Coal
Coal is so commonly used in life that we seldom notice it. We burn coal everyday to heat food and water. The first locomotive was driven by burning cml. In winter we feel a special need for coal. The coal gives us fire, light, heat and warmth. Industry cannot operate without coal; therefore,coal is called the food for industw.
Coal is not so treasured as gold. Gold is made into shining ornaments for the necks of heroes and heroines. Coal is shining in its own way. It burns silently for the benefit of all. In a sense it is black gold.
Coal underwent great change before it became the bright, brittle, black substance that we now use. During ancient times, when the earth enjoyed a very warm and moist climate, the land changed and began to sink. These enormous quantities of trees and vegetable matter were covered by a deposit of sand and clay. This layer of sand and clay pressed upon the layer beneath and prevented the contact with air. These trees and plants succumbed to the pressure and changed their appearance.
Generation after generation, as the ground kept gradually sinking, another layer of sand and clay was deposited above the layers already formed. A great pressure was thus exerted and the peat was changed into the black and brittle substance, which is known as coal.
From the formation of coal, I see that the coal has the same character as the pine trees in winter. The pines remain green when they are covered with snow. The coal remains of service to mankind after being buried underground for years and years.
文中第一段从介绍煤的用途展开，第二段使用比较法把煤和金子做一对比，第三、四段说明了煤的形成，最后一段用比较法赞美煤对人类的贡献。
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