36 Descriptive Arrangement of Volcanic Rocks. 

 A. Granitic ^ ■^ 



c' Compact l^ Massive, or granitoidal. I Loose a. 



d'. Resinous ^ ^^^'y' ""' ^'^''^^^ed. ^Close ^. 



E. Vitreous J J 



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 

 general 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 composing the sub- 

 stance, which forms another very characteristic distinction 

 amongst this family of rocks, and may with propriety be as- 

 sumed as the basis of their division \nto varieties. The prin- 

 cipal modes of mineral distribution are — 



1. Unifo7'm, when the minerals are generally intermixed 

 throughout the mass, as in granites, syenites, &c, 



2. Porphyritic, when large crystals, or grains, of one or 

 more minerals are dispersed throughout a base of very fine 

 texture, and uniform disposition, so as to strike the eye by 

 their prominence, as in porphyries. 



3. Globular concretionary, when some minerals have more 

 or less completely separated themselves from the remaining 

 mixture, and agglomerated into globular nuclei, as in pearl- 

 stone, variolite, orbicular granite, &,c. 



4. Nodular concretionary , when some minerals have sep- 

 arated in the form of irregular knots, as in the masegna of 

 the Euganean hills, in many granites and porphyries, or like 

 the flints in chalk. 



5. Lenticvlar concretionary, when the figure of the segre- 

 gated parts is much elongated in any one direction. 



6. Zoned concretionary, when they are elongated still more 

 into alternate stripes. 



7. Veined, when one or more species of minerals appear 

 to have occupied cracks in the rock. 



8. Amygdaloidal, when one or more minerals have occu- 

 pied vesicular cells 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 : — 



