VOLCANIC ROCKS. 



117 



some hornblende and some iron. It differs from 

 basalt in containing a larger proportion of feldspar 

 than augite, while the reverse is the case wiih the 

 latter. 



Porphyry is a form of rock very characteristic of 

 the volcanic formations. As we have already de 

 scribed it in a former chapter, we need only add, 

 that any rock possessing an earthy or compact 

 base, through which distinct crystals of one or more 

 minerals are scattered, is termed a porphyry. Thus 

 trachyte is a porphyry, because it contains distinct 

 crystals of feldspar, if the base be greenstone, ba- 

 salt, or pitchstone, the rock may be denominated 

 greenstone-porphyry, pitchstone-porphyry, &c. 



Amygdaloid. — This compound rock admits of ev- 

 . ery variety of composition. Indeed, it comprehends 

 any rock in which round or almond-shaped nodules 

 of some mineral, such as agate^ chalcedony^ calcareous 

 spar, or zeolite, are scattered through a base of vvacke, 

 basalt, greenstone, or other kind of trap. It is de- 

 rived from the Greek word amygdala,''^ an almond. 

 We can trace the origin of this structure in modern 

 lavas. Small pores or cells are caused by bubbles 

 of steam and gas confined in the melted matter. 

 After or during consolidation, these empty spaces 

 are gradually filled up by matter separating from the 

 mass, infiltered by water permeating the rock. The 

 almond shape of the globules is owing to the bub- 

 bles having been sometimes lengthened by the flow 

 of the lava before it finally cooled. Sometimes 

 these pores are empty, as in pumice-stone, giving it 

 a light, spongy appearance, and a capacity of float- 

 ing on water. 



Lava, — This is an indefinite term, having been ap- 

 plied to all melted matter which flows in streams 

 from volcanic vents. We usually find the upper 

 part porous or scoriaceous, from its consolidating in 

 the open air ; but, as we descend, the mass becomes 

 more s^Ud and ^tony, owing to the exclusion of 



