ICE AND GLACIERS. 129 



stantly falling on the edge, but cannot fall on the middle. 

 Thus are formed on the edge of the mass of ice the lateral 

 moraines, the boulders of which partly move along with 

 the ice, 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 grayer 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 some- 

 what 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 different parts 

 of its course. 



A very peculiar part is played by certain stones which 



