Agate is a rock consisting of cryptocrystalline silica, chiefly chalcedony, alternating with microgranular quartz. It is characterized by its fineness of variety of color. Although agates may be found in various kinds of host rock, they are classically associated with volcanic rocks and can be common in certain metamorphic rocks; the stone was given its name by Theophrastus, a Greek philosopher and naturalist, who discovered the stone along the shore line of the river Achates in Sicily, sometime between the 4th and 3rd centuries BC. Colorful agates and other chalcedonies were obtained over 3,000 years ago from the Achates River, now called Dirillo. Agate is one of the most common materials used in the art of hardstone carving, has been recovered at a number of ancient sites, indicating its widespread use in the ancient world. Most agates occur as nodules in volcanic rocks or ancient lavas, in former cavities produced by volatiles in the original molten mass, which were filled, wholly or by siliceous matter deposited in regular layers upon the walls.
Agate has been known to fill veins or cracks in volcanic or altered rock underlain by granitic intrusive masses. Such agates, when cut transversely, exhibit a succession of parallel lines of extreme tenuity, giving a banded appearance to the section; such stones are known as riband agate and striped agate. In the formation of an ordinary agate, it is probable that waters containing silica in solution—derived from the decomposition of some of the silicates in the lava itself—percolated through the rock and deposited a siliceous gel in the interior of the vesicles. Variations in the character of the solution or in the conditions of deposition may cause a corresponding variation in the successive layers, so that bands of chalcedony alternate with layers of crystalline quartz. Several vapour-vesicles may unite while the rock is still viscous, thus form a large cavity which may become the home of an agate of exceptional size; the most comprehensive review of agate chemistry is a recent text by Moxon cited below.
The first deposit on the wall of a cavity, forming the "skin" of the agate, is a dark greenish mineral substance, like celadonite, delessite or "green earth", which are rich in iron derived from the decomposition of the augite in the enclosing volcanic rock. This green silicate may give rise by alteration to a brown iron oxide, producing a rusty appearance on the outside of the agate-nodule; the outer surface of an agate, freed from its matrix, is pitted and rough in consequence of the removal of the original coating. The first layer spread over the wall of the cavity has been called the "priming", upon this base, zeolitic minerals may be deposited. Many agates are hollow, since deposition has not proceeded far enough to fill the cavity, in such cases the last deposit consists of drusy quartz, sometimes amethystine, having the apices of the crystals directed towards the free space so as to form a crystal-lined cavity or geode; when the matrix in which the agates are embedded disintegrates, they are set free.
The agates are resistant to weathering and remain as nodules in the soil, or are deposited as gravel in streams and along shorelines. Agates can be found in sedimentary rocks, they need a cavity to form, so they are seen in limestone and shale which may have shells, tree branches, or roots in them that decay away. With the void created, silica-rich fluids can enter the cavity and precipitate chalcedony in curved bands forming agates. Other quartz classes like amethyst or opal may form the inner-most band inside the geode. A Mexican agate, showing only a single eye, has received the name of cyclops agate. Included matter of a green, red, black or other color or combinations embedded in the chalcedony and disposed in filaments and other forms suggestive of vegetable growth, gives rise to dendritic or moss agate. Dendritic agates include fern-like patterns formed due to the presence of iron oxides. Other types of included matter deposited during agate-building include sagenitic growths and chunks of entrapped detritus.
Agate fills a void left by decomposed vegetatable material such as a tree limb or root and is called limb cast agate, due to its appearance. Enhydro agate contains tiny inclusions of water, sometimes with air bubbles. Turritella agate is formed from silicified fossil Elimia tenera shells. E. tenera are spiral freshwater gastropods with spiral shells composed of many whorls. Coral, petrified wood and other organic remains or porous rocks can become agatized. Agatized coral is referred to as Petoskey stone or agate. Coldwater agates, such as the Lake Michigan Cloud Agate, did not form under volcanic processes, but instead formed within the limestone and dolostone bedrock of ancient oceans. Like volcanic-origin agates, Coldwater Agates formed from silica gels that lined pockets and seams within the bedrock; these agates are less colorful, with banded lines of grey and white chalcedony. Greek agate is a name given to pale white to tan colored agate found in Sicily, once a Greek colony, back to 400 BC.
