316 



THE BONDI ANTICLINE. 



By C. Hedley, F.L.S. 



(Plates xxv.-xxvii.) 



To ascertain the quality and position of coal-seams beneath 

 and inland from Sydney, a series of bores were drilled to a great 

 depth. As a result, the conformation of the remotely underlying 

 strata is unusually well known in this neighboufhood . 



Sections* composed from these borings develop a central 1)asin 

 risintr to the coast on the one side, and to the Blue Mountains on 

 the other. This basin is here regarded as the lap of a fold. Had 

 the basin existed before the deposition of the strata it contains, 

 then salt would have accunuilated in an area of internal drainage 

 below sea-level. Further, the steep slope, on the western side, 

 of about five thousand feet in forty miles would have thrown 

 brisk streams, and would not have supported such swamps as 

 grew the coal. Consequently, the bowed strata were not laid 

 down in their present attitude, but on an almost level surface. 

 So considerable deformation of the original coal-horizon has 

 therefore happened. Since drawing the following sketch, it 

 occurs to me that the watershed, on which was laid down the 

 Hawkesl)ury Sandstone, might have desc-ended inland westwards, 

 while the granite mountain-range, whose waste supplied its 

 materials, was situated seawards and to the east. This would 

 harmonise with deeper, coarser deposits on the east becoming 

 finer and thinner on the west. 



Compressive crustal action has already been suggested {ante^ 

 xxxvi., p. 14) as an agent competent to effect the changes that 

 have taken place. On this hypothesis, both the coal and the 

 .succeeding shale and sandstone were spread evenly on an almost 

 level floor, and by subsequent earth-movements were compressed 

 and bent, first into smaller, then into larger, folds — was elets on 

 a wave (text-fig. 1). 



Came, Mem. Geol. Survey N.S.W., Geol. vi., 1908, p,160. 



