South Australian Dolomites. 499 



that the dominant meionite molecule is not the oxide type of 

 Tschermak, but of the type CaCOg, 3 CaAlgSijOg, it was of interest 

 to test these scapolites for COj. 



An uncovered thin section was treated firstly with dilute HCl, 

 and then observed under the microscope. No evolution of gas 

 occurred. More drastic treatment with a mixture of hydrochloric 

 and hydrofluoric acids failed to show evidence of the presence of 

 carbon dioxide. The grains as observed under the microscope 

 slowly dissolved without effervescence. 



As far as microscopic tests are available, therefore, there is no 

 evidence to indicate carbon dioxide as an essential constituent of 

 the scapolites of the rocks under discussion. The close similarity 

 in composition between anorthite and meionite, and the conditions 

 under which one or the other is produced in these rocks, are worthy 

 of remark. 



It is possible that chlorine of magmatic origin or derived from the 

 sediments themselves may be the determining factor. The suggestion 

 is that, acting catalytically, chlorine may induce the formation of 

 scapolite, which under other conditions would give place to 

 plagioclase. Nor is it, j)erhaps, essential under these circumstances 

 that scapolite should contain this element in its final constitution. 

 4 CaCOg + 3 (AlA, 2 SiO^, 2 H^O) = (4 CaO, 3 Al^O^, 6 SiO^) + 

 kaolin (Cy, scapolite 



6 H^O + 4 CO^. 



Titanite, which may be an important constituent, is doubtless 

 derived from the interaction of detrital rutile, calcite, and silica, 

 or the recrystallization of this mineral itself. 



Conclusion. 



Embraced within the oldest group of sedimentary rocks of 

 Southern Eyre Peninsula — the Hutchison series — is a series of 

 dolomites and calc-magnesian silicate rocks. These rocks represent 

 the metamorphosed equivalents of an original sedimentary series, 

 ranging from true dolomites to types in which silica and alumina 

 were abundant detrital constituents. 



The series has been invaded and metamorphosed by the 

 succeeding gneisses of the Flinders series. It is shown that for 

 the dolomites residuary crystallization alone can account for the 

 development of the metamorphic minerals. 



For the diopside rock masses there is clear evidence that siaper- 

 posed on the normal residuary crystallization is an additi've 

 metamorphism — (contact metasomatism of Barrell, or pneumato- 

 lytic contact metamorphism of Goldschmidt) — in the form of 

 silica solutions from the granites, and it is to these that the final 

 diopsidization is due. For the diopside-microcline rocks simple 

 residuary crystallization is adequate, but in some examples it is 

 probable that additive metamorphism is responsible for the removal 

 of the last traces of carbon dioxide. The addition of material — 



