FORMATION OF SOILS. 



tity of potash is set at liberty in the form of a carbonate 

 (Kuhlmann). 



The preceding considerations prove very clearly that arable 

 land has had its origin in the chemical and mechanical actions 

 exerted upon rocks and minerals rich in alkalies and alkaline 

 earths, by which means their coherence has been gradually 

 destroyed. It is scarcely necessary to furnish any further proofs 

 that all clays, whether they be pure or mixed with other minerals, 

 so as to form soils, suffer progressive and continued changes. 

 These changes consist in the giving of a soluble form to the 

 alkalies and alkaline bases, by the combined action of water and 

 of carbonic acid. This gives rise to the formation of soluble 

 silicates, or if these are decomposed by the carbonic acid, to the 

 hydrate of silica, which, being in its peculiar soluble condition, 

 may be taken up by the roots of plants. 



The influence of air, carbonic acid, and moisture, upon the 

 constituents of rocks, is best observed in certain uninhabited dis- 

 tricts of South America, where huntsmen and herds are the dis- 

 coverers of rich mines of silver. By the action of the weather 

 the constituents of the ores of silver are gradually dissolved and 

 carried away by winds and by rains ; the nobler metals resist the 

 destruction and remain on the surface. It is well known that 

 metallic silver veins are found in sharp angular projections from 

 the surface of the rock.* 



* Darwin states that the mine at Chanuncillo, from which silver to the 

 value of many hundred thousand pounds sterling has been obtained in a 

 few years, was discovered by a man who. in throwing a stone after a mule, 

 found it heavier than an ordinary stone ; it was a piece of solid silv< r, and 

 Was a fragment of a projecting vein of tl at metal. 



