THE CIIEMISTin OF I III. SOIL. 63 



Alluvial suits are found in the beds of old watercourses w un either Bide 

 of the banks of liowing streams, where they are deposited by the rise and 

 fall of the river, and especially during floods. The Hunter Valley, in X> w 

 South Wales, where the rivex is not only subject to periodic floods, but is 

 continually changing its course, affords an excellent example of a large area 

 of country covered by such alluvial soils. 



Alluvial soils are also met with in the plain country, where no rivers are 

 to be seen, and occupy the beds of dried-up watercourses, along which they 

 were originally deposited. 



Such alluvial soils are generally of great fertility, largely due to the fact 

 that the soil particles are exceedingly fine. Those formed by the deposition 

 of silt in successive floods upon the river banks are often of great depth, and 

 much of the best lucerne land of the State is found in such localities. 



Formation of Soils. 



Soils are produced in the first instance by the weathering of rocks. By 

 weathering is meant the continued action of wind, water, and changes of 

 temperature upon the rock, and the term includes the mechanical and 

 chemical changes brought about by these agencies, as well as by the action 

 of vegetable and animal life. 



The disintegration of rocks to soil is an exceedingly slow process. It has 

 been calculated that the disintegration of limestone rock (one of the most 

 readily-weathered rocks) to the depth of one metre would take over 70,000 

 years. Syenite would take ten times as long to disintegrate to the same 

 depth. 



Weathering of Rock. 



Action of Air — (a) Mechanical. — Air acts mechanically by the actual 

 pressure exerted by the wind upon exposed surfaces, and by the bombard- 

 ment of rock surfaces by wind-borne material such as fragments of rock and 

 pebbles. Its action is also of importance in the removal by wind of the dis- 

 integrated rock, leaving fresh surfaces exposed to the other disintegrating 

 agencies. It is also one of the principal factors in the formation of the 

 drift-soils mentioned above. 



(fe) Chemical. — The chemical disintegration of rock, due to the action of 

 the constituents of the atmosphere, is for the most part brought about by the 

 carbonic acid and oxygen. The action of carbonic acid is amongst the most 

 powerful of the agencies at work in the disintegration of rock, and since its 

 action is practically confined to the solution of the gas in water, either in 

 rain or in spring or in river water, it will be better discussed later under the 

 heading of water. 



The action of oxygen is practically confined to the further oxidation of 

 lower oxides of such metals as iron and manganese. A large number of 

 minerals contain ferrous oxide, to which they owe their dark colour. This 

 oxide really absorbs oxygen, being converted into ferric oxide (rust), the 

 change being accompanied by an increase in bulk which assists in shattering 

 the rock in which these minerals occur. 



Changes of Temperature. • 



The mechanical effects of alternate heat and cold are potent factors in the 

 disintegration of rocks, which, with few exceptions (such as limestone, 

 quartzite, &c), are made up of a mixture of different minerals. These 



