Makch 2, 1896.] 



KNOWLEDGE 



51 



jj / 7. Qiiartzites. 



"3 I 6. MalmaDi limestone. 



o I 5. Bokkeveld beds (wanting in the Transvaal). 



3 ^4. Table Mountain sandstone.* 



(5 I Unconformability. 



V 3. Malmesburv schists. 



2. Gneiss, and 



1. Granites. 



The auriferous beds or "reefs" are in a succession of 

 series, each series having two, three, or more reefs, 

 separated by the country rock of diflerent thicknesses 

 ranging up to one hundred and fifty feet, while the reefs 

 themselves have thicknesses up to six or seven feet. These 

 reefs consist of conglomerates, in which rounded pebbles 

 of white and tinted quartz are embedded in a quartzose 

 matrix, which, although originally sandy and loose, has 

 been, by the infiltration of silica, rendered a very hard 

 and compact mass. To the conglomerate the name 

 " banket " is given. The quartz pebbles range in size from 

 that of a pea to stones a couple of inches in diameter. It 

 is not, however, in the pebbles that the gold occurs, but 

 in the hardened quartzose matrix, and through this it is 

 disseminated in small particles — so small, indeed, that it 

 is often invisible to the eye ; but this is abimdantly 

 compensated for by its generally regularly continuous and 

 not intermittent dissemination. 



Associated with the gold — which is in a metallic or 

 "native" state, and not as an ore, or chemically combined 

 with some other element — are the following metallic 

 minerals : iron pyrites, with its variety, marcasite, hiema- 

 tite, ilmenite, magnetite, copper pj-rites, blende, galena, 

 and, more rarely, stibnite or antimonite, cobalt, and 

 nickel. Of all these, iron pyrites is the most abundant. 

 EatLle, zircon, corundum, mica, talc, and chlorite are also 

 met with. Besides " free gold," there are extremely minute 

 particles of gold contained in microscopical interstices of 

 the u-on pyrites, and which, accordingly, may escape 

 amalgamation and chlorination, and even the searching 

 cyanide of potassium, and so be lost to the miner. With 

 improved processes and methods the amount of gold thus 

 lost has been greatly reduced, by which much poorer 

 material is being rendered payable than was formerly the 

 case. 



to a certain depth from the surface, or without deep 

 shafthigs, and the earlier mines were along these outcrops. 

 It is obvious, however, that if the reefs dip from the 

 surface at an angle of forty-five degrees— the average dip^ 

 shafts at some distance from the outcrop in- the same 

 direction as the dip, if sufficiently deep, will strike the 

 reef, and thus there are three kinds of levels : upper, middle, 

 and lower ; but the lowermost levels as yet are only about 

 one thousand feet deep, whereas workings at five thousand 

 feet depth may possibly be attempted in the future. 



In the older goldfields of Lydenberg and the De Eaap 

 Valley the geological conditions are different from those 

 of the Witwatersrandt, for the gold occurs there in thin 

 " leaders," as they are termed, ranging from one-eighth of 

 an inch to eight or nine inches in thickness, which cut 

 through nearly horizontal more or less soft strata of a 

 different geological age from that of the quartzites and 

 conglomerate reefs of the Witwatersrandt. These leaders 

 chiefly consist of siderite or carbonate of iron, and quartz 

 with much oxidized iron pyrites. At Spitz Kop as many 

 as thirty such leaders have been recorded ; and sometimes 

 these, when in fine dove-coloured argillaceous shale, become 

 very thin, and then are unusually rich, in some cases being 

 formed of plates of solid gold from one to ten ounces in 

 weight. Here, too, u'on pyrites is abundant, sometimes in 

 large crystals and groups of crystals many pounds in weight. 



Again, gold is reported as occurring at King's Claim in 

 a soft breccia of sandstone, shales, etc., interpenetrated by 

 decomposed diorite ; and in SwazOand, gold associated with 

 native bismuth is found in the heart of quartz. In the 

 Blyde River Valley saccharoidal quartz is in parts richly 

 auriferous, whOe at Kantoor " flour-gold" is disseminated 

 not only through quartz veins, but also through their 

 enclosing decomposed diorites. 



With geological conditions such as those that obtain in 

 the Transvaal it is evident that the total yield of gold wiU 

 continuously increase with the increase of mining opera- 

 tions both in area and depth, until they are co-extensive 

 with the workable auriferous beds or reefs. Thus, with 

 the establishment of new claims and the working of deeper 

 levels the annual output of the precious metal may be 

 expected to show a yearly increase. What will be the 



Generalised Section showing details of a Witwatersrandt Keef Series. Length of Section, about 103 feet. 

 ° Ak B A _B AB__A B ^^ ^^_A_Jg-. 



A. Banket, beds of Gold-bearing Conglomerate. B. Country Rock. Qiuirtzites and Sandstones. 



Ecef Leader. 4. Main Ecef. 5. North Reef. 



1. South Reef. 



Middle Reef. 3. Main 



Although the gold is not chemically combined with any 

 other element, it is always mechanically intimately 

 associated with other substances, and is therefore never 

 obtained as absolutely pure gold. The proportion of 

 "fine gold," as perfectly pure gold is termed, is here 

 about from eight hundred and twenty to eight hundred 

 and fifty parts per thousand, with from one hundred 

 and twenty to one hundred and forty parts of silver, and 

 from thirty to forty parts of other impurities. 



Since the auriferous reefs cro p out they can be worked 



• The gi)ld-beariug rocks of the Witwatersrandt are assigned to this 

 formation. 



ultimate limit of that increase it is impossible to say ; bat 

 Mr. Hays Hammond, whose great practical knowledge of 

 the auriferous deposits of the Rand is so well known, says 

 he " would regard as well within the bounds of conservatism 

 the prediction that the annual output before the end of the 

 present century will exceed twenty millions sterling worth 

 of gold." 



The metallic riches of the Transvaal are not confined to 

 gold. 



Silver in association with copper and lead occurs in 

 granitic rocks in several parts of the country, as near 

 Malmani, Pretoria, Rustenberg, and north of Middleberg. 



