Geodes are rounded, generally hollow masses that occur mostly in limestones. They are scattered through the rocks and can be lifted or dug out. Some geodes are as small as walnuts, and some are as large as basketballs. Most of them have a rough, dull-looking outer surface. If you break geodes open, you will find that many are lined with beautiful crystals of [calcite], [celestite], or [quartz] that point inward toward the hollow center.
[Calcite] geode found in Lower [Cretaceous] strata of western Travis County, Texas.
It is thought that a geode forms when water, carrying dissolved mineral material, seeps into a cavity in the rock, then deposits the mineral material as a lining in the cavity. This lining becomes the outer part of the geode. Thus a geode—unlike a concretion, which grows from the center outward—forms from outside to inside.
Some of the Lower [Cretaceous] [limestone] rocks of Travis, Williamson, and Lampasas counties contain [calcite] and [celestite] geodes. Celestite geodes have also been found in [Permian] rocks in parts of Coke, Fisher, and Nolan counties.
Petrified Wood
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]).