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marked dolomitic character. As it is often impossible, in 

 the field, to distinguish a true dolomite from some dolomitic 

 limestones, it was considered better tO' describe this class of 

 rock, as a whole, as dolomitic limestones. There are bluish 

 limestones, exactly similar in appearance, to the Carboniferous 

 limestones of Europe; siliceous limestones, and arenaceous 

 limestones. Many of the thinner limestones have an oolitic 

 structure, and it sometimes happens that the oolitic grains 

 and rounded sand grains occur together in a rock in about 

 equal quantities. In one instance (in Wilkawillina Gap) an 

 oolitic limestone, at one particular zone, had become altered 

 to an oolitic flint. The Archaeocyathinae limestones form a 

 group by themselves. They are, usually, relatively pure, but 

 in places show siliceous and earthy veins and patches, which 

 weather into relief. The same happens when the included 

 fossils have undergone some measure of silicification, when they 

 make most interesting and showy faces on the weathered sur- 

 face of the rock. Occasionally the limestone partakes of the 

 nature of a marble, either dark coloured or nearly white. A 

 change of texture, of this kind, is generally destructive of the 

 organic remains, which become altered and indistinguish- 

 able from the cryptocrystalline matrix. The Archaeocya- 

 thinae outcrops in the district appear to be limited to two 

 distant localities, the one on the western side of Blinman and 

 the other on the eastern. In the western outcrops the beds 

 form the foot hills of the Flinders Ranges, facing the western 

 plains, where they extend both north and south of the Para- 

 chilna Gorge. In the eastern areas of outcrop they are more 

 irregularly placed. They follow, to some extent. The Bunkers, 

 running in a. south-eastern direction, where outcrops were 

 visited at the ''Big Hill," at the head of the Balcoracana 

 Creek, and in the Wilkawillina Gorge, measuring from point 

 to point a distance of 16 miles. There is, apparently, another 

 line of strike that passes in a north-easterly direction, 

 diverging from the ''Big Hill," passing by the old Wirrealpa 

 station; a small faulted patch occurs near Mount Lyall; 

 and then, following in the same direction (after a distance of 

 about 20 miles), there is another important outcrop of the 

 limestone in the Mount Chambers Creek. 



The Igneous Rocks are restricted, so far as the present 

 observations are concerned, to two localities; one of these is 

 in the neighbourhood of Blinman (especially developed on the 

 western side of the township), and the other oecurs on the 

 eastern side of the "Big Hill," nearer to Wirrealpa. The 

 occurrence of intrusive pipes, of a circular outline, are inter- 

 esting features as indicating ancient volcanic vents that have 

 been cut back by denudation. Petrographically, the rocks 



