OUTLINES OF GEOLOGY. 



41 



volcanic rocks have been ejected, are cavities at the summit 

 of conoidal mountains, and are called craters. 



88. The rocks of decided volcanic origin are : 



1. Leucostine, a rock which appears to be a compact 

 felspar, modified by igneous action ; 



2. Tephrine, a rock of cellular structure, also composed 

 of felspar. Both of these contain crystals of felspar, and of 

 other minerals, and go under the general name of lava ; 



3. Obsidian; 



4. Pumice ; 



5. Peperine, a conglomerate, in which pyroxene is a 

 constant ingredient ; 



6. Volcanic Pudding Stone, in which pebbles are ag^ 

 gregated by an igneous cement : 



7. Puzzolana, or Volcanic Ashes ; 



8. Moya, containing charred wood imbedded. 



89. The principal minerals disseminated in volcanic rocks 

 are— Felspar, Hornblende, Pyroxene, Olivine, Specular Iron, 

 Mica, Sulphur. There are found imbedded in them, and 

 probably of more remote origin — Quartz, Zircon, Corun- 

 dum, Spinelle, Idocrase, Garnets, &c. 



In some instances, organic remains, and the works of hu- 

 man art, have been found imbedded. The most familiar 

 instances are in the neighbourhood of Vesuvius. 



90. There is evidence of volcanic action in rocks, imbeds 

 ded minerals, streams of lava, and craters in many places 

 where no eruptions have taken place within historic times. 

 There are also instances where volcanic action has suddenly 

 commenced in positions where the most ancient traditions do 

 not reach back to similar phenomena. In no case, however 

 have eruptions taken place, except where there was distinct 

 evidence of more ancient volcanic action. Volcanoes are 

 therefore classed as extinct or active, and we rank with the 

 former all which have not been in activity for the last three 

 or four centuries. 4* 



