292 EVOLUTION AND ANIMAL LIFE 



in mineral matter. That is, the organic matter has been 

 slowly and exactly replaced by mineral. As each particle of 

 organic substance passed away by decay, its place was taken 

 by a particle of mineral matter. Such fossils are called petri- 

 factions. 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 a microscope, the exact details of its original 

 fine cellular structure. This substituted mineral matter may 

 be one of several minerals, but usually it is silica (quartz) or 

 carbonate of lime (limestone) 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 only 

 the organic matter is replaced by the petrifying mineral, 

 although sometimes the old inorganic matter is also replaced. 

 Finally, sometimes the organic matter and organic structure 

 are both lost, only the original outline of 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. 



Some traces even of the finest organisms occasionally 

 appear. 



"Conditions have sometimes permitted even the most delicate 

 structures, such as insects' wings and the impressions of jellyfishes 

 to become retained in the soft mud, which afterwards became solidi- 

 fied. Localities famous the world over for the beauty and delicacy 

 of their fossil remains are the lithographic stone quarries of Bavaria 

 and certain beds in France " (EASTMAN). 



These deposits were perhaps formed in the clear, quiet waters 

 of a coral lagoon. 



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 and rifts in the earth's 

 crust, as a red-hot, viscous liquid, which cools to form a hard 

 rock. No animal or plant caught in a lava stream will leave 

 any trace. Furthermore, fossils are not found in granite, nor 

 in ores of metals, nor in certain other of the common rocks. 



