60 THE DENUDANTS OF 



rapids and mountain torrents ; but it does not appear 

 capable of planing down a rock ; and ice, as icebergs, 

 may possibly be able to denude the sea-bottom, on 

 which none of the other forces can act, except, per- 

 haps, very strong sea currents. Many rocks, such as 

 doleryte, dioryte, diabase, granite, some limestones, 

 &c., are physically hard, while they are chemically 

 soft; others, such as slates, schists, clays, &c., are 

 physically soft, while they are very little susceptible 

 of chemical decomposition. The former, therefore, are 

 easily denuded by meteoric abrasion, and the latter 

 resist it. Also while a planing action, such as that 

 exercised by ice, would be resisted by physically hard 

 rocks, it could easily excavate and cut away the soft 

 ones ; and are not these the results found in nature ? 

 .In a country dressed, planed, and etched by ice, 

 all the physically hard rocks have formed features 

 from a few feet in height to hundreds or more ; but 

 since the ice has disappeared the meteoric agencies 

 have had their sway, and these rocks are now 

 weathering fast; while the chemically hard but 

 physically soft rocks those that under the old 

 regime suffered most are now almost perfectly 

 unmolested. 



Ice action, similarly to marine action, as before in- 

 timated, is principally a mechanical worker, wedging 

 out and quarrying masses of rock. It, however, seems 



