16 MANUAL FOB SUGAR GROWERS. 



depth of soil is formed. The action of the weather 

 alone might give rise to a clay, the presence of vege- 

 table matter being necessary to render this a true soil. 



Eocks are divided into two classes, according to 

 whether they owe their origin to volcanic agency or 

 have been deposited in water ; and these facts have 

 an important bearing on the nature of the soil and 

 on the agricultural operations to which it should be 

 subjected. Apart from their geologic origin, soils 

 may be divided into clays, marls, and sands, and 

 this division is of immense service in agriculture. 



Clay has its origin in the decomposition of rocks 

 containing felspar, and when pure consists of sili- 

 cate of alumina; it very rarely occurs in a pure 

 state, being usually mixed with varying amounts of 

 sand, marl, etc. It is characterized by its coherence, 

 plasticity, want of porosity, and slow permeability 

 by water; many of these properties are disadvan- 

 tageous from an agricultural point of view, and re- 

 quire to be modified by judicious operations of tillage, 

 etc. Pure clay also contains no plant-food, but the 

 clays usuall}^ existing as soils contain considerable 

 admixture of substances of value. Clays may have 

 their origin in both volcanic and sedimentary rocks. 



Chalk, — or, as it is frequently termed, marl, — 

 when pure, is carbonate of lime, but, as existing as 

 soil, it owes its origin to the decomposition of coral, 

 shells, and the like, and thus contains a variety of 

 substances valuable as plant-food, particularly phos- 

 phate of lime. Marls, not containing much sand or 

 stony matter, tend to set into a stifi", clay-like soil, 

 which may be termed coral clay or marly clay ; and 



