Shale is a compressed, and layered or laminated clay or mud rock. Consequently it will return to mud if it is wetted with water and rubbed. This may serve as a test for shale. It may occur in thick layers or formations, five, ten to fifty or more feet in thickness, and it ranges downward to paper-thin partings between beds of limestone. It is also commonly associated with coal beds. The color of shale varies from light gray to black, or it may be tan, yellow, red, rust, purplish, or green. It is platy, and these thin plates or laminae, piled on each other, make up the shale bed.

Some shales are hard, tough, and strong enough to serve as temporary mine roofs. Hard shales are sometimes called “slate” but this name is technically incorrect. True slate is a metamorphic rock, composed chiefly of the mineral mica in very fine flakes, and will resist the action of water (weathering) for a long time. Therefore, it is a good roofing material for buildings, whereas shale is composed chiefly of clay minerals, and despite the strength and compactness of the more “slaty” varieties soon disintegrates in water. Missouri “slaty” shale would not serve as satisfactory roofing material.

The red “burned” shale found on burned-out coal mine dumps is called “shale” locally. It is, of course, shale which has been fired more or less to the condition of building brick by the hot burning waste coal. The same original shale could be crushed, molded into brick, “burned” in a kiln, and become a satisfactory building brick. The “burned shale” of the coal mine dumps is used in many places as a drive-way covering.

“Soapstone” is a name applied by some persons to some soft, slippery to greasy shales, but this name is incorrect in a technical sense. True soapstone is a metamorphic rock (shale is sedimentary) which is composed chiefly of the mineral talc. Soapstone occurs abundantly in certain parts of the Appalachian Mountains but is exceedingly sparse in Missouri.

The chief commercial uses of shale are in the manufacture of common brick, building brick, building tile, drain tile, sewer pipe, Portland cement, and other ceramic products. Many shale beds and occurrences are technically suitable for these uses but have no real commercial value because other necessary factors are lacking. In order to make brick, tile, or cement there must be sufficient fuel available at low cost, low-priced bulk transportation of the raw and finished products, available labor, capital for the erection of a plant, and above all a large near-by, dependable market for the manufactured product. The value of a shale deposit, therefore, depends as much upon outside conditions as upon the properties of the rock (shale) itself.

The shales of Missouri were formed from deposits of mud that settled out in sea water which in the past covered this state. Fossil remains of sea-living organisms which are preserved in the shale give evidence of the marine conditions once existent here. Like the muds that are accumulating along the Atlantic coast and in the Gulf of Mexico, where the Mississippi River is discharging its load of silt and clay, so did mud form layers on the bottom of geologically ancient interior seas. In some cases sand was later washed in and covered the mud; in other cases limestone-forming material (like off the coast of Florida today) was deposited on top of the mud. The weight of the overlying beds and the slow movement which raised the sea bottom up to land squeezed out the excess water, compressed and compacted the muds into thin layers, and brought about the shale rock which is exposed to us today.

Black muds, rich in humus and other organic material, formed black shales; red and yellow clays colored by red and yellow iron oxides (iron rusts) formed red and yellow shales; and sandy muds were compacted into gritty, sandy shales. All of them were derived from eroding land and soils just as today our eroding soils contribute to the formation of more shale now in the long, slow process of formation.

The chemical composition of an average shale is not simple, as is shown by the subjoined composite analyses of sedimentary rocks taken from U. S. Geological Survey Professional Paper No. 127.

Many persons upon learning that average shale, and even “clay dirt,” may contain 15% alumina, Al₂O₃ (equivalent to almost 8% metallic aluminum), become thoughtlessly and erroneously enthusiastic about aluminum ore possibilities on their farms or properties. The aluminum is there all right, but it is so securely combined with silica and other elements that the cost of extraction is now greater than the price of aluminum obtained from less abundant ores. Until chemists find a method of extraction of the metal from ordinary clay or shale that can be carried out at considerably less expense than is now possible, the vast quantities of clay and shale on the earth’s surface must be considered a distant reserve of a prohibitively high cost aluminum.

