PYROGENIC OR ERUPTIVE ROCKS. 
61 
ment of the air, which is expanded by heat, forms the vesicles, 
the sides or w r alls of which are stiffened before the heated air 
escapes. The vesicles are large and small, and these may be 
arranged in stripes. Sometimes the vesiculation expands the 
mass sufficiently to render it buoyant on water as in pumice. 
Crystalline structure is wanting in the vesicular rocks. The 
fifth aspect which rocks of igneous origin present, is that 
of a glass; it is a vitrification of the rock; it is sometimes 
homogeneous or striped. To the eye it appears like a furnace 
production. Under certain circumstances the vitreous mass 
may be converted into fine glassy spiculae. These spiculae often 
cluster together, and form a flaxen appearance. The rock 
glasses contain less alumina than feldspar. Their compo¬ 
sition, though variable, may be represented as follows: 
1 
Obsidian. 
2 Otsidian. 
Silex, 
60-52 
84-00 
Alumina, 
19-05 
4-64 
Oxide of iron, 
4*22 
5-01 
do. manganese, 
0-33 
Lime, 
0-59 
2-39 
Magnesia, 
0-19 
Potash, 
10-63 
Soda, 
3-50 
3-55 
The 1st is from Teneriffe, the 2d from Iceland. 
The sixth aspect occurs in those rocks where heat and 
mechanical action is so combined as to reduce the mass to 
powder. Volcanic ashes are examples of this form. The 
particles are buoyant in the air, and are carried or transported 
by winds sometimes for hundreds of miles. The foregoing 
examples are all distinct in the extremes; indeed, except 
in a few cases, they may be recognized by the student 
without difficulty. They may, it is true, graduate into each 
other. It is sufficiently plain, however, that fire, acting with 
different degrees of intensity under different circumstances, and 
acting too on compounds variable in fusibility, must furnish a 
variety of results which are not perfectly classifiable. There 
will necessarily occur some intermediate results, belonging in 
