BE. F. H. HATCH — GEOLOGY OF THE [Feb. 1 898, 



Description of the Dolomite. 



This formation has an aggregate thickness, according to my 

 estimate, of from 6000 to 8000 feet. It is made up of two types 

 of rock : — 



(1) A compact bluish-grey magnesian limestone. 



(2) A hard white or grey chert. 



These two rocks appear in close association, the limestone almost 

 invariably having layers of chert interbedded with it. Sometimes, 

 however, the siliceous rock entirely replaces the dolomite, and 

 nothing but hard, smooth masses of chert are then seen outcropping. 

 It is this fact that gave rise, no doubt, to Penning's designation of 

 the formation as chalcedolite, and Alford's description as calcareous 

 quartzite. The cherty layers are occasionally much brecciated. 

 By weathering the dolomite acquires a whitish-brown crust and a 

 curiously wrinkled or corrugated appearance, which has given rise 

 among the Boers to the name of Elephant Kock (Olifants Xlip), on 

 account of its resemblance to the skin of an elephant. This appear- 

 ance is caused by the removal of the softer calcareous portions of 

 the rock, leaving the hard siliceous ribs and nodules sticking out. 

 The weathered surfaces are invariabl}^ coated with a brown dust, 

 consisting of hydrated oxide of manganese (wad). This manganese 

 is derived from the dolomite in which it is present in the form of 

 carbonate of manganese. 



A remarkable feature of the limestone is the frequent occurrence 

 of large swallow-holes, like those found in the Carboniferous Lime- 

 stone of this country. Another interesting feature is the great 

 quantity of water carried by the strata of this formation. The 

 beautifully clear water of the Mooi Eiver springs from this forma- 

 tion, the water welling out from the rocks in considerable abun- 

 dance. One of these so-called ' eyes ' or springs of the Mooi ^ 

 B/iver is situated 20 miles to the north-east of Ventersdorp and 

 another close to Prederikstad. These springs are probably connected 

 by an underground channel with the strong stream of water which 

 is seen underground in the Wonderfontein caves, 22 miles south- 

 west of Krugersdorp. Another similar spring or ' eye ' is the 

 source of the Schoon " Spruit, 6 miles north of Yentersdorp, and 

 there is one at the source of the Klip ^ Biver. 



With regard to the physical and chemical composition of the 

 dolomite, a section examined under the microscope shows it to be of 

 thoroughly crystalline character, having almost the appearance of 

 marble. The chemical composition is shown by the following 

 analysis, kindly made by Mr. G. T. Prior, of the Mineralogical 

 Department of the British Museum : — 



^ Mooi = beautiful. 

 ^ Schoon = fair, bright. 

 ' Khp— rock. 



