SUBGLACIAL AND ENGLACIAL STEEAMS. 301 



•exceptions to this rule are near the distal extremities of glaciers where the 

 ice is much shattered. 



Various physical causes can be assigned for the discharge of lateral 

 glacial lakes. Thus in the course of climatic changes or cycles it may 

 happen from time to time that crevasses open in new places, or they may 

 open wider and extend farther than usual toward the side of the glacier, or 

 there may be a larger supply of warm water in the lake to enable it to 

 melt its way farther into the glacier till an opening is made into a crevasse 

 connecting with a subglacial or englacial tunnel. So, too, by reason of its 

 greater specific gravity the water tends to float the ice in contact with it, 

 the buoyancy of the water being resisted not only by the weight of the 

 ice next the lake, but also by all the ice cohering to it. Again, the pressure 

 of the water is directly tending to rupture the ice. While these and other 

 physical agencies are operative in the discharge of glacial lakes, obviously 

 it is onl}^ by test and observation that we can determine the causes in any 

 particular case. 



While, then, the existence of so many lakes lateral to glaciers is proof 

 that waters can not find basal passage under glaciers in all directions 

 except under the most favorable conditions, yet the fact of occasional dis- 

 charge beneath the ice can be cited in favor of the hypothesis that subgla- 

 cial rivers can under some conditions flow transversely to the ice of evei:! 

 thick glaciers as well as the waters from glacial lakes. 



While the conclusions that can at present be drawn from existing gla- 

 ciers are rather ineager and demand further investigation as to the courses 

 of the internal streams, yet incidentally they fall in line with many other 

 indications as to the streams of the ice-sheet. The osars of Maine are often 

 for considerable distances moi'e or less transverse to the existing glacial 

 scratches, as well as to the bowlder trains and elongated drumlins, and 

 therefore probably transverse to the direction of glacial motion. The 

 known' instances of the subglacial flow of water transversely to the ice flow, 

 admitting the least allowable weight to analogies, indicate that the trans- 

 verse direction of the osars can not be held incompatible with their having 

 been subglacial. 



