The Ouachita Mountains referred to as the Ouachitas, are a mountain range in western Arkansas and southeastern Oklahoma. They are formed by a thick succession of deformed Paleozoic strata constituting the Ouachita Fold and Thrust Belt, one of the important orogenic belts of North America; the Ouachitas continue in the subsurface to the southeast where they make a poorly understood connection with the Appalachians and to the southwest where they join with the Marathon area of West Texas. Together with the Ozark Plateaus, the Ouachitas form the U. S. Interior Highlands; the highest natural point is Mount Magazine at 2,753 feet. Louis R. Harlan claimed that "Ouachita" is composed of the Choctaw words ouac for buffalo and chito for large, together meaning "country of large buffaloes". At one time, herds of buffalo inhabited the lowland areas of the Ouachitas. Historian Muriel H. Wright wrote that "Ouachita" is composed of the Choctaw words owa for hunt and chito for big, together meaning "big hunt far from home".
According to the article Ouachita in the Encyclopedia of Oklahoma History and Culture, "Ouachita" comes from the French spelling of the Caddo word washita, meaning "good hunting grounds". The Ouachitas are a major physiographic province of Arkansas and Oklahoma and are grouped with the Arkansas River Valley. Together with the Ozark Plateaus, the Ouachitas form the U. S. Interior Highlands, one of few mountainous regions between the Appalachians and Rockies; the Ouachitas are dominated by pine and hickory. The shortleaf pine and post oak are common in upland areas; the maple-leaf oak is found at only four sites worldwide. Some native tree species, such as the eastern red-cedar, are colonizers of human-disturbed sites; the Ouachita National Forest covers 1.8 million acres of the Ouachitas. It is one of the largest and oldest national forests in the Southern U. S. created through an executive order by President Theodore Roosevelt on December 18, 1907. There are six wilderness areas within the Ouachita National Forest, which are protected areas designed to minimize the impacts of human activities.
Bison and elk once have since been extirpated. Today, there are large populations of white-tailed deer and other common temperate forest animals. Though elusive, hundreds of black bear roam the Ouachitas. Several species of salamander are endemic to the Ouachitas and have traits that vary from one locale to another; the Athens Piedmont consists of a series of none exceeding 1,000 feet. It is located south of the Ouachitas and extends from Arkadelphia, Arkansas to the Arkansas-Oklahoma border; the Athens Piedmont runs through Clark, Howard and Sevier counties in Arkansas and McCurtain County in Oklahoma. The Caddo and Missouri mountains are a high, compact group of mountains composed of the weather-resistant Arkansas Novaculite, they are located in Montgomery and Polk counties, Arkansas. The highest natural point is Raspberry Mountain at 2,358 feet; the headwaters of multiple rivers are found in this area, including the Caddo and Little Missouri rivers. The Cross Mountains are located in Polk and Sevier counties, Arkansas and McCurtain County, Oklahoma.
The highest natural point is Whiskey Peak at 1,670 feet. The Crystal Mountains are located in Montgomery County, Arkansas, they are so named because of the occurrence of some of the world's finest quartz. The Crystal Mountains are taller than the nearby Zig Zag Mountains, achieving elevations over 1,800 feet; the Fourche Mountains are a long, continuous chain of mountains composed of the weather-resistant Jackfork Sandstone. They extend from Pulaski County, Arkansas to Atoka County and are home to several popular sites of interest, including Pinnacle Mountain State Park near Little Rock, Arkansas; the highest natural point is Rich Mountain at 2,681 feet, which intersects the Arkansas-Oklahoma border near Mena, Arkansas. The Fourche Mountains form a major watershed divide between the Arkansas River Basin to the north and the Red River Basin to the south; the Frontal Ouachita Mountains are located in the Arkansas River Valley and feature a number of isolated landforms. The highest natural point is Mount Magazine at 2,753 feet, the highest natural point of the Ouachitas and U.
