The cause or causes by which this change is produ- 

 ced, may be considered as two fold, (viz.) the one che- 

 mical, the other mechanical. 



One of the instances in which the decomposition of 

 mineral substances occur, by chemical causes, is that, 

 perhaps, of Kaolin, or rather Petunze, which, by the 

 quantity of potash it may contain, attracts moisture to 

 a degree sufficient to reduce the whole to the state of 

 Kaolin or a paste like substance. 



Another instance is such as is mentioned by Denon, 

 with respect to the mountains of the Lybian range, 

 where the decomposition of the rocks is promoted, it is 

 supposed, by the highly saline state of the air.* 



The decomposition of rocks is again, supposed to be 

 occasioned by a kind of lixivium that is formed by the 

 decomposition of vegetable substances, and which in- 

 filtrates through the superincumbent soil down upon 

 the rocks, producing thereby a kind of chemical de- 

 composition, differing in some degree from the common 

 process of disintegration. 



There are some instances which, at first view, are 

 calculated to favour this opinion, and give to it a great 

 degree of plausibility but a more general investiga- 

 tion of the subject will not justify the conclusion. 



For example It has often occurred that the soil, or 

 earth on the sides of mountains, being almost saturated 

 with water, slides down, in immense bodies covered 



* Denon's Travels, vol. 2. p. 2d. 



