EXERCISES 257 283. Exercises. 1. Clouds often move in a direction exactly opposite to that of the prevailing wind; what may be the explanation (cf. § 274)? 2. What is a weather vane? Its shape? Why has it this shape? Why does a windmill (cf. § 210) often have a vane? 3. Fishermen on the sea coast often sail out to the fishing grounds at or before daybreak, and return about noon. Do the land and sea breezes help them, or hinder them? 4. Suppose that a cyclone is 5 miles high, and has a diameter of 1,000 miles; how many times its height is its diameter? If, to show these proportions, I make a circle out of wooden boards one inch thick, how many inches in diameter must the circle be? How many feet? 5. If you were watching a barometer while a thunderstorm was passing, what change in its height would you observe as the center reached you? What would you observe in the case of a tornado? 6. Would your house be more likely to collapse inward, or outward, if a tornado were to pass near it? 7. What kind of an air current do you think there is at the "eye" of a hurricane? 8. In narrow bays a strong wind often "piles up" the water several feet; explain. 9. Find out, if possible, what are the various flags used as weather signals. 10. In what ways, in addition to making weather predictions, does the Government seek to prevent loss of life and property at sea and on the lakes? What recent invention has been of great assistance? CHAPTER XIV ROCKS AND SOIL 284. The Earth's Crust. We have already learned (cf. § 38, Fig. 30) about the four parts that appear in a cross section of the earth. The crust is merely that portion of the core that is known to us; so far as we know, the core contains the same materials as the crust, but in a more compact form. If we examine the crust, as it is exposed in a railroad cut, a cellar excavation, or, best, in a quarry or gorge, we find that it consists of two parts: (1) The surface materials, or mantle rock; (2) Bed rock. Mantle rock is made up of such materials as clay, sand, gravel, and pebbles, all of which are usually loose enough to be worked by the spade and the pick. That upper portion of the mantle rock which we can use for growing crops is called soil. Ordinary excavations may not be deep enough to go through the mantle rock; but everywhere, and at only a moderate depth, the mantle stops, and we have bed rock. When bed rock is exposed at the surface, we have an outcrop. 285. Some Common Rocks.— Bed rock usually consists of fragments of some mineral or minerals (cf. § 102) cemented together. The cementing material hardens, or SOME COMMON ROCKS 259 66 'sets," around and between the fragments, much as limestone holds sand together in mortar.. The most common rocks are sandstone and limestone. In sandstone, fragments of sand (quartz) are cemented by iron oxide or silicic (pronounced sil-is'-ik) acid. Limestone usually consists of masses, sometimes very small, having the shape of shells and the hard parts of water animals. These are compact, and cemented together by calcium carbonate. Animal and plant remains, as we find them in rocks, are called fossils. When the cementing materials of a rock are removed, the rock falls into its fragments. Sandstones are often found that can be crumbled in the hand, owing to the loss of cementing material. Shale is a soft rock that splits easily into thin leaves. It is made of flattened particles of clay, with cementing material. Granite consists of four minerals: quartz, feldspar, mica, and hornblende. Quartz looks like glass, but is harder. It crystallizes in hexagonal forms (cf. Fig. 75, § 95). In granite it forms small, shiny masses. The feldspar of granite is usually pink or white. The mica forms shiny masses that can be split off in very thin leaves (cf. § 250). The hornblende usually exists as black crystals. The most easily attacked of the ingredients of granite is feldspar; the carbonic acid of rain water decays it. So the granite's quartz becomes sand, and its feldspar becomes clay. Conglomerate is a gravel of various materials cemented together. We have artificial rocks as well as natural ones. In making articles out of artificial stone, men shape the rock material while it is soft and plastic, and then bake, or "fire" it. The people of ancient Babylonia and Assyria made their ordinary records, such as notes, deeds, and mortgages, as well as the records of their history and literature, upon clay tablets, and then baked the clay in the sun. Earthenware articles, such as bricks, jars, and tiles, are made of common clay, hardened by heat. If salt is put into the heating furnace (kiln), the articles receive a glazed surface. Porcelain and china are obtained by the "firing” of a mixture of kaolin (a pure, white clay), quartz, and feldspar. The feldspar melts. first, and cements the mixture together. The first "firing" of porcelain and china articles takes place at a rather low temperature; then the 286. Classes of Rocks. glaze is added to the surface, and the article is heated to a high temperature, often for several days. Stoneware is much like porcelain, but it has not been heated to so high a temperature. As a result, it is opaque, while porcelain is translucent. The science of making articles out of artificial stone is called keramics. Concrete is another artificial stone, made out of cement, gravel, and water. The cement and the water unite, and cause the mixture to "set." Cement is formed by the heating of a mixture of limestone and clay. If we examine many different rocks, we shall find that we can divide them into two great classes. One consists of rocks having a more or less CLASSES OF ROCKS 261 definite layer structure. A cross section, especially when polished, shows multitudes of parallel lines. These rocks are of different materials, a layer of one material covering a layer of another material. Thus, the rock under the Niagara River, at the Falls, is a hard limestone (Fig. 225). This rests upon a layer of soft shale. Sandstone also forms one of the layers. Each set of layers of rock of one kind is called a stratum. The plural is strata. Rocks made up of strata are called stratified rocks (cf. § 132, Fig. 107). They were deposited, as sediment, under water; hence they are also called aqueous rocks, and sedimentary rocks. Limestone, sandstone, shale, and conglomerate are stratified rocks. Fig. 226. Granite does not have this layer structure, and belongs to the class of unstratified rocks. The minerals present in granite are crystallized in separate masses, and the masses are closely interlaced. expect if quartz, feldspar, etc., had been melted together, and then allowed to cool very slowly. Giant's Causeway, Ireland; Made of Basalt. This is what we should Other common unstratified rocks are basalt, lava, and pumice. Basalt (Fig. 226) is often called "trap rock." Lava and pumice have a |