ever wonder how we happen to have coal? Yes, you know that coal is mined, or taken from the ground, and brought to you; but how did it get into the ground? It is not like the other rocks of the earth's crust. Then what is it, and how years ago great, luxuriant forests grew on the places where we now find coal fields. They grew up and lived and died and fell to the marshy ground on which they stood. Others grew up in their place; they, too, fell in turn into the marshy land and were covered by water. Some FIG. 101. What a cross section of a coal mine might look like. Note the great crack across the rock layers and the way in which the upper part has slipped down, producing a fault. times, probably, the land was lowered and the former forests were under water and covered with sediment and mud. This sediment afterwards became rock. Some of the gases escaped, but the rest of the carbon of the plants was held under the great pressure of the layers of rock and water above it, until it gradually hardened to form coal (Fig. 101). In swampy regions we can see soft peat being formed today. This was probably the first form of soft, or bituminous, coal. Hard coal is also called anthracite; it gives off much less gas than soft coal (cf. § 28) and so is harder to set on fire. Where are some of the coal fields of the United States? Did you ever see a piece of coal which had on it the imprint of the leaf, stem, or trunk of a plant or tree? 171. What are Our Building Stones? - Some of the stones used in building are granite, limestone, sandstone, and marble. The quarrying of stone furnishes employment for a great number of men in this country. Much granite is taken from New Hampshire, the "Granite State." It is very hard and will sustain a great weight. Limestone and sandstone are softer rocks, but very useful. When limestone is heated, carbon dioxide escapes and lime remains. Lime is used in making mortar and plaster. Limestone is also used in the smelting of iron (cf. § 164). Marble is perhaps the most beautiful of our building stones. It is of many different colors and may be highly polished. How does marble act with an acid? Read § 38. 172. Does Man Ever Make Stones? You have heard of artificial jewels, but have you ever seen artificial building stones? Of course you have. Man has not always been able to get, or been satisfied with, natural stones, so he has made certain ones for himself. Such are bricks and concrete. Bricks were made long ago in Egypt, Assyria, and Babylonia, and they are made in great quantities even now in our own country. Finely powdered clay is mixed with water and the mixture is either packed into molds or is cut into bricks by machinery. The bricks are then dried in the air for several days and later baked in immense ovens for several more days. Mortar, used to fasten bricks together, sets or hardens to form another artificial stone. Concrete is more durable even than brick. Men make it by mixing crushed stone, sand, and cement. Then they put it into molds and keep it from drying too rapidly by covering it. You have seen freshly-made concrete sidewalks covered with boards, or cloths, until the concrete has "set," or hardened. It hardens better if sprinkled with water occasionally. Concrete is used not only for sidewalks and floors, but for the foundations and walls of buildings, for bridges, and even for ships! 173. Exercises. 1. Why are layers of limestone and sandstone often found between layers of coal? 2. Is petroleum a mineral? Where is it found? For what is it used? 3. Why is iron ore shipped from the Lake Superior region to Pennsylvania to be smelted? 4. Name five or more great uses of some form of iron. 5. Why do we believe that coal is made up of the material of ancient forests? 6. What metals are called " precious "base" metals? "metals? What are the 7. Find out the value of the silver in a silver dollar. 8. What is solder used for? Pewter? 9. Why is iron often covered (plated) with tin? With a magnet test a pin and then a needle. Is either made of a form of iron? |