Strata
(plural of Stratum, meaning anything spread out, or strewn over a given surface). A term used to denote the layers in sand or in rocks, such as may be seen in a sand-pit, a slate-quarry, or a chalk-pit. One sees the layers in cuttings as thinner or thicker bands, one above the other; can be recognised if each layer is carefully removed from the surface of the one below it. By far the greater number of different kinds of rocks show layers, but some, such as granite and trap, do not. The layers, or Strata, are now believed by all geologists to be the result of the sand, or other substances of which rocks are made, having been at first strewed over the surface, on the bottom of seas, lakes, or rivers, less often by wind on the dry land. Gradually, the layers were pressed down by new layers above them, and, at last, often became converted into stone. The age of the Strata can often be determined from fossils of animals and plants that have been covered up in the soft mud, and turned into stone in it. The rocks that do not show Strata have been exposed to great heat, sufficient to melt them, so that any marks of Strata in them have disappeared where they once existed, e.g., in crystalline limestone.