8. Among these vegetable and animal fossils are those which have been penetrated throughout by siliceous or calcareous substances, which have transformed them into stone, of which some have the hardness of marble: these, properly speaking, are petrifactions. Others have simply been enveloped by matter in a soft state, and a few of them are found in a perfect state in the hardest stones. Others have left only imprints, but of a distinct perfect delicacy. In the interior of certain stones the imprint of feet have been discovered, also the form of fingers and nails, from which it is concluded that some animal has produced them.