340 



The easternmost beds have been invaded by igneous 

 intrusions which are comparatively absent from the western 

 side of the axis.( 2? ) 



These sedimentary beds contain an interstratified glacial 

 tillite. They have been designated as Lower Cambrian by 

 Professor Howchin, but the possibility of their being 

 Proterozoic must not be denied. 



Age of the Intrusion. — The granitic mass of Cape Wil- 

 loughby is intrusive into the eastern representatives of this 

 series. Howchin has shown that the late Palaeozoic (Permo- 

 Carboniferous) glacial deposits overlie the old metamorphic 

 rocks of eastern Kangaroo Island, and are represented near 

 Cape Willoughby itself. The presence of these glacial beds 

 indicates that already the granite was exposed in Permo- 

 Carboniferous times. If the intruded beds are to be relegated 

 to the Proterozoic, as has been suggested for their western 

 representatives, then the age of the granite can only be rigidly 

 defined as Post Proterozoic and Pre Permo-Carboniferous. 



On further analysis, however, it would appear that these 

 limits can be somewhat narrowed. It is clear that in Permo- 

 Carboniferous times the granite was exposed at the surface. 

 The vast amount of erosion that would be required for its 

 exposure indicates that it was intruded considerably prior to 

 Permo-Carboniferous times. On the other hand, it is clear 

 that the first great orogenic movements in this area subsequent 

 to deposition of the intruded beds developed only after 

 Cambrian time, for the Proterozoic age of these beds is 

 dependent on a disconformity between their western repre- 

 sentatives and the Cambrian Archaeocyathinae limestones. 



The absence of Ordovician beds at the edges of the Cam- 

 brian geosyncline points to the folding of this geosyncline at 

 the close of the Cambrian or in Ordovician time. 



The development of the Mount Lofty Ranges as a huge 

 anticlinorium with a pronounced westerly overthrust and the 

 occurrence of an eastern zone of igneous intrusion is sug- 

 gestive that these igneous intrusions are related to the folding. 



Since Ordovician times no orogenic movements have dis- 

 turbed this area to the present day. The evidence of 

 inclusions of country rock in the granite indicates that partial 



(27) This view of the structure of the Mount Lofty Ranges 

 has been denied by some observers, particularly W. G. Woolnough. 

 Remarkably clear evidence of the anticlinorial character of the 

 ranges can be obtained in the Inman Valley, where an easterly 

 succession beginning with the great angular unconformity of the 

 Grey Spur can be traced through to Victor Harbour. More com- 

 plicated but none the less clear is the succession in the Williams- 

 town-Mount Crawford area, Barossa. The possible Proterozoic 

 age of the westernmost beds affects this question not at all. 



