2 50 THE JOURNAL OF GEOLOGY. 



helps to enlarge the subglacial or englacial channels. But, in 

 the case of ice sheets covering all the land, the only heat avail- 

 able for enlarging the tunnels (omitting the small amount of 

 basal heat) is the heat absorbed by surface waters and carried 

 by them beneath the ice. It is known that the waters of surface 

 melting often collect in superficial brooks and torrents of con- 

 siderable size. The absorption of radiant energy from the sun- 

 light is instantaneous, or at least much more rapid than the 

 conduction of this energy as molecular heat from the water to 

 the ice with which it is in contact. Under sunlight all surface 

 waters become warmed a little above 32°, and, as they plunge 

 beneath the ice, they give up their surplus heat to help melt the 

 walls of the subglacial channels. This, I infer, is the most 

 efficient of all the agencies that help to enlarge the subglacial 

 tunnels. Enlargement of the subglacial tunnels is not uniform. 

 Thus, where a surface stream pours beneath the ice and brings a 

 fresh supply of heat into the tunnel, there would be more rapid 

 enlargement than elsewhere. For various reasons, not necessary 

 to be discussed here, the enlargement of the tunnels proceeds 

 unequally. 



Given a tunnel gradually enlarging till sedimentation begins, 

 this sedimentation will commence at the most favorable places, 

 as at the local enlargements, or at an obstruction. If, now, the 

 size of the tunnel, or rather the ratio between the tunnel capa- 

 city and amount of water increase, sedimentation will take 

 place at more frequent intervals, and if the tunnel becomes large 

 and rather uniform in size, the sediments will form a continuous 

 ridge. 



Various causes can be adduced why a glacial stream should 

 deposit a diminishing quantity of sediment, but the controlling 

 cause and almost the only one admissible under the peculiar 

 local circumstances is the following : We grant that as we go 

 southward toward the distal extremity of the glacier the supply 

 of drainage water will increase, as in all drainage systems. But 

 all these surface waters take with them heat as they pass beneath 

 the ice to help enlarge the tunnels. Thus, as it were, each 



