STRATUM, pl. strata (Lat. strewn or spread out), the term applied by geologists to the layers into which most of the rocks that form the crust of the earth are divided. It implies that the layers have been spread out over the surface, and that they were formed in this way we may infer from the deposits that are now taking place in lakes and seas into which rivers laden with muddy sediment empty themselves.
All the aqueous rocks, which cover so large a proportion of the earth's surface, are stratified. They were formed from the abraded materials of older rocks (aqueous or igneous), which have been washed down and rearranged. The kind of rock produced depended upon the material to which the carrying agent had access. Fine mud would produce shales, sand sandstones, and calcareous matter limestones. In a section, these different kinds of rocks are frequently found to interchange within a short space. This is produced either by the water obtaining different materials, or changing its velocity. Thus the fine sediment which has fallen from slowly flowing water may be covered by a layer of sand brought down by a flood, and this again may have spread over it a covering of shells and corals, and such changes may go on alternately for an indefinite period. Each of the different beds composed of the same kind of material is called a stratum. Thus, in the series mentioned, there would be a " stratum" of clay, one of sand, and then one of calcareous matter. An assemblage of strata having a com
mon age is called a "formation," and this term is also extended to rocks which agree in their composition or origin. Thus, we speak of stratified and unstratified, aqueous and igneous, fresh-water and marine, primary and secondary, metalliferous and non-metallif erous formations. As a formation is composed of many different beds, so a stratum is frequently made up of several "lamina;" or " layers." The lamina; have a more or less firm cohesion, but the strata easily separate from each other. Sometimes the cohe sion of the lamina; is so great that it is as easy to split the rock against as with the grain. In such compact rocks the lamination is obscure, or altogether imperceptible in fresh specimens, but whenever they are exposed to the influence of the weather, it becomes obvious. The laminae have been produced by short interruptions in the deposi tion. similar to what might be the result of tidal or other intermittent action. The degree of cohesion may be the result of rapid succession in the acts of deposition, but it is frequently produced by metamorphic action subsequent to deposition. The planes of stratification want the complete coalescence characteristic of lamination; when the contiguous layers are closely united, it is the result of the adhesion of two bodies, and not of their coalescence into one.