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What happens next depends not just on the pressure put on the skeleton, but also on the waters that slowly seep through as the rock forms. Hard parts, such as the shells of sea creatures, can remain unaltered, especially when the water is lime rich, as in the sediments that produce limestone rock.
If the water contains little lime, but is rich in other minerals, or if it is acid, the shells may dissolve away in the waters that seep through the rock. If the waters are acid, they can dissolve most skeletons. But if they are mineral rich, they may also bring new materials in solution that precipitate out in the exact place where the skeletons once existed. This process, called mineralisation, is possible for plants as well as
for animals.
Some of the most impressive of all fossils produced by replacement of one mineral by another, are the giant trees in petrified forests. The trees have had their bark and woody tissues entirely dissolved
(Below) Limestones often contain
a wealth of skeletons because the limy waters preserved them. This
is Silurian limestone and, although it is over 400 million years old, remains of many kinds of skeletons can be seen clearly.
(Below) The tree rings show up clearly in this cross-section of fossilised wood. This shows how mineralisation works at different rates in different tissues.
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