Gypsum. A sulphate of lime occurring in great beds. Burnt, it becomes plaster of paris.
Iron. This, the most important of all metals, is found in various forms. The ores of iron are:
| Name. | Sp. Gravity. | Hardness. | P. C. Fe. |
| Native ore | 7.7 | 4.5 | 100 |
| Magnetite | 5.1 | 6.0 | 72 |
| Hematite | 4.8 | 6.0 | 70 |
| Limonite | 3.8 | 5.2 | 60 |
| Siderite | 3.8 | 4.0 | 62 |
| Pyrite | 5.0 | 6.3 | 47 |
Native iron is only found in meteorites that have come from space.
Magnetite is loadstone ore; the powder is reddish black, and the ore, dark brown to black. It is found in the older rocks and is an important ore.
Hematite varies from metallic to dull in luster. There are many varieties of it, known as ironstone, ocher, needle ore, etc. Hematite may be slightly magnetic. Immense beds exist in the triassic sandstones, and in the secondary rocks below the coal measures. The powder and streak of limonite are always yellow; it is an important ore. Siderite assumes many forms. It is called spathic ore, clay-ironstone, carbonate of iron, black band, etc. Most of these carbonate iron ores only range between 30 and 40 per cent. of metallic iron, but are in demand as fluxes for other iron ores. The pyritic ores of iron, including marcasite, pyrrhotite and mispickel, are often taken for gold by the inexperienced. In an accessible region pyrites may be valuable, as they are bought by makers of sulphuric acid.
Iron is so low in price that vast deposits exist which cannot be made use of because they would be too expensive to mine. A deep bed, or a narrow one, or the slightest difficulty in transportation, would preclude any profitable development. It is known that enormous areas in northern Labrador, for instance, are full of iron deposits, yet there seems no chance of their having the slightest economic value for a long time, if ever. Conditions of commerce very different to those now obtaining will have to exist before they can be utilized.
Iron ore is most favorably situated for profitable extraction when it is near coking coal and beds of limestone; the former for fuel, the latter for flux. Occasionally such regions as that of Lake Superior may be able to compete successfully with others, although they do not possess the necessary smelting facilities, because these deficiencies are counterbalanced by inexhaustive stores of easily mined ores, and transportation facilities unrivaled in cheapness.
Lead. The two important sources of supply are galena and cerussite. The former contains 87 per cent. of lead, and frequently some silver and gold. It is so distinctive as to be easily recognized. Luster, metallic; opaque; lead-gray; harsh. Brittle to sectile (may be cut). Cleavage, perfect. Fracture, even to sub-conchoidal. Structure, granular or foliated, tabular, or fibrous. Specific gravity is 7.5, and hardness, 2.6.
The carbonate cerussite contains about 79 per cent. lead. Luster, vitreous to resinous. Translucent. Color, gray. Smooth. Brittle. Cleavage, perfect to imperfect. Fracture, conchoidal. Massive to granular. Rich carbonate ores look like clay, and are undoubtedly often passed by.