1906.] on Ore Dpjwsits and their Distribution in Depth. 321 



These three groups of ores hold reserves of ore, the deep working 

 of which is controlled by the elastic limits of expense. The assumed 

 boundary of practical mining is being continually pushed downward. 

 Once the final limit was accepted as 3000 feet, but there are already 

 gold mines working at the depth of 42.50 feet, and copper mines at 

 5000 feet, and preparations are being made for mining at 6000 feet. 

 The greatest obstacle to deep mining is the high temperature ; but 

 that can be reduced by ventilation with compressed air or by the 

 circulation of cold brine. It is unnecessary to suggest the powers 

 given to the miner by liquid air. It gives him means of cooling and 

 ventilation to which it is difficult to set a limit. There can be no 

 doubt that the ore-deposits below the present limits of mining, for 

 the existence of which the geologist can safely vouch, will one day be 

 accessible. The profitable working of these deeper ores will require 

 the close co-operation of the engineer and the geologist. Then the 

 miner, armed with the invincible ingenuity of the engineer, and 

 guided by the insight of the geologist, will follow the ore-shoots, in 

 their irregular courses, into ever deeper layers of the earth's crust, 

 and wrest from them their long hidden secrets and their well buried 

 stores of wealth. 



[J. W. G.] 



Vol. XVIII. (No. 100) 



