508 
ELEMENTS OE GEOLOGY. 
the crystals pre-existed, but were not melted, as being more 
infusible in their nature. Although melted matter rising in 
a crater, and even that which enters a rent on the side of a 
crater, is called lava, yet this term belongs 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 trachytie, as in the Peak of Tenerifle; a great num¬ 
ber are basaltic, as in Vesuvius and Auvergne; others are 
andesitic, as those of Chili; some, of the most modern in Ve¬ 
suvius consist of green augite, and many of those of Etna of 
augite and labrador-feldspar."^ 
Scorice and Pumice may next be mentioned, as porous 
rocks produced by the action of gases on materials melted 
by volcanic heat. Scorim 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 trachytie and other 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-feldspar is present. 
Volcanic Ash or Tuff^ Trap Tuff ,—Small angular fi*ag- 
ments of the scoriae and pumice, above-mentioned, and the 
dust of the same, produced by volcanic explosions, form the 
tufls which abound in all regions of active volcanoes, where 
showers of these materials, together with small pieces of oth¬ 
er rocks ejected from the crater, and more or less burnt, fall 
down upon the land or into the sea. Here they often be¬ 
come 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 lit¬ 
tle or no lime is present, there is a great tendency in the 
materials of ordinary tuffs to cohere together. The term 
volcanic ash has been much used for rocks of all ages sup¬ 
posed to have been derived from matter ejected in a melted 
state from volcanic orifices. We meet occasionally with ex¬ 
tremely compact beds of volcanic materials, interstratified 
with fossiliferous rocks. These may sometimes be tuffs, al¬ 
though their density or compactness is such as to cause them 
to resemble many of those kinds of trap which are found in 
ordinary dikes. 
Wache is a name given to a decomposed state of various 
trap rocks of the basaltic family, or those which are poor in 
silica. It resembles clay of a yellowish or brown color, and 
* G'^ Hose, Ann. des Mines, tom. viii., p. 32. 
