THE RIVER GRAVELS BETWEEN PENRITH AND WINDSOR. 255 



direction through Luddenham and Minchinbury to Prospect; 

 and many areas, such as the heights east of Mulgoa, Frog- 

 more Farms south-west of St. Mary's and others, were 

 elevated by the intrusion of laccolites at a depth. The 

 numerous basalt dykes on the surface in these places hint 

 of masses like that of Prospect below. We have reason to 

 believe that the basaltic eruptions of the County Cumber- 

 land belong to the late Pliocene or early Pleistocene period 

 of volcanicity — the period of the newer leads (Andrews, 

 'Geographical Unity,' this Journal, Nov. 1910.) 



The Warragamba River had prior to the commencement 

 of the monoclinal folding a small tributary which rose not 

 far from the old course of the Nepean and flowed westerly 

 into the main river at the Basin. The Warragamba River 

 was carried towards the west by the monoclinal fold in the 

 region of Mulgoa and the Basin, the line of maximum uplift 

 or axis of folding being east of the old river course. At 

 Glenbrook the old bed of the river lay east of the axis of 

 folding, so that the river was thrown to the east. 



After the fold movement had set in the intrusion of 

 basaltic laccolites commenced along an axis almost at right 

 angles to the axis of folding. The uplift caused by igneous 

 intrusions has been very considerable at Mulgoa and 

 between Mulgoa and St. Mary's, so much so that the 

 magnitude of the monoclinal fold has been considerably 

 obscured in this region by the intrusive uplift. 



The radical difference between the mode of action of an 

 igneous uplift and that of a tectonic uplift lies in the fact 

 that the former kind is rapid and sudden, whereas the latter 

 is very slow. The rapid elevation of a portion of the bed 

 of the old Nepean by the igneous uplift running nearly 

 east and west from the Basin towards Prospect enabled a 

 small tributary of the Warragamba to capture the Nepean 

 before the monoclinal uplift had neared its present mag- 



