Vol. 50.] DOLOMITE 1ST SOUTH AFRICA.. 5G3 



to Yredefort. This section shows the ' dolomite ' in its true position 

 with regard to the other rocks. A section from Pretoria to Vrede- 

 fort (south) would show a similar occurrence of the several strata. 



In ' Petermann's Mittheilungen,' vol. xxxiv. (18SS)p. 227, I find 

 the following description of this rock by Dr. A. Schenck : — 



" A characteristic rock which accompanies the strata of the Kaap 

 Formation throughout South Africa, and here and there extends over 

 a wide area, is a peculiar, blue-black, dolomitic limestone. It occurs 

 both on the Huib and the Han-ami plateaux in Great Namaqualand, 

 and also in Griqualaud West, where it forms the so-called Kaap 

 plateau, west of the Vaal, and extends from thence over a great part 

 of Bechuan aland anda great part of the Western and Central Transvaal 

 (Marico, Lichtenburg, Wonderfontein). Farther off it appears also in 

 the Northern Transvaal, and on the Drakensberg (Pilgrims' Pest, 

 Spitzkop, Krokodil River)." 



In the geological map, pi. xiii., accompanying Dr. Schenck's paper, 

 the position of the dolomite is marked ; and it appears to be of great 

 extent, especially in the Transvaal and Namaqualand. 



In the Malmani district of the Transvaal the dolomite is intersected 

 by numerous fissure-veins of quartz, bearing gold ; but, owing to 

 the immense difficulty experienced in draining the mines, the district 

 has been abandoned by the gold-miners. It was found that under- 

 ground channels of communication existed between the various 

 streams and pools in the neighbourhood, and all attempts to free 

 the mines from water proved abortive. 



Lead, zinc, cinnabar, silver, gold, and other metals have been 

 found in small quantities in the quartz-veins and pockets in the 

 dolomite at Malmani, and at Lydenburg the same rock is being- 

 worked for gold. 



In the Malmani district numerous large holes occur in the dolomite ; 

 some of these are of great extent ; for instance, the ' Baviaan Gat,' 

 near the village of Otto's Hoop, is about 100 yards in diameter at 

 the surface, narrowing to about 60 feet in a depth of 120 feet, and 

 at that depth is a pool of water 105 feet deep. These holes, ' Wonder 

 Holes ' they are called locally, have been caused by the disintegration 

 of the rock by natural causes, and the eventual sinking of the 

 surface-ground or roof, when too thin to support its own weight. 

 In one cave near Otto's Hoop the roof has fallen in and the trees 

 which grew on the surface are now found growing at the bottom of 

 the cave. 



Mr. Francis Galton, in his ' Tropical South Africa,' p. 200, 

 describes water-holes similar to those at Malmani, as occurring 

 at Otchikoto and Oriejo ; and, judging from his description, it 

 would appear that the dolomite occupies a large area in Ovampoland. 

 Pecent descriptions of the great caves in Mashunaland, and of the 

 rocks in which they are situated, lead to the supposition that this 

 rock extends to the neighbourhood of the Zambesi Piver. 



It will therefore be seen that it occupies an important position in 

 the geology of South Africa, and is worthy of more than a passing 

 notice. 



Q. J. G. S. No. 200. 2 r 