Missouri possesses a little bauxitic clay in the southeastern part of the state but unfortunately does not contain deposits of high grade bauxite, the chief ore of aluminum, and so does not contribute to the aluminum production of the United States (see the discussion under DIASPORE CLAY). Arkansas is a leading producer of bauxite, but the geological conditions present in that bauxite locality are so different from Missouri geology that little hope is held for finding bauxite in Missouri, except possibly in the extreme southeastern part.

Fire Clay

Fire clay resembles shale in that it is also a clayey rock and becomes muddy upon wetting and rubbing. It differs from shale at sight in that it (fire clay) is not laminated like shale, but occurs instead in a massive structure which is relatively uniform throughout. Fire clay fractures naturally into blocky or irregular fragments ranging in size from boulders to rough flakes, whereas shale weathers into layered, platy chips.

Shales are commonly buff, yellow, reddish, greenish, or brown in addition to gray in color, whereas good useable fire clay predominates in white, cream, and gray to almost black (if much organic matter is contained in it). Shale is ordinarily gritty with hard sand particles, but most good Missouri fire clay contains only a small amount of sand. Of course, fire clay may grade into sandstone through a sandy clay phase, but this part would not be confused with a layered, gritty shale.

The really determining characteristic of fire clay is its resistance to melting under high temperature. The most positive test for this property is to heat the fire clay to a white heat in comparison with standard preparations (Pyrometric Test Cones) whose fusion temperatures are known. Most of Missouri fire clay will withstand a clean oxidizing heat of over 3000° Fahrenheit without melting.

Clay minerals originate, in general, from the weathering of previously existing silicate rocks and have therefore been called, on occasions, “rotted rocks.” The writer has long insisted that clays, particularly fire clays, should be thought of instead as purified or refined rocks. The original silicate rocks and minerals, which were rich in constituents melting at low temperatures, have been soaked, leached, and washed by chemically active ground water and rain water until many of the undesirable elements have been carried away, leaving a refined material which we use and know as fire clay. Missouri possesses one of the largest reserves of finest quality fire clay in the world. Special bulletins on Missouri fire clay are published by the State Geologist, Rolla, Missouri, and may be obtained from his office.

Persons who have undeveloped fire clay deposits on their property frequently ask advice on whom to contact and how to arrange for sale of their fire clay, with the expectation of a fair return and fair treatment. The writer recommends in such cases that the owner of the clay dig into his deposit to obtain a fresh, clean, representative specimen of his fire clay (about one pound) and send it to one or more of the large substantial fire brick or refractories companies operating in Missouri. Obviously the company located nearest the deposit, or with the lowest-cost shipping facilities, will be in a favored position to purchase the clay. If the individual is skeptical about the trustworthiness of the company’s report, he may send opposite parts of the sample lumps to competitive companies. Of course, the individual may have his clay tested by an independent laboratory at his own expense, but this is ordinarily a useless, costly experience because a company will duplicate those tests in its own laboratory before purchasing the clay. If the refractories companies find the clay useful to them they will proceed with negotiations. If the clay is of inferior quality or if it is not needed by the particular company at that time, even though of acceptable quality, usually the company will return a truthful report at no cost to the clay owner.

The same general advice is given in regard to the development of any mineral deposit which the holder may have. The caution about obtaining a representative sample is especially to be emphasized. It applies to the metallic ores, mineral water, and common rock as well as to fire clay.

Plastic Fire Clay

Plastic fire clay forms a sticky, soft mass when wetted and kneaded with water, and will bond together other clays or rocks. Large plastic fire clay deposits occur in Audrain, Callaway, and St. Louis counties, and lesser quantities are known in Boone, Osage, Gasconade, and Phelps counties. The larger deposits assume a blanket shape with a highly irregular lower surface.

Flint Fire Clay

Flint fire clay is very fine-grained, smooth or slick, and breaks with a shell-like (conchoidal) fracture. It varies in color from white to black, but most flint fire clay mined is near to white. It is relatively non-plastic—that is, does not readily slake or form a sticky mass when worked a little in water. In fact, flint fire clay has been used locally as road surfacing because it does not become very muddy and sticky. Of course, it is inferior to black-top or concrete road surfaces and has too high a commercial value now to be used extensively as road metal.

