Becker — Auriferous Conglomerate of the Transvaal. 201 



occupy. The gold in the banket is not distributed in any 

 apparent relation to the dislocations. Not only do the dikes 

 cut the reefs in poor and rich patches indiscriminately, as was 

 pointed out above, but such of the dislocations as are in sand- 

 stone or in quartzite are, as a rule, absolutely barren. The 

 fissures are also almost altogether wanting in some localities 

 where the ore is nevertheless of good grade. This is the case 

 in portions of the Geldenhuis Deep and of the Simmer and 

 Jack. In these mines there is often an inconvenient absence 

 of any partings to which stoping can be carried, and this 

 "freezing" is not attended by any falling off in the assays. 

 On the other hand, the white quartz veins are manifestly 

 related to the dikes, often starting from them or accompanying 

 them, and themselves not dislocated by the fissure system to 

 any greater extent than is easily accounted for by slight 

 renewals of movement postdating the main upheaval. The 

 white veins, however, cut and fault the banket, but without 

 enriching or impoverishing it. The only natural conclusion is 

 that the dikes and white veins postdate not merely the forma- 

 tion of the conglomerate, but the deposition of gold in the 

 banket. The deep fissures which afforded channels for the 

 ore-bearing solutions might be narrow, as a rule ; but it is 

 hardly possible that all of them should be so, and it is most 

 improbable that the ore which they contained should not be 

 rich, as rich as the banket would be if all the pebbles were 

 removed from it. Now, no such "rake veins," as they would 

 be called, have been met with on the Rand. 



If the solutions rose through "rake veins," they would per- 

 meate all accessible permeable strata and partings. Now, it 

 seems to be generally assumed that a conglomerate without 

 matrix would be much more permeable to fluids than a sand- 

 stone. This is true if only rapid currents, such as might be 

 impeded by fluid friction, are to be considered. There is no 

 probability, however, that ores are deposited from rapid cur- 

 rents. For slow currents fluid friction is a vanishing quantity, 

 and then the permeability of a given rock may be considered 

 as measured by the interstitial space. Now, the interstitial 

 space in a conglomerate composed of pebbles of various sizes 

 is really less per cubic foot than that of a tine but even-grained 

 sandstone. If the material is of absolutely uniform grain, the 

 interstitial space is wholly independent of the size of the 

 component particles, and cannot fall below 26 per cent. It is 

 a fact well known to mill men that a cubic foot of sized tail- 

 ings weighs less than a cubic foot of unsized tailings. Sand- 

 stones used for building in some cases contain over 20 per 

 cent, of voids, and highly indurated sandstones contain 4 or 5 

 per cent. 



