ACTION OF THE WAVES AND OF TIDES. 131 



without friction to considerable distances from then place of origin. 

 These debris, from several causes, always accumulate on the late- 

 ral parts of the glacier, against the side of the valley, and fre- 

 quently in the middle also, from other valleys emptying laterally 

 into it, from which result long, slender hills, designated under the 

 term moraines. All these debris, having reached the inferior ex- 

 tremity of the glacier, tumble into the valley on its slope, and form 

 at its foot other moraines often of considerable height. If, after 

 having increased for a certain time in consequence of a series of 

 cold summers, the glacier diminishes again by a succession of 

 warm, prolonged summers, the moraines of different kinds, aban- 

 doned by the ice, are left on the soil ; some form dykes, of more 

 or less height, at the bottom and across the valley, and others long 

 lines on the flanks of the valley, at a greater or less elevation. 



19. It must be borne in mind that the slopes on which glaciers 

 move are always much greater than those of rivers, and that they 

 never descend at an angle of less than three degrees. This must 

 also be the minimum slope of masses of debris resting on the sides 

 of the valley, in consequence of the rapid melting of the glacier. 

 Thus we have a means of distinguishing the remains of lateral 

 moraines from deposits which may have been made by water-cur- 

 rents, the slopes of which are very much less. 



20. Strive, channels, polishing of rocks. Among the effects 

 produced by the motion of a glacier loaded with debris, and moving 

 slowly over the exposed face of a rock, is a rubbing, wearing, and 

 polishing of the surface which is passed over. The angles of the 

 rocks passed over are rounded ; deep undulating grooves, nearly 

 parallel and longitudinal, are cut in the surface, and the polished 

 surface of the rock passed over is scratched with fine striae, even 

 when it is of the hardest quartz. These effects are well known 

 to be produced by modern glaciers. 



21. Action of the waves and of tides. Waves exert an enor 

 mous power, particularly where rocks are abrupt and directly ex- 

 posed to the open sea. The shock is sometimes so great that the 

 earth trembles beneath the feet ; great blocks of stone are torn 

 up and carried far inland, pushed up against the inclination of 

 the shore, sometimes thrown up vertically on projecting points, 

 where they afterwards roll about like small pebbles : heavy banks 

 of sand and of shingle are often removed, and entire countries 

 have been in a moment destroyed. 



Chronology and tradition of maritime countries furnish numerous in- 

 stances of successive changes, of instantaneous disasters which have oc- 

 curred in a great many localities. Immense ones have taken place, and 

 every day new ones occur on low, sandy coasts, bordering the sea, in many 



19. What is the least slope or angle at which glaciers move ? 



20. What effects are produced on rocks by the movement of glaciers 

 loided with debris ? 



ai. What is the effect of the action of waves? 

 32* 



