THE ANIMALS OF THE PAST 421 



decay, its place was taken by a particle of mineral matter. 

 These are called petrifactions. This is beautifully shown 

 in the case of petrified wood. We can cut and grind thin 

 a bit of petrified wood, and see in it, with the microscope, 

 the exact details of its original fine cellular structure. 

 This substituted mineral matter may be almost any mineral, 

 but usually it is silica (quartz), or carbonate of lime (lime- 

 stone), or sulphide of iron (iron pyrites). In the case of 

 animal parts which were originally partly organic and 

 partly inorganic, as bones and teeth and shells, often the 

 organic matter only is replaced by the petrifying mineral, 

 although sometimes the old inorganic matter is also thus 

 replaced. Finally, sometimes the organic matter and 

 organic structure are both lost, only the original outline 

 or form of the whole part being retained. This occurs 

 when the organic matter imbedded in mud and clay decays 

 away, leaving a hollow which is filled up by some mineral 

 different from the matrix. In this case the fossil is simply 

 a cast of the original organic remains. 



336. Fossil-bearing rocks and their origin. Examination 

 and study of the rocks of the earth reveal the fact that 

 fossils, or the remains of animals and plants, are found in 

 certain kinds of rocks only. They are not found in lava, 

 because lava comes from volcanoes as a red-hot, viscous 

 liquid, which cools to form the hard lava. No animal or 

 plant caught in a lava stream will leave any trace. Fur- 

 thermore, fossils are not found in granite, nor in metals, 

 nor in certain other of the common rocks. Many rocks 

 are, like lava, of igneous origin ; others, like granite, 

 although not originally in melted condition, have been so 

 heated subsequent to their formation, that any traces of 

 animal or plant remains in them have been obliterated. 

 Fossils are found almost exclusively in rocks which have 

 been formed by the slow deposition in water of sand, clay, 

 mud, or lime. The sediment which is carried into a lake or 

 ocean by the streams opening into it sinks slowly to the 



