Fossils in the same layer of rock

   During the early part of the nineteenth century an engineer called William Smith made a valuable discovery. He was a surveyor of canals and coal mines. He noticed that certain kinds of fossils always turned up in the same layer of rock. From this he deduced that rocks containing similar fossils, although miles apart, were of the same age.
  Rocks belong to different ages which have been given names. From this we can construct a time chart. A palaeontologist can examine a fossil and say, for example, 'this is the leg bone of a dinosaur which lived during the Jurassic Period, 150 million years ago'. He can even tell from its shape that the dinosaur was slow and clumsy, or lively, that it walked on two or four legs, and how big it was. The teeth might tell him what it fed on, whether it was a flesh-eater or ate plants. Even footprints tell their story.
   Fossils are formed in different ways. In rare cases the soft parts form an impression in the rock. Even jellyfishes have been preserved in this way. Hard parts are partly strengthened by minerals or entirely replaced by them, that is, they become petrified (turned into stone). The famous Petrified Forest in Arizona, U.S.A., is an example. If a fossil disintegrates altogether, the space which it leaves becomes filled in with
the surrounding minerals. This forms a fossil cast. Ammonites and sea urchin fossils are commonly formed in this way, out of chalk or flint.
   Sometimes entire plants or animals are preserved. Insects and other small creatures were trapped in the gum which oozed out of a prehistoric tree. This gum hardened into amber which preserved the creature. In the far north, in Siberia and Alaska, tusks, teeth, and even bodies of mammoths and woolly rhinoceroses have been found in the frozen ground. Flesh, skin and hair are still present. It is like being preserved in the deep-freeze.