  Geologists classify rocks into these groups: igneous, sedimentary or metamorphic. Representatives of all three have been described in the preceding pages. Arrowheads made from white, gray, pink, and black chert. (Courtesy of Mr. A. A. Jeffrey, Columbia, Mo.) Sedimentary rocks are those whose particles settled down through the air or water to form rocks in layers or beds; hence layered, bedded, or so-called stratified rocks are sedimentary rocks. For instance, the sand and mud settling out of the Gulf of Mexico (or ocean) after being brought in by the Mississippi River is on its way toward becoming sandstone and shale. Limestone is forming off the coast of Florida now. All of these rocks are accumulating in layers. Where one sees regularly-layered or stratified rocks in streams, road cuts, quarries, bluffs or hillsides, he expects them to be sedimentary rocks. The sedimentary rocks that have been described herein include: Igneous rocks are those which solidified from a hot liquid which was either forced into older surrounding rocks (intrusive) or discharged on the earth’s surface as a lava flow or products from a volcano (extrusive). The examples given below illustrate the two types. Everyone knows about the extrusive forms from accounts of present-day volcanoes and occasional lava flows, like those of Vesuvius, Paricutin, and Mauna Loa. An intrusion was injected beneath the Yellowstone Park area years ago, and its heat, with steam and gases, is contributing to the unusual natural features which are found in the park and which make it famous. Igneous rocks in Missouri are: Metamorphic rocks are rocks which have been changed through the effects of tremendous pressure (enough to raise mountains) and high temperature while in the solid state. In most cases a banded rock results. The metamorphic rocks mentioned in this booklet are: 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. |
|