A hard, white variety of flint fire clay which breaks with numerous conchoidal fractures in appropriate shaped fragments has been called locally “pop-corn flint.” This clay, and other sand-free flint clay, when crushed between one’s teeth “goes to water” in the mouth. Many clay miners use the chewing test to establish the freedom of their clays from gritty sand, which renders flint clay inferior in quality.

Flint fire clays occur geologically in old land depressions and in roughly funnel-shaped pits surrounded by an enclosing layer of sandstone, the whole lying within limestone country rock. The most prominent flint fire clay deposits are found in Callaway, Warren, Lincoln, Osage, Gasconade, Maries, Franklin, and Phelps counties.

Diaspore Clay

Diaspore clay is a harsh, usually porous, earthy type of clay which has been found in Warren, Osage, Gasconade, Maries, Franklin, Phelps, and Crawford counties in Missouri. Some diaspore clay is mealy, or finely granular, some is chalky to compact, and much of it is more or less oolitic. Oolites (oolitic structure) are small rounded bodies varying in size from about bird shot to BB shot size, and those in diaspore may be solid or hollow. Their hollow structure contributes to the porous condition in diaspore clay. See page 29.

It is almost impossible to write a description of diaspore clay which can be used to determine it because the clay has so few individual characteristics. A person familiar with diaspore clay, however, can recognize it at a glance. Probably diaspore clay will not be found outside the counties listed above, and within those counties many persons know the clay from contact with the commercial production of it.

Diaspore clay occurs in old sink-hole, funnel-shaped pits which formed in the dolomite (limestone) underlying that region. A sandstone layer which lines the pit and commonly stands somewhat above the level of the clay because of the sandstones superior resistance to weathering is known as the “rim rock” of the pit. The diaspore clay may be thought of as an extra-refined type of fire clay from which silica has been leached during prolonged solution in swamps and ground water and the more stable alumina (Al₂O₃) left behind as the refined product.

Pits in the diaspore region may contain from a few truck loads of clay to over 50,000 tons of it, but a pit which produces 10,000 tons of good clay is a valuable and not very common deposit. A small fortune falls to the landowner who finds a large diaspore deposit (pit) on his farm, for royalty rates at this time are not less than $1.00 per ton for first grade, 70% Al₂O₃ clay.

Because of the high value of diaspore, a highly competitive prospecting, leasing, mining, and brokerage business has developed in the diaspore region. Practically all of the thrills, hopes, disappointments, and good fortunes that are associated with oil booms are found in this business and clay area; clay pits are only smaller in scale than wild oil gushers. Clay scouts work in secret, mining leases are contested in court, rumors fly fast in English, German, and German-Swiss over the country telephones, prospecting results may be hidden, personal pressure may be brought to influence a deal, and speedy salesmanship is employed when an exciting find is in the offing. When the legends, traditions, and facts of the diaspore region are collected and recorded, an interesting and essential part of Missouri history will have been written.

Missouri has the only locality in the entire world where relatively pure diaspore clay is now mined in commercial quantities. Because of its extreme resistance to fusion under very high temperatures, diaspore has been called the “aristocrat of fire clays.” Diaspore contains a higher percentage of aluminum than does bauxite, the chief ore of aluminum, but because of diaspore’s extremely refractory nature it is less easily reduced to aluminum metal than is bauxite, and therefore finds a more specialized use in the manufacture of refractory and super-refractory brick and tile which may even be used in furnaces to calcine aluminum ore. Where resistance to very high temperature has been required, diaspore super fire brick has been remarkably useful.

Burley Clay

Burley clay is a fire clay intermediate in alumina content between flint clay and first quality diaspore. It takes its name from the oolites (rounded pellets of diaspore) which are scattered through a flint clay base and which were called “burls” by the early clay miners. As the relative number of diaspore oolites increase, an otherwise flint clay becomes burley-flint, then typical burley, and finally grades into second quality and first quality diaspore. Clay in any stage of the variation may be found in some part of the diaspore region or pits. Most of the remarks written on diaspore apply as well to burley clay.

