448 



*' Jand," but Mr. Klrwan (hews, that foils are often of a different 

 nature from the flratum they reft upon; and I add, from my own 

 obfervation, that the foil covering our bleak Jhigh rocks (generally a 

 fpungy mofs) was not formed by the deftruftion of the materials it 

 refts upon, to wit, found bafalt. 



That this foil is the great fource of the detritus, or mud, carried 

 down by rivers, we all three agree ; but we totally differ from each 

 other as to the materials from which this foil is formed. 



Dr. Hut ton from his definition of a foil, and the paffages jufl: quoted, 

 fliews, that he confiders it as formed from the decompofition of the 

 fubjacent materials, regularly fupplying the place of the foil, or fuper- 

 ftratum, which is conftantly moving away to the unfathomable regions 

 of the ocean. 



Mr. Playfair derives it from fomewhat a different fource, admitting in 

 like manner, " that it is continually diminiflied ;" yet ftates it ^j «/a^, 

 " that the foil notwithftanding remains the fame in quantity," and pro- 

 ceeds, " the foil therefore is augmented from other caufes, jufl; as much 

 " as it is diminiflied, and this augmentation evidently can proceed 

 " from nothing but the conftant, and flow difintegration of the rocks ; 

 " in the permanence therefore of a coat of vegetable mould, on the 

 " furface of the earth ; we have a demonftrative proof of the continual 

 " deftruffion of the rocks." (Illus. Page io6.) 



I am afraid that in this paffage, Mr. Playfair miftakes ajfertion for 

 demonfiration. 



I confider that the fuperficial covering of the earth, called the foil, 

 is formed from the decayed parts of vegetables, and animals ; which 

 will be found to afford an ample fund for the repair of the loffes that 

 all parties agree it fuftains. "^^ 



Though the decayed parts of animals are known to pafs into 

 earth, and of courfe, to augment the foil, I will limit myfelf to ve- 

 getables alone, conceiving their contribution, on this occafion, to be much 

 more abundant. That 



