217 



With respect to the Cambrian limestones, those of Yorke 

 Peninsula bear a general resemblance to the corresponding 

 beds in the Flinders Ranges. The order of occurrence, as 

 dolomitic in the lower members, Archaeocyathinae reefs in 

 the middle portion, and pteropod-trilobite limestones in the 

 upper, is much the same in both cases, but the respective 

 beds are greatly attenuated in the Yorke Peninsula series as 

 compared with those in the Flinders Ranges. 



The Sellick Hill section of the same beds, with respect 

 to thickness, is intermediate in this respect, being a thinner 

 series than is developed in the Flinders Ranges, but thicker 

 than the Yorke Peninsula series, with purple slates under- 

 lying the Archaeocyathinae limestones and impure flaggy 

 limestones and purple slates above them. 



The fossils obtained from the Ardrossan beds are reserved 

 for further examination and description. 



2. Tectonic Problems. 



It is roughly estimated that the Lower and Upper Cam- 

 brian series combined have a thickness of not less than 15,000 

 feet.* 5 ) So extensive a sedimentation in one area implies, 

 as already stated, a geosynclinal fold which supplied the 

 necessary conditions by which the great Cambrian System 

 could be built up. Positive earth movements that occurred 

 subsequently to the filling up of the basin exercised a lateral 

 pressure on the sediments which threw them into folds, having 

 a north and south direction, and developed numerous small 

 overthrusts to the west. With respect to these movements, 

 the very strong and stable foreland of the Pre-Cambrian 

 massif, lying to the west, must have existed as a controlling 

 factor, presenting a barrier to the passage of the earth waves 

 in that direction, and compelling them to find relief in height 

 instead of horizontal extension. In this way the Pre-Cambrian 

 floor was raised nearly 2,000 feet higher in the centre of 



(5) In a paper read before the Australasian Association for the 

 Advancement of Science in 1907 (see Reports, vol. xi., p. 414), the 

 author mentioned 30,000 feet to 40,000 feet as the probable thick- 

 ness of the Upper and Lower Cambrian in South Australia. Since 

 then there has been a tendency among geologists, on various con- 

 siderations, to give a reduced estimate of the thicknesses of great 

 mountain masses. These considerations, together with a wider 

 knowledge of the tectonic structures of our highlands, led to a 

 modification of this estimate, so that, in 1914, when in a brief 

 outline of the Geology of South Australia, prepared for a visit of 

 the British Association to Australia, it was stated that "on a 

 moderate estimate the Cambrian System in South Australia 

 represented a thickness of from 15,000 feet to 20,000 feet" 

 (Handbook of South Australia, Brit. Assoc. Ad. Science, 1914,, 

 p. 212). 



