EFFECTS OF DECOMPOSITION. 253 



however, should appear to savour of volcanism, 

 ]et it be considered that we are on dangerous 

 ground ; for we now approach the volcanic 

 domain. 



The decomposition and ruin of mountains 

 forming one of the grandest features in the his- 

 tory of the earth, a few examples may be sub- 

 joined ; which shall be introduced by some ob- 

 servations of the greatest, of petralogists, upon 

 this singular and important topic. 



" Another fact, of which I discovered the J Nature of 



decomposition. 



solution by examining these granites close and 

 attentively, is that of those exfoliations which I 

 had observed in the upper valley. It is a fact 

 known by all mineralogists, that most rocks are 

 softer in the interior of mountains than at their 

 external part ; and that in the air they acquire 

 a considerable degree of hardness. It hence fol- 

 lows that the external part, or the edge of the 

 vertical section of a large layer of granite, ought 

 to harden by contact with the air, whilst the 

 interior of the same layer retains a certain de- 

 gree of softness. And so long as the lower 

 layers remain a little soft, the enormous weight 

 of all those that rest upon them, must in time 



