ENGLACIAL DRIFT OF THE MISSISSIPPI BASIN. 5 I 



some of the lower horizons and its lodgment on the crests of 

 ridges or the slopes or summits of mountains that lay athwart 

 its course. 



So again, it is certain that a considerable part of the periph- 

 eral drainage of glaciers takes place through'tunnels beneath 

 the ice. It is reasonable to suppose that during the winter 

 season, when the drainage is slack, these tunnels tend to collapse 

 in greater or less degree, under the continued pressure of the 

 ice and the "fattening" of the glacier, so that in the early part 

 of the next melting season the contracted tunnels may be over- 

 flooded by glacial waters. To the extent that these tunnels 

 become incompetent the water would become ponded back in 

 the crevasses and moulins by which the surface-water gains 

 access to them. They thus come to have something of the 

 force of water flowing in tubes, and may be presumed to be 

 capable of forcing rounded material to some considerable 

 height, and of carrying ice -imbedded boulders to any point 

 reached by the stream. These tunnels probably undulate with 

 the bottom, and lodgment along them takes place wherever 

 enlargement permits. 



Without, therefore, appealing to any upward cross currents 

 within the ice itself, it is possible to explain the transportation 

 of the drift from lower to higher altitudes. I have never seen 

 phenomena of this kind that seemed to call for any other ex- 

 planation than these. I am not prepared to say that there are no 

 such phenomena. One of the purposes of this article will have 

 been accomplished, if it shall call forth a critical statement of 

 phenomena that require the assumption of internal upward 

 movements of the ice to account for them, and of the criteria 

 which distinguish such phenomena from those that may be 

 referred to upward basal movements such as are common to all 

 streams or to the exceptionally conditioned subglacial streams. 

 That there are upward internal movements in most streams is as 

 much beyond question as the existence of upward basal currents 

 in rivers and glaciers, but they are dependent chiefly upon the 

 velocity of the current and the irregularity of the bottom. 



