(302 SCORLE— PUMICE— VOLCANIC TUFF. [Oh. XXVIIL 



longs more properly to that which has flowed either in the open air 

 or on the bed of a lake or sea. If the same fluid has not reached 

 the surface, but has been merely injected into fissures below ground, 

 it is called trap. 



There is every variety of composition in lavas ; some are trachytic, 

 as in the Peak of Teneriffe ; a great number are basaltic, as in Vesu- 

 vius and Auvergne ; others are Andesitic, as those of Chili ; some of 

 the most modern in Vesuvius consist of green augite, and many of 

 those of Etna of augite and Labrador-felspar.* 



Scorice and Pumice may next be mentioned as porous rocks, pro- 

 duced by the action of gases on materials melted by volcanic heat. 

 Scoriae are usually of a reddish-brown and black color, and are the 

 cinders and slags of basaltic or augitic lavas. Pumice is a light, 

 spongy, fibrous substance, produced by the action of gases on trachy- 

 tic and other lavas ; the relation, however, of its origin to the compo- 

 sition of lava is not yet well understood. Von Buch says that it 

 never occurs where only Labrador-felspar is present. 



Volcanic Tuff, Trap Tuff. — Small angular fragments of the scoriae 

 and pumice, above mentioned, and the dust of the same, produced by 

 volcanic explosions, form the tuffs which abound in all regions of 

 active volcanoes, where showers of these materials, together with 

 small pieces of other rocks ejected from the crater, fall down upon 

 the land or into the sea. Here they often become mingled with 

 shells, and are stratified. Such tuffs are sometimes bound together 

 by a calcareous cement, and form a stone susceptible of a beautiful 

 polish. But even when little or no lime is present, there is a great 

 tendency in the materials of ordinary tuffs to cohere together. Be- 

 sides the peculiarity of their composition, some tuffs, or volcanic grits, 

 as they have been termed, differ from ordinary sandstones by the 

 angularity of their grains, and they often pass into, volcanic breccias. 



According to Mr. Scrope, the Italian geologists confine the term tuff, 

 or tufa, to felspathose mixtures, and those composed principally of 

 pumice, using the term peperino for the basaltic tuffs.f The peperinos 

 thus distinguished are usually brown, and the tuffs gray or white. 



We meet occasionally with extremely compact beds of volcanic 

 materials, interstratified with fossiliferous rocks. These may some- 

 times be tuffs, although their density or compactness is such as to 

 cause them to resemble many of those kinds of trap which are found 

 in ordinary dikes. The chocolate-colored mud, which was poured for 

 weeks out of the crater of Graham's Island, in the Mediterranean, in 

 1831, must, when unmixed with other materials, have constituted a 

 stone heavier than granite. Each cubic inch of the impalpable pow- 

 der which has fallen for days through the atmosphere, during some 

 modern eruptions, has been found to weigh, without being com- 



* G. Kose, Ann. des Mines, torn. viii. p. 32. 

 f Geol. Trans., Second Series, vol. ii. p. 211 



