14 



An important factor in the tectonic phenomena has been^ 

 no doubt, the strong and unyielding Pre-Cambrian massif, 

 which led to differential movements and fractures in the over- 

 lying beds. The Cambrian rocks give evidence, in many 

 places, of powerful thrust movements operating from the east. 

 These arg well seen in the phyllites exposed in the cuttings on 

 the new road through the Torrens gorge. Large folds, over- 

 folds, and crush-zones occur, with plentiful splashes of quartz, 

 which often take the form of augen structure. The Cambrians 

 have been forced forward, and as the Pre-Cambrian floor 

 possessed greater stability and, consequently, less susceptibility 

 to movement, the newer series dragged on the surface of the 

 older, and became thereby much faulted and dismembered. 

 This seems to be the explanation of the exceptionally broken 

 condition of the rocks within the district, especially near the 

 basement. 



The two limestones (described in this paper) which occur 

 near the base of the Cambrian series form excellent horizons 

 for sectional determinations. The two principal quartzose 

 horizons within the area described are — (a) the basal grits 

 and quartzites, which underlie the Lower Torrens-limestone ; 

 and (b) a hard siliceous quartzite, which is inferior to the 

 Upper Torrens-limestone, and is separated from it by a slaty 

 zone of about 25 ft. in thickness. This quartzite is a useful 

 indicator for the upper or blue limestone, as, from the greater 

 solubility of the latter, it is sometimes inconspicuous or masked 

 in the outcrop. There are two smaller quartzites sometimes 

 seen in the phyllites which separate the two limestones, as 

 already described, but these are relatively thin and unim- 

 portant. Above the upper limestone the phyllites are very 

 thick, but it is diflicult to estimate their true thickness on 

 account of the crushed condition and frequent faulting of the 

 beds. 



The remarkable variations that occur in the strike of the 

 beds, within the district, are partly due to the comparatively 

 low angle of dip, in many places, together with the very hilly 

 nature of the ground. Narrow valleys and steep gorges inter- 

 sect the old peneplain, giving rise to tortuous outcrops and 

 repetitions. In consequence of this irregularity in the out- 

 crops the descriptive notes may appear, T fear, somewhat 

 laboured and tedious, but under the circumstances could 

 scarcely be avoided. Whatever difficulties might have been 

 experienced in following the descriptions, it is hoped will be 

 obviated by references to the map which accompanies this 

 paper. 



