296 FKAGMENTS OF SCIENCE. 



river more or less voluminous, charged with the matter 

 which the ice has rubbed from the rocks. Where the 

 rocks are soft, the amount of this finely pulverised 

 matter suspended in the water is very great. The 

 water, for example, of the river which flows from 

 Santa Catarina to Bormio is thick with it. The Ehine 

 is charged with this matter, and by it has so silted up 

 the Lake of Constance as to abolish it for a large 

 fraction of its length. The Ehone is charged with it, 

 and tens of thousands of acres of cultivable land are 

 formed by the silt above the Lake of Geneva. 



In the case of every glacier we have two agents at 

 work the ice exerting a crushing force on every point 

 of its bed which bears its weight, and either rasping 

 this point into powder or tearing it bodily from the 

 rock to which it belongs ; while the water which every- 

 where circulates upon the bed of the glacier continually 

 washes the detritus away and leaves the rock clean for 

 further abrasion. Confining the action of glaciers to 

 the simple rubbing away of the rocks, and allowing 

 them sufficient time to act, it is not a matter of opinion, 

 but a physical certainty, that they will scoop out valleys. 

 But the glacier does more than abrade. Rocks are not 

 homogeneous ; they are intersected by joints and places 

 of weakness, which divide them into virtually detached 

 masses. A glacier is undoubtedly competent to root 

 such masses bodily away. Indeed the mere d priori 

 consideration of the subject proves the competence of a 

 glacier to deepen its bed. Taking the case of a glacier 

 1,000 feet deep (and some of the older ones were 

 probably three times this depth), and allowing 40 feet 

 of ice to an atmosphere, we find that on every square 

 inch of its bed such a glacier presses with a weight of 

 375 Ibs., and on every square yard of its bed with a 

 weight of 486,000 Ibs. With a vertical pressure of 



