A _stratum_ will have its upper and lower surface apparently parallel, though not really so, for no stratum extends universally round the earth, and many of them disappear at no great distance when traced in any direction. Parts of one stratum may be composed of different materials from other parts when traced laterally, thus one stratum may be found composed essentially of sand in one place, of mud in another, and of a mixture of the two in an intervening locality. Whatever be the composition of a stratum it dies out eventually, owing to the coming together of the upper and lower bounding planes of stratification. The stratum is thickest at some spot, from that spot it becomes thinner in all directions, until it disappears at last by the coalescence of the bounding-planes. This is spoken of as _thinning-out_. Strata, then, consist of lenticular masses of rock, separated from the underlying and overlying strata by planes of stratification. The shape of the lenticle may vary immensely, the thickness bearing no definite relationship to the horizontal extent. Some strata, many feet in thickness, may thin out and disappear completely in the course of a few yards, whilst others an inch or two in thickness may be traced horizontally for many miles. We often find thin strata of coal and limestone, extending for great distances, strata of mud thinning out more rapidly, and sandstones still more rapidly, but no universal rule connecting rapidity of thinning-out with composition of the strata can be laid down. Having seen what a stratum is, it now remains to speak of the composition of the stratified rocks. They have been classified according to their composition, and according to their origin. According to composition they have been divided into: _Arenaceous_ rocks, composed essentially of grains of sand. _Argillaceous_ rocks, composed essentially of particles of mud. _Calcareous_ rocks, composed essentially of particles of carbonate of lime. _Carbonaceous_ rocks, composed largely of hydrocarbon compounds. _Siliceous rocks_, composed essentially of silica not in the form of grains; whilst according to their origin they have been separated into:-- _Mechanically-formed_ rocks, composed of fragments derived from other rocks by mechanical fracture. _Chemically-formed_ rocks, composed of particles which have been chemically deposited from a state of solution. _Organically-formed_ rocks, composed of materials which have been derived from a state of solution or from the gaseous condition by the agency of organisms.