THE NATURE AND ORIGIN OF GILGAI COUNTRY. 341 



larity from the hollows, and their precipitation by the 

 evaporation of the water in a zone where the effects of 

 dew and rainfall by downward percolation are negligible. 



The chemical nature of these soils is remarkable. They 

 are alkaline in reaction on the hummocks, but the hollows 

 may be acid. The gilgai soils proved on analysis (Table B.) 

 to be far from well balanced, the amounts of phosphoric 

 acid and sometimes potash being low in proportion to the 

 lime (Table B). They are very high in soda and magnesia, 

 and also in manganese oxide. The amount of soil insoluble 

 in acid is lower than in the case of normal black soils. 



The water extract (Table 0) contains in addition to 

 colloidal clay a considerable amount of salts, chiefly sodium 

 carbonate and common salt. Water-soluble magnesia and 

 lime are practically absent in the hummock soils, being 

 insoluble in alkaline solution. The sulphuric acid radicle 

 is quite absent, no reaction for that ingredient being 

 obtainable. 



The volatile matter in the gilgai soils ranges^ from 4*50 

 to 9*86 per cent., averaging 6*42 per cent., but very little 

 of this, probably not more than 3 per cent, is organic. The 

 balance is combined water contained in the colloidal clay. 

 Nitrogen is in most cases low, averaging *070 per cent. 



Occurrence of Manganese. — The manganese oxide 

 {Mn 3 4 ) not carried down with iron, was estimated in the 

 gilgai soils and is generally speaking high. Manganese 

 being one of the most soluble constituents of acid soils tends 

 to accumulate in undrained depressions. In the numerous 

 complete analyses of soils given by Hilgaard in his book on 

 the 'Soil,' the percentage of manganese seldom exceeds 

 '020 in leached soils like the Hawaiian volcanic soils or 

 those of the Oalifornian orchard slopes ; in other soils 

 manganese ranges from '010 to '300 per cent, averaging 

 *100, the humid soils giving a slightly higher average than 