Sandstone

Sandstone is a rock made of sand-size particles more or less well cemented. It is recognized by the grains of sand which are dislodged or scratched loose when the rock is broken, or when scraped with a piece of steel or another hard rock. The old-fashioned grindstone is a sandstone nicely cemented by nature.

Sandstone occurs in thin layers to thick massive beds and deposits which may exceed fifty feet in thickness. In addition to possessing horizontal bedding and parallel bedding planes, some sandstone displays beautiful, intricate cross-bedding or cross-lamination.

The grains of sand composing the stone may be either angular or rounded. They may sparkle in the light from reflections from their crystal faces, or they may have dull, frosted surfaces. Sandstones are ordinarily nearly white in color except where the grains are covered with coatings of yellow or red iron oxide (rust).

The grains themselves are predominantly particles of the mineral quartz, although any rock or mineral of sand size may be present in sandstone. The quartz (see discussion of quartz on page 41) may have been derived from pre-existing sandstones or more directly from granite, porphyry, or other igneous rocks in which quartz crystallized when the hot liquid rock solidified. Today quartz grains which are weathering out of igneous rocks and sandstones are being carried by the Missouri river and tributaries to the Mississippi river and thence to the ocean, where extensive deposits of sand are accumulating, probably destined to become widespread beds of sandstone.

The grains of sand may be broken and become angular during their long trip to the ocean, or they may become rounded by rubbing against each other. If they exist in sand dunes, blown about by the wind before being cemented into rock, the grains usually become somewhat rounded. Even after sandstones are buried beneath other rocks, silica, which is carried in solution by ground waters percolating through the sandstone, may crystallize out on the sand grains and restore some brilliant, angular crystal faces to the otherwise rounded grains.

Cementation of loose sand to more or less firm sandstone is due to the presence of clay, iron oxides, or calcite (mineral of limestone) which may be deposited with the sand. All of these cements are softer and weaker than quartz, thereby being broken first and freeing the harder quartz when the rock is scratched or crushed.

A variety of very hard sandstone called quartzite is one that is so strongly cemented that it breaks through the sand grains instead of around them as is the case with ordinary sandstone. This condition is brought about by their being cemented with silica (chemically the same as quartz), which makes for essentially uniform hardness throughout the rock.

Quartzites are, as previously noted, extremely hard, and resist abrasion and chemical weathering. Reddish quartzite boulders occur rather abundantly north of the Missouri River in the glacial clay, sand, and gravel which overlie the sedimentary rocks that form the bed rock or country rock there. Locally, the hard, red quartzite boulders may be called “red niggerheads”, although the term “niggerhead” is more often applied to black or dark greenish black boulders of basalt (see page 48) also present in the glacial drift. It is to be recalled that the distinguishing hardness of quartzite is due to the hardness of the quartz grains plus the equal hardness of the silica cement.

Asphaltic sandstone is a sandstone impregnated with a bituminous residue from the evaporation of petroleum which once occupied the pores of the rock. It has been reported from more than a dozen counties in western Missouri, but the most extensive deposits are probably in Barton, Vernon, and Lafayette counties.

Attention has been directed to the origin of sandstones from ocean deposits of sand and from sand dunes, but it should be recognized also that river channels and stream valleys which contain deposits of sand (such as those on floodplains, river bottoms, and sand bars) may be covered, and the sand consolidated to sandstone. Many years ago, even long ago geologically, a large river, almost comparable in size to the Missouri river, occupied a channel which is now represented by a long narrow sandstone deposit extending from a little north of Clinton through Warrensburg to Lexington and then east through Moberly almost to Paris. Smaller channel sandstones are abundant in other areas in Missouri.

The sandstones of the so-called Roubidoux formation, which occurs in south central Missouri, commonly show well-preserved ripple marks on the rock slabs. These marks were formed exactly as their name suggests—in sand which was thrown into ripples by the shallow water in which it accumulated and was covered and cemented so as to retain the ripple forms.

