538 POPULAR SCIENCE MONTHLY 



of the unaltered ore below. In many instances the ore discovered 

 from the outcroppings is gold ore. And gold mills are often erected 

 and operated for years upon such ore, without a suspicion arising that 

 extensive bodies of copper or lead sulphides occur at greater depths. 

 Such was indeed the history of Leadville, Colorado ; of Bingham, Utah ; 

 of Ely, Nevada, and of Mount Morgan, Australia. The latter is 

 one of the world's greatest gold mines; yet it is now producing copper 

 from its lower levels; and developments have proved it to be a great 

 copper mine. Immense low-grade deposits of copper ore are found below 

 the gossan at Ely and at Bingham, although it is doubtful whether the 

 most experienced geologist or keenest observer of mineralization phe- 

 nomena would in either place have felt justified in predicting the 

 existence of the wealth below. 



In other localities the metal values have either all been removed, 

 or else the primary sulphide ore was too poor in gold to leave oxidized 

 ores of value. In such cases the discovery of the subterranean treasures 

 is purely fortuitous. Butte may be considered the most conspicuous 

 example of this class. The outcrops of its copper veins contain the 

 merest traces of that metal; and there is seldom enough silver or gold 

 in them to justify mining even under the low costs obtaining there 

 to-day. The zone of oxidation is generally from one hundred to two 

 hundred feet deep; and if it had not been for the presence of another 

 system of veins carrying silver, veins of different age and origin, but 

 closely associated geographically, this greatest of copper camps might 

 not yet have been discovered. It was in the search for silver ore that 

 copper ore was discovered here, and one can not help wondering how 

 many more camps equal to Butte may be undiscovered and unsuspected 

 where no outcropping silver or gold mines attract the prospector, and 

 reward the efforts of the miner. Here is surely an important and un- 

 explored field for the geologist. The study of oxidized vein phenomena 

 may yield results thoroughly satisfactory from both material and 

 scientific points of view. 



Below the zone of oxidation the chemical reactions which take 

 place between the descending acid solutions and the unoxidized ores 

 result in the formation of more and richer sulphides, down at least to 

 the level of the lower limit of free circulation, and as far as surface 

 waters penetrate. And as erosion of the surface is continually bringing 

 deeper and deeper sulphides within the reach of oxidizing and dissolving 

 surface waters the operation is in constant progress, and these lower- 

 lying ores become more and more enriched until in some cases are 

 formed bonanzas of world renown, and almost inestimable value. It 

 is a fact of much significance that such bonanzas are generally limited 

 to depths where descending waters may have penetrated at one time 

 or another. Indeed the very channels through which the enriching 



