219 



directed from the east. This protective force exerted by the 

 stable basement is evidenced by the comparatively undis- 

 turbed condition of the Cambrian beds, not only in the 

 Ardrossan district, but in the extension of those beds in a 

 northward direction, as well as on the western side of Lake 

 Torrens and over the Pre-Cambrian plateau. 



(c) The relation of Yorke Peninsula to the great epeiro- 

 genic uplift, that isolated the interior of the continent and 

 rejuvenated the rivers and formed the prelude to the sculp- 

 turing of the South Australian highlands in their latest 

 aspects, cannot be clearly defined on account of the peculiar 

 position occupied by the region. It is almost surrounded by 

 sea, and as a fault-block its elevation would probably be 

 modified by the rift valleys by which it is bounded. There 

 is evidence, however, that the region at some comparatively" 

 recent date stood higher than it does at present, from the 

 fact that important river deposits occur in the present 

 topography of the country at sea level, particulars of which 

 have already been given (page 205 et seq.J. 



(d) The development of the great rift valley of South 

 Australia with its bifurcating branches, marked by the 

 drowned valleys of the two local gulfs, stands related to the 

 Peninsula in a peculiar way. On the Adelaide plains, within 

 4 miles of the sea, the Cambrian beds have dropped to a 

 depth of over 2,000 feet below sea level. As the sunken beds 

 form a faulted segment of the Mount Lofty peneplain, it is 

 not unreasonable to suppose that below this level the most of 

 the Lower Cambrian beds occur, and below these again the 

 Pre-Cambrian floor, at a much greater depth ; and yet that 

 Pre-Cambrian floor forms the main plateau of Yorke Penin- 

 sula as well as the axis of the Mount Lofty Ranges. The? 

 vertical displacement, as thus viewed, must be very great. 

 The relatively high dip of the Cambrian limestones at "Sliding 

 Rocks," where they pass below sea level at an angle of 40°, 

 may be caused by the proximity of the rift fault, as occurs 

 on the other side of the Gulf, at Sellick Hill, where both 

 the Cambrian and Tertiary beds are thrown down to the 

 west at a very high angle. In this instance at "Slidinq- Rocks," 

 however, the Tertiary beds, presumably of the same age as 

 those at Sellick Hill, and which here also overlie the Cam- 

 brian, do not participate in the downthrow. 



(e) The undisturbed condition of the Permo-Carboniferous 

 glacial beds in the southern portions of Yorke Peninsula, as 

 well as on the eastern side of Gulf St. Vincent, is proof that 

 this part of Australia has not been subjected to orogenic 

 deformations since Palaeozoic times. The only forms of 

 diastrophic modifications that have occurred in the interval 



