368 



T. W. E. DAVID. 



mines when coal comes to be worked in the vicinity, and 

 it is satisfactory to hear that the Geological Survey of 

 New South Wales will map the course of the fault and the 

 eastern fold, and show their further extent on the geological 

 map, now being prepared, of the Sydney and Blue Mountain 

 areas. From a geotectonic point of view the fault and 

 monocline are interesting illustrations of the principle 

 enunciated by Suess, 1 that where lateral pressure has pro- 

 duced a monoclinal fold, faulting usually takes place on the 

 inner limb of the fold, (i.e., on the portion nearest to the 

 direction from which the folding force is coming) after the 

 folding force has ceased to act, such faulting resulting in 

 downthrows of the inner limb. 



It would appear that the first stage in the evolution of 

 these geotectonic structures in the portion of the Blue 

 Mountains discussed in this paper was the formation of the 

 chief monocline between Kurrajong Heights and Lapstone 

 Hill, with the concurrent sinking of the coastal plane 

 between Penrith and the sea. It is not yet clear whether 

 the forming of the monocline at Mount Tomah was con- 

 temporaneous with the preceding or took place earlier or 

 later. The folding force came from a westerly direction, 

 and it pushed the strata eastwards forming the steep 

 easterly slopes of the Kurrajong and Lapstone Hill, a slight 

 fold facing the west developing to the west of this eastern 

 monocline. After the folding force had become less intense 

 or had ceased, a fracture formed along this western fold, 

 the plateau to the west subsiding over 400 feet as the 

 result, while the development of the monocline at Mount 

 Tomah depressed the western portion of the plateau by a 

 vertical amount of about 250 feet. 



The fact that the top of the eastern monocline eventually 

 attained to three times the height (1,964 feet) above sea- 



1 Das Autlitz der Erde, Vol. I., p. 142-307. 



