105 



terms of any subdivision of the Algonkian proposed in other 

 ai'eas, but as directly above the ilmenite-bearing intrusive 

 there lie the grits containing ilmenite derived from the for- 

 mer, it would appear probable that after the intrusion qf 

 the plutonic rock it had been bared by erosion during the 

 uplift and cycle of erosion that gave rise to the Cambrian 

 series. Whether plutonic activity ceased with the Algonkian 

 era, or whether products of the Houghton magma have sub- 

 sequently been injected into Cambrian strata, is not certain ; 

 but the probability is that the latter is the case. 



Cambrian. — -Upon the uplight and dissection of the Pre- 

 Cambrian complex a series of Cambrian strata was laid 

 down, beginning with a gritty sandstone or angular-grained 

 felspathic rock, often rich in ilmenite, derived from the in- 

 trusive rock. The ilmenite content of this basal grit varies 

 greatly. At one spot it is very abundant, the rock often 

 showing false bedding marked by black streaks of ilmenite ; 

 at another point the rock appears to be quite free from it. 

 This perhaps indicates that the formation of the ilmenite 

 grits was in valleys in the old range bordering the Cambrian 

 sea. One such would be represented by the ilmenite grits 

 near the junction of Deep Creek with the Torrens. The dip 

 of the Cambrian beds is here radial from the Pre-Cambrian to 

 the south along the Torrens Valley and to the west along 

 Deep Creek. As the average dip of the Pre-Cambrian schists 

 is to the east, well-marked unconformities occur, particu- 

 larly noticeable in the Torrens Valley. 



Above these grits comes the series of the lower phyllites 

 and Torrens limestones, the Mount Lofty or thick quartzites, 

 and above these the thick slates with their included blue 

 metal limestones. The general occurrence of each of these 

 beds is described by Mr. Howchin.(6) The map (plate v.) as 

 regards Cambrian details is based partly on his descriptions 

 and partly on observations of my own. A generalized sec- 

 tion showing the relationship of the rocks exposed along the 

 Torrens Valley is given in fig. 1 (p. 106). 



The general dip of the Cambrian series is, in the southern 

 portion of the Map, radially outwards from the Pre-Cambrian 

 rocks, but to the northern portion the dip is easterly towards 

 the Pre-Cambrian. Heavy faulting has taken place. The Tor- 

 rens limestone is cut out by a fault near Highercombe, and 

 does not reappear to the north of this in the present Map 

 (though found several miles north of this, as near Sampson's 

 Plat, and at the South Para C^)). Faulting also considerably 



(6) Trans. Roy. Sec, S.A., 1906, pp. 239, 242, 246. 

 (V)TV. Howchin, op. cit., p. 248. 



