Arrow heads, scrapers, rock knives and saws which were left by the Indians who formerly lived in Missouri may be found in moderate abundance in many parts of the state. Usually these artifacts are chert in its various colors, white, gray, mottled, reddish, or black (flint). See the discussion of chert on page 34. Chert, because of its conchoidal fracture, lack of cleavage, resistance to chemical weathering, and superior hardness, is an exceptionally useful rock for making tools and weapons. Hammers and axes of basalt, and arrow heads of rhyolite are less abundant than the chert artifacts. THE ROCKS OF MISSOURI Geologists classify rocks into these groups: igneous, sedimentary or metamorphic. Representatives of all three have been described in the preceding pages. [Illustration: 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: Limestone Dolomite Chert Shale Fire clay Flint fire clay Diaspore and Burley clay Sandstone Quartzite Travertine Coal 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: Granite Porphyry, Rhyolite, Rhyolite porphyry Gabbro and Diabase Basalt 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: Gneiss Marble