S. Interior Highlands; the Frontal Ouachita Mountains are structurally quite different from the rest of the Ouachitas and are sometimes considered a separate range. The Trap Mountains are located in Garland and Hot Spring counties, Arkansas; the highest natural point is Trap Mountain at 1,310 feet. The Zig Zag Mountains are located in Garland County and are home to the thermal springs of Hot Springs National Park, they are so named because of their unique chevron shape when viewed from above, the result of plunging anticlines and synclines. The Zig Zag Mountains do reach heights over 1,400 feet; the Ouachitas are formed by a thick succession of deformed Paleozoic strata constituting the Ouachita Fold and Thrust Belt, which outcrops for 220 miles in western Arkansas and southeastern Oklahoma. In a general sense, the Ouachitas are considered an anticlinorium because the oldest known rocks are located towards the center of the outcrop area; the Ouachitas continue in the subsurface to the Black Warrior Basin of Alabama and Mississippi where they plunge towards the Appalachian Mountains.
To the southwest, the Ouachitas join with the Marathon area of west Texas where rocks of the Ouachita Fold and Thrust Belt are exposed. Unlike many ranges in the United St
Chalcedony is a cryptocrystalline form of silica, composed of fine intergrowths of quartz and moganite. These are both silica minerals, but they differ in that quartz has a trigonal crystal structure, while moganite is monoclinic. Chalcedony's standard chemical structure is SiO2. Chalcedony has a waxy luster, may be semitransparent or translucent, it can assume a wide range of colors, but those most seen are white to gray, grayish-blue or a shade of brown ranging from pale to nearly black. The color of chalcedony sold commercially is enhanced by dyeing or heating; the name chalcedony comes from the Latin chalcedonius. The name appears in Pliny the Elder's Naturalis Historia as a term for a translucid kind of Jaspis; the name is derived from the town Chalcedon in Asia Minor. The Greek word khalkedon appears in the Book of Revelation, it is a hapax legomenon found nowhere else, so it is hard to tell whether the precious gem mentioned in the Bible is the same mineral known by this name today. Chalcedony occurs in a wide range of varieties.
Many semi-precious gemstones are in fact forms of chalcedony. The more notable varieties of chalcedony are as follows: Agate is a variety of chalcedony characterized by either transparency or color patterns, such as multi-colored curved or angular banding. Opaque varieties are sometimes referred to as jasper. Fire agate shows iridescent phenomena on a brown background. Landscape agate is chalcedony with a number of different mineral impurities making the stone resemble landscapes. Aventurine is a form of quartz, characterised by its translucency and the presence of platy mineral inclusions that give a shimmering or glistening effect termed aventurescence. Chrome-bearing fuchsite is the classic inclusion, gives a silvery green or blue sheen. Oranges and browns are attributed to goethite. Carnelian is a clear-to-translucent reddish-brown variety of chalcedony, its hue may vary to an intense almost-black coloration. Similar to carnelian is sard, brown rather than red. Chrysoprase is a green variety of chalcedony, colored by nickel oxide.
Blue-colored chalcedony is sometimes referred to as "blue chrysoprase" if the color is sufficiently rich, though it derives its color from the presence of copper and is unrelated to nickel-bearing chrysoprase. Heliotrope is a green variety of chalcedony, containing red inclusions of iron oxide that resemble drops of blood, giving heliotrope its alternative name of bloodstone. In a similar variety, the spots are yellow instead. Moss agate contains green filament-like inclusions, giving it the superficial appearance of moss or blue cheese. There is tree agate, similar to moss agate except it is solid white with green filaments whereas moss agate has a transparent background, so the "moss" appears in 3D, it is not a true form of agate. Chrome Chalcedony is a green variety of chalcedony, colored by chromium compounds, it is known as "Mtorolite" found in Zimbabwe and "Chiquitanita" found in Bolivia. Onyx is a variant of agate with white banding. Agate with brown, orange and white banding is known as sardonyx.
