350 H. I. JENSEN. 



shore and undergoes quiet evaporation. In this way the 

 knolls would grow in size especially as the fine matter 

 carried down by the small streams would tend to deposit 

 both its dissolved and suspended material chiefly on the 

 tussocks. 



One can state with absolute certainty that the gilgai 

 soils were not deposited by normal streams from the War- 

 rumbungles either in the late Tertiary wet period or in the 

 present semiarid period. Both the floods of the former and 

 of the latter are chiefly responsible for sandy material 

 such as we see along present creek beds or in the alluvial 

 strata underlying the gilgai at a depth of 30 to 40 feet. 



While I cannot agree that the hummocky surface of gilgai 

 country is due to subsidence of underlying strata, I am 

 strongly inclined to think that the formation of large basins 

 which developed into gilgai areas was partly due to sub- 

 sidence of late Tertiary sands and gravels of a loose nature 

 which have a great thickness under these areas. It is true 

 that these depressions always occupy sites along a belt of 

 our western slopes which is largely downfaulted, but the 

 faulting was probably in progress in the wet cycle of our 

 late Tertiary period and responsible for the thick accu- 

 mulation of detritus deposited along this belt at that time. 

 The amount of subsidence in the arid period was probably 

 very slight, so slight that it can easily be accounted for by 

 the settling down of loose sediments. 



Other Evidences. 

 (a) West of the Warrumbungles. — On the plain country 

 westward from Tooraweanah and Tundebrine, on the 

 western side of the Warrumbungle Mountains, it is a com- 

 mon thing to find the present creeks separated by slight 

 ridges, each of which is capped by an old creek bed con- 

 sisting of boulders and heavy gravel. Almost invariably 

 the highest ground is a gravel ridge marking the course of 



