101 



Petrographical notes on Certain pre-Cambrian 

 Rocks of the mount Lofty Ranges, with Special 

 Reference to the Geology of the houshton 

 District. 



By W. N. Benson, B.Sc, Acting Lecturer on Mineralogy 

 and Petrology, Adelaide University. 



[Read November 3, 1908.] 



Plates III. to V. 



Introduction. 



The dominant feature of South Australian geology is the 

 •occurrence of an immense series of lower Cambrian rocks. 

 These are most typically developed on the western flanks of 

 the Mount Lofty Ranges, and their stratigraphy has been 

 -ably demonstrated by Mr. W. Howchin, F.G.S.^i) He has 

 also shown that beneath them there lies a Pre-Cambrian com- 

 plex of schists, gneisses, and plutonic intrusive rocks. 



The present writer has made a detailed study of these 

 in the Houghton district, and more cursorily at other points 

 in a line seventy miles long on the western edge of the Mount 

 Lofty Range. 



It is the purpose of this paper to show that in these 

 localities the sedimentary rocks have a marked petrological 

 relationship with each other, and that the intrusive granites, 

 diorites, and syenites present the closest similarities in all 

 points, and are obviously all derived from a single Pre-Cam- 

 brian magma, for convenience termed the Houghton magma. 

 An examination of the published descriptions of rocks in other 

 portions of the State shows the similarity of many of these 

 to those derived from the Houghton magma, and points to 

 the probability of South Australia being a petrographical 

 province (in Judd's sense), in which the chief characteristic 

 is the presence of a large amount of titanium and to a less 

 'degree the rather high soda percentage. 



These chemical features give rise to unusual rock types 

 in the form of pegmatites, and diorites with very acid plagio- 

 clases ; these are described in detail. 



The Houghton district has been geologically mapped, the 

 Cambrian features being chiefly based on the work of Mr. 

 Howchin, to whom I am greatly indebted for very much valu- 

 able information and assistance during the preparation of 

 this paper. 



a) Trans. Roy. See, S.A.. 1904, 1906. 



