Stratigraphy

Stratigraphy, or STRATIGRAPHICAL GEOLOGY, that department of geology (q.v.) which arranges the rocks, especially the sedimentary ones, in chronological succession, and endeavours to trace the sequence of physical events of which they constitute the record. It thus, to a great extent, is the summation and application of the results of the other branches of the science. In establishing the order of superposition of strata, fossils (q.v.) are of the greatest utility, each species or group of organisms increasing from its first appearance to its culminating point of abundance, and then sooner or later declining, before the absence or higher types, until it may disappear, in which case it never reappears. Thus the relative abundance of a species of genus may be of the greatest use in demarcating a zone or horizon in a thick uniform series of rocks. The Ammonites (q.v.) have been largely employed for this purpose among thick uniform series of Secondary rocks, such as the Lias clays or the Ohalk. Similarly an abrupt change in the fossils, such as that between the lower and upper parts of the Elgin Sandstone, may represent a considerable break in the succession of the rocks (in this case, between Old Red and New Red Sandstone), though there may be no marked unconformity; and, vice versa, the absence of any such change may show that even a considerable local unconformity does not represent any great lapse of time. The geologieal record is necessarily so imperfect that, though we cannot doubt that the succession of physical events, of denudation and deposit, of depression and elevation, and of organic life on the globe, has been one continuous story, it is fairly easy to subdivide the rocks formed into successive divisions. It is proposed by the International Geological Congress to adopt the following terms for the various grades of subdivision among rocks and in the time taken to form them, beginning with the most comprehensive division :--