36 AGRICULTURE. 



the residuum of dead animal or vegetable matter,* 

 they become fertile, take the general name of soils, 

 and are again specially denominated after the earth 

 that most abounds in their compositions respective- 

 ly. If this be silica, they are called sandy ; if alu- 

 mina, argillaceous ; if lime, calcareous ; and if mag- 

 nesia, magnesian. Their properties are well known : 

 a sandy soil is loose, easily moved, little retentive 

 of moisture, and subject to extreme dryness ; an ar- 

 gillaceous soil is hard and compact when dry, tough 

 and paste-like when wet, greedy and tenacious of 

 moisture ; turns up, when ploughed, into massive 

 clods, and admits the entrance of roots with great 

 difficulty. A calcareous soil is dry, friable, and po- 

 rous ; water enters and leaves it with facility ; roots 

 penetrate it without difficulty, and [being already 

 greatly divided] less labour is necessary for it than 

 for clay. Magnesian, like calcareous earth, is light, 

 porous, and friable, but, like clay, when wet, takes 

 the consistency of paste, and is very tenacious of 

 water. It refuses to combine with oxygen or with 

 the alkalies^is generally found associated with 

 granite, gneiss, and schist, and is probably among 

 the causes of their comparative barrenness.! 



quartz, alumina, and sometimes of calcareous matter. Specu- 

 lative geology is romance, and does not merit the name 01 sci- 

 ence ; yet is science obliged to borrow her theory of soils. The 

 alternation of heat and cold, moisture and dryness, decomposed 

 the mountains of primitive, secondary, and tertiary formation ; 

 rains, and the laws of gravity, brought these broken parts from 

 places of more to those of less elevation ; where, by mechani 

 cal mixture and chymical combination, the present substrata 

 were formed. But these were yet naked and unproductive, 

 when the Cryptogamia family (mosses and lichens) took pos 

 session of them, and in due time produced that vegetable matter 

 which made the earth productive and the globe habitable ! 



* Dead animal and vegetable matter, in the last stage of de 

 composition, give a black or brown powder, which the French 

 chymists call terreau or humus, and which Mr. Davy calls an ex 

 tractive matter ; this is the fertilizing principle of soils and ma- 

 nures. 



t The opinion is general among the chymists of Europe, that 



