CHAP. V 
VOLCANIC NECKS 
57 
neighbourliood. But where the chimney has been filled with debris, there 
can be no doubt that it truly marks the site of a once active ‘volcano. The 
fragmentary material is an eloquent memorial of the volcanic explosions 
that drilled the vent, kept it open, and finally filled it up. These 
explosions could not have taken place unless the elastic vapours which 
caused them had found an escape from the pressure under which they lay 
within the crust of the earth. Now and then, indeed, wliere the outpouring 
of lava or some other cause has left cavernous spaces within the crust, there 
may conceivably he some feeble explosion there, and some trilling accumula- 
tion of fragmentary materials. But we may regard it as practically certain 
that the mass of tumultuous detritus now found in volcanic necks could 
not have been formed unless where a free passage hail been opened from 
the molten magma underneath to the outer surface of the planet. 
Considerable diversity may be observed in the nature and arrangement 
of the fragmentary materials in volcanic necks. Tlie chief varieties may be 
arranged in four groups: (1) Necks of non-volcanic detritus; (2) Necks of 
volcanic agglomerate or tuff; (3) Necks of agglomerate or tuff with a 
central plug of lava; and (4) Necks of agglomerate or tuff with veins, 
dykes or some lateral irregular mass of lava. 
(1) Necks of non-volcanic Detritus . — During tlie first convulsive eflorts of 
a volcanic focus to find a. vent at the surface, the explosions that eventu- 
ally form tlm orifice do so by blowing out in fragments the solid rocks of 
the exterior of the terrestrial crust. Of the detritus thus produced, shot 
up the funnel and discharged into the air, part may gather round the mouth 
of the opening and build up there a cone with an enclosed crater, while part 
will fall hack into the chimney, either to accumulate there, should the ex- 
plosions cease, or to lie thrown out again, should they continue. In the 
feeblest or most transient kinds of volcanic energy, the explosive vapours 
may escape without any accompanying ascent of the molten magma to the 
surface, and even withorit any sensible discharge of volcanic “ ashes ” from 
that magma. In such cases, as I have already pointed out, tlie detritus of 
the non-volcanic rocks, whatever they may be, through which volcanic energy 
has made an opening, accumulate in the pipe and eventually consolidate 
there. Examples of this nature will he adduced in later chapters from the 
volcanic districts of Britain. 
Where only non-volcanic materials fill up a vent we may reasonably 
infer that the eruptions were comparatively feeble, never advancing beyond 
the initial stage when elastic vapours made their escape with explosive 
violence, but did not lead to the outflow of lava or the discharge of ashes. 
In the great majority of necks, however, traces of the earliest eruptions have 
been destroyed by subsequent explosions, and the uprise of thoroughly 
volcanic fragments. Yet even among these fragments, occasional blocks 
may he detected which have been detached from the rocks forming the walls 
of the funnel. 
The general name of Agglomerate, as already stated, is given to all 
accumulations of coarse, usually unstratified, detritus in volcanic funnels. 
