520 A CORRELATION OP CONTOUR, CLIMATE AND COAL, 



Comparing the geosyncline with its beds to a " pile of saucers," 

 we note that the uppermost " saucer " consists of the hard 

 uniform Hawkesbury Sandstone, which, though not quite so large 

 as the lower "saucers," most effectually protects the centre of the 

 pile from atmospheric wear and tear. The peripheral portion of 

 the lower beds is exposed, however, and gives rise to physio- 

 graphical conditions eminently suited to river-deflection and 

 capture {vide fig. 2). 



The two facts which stand out pre-eminently in the stereogram 

 of the Coal Basin (see Plates xlv.-xlvi) are : — 



(1) The extraordinary shifting of the Main Divide by the 

 Hunter Goulburn Rivers system. 



(2) The curious bend of the Shoalhaven River near Marulan 

 and Barber's Creek. 



Can these be correlated with the geology of the areas in 

 question % 



It is suggestive that the rapid westerly cutting of the Goul- 

 burn River should have brought about one structure, while the 

 rapid westerly erosion of the Lower Shoalhaven would seem to 

 have given rise to the other. 



The common factor is seen to be (fig. 2) the outcrop of the 

 peripheral portions of the Permo-Carboniferous Marine Series (or 

 what is much the same thing — the thinning out of the overlying 

 harder Hawkesbury Sandstones). 



It is conceivable, and indeed probable, from study of the 

 current bedding, that the Trias sediments, deposited in the centre 

 of the basin, were derived to a great extent from the ancient 

 southern highlands around Queanbeyan ; and that the ancient 

 Shoalhaven flowed north along the present position of the 

 Wollondilly towards the centre of the Coal Basin.* It is certain 

 that the present Nepean River system antedated the folding of 

 the Blue Mountains, and the same holds good for the Shoalhaven. 



There is evidence, far to the south of the classical Lapstone Hill 

 occurrence, that the gradual earth-movements resulting in our 



* See the Paper, postea, p. 546. 



