166 BOARD OF AGRICULTURE. 



worked upon by water. You examine, for instance, the loose 

 materials wliich lie upon the beach, and which are tossed about 

 by the tides and by the currents. These loose shingles are water- 

 worn. They have smooth surfaces, it is^trne, but they are made 

 smooth by the sviccessive pounding of the fragments against one 

 another. Their surfaces become smooth as you might smooth 

 a stone by hammering it for a long time. You may continue 

 to hammer it until the surface is very smooth, and yet you will 

 not give it a polish in that way. In order to obtain your polish 

 you must have a rubbing over the surface by a body which 

 moves at a different rate from that of the body which is sub- 

 mitted to the process. That is what takes place under the ice. 

 The fragments of rock are held by the ice, at least for some 

 time, just as a precious stone is held in its setting, firmly, and 

 then moved over the rock, and the surface which is brought 

 into contact with the rock is thus polished. Those materials 

 which have already been ground furnish a kind of paste, which 

 adds to the polishing power of the rock, and the harder mate- 

 rials will produce scratches upon the polished surface. Now, 

 the loose pebbles which you find under glaciers have a polished 

 surface, and have scratches over them, in various directions, 

 owing to the fact that these loose materials may be turned in 

 their setting as the mass of ice moves on, and therefore every 

 side is in succession brought into contact with the immovable 

 rock, and tlnis they are equally polished on all sides, and 

 scratched in every direction, by lines which cross one another 

 in every direction. Such pebbles as you find with this appear- 

 ance under the surface of the glacier you never find in a stream 

 or current of water, because the action is different. There is 

 nothing in the current to hold the stone in a fixed position, and 

 tliercfore no possibility for the stone to be polished. But while 

 these loose materials are thus ground and polished and scratched, 

 they are working upon a solid, rocky foundation whicli is im- 

 movable ; and upon that immovable surface they in their turn 

 produce a mechanical action, and that mechanical action con- 

 sists in wearing the surface to a smooth, even plain, and in pol- 

 ishing it and furrowing it and grinding it in the direction in 

 which the motion takes place. In a valley which comes down 

 from a high elevation to a lower level the whole bottom and 

 sides occupied by the ice are filed away, polished, scratched 



