Vol. 54.] WITWATER8RAjS"D and other districts in S. TRANSVAAL. 89 



CaO 29 -ei per cent. 



MgO 19-71 



FeO 1-35 



MnO 118 



SiO, 0-94 



CO2+H2O 46-69 



99-48 



This analysis shows that the dolomite has the following percentage 

 proportion of carbonates : — 



OaCOg = 52-87 

 MgCOg =. 41-39 

 FeCOy = 217 

 MnCog = 1-99 



The specific gravity is 2'88. Ignited strongly on platinum foil, 

 the powdered dolomite turned chocolate-brown, due to the oxida- 

 tion of the manganese. 



Certain portions of the dolomite are highly impregnated with 

 silica ; and almost every graduation appears to exist between 

 dolomite and chert. A specimen of siliceous dolomite analysed by 

 Mr. Prior gave 6*84 per cent, of residue after treatment with 

 hydrochloric acid. The percentage of silica, however, was only 

 2-81 per cent. A siliceous layer in the dolomite analysed by Dahms 

 gave the following result^ : — 



SiOg 62-16 per cent. 



CaO , 9-07 



MgO 3-01 



Veins of quartz (sometimes carrying gold) are of fairly frequent 

 occurrence in the Dolomite Formation. Often these veins occupy 

 a horizontal position. They are always associated with deposits of 

 oxides of manganese (pyrolusite, wad, etc.). Other mineral deposits 

 are also found (galena, copper sulphides). 



Some interesting dykes occur in the dolomite. Two of these, 

 lying 5 miles apart at Wonderfontein, 22 miles south-west of 

 Krugersdorp, strike north and south. They are handsome pink 

 porphyry-rocks and are quarried for ornamental building-stone. A 

 section of the western dyke shows under the microscope porphyritic 

 crystals of orthoclase, plagioclase, and augite embedded in a holo- 

 crystalline and granophyric groundmass of quartz and felspar. It 

 may be termed a ' granophyric syenite.' This rock has been 

 described by Dahms ^ under the name of syenite-porphyry. The 

 eastern dyke is of similar composition, but contains no quartz. It 

 consists of green augite, brown mica, and large lath-shaped crystals 

 of orthoclase-felspar. It is an augite-syenite. 



' Neues Jahrb. Beilage-Bd. vii (1891) p. 118. 

 2 3id. p. 129. 



