PROCEEDINGS OF SECTION C. 419 



Hills, west of Port Augusta, or rest unconformably on the older series, 

 as at Ardrossan, Yorke Peninsula. This marks approximately the 

 western margin of the old Cambrian Sea, or at least its shallows, and 

 the beds of these districts were laid down by overlap and transgression. 

 At Ardrossan the Archceocyathince limestones and accompan-sang 

 dolomites (which in the Flinders Ranges are underlain bv a great 

 thickness of Cambrian deposits) are here only separated from the 

 Pre-Cambrian bv basal grits. 



Lower Cambrian. 



The lower beds of the Cambrian system are reckoned from the 

 horizon of the Brighton limestones, not, as has already been stated, 

 from the occurrence of any distinct unconformability in the series, 

 but conventionally on their lithological differences and color. This 

 lower series may be roughly estimated as comprising about one-third 

 of the total thickness of the Cambrian beds as developed in South 

 Australia. 



In the Mount Lofty series there are three well-marked limestone 

 horizons — (a) A Lower Limestone, which is within a few hundred 

 feet of the bise and about 150ft. in thickness, varying from a 

 blue limestone to marble or dolomite. (6) A Carbo- argillaceous 

 Limestone, situated near the centre of the series, forms a calcareous 

 zone about 100ft. in thickness. The stronger and more calcareous 

 beds of these outcrops are locally known as " blue metal " and are 

 extensively used for roads. These dark-colored calcareous beds are 

 particularly useful to the field geologist as supphdng a datum for the 

 determination of the order of succession in a much-faulted district. 

 (c) The Brighton or Upper Limestone, which in its highest beds is a 

 very pure oolitic limestone capped by a yellow dolomite, but passes 

 down to arsillo-siliceous limestones and calcareous slates, which merge 

 into the Taplev's Hill banded or ribbon slates. The latter (estimated 

 at about 1,500ft. in thickness) are composed of exceedingly fine clayey 

 material, laid down in very thin laminse, and marked by colored bands, 

 which are brought out strongly under weathering. The bed varies 

 from a laminated shale to a slate which splits readily at a high angle 

 to the bedding plane, but not sufficiently thin for roofing slate. Near 

 Adelaide it has a slaty structure, but further north it becomes a shale 

 and splits up into thin slabs. It is sometimes of a very dark color, 

 which is discharged by heat, indicating the presence of carbon. 



By far the greater portion of the Mount Lofty beds consists of 

 quartzites, slates, and phyllites, which make a very uniform succession 

 in the lower half of the series. The quartzites are characteristically 

 highly siliceous by infiltration of silica and the secondary development 

 of quartz. Most of the quartzites in this part of the series have a 

 macroscopic, granular, felspar constituent, which may be estimated 

 at about 30 per cent, of the whole. The felspar clastic material was 

 probablv derived from the granitic Pre- Cambrians, which carry a high 

 proportion of felspar, and would form the land surfaces from which 

 the fragmental beds would derive their material. Dr. Woolnough, 



