In the form of a thin layer(s) of [[sedimentary]] rock which is of different mineralization or [[lithology]] and is visually separable from the host rock or other laminae. Laminae of sand or other permeable sediment sometime show unusually high productivity. Laminae of clay, sandwiched between layers of a host rock, not only support overburden but also take the place of some of the host rock and whatever [[porosity]] the host rock would contain.