PRESIDENTIAL ADDRESS. 19 



There probably seems evidence in the Broken Hill region 

 for a considerable divergence between the strike of the 

 Archaean gneiss and that of the lower Cambrian rocks. 



This Broken Hill evidence suggests that Mr. Howchin's 

 conclusions as to the difference in strike of the Pre-Oam- 

 brian mountains of South Australia and the Cambrian may 

 be reconciled with the apparent conformity of the two 

 directions of strike in the Mount Lofty Ranges on the 

 assumption that in the Pre-Cambrian complex there are at 

 least two distinct groups, (1) an Archaean group chiefly 

 formed of gneiss and other coarsely crystalline rocks, (2) 

 an Algonkian (or Barossian) group, and that the folding of 

 group (1) took place much earlier than that of group (2) 

 and was divergent in direction from it. It may also be 

 assumed that group (2) was heavily folded before the depos- 

 ition of the lower Cambrian beds, so as to leave a consider- 

 able unconformity between it and the Cambrian system, 

 but that the direction of folding was less divergent from 

 that of the Cambrian than it was from that of the Archaean. 

 Mr. Howchin concludes that there is strong evidence of 

 the divergence of the Archaean folds of what he terms the 

 Willouran and Babbage line, and those of the Cambrian in 

 the region between Lake Torrens and Lake Eyre. The 

 other alternative is to assume that both Algonkian and 

 Archaean rocks have been folded on similar trend lines, 

 which mostly diverge from the later trend lines of the 

 Cambrian, though in places they coincide. 



On the whole it may be said that in the neighbourhood 

 of Spencer's Gulf, St. Vincent's Gulf and the Mount Lofty 

 Ranges the divergence between the trend lines of the 

 Barossian group and the Yorke Peninsula group on the one 

 hand and those of the Cambrian on the other, do not appear 

 to be very strongly marked. On the other hand there is 

 strong evidence of (1) a spiral structure, and (2) of virgation. 



