182 THE EARTH AS MAN'S ABODE present petroleum production. Limestone is another type of sedimentary rock formed from the remains of the shells of mollusks, coral, and other invertebrate animals, by the action of calcareous algae and bacteria, and also by chemical precipitation. Sedimentary rocks frequently contain fossils, which vary in size from the microscopic fossils to the huge fossils of trees or the dinosaurs. Some of the green and brown algae specialize in removing calcium carbonate from water to form external coatings for themselves. There are some large pinnacles of calcium carbonate at the lower end of Searles Lake, California, that were formed by such algae when the lake level was much higher than at present. Some of the Paleozoic limestone of the Rocky Mountains was formed by similar algae. One genus of green algae, the stonewort, is reported to deposit 100,000 tons of calcium carbonate yearly from a small lake in Wisconsin. Other algae, the diatoms, extract silica from the water, and their bodies form siliceous deposits which have formed layers three hundred feet in depth, extending over many square miles. Such deposits, called diatomaceous earth, are mined for use as mild abrasives. Limestone is used in metallurgy and as a building-stone. Limestone, impregnated with solutions of phosphates washed from thick deposits of bird droppings and other sources, serves as a useful fertilizer. Sedimentary rocks can be readily recognized as such because they are stratified and may be broken apart along the bedding planes separating the different layers. Metamorphic Rocks Are Produced by Changes in Igneous Rocks. Sedimentary and Metamorphic rocks are produced by the action of heat and high pressures on other forms of rocks. Slate is a good example of a metamorphic rock and is produced by the combined effect of heat and pressure on shale. It is easily split into sheets, which are valuable for blackboards and roofing. Marble is produced by the effect of high pressures and temperatures on limestone. In this process the fossil shells may lose their identity; and part of the calcium carbonate crystallizes, the small crystals giving marble its typical appearance. Marble is widely used as a buildingstone because of its durability and beauty. The different colors in marble, like those in rocks of other types, are produced by small amounts of different mineral and organic substances present as accidental impurities.
MANY DIFFERENT KINDS OF ROCKS 183 1. What is an igneous rock? STUDY QUESTIONS 2. Why are some granites fine-grained and other granites coarse-grained? 3. What are the characteristic features of igneous rocks? 4. What are the characteristic features of sedimentary rocks? 5. How may the age of a given sedimentary rock fragment be determined? 6. Outhne the most important type of rocks, and give one or more common examples of each type. 7. Give some examples of igneous rocks formed by volcanic activities. 8. What types of rocks might a kitchen cleanser contain? 9. In how many different ways may limestone be formed? 10. What is a metamorphic rock? Give some examples. Study the chart on p. 181 before attempting to answer the following questions: (1) Would rock salt be classified as a rock, and if so, what type of rock is it? (2) Name five useful sedimentary rocks. (3) Classify the following rocks: (a) granite, (b) soft coal, (c) limestone, (d) sandstone, (e) marble, (/) anthracite coal, (g) slate, (h) conglomerate, (i) pumice, (j) obsidian.