VOLCANIC ROCKS. 601 



THIRD DIVISION. 



With a base of basalt, or some analogous sub- 

 stance, simple to the eye. 



A. Simple: dark lava, and scoriform lava. 



a. Compact : compact lava. 



b. Porous : cavernous lava ; or scoria of 



some authors. The caverns are some- 

 times partially filled with some of the 

 minerals which constitute the volcanic 

 amygdaloids, into which this variety 

 passes. 



c. Prismatic, or columnar: volcanic basalt 



of authors. 



d. Concretionary on a smaller scale : 



spheroidal or otherwise. 



B. Compound: containing felspar: porphyries. 

 This variety may also contain many minerals 



besides felspar. 



C. Compound : containing amygdaloidal nodules : 

 volcanic amygdaloid. 



The most remarkable of the imbedded no- 

 dules are, calcareous spar, fluor, the zeolitic 

 minerals, and chalcedony. These appear to be 

 the produce of infiltration, as they are sometimes 

 accompanied by water. The imbedded minerals 

 not properly amygdaloidal, and which may exist 



