GEOLOGY OF SOIL. 29 



marks the isotheral line. In the Swiss Alps, the grains 



cease growing at the following heights : 



Wheat at 3400 feet, corresponding to lat. 64 degrees. 



Oats " 3500 " " " 65 



Rye " 4600 " " " 67 " 



Barley " 4800 " " " 70 



This shows a beautiful correspondence between latitude 



and altitude, and leads a step farther in the proof of this 



principle, that rocks do not affect the vegetation which covers 



them. 



29. The space which has thus been surveyed, presents, 

 amid great variety of rocks, a singular similarity in chem- 

 ical composition of the soil. 'J'hese facts lead to the third 

 principle of agricultural chemistry, rocks have not formed 



THE SOIL WHICH IMMEDIATELY COVERS THEM. 



30. Everywhere, with the exception of the tops of some , 

 mountains, the rocks of the globe are covered, from a fewj 

 inches, to some hundred feet in depth, with gravel, sand, 

 clay, rolled stones, sometimes alternately with each other, 

 sometimes in confused heaps. The best attested and most 

 universally admitted fact of geology, is, that the loose 

 materials of our globe have been transported, from a few, to 

 many hundred miles from their original situation. With a 

 few exceptions, the soil, which now covers rocks, has been 

 derived from places distant, and from rocks distinct, from 

 those on which it now reposes. This is peculiarly true of 

 soil on limestone districts, which does not contain more lime 

 than the soil reposing^ on granite. 



31. Transportation of soil is a fact so well established, 

 that it needs only to be mentioned. There has been a uni- 

 versal mingling of the loose material, soil, derived from 

 worn-down and mingled rocks. 



32. The same uniformity of chemical composition charac- 



