be regarded as hydrated silicates of aluminium, iron and magnesium. They crystallise in the monoclinic system and are green in colour. They occur as alteration products of such minerals as biotite and hornblende, and also in schistose rocks.Clast - An individual constituent grain or fragment of a sediment or rock produced by mechanical weathering of a rock mass.Clastic rocks - Mechanically redeposited remains of eroded older rocks. Rocks formed of fragments, or clasts, of pre-existing rocks.Cleavage - in structural geology and petrology, term describing the tendency of a rock to break along preferred planes of weakness, caused by the development of a planar fabric as a result of deformation.Coccolith - individual plates of calcium carbonate formed by coccolithophores (single-celled algae such as Emiliania huxleyi) which are arranged around them in a coccosphere.Coccolithophore - (also called coccolithophorid) - important microfossils: single-celled algae, protists and phytoplankton belonging to the division of haptophytes. They are distinguished by special calcium carbonate plates called coccoliths.Compactions - process by which a newly deposited sediment progressively loses its original water content due to the effects of loading, this forms part of the process of lithification.Compression - system of forces that tend to decrease the volume of or shorten rocks.Conchoidal - Type of fracture that gives smoothly curved surface.Concretion - volume of sedimentary rock in which a mineral cement fills the porosity (i.e. the spaces between the sediment grains).Conglomerate - rock consisting of individual stones that have become cemented together.Contact metamorphism - Metamorphism due to the local heating of rocks by the intrusion of magma nearby.Continental crust - layer of granitic, sedimentary, and metamorphic rocks which form the continents and the areas of shallow seabed close to their shores, known as continental shelves.Continental margin - zone of the ocean floor that separates the thin oceanic crust from thick continental crust.Continental shelf - extended perimeter of each continent and associated coastal plain, which is covered during interglacial periods such as the current epoch by relatively shallow seas (known as shelf seas) and gulfs.Convergent boundary - boundary between two plates that are moving toward each other.Copal - type of resin produced by plant or tree secretions, particularly identified with the forms of aromatic tree resins used by the cultures of pre-Columbian Mesoamerica as a ceremonially burned incense, as well as for a number of other purposes.Coprolites - fossil that results when human or animal dung is fossilized.Cordierite - A silicate of aluminium, iron and magnesium with water, which crystallises in the orthorhombic system, occurs mainly in metamorphic rocks.Core - innermost layer(s) of a planet.Corestone - An ellipsoidal or broadly rectangular joint block of granite formed by subsurface weathering in the same manner as a tor but entirely separated from bedrock.Craton - old and stable part of the continental crust that has survived the merging and splitting of continents and supercontinents for at least 500 million years.Cross-bedding - inclined sedimentary structures in a horizontal unit of rock; such tilted structures indicate the type of depositional environment, not post-depositional deformation.Crude oil - a liquid mixture of naturally occurring hydrocarbons.Crust - outermost solid shell of