Mac Highlighter In Shell

important fossils in the rocks?

rocks where it is possible to find fossil remains of animals and plants are the sedimentary , ie those resulting from sediments of mud and sand in remote times filled the basins of the seas, becoming stratified and compacted stone.
Animals were accidentally buried in mud that under certain environmental conditions immediately decomposed, but their bodies were impregnated with mineral salts. Gradually they were replaced by cells of organic substances without altering its shape so that when cutting the rock now found in it the bodies of ancient living beings, whose features are preserved turned to stone.

is what happens in the case of fossil shells and many other marine animals.
Since all animals (and all plants) lived in a rather long time but always well defined, to find fossils in rock strata can be established at what time their training dates , based on just the animal that he became trapped.

There fossils of animals that lived in all areas of the earth, but only for relatively short periods and well defined. Geologists call them " guide fossil", and its presence in a field indicates with absolute certainty that these rocks were formed in a given period Earth history .