690 GEOLOGICAL SUEVEY OF THE TERRITOKIES. 



Cupellation gave, gold. 4.40 per cent. ; and silver, 50.90 per cent. 



By a partial analysis I found, in the wet way, gold, 7.131; silver, 51.061 

 per cent. Understanding from Dr. Genth that he is engaged in the 

 chemical investigation of this species, as well as of the other tellurium 

 minerals of the Red Cloud mine, with abundant material, I have will- 

 ingly abandoned this work to him, satisfied that it cannot be in better 

 hands. 



Sylvanite. — This species from the Eed Cloud mine yields in the open 

 tube a. faint odor of selenium, and the gray ring of tellurium is preceded 

 by a slight reddish ring of selenium. In the closed tube, the ring of tellu- 

 rium is more distinct, and the deep yellow-brown vapor of the metal is 

 clearly seen, but the selenium is not evident. 



Alone on the coal it fuses with exhalation of the odor of selenium and 

 its well-characterized blue flame. The first touch of the outer flame 

 causes a liquid fusion, coating the coal, like argentic nitrate, with a 

 silver film, and a yellow areola appears before the white film of tellu- 

 rium-oxide. Continuous flaming in the reducing-flame produces a well- 

 marked yellow-brown areola within the tellurium-ring, becoming, as it 

 cools, much more brown. It probably contains lead and antimony. Its 

 reactions are not those given by Berzelius for sylvanite. It contains by 

 assay gold and silver in the proportion 1.7 to 1. In the formula, (Au 

 28.5, Ag 15.7,) the ratio of the gold and silver is 1.8 :1. My stock of 

 this species was not sufficient to permit a determination of the specific 

 gravity. 



Professor Hill, who has smelted large quantities of the ores of the Red 

 Cloud mine, informs me that "these minerals exist in this ore as minute 

 particles, or so finely divided as to produce the effect of a stain in the 

 rock. One of the specimens sent — the darkest colored — assayed here 

 was found to contain 1,890 ounces of gold and 5,300 ounces of silver to 

 the ton of 2,000 pounds"— about $50,000 in value. 



Comparing what is known of the mineral associates of the tellurium- 

 ores of Colorado with those of Transylvania, as described by von Cotta, 

 the great simplicity of the mineralogy of the Colorado veins becomes 

 very conspicuous. The age of the porphyry-dikes which cut the Ar- 

 cheean rocks of Colorado has not been determined; but it is probable 

 that they are more recent tlian the Triassic rocks which flank the base 

 of the mountains. The tellurium-veins of Offenbanya are accompanied 

 by igneous rocks of more recent date tlian the Eocene sandstones, and 

 those of l^agyag exist only in connection with igneous rocks, also of 

 probable Eocene age, (called by von Hingenau "greenstone-porphyry,") 

 and comiDosed of feldspar and amphibole, which have broken through, 

 sandstone and argillaceous shales. 



In Oifenbanya, the tellurium-ores occur under very peculiar geologi- 

 cal conditions; that is, in veins in igneous rocks and in segregated 

 masses in granular limestone. The veins occupy thin clefts, fifteen of 

 which on one property are tolerably parallel to each other, (east and 

 west, dip 30'^-40o north,) with an average width of about an inch, and 

 they carry chiefly sylvanite, and nagyagite, sparingly distributed, and 

 more rarely native gold. The chief matrix is quartz and diallogite, as- 

 sociated with pyrite, galenite, sphalerite, stibnite, native silver, and 

 pyrargyrite. 



The gangue of the Nagyag lodes is diallogite, or brown spar, or cal- 

 cite, or hornstone and quartz; it varying in the dilierent lodes and in 

 different parts of the same lode. The gold-bearing tellurium-ores are 

 scattered through this gangue with mangan-blende and pyrite. The 

 chief ores worked are nagyagite, sylvanite, gold, auriferous iron pyrites, 