Sandstone is used for building stone, walks, grindstones, furnace linings, and rock gardens. Large quantities are mined each year near Pacific, Festus and Crystal City, Klondike, and Hermann, for the manufacture of glass and other uses. Common glass is a cooled melt of relatively pure silica sand, soda ash, and lime. Asphaltic sandstone is used in road building. Sand-lime brick are made of sand. Sand is used as a molding material for metal castings, a parting substance between brick in kilns, and in large quantities in concrete and mortar mixtures.

Chert, Flint

The names chert and flint have in some regions been used for the same hard, fine-grained rock found so abundantly in Missouri, but correct usage employs chert for the white and gray varieties, and flint for the black variety. Flint may be thought of as slightly impure chert, a chert which is colored black by a small amount of pigment, usually fine carbon, or perhaps iron sulphide, scattered through it like fine dust.

Chert is characterized by being harder than glass, brittle, very fine-grained, and by breaking with a smooth, rounded or hollowed clam shell-like (conchoidal) fracture and sharp edges. It was used by Indians to make arrow heads. It accumulates in abundance both in stream beds as gravel which has been more or less rounded by wear, and on the hillsides within the soil and sub-soil. Yellow and red iron oxides may stain and penetrate weathered chert gravel so that it becomes reddish, rusty, tan, yellow or brown.

Chert remains abundant because of its extreme resistance to weathering. It is so hard that stream action wears it only very slowly. Its chemical composition is silica, SiO₂, a substance which is but little affected chemically by ground water. Where chert has contained fine grains of calcite scattered through it, the calcite may be removed in solution, leaving pores, and a zone of porous, light weight, tripolitic chert, harsh to the feel and enveloping an unaltered interior (See WEATHERED CHERT). Not uncommonly, fossil remains of calcite which were embedded in chert have been dissolved, leaving their hollow impressions.

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.

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”

“Kaoleen” is a term used locally in part of south-central Missouri to refer to a chalky, white to tan or buff, porous weathered chert, but the name should be dropped because it is unnecessary (use weathered chert), confusing, and not recognized elsewhere. Most probably the term arose in corruption of the word kaolin, which is the name for a true, high-quality clay, to which the leached and weathered chert bears a slight resemblance. Kaolin has the chemical composition of clay (hydrous aluminum silicate), whereas “kaoleen” is impure silica. See the discussion on Weathered Chert.

Tripoli

Tripoli occurs in the vicinity of Seneca, Newton County, Missouri. It is a light-weight, porous, white to creamy, siliceous rock, which may be scratched because of its softness. Tripoli represents the porous insoluble residue of an earlier rock, which was composed of skeletal insoluble silica and interstitial soluble calcium carbonate (calcite), the latter having been dissolved away by ground water. Tripoli has a chalky appearance but is totally unlike chalk chemically. Tripoli is nearly pure silica, whereas chalk is calcium carbonate. Any tripoli-like rock found in Missouri outside the region of tripoli mines is likely to be a fragment of weathered chert which is described above.

Tripoli has been used as an abrasive, a polishing agent, a parting material in molding sand, and a filter rock.

Agate

Agate is a banded variety of chert. Although the chemical composition of agate is SiO₂, the same as chert, a microscopically fibrous part of it having a waxy luster or varying in color or translucency may give the appearance to the rock that we associate with the name agate. The mineral name chalcedony is given to the fibrous, waxy material.

Typical agates are most abundant in Missouri in the glacial and stream gravels in the northern part of the state, although part of the Potosi drusy quartz and chalcedony in the southeast is also prized. The large gravel pit near LaGrange, in the northeast, has furnished many beautiful specimens, not only of agate, but also of petrified wood and fossils.

Missouri lapidists and collectors of semi-precious stones find plenty of interesting raw material within their own state.

Jasper

Jasper is chert which is colored red or yellowish brown by iron oxides.