As early as the Bronze Age chalcedony was in use in the Mediterranean region. People living along the Central Asian trade routes used various forms of chalcedony, including carnelian, to carve intaglios, ring bezels, beads that show strong Greco-Roman influence. Fine examples of first century objects made from chalcedony Kushan, were found in recent years at Tillya-tepe in north-western Afghanistan. Hot wax would not stick to it so it was used to make seal impressions; the term chalcedony is derived from the name of the ancient Greek town Chalkedon in Asia Minor, in modern English spelled Chalcedon, today the Kadıköy district of Istanbul. According to tradition, at least three varieties of chalcedony were used in the Jewish High Priest's Breastplate.. The Breastplate included jasper and sardonyx, there is some debate as to whether other agates were used. In the 19th century, Idar-Oberstein, became the world's largest chalcedony processing center, working on agates. Most of these agates were in particular Brazil.
The agate carving industry around Idar and Oberstein was driven by local deposits that were mined in the 15th century. Several factors contributed to the re-emergence of Idar-Oberstein as agate center of the world: ships brought agate nodules back as ballast, thus providing cheap transport. In addition, cheap labor and a superior knowledge of chemistry allowed them to dye the agates in any color with processes that were kept secret; each mill in Idar-Oberstein had five grindstones. These were of red sandstone, obtained from Zweibrücken.
Everett is a borough in Bedford County, United States. The population was 1,834 at the 2010 census. Everett's original name was Bloody Run, after a creek, the site of a battle between settlers and Native Americans; the town was renamed in honor of Massachusetts politician and orator Edward Everett. Bestselling American novelist Dean Koontz was born in Everett. Over 200 years ago, the sun pierced through the thick forest on a small Indian village and trading post known as Bloody Run, located on a wagon road headed to Fort Dusquesne in south central Pennsylvania. In 1787, Michael Barndollar purchased the land in this area, laid out a town, called Waynesburg; this name was never used and this small village was incorporated as a borough in November 1860, to be known as Bloody Run. While this name carries with it many interesting stories and much history, the name was changed in February 1873 to Everett; the Everett Historic District was added to the National Register of Historic Places in 2003. The town came to national attention in 2014 when a 14-year-old boy was arrested for simulating a sex act on a statue and the district attorney defied outcry and asserted his intention to have the boy sentenced to two years of detention for the prank.
Everett is located in eastern Bedford County at 40°0′51″N 78°22′24″W. It is bordered on the south by the unincorporated community of Earlston. According to the United States Census Bureau, the borough has a total area of 1.1 square miles, all land. Located in a valley of the Allegheny Mountains, Everett sits within a natural transportation corridor where the Raystown Branch of the Juniata River has carved a water gap, called The Narrows, through Tussey Mountain; the Alleghenies are a sub-region of the much larger Appalachian Mountains, cover an area of central Pennsylvania, western Maryland and northern West Virginia. The countryside surrounding Everett is composed of large forested areas, extensive agricultural fields, small villages, woodlots. U. S. Route 30 bypasses the borough along its north edge; the highway's former route, the Lincoln Highway, passes through the center of town as Main Street. Interstate 76, the Pennsylvania Turnpike, passes just south of the borough but does not provide access, with the nearest exits being Bedford 10 miles to the west and Breezewood 8 miles to the east.
Pennsylvania's longest hiking trail, the Mid State Trail, passes directly through the center of town. As of the census of 2000, there were 1,905 people, 876 households, 515 families residing in the borough; the population density was 1,773.4 people per square mile. There were 967 housing units at an average density of 900.2 per square mile. The racial makeup of the borough was 98.43% White, 0.52% African American, 0.26% Asian, 0.26% from other races, 0.52% from two or more races. Hispanic or Latino of any race were 0.52% of the population. There were 876 households, out of which 27.6% had children under the age of 18 living with them, 43.5% were married couples living together, 12.0% had a female householder with no husband present, 41.1% were non-families. Of all households 38.5% were made up of individuals, 18.3% had someone living alone, 65 years of age or older. The average household size was 2.15 and the average family size was 2.79. In the borough the population was spread out, with 22.9% under the age of 18, 8.6% from 18 to 24, 26.3% from 25 to 44, 21.6% from 45 to 64, 20.6% who were 65 years of age or older.
