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. Hardness, which indicates the mineral composition. 



3. Solidity, or the coherence of its integrant parts, which 

 depends upon texture. 



4. Fusibility, which varies somewhat with the size of the 

 grain ; the smallest grain melting most readily cceteris pari- 

 bus ; ijut 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, 

 unless it is stained by metallic oxides, or other accidental 

 modifications, which are in general easily distinguishable 

 from the genuine tint of the component minerals. 



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 — 



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 

 of— 



1 . The average size of the fragments ; which may be 

 called, 



A. Coarse, 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 

 occasionally of crystalline texture. 



