LATE PALAEOZOIC FOLDING IN THE HUNTER RIVER DISTRICT. 127 



it was at the completion of the folding, before erosion had 

 been able to make any impression on the newly risen area. 

 From such a restored section the crustal shortening can be 

 determined by choosing some suitable competent horizon, 

 which has behaved uniformly throughout the folding, and 

 tracing along it through all the corrugations, thus obtain- 

 ing its original length which is now squeezed into a smaller 

 horizontal extent. 



The elevation of the region due to folding is determined 

 after the selection of a datum plane, above which to 

 measure the average vertical height of the folds, this datum 

 plane being the present day expression of the plane from 

 which the folds rose. 



Any regional epeirogenic uplift, rise of sea-level, etc., 

 must be taken into account in choosing this datum line, 

 and it is often found that a study of the physiography of 

 the district enables one to unravel the effects of post-folding 

 processes. It is clear that the more remote, geologically, 

 the diastrophism has been, the more difficult will be the 

 endeavour to arrive at the position of such a datum plane. 



The above considerations are admittedly theoretical, and 

 numerous small modifications come into the practical 

 investigation. For a clear exposition of the factors which 

 enter into the work and tend to affect the quantitative 

 determinations, the reader is referred to the papers by 

 Van Hise and Ohamberlin, (see Bibliography). 



In all work of this sort the investigator has to use his 

 discretion and be guided by features peculiar to the area 

 under his consideration. 



(ii) Outline of the Work. 

 The section chosen for the present study is that across 

 the Hunter district from Port Stephens to Rix's Oreek, 

 near Singleton, a section in a general east and west direc- 

 tion, traversing the folds more or less]normal to their strike. 



