Descriptive Arrangement of Volcanic Rocks. 37 



1. Massive, or compact. 



2. Porous, as are all loose-textured, earthy, and bibulous 

 rocks. 



3. Cellular, when the cavities are visible to the eye, but 

 irregular and angular. 



4. Vesicular, when the cells are more or less spheroidal. 



5. Cavernous, 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-like fi- 

 bres. 



The last head to be noticed in the description of this class 

 of rocks is their divisionary 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 divisionary structure may be classed as — 



L The bedded structure, when divided into massive beds. 



2. Stratified, when the beds are less bulky, from the great- 

 er frequency of the seams. 



3. Tabular, when the separate divisions are still thinner, 

 flat, and of no great longitudinal extent, 



4. Laminar, when still thinner. 



5. Schistose, lamellar, or slaty ; a well known structure. 



6. Columnar, when the divisions are regular many-sided 

 prisms of considerable length. 



7. Prismatic, when the form of the prisms is less regular, 

 and the transverse joints more frequent. 



8. Rhomboidal, when there exists a double system of par- 

 allel seams, dividing the mass into portions approaching in 

 figure to cubes or ihomboids. 



9. Conchoido-prismatic, when the boundaries of these por- 

 tions are curvilinear. 



10. The glohiform, when the rock is divided into globular 

 masses of a large size. These are often subdivided into con- 

 centric laminae, less frequently into radiating prisms, or even 

 columns. 



1 1 . The globular, when the spherical concretions are very 

 small. 



