﻿510 DE. F. H. HATCH AND MR. E. H. RASTALL ON [Nov. ICJIO, 



IY. Petrographical Description of the Dolomite, its 

 Inclusions, and its Alteration- Products. 



(1) The Dolomite. 



The dolomite, away from the sphere of influence of the inclusions, 

 is a beautiful white marble of very coarse texture : in the hand- 

 specimens scarcely any coloured minerals are observable. The 

 following che'mical analysis of a typical specimen, made by Mr. Camp- 

 bell Smith, shows that the rock is composed of 74*56 per cent, of 

 dolomite and 21*43 per cent, of calcite, with 2*46 per cent, of 

 insoluble matter and 1*34 per cent, of water : — 



Calcium carbonate 6P95 



Magnesium carbonate 34'04 



Ferrous carbonate ... 021 



Insoluble matter 2 - 46 



Water and loss 1*34 



Total 100-00 



To gain an idea of the average composition of the dolomite in the 

 Marble Delta Company's Quarry, a sample was taken by chipping off 

 a great number of small pieces from the quarried material, of which 

 a large stack had been accumulated. This sample was analysed in 

 Pietermaritzburg by Mr. F. W. Penny, and was found to have the 

 following composition * : — 



Calcium carbonate 57*75 



Magnesium carbonate 37"46 



Ferrous oxide 0*50 



Insoluble matter 316 



Water and loss 1'13 



Total 100-0 



Ratio of CaC0 3 : MgC0 3 = 3:2. 



An examination of a thin section shows that, besides carbonates, 

 the rock contains a small quantity of a colourless augite (diopside) 

 showing the characteristic high refractive index, strong birefrin- 

 gence (about # 02), and a maximum extinction-angle of 36°. 



(2) The Granite Inclusion and its Reaction-Border. 



(a) The granite inclusion. — The granite inclusion consists 

 of a subangular block, the longest dimension of which is roughly 

 3 feet (see photograph, fig. 3, p. 512). The rock is of a light- 

 grey colour and of fairly fine texture. Under the microscope it 

 is seen to be a holocrystalline, even-grained aggregate of quartz, 

 perthite, and a green ferromagnesian mineral, the felspar being the 

 predominant constituent. Besides perthite, there is a very small 



1 F. H. Hatch, ' Report on the Mines & Mineral Resources of Natal ' 

 Published by order of the Natal Government, London, 1910, p. 99. 



