GLACIAL EROSION AND TRANSPORTATION. 251 



and that, after moving a sufficient distance, they appeared 

 upon its surface ; and we can easily believe that many bowl- 

 ders have thus been repeatedly transferred from beneath the 

 ice to its surface, and thence projected by the more rapid 

 superficial motion to the front to be reincorporated into the 

 lower strata of the mass, and re-elevated to the surface again. 

 Upon this subject Professor Lesley offers the following 

 ingenious suggestions : 



When two equal solid bodies descending opposite slopes 

 meet, they arrest and support each other. 



Imagine myriads of cannon-balls rolled from both sides to 

 meet in the middle of a symmetrical valley. Those arriving 

 first would remain ever afterward the lowest stratum ; those 

 which followed would arrange themselves in higher and higher 

 layers until the valley was full or the supply exhausted. No 

 shifting of places would take place after each had found its 

 lowest place. 



But suppose opposite descending quantities of pitch, or 

 moist clay, through which cannon-balls were scattered, to meet 

 along the middle line of a valley ; the two advancing fronts 

 would mash against each other, and thicken upward, the in- 

 cluded cannon-balls rising vertically in the thickening mass, 

 the thickening being in proportion to the height and weight 

 and rigidity of the masses of clay pressing from the side slopes 

 upon the middle line which had come to rest. 



By substituting plastic ice for moist clay, and rock-bowlders 

 for cannon-balls, we get an idea of how the American ice-sheet 

 may have carried up (diagonally) the masses which it tore 

 from low-lying outcrops to higher levels, and even over mount- 

 ain-crests. . . . 



A terminal moraine is, often described as if it were merely 

 the tumbled-off accumulation of the medial and lateral mo- 

 raines which cover the surface and sides of a glacier. But the 

 fact is, that a glacier is like a plum-pudding— full of scattered 

 sand and stones from top to bottom and from side to side, all 

 of which are delivered at its front end down the valley. The 

 surface exhibition is made much stronger than it would other- 

 wise be by the perpetual melting of the upper surface and sides 



