Perhaps you'll find rocks containing fossils--or even fossils
by themselves. They should form a separate part of your collection.
Fossils are the remains--or the outlines--of former plant or
animal life buried in rock. The older the rock, the simpler the
plant and animal life it contains. Thus fossils can give a clue
to the age of the rock strata.
Fossils can teach history. They tell us about plants and animals
that are now extinct--the dinosaur, for example. They can also tell
of ancient climates. Coral found in rocks in Greenland suggests it
must have once been warm. Remains of fir and spruce trees have been
found in the tropics.
How are fossils formed? Teeth, bone and wood don't last long in
their original state. However, buried materials decompose, leaving
a film of carbon as a fossil. This results in a leaf tracery, or
the outlines of some simple animal. On a gigantic scale, this process
of forming carbon has resulted in our great coal deposits.
Sometimes the buried material is gradually replaced by silica or other
substances, making petrified objects. Wood can be replaced--cell by
cell--by agate or opal from silica-bearing water. The result is
petrified wood, the finest examples of which can be found in our
Petrified Forest National Park in Arizona. This can happen to
shells, too.
How about molds and casts of footprints of ancient animals? A
brontosaurus might have stomped along in soft, warm mud eons ago.
The mud hardened and later another layer of soft earth covered the
print, preserving it.