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 
