THE RIVER GRAVELS BETWEEN PENRITH AND WINDSOR. 257 



The extrusion of alkaline basalt in the Sydney district 

 may be here as elsewhere a phase, function or effect of 

 senkungsfeld formation. x 



Conclusions. 



1. It has been shown in this note that an old river 

 channel representing the combined course of the Nepean 

 and Warragamba extended from the Penrith Plain to 

 Windsor along Rickaby's Greek. 



2. That the gravels at Mulgoa overlooking the Basin and 

 at Glenbrook belong to the Warragamba stream. 



3. That the old Nepean flowed along the Mulgoa Greek 

 valley due north and south. 



4. That this river was probably captured by the Warra- 

 gamba because of the igneous uplift at right angles to the 

 monocline forcing it back. 



5. That the St. Mary's gravels belong to an older period 

 in the history of the Nepean. 



6. That depression of the Richmond area caused a tilting 

 of the combined river to the west from its old course along 

 Rickaby's Greek. 



7. That the Warragamba was tilted to the west at the 

 Basin because it was slightly west of the monoclinal axis 

 and was perhaps partly coincident with the line of depres- 

 sion which is expressed in the Kurrajong fault and the fold 

 at Glenbrook railway station described by Prof. David, this 

 Journal, 1902. 



1 See The Age, Origin and Relationship of Alkaline Rocks, Proc. Linn. 

 Soc. N.S.W., 1908. 



Q— Oct. 5, 1911. 



