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The ages of rocks measured
In the 19th century, geologists could tell whether one rock layer was older or younger than another by the fossils they contained. But they could not measure the absolute ages of rocks until after the discovery of radioactivity in the 1890s.
Radioactive substances, which are occasionally found in rocks, give off high-energy rays and decay (break down) at a fixed rate. For instance, as the radioactive element uranium--decays it chqpges into lead. If you measure the amount of lead in a uranium sample, you can then work out its age the uranium-lead' method is used for extremely old rocks.
For substances up to 50,000 years old, scier scientist use carbon-14 dating. Carbon-14 is a radioactive element that is present in all living things. When an organism is die the carbon-14 in the tissues decays. Half of the carbon-14 decays every 5,700 years. Knowing this, scientists can work out the ages of old bones or logs.
Coral
The hard, dry corals seen in stores and museums are made up of the combined skeletons of tiny creatures which once lived joined together as a colony in the sea. The coral animal, or polyp, measures only about a fifteenth of an inch in diameter. It is a primitive type of plant-like creature closely related to the sea anemone. But, unlike the sea anemone, the polyp has a stony skeleton and, once it has made its home, cannot move. Each polyp is like a tiny tube, closed at one end and with a mouth, fringed with tentacles, at the other.
The tentacles have a sting and capture minute sea creatures for food most polyps multiply by budding. A small bud grows from the animal's body and finally becomes as big as the parent with a mouth and tentacles of its own. Gradually the corals build up into colonies of many millions.
Coral animals inhabit shallow water in warm seas. In tropical waters colonies s form reefs round islands which can be dangerous to slims. Eventually new islands are formed by the accumulated skeletons of billions of dead polyps.
But the romantic coral islands and reefs of the South Seas do not supply the preciaiis material used for beads and brooches. These are made from the red and pink coral found in the Medfterranean off the coasts of Africa and Italy.
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