Graptolite. Extinct marine organisms without known close living relatives, with small black sawblade-like chitinous hard parts preserved as [fossils].
Hematite. A steel gray or metallic grayish black or reddish gray mineral (Fe₂O₃) that is an important ore of iron.
Hogback. A sharp-crested ridge formed by a resistant layer of steeply dipping rock.
Huebnerite. A heavy reddish brown mineral (MnWO₄) that is a major ore of tungsten.
Igneous rocks. Rocks formed by solidification from a molten state, either at the surface (extrusive) or below the surface (intrusive).
Intrusive rocks. [Igneous rocks] formed when molten rock material solidifies without reaching the surface.
Joint. A fracture in the rock, along which no discernible movement has taken place.
Kerogen. Solid bituminous material in oil shales.
Laccolith. A lens-shaped mass of igneous rock intruded into layered rocks.
Lava. Fluid or molten rock such as that which issues from a volcano.