by the passage over them of Sharp Detrital Matter. 1 19 



thickness, contributes more or less to the formation of ter- 

 minal moraines. This bed proceeds from the fragments of 

 rocks which fall under the glacier, either through its nume- 

 rous cracks or by its margins, and are triturated into minute 

 particles by the grinding action of the glacier as its moves 

 along its channel. When glaciers move over granitic rocks, 

 this bed is composed of very fine, white, and very loose sand ; 

 when, on the contrary, the moraines, which supply the mate- 

 rials, have proceeded from calcareous or slaty rocks, the bed 

 is dark and pasty. It is to the small gravel contained in this 

 intermediate bed that the characteristic striae of polishing 

 rocks must be ascribed. In the upper valleys, this bed is 

 frozen and adherent to the ground, whilst in the lower it is 

 thawed. Independently of this bed of sand or mud, it is not 

 unusual to meet under the glaciers a bed, more or less con- 

 siderable, of small rounded blocks, varying from the size of an 

 ordinary pebble to a diameter of six inches or a foot. These 

 rounded boulders, which have been evidently triturated and 

 worn like the sand by the movement of the glacier, strongly 

 resemble the gravel beds of what have been called diluvial de- 

 posits, and were they not so clearly connected with the glacier 

 action, would be ascribed to powerful torrents. These beds 

 of pebbles vary considerably in different glaciers, and are 

 specially well exhibited under the glacier of the Trienfc, where 

 it is manifest that they proceed from the detritus of the 

 sides of the valley, and are renewed continually as the more 

 ancient portion of the bed is pushed forward to the lower 

 part of the valley — a fact which negatives the supposition 

 that the glacier may have been formed on a tertiary de- 

 posit." 



M. Agassiz also points out that fragments encased in the 

 ice act as files or rasps upon the rock when the glacier is in 

 actual contact with it — a mode of operation which has been 

 reasoned upon, whilst the other, or that in which a mass of 

 detritus is moved with and under the glacier, has almost 

 escaped notice. 



Agassiz indeed says — " We have seen that the bed of mud 

 and gravel, which is intermediate between the glacier and the 

 bottom of the valley, contains a quantity of small fragments 



