MINERAL OCCURRENCES 133 
The chief selenide veins lie on the eastern slope of the Cerro 
Cacho, some leagues to the west of the Puesto Umango. The veins 
have been exploited some ten years ago by an Argentine mining 
- prospector. The claims are still visible and well accessible, but all 
valuable parts of the mineral mass have been quarried. Without 
operating with dynamite it seems impossible to obtain any good 
specimens of the minerals. The selenides, eukairite and umangite, 
have been exploited for their high value in silver and copper. The 
_ ore was brought on muleback over the Sierra de Umango ridge 
and the Paso de Tocino over the Sierra de Famatina down to the 
town of Chilecito along asperous and even dangerous trails. The 
mines lie very high, about 3000 m and the icecold storm from the 
Cordillera sweeps nearly the year round over these heights. 
The selenide minerals mentioned occur in thin fissure veins 
transversally cutting across the crystalline schists. These last named 
consist principally of limestone and amphibolite, contorted in many 
‘ways. The gangue mass is for the most part calcite and quartz. 
The metalliferous minerals occur as scattered grains or irregularily 
formed dense masses. According to Klockmann (I. c.) these mi- 
nerals are, as already stated, eukairite and umangite. The former 
mineral has the composition AgCuSe, the later Cu,Se,. The eukai- 
tite is as specimen known already many years ago, principally from 
Småland, Sweden.! The Umango-eukairite was, as said, first found 
and examined by Hiinicken. The umangite was, however, fixed 
by this explorer as bornite, but the true nature was then stated by 
Klockmann. The chemical composition of the two minerals is 
according to him as follows: 
Eukairite: Umangite: 
N na er ADs OS SD ER RE 54 Yo 
Sie ea tile DAs eee AAS) chr ak pes ae oat OR a bref del Aes 
SIR N et RE SOE PR AOL a RRL tach all ou traces. 
1A. E. Nordenskiöld: Ofversigt af Kongliga Vetenskapsakademiens Hand- 
lingar. Stockholm 1886. 
