66 
SUBTERRANEAN VOLCANIC ACTION 
BOOK I 
granophyre. Many intermediate varieties may be found between these 
extremes. It is notew'orthy 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. 
(4) iVec/iJs with Dykes, Veins, or irregular intr%isions 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 
anotlrer structure which seems to 
point to the ascent of lava after 
the funnel has been choked up. 
N’umerous instances have been 
observed where lava has been 
forced upwmrd 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 Car- 
boniferous and I’ermiaii 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 theii’ forms, the 
lava having evidently been driven through a mass of material which, not 
having yet consolidated sxrfficiently to acquire a jointed structure, afforded 
few dominant lines of division along which it could ascend. How 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 frag- 
mentary 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 Buy 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 
Fig. 27. — Section of agglomerate neck {aa) with 
dykes and veins {b h). 
