VOLCANOES. 707 



nate in a lofty cone of cinders, of forty degrees. The crater may be 

 nearly like that of the cinder-cone, — a deep cavity, with the walls 

 thin, compared with those of the simple lava-cone. There is no fixed 

 order in the alternations of lavas and cinder or tufa layers ; the lavas 

 are apt to prevail most in the early stages of a volcano. 



3. Volcanic Action. 



The agents concerned in volcanoes are (1) lava, and (2) over- 

 heated steam and atmospheric air, with vapors of sulphur and some 

 other gases. 



The phenomena are (1) Rising and projectile effects, from escaping 

 vapors ; (2) Movements of the lavas in the crater ; (3) Eruptions. 



The facts presented in illustration of this subject are taken mainly from the volcanoes 

 of Kilauea and Vesuvius, both of which have been visited by the author. 



1. Agents. 



1. Kinds of Volcanic Rocks, or Lavas. — The fused rock-material is 

 called lava. When solidified, it is lava still, and is often so termed, 

 whatever its texture ; but in general the name is restricted to those 

 volcanic rocks which are more or less cellular. The cellules are usu- 

 ally ragged, and not smooth and almond-shaped like those of an 

 amygdaloid. The solid kinds, with rarely a cellule or with none at all, 

 come under the general designation of volcanic rocks. A very light 

 cellular lava is a scoria, or volcanic slag, or is said to be scoriaceous. 



The principal kinds of volcanic rocks and lavas have been described on pp. 76-79, to 

 which reference may here be made. The most common are doleryte, which takes on the 

 form of lava, and is then often called basalt; peridotyte, or a doleryte or basalt contain- 

 ing chrysolite ; trachyte and phonolyte. The. rock of Vesuvius is amphigenyte, it contain- 

 ing the white mineral leucite (or amphigene) disseminated through it; that of Mount 

 Loa is mostly of the first three of the kinds just mentioned. But, about some parts, 

 and even at the summit, of Mount Loa, there is phonolyte. — a compact light colored 

 feldspathic rock without cellules. It is not an uncommon fact, that, while the ordinary 

 rocks of the exterior of a volcanic mountain are the heavy cellular dolerytes or basalt 

 or peridotyte, 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 

 ordinary 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 condi- 

 tion ; a portion, in unfused grains, is suspended in a fused portion. 

 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 



