Petrifaction
the act or process of petrifying or changing into a stone; the state of being petrified; conversion of any organic matter, animal or vegetable, into stone, or a substance of stony hardness. A "petrifaction" is not, strictly speaking, a transformation of the original animal or plant into stone.
It is merely a replacement of the organic tissue by mineral substance. As each particle of the plant or animal decays and disappears its place is taken, usually in water or mud, by a particle of mineral PETROLEUM matter deposited from the water which has held it in suspension. Thus the perishable original is changed into imperishable stone. By such means have the skeletons of animals millions of years old been preserved in the rocks of the everlasting hills. In the same way whole forests of trees in the Yellowstone region and elsewhere are changed into agate and other forms of stone, the hollow logs of the forest primeval being often found filled with beautiful crystals of quartz and amethyst.