CHAP. 
NECKS WITH LA VA-PLUG 
65 
of tliG liiciblo tuffs uro crowiiGci or iiitGrru^Dtcd. by more craggy fotiturcs, 
whicli mark the position of the liarder intrusive rock. 
The plug, like the pipe up which it has risen, is in general irregularly 
circular in ground-plan. It may be conceived to be a column of rock, 
descending to an unknown depth into the interior, with a easing of 
pyroclastic debris surrounding it. It may vary considerably in the pro- 
portion which its cross-section bears to that of the surrounding fragmental 
material. Sometimes it does not occupy more than a small part of the 
whole, often appearing in the centre. 
In other cases, it more than equals 
all the rest of the material in the 
vent, while instances may he noted 
where only occasional jjatches of 
tuff or agglomerate are visible be- 
tween the lava-plug and the wall 
of the pipe. From these we natur- 
ally pass to the second type of vent, 26.— Suction of neck of agglomerate (a a) with 
where no fragmentary material is to 
be seen, but where the chimney is now entirely filled with some massive 
once-molten rock. 
A neck with a lava-plug probably contains the records of two stages in 
volcanic progress, the first of which, indicated by the tuff or agglomerate, 
was confined to the discharge of fragmentary materials ; while the second, 
showm by the lava -plug, belonged to the time when, after the earlier- 
explosions, lava ascended in the vent and solidified there, thus bringing 
the eruptioirs from that particular orifice to an end. Where a small central 
column of lava rises through the tuff, we may suppose that the funnel had 
been mainly choked up by the accumulation in it of ejected detritus, which 
was compacted to a solid mass adhering to the wall of the funnel, but 
leaving a central orifice to be kept open by the gradually waning energy of 
the volcano. By a final effort that impelled molten rock up that duct and 
allowed it to consolidate there, tlie operations of the vent were brought 
to a close. 
Where, on the other hand, only occasional strips of tuff or agglomerate 
are to be found between the lava-plug and the wall of the pijie, the last 
uprise of lava may be supposed to have been preceded by more vigorous 
explosions which cleared the throat of the volcano, driving out the accumu- 
lated detritus and leaving only scattered patches adhering to the sides of 
the funnel. 
There is, 110 doubt, some downward limit to the production of fragmentary 
material, and if we could lay bare successive levels in the chimney of 
a volcano ve should find the agglomerate eventually replaced entirely 
by lava. 
The materials of the lava-plugs vary widely in composition. Sometimes 
they are remarkably basic, and present rocks of the picrite or limburgite 
type ; in other cases they are thoroughly acid rocks such as felsite and 
VOL. I If 
