Is a fossil in a rock a grain?
Sometimes: Some fossils are clasts, or grains, in a rock. However, often a fossil takes the form of a mould or cast – an impression of the item that was there, but has since been dissolved, and in this case it is not a grain. A fossil is any trace of organic life in a rock – including tracks and burrows, impressions like leaf impression and shells and other hard parts. A fossil can only be regarded as a clast or grain if it still has its original form, as in shells and shelly debris which persist in the rock. Some limestones for instance consist almost entirely of the tests of micro-organisms like foraminifera – in this case the microfossil is also a grain.
