42 MINERALOGY. 



to about 14 per cent. One of its forms is yellow ochre, 

 which is used as a common material in paint. The pow- 

 der of this ore is also used for polishing metallic surfaces. 

 75. Chromic Iron. This mineral is composed chiefly 

 of the oxyds of two metals, iron and chromium. The 

 oxyd of the latter acts as an acid, as mentioned in 301, 

 Part II., and the mineral is therefore said to be a chro- 

 mate of iron. There are, however, two other compo- 

 nents, alumina and magnesia, which vary in quantity in 

 different specimens. This mineral is quite abundant at 

 Barehills, near Baltimore, in Lancaster county, Pennsyl- 

 vania, and several other places in this country. Its crys- 

 tals are octahedrons. It is valuable in the manufacture 

 of the chrome pigments, of which the chrome yellow is 

 the principal. 



76. Carbonate of Iron. This miner- 

 al varies^ in color from light gray to 

 dark brown or nearly black on ex- 

 posure. Its crystals sometimes have 

 curved faces, as represented in Fig. 

 21. Metallic iron is extensively ob- 

 tained from this ore. 

 77. Native Copper. This occurs in company with ores 

 of the metal, commonly in the neighborhood of igneous 

 rocks that is, rocks which have been made and thrust 

 upward in the crust of the earth by the agency of heat. 

 There is often silver with the copper, either intimately 

 mixed with it, making an alloy, or collected by itself in 

 small masses or in strings. Copper is next to iron in 

 abundance. There are famous mines in Cornwall, En- 

 gland, in Brazil, and in Siberia. Perhaps the most ex- 

 traordinary copper region in the world is in the vicinity 

 of Lake Superior. It is found there in veins filling up 

 fissures in the rocks, and it is cut out in monstrous blocks 

 with chisels and drills. One mass, weighing nearly 4000 

 pounds, was carried to Washington, and a large mass has 

 been since got out which was estimated to weigh 200 



