round and spiny, typically from 3 to 10 centimetres (1.2 to 3.9 in) across.Urgonian - shallow-water carbonate facies deposited along the northern margins of the Tethys Ocean during the Barremian and AptianV[edit]Vacuoles - Bubble inclusions within mineral grains (typically monocrystalline quartz), filled with liquid, liquid and gas, or gas alone. They are randomly distributed in contrast to the oriented bubble trains of 'Boehm Lamellae'Variscan orogeny - or Hercynian orogeny - is a geologic mountain-building event caused by Late Paleozoic continental collision between Euramerica (Laurussia) and Gondwana to form the supercontinent of Pangaea.Varve - an annual layer of sediment or sedimentary rock.Vein (geology) - Mineral filling of a fracture or other crack within a rock in a sheet-like or tabular shape.Vermiculite - a hydrous, silicate mineral that is classified as a phyllosilicate and that expands greatly when heated. Exfoliation occurs when the mineral is heated sufficiently.Vergence (geology) - direction of overturning of asymmetric folds - matches direction of thrusting.Vesiculated -Vitrinite - a group of macerals, that are the most common component of coalVitrophyre - see PitchstoneVolcanic - Rocks that have crystallised from magma poured out at the surface or introduced at shallow depth. They have cooled relatively rapidly, grain size of crystals is small, some part of the melt may solidify as glass, volatiles are lost and anhydrous minerals of high temperature of crystallisation are present.Volcanic bomb - Rounded or spindle-shaped rock of mainly basaltic composition ejected during eruptions.Vug - Small cavity in a rock filled or lined with crystals/minerals that are different from the host rock.W[edit]Wiggle trace - a graph that plots wave amplitudes (recorded by seismic reflection and borehole logging) as a function of time, with the positive peaks shaded in a single dark color.Wolframite - (Fe,Mn)WO4 - an iron manganese tungstate mineral.X[edit]Xenolith - a rock fragment which becomes enveloped in a larger rock during the latter's development and hardening. In geology, the term xenolith is almost exclusively used to describe inclusions in igneous rock during magma emplacement and eruption.Xenotime - a rare earth phosphate mineral, whose major component is yttrium orthophosphate (YPO4).XRD - X-ray diffraction - a method of determining the arrangement of atoms within a crystal, in which a beam of X-rays strikes a crystal and diffracts into many specific directions.XRF - X-ray fluorescence - the emission of characteristic "secondary" (or fluorescent) X-rays from a material that has been excited by bombarding with high-energy X-rays or gamma rays. The phenomenon is widely used for elemental analysis and chemical analysis of minerals.Xyloid coal - also known as lignite, often referred to as brown coal, is a soft brown fuel with characteristics that put it somewhere between coal and peat.Y[edit]Yellow cake - or Yellowcake (also called urania) - a kind of uranium concentrate powder obtained from leach solutions, in an intermediate step in the processing of uranium ores.Young's modulus - in solid mechanics, Young's modulus, also known as the tensile modulus, is a measure of the stiffness of an isotropic elastic material. It is defined as the ratio of the uniaxial stress over the uniaxial strain in the range of stress