370 M. Agassiz on Glaciers, Moraines, and Erratic Blocks. 



are produced by faults, or by the sliding of strata upon one 

 another. The former of these, penetrating vertically or ob- 

 liquely across many strata, are not visible except when one of 

 the sides of the ruptured rock is sunk ; they are, moreover, 

 never seen over a somewhat extended surface, as are the laves : 

 the latter, or sliding strata, sometimes present surfaces which 

 are pretty extensive, when the beds above the slide have been 

 removed ; but then the grooves or furrows produced by the 

 sliding are in the direction of the slope, which is by no means 

 one of their characters on the surface of the laves. Surfaces 

 which are polished by the action of water also possess a pecu- 

 liar character, whether produced by running streams or by 

 more considerable masses of water contained in a lake, &c. 

 In the former case the sinuous furrows always descend ; whilst 

 the furrows and the gibbosities of the laves ascend and descend 

 with the accidental surface of the polished rock. In the latter 

 case, the waters, agitated near the shoi'e by the winds, and 

 raised above their habitual level, always regain their equili- 

 brium, and thus form unequal furrows, more or less deep, which 

 usually follow the line of the greatest inclination, at all events 

 when there are no local peculiarities to impart to them a dif- 

 ferent direction. This occurs when the waters of the lake are 

 high or low, in spring-time and autumn. All these difFei*ences 

 may be studied in the immediate neighbourhood of this city, 

 by comparing the polished surfaces of the Mail with the ero- 

 sions produced by the lake in the prolongation of the same 

 beds, or with the sinuosities which have been produced by the 

 Seyon in its ravines. I may add, that the polished surfaces 

 produced by the action of water are never so smooth as these 

 laves, or as the surfaces polished by the glaciers. When the 

 water is charged with sand or mud, the same effects are pro- 

 duced, with this modification, that they are not quite so marked. 

 I have not hitherto had occasion particularly to study the effects 

 produced by great masses of water charged with ice ; I believe, 

 however, they are much the same as those prt^duced by simple 

 water. One thing is clear, viz. that in the beds of our rivers, 

 and at the margins of our lakes, both of these effects are pro- 

 duced ; and it is m^oreover evident, that floating ice can pro- 

 duce no action at the under surface of the water that supports 



