Niles.] 334 [March 20, 



glacier in summer has not failed to notice the small fragments of stone 

 which often darken and sometimes cover the surface. When these 

 are examined they are found to be sharply angular; and if the exam- 

 ination is extended to the medial and. lateral moraines, they will be 

 found to contain immense quantities of similar materials. These 

 small fragments, as well as large ones, find their way into the sub- 

 glacial streams, in which by the sharpness of their angles they become 

 most effective instruments in the work of erosion. The materials 

 transported by ordinary streams, even when swollen by heavy rains, 

 are of a different nature. The small stones and gravels which they 

 receive are usually more or less rounded, while the finer materials are 

 chiefly loam, clay, soil, or well-worn sand. The erosive power of a 

 current carrying such old, worn, and often soft materials, is much 

 less than that of one charged with the new and sharp instruments of 

 the sub-glacial streams; hence the denuding agency of the latter 

 should not be estimated by observations upon the former. 



The excavating power of these streams is shown in the number of 

 pot-holes which they produce. The steepness and irregularity of 

 their courses, the abundance of water with stones and sand, and in 

 many places the presence of ice causing gyratory movements of the 

 water, make these streams peculiarly efficient in this work. Some- 

 times these pot-holes succeed each other so closely in the course of 

 the stream, that as they increase in size they unite and form a deep, 

 narrow gorge, whose walls present a succession of their concave 

 surfaces. 



Furthermore, the ice of the glaciers often exercises a controlling 

 influence upon the positions and courses of these streams. It is not 

 uncommon to find a stream flowing along the edge of the glacier 

 considerably below its surface, in a channel one side of which is ice 

 and the other side rock. In such instances the streams are often sup- 

 ported by the ice at a considerable elevation above the bottom of 

 the valley where they would otherwise be. The power which a gla- 

 cier may have for preventing water from flowing directly into the 

 lower portion of its valley, is well illustrated by the Marjelen See, a 

 lake which owes its existence to the ice- wall of the side of the Great 

 Aletsch Glacier which forms one end of the basin which it occupies. 



The lateral streams above described are abundantly supplied with 

 small and large pieces of stone from the lateral moraines, and they 

 thus become agents in the erosion of the sides of the valleys. It 

 will probably be remarked that such streams must naturally erode 



