CHAP. 
VOLCANIC VENTS 
55 
“ necks ” now visible at the surface. Many striking examples of these 
features may be seen among the Carboniferous and Permian volcanoes to be 
afterwards described. Occasionally where an open fissure has served as a 
vent it has given rise to a long dyke-like mass {No. 1 in Fig. 22). 
The size of a volcanic vent may vary indefinitely from a diameter of 
not more than a yard or two up to 
one or two or more miles. As a 
rule, the smaller the vents the more 
numerously are they crowded to- 
gether. In the case of large central 
volcanoes like Etna, where many 
subsidiary vents, some of them form- 
ing not inconsiderable hills, may 
spring up along the sides of the 
parent cone, denudation will ulti- 
mately remove all the material tliat 
was heaped up on the surface, and 
leave the stumps or necks of the 
pai’asitic vents in groups around the 
central funnel. 
Each volcanic chimney, by whicli 
va])ours, ashes or lava are discharged 
at the surface, may be conceived to 
descend in a more or less nearly 
vertical direction until it reaches the 
surface of the lava whence the erup- 
tions proceed. After the cessation 
of volcanic activity, this pipe will be 
left filled up with the last material 
discharged, which will usually take 
the form of a rudely cylindrical column reaching from the bottom of the crater- 
down to the lava-reservoir. It will be obvious that rro matter how great may 
be the derrudation of the volcarro, or how extensive rrray be the removal of 
the various nraterials discharged over the surrounding grourrd, the pipe or 
funnel with its column of solid rock must still renrain. No amount of 
waste of the surface of the land can efface that colurirn. Successively lower 
and yet lower levels may be laid bare in it, but the colurrrn itself goes still 
fui’ther down. It will contirrue to rrrake its appeararrce at the surface until 
its roots are laid bare in the lava of the subterranean magma. Hence, of 
all the relics of volcarric action, the filled-up chunney of the eruptive vent 
is the most enduring. Save where it may have been of the less deep-seated 
nature of a “ hornito ” upon a lava-stream, we may regard it as practically 
permanent. The full meaning of these statements will be best understood 
from a consideration of the numerous illustrations to be afterwards given. 
Tlie stumps of volcanic columns of this nature, after prolonged denuda- 
tion, generally project above the surrounding ground as rounded or conical 
1. Linhope Burn, near Mosspaul, Roxburghshire ; the 
shaded parts are intrusioufi of trachytie luaterial. 2. 
Hazelside Hill, two miles W. from Newcastleton, 
Roxburghshire. 3. St. Magdalen's, Linlithgow. 4. 
South- west side of Coom’s Fell (see-Fig. 174). 5. Neck 
on Greatmoor, Roxburghshire. 6. Pester Hill, Tarras 
Water. 7. Heail of Routing Burn, S.E. side of Harts- 
garth Fell, Liddesdale. 8. Hart.sgarth Flow, Liddes- 
tlale. 
