[Illustration: Petrified wood from Texas Gulf Coastal Plain.]
We often find some minerals occurring as petrified wood. (Petrified wood
includes silicified wood, opalized wood, agatized wood, and carbonized
wood.) Petrified wood forms when plant material, such as a tree or a
bush, is replaced by a mineral. It is formed by underground water
carrying dissolved mineral matter. As this water seeps through sediments
in which the plants are buried, it gradually deposits agate, chalcedony,
calcite, opal, chalcocite, or some other mineral in the place of each
fiber of the wood. By this slow change from plant to mineral matter, the
original shape and structure of the wood remain unchanged.
Petrified wood is commonly found in some of the Tertiary, Permian, and
Lower Cretaceous rocks of Texas. (_See_ Opal, Quartz, Copper Minerals,
pp. 78, 84, 52).