PRESERVATION 13 



substances. Take the case of a dead leaf falling into a lake, 

 or some quiet pool in a river. It sinks to the bottom, and is 

 buried up in gravel, mud, or sand. Now, our leaf will stand a 

 very poor chance of preservation on a sandy or gravelly bottom, 

 because these materials, being porous, allow the water to pass 

 through them easily. But if it settles down on fine mud it may 

 be covered up and become a fossil. In time the soft mud will 

 harden into clay or shale, retaining a delicate impression of the 

 leaf; and even after thousands of years, the brown body of the 

 leaf will be there only partly changed. In the case of the plants 

 found in coal, the lapse of ages since they were buried up has 

 been so great (and the strata have been so affected by the great 

 pressure and by the earth's internal heat) that certain chemical 

 changes have converted leaves and stems into carbon and some 

 of its compounds, much in the same way that, if you heat 

 wood in a closed vessel, you convert it into charcoal, which 

 is mostly carbon. The coal we burn in our fires is entirely of 

 vegetable origin, and every seam in a coal-mine is a buried 

 forest of trees, ferns, reeds, and other plants. 



The reader will understand how it is that rocks composed of 

 hardened sand or gravel, sandstones and conglomerates, contain 

 but few fossils ; while, on the other hand, such rocks as clay, 

 shale, slate, and limestone often abound in fossils, because they 

 are formed of what was once soft mud, that sealed up and 

 protected corals, shell-fish, sea-urchins, fishes, and other marine 

 animals. Had they been covered up in sand the chances are 

 that percolating water would have slowly dissolved the shells 

 and corals, the hard coats of the crabs, and the bones of the 

 fishes, all of which are composed of carbonate of lime; and we 

 know that is a substance easily dissolved by water. 



It is in the rocks formed during the later geological periods 



