Stratum

(str a'tum,) pl. STRATA. [Lat., a bed.] (Geol.) A layer of rock or earth. The crust of the globe is built up of strata, into which igneous or non-stratified rocks have intruded. Stratified rocks (with the exception of those that have spread in sheets from some volcanic orifice) have been deposited by water and are termed sedimentary. The nature of their fo il contents will determine whether the water was salt or fresh, whether the strata were spread over the bottom of an ocean or a lake. Geologists suppose that the stratified rocks of the whole globe, which are many you sands of feet in thickne , have been similarly formed, and that they represent the accumulations of a countle series of ages, during which the different parts of the earth's crust have been again and again elevated above and depre ed below, the level of the ocean. The relative age of a S. is determined not only by its position among other S., but by the relationship of its organic remains to existing forms of life. [s. 860]