BY E. C. ANDREWS. 795 



and towards the interior west of a meridional line passing through 

 Blayney or Orange. To the 3,100 feet level and its coastal slopes 

 we propose the name of the Lithgow Peneplain; to the 3,500 

 (approximate) feet level the name of the Blue Mountain Plain.* 

 To tlie 4,100-l:,200 feet level the name Jenolan Plain seems 

 appropriate from the occurrence there of numerous residuals of 

 that old high-level surface. Above the Jenolan level very small 

 hills are found, such as the Sunny Corner Mountain (Plate 

 xxxix). 



Fig. g of Plate xxxix represents the relative slopes and mesas 

 of each surface approximately. 



More detailed observation may hereafter demand the fusion of 

 the Blue Mountain and Lithgow Plains, with reference of the 

 3,000 feet level at Lithgow, Bowenfels, Rydal and in the neio-h- 

 bourhood of Mount King George to "benching" agencies. The 

 great extent of the 3,000 feet surface, however, in the contorted 

 Palasozoic rocks to the near west is against this idea of a common 

 age for the two surfaces. 



Thus it would appear that three peneplains have been developed 

 near sea-level, and successively raised 700, 400 and 3,100 feet 

 approximately above this datum line.f 



From a study of the present disposition of the streams we 

 should feel constrained also, even should the evidence of the 

 mesas be withheld, to postulate at least a double cycle of erosion 

 for the area under consideration, so marvellousl}^ are the streams 

 adjusted to the rock structures. Reference in detail to this will 

 be made in the proper place. Subaerial erosion and elevation 

 thus appear to be the key to the formation of the successive 

 plateaus. 



At present the exact ages of these elevations and the cycles of 

 erosion initiated thereby cannot be fixed. Each cycle, especially 



* From the more commonly known peaks in the Blue Mountains which 

 form mesas of this level. 



t Allowance must be made in these figures in the central areas for the 

 incomplete reduction of a plateau to base-level. 



