THE NATURE AND ORIGIN OP GILGAI COUNTRY. 343 



accumulates in them, the manganese may creep up too, 

 but is washed back to the depressions every time a spell of 

 wet weather brings about a decay of leaves and a restor- 

 ation of surface acidity. The manganese therefore tends 

 to accumulate in the less alkaline depressions. The accu- 

 mulation of manganese in these is evidence of absence of 

 subdrainage, for if any escape existed the manganese 

 would be carried away in the faintly acid soil water which 

 may accumulate here. 



The capillary power of most of the gilgai soils is very 

 poor, though the small patches of faintly acid red gilgai 

 country may have very good capillary power. Small as 

 the amount of sodium carbonate is in the alkaline soils it 

 is yet sufficient to destroy their mechanical condition. The 

 water capacity is highest in the case of the alkaline, 

 colloidal clay soils. 



V. Origin of Gilgai Country. 



A number of theories have been advanced to explain the 

 uneven nature of gilgai country. The most accredited are : 



1. Collapse of substrata causing a breaking up and 



partial subsidence of the top soil. 



2. Expansion and contraction due to alternate wetting 



and drying. 



3. Removal of soluble soil ingredients, as for instance 



lime, by percolation into the underlying sandy, 

 subartesian strata. 



4. Wind action. 



5. Effect of vegetation. 



6. Effect of sodium carbonate in destroying soil crumbs, 



and causing partial collapse of the soil. 



7. Mud springs. 



1. The theory that the irregularites in surface are due 

 to faulty substrata, or subsiding substrata, is widely 



