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 lava-plugs, that the injections into the necks may be much more acid than any of the superficial 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. After 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_ The 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. Where 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 or elliptical ground-plan, and to descend vertically like a huge pillar into the crust 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 series, it might with some confidence be set down as marking the place of one of the active vents from which that series was ejected.