HEAT — VOLCANOES. 691 



The rock of Vesuvius is leucitophyr, it containing the white mineral leucite 

 disseminated through it; that of Mt. Loa is mostly of the first four of the kinds 

 just mentioned. But about some parts, and even at the summit, of Mt. Loa, there 

 are clinkstone and porphyry, — compact light-colored feldspathic rocks without 

 cellules. It is not an uncommon fact that, while the ordinary rocks of the ex- 

 terior of a volcanic mountain are the heavy cellular dolerites and basalt, those 

 of the interior (as best seen when the mountain-mass is intersected by profound 

 gorges) are of these compact feldspathic kinds having no resemblance to ordi- 

 nary lavas. 



2. Liquidity of lava. — The liquid lava flows usually with nearly 

 the mobility of melted iron or glass. The whole of the flowing 

 mass does not, however, appear to be properly in a liquid or melted 

 condition ; a portion, in unfused grains, is suspended in a fused por- 

 tion. As the heat just below the surface has the intensity of what 

 is called white heat, any part of the rock-material which is fusible 

 at this temperature, or, rather, which is not consolidated at this 

 temperature (for the material has come from the depths below, 

 where the heat is much greater, it increasing with the depth or 

 pressure), will be in a melted state. In the crater of Kilauea, the 

 liquid lava cools at surface into a scoriaceous glass, and this glass was, 

 beyond doubt, in fusion, like the glass of a glass-furnace, — though 

 perhaps less perfectly, as stony unfused grains may be disseminated 

 through it. Below the surface, six inches more or less, the lava 

 has the aspect of a cellular rock ; but even glass takes this form if 

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

 tained 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 Pete'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. This is considered its 

 sole cause by Scrope. 



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

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

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

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

 water, but probably not exclusively. 



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

 mogtly emanations from fumaroles, — vents of hot air, steam, or sulphurous fumes, 

 in the neighborhood of a volcano, — rather than from the liquid lava. Further 

 examinations of the gases which escape from the liquid lavas in the crater are 



