216 



On the Causes of Glacier-Motion. 



even in the hard hypersthene of Skye, which were a considerable 

 fraction of an inch in depth. When we consider the enormous force 

 necessary to plough out such a furrow in hard rock, it is almost 

 impossible to believe that it was done by the simple passage over it, 

 once for all, of a stone imbedded in the ice. If, however, the stone 

 descended by a series of oscillations, so that it passed many times over 

 the same spot, this difficulty is greatly lessened. 



(4.) In conclusion I may point out that the advocates of the gravi- 

 tation theory are bound to explain what becomes of the heat energy 

 which is poured into a glacier. When the sun is shining this radiant 

 energy is always very large, although the temperature of the air may be 

 low. In such cases the glacier does not melt ; it is perfectly clear that it 

 must expand, as any other solid must expand under the action of heat. 

 If so, it seems unreasonable not to hold that the gradual descent by 

 alternate expansion and contraction must follow, as it is known to 

 follow in the case of other materials. 



On the subject of the motion of Arctic ice, Dr. Rae, F.R.S., has 

 kindly permitted the publication of the following particulars : — 



" When in Greenland, in the autumn of 1866, I was ice-bound at the 

 head of one of the fiords, and slept a couple of nights at an Eskimo's 

 house. A glacier about half a mile distant was then in full activity, 

 the movement of which might, I believe, have been as visible to the 

 eye as it certainly was audible to the ear. 



" My own idea is that Arctic glaciers must have a downward motion 

 more or less during the whole year, summer and winter. I believe the 

 alternations of heat and cold — or, I should rather say, of temperature 

 — would of itself cause motion, especially near the upper surface. 



44 We know that ice 2 or 3 feet or more thick, contracts very 

 considerably in a few hours by a sudden fall of fifteen or twenty 

 degrees of temperature. I have found cracks in Lake Winnipeg 

 3 or 4 feet wide, formed by this cause during a single night, almost 

 stopping our sledge journey. This gap soon freezes up. Then the 

 weather gets milder, the ice expands, and with the new additional 

 formation is too large for the lake, and is forced up into ridges. This 

 process goes on at every ' cold snap,'* alternating with milder weather. 

 Now supposing a glacier for 10 or more feet of its depth contracts by 

 cold, as lake ice is known to do, it will get a series of cracks probably 

 in its longest axis, say from inland seaward ; the first snow-drift 

 will fill up these cracks or some of them, and this filling up will to 

 some extent perform the same office as the freezing of the cracks in 

 the lakes. The longitudinal extent of the glacier will be increased. 

 A snow-storm always brings milder weather, which would expand the 

 glacier, but as this expansion would naturally tend downhill, instead 



* " Cold snap," an American term meaning a rather sudden increase of cold. 



