118 VOLCANIC PRODUCTS. 



the air and the superincumbent pressure. It is not, 

 however, unusual to find scoriaceous rock at the 

 bottom, owing to the first ejected matter having 

 been spread out in the form of a thin sheet, or, per- 

 haps, from its contact with water in a damp soil. 



Lava is often porphyritic, having imbedded in it 

 crystals which have been derived from some older 

 rock, but which, being more infusible than the 

 other ingredients of the same rocks, have been 

 thrown out unmelted. 



The term lava is generally confined to such vol- 

 canic rocks as have actually 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. 

 Some lavas are trachytic, as in the Peak of Tene- 

 rirTe ; some basaltic, as in Vesuvius ; while others 

 may be a compound of both. In the volcanic dis- 

 tricts of Auvergne, scoriaceous lava becomes com- 

 pact, and at length passes into well-characterized 

 black basalt, with the columnar structure. In some 

 places currents of lava form what is called obsidian 

 or volcanic glass. In Christiana, in Norway, trap 

 rock may be seen passing distinctly into true gran- 

 ite. 



Scoria and Pumice are porous rocks, produced by 

 the action of gases on materials melted by volcanic 

 heat. Scoriae are usually of a reddish brown and 

 black colour, and are the cinders and slags of ba- 

 saltic or augitic lavas. Pumice is a light, spongy, 

 fibrous substance, produced by the action of gases 

 on different kinds of lava, precisely as the porous 

 character of bread is caused by the evolution of 

 carbonic acid gas by fermentation. 



Tuff. — Trap tuff or volcanic tuff, so called, is form- 

 ed by a mixture of small angular fragments of sco- 

 riae and pumice, together with the dust of the same, 

 which, ejected from the crater, fall in showers, and, 

 mingling with the shells, often become stratified. 



