It is believed that some concretions form at the same time as the rocks in which they occur. Other concretions develop after the rocks themselves have formed. These are deposited by underground water that contains dissolved mineral matter. The water seeps through the rocks and deposits mineral matter around an object in the rock, such as a fossil or a grain of sand, to form a concretion. Geodes 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. [Illustration: 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 [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). COLLECTING ROCKS AND MINERALS Perhaps you would like to start your own collection of rocks and minerals. For this purpose you will need a _hammer_ (a prospector’s hammer with a pick on one end of it is a good tool), some _newspapers_ to wrap around the specimens to keep them from breaking, and a _cloth bag_ in which to carry the specimens. [Illustration: Prospector’s hammer.]