Yol. 50.] OCCURRENCE OF DOLOMITE IK SOUTH AFRICA. 561 



36. The Occurrence of Dolomite in South Africa. 

 By David Praper, Esq., F.G.S. (Read May 23rd, 1894.) 



[Plate XXIII.] 



Mr. W. H. Penning, F.G.S., examined a certain rock near Lyden- 

 burg, when in company with Mr. A. C. Crutwell, and fchey 

 described it in the Quart. Journ. Geol. Soc. vol. xli. (1885) p. 57ti. 

 as " a peculiar blue, hue-grained, calcareo-siliceous rock." This 

 they named " ' chalcedolite,' in consequence of the chalcedonic texture 

 frequently displayed — indeed, some portions of the rock are true 

 chalcedony. . . Sometimes it occurs in amorphous masses, weathered 

 to a grey colour, and to a peculiar, rough, trachyte-like surface; 

 but mostly in thin beds, 2 or 3 inches in thickness, with earthy 

 partings." 



Mr. Penning in another contribution, Quart. Journ. Geol. Soc. 

 vol. xlvii. (1891) p. 456, mentions the same rock (chalcedolite) as 

 immediately overlying the ' Black Reef series ' of what he calls the 

 Megaliesherg Beds.' 



Mr. C. J. Alford, F.G.S., in his ' Geological Features of the Trans- 

 vaal,' 1891, mentions the occurrence and peculiarities of this rock 

 as follows : — 



Page 3. Referring to ' alluvial deposits,' he states : — " In several 

 places deposits of a fairly pure crystalline carbonate of lime are met 

 with, those in the vicinity of the Six-mile Spruit, between Pretoria 

 and Johannesburg, being some of the most remarkable ; these appear 

 to result from the disintegration of a hard blue quartzite plentifully 

 veined with lime, which occurs in the neighbourhood." 



Page 6. " The metamorphism of these beds [' schistose rocks '1, 

 where in contact with, or adjacent to, the irruptive rocks, results 

 in several somewhat complicated products ; amongst others the cal- 

 careous quartzite before mentioned, which in some places passes into 

 dolomite, and on exposure to atmospheric influence yields up its 

 lime and becomes, on the surface, altered into a species of chert. 

 This calcareous quartzite is locally known as elephant-rock, from the 

 resemblance which its weathered surface bears to the hide of that 

 animal. In other places, as on the hill near the junction of the 

 Olifants and Blyde rivers, it has resulted in very calcareous beds of 

 granular quartzite, bearing in structure a marked resemblance to 

 dolomite, in some of which the siliceous and calcareous matter 

 has assumed a peculiar banded arrangement." 



And finally, at p. 25 : — ''-In the neighbourhood of Wonderfontein 

 and in other places, the erosion of the exposed surface of the altered 

 sandstone has produced very peculiarly weathered rock-masses, and 

 the underground watercourses have resulted in the formation of 

 many a series of extensive caves, the tortuous windings of which 

 can be followed for a longdistance, and which form the underground 



