

 however, is a hard continuous mass, with sufficient cohesion to 

 oppose for a time this pressure, and hence the glacier crawls 

 on to a depth far beyond the point at which, had it been free, 

 it would have risen to the surface and floated. If at this great 

 depth the whole mass of the glacier could be buoyed up with- 

 out breaking off, it would certainly go to prove that the ice of 

 Arctic regions, unlike ice anywhere else, had the property 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 stretching, but by 



