690 DYNAMICAL GEOLOGY. 



crater with a narrow throat, a narrow rim above, steep sides, the 

 slope 35 to 45 degrees (fig. 972). If the volcano is in brisk action, 

 the space within the crater is dark with the rising vapors, and 

 the explosions attending the ejection of cinders occur usually at 

 short intervals. 



The cone is at first nearly black or brownish black, but, if not soon covered 

 with vegetation, it often becomes, through atmospheric agencies, of a red color, 

 from the peroxydation of the protoxyd of iron in the lava : the peroxyd of iron 

 formed differs from that of the tufa-cone in not containing water, and hence 

 the difference of color. The growth of vegetation tends to change back the 

 red color to brownish black, since the carbon deoxydizes the peroxyd, making 

 protoxyd and carbonic acid. 



4. Mixed cones. — The cones which, like Vesuvius, are formed partly 

 of lava and partly of cinders or tufa, may have any angle of slope 

 up to 35 degrees. They may be lava below, and terminate in a 

 lofty cone of cinders of 40 to 45 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) Eising and projectile effects of 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-mate- 

 rial is, in all cases, 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 usually 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. 

 87-89, to which reference may here be made. The most common are dolerite, 

 doleritic lava, basalt, basaltic lava, clinkstone, trachyte. 



