83 



to an Archaean, or at least Pre-Cambrian, age.* This hypothesis 

 rested for support mainly on three considerations : — 



1. Analogical. Rocks of Pre-Cambrian age are known to occur 

 on Yorke's Peninsula, which exhibit lithological resemblances to 

 many of the rocks in the Mount Lofty Ranges. 



2. The discordance in the angle of dip between the Cambrian 

 outcrops (especially those of Yorke's Peninsula) and that which 

 is characteristic of the Mount Lofty Ranges. 



3. The fossiliferous features of the Cambrian limestone, com- 

 pared with the azoic features of the comparatively little altered 

 and pure limestones which are abundantly developed in the 

 Mount Lofty series. 



It must be conceded that these considerations have been con- 

 siderably weakened by the discoveries now placed before the 

 Society, and requires a re-consideration of the whole question. 

 The Cambrian beds at Sellick's Hill occupy a position which has 

 been regarded as near the base of the Mount Lofty series, so that 

 unless the accepted order of succession is to some extent reversed 

 (or otherwise it can be proved that extensive faulting in the 

 rocks has occurred) the Cambrian age of the Mount Lofty Ranges, 

 as a whole, must be accepted. 



With regard to the supposed analogy on lithological grounds the 

 Pre-Cambrian rocks of Yorke's Peninsula are uniformly highly 

 metamorphic and igneous in their features and have their analo- 

 gues, in the Mount Lofty succession, .only on the eastern flanks. 

 The value of the analogical argument is entirely dependent on 

 the assumption that the more highly metamorphosed beds of the 

 eastern outcrops are superior in position, and therefore newer in 

 point of age, to the less altered beds of the western side. For, 

 otherwise, if they underlie instead of overlie the latter, they pro- 

 bably represent an older uncomformable series with the less 

 altered shales, quartzites, and limestones of the western portions 

 as a newer, or Cambrian, formation. 



Again, little weight can be given to the consideration of dis- 

 cordance in angle of dip when we take into account that the 

 Cambrians near Ardrossan pass rapidly from a dip of 15° to 40°, 

 and in the Sellick's Hill outcrop in a series of anticlinal and 

 synclinal folds with angles of dip varying from 60° to 90° 



Further, the discovery of characteristic fossils in these ranges 

 has not only clearly defined the geological age of the beds in 



* As an exception to this opinion, Mr. H. Y. L. Brown, Government 

 Geologist, in his geological map of the colony (1886), divided the Mount 

 Lofty Ranges into three main geological divisions, marked by the degree 

 of metamorphism exhibited. In descending order the divisions are repre- 

 sented as follows : — 1. Palaeozoic (Lower Silurian). 2. Pakeozoic, or Azoic. 

 3. Archaean. 



