304 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [June 1, 



the result when a protruding mass is sHpping imperceptibly over a 

 convex or ledged surface * . 



Although there certainly is a relation between the upper and lower 

 surfaces of a plastic glacier, even when it may be upwards of 2000 

 feet in thickness, stUl I must confess that in my opinion we can scarcely 

 attribute the regularly pinnacled appearance of many a large iceberg 

 and magnificent glacier to this cause. Some glaciers and icebergs, 

 again, are so flat and smooth on the upper surface, that one can 

 hardly conceive a rocky bottom beneath a glacier to be equally 

 smooth. On the north side of Cape Clarence, in the north shore 

 of Jones' Sound, during the late voyage of the " Isabel," I observed 

 that one portion of the surface of a flat but extensive glacier, that 

 protruded several miles into Glacier Strait, was exceedingly smooth, 

 while another portion of it was so rough and pinnacled that to walk 

 over it would have been impossible. This roughness must be attri- 

 buted to some peculiar atmospheric cause, or to the diflFerence of 

 temperature between the surface and the interior of the glacier. 



Eight feet is the depth to which a minimum temperature of — 45°, 

 a monthly mean of —30°, or an annual mean temperature of +2'5° 

 extended the freezing-point of water through freshwater ice on a 

 lake of two fathoms' depth (Kate Austin's Lake) in lat. 74° 40' and 

 long. 94° 16'. If we can presume the heat-conducting power of ice 

 formed on the surface of water, and of glacier-ice, to be the same, 

 then the temperature of the interior of the glacier below the above 

 depth, with the same minimum or mean annual degree of cold, 

 would be about -|-32°. The surface exposed to any alternation of 

 heat and cold, from the freezing-point to —45° or many degrees 

 lower, would necessarily become contorted and disturbed by contrac- 

 tion and expansion, even supposing its base or supporting part were 

 standing still. Of this we had unexceptionable proofs in the condi- 

 tion of the surface of the ice on the lake already noticed. But when 

 we take into account that the whole bulk of the glacier, except a few 

 feet of its upper surface, retains its plasticity and continues its down- 

 ward motion, it need not be wondered that the latter, hard and friable, 

 assumes a broken-up appearance. This view, however, does not 

 fully satisfy us, not being universally applicable. 



Following the example of Mr. Christie, one of the Secretaries of 

 the Royal Society f , during a winter in Barrow Straits, I performed 

 a number of experiments by submitting water in a strong iron bottle 

 to various temperatures, from -|-32° to —45°. While the tempera- 

 ture to which the bottle containing the water was exposed did not 

 descend more than eight or ten degrees below the freezing point, the 



* This is very well seen in the glacier of Petowak, and also in a glacier at 

 Cape Fitzroy, on the south shore of Jones' Sound. These crevasses are not un- 

 frequently filled up with mud, &c. brought down by debacles and other means 

 from the land on either side, and then they become frozen, thus cementing the 

 whole mass firmly together, and perhaps forming part of the future iceberg so 

 long as a few cubic feet of it remain undissolved 



t Sir Charles Lyell's Principles of Geology, Seventh Edition, p. 226. 



