Stratum

Collier's New Encyclopedia (1921)

a bed or mass of matter spread out over a certain surface, in most cases by the action of water, but sometimes also by that of wind. The method in which stratification by the agency of water has been effected in bygone times may be understood by a study of the manner in which successive layers of gravel, sand, mud, etc., are depos posited in a river or ru running brook. The same process has been at work through untold periods of time. The greater part of the earth's crust, in nearly every land, is found to be thus stratified. Strata may be CONFORMABLE (q. v.), or UNCONFORMABLE (q. v.). In the former case there generally is a considerable approach to parallelism among them. It is, however, inferior in xactness to that of cleavage planes. Strata laid down by water, as a rule, retain fossil remains of the animals and plants imbedded in them when they were soft and plastic. Metamorphism generally destroys these organic remains, but leaves the stratification undisturbed; thus there are two kinds of stratasedimentary and metamorphic-nearly synonymous with fossiliferous and nonfossiliferous stratified rocks. Most strata have a dip and a strike. The fossils will in most cases show whether strata are lacustrine, fluviatile, or marine. They prove that deposit was very slow. One stratum may overlap another, or a stratum may thin out, or an outcrop of it may exist As a rule, the lowest are the oldest, but some great convulsion may have tilted over strata in limited areas, so that the oldest have been thrown uppermost. A study of the same beds over a wide expanse of country prevents error in estimating the relative age of strata thus reversed. The thickness of the stratified rocks is believed to be about 20 miles, or 100,000 feet. They are not all present at one place, or even in one country. Though a surprisingly large number are to be found in America, yet some foreign beds require to be inserted in the series, and even then great gaps remain, each representing a lapse of time. For the order of superposition, see FOSSIL: GEOLOGY.