Chert in Missouri originally occurs chiefly in limestone formations, where it is found as nodules, lenses, stringers, and irregular forms in and between the limestone beds. Chert and flint may be deposited directly from silica in solution, or they may replace (substitute for) wood, fossils, or older rock where silica-bearing solutions contact and react with the replaced substance. For example, petrified wood usually is wood which has been replaced molecule by molecule with silica. This statement applies equally to the brightly colored petrified wood in the Petrified Forest in Arizona and to that with comparatively drab coloring in Missouri. Many other siliceous fossils, notably animal remains, are replacements of calcite (limestone) by silica. In anticipation that the reader may have difficulty understanding how silica may go into solution if chert (silica) is hardly attacked by the weathering process, it should be explained that silica is freed in solution predominately during the weathering of complex silica-combinations, silicates, rather than from uncombined silica. For instance, feldspar and pyroxene from granite or gabbro weather in ground water to a soil-forming clay mineral and release some silica in solution in the ground water. After this silica is redeposited in an uncombined form, like chert, it becomes highly insoluble. An observation in regard to flint is that the metallic “flints” which are used to ignite gas burners or cigarette lighters are not black chert, SiO₂. Instead, they are special alloys containing rather uncommon elements which possess the useful characteristic of emitting a brilliant hot spark when harshly scratched. Chert is the chief source of natural gravel in Missouri because it accumulates in stream beds and on hillsides on account of its resistance to weathering. The piles of “chats” in the Joplin region, containing thousands of tons of crushed chert, have been used in part in road surfacing material. Weathered Chert Weathered chert, or leached chert, is a white to gray, or yellowish, porous, light-weight, harsh to feel, chalky-appearing rock which occurs over much of the southern half of Missouri. It does not effervesce in acid. Usually it occurs as a zone from a fraction of, to more than an inch in thickness, about a denser core of hard, compact chert (flint), or makes up an entire small rock fragment or gravel. [Illustration: Chert hand specimen showing quartz-lined fossil cavity in center, compact fresh chert in interior, and chalky-appearing weathered outside margins. From near Columbia.] It develops as a relatively insoluble residue left when the more soluble rock material in association has been leached away during the weathering process. Its composition approaches pure silica. It has no established use and no commercial value. “Kaoleen”