November i, 1900] 



NATURE 



19 



required merely to define the deposit by its outward form. Any 

 efforts to introduce improvements in mining must, however, 

 subordinate theory to practical requirements. 



In consequence of the difference of form in beds, veins and 

 masses, various methods of working have to be employed. 

 Underground mining is not necessary with all ore deposits. The 

 iron ore beds of Northampton, for example, and the alluvial beds 

 of river gravel containing gold are worked open-cast. 



ferous gravel 45 feet below the water, and stacking it 24 feet 

 above. 



The gradual increase in the world's annual production of gold 

 is shown in the accompanying diagram (Fig. 2). 



The value of the world's gold production in 1898 was 

 57,500,000/., of which the Transvaal produced 27'6 per cent., 

 Australasia 225 per cent., the United States 22'i per cent., 

 Russia 8*8 per cent., Canada 4*8 per cent , Mexico 3 per cent.. 



Fig. I. — Gold Dredge at work excavating auriferous Gravel 45 feet below the Water Surface. 



Of late years very successful results have been obtained by 

 extracting auriferous gravels from the beds of rivers by dredges. 

 The practice of dredging originated and has been brought to its 

 present state of perfection on the Clutha river, in the province 

 of Otago, New Zealand. Ground containing only a grain or a 

 grain and a half of gold per cubic yard can now be worked at a 

 profit. The remarkable yield of a dredge working at Cromwell, 

 on the Clutha river, which cost 5,000/. to build and launch, and 

 obtained more than that amount of gold within seven weeks after 

 starting, shows how quickly the capital sunk in the industryhas, 

 in some instances, been returned. Experience in Montana, 

 United States, shows that with a bucket-dredge 98 per cent, of 

 the gold in the gravel is extracted. The cost of dredging when 

 steam is employed is i,\d. per cubic yard, and when electricity 

 is employed for power 2\d. per cubic yard. The practice of 

 dredging is coming into increasing use in New Zealand, Canada, 

 California, Montana, the Republic of Colombia, and elsewhere. 

 It represents an important advance in the working of alluvial 

 deposits, and if the yields of gold in the future are not likely to 

 be so sensational, tHey will probably be more regular than they 

 have been in the past. The accompanying illustration (Fig. i) 

 shows the latest type of gold dredge made by the Risdon Iron- 

 works ol San Francisco. As represented, it is excavating auri- 



NO. 1618, VOL. 63] 



India 27 per cent., and China 2'l per cent. Thus the Trans- 

 vaal, Australasia, and the United States together produced 72 

 per cent, of the total. The production last year was even 

 greater, amounting probably to 62,700,000/., notwithstanding 



Fig. 2. — World's gold production (in million.s of pounds). 



the interruption in Transvaal mining. From the present con- 

 dition and prospects of the more important mines in Africa, 

 Australasia, the United States, Canada and other countries, 

 it seems that there are no signs of falling off in the world's gold 



