G2 M. Ch. Martins on the 



2. Rocks Polished and Striated hy existing Glaciers. 



Tlie friction of the glacier on its bottom and walls is too 

 considerable not to leave traces on the rocks with which it 

 comes in contact ; but its action is different according to the 

 mineralogical nature of these rocks, and the configuration of 

 the bed it occupies. If we penetrate between the ground and 

 the lower surface of the glacier, taking advantage of the ca- 

 verns of ice which sometimes open on its sides and extremity, 

 we creep on a bed of pebbles and fine sand mixed with water. 

 If we remove this layer, we find that the subjacent rock is 

 levelled, polished, worn by the friction, and covered with 

 rectilinear striae, sometimes resembling small furrows, more 

 frequently perfectly straight rays which have been cut by a 

 graving tool, or even a very fine needle. The mechanism 

 by which these striae have been engraved is the same that 

 human industry employs to polish stones and metals. By 

 means of a fine powder called emery we rub the metallic sur- 

 face, and give it a brilliancy which arises from the reflection 

 of the light from an infinite number of excessively fine striae. 

 The layer of pebbles and mud interposed between the glacier 

 and the subjacent rock, represents the emery. The rock is 

 the metallic surface, and the mass of the glacier, which presses 

 and displaces the bed of mud by continually descending to- 

 wards the plain, represents the action of the polisher's hand. 

 Accordingly the striae of which we speak are always in the 

 same direction as the progress of the glacier ; but as the latter 

 is liable to small lateral deviation, the stride sometimes cross 

 each other, and form with each other very small angles. If 

 we examine the rocks which border the glacier, we find the 

 same striae engraven on the parts which have been in contact 

 with the congealed mass. I have often taken pleasure in 

 breaking the ice which pressed upon the rock, and under the 

 ice I found the surfaces polished and covered with striae. The 

 pebbles and grains of sand which had engraved them were 

 still fixed in the glacier, as the diamond of the glazier is fixed 

 at the extremity of the instrument which he uses to cut glass. 



The distinctness and depth of the striae depend on many 

 circumstances. If the fixed rock be calcareous, and the 



