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 pipe, 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 accumulated detritus and leaving only scattered patches adhering to the sides of the funnel.
There is, no doubt, some downward limit to the production of fragmentary material, and if we could lay bare successive levels in the chimney of a volcano we 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 granophyre. Many intermediate varieties may be found between these extremes. It is noteworthy that, in districts where the lavas erupted to the surface have been andesitic or basaltic, the material which has finally solidified in the vents is often more acid in composition, trachytic rocks being specially frequent.
Fig. 27.—Section of agglomerate neck (a a) with dykes and veins (b b).
(4) Necks with Dykes, Veins, or irregular intrusions of Lava.—While the presence of a central plug of lava in a neck of fragmental material may indicate that the vent was still to some extent open, there is another structure which seems to point to the ascent of lava after the funnel has been choked up. Numerous instances have been observed where lava has been forced upward through rents in a mass of tuff or agglomerate, and has solidified there in the form of dykes or veins ([Fig. 27]). Illustrations of this structure abound among the Carboniferous and Permian necks of Britain. Here, again, though on a less marked scale, the contrast in the amount and character of the weathering of the two groups of rock gives rise to corresponding topographical features, which are especially observable in cliffs and coast-sections, where the dykes and veins project out of the tuffs as dark prominent walls (Figs. [135], [149], [166], [168], [219], [221], [222]).
These intrusive injections are generally irregular in their forms, the lava having evidently been driven through a mass of material which, not having yet consolidated sufficiently to acquire a jointed structure, afforded few dominant lines of division along which it could ascend. Now and then, however, sharply defined dykes or veins, which at a distance look like dark ribbons, may be seen running vertically or at a high angle, and with a straight or wavy course, through the fine compacted tuff of a vent. Frequently the injected material has found its readiest line of ascent along the walls of the funnel, between the tuff and the surrounding rocks. Occasionally it has made its way into rents in these rocks, as well as into the body of the neck.
It is worthy of remark in passing that complete consolidation of the fragmentary material does not appear to be always requisite in order to allow of the formation of such fissures as are needed for the production of dykes. A singularly interesting illustration of this fact may be seen on the northern crest of the outer crater of the Puy Pariou in Auvergne. A dyke of andesite 8 or 10 feet broad may there be traced running for a distance of about 300 yards through the loose material of the cone. The rock is highly vesicular, and the vesicles have been elongated in the direction of the course of the dyke so as to impart a somewhat fissile structure to the mass.
There can be little doubt that the dykes and veins which traverse necks of agglomerate belong to one of the closing phases in the history of the vents 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 consequently become nearly or quite impossible. They 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 been 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 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.