The median age was 39 years. For every 100 females there were 83.2 males. For every 100 females age 18 and over, there were 76.9 males. The median income for a household in the borough was $23,919, the median income for a family was $33,819. Males had a median income of $26,953 versus $16,196 for females; the per capita income for the borough was $15,841. About 13.3% of families and 19.2% of the population were below the poverty line, including 22.7% of those under age 18 and 18.7% of those age 65 or over. Residents of Everett may attend the local, public schools operated by Everett Area School District which provides full day kindergarten through 12th grade. In 2014, the Everett Area School District's enrollment declined to 1,293 students. In 2011, Everett Area School District enrollment was 1,422 pupils; the District's enrollment was 1,447 pupils in 2005–2006. Everett Area School District operates: Breezewood Elementary School; the high school and middle school share a single school building. In 2014, Everett Area School District’s graduation rate was 87%.
In 2015, the Pittsburgh Business Times ranked Everett Area School District 340th out of 493 public schools for academic achievement of its pupils. In 2012, Everett Area School District achieved Adequate Yearly Progress despite the chronic, low academic achievement at the high school. High school students can attend the Bedford County Technology Center for training in the construction trades, child care, allied health careers as well as cosmetology. Everett residents may apply to attend any of the Commonwealth's 14 public cyber charter schools at no additional cost to the parents. Tuition is paid by the state and local school district; the cyber school provides a internet access. In 2013 the tuition rate that Everett Area School District must pay was $8,864.08 elementary school, $9,725.26 for middle and high school students. By Commonwealth law, if the District provides transportation for its own students the District must provide transportation to any school that lies within 10 mi
Flint is a hard, sedimentary cryptocrystalline form of the mineral quartz, categorized as a variety of chert. It occurs chiefly as nodules and masses such as chalks and limestones. Inside the nodule, flint is dark grey, green, white or brown in colour, has a glassy or waxy appearance. A thin layer on the outside of the nodules is different in colour white and rough in texture. From a petrological point of view, "flint" refers to the form of chert which occurs in chalk or marly limestone. "common chert" occurs in limestone. Flint is durable and can be found along streams and beaches, its use to make stone tools dates back millions of years. Due to some properties of flint it breaks into sharp edged pieces making it useful for knife blades and other sharp tools. During the Stone Age access to flint was so important for survival that people would travel or trade to obtain flint. Flint Ridge in eastern Ohio was an important source of flint and Native Americans extracted the flint from hundreds of quarries along the ridge.
This "Ohio Flint" was traded across the eastern United States and has been found as far west as the Rocky Mountains and south around the Gulf of Mexico. The exact mode of formation of flint is not yet clear, but it is thought that it occurs as a result of chemical changes in compressed sedimentary rock formations, during the process of diagenesis. One hypothesis is that a gelatinous material fills cavities in the sediment, such as holes bored by crustaceans or molluscs and that this becomes silicified; this hypothesis explains the complex shapes of flint nodules that are found. The source of dissolved silica in the porous media could be the spicules of silicious sponges. Certain types of flint, such as that from the south coast of England, contain trapped fossilised marine flora. Pieces of coral and vegetation have been found preserved like amber inside the flint. Thin slices of the stone reveal this effect. Puzzling giant flint formations known as paramoudra and flint circles are found around Europe but in Norfolk, England on the beaches at Beeston Bump and West Runton.
