The majority of fossils are found in marine _sedimentary rocks._ These
are rocks that were formed when salt-water sediments, such as limy muds,
sands, or shell beds, were compressed and cemented together to form
rocks. Only rarely do fossils occur in igneous and metamorphic rocks.
The _igneous rocks_ were once hot and molten and had no life in them,
and _metamorphic rocks_ have been so greatly changed or distorted that
any fossils that were present in the original rock have usually been
destroyed or so altered as to be of little use to the paleontologist.
But even in the sedimentary rocks only a minute fraction of prehistoric
plants and animals have left any record of their existence. This is not
difficult to understand in view of the rather rigorous requirements of
fossilization.