470 VOLCANIC TUFF — PALAGONITE TUFF. [Ch. XXVIII. 



lavas ; the relation, however, of its origin to the composition of lava is not 

 yet well understood. Von Buch says that it never occurs where only 

 Labrador-felspar is present. 



Volcanic tuff, Trap tuffi — 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 vol- 

 canoes, 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. Besides the peculiarity of their composition, 

 some tuffs, or volcanic grits, as they have been termed, differ from ordi- 

 nary sandstones by the angularity of their grains, and they often jjass 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.* The pepeiinos thus dis- 

 tinguished are usually brown, and the tuffs gray or white. 



We meet occasionally with extremely compact beds of volcanic ma- 

 terials, interstratified with fossiliferous rocks. These may sometimes 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 powder which has 

 fallen for days through the atmosphere, during some modern erup- 

 tions, has been found to weigh, without being compressed, as much 

 as ordinary trap-rocks, and to be often identical with these in mineral 

 composition. 



Palagonite-tuff. — The nature of volcanic tuffs must vary according 

 to the mineral composition of the ashes and cinders thrown out of 

 each vent, or from the same vent, at different times. In descrip- 

 tions of Iceland, we read of Palagonite-tuffs as very common. The 

 name Palagonite was first given by Professor Bunsen to a mineral 

 occurring in the volcanic formations of Palagonia, in Sicily. It is 

 rather a mineral substance than a mineral, as it is always amorphous, 

 and has never been found crystallized. Its composition is variable, 

 but it may be defined as a hydrosilicate of alumina, containing oxide 

 of iron, lime, magnesia, and some alkali. It is of a brown or black- 

 ish-brown color, and its specific density, 2 -43. It enters largely into 

 the composition of volcanic tuffs and breccias, and is considered by 

 Bunsen as an altered rock, resulting from the action of steam on vol- 

 canic tuffs. 



* Geol. Trans. 2d series, vol. ii. p. 211. 



