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XXIII. Proceedings of Learned Societies. 



GEOLOGICAL SOCIETY. 

 [Continued from vol. xx. p. 1009.] 



May 25th, 1910.— Prof. W. W. Watts, Sc.D., M.Sc, F.R.S.,. 

 President, in the Chair. 



HHHE following communications were read :— 



1. ' Dedolomitization in the Marble of Port Shepstone^atal)/ 

 By F. H. Hatch, Ph.D., M.Inst.C.E., F.G.S., and E. H. Rastall, 

 M.A., F.G.S. 



The Port Shepstone marble is shown by chemical analysis to be 

 a dolomite (the molecular ratio of calcium carbonate to magnesium 

 carbonate being as 3 : 2). It owes its marmorization to thermal 

 metamorphism by an extensive intrusion of granite, which completely 

 surrounds it and penetrates it in broad dykes. This intrusion took 

 place at some time prior to the deposition of the Table Mountain 

 or Waterberg Sandstone, and is therefore pre-Devonian. The 

 dolomite is relegated to the Swaziland Period. 



The metamorphism of the dolomite under normal conditions is 

 shown to have produced a saccharoidal marble of coarse texture, 

 consisting almost entirely of carbonates ; and the fact that neither 

 periclase nor brucite has been produced in the normal ma.rble is 

 taken to indicate that the high-pressure conditions obtaining during 

 the metamorphism precluded dedolomitization. In those places, 

 however, where the dolomite contains blocks or boulders of earlier 

 granitic rocks, interaction took place between the magnesium and 

 calcium carbonates of the dolomite and the silica and alumina pro- 

 vided by the inclusions, resulting in the production, in the zone of 

 marble immediately surrounding the inclusions, of a number of 

 interesting silicates of magnesium, calcium, and aluminium, such as 

 olivine, forsterite, diopside, wollastonite, and phlogopite, as well as 

 the oxides brucite and spinel. Magnesian compounds predominate,, 

 the excess of lime recrystallizing as calcite. A noteworthy feature 

 is the absence of minerals such as garnet and cordierite, which are 

 especially characteristic of low-temperature metamorphism, thus 

 indicating the prevalence of a high temperature during the meta- 

 morphism of the dolomite. 



The paper concludes with a reference to the occurrence of granite 

 boulders as foreign inclusions in other limestones, and a discussion 

 of the chemical reactions by which the formation of the above- 

 mentioned minerals may be theoretically explained as a result of 



