
STRUCTURE AND SURFACE FEATURES 395 
the phenomena characteristic of this group., It is typically 
represented by volcanoes, the structure and origin of which 
are familiar. Volcanoes may occur singly or in irregular 
groups, or they may be distributed along definite lines or 
zones. They vary in size from mere monticles up to vast 
cones. Sometimes they appear in low-lying regions, at other 
times they occur upon the flanks or are strung along the 
crests of lofty mountain ranges. Built up, as they are, of 
materials ejected from below—of molten rock alone, or of 
loose stones, dust, and ashes alone, or, as is most frequently 
the case, partly of one and partly of the other—they all have 
essentially the same structure. They consist of successive 
layers of variable thickness, sloping outwards in all directions 
from the centre or centres of eruption. The actual form of a 
erowinge cone depends very largely upon the nature of the 
materials of which it is composed. Thus a volcano which 
emits highly fluid lavas does not, as a rule, throw out loose 
materials in much abundance. Hence such volcanoes have 
usually the form of more or less depressed cones—the liquid 
lavas flowing away and spreading out rapidly, while the 
limited supply of loose ejecta does not favour rapid growth in 
the vicinity of the crater. Viscous lavas, on the other hand, 
do not flow so rapidly, and tend, therefore, to coagulate at no 
great distance from the focus of eruption. And as they are 
generally accompanied by abundant discharges of loose 
ejecta, the resulting cone is usually more or less abrupt. 
Hence, in the case of active volcanoes, the external form is an 
expression of the internal or geological structure. 
Volcanoes are subject, like all other portions of the land- 
surface, to the modifying influence of the superficial or 
epigene agents of change. While in a state of activity they 
are worn and degraded by rain and torrents, by which they 
are often deeply scarred and furrowed. But such ravages 
are more than compensated for during times of eruption and 
accumulation. When volcanic action ceases, however, there 
can be no such compensation—degradation then proceeds 
apace. -The more or less steep inclination of the surface and 
the weak geological structure favour the action of the epigene 
agents. -Gullies and ravines are rapidly deepened and 
widened ; rock-falls and landslips ever and anon take place ; 