The Greeks used it for making jewelry and beads. Brazilian agate is found as sizable geodes of layered nodules; these occur in brownish tones interlayered with gray. Quartz forms within these nodule
A crystal or crystalline solid is a solid material whose constituents are arranged in a ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macroscopic single crystals are identifiable by their geometrical shape, consisting of flat faces with specific, characteristic orientations; the scientific study of crystals and crystal formation is known as crystallography. The process of crystal formation via mechanisms of crystal growth is called crystallization or solidification; the word crystal derives from the Ancient Greek word κρύσταλλος, meaning both "ice" and "rock crystal", from κρύος, "icy cold, frost". Examples of large crystals include snowflakes and table salt. Most inorganic solids are not crystals but polycrystals, i.e. many microscopic crystals fused together into a single solid. Examples of polycrystals include most metals, rocks and ice. A third category of solids is amorphous solids, where the atoms have no periodic structure whatsoever.
Examples of amorphous solids include glass and many plastics. Despite the name, lead crystal, crystal glass, related products are not crystals, but rather types of glass, i.e. amorphous solids. Crystals are used in pseudoscientific practices such as crystal therapy, along with gemstones, are sometimes associated with spellwork in Wiccan beliefs and related religious movements; the scientific definition of a "crystal" is based on the microscopic arrangement of atoms inside it, called the crystal structure. A crystal is a solid where the atoms form a periodic arrangement.. Not all solids are crystals. For example, when liquid water starts freezing, the phase change begins with small ice crystals that grow until they fuse, forming a polycrystalline structure. In the final block of ice, each of the small crystals is a true crystal with a periodic arrangement of atoms, but the whole polycrystal does not have a periodic arrangement of atoms, because the periodic pattern is broken at the grain boundaries.
Most macroscopic inorganic solids are polycrystalline, including all metals, ice, etc. Solids that are neither crystalline nor polycrystalline, such as glass, are called amorphous solids called glassy, vitreous, or noncrystalline; these have no periodic order microscopically. There are distinct differences between crystalline solids and amorphous solids: most notably, the process of forming a glass does not release the latent heat of fusion, but forming a crystal does. A crystal structure is characterized by its unit cell, a small imaginary box containing one or more atoms in a specific spatial arrangement; the unit cells are stacked in three-dimensional space to form the crystal. The symmetry of a crystal is constrained by the requirement that the unit cells stack with no gaps. There are 219 possible crystal symmetries, called crystallographic space groups; these are grouped into 7 crystal systems, such as hexagonal crystal system. Crystals are recognized by their shape, consisting of flat faces with sharp angles.
These shape characteristics are not necessary for a crystal—a crystal is scientifically defined by its microscopic atomic arrangement, not its macroscopic shape—but the characteristic macroscopic shape is present and easy to see. Euhedral crystals are those with well-formed flat faces. Anhedral crystals do not because the crystal is one grain in a polycrystalline solid; the flat faces of a euhedral crystal are oriented in a specific way relative to the underlying atomic arrangement of the crystal: they are planes of low Miller index. This occurs; as a crystal grows, new atoms attach to the rougher and less stable parts of the surface, but less to the flat, stable surfaces. Therefore, the flat surfaces tend to grow larger and smoother, until the whole crystal surface consists of these plane surfaces. One of the oldest techniques in the science of crystallography consists of measuring the three-dimensional orientations of the faces of a crystal, using them to infer the underlying crystal symmetry.
A crystal's habit is its visible external shape. This is determined by the crystal structure, the specific crystal chemistry and bonding, the conditions under which the crystal formed. By volume and weight, the largest concentrations of crystals in the Earth are part of its solid bedrock. Crystals found in rocks range in size from a fraction of a millimetre to several centimetres across, although exceptionally large crystals are found; as of 1999, the world's largest known occurring crystal is a crystal of beryl from Malakialina, Madagascar, 18 m long and 3.5 m in diameter, weighing 380,000 kg. Some crystals have formed by magmatic and metamorphic processes, giving origin to large masses of crystalline rock; the vast majority of igneous rocks are formed from molten magma and the degree of crystallization depends on the conditions under which they solidified. Such rocks as granite, which have cooled slowly and under great pressures, have crystallized.
