45 



that a large area of South Australia was, during the 

 epoch of the Encounter Bay intrusions, underlain by 

 a magma, differentiation of which was expressed both by 

 plutonic rocks of granitic and syenite types, and by doleritic 

 and aplitic dyke intrusions perhaps representing complement- 

 ary differentiates. These dykes may well be satellitic to larger 

 plutonic masses in every case, the latter being not always 

 visible at the surface. 



VI. Age of the Intrusions. 



To the geological age of the Encounter Bay rocks only 

 very wide limits can be assigned on the local evidence. After 

 the close of the Cambrian this part of South Australia appears 

 to have become a land mass, remaining as such during 

 Palaeozoic times. There is no existing record of any further 

 deposition of sediment until the Permo-Carboniferous glaciers 

 passed over the region and the older rocks were mostly buried 

 under morainic material and till. The igneous rocks are 

 definitely intrusive into the Cambrian sediments, and by 

 Permo-Carboniferous times had already been laid bare by 

 denudation. Indeed, the old granitic range, of which the 

 present outcrops form the northern remnant, may have formed, 

 the gathering ground for the glaciers, ^^''^ 



The intrusions then are to be assigned to the Palaeozoic, 

 somewhere between the Cambrian and the Permo-Carbon- 

 iferous. 



It appears that at the close of the Cambrian, when the 

 great basin had filled with sediment, tangential earth move- 

 ments converted the geosyncline into a geanticline (or 

 anticlinorium rather) and caused the beds to be to some extent 

 metamorphosed, especially on the eastern side of the axis. 



In his paper on the Cape Willoughby intrusion Tilley 

 argues that it is to be assigned to a late period of this folding, 

 and it is true that, if the injection of igneous magma was con- 

 nected with earth movements at all, then it was probably a 

 concomitant of these post-Cambrian disturbances, for there is 

 no evidence of any subsequent Palaeozoic movements. 



Tlie records of other granitic intrusions in South Australia, 

 which probably synchronized with those of Encounter Bay, do 

 not help in fixing the age; they are just intrusive into Cam- 

 brian or pre-Cambrian rocks. But the occurrence of the 

 dolerite dyke intrusions following the axis of foldinor of the 

 Cambrian beds through the Mount Lofty and Flinders 

 Ranges, and as far away as Broken Hill, would lend support 

 to Mr. Tilley's contention. 



(37) Howchin : Trans. Roy. Soc. S. Austr., vol. xxxiv., 1910. 



