THE NATURE AND ORIGIN OF GILGAI COUNTRY. 355 



Secondly — There appears to be good reasons for believing 

 that in quaternary times a number of semiarid periods and 

 arid periods have alternated, just as did the glacial and 

 inter-glaeial periods of the last Ice Age. For some of the 

 red clay bands in the upper clay bands interbedded with 

 the late Tertiary alluvials and drifts underlying the plains 

 might well be wind blown deposits of arid intervals between 

 the wet periods which gave us the gravels. Again the 

 numerous 'monkeys' of the Castlereagh and other rivers 

 may each have had its course formed in a wet period, and 

 might have been filled with drift sand in the succeeding 

 dry period. This too is a plausible theory. 

 VII. Origin of Gilgais. 



It is possible that a tussocky herbaceous growth has 

 been aided by the contraction and expansion of clay soils 

 with wetting and drying, and by the chemical precipitation 

 of lime and maguesian salts on plant roots during periods 

 of drying up in the formation of a hummocky surface over 

 the gilgai area of the Pilliga Scrub. 



The question arises, if such an origin be assumed, why 

 did the inequalities not vanish when marshy conditions 

 disappeared with the restoration of integrated drainage ? 



Not only have the hummocks remained, but they have 

 become enlarged and the depressions have relatively 

 deepened. There can be little doubt that the salinity of 

 the Pilliga Scrub gilgais has been reduced by repeated floods 

 in the recent period. There can be little doubt that the 

 present flora of belar, brigalow and saltbush took root as 

 soon as moister conditions commenced and that these plants 

 have helped to preserve the hummocks and to resist their 

 collapse. 



But the vegetation could not have performed this work 

 without the assistance of another factor which has done 

 much to produce an accentuation of the hummocky surface. 



