Ground water, carrying calcium carbonate, silica, or iron compounds in solution, is a great concretion builder. It percolates through sandstone or other permeable rock and slowly leaves behind enveloping layers or additions of mineral matter, until a concretion is formed, to remain hidden from view until its host rock is softened and removed by the action of the weather. Geodes A geode is usually a hollow, more or less spherical or ball-shaped shell of mineral and crystal growth which has formed within surrounding rock. Missouri geodes commonly vary in size from hickory nuts to small watermelons, although neither direction of variation is limited. They weather out abundantly at several localities in northeast Missouri from the so-called Warsaw formation, a limy shale. Here they are dark brown, rough and irregular on the outside, but where broken open show many brilliant, glistening faces of intergrown quartz crystals. Less frequently calcite, chalcedony, kaolinite, and rarely millerite (nickel sulphide) may occur in Missouri geodes. [Illustration: Typical small geode from northeastern Missouri.] The minerals and crystals of a geode grow inward from the walls of a cavity in the rocks. The mineral matter is carried there in solution by ground water and crystallizes out very much more slowly but in the same manner that sugar or salt crystals develop in a saturated solution of those substances. If crystal growth continues until the geode is solid, it may bear superficial resemblance to a concretion, but the latter structure is one which has grown outward. The idea of “growth” in either case is that of mineral crystallization and enlargement, but does not in the least involve life like that of a plant or animal. Geodes have no value or use other than for ornamental purposes. Fossils