132 DE. A. L. DU TOIT ON THE [vol. lxXV, 



VI. The Calcttization oe Dolomitic Limestone. 



The most curious phenomenon at the Marble Delta is the 

 conversion under the influence of certain of the granite intrusions 

 of the dolomitic marble into calcite through the complete removal 

 of the magnesia present. 



This action, here referred to as calcitization, though akin 

 to the well-known phenomenon of dedolomitization, is some- 

 what different, for in the latter the magnesia merely becomes 

 segregated within the scattered silicate-crystals developed through- 

 out certain parts of the metamorphic aureole — a concentration in 

 situ, it may be termed. 



The initial stage in the withdrawal of the magnesia is seen in 

 the case of small granite- veins, where the reaction-zone of 

 magnesium-bearing silicates is separated from the dolomitic rock 

 by a few inches of coarsely-crystalline calcite, this indicating 

 presumably the migration of the magnesia towards the intrusion. 



On a much bigger scale is the feature to which I now direct 

 attention. For example, along the tramway immediately be} r ond 

 the fourth quarry on The Glen, the marble beside the upper- 

 most of the two nearly vertical granite-dykes has acquired a 

 coarse texture, and consists of interlocking crystals of semi- 

 translucent calcite from 06 to 12 cm. across, while the bedding- 

 planes have become less distinct. Hardly represented on the lower 

 side of the intrusion, this type of alteration extends for a distance 

 of from about 10 to perhaps as much as 30 feet from the upper 

 wall, the limit being ill-defined, and can be traced in contact 

 with the dyke up the steep hillside to the old tramway about 50 feet 

 above ; it transgresses the bedding-planes of the marble, which 

 are here dipping towards the dyke. 



The body is large enough to serve as a source of agricultural 

 limestone, and, whereas the adjacent rock averages from 15 to 

 20 per cent, of magnesium carbonate, the more coarsely crystalline 

 and almost semi-translucent product contains from under 2 to 

 about 6 per cent., the silica, iron, and alumina being low in both. 



The 45 -foot granite dyke near by includes a large xenolith in 

 which the cleavage-faces of calcite are well over an inch across. 

 A similar occurrence was recorded earlier, in § III, p. 124. 



Again, on the spur descending to the bed of the Umzimkulu 

 on Westlands there is a large body of similar rock along the 

 main gneiss-contact, the general zone of alteration having a length 

 of over 150 yards at least, while the depth, though uncertain owing 

 to vegetation and talus, is probably not less than 40 feet. Six 

 samples were taken from along this spur, the percentage of 

 magnesium carbonate being 1*36 in three of them and 7*4, 8*7, 

 and 106 in the others ; the impurities ranged from 1'5 to 4 per 

 cent., represented by minute grains of silicates and specks of 

 graphite. 



Another case is the strip flanked by gneiss on the farm F.O., on 

 the right bank of the Umzimkulu, and dipping nearly vertically. 



