AVITH AGRICULTU11E AND PLANTING. 473 



felspar is commonly converted by decomposition into a 

 hydrate or ochre. The carbonate of iron, as well as of 

 manganese, which sometimes occur in rocks, in limestone 

 rock for example, are deprived of carbonic acid by the 

 oxidation of their bases. Charcoal and bitumen, which 

 are sometimes contained in rocks, limestone and argil- 

 laceous ones for example, are dissipated by the contact 

 of air, so that rocks which were originally of a dark 

 colour, lose it, and become whitish. Water, as a chemi- 

 cal agent, contributes so much to the decomposition of 

 certain rocks, that, either in a pure state, or in combina- 

 tion with carbonic acid, it dissolves their parts, of which 

 gypsum and limestone afford examples. In certain other 

 minerals, in felspar for instance, a separation of the consti- 

 tuent parts, produced by the contact of air and water, is 

 observed, the proximate cause of which has not hitherto 

 been discovered. The mass is decomposed, its lamellar 

 structure is converted into an earthy nature, the alkali 

 contained in the felspar is extracted by the water, a mine- 

 ral is produced, to which the Chinese have given the name 

 of Kaolin, and which is adapted for the manufacture of 

 porcelain. Granite and gneiss occur in some districts, 

 the felspar of which is decomposed in this manner 

 through the whole mass, a circumstance which must be 

 of great importance in regard to the formation of pro- 

 ductive soil. 



Cryptogamic plants covering the surface of rocks, and 

 thriving well in this situation, where more perfect vege- 

 tables could not grow, seem also destined to promote the 

 chemical decomposition of rocks, an effect which they 

 produce both directly and indirectly. As they imbibe 

 the water of the atmosphere, and retain it like a sponge, 



