South Australian Dolomites. 



461 



It can be considered that the veins have arisen as a consequence 

 of the stress set up during the hydration by probably meteoric waters 

 of the original forsterite to compact serpentine. Potential fractures 

 arising through this agency, the minute fibres of heterogeneous 

 orientation, are subject to an active uneqiial pressure in the vicinity 

 of the points of potential fractures, such that in recrystallizing 

 solution takes place in those fibres oriented across the direction of 

 minimum pressure, only to be redeposited on those fibres whose 

 elongation is parallel to the direction of minimum pressure, uniform 

 cross fibre veins thus arising. 



The relationship of the new-formed calcite to forsterite. 



Teall,^ first pointed out that forsterite formed during metamorphism 

 was intimately associated with the new-formed calcite and usually in 

 direct contact. It has already been remarked that the same features 

 are to be seen in the rocks under discussion, and in many cases the 

 association is very striking. The forsterite may be wholly embedded 

 in calcite or partly surrounded by an aureole of this mineral. Crook ^ 

 has pointed out that in many examples of forsterite marble studied 

 by him the silicate is separated from the calcite by a thin ring 

 of dolomite. No cases of this nature have been found in these 

 marbles. The distribution of dolomite, calcite, and forsterite is 

 often best seen by staining a thick slice of the rock and then covering 

 with a cover-glass. The individual grains can then be easily made out 

 with a lens. 



Dolomite band 



Forsterite. 



Fig. 2. — Diagrammatic sketch of portion of a flat surface of dolomite. 



A dolomite from the Hutchison area treated in this way was seen 

 to consist of bands of unstained dolomite, alternating with a band in 

 which the forsterite grains are concentrated, and of calcite (see Fig. 2). 



This appears to afford evidence that we have here to deal 

 with bands of originally difiering composition, viz. dolomite bands 

 free from impurities and containing detrital silica respectively. 

 As a result of the formation of forsterite, the siliceous band is 

 dedolomitized, the newly formed forsterite being surrounded by 

 recrystallized calcite derived from the reaction. These examples 

 serve to confirm the view that the radius of diffusion in thermal 

 metamorphism is confined within comparatively narrow limits. 



1 J. J. H. Teall, Geol. Mag., 1903, pp. 513-14. 

 '^ T. Creole, Geol. Mag., 1911, pp. 339^5. 



