522 A CORRELATION OF CONTOUR, CLIMATE AND COAL, 



gloomy gorge with walls several hundred feet above its source. 

 It continues through similar gorges for many miles.* 



Consider now the action of a small consequent river while 

 cutting back through the comparatively soft strata at the limits of 

 the hard Triassic sandstones in the south of the Geosyncline. 

 We will suppose its mouth lies north of Jervis Bay. Tt gradually 

 erodes the rocks, striving to reach its base-level, while its bed 

 has a lower " coefficient of resistance " (to borrow an engineering 

 term) than that of the ancient Shoalhaven-Wollondilly River 

 which flowed chiefly through an area of hard Triassic sandstones. 

 There results a typical example of river-capture; the headwaters 

 of the northward-flowing river (Upper Shoalhaven) are deflected 

 into the young pirate stream (Lower Shoalhaven) and we get the 

 present Shoalhaven with its abrupt change of course as mentioned 

 above. 



Turning now to the northern exposure of the Coal Measures, 

 we see traces of a similar differentiation in river-erosion exem- 

 plified by the Upper Hunter and Goulburn Rivers. The Goul- 

 burn is the longer branch above Denman, its development being 

 due to the lesser resistance of the Permo-Carboniferous marine 

 strata. The Upper Hunter flows over older (Carboniferous Age) 

 rocks within a. comparatively short distance of Denman (the 

 junction town), while, as seen from the model, the Goulburn is 

 steadily eating into the main divide along the centre of the 

 Permo-Carboniferous basin. 



It is not difficult to predict the formation of an even more 

 definite " geocol " to the west of Cassilis before no very great 

 length of time (geologically speaking) has elapsed. It is pos- 

 sible that the New England massif originally extended towards 

 the south and joined the Blue Mountains massif with but little 

 lowering of the general 3,000 ft. contour. There is little doubt 

 at any rate as to the action of the Hunter and its tributaries in 

 materially reducing the level of this region and thus leading to the 



* A figure illustrating this feature will be found at the end of a paper by 

 Mr. Mawson and myself, opposite Plate xxv. Journ. Proc. Roy. Soc. N. S. 

 Wales, Vol.xxxvii. 



