BY H. I. JENSEN. 233 



long periods of dry weather, are characteristics common to 

 both regions. The only difference is that the Ortiz Mountains 

 are a dissected laccolite, whilst the Warrumbungles form a 

 dissected volcanic dome — a distinction of no importance. The 

 Warrumbungle Mountain district is, therefore, best regarded as 

 a conoplain. Like that of the Ortiz Mountains, it has been 

 formed by arid agencies. 



Professor W. M. Davis in the " Journal of Geology," ( Yol. xiii., 

 No. 5, July- August, 1905), has described the characteristic 

 features of arid erosion. 



In a mountainous district they are exactly the topographical 

 features met with in the Ortiz Mountains in the United States, 

 and in the Warrumbungle and Nandhewar Mountains in this 

 State. 



T have already shown that the Coonabarabran tableland, the 

 adjacent parts of the Pilliga Scrub, and the plain country to the 

 south and west of the mountains also, have the features of a 

 " desert plain," in whose degradation sand-blast action has played, 

 and is playing, an important part. This is proved by the 

 existence of " mesas," "buttes," and bad lands. 



An elevated "desert plain," according to Professor Davis, is 

 best regarded as having formed in situ by arid agencies. As no 

 evidences to the contrary have been found, such as Tertiary 

 marine deposits, it is best to consider the Warrumbungle 

 Mountains and the surrounding district to be a conoplain 

 ■dissected by arid agencies in Post-Tertiary and recent times. 



The plains west of the Warrumbungles owe their table-like 

 flatness to the fact that only wind erosion is going on. They 

 have the features characterising the "old age" of the "arid 

 cycle," whilst the Coonabarabran tableland displays those of 

 " maturity," and the mountains themselves those of " youth." 

 It is indeed highly probable that the peneplain, of which we have 

 evidence in the flat-topped mesas of Coonabarabran and the even 

 2,000 feet skyline of the Warrumbungle Range, was developed 

 itself by means of arid erosion. 



