38 Descriptive Arrangement of Volcanic Rocks. 
12. The angulo-globular, when the rock separates into 
small angular divisions rudely approaching to a globular 
form. It resembles the conchoido-prismatic structure on a 
very small scale. 
The secondary characters of these rocks are often of ser- 
vice towards ascertaining with greater precision their prima- 
ry qualities, and thus accurately defining the rock. They 
consist chiefly of— 
1. The lustre and fracture ; both of which depend upon, 
and consequently disclose, the texture of the rock. 
2. Harilness, which indicates the mineral composition. 
3. Solidity, or the coherence of its integrant parts, which 
depends upon texture. = . 
4. Fus , which varies somewhat with the size of the 
grain; the smallest grain melting most readily ceteris pari- 
bus ; but it depends chiefly on the mineral composition of 
the rock, of which it becomes a serviceable test. 
5. Colour usually indicates the mineral nature of the rock, 
Each of the three genera of lithoidal volcanic rocks pos- 
sesses their conglomerates, which may be referred to any of 
them, according to the mineral nature of the composing frag- 
ments. They thus are divided into— ae 
1, Trachytic conglomerates. 2. Greystone ditto. 3. Ba- 
saltic ditto. : 
'_ The primary characters of these conglomerates, by which 
they are most distinctly recognized and described, consist 
& ‘The average size of the fragments; which may be 
A. rse, when of a considerable size. 
B, Gravelly, when of a medium size. 
C. Sandy, or arenaceous. 
D. Fine. 
E. Argillaceous. 
F. Mixed, when fragments of one size or more are imbed- 
ded in a base or cement of finer materials. The cement is 
Cc 
occasionally of crystalline texture 
