Stratum (Lat., 'spread out'), equivalent in Geology to the term bed or layer, but implying that the beds or layers of rock have been spread out over the surface. Rocks so arranged are said to be stratified. The stratified rocks include all those that are of derivative origin, such as conglomerate, sandstone, shale, &c. Many igneous rocks, however, are also arranged in layers or beds, as in the case of the basalt plateaus of Antrim, the Inner Hebrides, the Faroe Islands, Iceland, &c. In these regions we encounter a great succession of sheets of basalt with inter-bedded layers of fragmental materials (tuff, &c.). Such consecutive series of igneous rocks are truly stratified. But when a geologist speaks of 'the stratified rocks' he is understood to refer more particularly to the derivative or aqueous rocks, the most important characteristic of which is their bedded or stratified arrangement. In a series of stratified rocks each individual layer of sandstone, shale, limestone, &c. is a stratum, which may or may not be homogeneous in structure. For while some beds consist of a series of thinner layers or laminae, others show no such subordinate divisions. Thus, the particular variety of sandstone which is called freestone is not laminated, but of homogeneous structure, while a stratum of shale is composed of numerous thin laminae. Such laminae have a more or less close cohesion, which is sometimes so great that it is almost as easy to break the rock against as with the grain.
Individual strata are more readily separated from overlying and underlying beds. The degree of cohesion between laminae probably depends upon the rate at which sedimentation took place. If deposition was comparatively rapid the successive laminae would tend to cohere more readily than would be the case where each individual layer had had time to become more or less solidified before the deposition of the succeeding laminae. But in very many cases the cohesion of laminae has been effected by subsequent pressure, and sometimes by infiltration of cementing material. The planes of stratification are always more strongly pronounced than those of lamination, and generally point to some lapse of time (longer or shorter as the case may be)—to a pause in the deposition of sedimentary matter. For further remarks, see GEOLOGY.