344 H. I. JENSEN. 



accepted by surveyors. Support is lent to this theory by 

 the fact that gilgai country occurs only along the belt of 

 heavy Tertiary and Post-Tertiary alluviation, at the western 

 base of our western slopes. The deep detrital accumula- 

 ions of this belt are no doubt settling down and being 

 rendered more compact by their own weight. But a sub- 

 sidence due to this cause can hardly be expected to give 

 rise to the vermiform depressions and tortuous hummocks 

 observed in the gilgais. The whole surface would be 

 expected to subside at a uniform rate, and if ridges were 

 formed at all, one would expect them to have a definite 

 alignment and to be of a gentle nature. 



It has been suggested that the depressions are due to an 

 underlying limestone formation, which is being removed by 

 subterranean waters and causing overlying soils to sink 

 into the hollows so formed. This suggestion can be dis- 

 missed with the statement that borings in the district show 

 no limestone in the substrata but only sands and clays. 

 Nor is there a scrap of geological evidence to favour the 

 suggestion. It has also been suggested that subterranean 

 streams in the underlying subartesian strata might be 

 undermining the surface soil, bringing about its collapse. 

 No such streams have been met with, nor could such a 

 cause produce the tortuosity of the gilgais over such a 

 wide area. 



2. Alternate expansion and contraction of clay lands from 

 wetting and drying is known to produce an uneven surface. 

 The inequalities produced in this way seldom exceed six or 

 twelve inches. On p. 114, 'The Soil,' by Hilgaard, such 

 country is described under its American name ' hogwallows.' 

 This cause by itself could not produce the gilgais, though 

 it might give a start to the formation of a hummocky sur- 

 face which might develop into gilgai from other causes. 



