310 



GEOLOGY AKl) I'ETKOLOGV Oi' TUt; GREAT SEBPE^TI^E BELT OF X.S.W. 



hard rock. Most striking are the "cuesta" ridges of tlie northeru part of the 

 eastern zone of pyroxene •ludesite, the main zone, Duri and ICingsmiU's Peaks. 

 Here and there for reasons not yet apparent, these hard rocks have been reduced 

 to rounded hills, as in .Sandy Creek valley or the low knoll in the valley of Duri 

 Creek, or they have even been reduced to lowland as in the valley of Chinaman's 

 Creek, just north of the region mapped. (Perhaps this la.st is an old water gap.) 

 West of these rise the complex ridges cut from the Middle and Upper portions of 

 the Kuttung rocks and moulded in a great degree by the resistant masses of the 

 lower tillites and the main felspathie grit. Between this is the softer horizon of 

 the main ''varve'" zone, and overlying mudstone, in which extend tributary heads 

 of the small streams, the adjustment of streams to structures being as yet far from 

 complete. Westward again the hills around Warragundi, made up of resistant 

 trachytic breccia or strengthened liy sills and dykes of dolerite and andesite, rise 

 above the surrounding easily-eroded Werrie basalts. These greater elevations all 

 rise approximately to a height of 3000 feet or rather less, or nearly two thousand 

 feet above the general level of the lower land. 



In minor topographic features, the adjustment of streams to structures is no 

 less marked. Nearly every stream which crosses a resistant band or zone of roek 

 follows through a belt of weakness, a line of faulting or crush brecciation, or a 

 dyke, generally one of the very readily decomposed basic dykes, often in a fault- 

 fissure. Attention may be called to several instances of this feature. The main 

 transverse valley of the district, that by which Currabubula Creek passes through 

 the Kuttung Series follows the fracture line, the existence of which is shown by 

 the displacement of the rocks of this Series, and by other features. To the 

 same direction are parallel the lower portion of Rocky Creek and the fault and 

 dyke extending beyond it towards the gap in the main zone of pyroxene ande- 

 site. The main transverse valley in the district thus follows a marked and ancient 

 zone of weakness. 



Of the gaps determined by the presence of decomposed basic dykes we have 

 already indicated that that between Mts. Cobla and Sugarloaf is the most marked. 

 To the same cause, however, must be assigned the form of Duri Peak, a striking 

 landmark seen from most points between Barral)a and Hanging Rock {see Text-fig. 

 .3) . A decomposed basalt dyke crosses the ridge between the two summits, and 

 may be traced down cither side. Though only a few feet wide, it has in all 

 probability determined the depression in the ridge between the summits. 



Text-fig.7. Block diagram of Rocky Creek Valley. 



