318 Professor John W. Gregory [April 27, 



the lithosphere have a specific gravity of 2*5; the barysphere must 

 contain masses of a specific gravity of at least 6 and 7, and the 

 average specific gravity of the average zone at the depth of 37,000 

 feet will be only 2 "52. But as ore formation probably takes place 

 where the barysphere is nearer the sm-face, its specific gravity at the 

 critical temperature of water beneath a mining field in process of 

 formation will proljably Ije higher. But it can hardly be more than 

 3*0. So the metals in this zone of the crust will l)e more diffused 

 than in a lode. The process of ore formation is the collection of the 

 scattered particles, their concentration in a lode, and then the sorting 

 out of the mixed, complex, primary constituents into purer, simpler, 

 secondary minerals. 



Thus, gold in a deep lode is always alloyed with silver, and pro- 

 bably with lead, zinc, and antimony as well. It is only near the 

 surface that gold occurs with only 3 parts in the 1000 of silver ; there 

 are veins of native metals. 



Similarly with silver : its deep ores are always alloyed with lead. 

 It occurs as native silver and as horn-silver only in shallow ores. 



Again, with copper, the primary ores are complex sulphides 

 combined with iron. Thus the commonest primary ore is of copper 

 chalcopyrite with 34*5 per cent, of copper, and 30*5 per cent, of 

 iron ; and the mineral occurs as streaks in iron pyrites, so that the 

 copper is often only 1 or 2 per cent, of the ore. This material is 

 attacked, its constituents dissolved and re-deposited, probably as 

 veins of bornite, which contains 55*5 per cent, of copper, and only 

 16*4 per cent, of iron ; and bornite often occurs in comparatively pure 

 veins, the great bulk of the iron having been deposited apart as veins of 

 haematite. The secondary bornite is then in turn attacked, and the 

 copper re-deposited as the tertiary minerals, covellite, with 66*4 

 per cent, of copper, or cuprite, with 88 * 8 per cent, and no iron ; and 

 in the last stage in the process the copper occurs in veins or masses 

 of native copper. 



The depths to which ores will descend depend on the factoi*s 

 that control l)oth their deposition and concentration. In considering 

 the future of any mining field, the question of primary importance 

 is whether the ores have been deposited l)y ascending or descending 

 solutions, and we are beginning to acquire sulficient knowledge to 

 form some idea as to the probable depths to which various types of ore- 

 deposits will go. 



Thus, there is no reason why gold quartz lodes should not go down 

 to depths of about 18,000 feet below the level of the surface at the 

 time of their formation. Hence, in estimating the depth to which any 

 particular lode will go, we have to consider how much of the upper part 

 of the lode may have been destroyed by denudation. In two adjacent 

 mining fields, in one the lodes may continue to be auriferous to the 

 depth of 5000 or 7000 feet from the present surface, whereas, in 

 the other, where the lodes were formed in precisely the same way, 



