19 



or 12 ft. above sea-level. At the Croydon bore, two and a 

 half miles west of Adelaide, the upper limit of the same beds 

 occurs at about 700 ft. below sea-level. At Gawler, they 

 have an elevation of about 400 ft. above sea-level ; and at 

 the Red Banks (as determined by aneroid) they are about 

 230 ft. above sea-level. 



This great discordance, within short geographical dis- 

 tances, can only be reasonably explained by the occurrence 

 of step-faulting, probably more or less influenced by differential 

 erosion of the beds. It has been demonstrated 0) that such 

 step-faulting has occurred in localities, further south, since 

 the period when the marine beds of this age were laid down, 

 and it is highly probable that the effects of such earth- 

 movements would be felt marginal to Gulf St. Vincent 

 throughout its entire length. 



The results of step-faulting would be to produce a series 

 of distinct shelvings of the older rocks at different depths. 

 It is believed that the Red Banks Cainozoics rest on such a 

 shelf or platform of Cambrian rocks. That is the usual order 

 in which the geological formations mentioned occur in this 

 part of the country. Moreover, about a mile to the north 

 of the Red Banks, the Cambrian slates outcrop in a small 

 gully near the middle of Section 1, Hundred of Grace. This 

 is the first evidence of Cambrian outcrops in the Lower 

 Light, but about two and a half miles further up the valley, 

 to the north-east, in Sections 151 and 153, strong outcrops of 

 these rocks occur in the bed of the river and continue from 

 this point to Hamley Bridge. I was informed by Mr. Marsh- 

 man, of Mallala, that in a well-sinking near the Woolsheds 

 Methodist Church, at the five cross-roads, about three miles 

 out from Wasleys, the old "blue-rock" (Cambrian) was struck 

 at a depth of 40 ft., and no water tapped. 



It is therefore certain that each of the Cainozoic out- 

 liers, included within the Adelaide and Red Banks groups, 

 rests on Cambrian steps or platforms. The highest of these 

 steps is that which carries the Gawler fragment, at an eleva- 

 tion of about 400 ft., above present sea-level. The Red Banks 

 platform is about 200 ft. above sea-level. The Kent Town 

 bore proved the Cambrians at 221 ft. below sea-level; and 

 at the Croydon bore at 2,206 ft. below sea-level. It is pos- 

 sible that the Cainozoic sea-floor was uneven originally, which 

 might account for some differences of level, but can scarcely 

 account for the extraordinary disparity, within short dis- 

 ci) Howchin, Trans., Proa, and Rep. Roy. Soc, S.A., vol. 

 xxxv., 1911, p. 47. 

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