578 CASTS OP RADIOLARIA IX PRE-CAMBRIAN ROCKS, 



Field River. A few miles further south the rocks forming the 

 sea cliffs are contorted and overthrust from E. to W. in a very 

 striking manner. If the coastline be followed to Normanville, 

 48 miles south from Adelaide, the crystalline and highl}^ meta- 

 morphic beds of the eastern flanks of the ranges are met with. 

 The marked lithological distinction between the western and 

 eastern sides of the Mt. Lofty Ranges is an interesting feature. 

 The greater part of the ranges, including the western flanks and 

 highest portions of the watershed, show a series of sedimentary 

 rocks metamorphosed to only a slight degree, with a general 

 easterly dip at a steep angle of from 40'' to 80°. The eastern 

 flanks are composed of highly crystalline metamorphic rocks, 

 felsites, hornblendic and micaceous schists, gneiss and granites, 

 which give distinctive features to this side of the ranges for over 

 200 miles in length. Intrusive granites are extensively associated 

 with this zone of extreme metamorphism. 



Professor R. Tate * regards the Mt. Lofty Ranges throughout 

 their entire width as forming one great conformable system, the 

 aggregate thickness of which he estimates cannot be less than ten 

 miles. Further, as the dip of these beds is in the main a south- 

 easterly one, it follows ujaon the above assumption that the highly 

 crystalline rocks of the eastern side of the watershed are actually 

 superimposed on the less metamorphosed shales, limestones, and 

 quartzites of the western portions. If this reading of the strati- 

 graphical features be the correct one, the Brighton limestones 

 must rank amongst the oldest rocks exposed in the Mt. Lofty 

 series, as shown on Fig. 1, Plate XL. 



The geological age of these old rocks is a subject of great 

 interest. Selwyn, and other early observers, regarded them as 

 Silurian, although the entire absence of fossils from the series 

 left the question an open one. The discovery by Mr. Otto 

 Tepper and Professor R. Tate in 18791 of a fossiliferous horizon 

 near Ardrossan, Yurke's Peninsula (subsequently determined by 



Presidential Address Aust. Assoc. Ad. Sc. Vol. V. (1893), p. 47, et seq. 

 + Trans. Philosop. (Royalj Society S. Aust. Vol. ii. 1879, p. 71. 



