Tuff

Tuff, or VOLCANIC TUFF, in Geology, the name given to the comminuted rock-debris ejected from a volcanic orifice. The term is usually restricted to the finer kinds of volcanic detritus, or to fragmental igneous rocks in which such fine-grained materials predominate. Thus some tuffs may be described as masses of finely comminuted debris through which are scattered, more or less abundantly, lapilli and angular or subangular blocks of volcanic or other rocks. Other tuffs may consist wholly of very fine-grained materials or of the most impalpable volcanic dust. Most tuffs, whether deposited on a land-surface or accumulated under water, are stratified. Subaqueous tuffs shade off gradually into ordinary aqueous rocks. Thus we have tuffaceous sandstones and tuffaceous shales, made up of mixtures of volcanic detritus and the ordinary products of aqueous erosion; and many of these rocks are fossiliferous. Subaerial tuffs, likewise, not infrequently contain relics of land plants and animals. Many varieties of tuff are known by special names. Thus trachyte-tuffs, basalt-tuffs, &c. are rocks composed essentially of the debris of trachyte, or of basalt, &c. Pumice-tuff consists mainly of pumiceous materials—Trass being the name given to a variety of pumice-tuff met with in the Eifel. Peperino is an earthy granular tuff, containing abundant crystals of various volcanic minerals, which is well developed in the Alban Hills near Rome. Palagonite-tuff is a fine-grained tuff. The term tufa, once synonymous with tuff, is now restricted to Calcareous Tufa (q.v.).