12 EXTINCT MONSTERS. 



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 

 that we find fossils least changed from their original state ; for 

 time works great changes, and too little time has elapsed since 

 those periods for any considerable alterations to have taken place. 

 But when we come to examine some of the earlier rocks, which 

 have been acted upon in various ways for long periods of time, 

 such as the pressure of vast piles of overlying rocks, and the 

 percolation of water charged with mineral substances (water 

 sometimes warmed by the earth's internal heat), then we may 

 expect to find the remains of the world's lost creations in a much 

 more mineralised condition. Every fossil-collector must be familiar 



