118 
VOLCANIC PROBUCTS. 
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 ^ciusWy 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- 
rifl*e ; 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. 
ScoricB 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 volcunic 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. 
