AGE OP ORE DEPOSITS. 



apparent freaks in the deposition of mineral matter in such centers of 

 thermal activity as the Yellowstone Park realizes how little it takes to 

 deflect the course of ascending aqueous or gaseous currents and how, under 

 varying conditions, mineral deposition is liable to undergo change within 

 restricted areas. The occurrences of ore bodies on Ruby Hill and Pros- 

 pect Mountain are variable and uncertain, but such as one might anticipate 

 from their mode of formation. 



Relative Age of Rhyolite and Ore. The best example Oil Ruby Hill showing 



direct contact between the ores and rhyolite bodies was observed in the 

 Jackson mine, but probably a still finer illustration of the relationship 

 between the two with the ore lying undisturbed along the under side of a 

 highly inclined dike was seen in the Dunderburg mine on Hamburg Ridge. 

 It was exposed on the third level of the mine near the main shaft which 

 had been sunk all the way in hard limestone. The rhyolite dike varied 

 from 1 to 8 feet with an average width of a little more than 2 feet. The 

 strike of the dike was approximately east and west with a dip to the north, 

 whereas the course of the ore channel stood nearly at right angles to it. 

 The ore never penetrated the rhyolite, its course being deflected on 

 approaching the intrusive dike. At the shaft house the ore body 

 measured 50 feet in thickness. Opportunity for examining the contact 

 was excellent as much of the ore still remained in place, while over 

 other areas along the contact the ore had been stripped off, rendering it 

 possible to observe the relationship between it and the dike, as well as the 

 position of both to the inclosing limestone. Nowhere did the ore penetrate 

 the rhyolite, and nowhere had any ore been found inclosed within the dike. 

 In like manner the ore was wholly free from rhyolite. Nothing could seem 

 more clear than that the mineral matter had been quietly deposited from 

 solution along the under side of a highly inclined dike, neither could any- 

 thing be seen suggesting a replacement of rhyolite by ore, although imme- 

 diately along the contact there is considerable kaolinization of rhyolite. 

 Such instances as the Jackson mine on Ruby Hill, the Dunderberg on 

 Hamburg Ridge, and the position of the rhyolite and ore at the Geddes 

 and Bertrand mine in Secret Canyon, furnish strong proof corroborating 

 other evidence that the ore followed the rhyolite. 



