
Parr I. Sxcr.i.§3.] | VOLCANIC CONES. 247 
: up with a fluidity more like that of water than what is commonly 
shown by molten rock, surges against the surrounding terrace walls. 
Large segments of the cliffs undermined by the fusion of their base, 
fall at intervals into the fiery waves and are soon melted. 
5. Cones of Tuff and Lava.—tThis is by far the most abundant 
type of volcanic structure, and includes the great volcanoes of the 
globe. Beginning, perhaps, as mere tuff-cones, these eminences have 
eradually been built up by successive outpourings of lava from 
different sides, and by showers of dust and scorie. At first the lava, 
if the sides of the cone are strong enough to resist its pressure, may 
rise until it overflows from the crater. Subsequently, as the funnel 
becomes choked up, and the cone is shattered by repeated explosions, 
the lava finds egress from different fissures and openings on the cone. 
As the mountain increases in height, the number of lava-currents 
from its summit will usually decrease. Indeed, the taller a volcanic 
cone grows the less frequently as a rule does it erupt. The lofty 
volcanoes of the Andes have each seldom been more than once in 
eruption duringacentury. The peak of Teneriffe (Fig. 54) was three 







i) 
Hu 
{/ 
WO 
zz 
Hid 
Wy if 
S 
=> 
= 

Fig. 54.—PLAN oF THE SUMMIT or THE PEAK OF TENERIFFE, SHOWING THE LARGE 
CRATER AND MINOR CONES. 
times active during 370 years prior to 1798. The earlier efforts of a 
volcano tend to increase its height, as well as its breadth; the later 
eruptions chiefly augment the breadth, and are often apt to diminish 
the height by blowing away the upper part ofthe cone. The forma- 
tion of fissures and the consequent intrusion of a network of lava- 
dykes, tend to bind the framework of the volcano and strengthen it 
against subsequent explosions. In this way a kind of oscillation is 
established in the form of the cone, periods of crater eruptions being 
