462 Metamorphism of South Australian Doloviites. 



The occurrence of small grains of spinel embedded in dolomite 

 without proximity to calcite may perhaps be explained by assuming 

 that the ne'\f-formed calcite is completely absorbed within the 

 dolomite molecule— the evidence pointing to the view that there is 

 a limited miscibility of these two minerals, conforming, presumably, 

 to Type 5 of Roozeboom's classification.^ 



TJie criteria for the distinction of dolomite and calcite. 



The three criteria which are available for the distinction of 

 dolomite and calcite in some of these rocks have already been 

 mentioned. Of these the most important is the distinction based 

 on the turbidity of the new^ formed calcite. The dolomite, on the 

 other hand, is remarkable for its clarity and limpid character. The 

 turbidity of the calcite, when examined under the high power, is 

 seen to be related to the presence of minute inclusions of two types, 

 one colourless to greyish particles of high refractive index, and certain 

 opaque particles of uncertain nature, but perhaps carbonaceous. 

 However this may be, it is evident that the turbidity is of general 

 occurrence. Some of the particles may be of detrital origin, and 

 their presence in calcite and absence in dolomite may stand related 

 to the position of these minerals in the crystalloblastic series. 



Dolomite is the stronger mineral, and is usually idioblastic towards 

 calcite. During recrystallization fqreign material may be discharged, 

 only to be absorbed within the weaker and more yielding calcite. 



All these criteria are to be found in the literature on the con- 

 stitution of the crystalline limestones, but the data are somewhat 

 confusing. The criteria as noticed here are somewhat similar to 

 those advanced by Lacroix ^ for the limestones of Ceylon, and 

 later by Fermor ^ for the crystalline limestones of the Chhindwara 

 district, India. 



Some of them are diametrically opposed to the criteria summarized 

 by Eenard,^ and reprinted in the textbooks. Fermor would explain 

 his own examples (apparently as abnormal), and presumably those 

 of Lacroix, by the suggestion that the carbonates have been formed 

 direct from silicates, and not by the alteration of pre-existing 

 calcite. Whilst it is true that the carbonates in the rocks under 

 discussion have been recrystallized, there is not the slightest evidence 

 that this silicate derivation obtains. They are recrystallization 

 products under the influence of thermal metamorphism, the majority 

 of the calcite being released from original dolomite molecules. 

 (To be contimied.) 



^ R. C. Wallace, Compte-Rendu XII« session Congres Geologique Intei- 

 national, Canada, 1913, pp. 882-0. 



2 M. Lacroix, Rec. Geo). Suiv. India, vol. xxiv, 1891, pt. iii, pp. 191-2. 



* L. L. Fermor, ibid., vol. xxxiii, 1906, pt. iii, pp. 195-7. 



* A. F. Renard, Bull. Acad. Royale de Belgique, vol. xlvii, 1879, No. 5, 

 p. 557. 



