. Massive, or compact.” Z 
E ee font as are all Sota on He a bibalou 
3 Cellular, when the cavities are visible to the eye, but 
ee and angular. 
. Vesicular, when the cells are more or less spheroidal. 
* Coxennites when the blisters or air-cells are of a very 
large size, and very numerous 
6. Spumous, when the air-cells are so numerous as to give 
a lightness and frothy appearance to the rock, as in some 
varieties of pumice and scoria. 
7. Filamentous, when composed of twisted thread-tike fi- 
bres. 
The last head to be noticed in the description of this class 
of rocks is their dwvistonary structure ; by which is meant the 
figures or the parts into which the rock is divided by seams 
or natural clefts. Frequently there are no such separations 
of continuity, and the rock is then pronounced amorphous. 
The varieties of structure structure may be classed as— 
e bedd ructure, when divided into massive beds. 
2. Stratified, w set beds are less bulky, from the great- 
= ney of “oe ‘ 
abular, when the separate divisions are still thinner, 
a, and of no great longitudinal extent. — 
. Laminar, when still thinner. side gaan 
: Schistose, lamellar, or slaty ; a well cow dractire 
6. Columnar, when the divisions are regular many-sided 
prisms of considerable lengt 
7. Prismatic, when the form of the prisms is less regular, 
= the transverse joints more frequen 
- Rhomboidal, when there exists a double system of par- 
alle seams, dividing the an $ into portions approaching in 
figure to cubes o mboi 
v: ConchSite:priewetie ae the boundaries of these por- 
tions are curvilinear. 
10. The globiform, when the rock is div — into — 
masses of a large size. These are often subdivided in 
centric lamine, less frequently into radiating prisms, r ren 
columns. 
11. The globular, when the spherical concretions are very 
a 
s ie 
