728 DYNAMICAL GEOLOGY. 



it graduates through pitchstone and pearlstone into trachyte — slow- 

 cooling making the material stony. Daubree has converted obsidian 

 into trachyte (by subjecting it in a confined vessel to heat and steam), 

 producing orthoclase in crystals. Augitic lavas are less commonly in 

 the glassy state than the orthoclase lavas. 



2. Volcanic Vapors or Gases. — The material escaping in the state 

 of vapor or gas is almost wholly (1) the vapor of water. Besides this, 

 there are (2) atmospheric air, and (3) sulphurous acid ; and sometimes 

 also (4) carbonic acid gas, derived, probably for the most part, from 

 limestone beds in the vicinity of the vent, and (5) hydrochloric acid; 

 derived mostly from sea-water ; also, in the neighborhood of volca- 

 noes, but not ordinarily from the lava-vents, (6) hydrogen, (7) nitro- 

 gen, and (8) sulphuretted hydrogen. Flames are never seen rising 

 from the lavas, except through the imagination of the beholder ; and 

 the so-called " thick smoke " is vapor of water, with usually a little 

 sulphurous acid, and some atmospheric air. Boussingault detected no 

 hydrochloric acid in the vapors of the South American volcanoes. 



Hydrogen was detected by Deville and Le Blanc in fumaroles nearer the lava-vent 

 than those affording the sulphuretted hydrogen, but not in vapors from the melted 

 lavas ; they also detected it in vapors from the old lavas of Torre del Greco, and in the 

 Tuscan lagoons; and Bunsen, in the Solfataras of Iceland. 



2. VOLCANIC PHENOMENA. 



The more common phenomena are, in brief, as follows : — 



(1.) Movements resembling sometimes those of a boiling liquid in 

 the lavas of the crater, with the escape of steam, and other vapors ; 

 (2) rising and projectile effects caused by the confined vapors ; (3) frac- 

 turing of the volcanic mountain or the region around ; (4) eruptions of 

 lava from the crater or from the fissures opened, sometimes flooding the 

 region with lava streams ; (5) making of cinder-cones from the dry cin- 

 ders falling about a vent ; (6) making of tufa-cones and flowing streams 

 of cinders, when water is present to wet the falling cinders, — either 

 the waters of the ocean or of subterranean fresh-water streams, or 

 those of the clouds precipitated in rain during the eruption ; (7) un- 

 derminings of the region by the outflows, leading to subsidences of 

 wide extent. (8) In addition, there are often, in the vicinity of vol- 

 canoes, fissures through which steam rises, with sulphur and other va- 

 pors, as explained on pp. 736, 737. 



Cinders are cooled fragments of lava-bubbles. The bubbles made 

 in the lava of a vent by the escaping vapors, finally burst, and the 

 material thrown up comes down cinders. 



The further explanations of these phenomena are best appreciated 

 from the facts connected with particular volcanoes ; and for this pur- 



