No doubt it has become apparent to the reader that rocks ordinarily occur in great quantities, that they are composed of multitudes of grains (mineral grains), and that their properties and compositions vary with the different minerals which are present in the grains of the rock. A _rock_ can, therefore, be different from a _mineral_. In fact, a rock may be defined as “an aggregate of mineral particles,” or more broadly “a typical part of the earth.” To focus closer attention on minerals we may discuss them for their own sake below. MINERALS OF MISSOURI A mineral is characterized by a constancy of composition and of properties which sets it apart from rocks which vary widely. Minerals may be metallic, like pyrite, or non-metallic, like barite; they may be ore, like galena, or rock-forming, like quartz or feldspar; they may show crystal faces, or they may be fragments with rounded or broken surfaces. A favored definition teaches that “a mineral is a naturally occurring, inorganic substance having a definite chemical composition and definite physical properties, within limits.” The Missouri minerals listed herein include: Quartz Mica Calcite Dolomite Sphalerite Galena Barite Marcasite Carnotite Tyuyamunite Pyrite Hematite Limonite Gypsum Pyroxene Orthoclase feldspar Microcline feldspar Plagioclase feldspar Metatorbernite GEOLOGIC VALUES Although the emphasis in this pamphlet has been on the recognition of Missouri rocks and minerals, it is not out of order to consider the broad values that they contribute to our civilization. Their use as building materials has been noted, but it should be further recognized that as our timber is being rapidly depleted more and more structures will be built out of earth materials. Missouri possesses a wealth of beautiful limestone that is serviceable and readily quarried. Where limestone is not near, there is usually shale or glacial clay which can be used in the manufacture of brick and tile. Permanency will be the keynote of the rock and ceramic structures. Gravel and sand are abundant in Missouri for concrete and other varied uses. The soil is Missouri’s most valuable earth material. Hundreds to thousands of years of normal weathering are required to develop the rocks and minerals and texture of the inorganic fraction of the soil. We should preserve it and prevent disastrous soil erosion.