COLLECTIONS IN ECONOMIC GEOLOGY AND METALLURGY. 77 



It crystallizes in the monoclinic system, showing especially the basal 

 prism and both octahotlral planes; twin crystals are common and the 

 groups of crystals are very beautiful. It also occurs in columnar forms 

 and massive. It varies in hardness from 3.5 to 4.25 and in specific 

 gravity from 3.5 to 3.83. The color is bright blue of several shades and 

 the luster vitreous, while it is transparent to subtranslucent. 



Azurite occurs over a very wide extent of country and is frequently 

 associated with malachite and other copper minerals, but it seldom oc- 

 curs in large amounts. In Arizona and New Mexico, however, it is very 

 abundant. To illustrate the occurrence of azurite two specimens are 

 shown : 



(1) Crystallized, a cotuplicated group of crystals on massive azurite. Copper Queea 

 Mine, Bisbee, Cochise County, Arizona. (5(5054.) 



(2) Crystallized, twin crystals on ferruginous fxuartz. Longfellow Mine, Clifton 

 district, Arizona. (33382.) 



Copper Ores. 



The ores of copper may be divided into three classes: Metallic, oxi- 

 dized, and sulphide ores. 



The first contain copper in the metallic form and are treated by a pro- 

 cess peculiar to themselves. They are well illustrated in the collections 

 from the Lake Superior copper region. 



The second and third classes graduate into each other and many 

 mines i)roduce both characters of ores at difierent levels. Occasionally, 

 however, a mine will produce only sulphide ores, and many of the Ari- 

 zona and New Mexico mines have produced only oxidized ores. The 

 treatment of the two kinds of ores for the production of the metal is 

 essentially the same, except that the oxidized ores do not have to be 

 treated for the removal of sulphur. These varieties are illustrated in 

 the collections from the various smelting works. 



LAKE SUPERIOR COPPER REGION. 



This region occupies a peninsula jutting into Lake Superior, and 

 embraces Keweenaw, Houghton, and Ontonagon Counties, of the State 

 of Michigan. Copper was mined here by the aborigines, and various 

 traces of their work yet remain; but it was not until after 1840 that 

 white men were enabled to explore the region carefully, and it was a num- 

 ber of years more before any active mining operations were undertaken. 



The discovery of large masses of pure copper (which was very fre- 

 quent in the early days) attracted a great deal of attention, and the 

 first attempts at mining were devoted almost altogether to this mate- 

 rial. It was found, however, that the diflficulty of getting out these 

 large masses and getting them transported to manufacturing centers 

 was very great, and as a result the early mining was generally unprofit- 

 able. These large masses sometimes contained many tons of metal, 

 and in order to take them out of the mine it is necessary to divide 