Polarization is a property applying to transverse waves that specifies the geometrical orientation of the oscillations. In a transverse wave, the direction of the oscillation is perpendicular to the direction of motion of the wave. A simple example of a polarized transverse wave is vibrations traveling along a taut string. Depending on how the string is plucked, the vibrations can be in a vertical direction, horizontal direction, or at any angle perpendicular to the string. In contrast, in longitudinal waves, such as sound waves in a liquid or gas, the displacement of the particles in the oscillation is always in the direction of propagation, so these waves do not exhibit polarization. Transverse waves that exhibit polarization include electromagnetic waves such as light and radio waves, gravitational waves, transverse sound waves in solids. In some types of transverse waves, the wave displacement is limited to a single direction, so these do not exhibit polarization. An electromagnetic wave such as light consists of a coupled oscillating electric field and magnetic field which are always perpendicular.
In linear polarization, the fields oscillate in a single direction. In circular or elliptical polarization, the fields rotate at a constant rate in a plane as the wave travels; the rotation can have two possible directions. Light or other electromagnetic radiation from many sources, such as the sun and incandescent lamps, consists of short wave trains with an equal mixture of polarizations. Polarized light can be produced by passing unpolarized light through a polarizer, which allows waves of only one polarization to pass through; the most common optical materials are isotropic and do not affect the polarization of light passing through them. Some of these are used to make polarizing filters. Light is partially polarized when it reflects from a surface. According to quantum mechanics, electromagnetic waves can be viewed as streams of particles called photons; when viewed in this way, the polarization of an electromagnetic wave is determined by a quantum mechanical property of photons called their spin.
A photon has one of two possible spins: it can either spin in a right hand sense or a left hand sense about its direction of travel. Circularly polarized electromagnetic waves are composed of photons with only one type of spin, either right- or left-hand. Linearly polarized waves consist of photons that are in a superposition of right and left circularly polarized states, with equal amplitude and phases synchronized to give oscillation in a plane. Polarization is an important parameter in areas of science dealing with transverse waves, such as optics, seismology and microwaves. Impacted are technologies such as lasers and optical fiber telecommunications, radar. Most sources of light are classified as incoherent and unpolarized because they consist of a random mixture of waves having different spatial characteristics, frequencies and polarization states. However, for understanding electromagnetic waves and polarization in particular, it is easiest to just consider coherent plane waves. Characterizing an optical system in relation to a plane wave with those given parameters can be used to predict its response to a more general case, since a wave with any specified spatial structure can be decomposed into a combination of plane waves.
And incoherent states can be modeled stochastically as a weighted combination of such uncorrelated waves with some distribution of frequencies and polarizations. Electromagnetic waves, traveling in free space or another homogeneous isotropic non-attenuating medium, are properly described as transverse waves, meaning that a plane wave's electric field vector E and magnetic field H are in directions perpendicular to the direction of wave propagation. By convention, the "polarization" direction of an electromagnetic wave is given by its electric field vector. Considering a monochromatic plane wave of optical frequency f, let us take the direction of propagation as the z axis. Being a transverse wave the E and H fields must contain components only in the x and y directions whereas Ez = Hz = 0. Using complex notation, the instantaneous physical electric and magnetic fields are given by the real parts of the complex quantities occurring in the following equations; as a function of time t and spatial position z these complex fields can be written as: E → =
Sedimentary rocks are types of rock that are formed by the accumulation or deposition of small particules and subsequent cementation of mineral or organic particles on the floor of oceans or other bodies of water at the Earth's surface. Sedimentation is the collective name for processes; the particles that form a sedimentary rock are called sediment, may be composed of geological detritus or biological detritus. Before being deposited, the geological detritus was formed by weathering and erosion from the source area, transported to the place of deposition by water, ice, mass movement or glaciers, which are called agents of denudation. Biological detritus was formed by bodies and parts of dead aquatic organisms, as well as their fecal mass, suspended in water and piling up on the floor of water bodies. Sedimentation may occur as dissolved minerals precipitate from water solution; the sedimentary rock cover of the continents of the Earth's crust is extensive, but the total contribution of sedimentary rocks is estimated to be only 8% of the total volume of the crust.
Sedimentary rocks are only a thin veneer over a crust consisting of igneous and metamorphic rocks. Sedimentary rocks are deposited in layers as strata; the study of sedimentary rocks and rock strata provides information about the subsurface, useful for civil engineering, for example in the construction of roads, tunnels, canals or other structures. Sedimentary rocks are important sources of natural resources like coal, fossil fuels, drinking water or ores; the study of the sequence of sedimentary rock strata is the main source for an understanding of the Earth's history, including palaeogeography and the history of life. The scientific discipline that studies the properties and origin of sedimentary rocks is called sedimentology. Sedimentology is part of both geology and physical geography and overlaps with other disciplines in the Earth sciences, such as pedology, geomorphology and structural geology. Sedimentary rocks have been found on Mars. Sedimentary rocks can be subdivided into four groups based on the processes responsible for their formation: clastic sedimentary rocks, biochemical sedimentary rocks, chemical sedimentary rocks, a fourth category for "other" sedimentary rocks formed by impacts and other minor processes.
