South Australian Dolomites. 457 



amount of calcite. The specific gravity is in conformity with this 

 determination, being 2 '84. There are a few flakes of phlogopite and 

 a few grains of an amphibole, possibly edenite. 



Nodular masses of yellow-green serpentine are present in a zone 

 of the same outcrop up to 2 inches in diameter, and occasionally 

 larger masses are met with. It is with these that the chrysotile 

 veins are associated. The specific gravity of homogeneous fragments 

 of the serpentine varied from 2'55 to 2-56. The specific gravity of 

 the ophicalcite rocks is close to 2*70, and vary from 2"69 to 2"72. 

 This can be ascribed to a changing distribution of serpentine and 

 concurrently the coming in of dolomite. 



Chemical Changes involved in Metamorphism. 

 The mineralogical features of this suite of rocks are those 

 characteristic of metamorphism of the thermal type, and there is 

 no evidence that shearing stress has played any important role. 

 In the rocks under discussion we can also exclude magmatic additions 

 as being inoperative, with the possible exception of such a volatile 

 constituent as water. It is clear that apart from detrital impurities, 

 like sihca and alumina, the sediments were true dolomites or 

 dolomitic limestones, with a high percentage of magnesia. As to 

 the origin of the dolomitization it is clearly anterior to this epoch 

 of metamorphism, and there seems little reason to doubt that it is 

 the result of metasomatic processes subsequent to the deposition of 

 the rocks. At the time of metamorphism we can safely assume that 

 they were dolomitic sediments with a certain amount of silica and 

 alumina of detrital origin. In the first place, the chemical changes 

 involved have been the elimination of carbon dioxide in so far as 

 this has been possible by reactions, involving the material of detrital 

 origin. In the absence of such reactions there has been a re- 

 crystallization of the carbonate minerals. In no case has the simple 

 decarbonation of the magnesite molecule of dolomite taken place to 

 periclase. It is to be inferred that at the temperature prevailing 

 the pressure was in excess of that necessary to efiect any appreciable 

 disruption of the dolomite molecule, in the absence of silica or 

 alumina. 



At a given pressure the temperature of decomposition must be 

 considerably lower for dolomite in the presence of silica or alumina. 

 In these rocks reactions involving these constituents have proceeded 

 to completion. The reactions involved bring out very clearly the 

 greater chemical reactivity of magnesia as compared with lime, 

 a point which is, of course, the basis of Teall's dedolomitization 

 principle. Dominant lime-bearing minerals, as the garnets, 

 vesuvianite, wollastonite, etc., are conspicuous by their absence, 

 and when they do appear it is usually after all magnesia has 

 been satisfied. For example, scapolite and bytownite appear in 

 associated rocks originally so impure that their metamorphism is 

 accompanied by complete ehmination of carbon dioxide. These 



