118 EEPOET UNITED STATES GEOLOGICAL SURVEY. 



mous yield, and many of them may be classed as " very good." Galenite, 

 sphalerite, vseveral varieties of fahlerz and pyrargyrite are the silver- 

 bearing minerals. Quantitatively they occur as enumerated. In the 

 direction of Gray's Peak the ore-bearing "belt" extends, and there we 

 find mines located above timber-line, at an altitude of about 12,000 feet. 

 I may here mention a curious occurrence that has excited considerable 

 comment. In the report of 1873 I alluded to the frozen condition of the 

 ore In the International mine on Mount McOlellan, nine miles west of 

 Georgetown. A tunnel has been driven into the side of the mountain 

 for the distance of 140 feet. There it is reached by a vertical air-shatt. 

 It was found that the ore was all frozen from the very entrance of the 

 tunnel to the depth reached at the time of my visit (June 18, 1873). 

 I have learned that the same characteristic holds good to the present 

 day, although much work has since been done. While there I satisfied 

 myself that the frozen condition of the ore was not owing to any draught 

 of very low temperature that might be created by the air-shaft. Va- 

 rious views have been promulgated tending to explain the presence of 

 solidly-frozen masses to a depth of more than 200 feet, but none of them 

 appear to be satisfactory. I presume that we have, in this instance, a 

 case analogous to that of the " frozen caves " of other parts of the world. 

 It may be that some chemical change at present going on in the sur- 

 rounding rocks causes a diminution of heat. This occurrence is one of 

 very great interest, but it is my opinion that the proper solution of the 

 question cannot be reached except by a series of observations extending 

 over a long period of time. Meteorological conditions, hygroscopic va- 

 riations at different seasons, as pertaining to the humidity of the rocks, 

 chemical and physical activity of the minerals constituting the rocks 

 and the ore, besides other factors involved, must be taken into consid- 

 eration before any acceptable theory can be established. Within the 

 tunnel the sight is one of the most beautiful imaginable. Thousands of 

 thin, transparent ice-crystals line walls and roof, reflecting with myriads 

 of sparkling flashes the light of the miner's lamp. It is truly a magical 

 scene, transporting the visitor to the fairy palaces of the "Arabian 

 Nights." 



At Idaho a number of silver lodes are worked. They are well devel- 

 oped, and furnish a good yield. Near Empire, in the same county, gold 

 mines are worked to some extent. Throughout both Gilpin and Clear 

 Creek Counties there are scattering locations of lodes at many localities. 

 Many of the gulches are "worked out," but others still afford sufficient 

 pay to tempt the miner. Gulch-work, though physically, perhaps, more 

 severe, has the popular advantage over mining proper that every day or 

 every week the workman can perceive the reward for his labors in tan- 

 gible gold, without the interference of a mill or smelting-works. 



Caribou District. — Mining in this district is comparatively young, as 

 yet. Almost all the ores are typical silver ores, consisting of galenite, 

 sphalerite, fahlerz, argentite, and pyrargyrite, mainly. From the sur- 

 face down the indications have been favorable, and, as far as developed, 

 the mines show good results. Many lodes have been located, and the 

 mining-camp that at the time of my visit (June, 1873) was in its Incipi- 

 ency is to-day an active one. Further development of the mines already 

 started will no doubt result in very satisfactory returns of the iDrecious 

 metal. 



Sunsliine District. — In 1874 the first discovery was made by D. O. 

 Patterson. At first the nature of the ore was not fully recognized, but, 

 as numerous other discoveries soon followed the first, that of the Sun- 

 shine lode, the great value of the rich ores was soon established. Prof. 



