Siliceous petrefactions are ſometimes met with, but, in general, this material forms only nuclei (§ 264). Trunks of trees are ſometimes found changed into agate. The celebrated Ferber has ſeen petrefactions in chert and jaſper, and the illuſtrious Born mentions corallines (porpitæ) in ſinople or martial jaſper.
Earth organic.
Animals and vegetables are reſolved by putrefaction into an earth, which may be regarded as forming a peculiar genus, until every appearance of organization being obliterated, at length it comes to be conſidered as common earth.
Petroleum impregnating organic Bodies.
Wood, penetrated by indurated petroleum, forms a remarkable variety of coal.