213 



clear evidences of alternations of movement, up and down, 

 within recent times. The evidences of subsidence are especi- 

 ally marked within the limits of the rift valley. 



General Considerations. 



1. Stratigraphical and Lithological Characteristics. 



Yorke Peninsula, as a geological province, stands related 

 to the South Australian highlands, on the east, and also to 

 Eyre Peninsula, on the west, and yet is different from both. 

 In common with the land lying to the eastward it possesses 

 a Cambrian fauna, which, so far as known, is not shared by 

 the country lying to the westward ; but, like Eyre Peninsula,, 

 its dominant geological feature is that of a Pre-Cambrian 

 massif. There is reason to think that the region now known 

 as Yorke Peninsula has existed as a stable horst from Pre- 

 Cambrian times, and has received less sedimentation in the 

 interval than the country on the eastern side of Gulf St. 

 Vincent, and has also been for longer periods under the 

 influence of subaerial waste. 



The basement (Pre-Cambrian) series, in its highly fel- 

 spathoid rocks of pegmatites, syenites, porphyries, schistose 

 rocks, and basic igneous dykes, show a striking similarity 

 to the Pre-Cambrian complex that forms the axes of the 

 orogenic deformations of the Mount Lofty and associated 

 ranges. Professor Tate recognized the existence of two series 

 of ancient rocks, separated by an unconformity, on Yorke 

 Peninsula, but failed to see that a similar order of occurrence 

 existed in the Mount Lofty Ranges, and the whole section, 

 from the sea to the River Murray, was classed by him under 

 one system as Archaean. That a major unconformity exists 

 in the Mount Lofty Ranges, as it does on Yorke Peninsula, 

 can be clearly demonstrated ; and that the basement rocks, 

 on either side of the Gulf, are of the same order and corre- 

 sponding age, also admits of no doubt; but in the case of 

 the Cambrian beds which rest immediately upon the Pre- 

 Cambrian floor the order of occurrence is strikingly different 

 in the respective areas. 



Between Aldgate and Brighton, in a direct line of 12 

 miles, taking in Mount Lofty and the foothills on its western 

 side, there is a series of grits, phyllites, slates, quartzites, 

 tillites, and limestones of Lower Cambrian Age (fig. 3), hav- 

 ing a thickness of many thousands of feet that are entirely 

 wanting on Yorke Peninsula; and not only these, but thou- 

 sands of feet of purple slates and quartzites of Upper Cam- 

 brian Age, which in other places overlie the Lower Cambrian 

 beds and underlie the Archaeocyathinae limestones, are also 