Flint sometimes occurs in large flint fields for example, in Europe. The "Ohio flint" is the official gemstone of Ohio state, it is formed from limey debris, deposited at the bottom of inland Paleozoic seas hundreds of millions of years ago that hardened into limestone and became infused with silica. The flint from Flint Ridge is found in many hues like red, pink, blue and gray, with the color variations caused by minute impurities of iron compounds. Flint was used in the manufacture of tools during the Stone Age as it splits into thin, sharp splinters called flakes or blades when struck by another hard object; this process is referred to as knapping. The process of making tools this way is called "flintknapping". In Europe, some of the best toolmaking flint has come from Belgium, the coastal chalks of the English Channel, the Paris Basin, Thy in Jutland, the Sennonian deposits of Rügen, Grimes Graves in England, the Upper Cretaceous chalk formation of Dobruja and the lower Danube, the Cenomanian chalky marl formation of the Moldavian Plateau and the Jurassic deposits of the Kraków area and Krzemionki in Poland, as well as of the Lägern in the Jura Mountains of Switzerland.
Flint mining became more common since the Neolithic. In 1938, a project of the Ohio Historical Society, under the leadership of H. Holmes Ellis began to study the flintknapping "methods and techniques" of Native Americans. Like past studies, this work involved experimenting with actual flintknapping techniques by creation of stone tools through the use of techniques like direct freehand percussion, freehand pressure and pressure using a rest. Other scholars who have conducted similar experiments and studies include William Henry Holmes, Alonzo W. Pond, Sir Francis H. S. Knowles and Don Crabtree; when struck against steel, a flint edge produces. The hard flint edge shaves off a particle of the steel that exposes iron, which reacts with oxygen from the atmosphere and can ignite the proper tinder. Prior to the wide availability of steel, rocks of pyrite would be used along with the flint, in a similar way; these methods are popular in woodcraft and amongst people practising traditional fire-starting skills.
A major use of flint and steel was in the flintlock mechanism, used in flintlock firearms, but used on dedicated fire-starting tools. A piece of flint held in the jaws of a spring-loaded hammer, when released by a trigger, strikes a hinged piece of steel at an angle, creating a shower of sparks and exposing a charge of priming powder; the sparks ignite the priming powder and that flame, in turn, ignites the main charge, propelling the ball, bullet, or shot through the barrel. While the military use of the flintlock declined after the adoption of the percussion cap from the 1840s onward, flintlock rifles and shotguns remain in use amongst recreational shooters. Flint and steel used to strike sparks were superseded by ferrocerium; this man-made material, when scraped with any hard, sharp edge, produces sparks that are much hotter than obtained with natural flint and steel, allowing use of a wider range of tinders. Because it can produce sparks when wet and can start fires
Weathering is the breaking down of rocks and minerals as well as wood and artificial materials through contact with the Earth's atmosphere and biological organisms. Weathering occurs in situ, that is, in the same place, with little or no movement, thus should not be confused with erosion, which involves the movement of rocks and minerals by agents such as water, snow, wind and gravity and being transported and deposited in other locations. Two important classifications of weathering processes exist – physical and chemical weathering. Mechanical or physical weathering involves the breakdown of rocks and soils through direct contact with atmospheric conditions, such as heat, water and pressure; the second classification, chemical weathering, involves the direct effect of atmospheric chemicals or biologically produced chemicals known as biological weathering in the breakdown of rocks and minerals. While physical weathering is accentuated in cold or dry environments, chemical reactions are most intense where the climate is wet and hot.
However, both types of weathering occur together, each tends to accelerate the other. For example, physical abrasion decreases the size of particles and therefore increases their surface area, making them more susceptible to chemical reactions; the various agents act in concert to convert primary minerals to secondary minerals and release plant nutrient elements in soluble forms. The materials left over after the rock breaks down combined with organic material creates soil; the mineral content of the soil is determined by the parent material. In addition, many of Earth's landforms and landscapes are the result of weathering processes combined with erosion and re-deposition. Physical weathering called mechanical weathering or disaggregation, is the class of processes that causes the disintegration of rocks without chemical change; the primary process in physical weathering is abrasion. However and physical weathering go hand in hand. Physical weathering can occur due to temperature, frost etc. For example, cracks exploited by physical weathering will increase the surface area exposed to chemical action, thus amplifying the rate of disintegration.
