42 THE FARMEKS' HANDBOOK. 



Geological Notes on the Black soil Plains. 



With regard to the origin and general geological features of black-soil 

 plains, a few notes may be of interest. Black soils may be divided broadly 

 into four classes, namely — (a) Peaty black soils, (b) basaltic black soils, 

 (c) alluvial black soils, (d) black limestone soils. These classes are not 

 sharply differentiated, but intermediate varieties occur. 



(a) The peaty black soil occurs in swamps, and owes its dark colour 

 essentially to the presence of organic matter. If such a soil occurs in a 

 sandstone area it may consist almost wholly of quartz sand (silica), and 

 carbonaceous material. On ignition it will, in such a case, become ligin 

 grey or white in colour. If the soil occurs in any formation abounding in 

 iron-bearing minerals it will, of course, take a red colour on burning. 



(b) Basaltic black soils occur on basaltic plains and tablelands, and 

 often cover very extensive areas, as on the Darling Downs of Queensland, 

 and on the Liverpool Plains of New South Wales. The presence of an 

 abundance of lime-bearing minerals in the parent rock has, probably, much 

 to do with the formation of the black colour, since the lime is readily 

 liberated on the weathering of the rock, and its presence in the soil as 

 carbonate would favour nitrification and accelerate plant growth, and the 

 humus formed by decaying vegetation partly tends to blacken the soil in 

 itself, and partly acts so as to reduce iron oxide to the black ferroso-ferric 

 condition (magnetite), and also so as to form black organic salts with iron. 

 Generally basaltic soils tend to posssess a brownish colour on hillsides, and 

 a black colour on flats and in valleys, where vegetation is more luxuriant. 



(c) Alluvial black soils, comprising those here tabulated and discussed, 

 known in the west as river country, are generally formed of sediments carried 

 hundreds of miles by rivers from various geological formations. Their 

 black colour is probably due to the same cause as in the case of basaltic soils. 

 They differ from the latter in being more loamy in character, and more 

 pervious to water. 



West of the Warrumbungle Mountains, at Tundebrine, Tooraweenah, 

 Tenandra, &c, and west of the Nandewar Mountains, near Narrabri, at 

 Maule's Creek, Spring Creek, Bobbiwaa Creek, &c, occur miniature black- 

 soil plains, whose black soils belong to a type intermediate between (&) and 

 (c), although in general appearance and in flora they are very like the typical 

 alluvials (river country) of the Namoi and Castlereagh. These soils are 

 composed mainly of volcanic detritus, which has not formed by the decom- 

 position of volcanic rock in situ, but which has been carried there by streams 

 from the adjoining volcanic mountains. 



(d) Limestone formations are also not infrequently covered with black 

 soil, the colour of which may be due to the causes already mentioned. 



In addition to the four classes mentioned, the soils on diorite and 

 gabbro formations are also frequently black loams. They are like the basalt 

 soils — brownish on the hills, becoming black on plains and in valleys, and 

 differ only from basaltic soils in being less clayey. 



SOILS FROM THE SEMI ARID REGION. 



In the table on page 34 is given the general characteristics of an average 

 soil (No. 6) from this area, which includes a large portion of the present and 

 the potential wheat-growing area of the State. In the following table the 

 nature of these soils is set forth in more detail. 



