with quartz, mica, hornblende and garnet.Gondwanaland - the southern part of Pangaea that formed South America, Africa, India, Australia and Antarctica.Granite - A coarse grained, often porphyritic, intrusive, felsic, igneous rock containing megascopic quartz, averaging 25%, much feldspar (orthoclase, microcline, sodic plagioclase) and mica or other coloured minerals. Rhyolite is the volcanic equivalentGranitoid - general term for a granite-like rock, including granite, granodiorite, diorite, monzonite, etc.Granoblastic - An arrangement of mineral grains in a rock of metamorphic origin similar to that of a normal granite, but produced by recrystallisation in the solid and not by crystallisation from a molten condition.Granodiorite - intrusive felsic igneous rock similar to granite, but contains more plagioclase than potassium feldspar. Volcanic equivalent DaciteGraywacke - variety of sandstone generally characterized by its hardness, dark color, and poorly-sorted, angular grains of quartz, feldspar, and small rock fragments (lithic fragments) set in a compact, clay-fine matrix. From German Grauwacke.Greenstone - An omnibus term lacking precision and applied indiscriminately to basic and intermediate igneous rocks of Lower Palaeozoic age in which much chlorite has been produced at the expense of the original coloured minerals, staining the rocks green.Grus - freshly-eroded and angular grains of quartz and feldspar derived from a granitoid.H[edit]Half-life - the time it takes for a given amount of a radioactive isotope to be reduced by one-half.Hardpan -Hemicrystalline - rocks of igneous origin which contain some interstitial glass, in addition to crystalline minerals.Hinge - zone of maximum curvature of a fold.Hinge line - a line joining the points of maximum curvature along the hinge of a fold.Holocrystalline - those igneous rocks in which all the components are crystalline; glass is absent.Hornblende - an important rock-forming mineral of complex composition, essentially silicate of calcium, magnesium and iron, with smaller amounts of potash, soda and hydroxyl; crystallises in the monoclinic system; occurs as black crystals or grains in many different types of igneous and metamorphic rocks, including hornblende-granite, syenite, diorite, andesite, etc., and hornblende-schist and amphibole.Hornfels - hard, compact, fine textured contact altered argillaceous [from clay] rocks that break into splintery fragmentsHot springs - spring that is produced by the emergence of geothermally-heated groundwater from the Earth's crust.Hydrothermal - Pertaining to heated water. The action of or products of heated water.Hydrothermal vent - fissure in a planet's surface from which geothermally heated water issues.Hypersaline - saltiness or dissolved salt content of a body of water greater than that of seawater.I[edit]Ichnology - branch of biology that deals with traces of organismal behavior.Idiomorphic (crystals) - Rock minerals which are bounded by the crystal faces peculiar to the speciesIgneous rock - rocks formed by solidification of cooled magma (molten rock), with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks.Ignimbrite - Fine-grained to aphanitic, buff to dark brown compact rock with parallel streaks or lenticles of black glass, produced by violently explosive volcanoes.Ilmenite - An oxide of iron and titanium, crystallising in the trigonal system; a widespread accessory mineral in igneous rocks, especially those of basic composition.Indurated - Made hardInterbedded - beds (layers) of