810 GEOCxRAPHY OF BLUE MTS. AND SYDNEY DISTRICT, 



adjustment of coastal masses to one set of conditions and later 

 reversal of the motion. The fold and fault appear to date back 

 at least to the closing stages of the plateau (Lithgow) C3'cle. 

 The establishment of the old plain stream, before the canon cycle 

 commenced, along a line fairly coincident with the general direc- 

 tion taken by the great fold, would at first sight appear to throw 

 the age of the movement back well into the plateau cycle, on the 

 ground that the fault line predetermined the river course rather 

 than that the fault followed the stream. The Wianamatta Shales 

 themselves, however, by their position appear competent to bring 

 about such stream-deflection, and until further observations are 

 taken along the line of folding and faulting the writer would be 

 inclined to refer the age of the movements to the early caiion 

 stage,* and assign the prime cause of the longitudinal course of 

 the river to the instability of the shales and the prevailing rock 

 strike. A fine problem for future study is thus opened up, viz., 

 as to the relative age of river and folding, for faults rarely assume 

 accidental parallelism with streams. 



Possibly the movement described by Prof. David t may be but 

 a fresh slip on an ancient line of fault, but it seems absolutely 

 demonstrated, howe^■er, that the fault scarp as seen to-day does 

 not antedate the caiion cycle. River-develojDment must be the 

 criterion of age in the present state of our knowledge. 



On the other hand, assuming the age of the folding to post- 

 date the plateau cycle, it appears that the subsequent Hawkes- 

 bury had practically determined its course before the folding took 

 place; thus the influence of the fold would not be marked, though 

 its guiding action appears manifest in places. 



The Wianamatta Shales are doubtless responsible in great 

 measure for the lengthwise course of the river. On the great 



* Very probably the old Hawkesbury Kiver bed of Lapstone Hill belonged 

 to the early canon cycle, and the post-basaltic period of elevation revived the 

 old stream which had previously been flowing over an area of but slight 

 elevation. 



t Loc. cit. 



