and/or temperature conditionsProtolith - the source rock from which a metamorphic, or in some rare cases a sedimentary, rock was formed. In most cases the appropriate sedimentary term is "provenance" rather than "protolith", since the material has been transported.Psammite - (Greek Psammos, Sand) - a general term for a sandstone, most often used to describe a metamorphosed rock unit with a dominantly sandstone protolith.Pseudomatrix - weaker material (mainly lithic fragments) that become crushed and become matrix in a rock.Pumice - Light-coloured highly vesicular volcanic rock of very low density.Pyroclastic flow - a fast-moving current of hot gas and rock (collectively known as tephra), which normally hugs the ground and travels downhill, or spreads laterally under gravity.Pyroclastics - volcanic fragments - Volcanic bomb, Breadcrust bomb, Achnelith.Pyroxene group - A number of mineral species which although falling into different systems (orthorhombic, monoclinic and triclinic) are closely related in form and structure. They are metasilicates of Calcium, magnesium and iron with manganese, and less often with sodium, potassium, zirconium and fluorine.Pyroxenite - A coarse grained holocrystalline igneous rock, consisting mainly of pyroxenes. It may contain biotite, hornblende or olivine as accessoriesQ[edit]Quartzite - Compact, hard, very fine-grained white to creamy white rock, which breaks into sharp angular fragments. Quartzite is always associated with other metamorphic rocks, while cemented sandstone is always associated with other sedimentary rocks.Quaternary - The Quaternary Period is the most recent of the three periods of the Cenozoic Era in the geologic time scale of the ICS. It follows after the Neogene Period, spanning 2.588 0.005 million years ago to the present.R[edit]Regional metamorphism - Over wide areas resulting from deep burial with consequent rise in temperature and static pressure, usually with the help of folding movements that accompany the formation of mountain rangesReticulite - Lightest rock known. A basaltic pumice in which the walls of the vesicles have collapsed, leaving a network of fine, interconnecting glass threads.Retrograde metamorphism - the reconstitution of a rock via revolatisation under decreasing temperatures (and usually pressures), allowing the mineral assemblages formed in prograde metamorphism to revert to those more stable at less extreme conditions.Reverse fault - Dip-slip faults can be sub-classified into the types "reverse" and "normal". A reverse fault (or thrust fault) occurs when the crust is compressed. The hanging wall moves upward, relative to the footwall.Rhyolite - Aphantitic, buff to greyish flow-banded rock, often containing spherulites or phenocrysts of quartz and feldspar.Rift -Roche moutonnee - a elongated post glacier rock shape with smoothed surface on the uphill side and a "plucked" surface on the downhill side.Rudaceous - Clastic rocks. Coarse grained sedimentary rocks, conglomerates, breccias. Particle size >2mmRudite - a general term for sedimentary rocks that are composed of rounded or angular detrital grains, i.e. granules, pebbles, cobbles, and boulders, which are coarser than sand in size.S[edit]Saccharoidal - Having a texture similar to that of granulated sugar.Saldanian (orogeny) -Sandstone - Sand (with grains up to 2 mm in diameter) in which the grains are cemented together by secondary silica or calcite. Maybe loosely cemented and soft or well cemented and hard. Buff to brownish; sometimes