glossary

Accessory mineral	A mineral whose presence or absence does not define the rock type.
Alkali feldspar	The Potassic feldspars (K) and Sodic feldspars (Na) and several intergrowths. Common type Orthoclase KAlSi3O8.
Aplite	Fine grained felsic igneous rock, found in vains or dykes, most abundantly in parent granite, but may intrude surrounding rocks. High in quartz and alkalies, they are depleted in mafic minerals such as biotite mica.
Batholith	A very large intrusive mass of igneous rock, large outcrop with no observable bottom.
Biotite	A mafic mineral which is a type of mica, or sheet silicate. important essential mineral in granite, but may occur in a whole range of rocks from felsic to ultramafic. back to photo
Breccia	A sedimentary rock containing coarse angular clasts or fragments of pre-existing rocks or rock. Often formed by flash flood events in semi-arid environments.
Caledonian continent	Another name for Laurasia, that part of the continent formed after the Caledonian mountain building episode.back
Carboniferous	Name given to a period of geological time from 363 to 290 Ma. Conybeare, 1822, from widespread occurrence of carbon in the form of coal in sedimentary rocks of this age..
Carlsbad twinning	Crystal form common in orthoclase feldspar, usually interpenetrant and resulting from rotation about the c-axis.
Devonian	Name given to a period of geological time from 408 to 363 Ma. Sedgwick & Murchison 1840 from the occurrence of rocks of this age in Devon.
Dyke	Intrusion of igneous rock which cuts across bedding. Often feeder pipes for volcanoes.
Essential mineral	A mineral whose presence defines the rock type.back
Euhedral (idiomorphic)	Textural descriptive term used to illustrate the degree of crystallinity of a rocks minerals. Showing perfect or near perfect form.
Feldspar	Most important rock forming silicate mineral, aluminosilicates of potassium,sodium,calcium and rarely barium. Crytallize in the triclinic and monoclinic systems. An essential mineral of granite.
Felsic	A mnemonic covering the minerals of the Feldspar and feldspathoid groups along with the silica group. Igneous rocks are classified felsic if they are high in these minerals, e.g. Quartz and feldspar.
Granite	A felsic igneous rock type, coarse grained, and light in colour. essential minerals quartz, feldspar and mica.
Granular (granitic)	A general textural term used to describe igneous rocks whose constituent crystals are anhedral to sub-hedral and equidimensional.
Gondwanaland	The large southern continent made up of Africa, South America, Antarctica, Australia and India, during the Palaeozoic era of earth history. back
Holocrystalline	Textural descriptive term meaning entirely composed of crystals
Igneous rocks	One of the three main groups of rocks, those rocks which are produced from molten magma, they may be intrustive or extrusive,
Ilmenite	An accessory mineral found in igneous rocks. FeTiO3
Laurasia	The large northern continent made up of North America, Europe and parts of Asia, during the Palaeozoic period of earth history.
Leucocratic	Term used to describe generally light coloured igneous rocks
Lizard Complex	Part of Cornwall, which represents a slice of ocean crust, obducted by Variscan crustal movements.
Luxullianite	Granite altered by boron rich gases. Biotite is replaced by secondary tourmaline and with increasing flux-concentration so too are the feldspars. Luxulliante represents an arrested stage of tourminlinization where corroded brick red feldspars remain.
Mafic	A mnemonic covering the minerals of the ferro-magnesian silicate groups, olivines, pyroxenes, amphiboles and micas. Dark in colour. Rocks with a high proportion of these minerals are said to be mafic.
Metamorphic rocks	Rocks which have been changed or altered by heat and pressure, due to deep burial or contact with an igneous intrusion. back
Metamorphic aureole	Zone of rock around an igneous intrusion, which is altered or changed by contact with the magma.
Mica	A mafic group of sheet silicate minerals high in ferro-magnesium. Biotite and muscovite.
Microgranitic	Textural descriptive term to describe medium grained granitic rocks
Millstone Grit	Coarse grained sedimentary clastic rock deposited in the Carboniferous period, outcropping in northern England. Used in the industrial revolution for grinding wheels. Formed in deltaic environment.
Muscovite	White mica , a mafic mineral, secondary in the Dartmoor granites.
Nappe	Large scale overfold structure common in mountain building, associated with thrust faulting.
Normannia	The name given to part of the continental plate which moved northwards to hit the southern shores of Laurasia. May have been a tectonic micro-plate.
Old Red Sandstone	Alternative name given to the Caledonian continent.back
Palaeozoic (Paleozoic USA)	A name given to an era of earths history meaning ancient life, 600-230m.a. The Cambrian through to end Permian.
Pangaea	The Super continent created when Gondwanaland and Laurasia collided about 290 million years ago.
Permian	Name given to the period of Earth history from 280 to 230 Ma. Murchison 1841. Named after the Perm region of Russia.
Phenocryst (megacryst)	Large crystal within a finer groundmass.
Pneumatolytic	Processes leading to changes in constituent minerals of rocks by hot gaseous substances (other than water) associated with igneous activity.
Porphyritic	Textural descriptive term for igneous rocks which display large crystals set in a coarse goundmass. Often feldspars form these crystals. May show alignment.
Quartz	Silicate mineral SiO2 essential in granite. Quartz is said to be free silica, because it does not form part of the other rock forming silicate minerals which are satisfied. In other words it is not used up in the chemical make up of the other minerals in the rock.
Rheic Sea	The sea which separated the super continents of Gondwanaland and Laurasia during the Devonian and Carboniferous periods of earth history.
Saccharoidal	Sugary texture in an igneous rock back
Secondary mineral	Mineral which has resulted from the alteration or reconstruction of primary minerals
Schorl	Rock formed of only free quartz and tourmaline, where all other minerals have been altered to tourmaline by pneumatolytic processes.
Sub-hedral	Textural descriptive term used to illustrate the degree of crystallinity of a rocks minerals. Showing recognizable but imperfect crystal form.
Tourmaline	Mafic mineral common in granite as a secondary mineral replacing other minerals.
Triassic	Name given to the period of Earth history from 230Ma to 202Ma. von Alberti in 1834. Taken from the three fold division of the period which can be taken in Germany. It is the first period of the Mesozoic or middle life era of Earth history.
Turbidite	Coarse clastic sedimentary rock with a muddy matrix, produced by underwater marine avalanches at continental slope margins.back
Variscan	Name given to the mountain building phase which took place in the late Palaeozoic. As the mountain chain crosses several modern countries it has ben given three names, may also be called Hercynian or Armorican.

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