130 TRANSPORTATION BY ICE AND GLACIERS. 



All the lower part of torrents, where the soil is sufficiently flattened, 

 or the enlargement of the valley permits the waters to expand, 

 diminishing their depth, and consequently their rapidity, is gene- 

 rally found covered with these flints, which are sometimes accumu- 

 lated in immense quantities, and through which, in its ordinary 

 course, the stream meanders in different ways, in a bed it forms 

 and often changes. Rivers and lakes into which torrents empty, 

 tind where they consequently lose their swiftness, are often loaded 

 with these flints ; and this is the cause of the constant elevation of 

 the bed of the river Po (see page 15). Gravel and sand, which 

 are merely small flints, the mud which results from their friction, 

 and the earthy particles removed, are always transported far, either 

 immediately into lakes, or seas, or rivers, which deposit them on 

 their banks, and especially at their mouths, which they more or 

 less obstruct. 



16. Rolled flints, or pebbles, are also formed by the action of the 

 waves on fallen rocks. In this way. on the coasts of France and 

 England, the silex, or flints of the chalk, are rounded, by being 

 rubbed against each other, and constitute considerable banks of 

 pebbles or shingle. Something similar must have taken place at 

 points now far inland, where we find blocks round and smooth, at 

 a short distance from rocks from which they were evidently de- 

 tached. 



17. Transportation by ice and glaciers. On the shores of 

 northern seas, the ice envelopes blocks and masses of rock, which, 

 at the breaking up, are floated away on ice-cakes in all directions, 

 and deposited here and there, wherever they may ground, or fall, 

 to the bottom of the sea. In this way, in Canada, Greenland, and 

 on the coasts of Nova Zembla, &c., very voluminous blocks are 

 transported from one place to another, and often to very conside- 

 rable distances from the point of departure. There is no doubt 

 that many small debris, embedded in the ice, are transported in the 

 same way, and form adventitious deposits of more or less extent. 



18. Glaciers, that is, beds of ice occupying the high valleys^ of 

 lofty mountain chains, are also very remarkable means of trans- 

 portation. Various circumstances (their great weight chiefly) 

 keep these deposits in constant, though very slow motion, from 

 half an inch to an inch an hour, descending along the slopes on 

 which they rest ; now, the surface of these glaciers is found to be 

 covered with fragments and blocks which have fallen from the 

 surrounding mountains, and the whole is conveyed from the upper 

 to the lower part ; and blocks, often of enormous size, are carried 



1 6. Are rolled flints, or pebbles, produced by running water exclusively ? 

 What is shingle ? 



17. How are rocks transported by ice? 



18. What are glaci.rs? At what rate do they move? What are 

 moraines ? 