Abrasion by water and wind processes loaded with sediment can have tremendous cutting power, as is amply demonstrated by the gorges and valleys around the world. In glacial areas, huge moving ice masses embedded with soil and rock fragments grind down rocks in their path and carry away large volumes of material. Plant roots pry them apart, resulting in some disintegration. However, such biotic influences are of little importance in producing parent material when compared to the drastic physical effects of water, ice and temperature change. Thermal stress weathering, sometimes called insolation weathering, results from the expansion and contraction of rock, caused by temperature changes. For example, heating of rocks by sunlight or fires can cause expansion of their constituent minerals; as some minerals expand more than others, temperature changes set up differential stresses that cause the rock to crack apart. Because the outer surface of a rock is warmer or colder than the more protected inner portions, some rocks may weather by exfoliation – the peeling away of outer layers.
This process may be accelerated if ice forms in the surface cracks. When water freezes, it expands with a force of about 1465 Mg/m^2, disintegrating huge rock masses and dislodging mineral grains from smaller fragments. Thermal stress weathering comprises thermal shock and thermal fatigue. Thermal stress weathering is an important mechanism in deserts, where there is a large diurnal temperature range, hot in the day and cold at night; the repeated heating and cooling exerts stress on the outer layers of rocks, which can cause their outer layers to peel off in thin sheets. The process of peeling off is called exfoliation. Although temperature changes are the principal driver, moisture can enhance thermal expansion in rock. Forest fires and range fires are known to cause significant weathering of rocks and boulders exposed along the ground surface. Intense localized heat can expand a boulder; the thermal heat from wildfire can cause significant weathering of rocks and boulders, heat can expand a boulder and thermal shock can occur.
The differential expansion of a thermal gradient can be understood in terms of stress or of strain, equivalently. At some point, this stress can exceed the strength of the material. If nothing stops this crack from propagating through the material, it will result in the object's structure to fail. Frost weathering called ice wedging or cryofracturing, is the collective name for several processes where ice is present; these processes include frost frost-wedging and freeze -- thaw weathering. Severe frost shattering produces huge piles of rock fragments called scree which may be located at the foot of mountain areas or along slopes. Frost weathering is common in mountain areas where the temperature is around the freezing point of water. Certain frost-susceptible soils expand or heave upon freezing as a result of water migrating via capillary action to grow ice lenses nea
Limestone is a carbonate sedimentary rock, composed of the skeletal fragments of marine organisms such as coral and molluscs. Its major materials are the minerals calcite and aragonite, which are different crystal forms of calcium carbonate. A related rock is dolostone, which contains a high percentage of the mineral dolomite, CaMg2. In fact, in old USGS publications, dolostone was referred to as magnesian limestone, a term now reserved for magnesium-deficient dolostones or magnesium-rich limestones. About 10% of sedimentary rocks are limestones; the solubility of limestone in water and weak acid solutions leads to karst landscapes, in which water erodes the limestone over thousands to millions of years. Most cave systems are through limestone bedrock. Limestone has numerous uses: as a building material, an essential component of concrete, as aggregate for the base of roads, as white pigment or filler in products such as toothpaste or paints, as a chemical feedstock for the production of lime, as a soil conditioner, or as a popular decorative addition to rock gardens.
