THE NATURE AND ORIGIN OF GILGAI COUNTRY. 349 



silting up of the outlets of numerous swamps and marshes 

 along the courses of stream channels. Subsequently wind- 

 action filled in these channels. In the Pilliga Scrub the 

 gilgai area, originally a depression, which was during the 

 wet cycle receiving Warrumbungle detritus carried down 

 by large rushing streams, became a sea without outlet, 

 and later, as the streams became extinct, a series of salty 

 marshes into which little trickles of water, highly charged 

 with solids, came from a great abundance of mudsprings in 

 the back country. 



The dark (black and dark grey) gilgai soils w T ere laid 

 down at this time. Their peculiar chemical and mechanical 

 nature shows that they are not true alluvial soils like those 

 of the Black Soil plain. Their high lime and magnesia, 

 without correspondingly high potash and phosphoric acid, 

 suggest that they were derived from the evaporation of 

 spring waters. Their salt and soda contents show accu- 

 mulation in an undrained basin, and this conclusion is 

 strengthened by the high manganese. 



Mudsprings might have existed locally over the gilgai 

 area, but probably the muds were chiefly supplied by springs 

 some little distance away on the flanks of the Ooghill hills 

 in the centre of the Pilliga Scrub. 



In these saline marshes various coarse grasses would 

 grow in tufts and give rise to inequalities of surface, which 

 would be increased by, (1) the greater shrinkage and crack- 

 ing on drying of the intervals between the tufts than of 

 the knolls which the roots would tend to hold together, 

 and by (2) the chemical deposition of carbonate of lime 

 round the grass roots and stems on every occasion that a 

 particularly droughty spell caused the evaporation of a 

 marsh. This kind of precipitation of lime we often see 

 w r here sea- water has been carried by spring tides into the 

 small swampy depressions behind the hurricane bank of the 



