SUBGLACIAL AND ENGLACIAL STREAMS. 299 



But second, we may consider the ice of glaciers as in motion. While 

 portions of the land are being upheaved the rising terranes are brought 

 under the sharper rasp of swifter streams, the earth by its internal move- 

 ments thus guiding the development of the erosion. In like manner we 

 may view the glacier as in motion, a sort of organism having its internal 

 motion so far determined by its environments that it has a systematic devel- 

 opment, and each part of the ice must be considered not alone with respect 

 to the forces now acting on it, but as having a history, and as often retain- 

 ing the forms or structures it obtained long before. This is obviously true 

 of the banded structure and other features visible on the surface, and ought 

 equally to be true of unseen parts Thus if the basal ice is hollowed out 

 by the water that falls down a crevasse at a moulin, the forward motion of 

 the ice will cause each successive portion of the ice as it advances to that 

 place to be also hollowed — the mechanical equivalent of a forward prolon- 

 gation of a series of hollows that together make a tunnel but are subse- 

 quently modified by the tendency of the stream to enlarge the channel and 

 of the antagonistic upward flow of the ice to cause its collapse. Now if 

 the ice, having thus, so to speak, gotten the stream in its power, shall 

 continue to carry it along the same tunnel prolonged by the ice movement, 

 we must consider the ice as having more than obstructive power. By 

 virtue of its motion it so exerts its obstructive power in the direction or 

 along the line of its motion, that it can be said to have a constructive power 

 to help build its own tunnels and determine their courses and develop- 

 ment. The moving ice tends to the maintenance of all subglacial and 

 englacial tunnels parallel to its flow, while the water with eqiial pertinacity 

 strives to follow the slopes of the underlying land. When the movement 

 pushes the tunneled ice over rising ground the water bides its time, and at 

 the first eligible transverse crevasse it steals off sidewise toward the lower 

 ground. The ice moves onward and prolongs the now unused tunnel 

 until it becomes filled by subglacial till or disappears by the collapse of its 

 sides and roof. On this conception the actual course of a subglacial or 

 englacial river is the resultant of two forces which may or may not be 

 antagonistic, viz, the movement prolonging the tunnel in its own direction, 

 and the water tending to follow the slopes of the underlying land wherever 

 practicable. In this discussion we assume the tunnels; we do not account 

 for their origination. 



