350 ELEMENTARY GEOLOGY 
History of the Volcanic Cone. — Commencing with a single erup- 
tion, during which a cone was built about the point of ejection, 
the voleano grows with each successive outbreak (Fig. 214). If 
in the sea, it will grow rapidly until the surface is reached; for 
here there is nothing to remove the erupted material. Such a 
cone will be steep, because there is no strong action which would 
tend to distribute the materials. 
On the land, however, as the voleano grows upward, denudation 
at the same time attacks it, spreading some of the materials 
about, and thus lowering the level of the cone. Not only is the 
height reduced and the breadth increased, but the surface is 
carved and gullied (Fig. 198). Also, if ash is erupted, much of 
it will be carried away in the air. So long as eruptions are 
frequent, the additions from within will exceed this tendency to 
Fia. 214, 
Diagram to illustrate life history of a voleano. 
destruction, and the cone will grow, although temporarily it may 
be partially destroyed by a violent eruption (Fig. 208). 
However, when eruption ceases, and there are therefore no 
further additions from below the surface, denudation acts in 
excess (Fig. 205), and the cone becomes more and more gullied, 
and lower and lower, finally losing all semblance to the volcanic 
cone; and as a last stage, with the cone gone, the neck or plug 
may stand up as a peak (Fig. 215), while the dykes of hard lava 
rock project above the surface. The voleano becomes extinct, then 
loses form and size, and finally is entirely destroyed. Among the 
Cordilleras of the west there are illustrations of every gradation 
between the young cone, fresh and perfect in outline, as if formed 
yesterday, and the entirely extinct and nearly destroyed volcano, 
whose form tells us nothing concerning its history. 
