CLASSIFICATION. 



229 



M. Gasparin has published analyses of the soils of the south of 

 France. Instead of destroying the humus and organic matter by 

 calcination, as Davy and the generality of analysts did, M. Gasparin 

 dissolved out the humus by means of a strong alkaline solution. 

 This method of procedure is, however, at least as liable to objection 

 as the other. 



There is an important element which must always be taken into 

 the account in estimating the value of soils, no matter what their 

 special composition ; this eleirient is their depth, or thickness. In 

 running a deepish furrow in a cultivated field, we generally distin- 

 guish at a glance the depth of the superficial layer, which is com- 

 monly designated as the mould or vegetable earth ; this is a layer 

 generally impregnated with humus, and looser and more friable than 

 the subsoil upon which it rests. The thickness of this superficial 

 layer is extremely variable ; it is frequently no more than about 3 

 inches ; but it is also encountered of every depth from 3 or 4 to 12 

 or 13 inches. It must be held an exceptional and unusual case when 

 it has a depth of 3 feet or more. Nevertheless "we do meet with 

 collections* of vegetable soil of great depth, deposited by rivers, 

 washed down into the bottoms of valleys, or accumulated on the 

 surface, as in the virgin forests or vast prairies of America. Depth 

 of mould, or vegetable soil, is always advantageous ; it is one of the 



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