Granite

Granite is a granular (coarse-grained) rock which has a glassy luster and is too hard to be scratched appreciably by steel. It may be white to gray, tan, brown, or pink to red in color, but pinkish to red granite predominates in Missouri. Some black stone, referred to locally as “black granite,” is usually a variety of gabbro. Most Missouri granite is coarse-grained, so that the constituent mineral grains—quartz, feldspar, and (less frequently) mica—can be readily recognized by anyone familiar with those minerals. It makes up many of the mountains and hills in Iron, Madison, and St. Francois counties and adjacent regions. North of the Missouri River, or where the glacial deposits remain, granite boulders may occur in the sandy and clayey glacial drift.

The mineral quartz is recognized in granite by its glistening, oily luster, really more brilliant than the luster of glass, and by its curved to irregular broken surface. Furthermore, the brilliant luster of quartz is not dulled by exposure to weather.

The mineral feldspar, in granite, has a glassy luster on the tiny flat cleavage faces where the individual grains are broken. Where weathered, feldspar becomes dulled, and chalky to dusty or clayey. Fresh feldspar may be glassy, white, buff, pink, red, in intermediate shades in color. With the mica, it imparts most of the color to granite.

Mica is recognized by its softness and its ability to be split very easily into tiny flakes. Other minerals may be found in granite under the microscope, but they have little importance or significance.

Granite is an intrusive igneous rock; that is, it solidified from a hot liquid state (like lava) in a large body, beneath, or surrounded by pre-existing rocks. Because of slow solidification a coarse-grained texture was developed. In southeastern Missouri where granite is now exposed at the surface (for example, the Elephant Rocks State Park at Graniteville), that granite was covered originally by hundreds of feet of rock at the time it solidified from a liquid. During the millions of years which have elapsed since the granite solidified, its cover and the upper part of the granite have been eroded away by streams and rain after weathering to soil material. In fact, the ocean has covered the area several times during its long geological history.

Missouri has a fine quality of granite in large quantity in southeastern Missouri. Granite is used for building, structural and monument purposes (see discussion under marble), for rubble stone, rip-rap, ballast, gravel, paving blocks, crushed chicken gravel, and for other specialized uses where favorably located. Chemical analyses of granite and porphry, taken from Missouri Geological Survey Report, Volume VIII, 1895, follow.

The glacial granite boulders found in central to northern Missouri also solidified as intrusive rock in the northern United States or in Canada. After being exposed at the surface they were picked up and carried down by the geologically recent, continental ice sheet (glacier) that moved down from Canada to across the northern half of Missouri. Scratches and grooves may have been cut in some of these boulders, or flat faces scoured and planed off as they were scraped against other hard rocks. Quartz gravel is usually present, often in abundance, in glacial deposits. Small specimens of native metallic copper, which come from near Lake Superior, have been found in Missouri glacial deposits. Even diamonds from an unknown source in the north were carried by the ice down into the United States. The history of the glaciation is a spectacular account of changes which our continent has undergone in the geological past.

Quartz

Quartz is a mineral of wide-spread occurrence which is characterized by the following properties: (1) it is considerably harder than glass or steel, (2) it has a high luster, glassy to oily, (3) it breaks with an irregular or rough glistening fracture, and (4) it crystallizes in six-sided crystals when it grows unobstructed. Ordinary acids do not attack quartz, and it is relatively unaffected by chemical weathering in Missouri. Its composition is silicon dioxide, SiO₂.

Quartz occurs in granite as the lustrous, partially rounded grains which constitute perhaps 20% of the rock (feldspar makes up most of the more opaque remainder which breaks with many small flat faces), and is recognized in the small glistening grains in the porphyry. Hence it is an important igneous rock-forming mineral.

Sandstone in Missouri is made almost entirely of quartz grains which have been broken, worn, and more or less rounded during their long travel history. The sandstone formation quarried and mined near Pacific and Crystal City, named the St. Peter sandstone formation, is an outstandingly pure quartz sandstone and therefore usable in the glass industry. It is obvious that quartz is an important constituent of sedimentary rocks.

Further, in the sedimentary dolomite formation near Potosi, fine to coarse quartz crystals line the surfaces of cavities and pockets in the stone. This cavity coating of quartz which reflects light brilliantly from the many small crystal faces is called drusy quartz by the mineralogist, but is locally and popularly known as “blossom rock.” Thousands of pounds of “blossom rock” are sold each year for rock gardens and various ornamental purposes.

