South Australian Doloraites. 455 



unstained grains in certain of the pseudomorplis indicate dolomite 

 or even magnesite, the distinction between these two minerals not 

 being possible. 



(115) is a good example of the replacement by opal and calcite. 

 The opal generally forms a border to the pseudomorph, and by its 

 extension across the grain often separates the calcite into isolated 

 grains. The relative positions of forsterite with regard to calcite and 

 dolomite call for remark. 



An examination of the sHdes shows that in the great majority of 

 cases the forsterite is embedded in calcite. In almost all cases there 

 is present some calcite abutting forsterite crystals. 



(6) Dolomites of the Hutchison Area. 



The dolomites of the Hutchison area are well developed in the 

 hilly district behind Tumby Bay, and good exposures are met with 

 in the gorge of the Mine creek. 



Serpentinous marbles outcrop in the north-eastern corner of 

 section 63 in the hundred of Hutchison, and are here striking east 

 with an almost vertical dip. Marbles are again met with near the 

 Copperinga mine in section 804. Here serpentine occurs in bands 

 and lenses through the rock. In a tributary stream — the Waterfall 

 Creek — the marble makes a prominent cHff-face with a large angle 

 of dip, and striking approximately north-east. A drive into the side 

 of the hill shows veins of chrysotile asbestos up to half an inch in 

 width. The associated rocks are garnet gneisses. On the west the 

 marbles are bounded by mica schists and pegmatized schistose rocks. 

 Graphite has been observed in these schists. The whole series has 

 been invaded by the granite gneisses, which have their best develop- 

 ment in the Mine Creek to the east. These rocks present the same 

 characteristics as those at Sleaford Bay, and represent the same 

 series of sediments now highly metamorphosed. 



To the north-east, in the adjoining county of Jervois, R. L. Jack ^ 

 has noted crystalline dolomites, associated with quartzites, schists, 

 and gneisses in the hundreds of Dixon and Miltalie. These rocks 

 outcrop along the strike extension of the " graphite line " in the 

 county of Flinders. No detailed petrological examination of these 

 dolomites is reported, but Jack indicates the presence of forsterite, 

 tremolite, and diopside. 



Analyses of three dolomites from different localities in this area 

 :show a range of the CaO/MgO ratio from I'O to 1-9. The series 

 has been invaded by granitic and pegmatitic intrusions. 



In addition to the minerals found in the Sleaford Bay exposures, 

 there are rocks in this district containing spinel, phlogopite, apatite, 

 diopside, chrysotile-asbestos, talc, and magnetite. Some of the 

 dolomites are now represented by ophicalcites, in which dolomite 

 is only sparingly present, and may be absent. There has in these 



1 R. L. Jack, Bull. No. 3 Geol. Surv. S. Austr., 1914, pp. 11-12. 



