ICE AND GLACIERS. 115 



partly glide over its surface, and partly rest on the solid rocky 

 base near the ice. But when two glacier streams unite, their 

 coinciding lateral moraines come to lie upon the centre of the 

 united ice-stream, and then move forward as central moraines 

 parallel to each other and to the banks of the stream, and they 

 show, as far as the lower end, the boundary -line of the ice which 

 originally belonged to one or the other of the arms of the glacier. 

 They are very remarkable as displaying in what regular parallel 

 bands the adjacent parts of the ice-stream glide downwards. A 

 glance at the map of the Mer de Glace, and its four central 

 moraines, exhibits this very distinctly. 



On the Glacier du Geant and its continuation in the Mer de 

 Glace, the stones on the surface of the ice delineate, in alternately 

 greyer and whiter bands, a kind of yearly rings which were 

 first observed by Forbes. For since in the cascade at g, Fig. 21, 

 more ice slides down in summer than in winter, the surface of 

 the ice below the cascade forms a series of terraces as seen in. 

 the drawing, and as those slopes of the terraces which have a 

 northern aspect melt less than their upper plane surfaces, the 

 former exhibit purer ice than the latter. This, according to 

 Tyndall, is the probable origin of these dirt bands. At first 

 they run pretty much across the glacier, but as afterwards their 

 centre moves somewhat more rapidly than the ends, they 

 acquire farther down a curved shape, as represented in the 

 map, Fig. 19. By their curvature they thus show to the 

 observer with what varying velocity ice advances in the dif- 

 ferent parts of its course. 



A very peculiar part is played by certain stones which are 

 imbedded in the lower surface of the mass of ice, and which 

 have partly fallen there through crevasses, and may partly have 

 been detached from the bottom of the valley. For these stones 

 are gradually pushed with the ice along the base of the valley, 

 and at the same time are pressed against this base by the 

 enormous weight of the superincumbent ice. Both the stones 

 imbedded in the ice as well as the rocky base are equally 

 hard, but by their friction against each other they are ground 

 to powder with a power compared to which any human exertion, 

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