Glossary of geological terms

Basalt Dark-coloured, fine-grained basic igneous rock

Basic igneous rock Containing a low proportion of silica and a high proportion of calcium, iron and magnesium

Brackish Saline water with a salt content less than that of seawater

Breccia Sedimentary rock containing large, angular blocks

Calcite Common crystalline form of calcium carbonate

Conglomerate Sedimentary rock containing large, rounded cobbles or boulders

Cretaceous Period Sub-division of geological time; see Eigg timescale (inside back cover) for details

Dyke Tabular body of igneous rock intruded into surrounding rocks; generally vertical or nearly so

Erosion Wearing away of the land surface as a result of weathering and transport of material by rivers, glaciers, etc.

Feldspar A group of pale-coloured aluminium silicate minerals forming tabular crystals; the most common minerals in igneous rocks

Igneous rock Rock formed from the solidification of molten magma

Ignimbrite Rock formed from material deposited by a pyroclastic flow

Intrusive igneous rock Rock formed by crystallisation of magma injected into pre- existing rocks

Jurassic Period Sub-division of geological time; see Eigg timescale (inside back cover for details (Figure 43)).

Limestone Sedimentary rock formed mainly of calcium carbonate

Metamorphic rock Rock that has been changed from its original state by the action of heat and pressure

Mudstone A sedimentary rock formed from the consolidation of clay or silt

Peperite Sedimentary rock that contains fragments of igneous material that is formed when magma comes into contact with wet sediments

Pitchstone A glassy, i.e. largely non-crystalline, silicic igneous rock

Palaeogene Period Sub-division of geological time; see Eigg timescale (inside back cover) for details

Pumice Silicic volcanic rock containing numerous gas bubbles

Pyroclastic flow A fast-moving current of hot gas and rock resulting from an explosive volcanic eruption, usually of silicic magma

Quartz The common crystalline form of silica (silicon dioxide)

Sandstone A sedimentary rock consisting of sand-sized grains, generally predominantly quartz

Sedimentary rock Rock formed from the consolidation of sediments, such as sandstone, mudstone and limestone

Silicic igneous rock Containing a high proportion of silica, therefore formed mainly of high-silica minerals such as quartz and feldspar e.g. granite

Sill A tabular body of igneous rock, formed from magma intruded into sedimentary rocks parallel to their bedding; often near horizontal

Till Glacial debris deposited beneath an ice sheet (formerly known as boulder clay)

Weathering The fragmentation and alteration of rocks by physical means such as frost action or by chemical change due to attack by water and the atmosphere, without transport (cf. erosion)