126 G. D. OSBORNE. 



cline was already beginning to rise, but the final phase of 

 it took place subsequent to the long period of crustal 

 stability which obtained during the development of the 

 Upper Goal Measures. 



It is with this last orogenic folding of New South Wales 

 that the present paper is concerned. The writer has been 

 able at intervals to examine many sections which display 

 this folding, and a perusal of the papers by Van Hise (8) and 

 R. T. Ohamberlin/ 9 ' 10) especially those of the latter on the 

 diastrophism of the Appalachians and the Colorado Rockies, 

 prompted him to apply to the Hunter district, the methods 

 so clearly discussed by Chamberlin. 



II. General. 



(i) Principles involved. 

 Students of diastrophism agree that, in general, the 

 determination can be made of the nature of an earth block 

 subjected to compressional forces; provided certain ele- 

 mental features of the resultant folding are known. Assum- 

 ing that there is no volume change consequent upon the 

 impression of the diastrophic forces, then the depth of the 

 folded earth segment can be computed in the following 

 manner ; 



If A = The present length of the section in question, 

 B = The original length of the section, so that (B - A) 



= the crustal shortening, 

 C = The elevation produced by the lateral stress, and 

 D = The depth of the folded block, 



Then D _ A X C 

 B - A 



It will be readily seen that the first pre-requisite to 

 form a basis for any investigation of this sort is a detailed 

 geological section across the folded region, in a general 

 direction perpendicular to the strike of the folds. From 

 this it is necessary to restore the folded structure to what 



