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. Row, 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 
