64 THE DENUDANTS OF 



pens in polishing a stone. At first the polishing putty 

 removes all loose particles, but afterwards it not only 

 smooths but also hardens the surface, and we find 

 that polished marble bears the weather longer than 

 a plainly cut limestone, or a sawed sandstone better 

 than a chisel-dressed one. 1 The latter case is well 

 exemplified in buildings, such as those of the colleges 

 in Oxford. There it may be observed that the saw- 

 ing of the sandstones has formed on the outside of 

 the blocks a thin sheet or coat, impervious to the 

 weather ; consequently the weathering does not take 

 place from the outside surface, but has to work side- 

 ways in under this shell of rearranged particles ; but 

 once they peel off, meteoric abrasion gradually wears 

 away the rest of the stone. The action of ice, how- 

 ever, would seem to be more like that of polishing, 

 as the hardening process appears to extend much 

 deeper into the rocks, and instead of forming only a 

 hardened shell or envelope, the effect seems gradu- 

 ally to die out in depth. If, therefore, a glacier is 

 formed in a valley, the rocks of which have been for 

 years exposed to meteoric abrasion, the work done 



1 In burnished metal there is undoubtedly a rearrangement of 

 particles, and 'this seems to be so in highly-polished marble also, but it 

 has been questioned if such can be in ice-smoothed surfaces. We 

 have, however, carefully examined into the subject, and find that in 

 sandstones, such as the carboniferous sandstone of Mayo, a thin shell 

 of rearranged particles forms on the surface of the rock. In such rocks 

 as limestone there is a rearrangement and hardening gradually dying 

 out downwards, while in such rocks as granite we could detect none. 



