BY LOFTUS HILLS, M.B.E., M.SC. 131 



rogenic period occurred at the close of the Epi-Silurian 

 erogenic paroxysm. 



No more definite determination of the Diabase has been 

 made than that it is Post-Trias-Jura and Pre-Tertiary. It 

 is generally referred to the Cretaceous, but there has been 

 no further evidence of this adduced since the late W. H. 

 Twelvetrees wrote his summary in 1902. 



An important discovery was made by Professor E. W. 

 Skeats in 1916 when he located a dyke of the Port Cygnet 

 Alkaline Series cutting the diabase near Woodbridge. In a 

 paper read before the Royal Society of Victoria (2) Profes- 

 sor Skeats discusses this discovery and demonstrates the 

 Tertiary age of this most interesting alkaline series. Up to 

 the time of this discovery the series was regarded as of 

 Permo-Carboniferous age. 



No data whatever have been obtained from which to 

 determine the relative ages of our olivine basalts, limburgite, 

 trachydolerite, and melilite basalt. We know that they are 

 Tertiary, but our knowledge has not advanced in this direc- 

 tion during the period under review. 



In only one district have sufficient investigations of the 

 structural features of the Porphyroid Igneous Complex been 

 carried out to enable definite conclusions to be drawn in re- 

 gard to the actual structure of the masses of various rock- 

 types of the series. The area referred to is the Read-Rose- 

 bery district, in which the writer has mapped the fold axes 

 of the Cambro-Ordovician sediments and the associated 

 igneous rocks, and demonstrated that the falsite or kerato- 

 phyre which is so well developed in that locality is in the 

 form of an extrusive sheet now characterised by a complex 

 series of folds. Evidence gathered at other localities on the 

 West Coast gives confirmation of this effusive character 

 of many of the porphyries, porphyrites, spilites, etc , but 

 the mapping of the structural geology has not advanced far 

 enough to allow of the definite demonstration of the struc- 

 ture as that of contemporaneous extrusive sheets in the same 

 detail as in the Read-Rosebery district. Neither is it yet 

 possible to give any indication of the order of succession 

 within this petrogenic cycle — an achievement which will only 

 be possible when the structural features of the whole Cam- 

 bro-Ordovician system have been elucidated. 



As indicated above the end-point of the Cambro-Ordovi- 

 cian petrogenic cycle, as well as that of the Epi-Cambro- 



(2) Proc. Roy. Soc. Vic, VoL XXIX., Part IL (1917), pp. 155-164. 

 J 



