1906] Professor Gregory on Ore Deposits. 805 



WEEKLY EVENING MEETING, 

 Friday, April 27, 1906. 



Sir Williax Crookes, D.Sc. F.R.S., 

 Honorary Secretary and Vice-President, in the Chair. 



Professor John W. Gregory, D.Sc. F.R.S. 



Ore Deposits and their Distribution in Depth. 



Primitive man obtained his limited store of metal by gathering 

 scanty grains and pebbles of ore from the beds of streams ; but even 

 in pre-historic times he worked shallow mines, for Job tells us that 

 the gold miners of his day not only diverted rivers from their courses, 

 as in modern alluvial practice, but they put forth their hands upon 

 the flinty rock, by which he doubtless meant quartz. Still less do 

 alluvial ores satisfy modern requirements, even though the great 

 metal-using countries place the rest of the world under tribute of its 

 mineral wealth ; and owing to the exhaustion of the richer ores in 

 the more accessible mining fields, miners are now thawing the frozen 

 gravels of Klondike, washing the gold-bearing loams in the Siberian 

 rivers during their short summer flow, mining in the malarial jungles 

 of West African coast lands, and sluicing low grade drifts in the 

 heart of Africa, in Katanga. 



We caimot expect many more discoveries of alluvial deposits as 

 rich and as extensive as those of California and Australia ; nor can 

 we go back to the low metal output of a century ago. The exhaus- 

 tion of coal we destroy so wastefully might not be an irremediable 

 disaster, for there are other sources of heat and power ; but the metal 

 famine that would follow a return to the metal supplies of 1800, or 

 even 1850, would destroy the whole fabric of our civilisation. 



The supply of metals can be maintained in two ways. We may 

 work superficial alluvial deposits with more refined methods of 

 metal recovery ; or we may mine the more deeply buried primary 

 ore-deposits. 



The simplest method of collecting ores is to wash the gravels that 

 contain them in a tin dish ; but this method is only profitable with 

 ground so rich that it can be handled in small quantities. Poorer 

 material can be washed in simple machines, but they are only 

 successful economically, when the ore has undergone preliminary 

 concentration by nature. Sluicing and dredging recover profitably 

 the minute grains of ore scattered through thick sheets of barren 

 material. Dredges work with marvellous economy. A dredge will 



Vol. XVIII. (No. 100) _ x 



