COLLECTfONS IN ECONOMIC OKOLOdV AND MI-yrAI.LIMvNiV. 5 



is uotliing- to indicate the presence of ooi,i ii, these .speciuieii.s. Further 

 illustrations of this occurrence will be found in the ore series. Aurifer- 

 ous sulphurets consist of a more or less intimate mixture of tlie sulphide 

 of iron (pyrite) and the double sulphide of iron and copi)er (chalcopyr- 

 ite). Both of these sulphides are frequently supposed to be free gold, 

 and pyrite is commonly known as "fool's ^old." 



(1) Aiuiferoiis pyrite, crystallizod in quartz. Keating locL;, Cedar }*lain.s mining 

 district, Kcatingville, Jetfersoii County, Montana. (548:?.) 



(2) Auriferous pyrite, crystallized in quartz. Bates Mine, Nevada District, Gilpin 

 County, Colorado. (19-2:?0.) 



AUKIFEKOIS SLATK. 



A single illustration of the occurrence of free gold in slate is shown. 



(1) Free gold and pyrite in slate, witii a little quartz. Gustin Mine, Lawrence 

 County, Dakota. (SOir.^.) 



SYLVANITE. (GRAPHIC TELLURIUM.) 



Composition (Au. Ag.), Te2. Telluride of gold and silver, containing 

 gold 25 to 30 per cent., silver 7.5 to 15 per cent., and tellurium 45 to 

 ()0 per cent.; frequently antimony replaces the tellurium and lead the 

 silver. 



It crystallizes in the mouoclinic system in complicated crystals ; also 

 occurs massive. It varies in hardness from 1.5 to 2, and in specific 

 gravity from 7.9 to 8.3, has a steel-gray color and a metallic luster. 



A somewhat rare mineral, although it has been found in large 

 amounts in a few Colorado mines. A single illustration of telluride of 

 gold is shown here ; others will be found in the ore series. 



(1) With pyrite, in quartz. American Mine, .Sunshine district, Boulder County, 

 Colorado. (l!»0i)2.) 



Gold OrEvS. 



For the purpose of extracting the metal the ores of gold may be 

 divided into placer, including hydraulic, quartzose, sulphuret, tellu- 

 ride, and fusion ores. 



PLACER GOLD. 



Placer gold ore consists of tiie ordinary sands and gravels of rivers 

 carrying free gold, and sometimes auriferous suli)hurets, which have 

 been deposited in the channel of the streams. Many of these beds 

 are very old, and it is long since water flowed over them. Some are 

 overlaid by recent lava flows. Where these ores occur in vast depos- 

 its they are called hydraulic ores, from the methods used in extracting 

 the gold. Placer ores also occur on the seashore. The gold is scat- 

 tered very irregularly through the sand, being found in largest amounts 

 where eddies in the streams favored its deposition. 



The gold is separated from the sand by washing, which consists in 

 keeping the material agitated in water, m a suitable apparatus, so that 