Like most other sedimentary rocks, most limestone is composed of grains. Most grains in limestone are skeletal fragments of marine organisms such as foraminifera; these organisms secrete shells made of aragonite or calcite, leave these shells behind when they die. Other carbonate grains composing limestones are ooids, peloids and extraclasts. Limestone contains variable amounts of silica in the form of chert or siliceous skeletal fragment, varying amounts of clay and sand carried in by rivers; some limestones do not consist of grains, are formed by the chemical precipitation of calcite or aragonite, i.e. travertine. Secondary calcite may be deposited by supersaturated meteoric waters; this produces speleothems, such as stalactites. Another form taken by calcite is oolitic limestone, which can be recognized by its granular appearance; the primary source of the calcite in limestone is most marine organisms. Some of these organisms can construct mounds of rock building upon past generations. Below about 3,000 meters, water pressure and temperature conditions cause the dissolution of calcite to increase nonlinearly, so limestone does not form in deeper waters.
Limestones may form in lacustrine and evaporite depositional environments. Calcite can be dissolved or precipitated by groundwater, depending on several factors, including the water temperature, pH, dissolved ion concentrations. Calcite exhibits an unusual characteristic called retrograde solubility, in which it becomes less soluble in water as the temperature increases. Impurities will cause limestones to exhibit different colors with weathered surfaces. Limestone may be crystalline, granular, or massive, depending on the method of formation. Crystals of calcite, dolomite or barite may line small cavities in the rock; when conditions are right for precipitation, calcite forms mineral coatings that cement the existing rock grains together, or it can fill fractures. Travertine is a banded, compact variety of limestone formed along streams where there are waterfalls and around hot or cold springs. Calcium carbonate is deposited where evaporation of the water leaves a solution supersaturated with the chemical constituents of calcite.
Tufa, a porous or cellular variety of travertine, is found near waterfalls. Coquina is a poorly consolidated limestone composed of pieces of coral or shells. During regional metamorphism that occurs during the mountain building process, limestone recrystallizes into marble. Limestone is a parent material of Mollisol soil group. Two major classification schemes, the Folk and the Dunham, are used for identifying the types of carbonate rocks collectively known as limestone. Robert L. Folk developed a classification system that places primary emphasis on the detailed composition of grains and interstitial material in carbonate rocks. Based on composition, there are three main components: allochems and cement; the Folk system uses two-part names. It is helpful to have a petrographic microscope when using the Folk scheme, because it is easier to determine the components present in each sample; the Dunham scheme focuses on depositional textures. Each name is based upon the texture of the grains. Robert J. Dunham published his system for limestone in 1962.
Dunham divides the rocks into four main groups based on relative proportions of coarser clastic particles. Dunham names are for rock families, his efforts deal with the question of whether or not the grains were in mutual contact, therefore self-supporting, or whether the rock is characterized by the presence of frame builders and algal mats. Unlike the Folk scheme, Dunham deals with the original porosity of the rock; the Dunham scheme is more useful for hand samples because it is based on texture, not the grains in the sample. A revised classification was proposed by Wright, it adds some diagenetic patterns and can be summarized as follows: See: Carbonate platform About 10% of all sedimentary rocks are limestones. Limestone is soluble in acid, therefore forms many erosional landforms; these include limestone pavements, pot holes, cenotes and gorges. Such erosion landscapes are known
Radiolarite is a siliceous, comparatively hard, fine-grained, chert-like, homogeneous sedimentary rock, composed predominantly of the microscopic remains of radiolarians. This term is used for indurated radiolarian oozes and sometimes as a synonym of radiolarian earth. However, radiolarian earth is regarded by Earth scientists to be the unconsolidated equivalent of a radiolarite. A radiolarian chert is well-bedded, microcrystalline radiolarite that has a well-developed siliceous cement or groundmass. Radiolarites are biogenic, finely layered sedimentary rocks; the layers reveal an interchange of clastic mica grains, radiolarian tests and organic pigments. Clay minerals are not abundant. Radiolarites deposited in shallow depths can interleave with carbonate layers, yet most radiolarites are pelagic, deep water sediments. Radiolarites are brittle rocks and hard to split, they break conchoidally with sharp edges. During weathering they decompose into rectangular pieces; the colors range from light to dark via red and brown hues.
