264 The Realm of Nature CHAP. 



,ing of the plot if it were a novel. A few consecutive pages 

 found in their proper order would give a key to arranging 

 the rest, and although uncertainty as to the precise sequence 

 of some parts of the narrative would remain, the patient 

 reader could in time obtain a fair idea of the nature and 

 order of the contents. If it is possible to find a narrative 

 showing a regular development of events written in charac- 

 ters with which we are familiar on the sheets of rock, the 

 order and circumstances in which these rocks were formed 

 can be got at, however confusedly they may now lie. Sedi- 

 mentary rocks are full of picture-writings recording the 

 history of successive races of living creatures, and the 

 writings are very legible, being the actual mummies or 

 casts of the creatures themselves. 



343. Fossils. All remains and traces of living creatures 

 preserved in rocks are called fossils. Some of the traces 

 are only footprints, or worm-tracks that have been impressed 

 on an ancient surface of clay or wet sand, and after harden- 

 ing have been filled in by finer sediment. Plants and 

 animals are usually represented only by their hardest 

 parts, such as bark, shells, teeth, or bones. But often the 

 whole organism was surrounded by compact sediment, in 

 which, as it decayed away, a hollow was left exactly corre- 

 sponding with its outer surface. This mould became filled 

 in turn with fine sediment, or impregnated with carbonate 

 of lime or silica deposited from solution in the water which 

 percolated through, and thus a perfect cast or model has been 

 produced. The most complete fossils preserve not only the 

 external form but the minutest internal structure, every 

 part being individually turned into stone by the exchange 

 of animal or vegetable substance, molecule by molecule, for 

 some mineral such as pyrite (sulphide of iron), calcite 

 (carbonate of lime), or one of the many forms of silica. 

 Other fossils are simply shells or skeletons closely com- 

 pacted together, such as chalk, made up of foraminifera 

 like the deep-sea oozes ( 275), coral limestone ( 280), 

 and siliceous earth composed of the cases of diatoms, 

 Sometimes organic substance undergoes only partial de- 

 composition while retaining much of its original form. 



