  A concretion is an aggregate of inorganic matter in the shape, roughly, of a ball, disc, rod, or irregular nodular body. Usually the aggregation or accumulation started around a small center grain or particle and continued in the growth of layers about it like the shells of an onion, or in the growth of needle-like fibers which radiate from the center like pins stuck into a spherical pin cushion. Concretions vary in size from buck-shot (buck-shot concretions in the soil) to oddities ten or twenty feet in diameter, or even longer in elongate forms. The variety about one-sixteenth or one-thirty-second of an inch in diameter is called an oolite (pronounced oh-oh-lite). Some chert of southern Missouri, most of the diaspore and burley clay, and a limestone cropping out near Louisiana, Missouri, are made up partly to almost entirely of oolites. See page 29. Concretions may be composed of pyrite, calcite, limonite, chert, cemented sandstone, or even cemented clay. They are usually recognized by their structure after the previously enclosing rock has been eroded. Thus pyrite, limonite, or calcite (limestone) concretions remain after shale has softened and washed away, chert remains after limestone, and strongly cemented “irony” (limonite or hematite) or siliceous sandstone concretions may be found on the outcrop where the softer or less resistant host rock has been carried off. The irregular-shaped, intergrown, nodular limestone concretions (sometimes called “loess-kinder”, or loess dolls) in the upper part of loess deposits along the Missouri and Mississippi Rivers can be found remaining on rain-washed slopes. Limy, mudstone concretions and brown iron carbonate concretions are abundant in certain localities in northwestern and southeastern Missouri, where they are used as oddities in rock gardens or walls. Concretions. Dark, limy concretion at left and brassy pyrite at center and right. Note the inter-grown pair in center. Some concretions are formed at the same general time as the surrounding rock accumulates, but others may be formed years after the surrounding rock has been buried or removed from the environment of its formation. In either case, deposition of the mineral matter follows the pattern of addition or “growth” from inside out. This growth, of course, does not involve a life process like that of a plant or animal. If two or more centers of deposition occur close together, the several growing concretions may touch, intergrow, and develop some weird forms, suggesting organic growth. However odd these curiosities may become, there is no question that they are not fossils, or evidence of life. Probably concretions excite the interest of persons more than any other rock structure. 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. GeodesA 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 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. FossilsFossils are also found and collected by persons who are interested in rocks and minerals. The varied remains of plants and animals long since petrified or replaced by mineral matter have stimulated the curiosity and become a source of enjoyment to many persons, from those who merely give a passing glance to the peculiar organic structures in the rocks to those who make a serious hobby or business of collecting and classifying the unreplaceable heritage from the ancient rocks. Fossils are interesting in part because of their variety, for they include petrified wood, shells like those of oysters, fish teeth, foot-prints, amber, dinosaur eggs, coal, imprints of fern leaves, of insects, and of fishes, and the bones of small and gigantic dinosaurs and elephants. In fact, a fossil is any evidence of life in the geologic past preserved in the rocks. Missouri rocks furnish fossils ranging in size from microscopically small fish teeth to the big skeletal remains of the mastodon, an ancestor of the elephant; but the most common ones are the structures and shells of ancient clams, corals, brachiopods, crinoids, and trilobites. A tooth of a mastodon, about one-half natural size. (Photo courtesy of Mr. J. R. Morrison, Louisiana, Missouri.) Fossils. Upper row, coral on left, trilobite on right; center row, brachiopods; lower row, coiled cephalapod, crinoid head, and a bryozoan spiral. The accompanying photographs illustrate a few fossils that may be found within our state, but a thorough, non-technical treatment of Missouri fossils is available in a companion volume to this booklet, “The Common Fossils of Missouri” by Prof. A. G. Unklesbay, Missouri Handbook No. 4. The study of fossils, or paleontology, is a fascinating branch of geology which extends far beyond the recognition and cataloging of the specimens. It has been found that certain particular fossils occur in rocks of the ages which produce petroleum, and the search for that valuable substance has been directed in many instances by the fossil content of the rocks. Rocks of different ages carry different fossil assemblages, and a man skilled in paleontology utilizes the fossils in dating geologic history like the page numbers in a book of human history. Further, any student of present-day animals and plants is aided in his understanding of them if he knows the fossil record of their ancestors of the long geologic past. Arrow Heads and Other Indian ArtifactsArrow 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. |
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