In northeastern Missouri quartz crystals line the hollow, more or less spherical bodies called Geodes, which vary in size from small nuts to melons, and weather out of the so-called Warsaw formation. Other types of hollow cavities in many Missouri rocks may contain quartz growing inward from their walls.

Missouri chert is composed primarily of quartz in microscopically fine grains; likewise, agate and petrified wood may contain abundant quartz. Other varieties are rock crystal, rose quartz, amethyst, false topaz, bloodstone, carnelian, and onyx.

Quartz crystallized in an igneous rock as the hot fluid cooled through its “freezing” temperature interval, which was probably not below 1000°F. In the cases of the quartz in geodes, the drusy quartz, or that in cavities within petrified wood, quartz crystals grew from ground water solutions which must have carried very low concentrations of silica in solution, and whose temperatures did not depart far from that of rocks buried at various depths today. Although quartz is a very common and abundant mineral, our specific knowledge about its transport and deposition is surprisingly meager.

Quartz crystals are used in large quantities in radio apparatus where it is necessary to maintain very close control on the tuning of a circuit. This use requires quartz of highest quality and crystals above minimum size, which have never been found in Missouri and probably are not present. Silica production from this state is in its sandstone, tripoli, chert chats, and rock garden ornamental stone.

Feldspar

Feldspar is a white to pink or red mineral having a glassy luster on its flat broken surfaces (cleavage faces). It will scratch window glass.

It is the most abundant mineral in granite and usually controls the color of that rock; for example, the red granite at Graniteville contains red feldspar, and the pink-gray granite in the Knoblick region has feldspar of those colors. Small bodies or bands of very coarse feldspar, quartz, and mica (pegmatite dikes) which cut the granite may contain crystals of feldspar large enough to be recovered as small, hand specimens, but otherwise it does not occur in coarse fragments. The recognizable crystals, or phenocrysts, in the porphyry are mainly feldspar.

Feldspar is really a family name for a group of several minerals, all of which are crystallized in the igneous rocks. The potassium (potash)-containing varieties, named orthoclase and microcline, occur in the granite and porphyry, whereas plagioclase, a calcium-sodium (lime-soda) feldspar is in gabbro, diabase, and basalt.

Plagioclase commonly has a thin, lath shape, is a shade of gray, and makes up the lighter colored part of the greenish to dark gray igneous rocks. Further differences between it and orthoclase may interest the mineralogist but are of little concern to the non-technical person.

Pulverized feldspars are used extensively in the ceramic industries, but Missouri does not have any productive deposits. Under natural, long-time weathering processes feldspar usually decomposes to clay which may be used technically, but the usual fate of it is soil formation.

Mica

Mica, incorrectly called isinglass, is an elastic, fairly soft, platy mineral, which may be split into flakes of paper thinness. The relatively clear variety is called muscovite, and the brownish black to black variety is biotite, both being members of the mica family. They may occur in Missouri in small grains in the igneous rocks, except that muscovite may be present in sandstone, where it was deposited along with the quartz sand.

Mica is used chiefly as insulating material in the electrical industry where large sheets are required. Another use is as window or chimney material in stoves or lanterns. Missouri has no mica which is satisfactory for these purposes.

Porphyry, Rhyolite, Rhyolite Porphyry

Porphyry and granite are the two most abundant igneous rocks in southeastern Missouri (Iron, Madison, and St. Francois counties, and adjacent country). The porphyry there is a compact, very fine-grained, almost glassy, hard, brittle rock that varies in color from light gray through pink and red to dark purplish red and almost black. It always breaks with a horny, flinty fracture. Small mineral crystals of glistening quartz and usually reddish feldspar are generally scattered throughout the dense background (groundmass). The crystals are commonly about one-sixteenth of an inch in cross section and ordinarily constitute from about ten to twenty per cent of the rock. Other names, somewhat more specific than simple porphyry, which are applied technically to certain phases of the rock are rhyolite, and rhyolite porphyry.