Clastic sedimentary rocks are composed of other rock fragments that were cemented by silicate minerals. Clastic rocks are composed of quartz, rock fragments, clay minerals, mica. Clastic sedimentary rocks, are subdivided according to the dominant particle size. Most geologists use the Udden-Wentworth grain size scale and divide unconsolidated sediment into three fractions: gravel and mud; the classification of clastic sedimentary rocks parallels this scheme. This tripartite subdivision is mirrored by the broad categories of rudites and lutites in older literature; the subdivision of these three broad categories is based on differences in clast shape, grain size or texture. Conglomerates are dominantly composed of rounded gravel, while breccias are composed of dominantly angular gravel. Sandstone classification schemes vary but most geologists have adopted the Dott scheme, which uses the relative abundance of quartz and lithic framework grains and the abundance of a muddy matrix between the larger grains.
Composition of framework grains The relative abundance of sand-sized framework grains determines the first word in a sandstone name. Naming depends on the dominance of the three most abundant components quartz, feldspar, or the lithic fragments that originated from other rocks. All other minerals are considered accessories and not used in the naming of the rock, regardless of abundance. Quartz sandstones have >90% quartz grains Feldspathic sandstones have <90% quartz grains and more feldspar grains than lithic grains Lithic sandstones have <90% quartz grains and more lithic grains than feldspar grainsAbundance of muddy matrix material between sand grains When sand-sized particles are deposited, the space between the grains either remains open or is filled with mud. "Clean" sandstones with open pore space are called arenites. Muddy sandstones with abundant muddy matrix are called wackes. Six sandstone names are possible using the descriptors for grain composition and the amount of matrix. For example, a quartz arenite would be composed of quartz grains and have little or no clayey matrix between the grains, a lithic wacke would have abundant lithic grains and abundant muddy matrix, etc.
Although the Dott classification scheme is used by sedimentologists, common names like greywacke and quartz sandstone are still used by non-specialists and in popular literature. Mudrocks are sedimentary rocks composed of at least 50% silt- and clay-sized particles; these fine-grained particles are transported by turbulent flow in water or air, deposited as the flow calms and the particles settle out of suspension. Most authors presently
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.
Glass is a non-crystalline, amorphous solid, transparent and has widespread practical and decorative uses in, for example, window panes and optoelectronics. The most familiar, the oldest, types of manufactured glass are "silicate glasses" based on the chemical compound silica, the primary constituent of sand; the term glass, in popular usage, is used to refer only to this type of material, familiar from use as window glass and in glass bottles. Of the many silica-based glasses that exist, ordinary glazing and container glass is formed from a specific type called soda-lime glass, composed of 75% silicon dioxide, sodium oxide from sodium carbonate, calcium oxide called lime, several minor additives. Many applications of silicate glasses derive from their optical transparency, giving rise to their primary use as window panes. Glass will transmit and refract light. Glass can be coloured by adding metallic salts, can be painted and printed with vitreous enamels; these qualities have led to the extensive use of glass in the manufacture of art objects and in particular, stained glass windows.
Although brittle, silicate glass is durable, many examples of glass fragments exist from early glass-making cultures. Because glass can be formed or moulded into any shape, it has been traditionally used for vessels: bowls, bottles and drinking glasses. In its most solid forms it has been used for paperweights and beads; when extruded as glass fiber and matted as glass wool in a way to trap air, it becomes a thermal insulating material, when these glass fibers are embedded into an organic polymer plastic, they are a key structural reinforcement part of the composite material fiberglass. Some objects were so made of silicate glass that they are called by the name of the material, such as drinking glasses and eyeglasses. Scientifically, the term "glass" is defined in a broader sense, encompassing every solid that possesses a non-crystalline structure at the atomic scale and that exhibits a glass transition when heated towards the liquid state. Porcelains and many polymer thermoplastics familiar from everyday use are glasses.
