242 DYNAMICAL GEOLOGY. —— [Boox IIT. 
The crater doubtless owes its generally circular form to the equal — 
expansion in all directions of the explosive vapours from below. In 
some of the mud-cones already noticed the crater is not more than a 
few inches in diameter and depth. From this minimum every 
gradation of size may be met with, up to huge precipitous depres- 
sions, a mile or more in diameter, and several thousand feet in depth. 
In the crater of an active volcano, emitting lava and scoriz, like 
Vesuvius, the walls are steep, rugged cliffs of scorched and blasted 
rock—red, yellow, and black. Where the material erupted is only 






q 
Fia. 47,—DIAGRAM-SECTION OF A NORMAL VOLCANO. 
2 
x «, Pre-voleanic piatform, supposed here to consist of upraised stratified rocks, broken 
through by the funnel /, from which the cone of volcanic materials ¢ ¢ has been 
erupted. Inside the crater v, previously cleared by some great explosion, a 
minor cone may be formed during feebler phases of volcanic action, and this 
inner cone may increase in size until the original cone is built up again, as 
shown by the dotted lines. . 
loose dust and lapilli, the sides of the crater are slopes, like those of 
the outside of the cone. “The 
The crater bottom of an active volcano of the first class forms a 
rough plain dotted over with hillocks or cones, from many of which 
steam and hot vapours are ever rising. At night the glowing lava 
may be seen lying in these vents, or in fissures, at a depth of only a 
few feet from the surface. Occasional intermittent eruptions take 
place and miniature cones of slag and scorie are thrown up. _ In some 
instances, as in the vast crater of Gurung Tengger, in Java, the 
crater bottom stretches out into a wide level waste of volcanic sand, — 
driven by the wind into dunes like those of the African deserts. 
A volcano commonly possesses one chief crater, often also many 
minor ones, of varying or of nearly equal size. ‘The volcano of the 
Isle of Bourbon has three craters. Not infrequently craters appear 
successively, owing to the blocking up of the pipe below. Thusin the 
accompanying plan of the volcanic cone of the island of Volcanello 
(Fig. 48), one of the Lipari group, the volcanic funnel has shifted its 
position twice, so that three craters have successively appeared upon 
the cone, and partially overlap each other. It may be from this 
cause that some volcanic mountains are now destitute of craters, or 

