40 
TYPES OF VOLCANOES 
KOOK I 
distance from the crater, these volcanic materials might he seen to include 
layers of soil and remains of land-vegetation, marking pauses between the 
eruptions, during which soil accumulated and plants sprang up upon it. Where 
the lavas and ashes had made their way into sheets of fresh water or into 
the sea, they would probably fje found interstratilied with layers of ordinary 
sediment containing remains of the animal or vegetable life of the time. 
f’ozrceive now the effects of prolonged denudation upon such a pile of 
volcanic rocks. The cone will eventually be worn down, the crater will 
disappear, and the only relics of the eruptive orifice may be the top of the 
central lava-column and of any fragmental materials that solidified within 
the vent (Fig. 16). The waste will, on the whole, he greater at the cone 
than on the more level areas beyond. It might, in course of time, reach 
the original surface of the ground on which the volcano built up its heap 
of ejected material. The central lava-plug might thus be left as an isolated 
l a I (I pad/ p / p 
Fig. 16. — EtTects of d«TiiuUition on a Vesuvian cone. 
eminence rising from a platform of older non-volcanic rocks, and the distance 
between it and the dwindling sheets of lava and tuff which came out of it 
would then be continually increased as their outcrop receded under constant 
degradation. 
This piece of volcanic history is diagranimatically illustrated in Fig. 16. 
The original forms of the central volcano and of its parasitic cones are 
suggested by the dotted lines in the upper half of the Figure. All this 
upper portion has been removed by denudation, and the present surface of 
the ground is shown by the uppermost continuous line. The general structure 
of the volcanic pile is indicated underneath that line — the lenticular sheets 
of lava and tuff (/, /), the dykes id,d), and the lavas and agglomer- 
ates (a, a) of the central vent and of the subordinate cones. 
I’he waste, though greatest on the higher ground of the great cone, 
would not stop there. It would extend over the flatter area around the 
volcano. Streams flowing over the plain would cut their way down through 
the lavas and tuffs, eroding ravines in them, and leaving them in detached 
and ever diminishing outliers on the crests of the intervening ridges. AVe 
can easily picture a time when the last of these relics would have been worn 
away, and when every vestige of the volcanic ejections would have been 
removed, save the lava-column marking the site of the former vent. 
