64 
SUBTERRANEAN VOLCANIC ACTION 
KOOK I 
Wliere vents have been tilled up with tuff rather than with agglomerate, 
the stratified structure is best developed. Alternations of coarser and 
finer detritus give rise to more or less definite layers, which, though in- 
constant and irregular, serve to impart a distinctly stratified character 
to the mass. Where there has been no subsequent disturbance within a 
vent, these layers show the same inward dip towards the centre just referred 
to, at the ordinary angles of repose. Now and then, where a neck with 
Fig. 25. — Neck filled with stratified tufi'. A. gronud plan ; B. transverse section. 
this structure has been laid bare on a beach, its denuded cross-section presents 
a series of concentric rings of strata from the walls towards the centre. 
Good illustrations of these features are supplied by tlie probably Permian 
necks of eastern Pile (Figs. 25 A and 217).^ 
It has frequently happened, however, that, owing to subsidence of the 
materials filling up the vents or to later volcanic disturbances, the compacted 
tuffs have been broken up and tlirowu into various positions, large masses 
being even placed on end. Among the Carboniferous and Permian necks 
of Central Scotland such dislocated and vertical tuffs are of common occur- 
rence (see Figs. 145, 218). If, as is probable, we arc justified in regarding 
the stratified parts of necks as indicative of the uppermost parts of vol- 
canic funnels, not far from the surface, the importance of this inference 
will be best understood when the Carboniferous and Permian volcanoes are 
described. 
(o) Necks with a central Lava-phuj . — Some vents of agglomerate or tuff 
are pierced by a plug of lava, as may be instructively seen in many of the 
Carboniferous and Permian necks of the centre and south of Scotland (Fig. 
20; compare also Figs. 148, 174, 207, and 226). Where this structure 
shows itself, the contrast in hardness and durability between the more 
destructible fragmentary material and the solid resisting lava leads to a 
topographical distinction in the outer forms of necks. The smooth declivities 
^ See also the sections of vents on the west coast of Stromo Faroes, above referred to. 
