i 
36 Descriptive Arrang gement of Volcanic Rocks. 
A. Granitic ) | 
B. Granular “| 
a Massive, or granitoidal. | Loose «. 
5 Mel ne rb Scaly, or foliated. Close 8. 
ou 
The texture is also either, 1, Uniform, which needs no ex- 
planation, or, 2. Varied, when consisting of parts of different 
texture. Epithets may be also made use of to describe the 
aon form and disposition of these parts, as, 1. Nodular. 
2. Lenticular. 3. Zoned. 4. Brecciated. 
“These varieties of texture in the same mass are generally 
connected with, and in all probability owing to, an unequal 
distribution of the different minerals com the sub- 
stance, which forms another very characteristic distinction 
amongst this family of rocks, and may with propriet be as- 
sumed as the basis of their division nto varieties. The prin- 
“ modes of eer Simation e— 
he 
the flints in chalk. 
= 5. Lenticular coneretionary, when the Bgure of an segre- 
ongated in any one dire 
concr cotar y, when they are stongated still more 
Veined, w one or more species of minerals appear 
to have poses ati: in the ro 
ygdaloidal, when one or =n minerals have occu- 
Hed vesicular cells j in it. 
~ The next head under Which it has been thought right to 
class the characteristic qualities of these rocks, is their inter- 
nal structure, which comprehends the following varieties :— 
