ICE AND GLACIERS ftf 



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 



FIG. 112 



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 cas- 

 cade at g, FIG. 112, 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. no. 

 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 are 

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



