CHAP. V 
NECKS FILLED WITH LAVA 
67 
of agglomerate belong to one of the closing phases in tire history of the 
vent.s in which they occur. They could only have been injected after the 
pipes had been so choked up that explosions had almost or entirely ceased, 
and eruptions had consecpiently become nearly or quite impossible. ThcA^ 
show, however, that volcanic energy still continued to manifest itself by 
impelling the molten magma into these extinct funnels, while at the same 
time it may have ))een actively discharging materials from other still open 
vents in the same neighbourhood. 
With regard to the composition of these dykes and veins, it may be 
remarked that in a district of acid lavas they may be expected to be felsitic 
or rhyolitic, sometimes granophyric. Where, on the other hand, the lavas 
poured out at the surface have been intermediate or basic, the veins in the 
necks may be andesites, basalts or other still more basic compounds. But 
it is observable, as in the case of the la\n-plugs, that the injections into the 
necks may be much more acid than any of the superhcial lavas. The 
advent of acid material in the later part of a volcano’s history has been 
already alluded to, and many examples of it will be given in this work. 
Aftei all explosions and eruptions have ceased, heated vapours may 
still for a long period continue to make their way upward through the 
loose spongy detritus filling up the vent. The ascent of such vapours, and 
more particularly of steam, may induce considerable metamorphism of the 
agglomerate, as is more particularly noticed at p. 71 . 
ii. Necks of Lava-form Material 
ihe second type of neck is that in which the volcanic pipe has been 
entirely filled up with some massive or crystalline rock. As already 
remarked, it is not always possible to be certain that bosses of rock, having 
the external form of necks of this kind, mark the sites of actual volcanic 
orifices. Eruptive material that has never reached the surface, but has been 
injected into the crust of the earth, has sometimes solidified there in forms 
which, when subsequently exposed by denudation, present a deceptive 
resemblance to true volcanic necks. Each example must be examined by 
itself, and its probable origin must be determined by a consideration of all 
the circumstances connected with it. IWiere other evidence exists of 
volcanic activity, such, for instance, as the presence of bedded tuffs or 
intercalated sheets of lava, the occurrence of neck-like eminences or bosses 
of felsite, andesite, dolerite, basalt or other eruptive rock, would furnish a 
presumption that these marked the sites of some of the active vents of the 
period to which the tuffs and lavas belonged. 
If a neck-like eminence of this kind were found to possess a circular 
01 elliptical ground-plan, and to descend vertically like a huge pillar into 
the Cl list of the earth ; if the surrounding rocks were bent down towards 
it and altered in the manner which I shall afterwards describe in detail ; 
if, moreover, the material composing the eminence were ascertained to be 
closely related petrographically to some parts of the surrounding volcanic 
