436 MK. W. GIBSON ON THE GEOLOGI OF THE GOLD-BEARING 



paper which had just been read ; but as he had inspected many gold- 

 lields in California, South America, and in different parts of the 

 world, and obtained a practical knowledge of their surroundings, it 

 appeared to him evident that the district now described by Mr. 

 Gibson bore no resemblance, geologically speaking, to anything 

 hitherto discovered, and therefore it was one of especial interest. 

 The speaker did not think that the gold in the quartzites and conglo- 

 merates could be called alluvial gold, because from all the information 

 he had on the subject the gold was found in a state of fine division, 

 whilst in all true alluvial deposits the gold was found in various forms 

 and sizes, varying from lumps of many pounds in weight to the 

 fi.nest dust. The only case which had come under his notice of metals 

 being found in conglomerates (ancient river-beds being excepted) 

 was that of the copper mines on the southern shore of Lake Superior, 

 in Michigan, U.S.A. There the copper was found in a metallic state, 

 sometimes in very fine dust and grains, and often in masses weigh- 

 ing many tons. It was most likely that the gold had been deposited 

 in the quartzites and conglomerates, and was not derived from allu- 

 vial sources, as some would suppose. 



Mr. ToPLEY remarked that the Banket deposits of the Gold Coast 

 seemed to contain gold introduced into the conglomerate at the time 

 of its formation ; whilst in the Witwatersrandt area the gold had 

 frequently been deposited around the pebbles after the formation of 

 the conglomerate. 



Mr. Alford remarked that the paper dealt only with the gold- 

 bearing deposits of the Witwatersrandt, and that nothing had been 

 said about the very important series of auriferous rocks of the Bar- 

 berton district, which were of an entirely different nature. The 

 coal-bearing sandstones were by no means small deposits, as they 

 occurred over an area of about 1000 square miles, and, although 

 the coal-beds had not been proved throughout, there were strong 

 indications of their existence. The coal-seam at Brakpan Colliery 

 was 21 feet in thickness, and the output of coal at that coUiery 

 exceeded 13,000 tons per month. The outcrops of the conglomerate- 

 reefs, though in places faulted a few feet, were traceable east and 

 west of Johannesburg for a total distance of nearly 40 miles. They 

 were by no means regular in gold-bearing value, which appeared 

 generally to be greater where the beds had a high angle of dip and 

 in the proximity of the intrusive igneous rocks. He did not con- 

 sider the thickness of the quartzite-and-conglomerate series to 

 be very great, but thought that the general break-up of the strata 

 by the igneous intrusions had brought portions of the same beds re- 

 peatedly to the surface ; and he entirely concurred with Prof. Green 

 in his description of the multitude of trap dykes in the Orange Pree 

 State, which also agreed with many parts of the Transvaal. The 

 origin of the. gold in these conglomerate-beds was a most interesting, 

 but at the same time a most intricate subject. The gold occurred in 

 the matrix of the conglomerate, and seldom in the quartz-pebbles. 

 On taking a pebble from its resting-place in the conglomerate, both 

 the surface of the pebble and also that of the cavity from whence it 