Radiolarites are composed of radiolarian tests and their fragments. The skeletal material consists of amorphous silica. Radiolarians are planktonic protists with an inner skeleton, their sizes range from 0.1 to 0.5 millimeters. Amongst their major orders albaillellaria, the spherical spumellaria and the hood-shaped nassellaria can be distinguished. According to Takahashi radiolarians stay for 2 to 6 weeks in the euphotic zone before they start sinking, their descent through 5000 meters of ocean water can take from two weeks to as long as 14 months. As soon as the protist dies and starts decaying silica dissolution affects the skeleton; the dissolution of silica in the oceans parallels the temperature/depth curve and is most effective in the uppermost 750 meters of the water column, farther below it diminishes. Upon reaching the sediment/water interface the dissolution drastically increases again. Several centimeters below this interface the dissolution continues within the sediment, but at a much reduced rate.
It is in fact astonishing. It is estimated that only as little as one percent of the original skeletal material is preserved in radiolarian oozes. According to Dunbar & Berger this minimal preservation of one percent is due to the fact that radiolarians form colonies and that they are embedded in fecal pellets and other organic aggregates; the organic wrappings act as a protection for the tests and spare them from dissolution, but of course speed up the sinking time by a factor of 10. After deposition diagenetic processes start affecting the freshly laid down sediment; the silica skeletons are etched and the original opal A commences to transform into opal CT. With increasing temperature and pressure the transformation proceeds to chalcedony and to stable, cryptocrystalline quartz; these phase changes are accompanied by a decrease in porosity of the ooze which becomes manifest as a compaction of the sediment. The compaction of radiolarites is dependent on their chemical composition and correlates positively with the original SiO2-content.
The compaction factor varies between 3.2 and 5, which means that 1 meter of consolidated sediment is equivalent to 3.2 to 5 meters of ooze. The alpine radiolarites of the Upper Jurassic for instance show sedimentation rates of 7 to 15.5 meters/million years, which after compaction is equivalent to 2.2 to 3.1 meters/million years. As a comparison the radiolarites of the Pindos Mountains in Greece yield a comparable value of 1.8 to 2.0 meters/million years, whereas the radiolarites of the Eastern Alps have a rather small sedimentation rate of 0.71 meters/million years. According to Iljima et al. 1978 the Triassic radiolarites of central Japan reveal an exceptionally high sedimentation rate of 27 to 34 meters/million years. Recent non-consolidated radiolarian oozes have sedimentation rates of 1 to 5 meters/million years. In radiolarian oozes deposited in the equatorial Eastern Atlantic 11.5 meters/million years have been measured. In upwelling areas like off the Peruvian coast high values of 100 meters/million years were reported.
The view that radiolarites are deposited under pelagic, deep water conditions cannot be asserted any longer. Layers enriched in radiolarians do occur in shallow water limestones like the Solnhofen limestone and the Werkkalk Formation of Bavaria. What seems to be important for the preservation of radiolarian oozes is that they are deposited well below the storm wave base and below the jets of erosive surface currents. Radiolarites without any carbonates have most been sedimented below the calcite compensation depth. One has to bear in mind that the CCD has not been stationary in the geological past and that it is a function of latitude. At present, the CCD reaches a maximum depth of about 5000 meters near the equator; the characteristing banding and ribbon-like layering observed in radiolarites is due to changing sediment influx, secondarily enhanced by diagenetic effects. In the simple two component system clay/silica with constant clay supply the rhythmically changing radiolarian blooms are responsible for creating a clay-chert interlayering.
These purely sedimentary differences become enhanced during diagenesis as the silica leaves the clayey layers and migrates towards the opal-rich horizons. Two situations occur: with high silica input and constant clay background sedimentation thick chert layers form. On the other