FORMATION OF ARABLE LANDS. 27 



yard manure will correct for a short time only this state of 

 a soil, and it is better to mix with it the sand, and coarse 

 gravel, which are necessary to restore it to fertility. 



It appears that the three earths, which form the basis of 

 the most fertile soil, enter into the composition of plants ; 

 Bergmann has proved this by analysis of several kinds of 

 grain; and Ruckert by the results of his experiments 

 upon a variety of vegetable productions, in a way to put 

 it beyond doubt. About 100 parts of ashes well 



All soils are not composed of the mixture of the three 

 most important earths ; some of them are formed by the 

 union of two, as of silica with alumina, or of carbonate 

 of lime with alumina, 6lc., and we occasionally find each 

 one of them combined separately with quartzeous or cal- 

 careous gravel, forming land which may be cultivated. 



It is seldom that the soils of which we have spoken in 

 the preceding paragraph, are composed solely of the two 

 substances referred to; but these so far exceed in impor- 

 tance all the others which enter into the mixture, as to give 

 a character to the whole, which the latter cannot much 

 affect. 



The mixture of silex with alumina forms that soil called 

 clayey, argillaceous, or simply clai/. The properties of 

 the alumina predominate in all clayey soils, which are 

 less fertile in proportion to the increased quantity of it 

 which they contain ; when it equals or exceeds one half, 

 they are only fit to be employed as the basis of some kinds 

 of earthen ware ; especially if the other moiety consist of 

 silex finely pulverized. 



I have had occasion to analyze three specimens of clay 

 taken from three fields situated upon a plain, formed al- 

 most wholly of argillaceous marl ; the first afforded 



Silex in grains 17 



Alumina 47 



Silica 21 



