708 DYNAMICAL GEOLOGY. 



melted state. In the crater of Kilauea, the liquid lava cools at sur- 

 face into a scoriaceous glass ; and this glass was, beyond doubt, in 

 fusion, like the glass of a glass-furnace, — though perhaps less per- 

 fectly so, as stony unfused grains may be disseminated through it. Be- 

 low the surface, six inches more or less, the consolidated lava has the 

 aspect of a cellular rock ; but even glass takes a stone-like texture, if 

 very slowly cooled, and would do so all the more readily if it contained 

 a large amount of unmelted grains of any stony material. 



At Kilauea, the liquidity is so complete that jets, but a quarter of an inch through, 

 are sometimes tossed up from a tiny vent, and, as they fall back on one another, make 

 a column of hardened tears of lava. Again, the winds draw out the glass of the lava- 

 jets, in the boiling pools, into fine threads, by carrying off small fragments, and thus 

 make what is called Pele's hair: the crater being the residence, in native mythology, of 

 the goddess Pele. 



The mobility is also very largely promoted by the vapors rising in 

 the lava, especially the overheated steam. Scrope considers this its 

 sole cause. 



3. Vapors or Gases. — Besides air, steam (vapor of water), and 

 sulphurous vapors (either sulphurous acid or sulphur), there are some- 

 times (1) Carbonic acid gas, derived from limestone, and perhaps 

 from other sources below ; (2) Ghhrhydric acid gas, derived from sea- 

 water, but probably not exclusively. 



But these two gases, along with nitrogen and sulphuretted hydrogen, are mostly em- 

 anations from fumaroles, — vents of hot air, steam, or sulphurous fumes, in the neigh- 

 borhood of a volcano, — rather than from the liquid lava. Aqueous vapor exceeds 

 vastly in amount all other vapors, and, at Vesuvius, is the first that issues from newly 

 opened fumaroles; afterward follow in some cases carbonic acid, generally hydrochloric 

 acid, sulphurous acid, and also common salt, with often oxyds or clilorids of copper, 

 lead, etc. The hydrochloric acid changes the oxyds to chlorids, and thus the chlorids 

 originate; and sulphurous acid is the means of changing them to sulphates. 



These facts appear to show that sea-water gains access to the lavas. The steam 

 comes mainly from superficial waters. 



2. Volcanic Phenomena. 



1. Rising and Projectile Effects of escaping Vapors. — The water 

 and other vaporizable substances within the lava are under a pressure 

 of about 125 pounds to a square inch, for every 100 feet of depth. 

 Owing to the heat and their consequent expansion, they slowly rise in 

 the heavy, viscid liquid ; as they rise, they keep expanding, until, 

 nearing the surface, they begin to take the form of vapors, and finally 

 break through. 



The bubble or vapor in boiling water has projectile force enough, 

 as it breaks at the surface, to throw up water in jets to a height of two 

 or three inches. In lavas which have the freest liquidity, as those of 

 Kilauea, the jets are thirty to forty feet high. Consequently, a surface 



