Fossils and How They Are MadeFossils are traces of the living things of long ago. All that we know about the plants and animals that lived on the earth before there were any people on the earth has come from fossils. Let us find out how the fossils which tell the stories of the animals of yesterday were made. An animal fossil may be a whole animal which, when it died, was buried in such a way that its body was kept almost as it was when the animal was alive. Nearly so million years ago, for example, many insects were trapped in the sticky resin which came from the pine trees of that time. The resin kept the bodies of the insects from decaying. Some of this resin was buried under the ground. There it hardened into amber. In the amber the insects trapped in the resin have kept perfectly. Often an animal fossil is simply some hard part of an animal's body — its shell, perhaps, or a bone. The soft parts of an animal's body usually decay very fast. The hard parts often last much, much longer. Some animal fossils are hard parts of animals that have been petrified, or remade in stone. If a bone, for example, is buried, water in the ground may take away, particle by particle, the material of which the bone is made and leave some mineral, such as lime, in its place. At last none of the real bone is left. Instead, there is a "bone" of stone —a petrified bone.A fossil may be only a footprint of an animal. A dinosaur, for example, may have wandered along the shore of a pond one day and left deep footprints in the mud. The waves may have washed sand or mud into the footprints so gently that they were not disturbed. The layer of mud on whichthe footprints of the gaint riptile were made may, as the ages passed, have hardened into solid rock and kept the footprints as they were.A fossil may be a cast. Have you ever made lead soldiers by pouring lead into a mold? If you have, it will be easy for you to understand how a fossil cast may be made. The shell of a snail of long ago, let us say, was aressed down into mud. The material in the shell was dissolved by water, but a hole, or mold, was left where the shell had been. Later the holes was filled with lime and mud, and the lime and mud hardened into a cast of the shell. As you probably know, much of the dry land of today wa's once covered with water. Layers of such rocks as sandstone, limestone, and shale -- rocks that were made under water -- show that it was. Such layers of rock furnish most of the fossils of the living things of past ages.化石及化石的形成化石是生活在很久以前生物的痕迹。我们所知道的所有关于动植物的化石都是在人类出现以前就已经形成的。让我们一起探索化石是怎么形成的和这些化石所告诉我们的远古动物的故事。一个动物的化石有可能是完整的,就是当它死的时候,它身体所摆出来的被埋藏的姿势正如它活着的姿势一样。例如,大约百万年以前,许多昆虫被那个时代的松树所产出的粘稠的松脂粘住。这些松脂使昆虫免受腐烂。一些松脂被深埋地下,它们便的坚硬形成琥珀。在琥珀中,这些昆虫被松脂包裹着保存的非常完整。通常动物的化石是动物身体比较坚硬的部分——外壳或者骨骼。动物身体的柔软部分常常腐烂的很快。坚硬的部分通常要很长很长的时间才能够腐烂。一些动物的化石是动物身体坚硬部分被石化或形成岩石。例如,假如一块骨头埋在地下,水在底下流过,慢慢地慢慢的,形成骨头的骨质被带走,而另一些东西保留下来,就像石灰充满里面。最后没有骨头被保留下来,相替代的是那些石头的“骨头”——石化的骨头。 化石有可能是一个动物的足迹。例如,有一天一只恐龙在池塘边散步,在泥沼中留下一个深深地脚印。水的波浪有可能使沙子或者泥沼进入恐龙的脚印中并且脚印没有被破坏掉。随着时间的推移,爬行类动物泥沼足迹的地层变得越来越硬,形成砂岩保留了他们原有的足迹。 化石有可能是一个模子。你怎经是否将铅倒入一个模子中做过铅质的士兵?如果你做过,那么模铸化石是形成的对你来说是很容易理解的。例如,很久以前蜗牛的硬壳被困在泥沼中,硬壳里的物质被水逐渐的冲蚀掉,但是那个洞或者是那个模子被保留下来。渐渐地那个洞里填充了石灰或泥,石灰和泥逐渐变硬形成了外壳的模子。你或许知道,现在很多干燥的大陆在以前都曾被水覆盖。许多岩石地层像砂岩,石灰岩,泥岩,都是在水下形成的可以说明这一点。这些岩石的地层提供了大部分过去生物的化石。