GEOLOGICAL FAULT AT KURRAJONG HEIGHTS. 369 



level at the Kurrajong as at Lapstone Hill (620 feet) 

 explains why the Grose River has been pushed further and 

 further south, the water naturally making for the lower 

 end of the embankment or fold, just as when a dam is 

 thrown across a river flowing from west to east, if the 

 north end of the dam is higher than the southern end the 

 river will establish a bywash for itself over the lower end 

 of the dam, which in this case would lie on the south side 

 of the valley. The above I think is a reasonable explana- 

 tion of the great bend towards the south-east in the Grose 

 River as it sweeps through the parishes of Grose, Burralow,. 

 and Ooomassie just before it crosses the line of fold. It 

 will be noticed on reference to the map (Plate 16) that 

 there are bends, almost concentric to this river bend, in 

 the main line of water parting to the north of the Grose 

 Valley near Bilpin (on ' Bell's Line ') and on the main 

 water parting to the south, viz., the line of the Main 

 Western Road and Western Railway. (See plan, Plate 16.) 



Suggestions for further explorations. — Obviously a very 

 important question to be further studied in connection with 

 the subsidence area between the Kurrajong and Mount 

 Tomah, is its relation to the volcanic outbursts of Mounts 

 Tomah, Bell, Hay, Tootie, Irvine, King George, Wilson, 

 etc. Basalt is said to occur close to the fault plane at the 

 * Mountain Lagoon,' and this should be worth examining 

 to determine its relation to the faulting. 



The points where the fault crosses the Grose and Colo 

 Valleys should be worth exploring, and in these valleys, as 

 well as in that of Wheeny Greek, searcli might be made for 

 the chocolate shales, the topmost beds of the Narrabeen 

 Series, (the third and lowest member of the local Trias). 

 An explanation is much needed of the remarkable horizontal 

 displacement of the fold and fault as they cross the Grose 

 Valley. The question also suggests itself, was the fault- 



X— Dec. 3, 1902. 



