204 



SOIL. 



esting to know the composition of the minerals which are the most 

 abundantly dispersed in the solid mass of the globe. 



The solid part of our planet, as is well known, occupies but one 

 third of its whole surface. The ocean occupies two-thirds, and the 

 majority of the rocks of sedimentary formation must have been pri- 

 marily deposited at the bottom of the sea. These rocks will there- 

 fore be apt to contain the saline substances which are met with in 

 sea-water, and it is a fact that many of the secondary sandstones 

 show unequivocal traces of these substances. Deltas and low downs, 

 left by the ocean, are constantly being brought under tillage, and the 

 fierce winds of the sea frequently carry saline matters to vast dis- 

 tances, even to the centre of great continents; lastly, .as we shall 

 see by and by, the ocean supplies agriculture with powerful manures. 

 Analysis shows that sea-water contains, besides chloride of sodium 

 or common salt, hydrochlorate of magnesia, sulphate of snd^ sul- 

 phate of magnesia, sulphate of lime, carbonate of lime, caroonate of 

 magnesia, and a quantity of carbonic acid, to which must be added 

 the substances discovered in the mother waters of salt marshes, and 

 which occur with reference to the others in quantities so small as to 

 escape direct analyses of any moderate portions of sea-water : these 

 substances are iodides, bromides, and certain ammoniacal salts. 



The minerals most generally found in rocks are quartz, felspar, 

 mica, amphibolite, pyroxenite, talc, serpentine, and diallage. 



Quartz is frequently composed of silica nearly in a state of purity ; 

 but I may save time by presenting in a single table the composition 

 of the principal mineral species such as we find it indicated by the 

 best chemical analysts : 



If we now compare the analyses of the ashes of vegetables which 

 we have already given with those just indicated, we see that the 

 mineral substances which meet us in plants ilsc exist in the soil in 

 dependenlly of any addition from manure. We nay therefore lay jl 



