54 
SUBTERRANEAN VOLCANIC ACTION 
liOOK I 
plete the formation of a volcano by opening a passage to the surface. But 
where the thickness of rock above the end of the fissure is not too great, 
the expansive energy of the vapours absorbed in the magma may overcome 
the resistance of that cover, and blow out an orihee by which the volcanic 
materials can reach the surface. In the formation of new cones within the 
historic period at a distance from any central volcano, the existence of an 
opep fissure at the surface has not been generally observed. When, for 
example, Monte Nnovo was formed, it rose close to the shore among fields 
and gardens, but without the appearance of any rent from which its 
materials were discharged. 
That in innumerable instances during the geological past, similar vents 
have been opened without the aid of fissures that readied the surface, will be 
made clear from the evidence to be drawn from the volcanic history of the 
British Isles. So abundant, indeed, are these instances that they may be 
taken as proving that, at least in the Buy type of volcanoes, the actual vents 
have generally been blown out by explosions rather than by the ascent of 
fissures to the open air. 
In cases where, as in Iceland, fissures open at the surface and discharge 
lava there, the channel of ascent is the open space between the severed walls 
of the rent. Within this space the lava will eventually cool and solidity as 
a dyke. It is obvious that a comparatively small amount of denudation 
will suffice to remove all trace of the connection of such a dyke with the 
stream of lava that issued from it. Among the thousands of dykes belong- 
ing to the Tertiary period in the British Islands, it is probable that many 
may have served as lines of escape for the basalt at the surface. But it is 
now apparently impossible to distinguish between those which had such a 
communication with the outer air and those that ended upward within the 
crust of the earth. The structure of dykes will be subsequently discussed 
among the subterranean intrusions of volcanic material. 
In an ordinary volcanic orifice the ground-plan is usually irregularly 
circrdar or elliptical. If that portion of the crust of the earth througli 
which the vent is drilled should be of uniform structure, and would thus 
yield equally to the effects of the volcanic energy, we might anticipate that 
the ascent and explosion of successive globular masses of highly heated 
vapours would give rise to a cylindrical pipe. But in truth the rocks ot 
the terrestrial crust vary greatly iu structure ; while the direction and force 
of volcanic explosions are liable to change. Hence considerable irregularities 
of ground-plan are to be looked for among vents. 
Some of these irregularities are depicted in Fig. 22, which represents 
the ground plan of some vents from the Garboniferous volcanic districts of 
Scotland. They are all drawn on the same scale. Other examples will be 
cited in later chapters from the same and other parts of the British Isles. 
Some of the most marked departures from tlie normal and simple type 
of vent occur where two orifices have been opened close to each other, or 
where the same vent has shifted its position (Figs. 29, 125, 205, and 214). 
Curiously irregular or elongated forms may thus arise in the resultant 