These sorts of glasses can be made of quite different kinds of materials than silica: metallic alloys, ionic melts, aqueous solutions, molecular liquids, polymers. For many applications, like glass bottles or eyewear, polymer glasses are a lighter alternative than traditional glass. Silicon dioxide is a common fundamental constituent of glass. In nature, vitrification of quartz occurs when lightning strikes sand, forming hollow, branching rootlike structures called fulgurites. Fused quartz is a glass made from chemically-pure silica, it has excellent resistance to thermal shock, being able to survive immersion in water while red hot. However, its high melting temperature and viscosity make it difficult to work with. Other substances are added to simplify processing. One is sodium carbonate; the soda makes the glass water-soluble, undesirable, so lime, some magnesium oxide and aluminium oxide are added to provide for a better chemical durability. The resulting glass is called a soda-lime glass. Soda-lime glasses account for about 90% of manufactured glass.
Most common glass contains other ingredients to change its properties. Lead glass or flint glass is more "brilliant" because the increased refractive index causes noticeably more specular reflection and increased optical dispersion. Adding barium increases the refractive index. Thorium oxide gives glass a high refractive index and low dispersion and was used in producing high-quality lenses, but due to its radioactivity has been replaced by lanthanum oxide in modern eyeglasses. Iron can be incorporated into glass to absorb infrared radiation, for example in heat-absorbing filters for movie projectors, while cerium oxide can be used for glass that absorbs ultraviolet wavelengths; the following is a list of the more common types of silicate glasses and their ingredients and applications: Fused quartz called fused-silica glass, vitreous-silica glass: silica in vitreous, or glass, form. It has low thermal expansion, is hard, resists high temperatures, it is the most resistant against weathering. Fused quartz is used for high-temperature applications such as furnace tubes, lighting tubes, melting crucibles, etc.
Soda-lime-silica glass, window glass: silica + sodium oxide + lime + magnesia + alumina. Is transparent formed and most suitable for window glass, it has a high thermal expansion and poor resistance to heat. It is used for windows, some low-temperature incandescent light bulbs, tableware. Container glass is a soda-lime glass, a slight variation on flat glass, which uses more alumina and calcium, less sodium and magnesium, which are more water-soluble; this makes it less susceptible to water erosion. Sodium borosilicate glass, Pyrex: silica + boron trioxide + soda + alumina. Stan
The matrix or groundmass of a rock is the finer-grained mass of material in which larger grains, crystals or clasts are embedded. The matrix of an igneous rock consists of finer-grained microscopic, crystals in which larger crystals are embedded; this porphyritic texture is indicative of multi-stage cooling of magma. For example, porphyritic andesite will have large phenocrysts of plagioclase in a fine-grained matrix. In South Africa, diamonds are mined from a matrix of weathered clay-like rock called "yellow ground"; the matrix of sedimentary rocks is finer-grained sedimentary material, such as clay or silt, in which larger grains or clasts are embedded. It is used to describe the rock material in which a fossil is embedded. All sediments are at first in an incoherent condition, they may remain in this state for an indefinite period. Millions of years have elapsed since some of the early Tertiary strata gathered on the ocean floor, yet they are quite friable and differ little from many recent accumulations.
There are few exceptions, however, to the rule that with increasing age sedimentary rocks become more and more indurated, the older they are the more it is that they will have the firm consistency implied in the term "rock". The pressure of newer sediments on underlying masses is one cause of this change, though not in itself a powerful one. More efficiency is ascribed to the action of percolating water, which takes up certain soluble materials and redeposits them in pores and cavities; this operation is accelerated by the increased pressure produced by superincumbent masses, to some extent by the rise of temperature which takes place in rocks buried to some depth beneath the surface. The rise of temperature, however, is never great; the redeposited cementing material is most calcareous or siliceous. Limestones, which were a loose accumulation of shells, etc. become compacted into firm rock in this manner. The cementing substance may be deposited in crystalline continuity on the original grains, where these were crystalline, in sandstones, a crystalline matrix of calcite envelops the sand grains.
The change of aragonite to calcite and of calcite to dolomite, by forming new crystalline masses in the interior of the rock also accelerates consolidations. Silica is less soluble in ordinary waters, but this ingredient of rocks is dissolved and redeposited with great frequency. Many sandstones are held together by an infinitesimal amount of cryptocrystalline silica. Others contain fine scales of mica. Argillaceous materials may be compacted by mere pressure, like graphite and other scaly minerals