
200 STRUCTURAL AND FIELD GEOLOGY 
denudation would well account for their disappearance, for not only 
have the volcanic cones been removed, but the surface upon which these 
were built up has also often been carried away. In the case of those 
necks which contain no igneous materials, but are filled exclusively with 
the débris of derivative rocks, it 1s clear that if at the time of eruption 
molten matter was present at all, it could only have been at a relatively 
great depth. The character of the débris, at all events, shows that only 
explosive vapours escaped by such pipes and funnels. While there is 
reason to believe that some necks may represent subaérial volcanoes, 
not a few are certainly of subaqueous origin. In the former case no 
trace of the old cones or the surface upon which they were accumulated 
has been preserved, the pipes alone remain to tell their tale. From the 
fact, however, that these sometimes contain quantities of coniferous wood, 
which from its appearance must have been buried in a fresh state, it has 
been inferred that some volcanoes were probably subaérial, and that 
after their extinction they became clothed with a coniferous vegetation. 
The majority of the necks met with in Scotland, however, would seem 
to represent subaqueous volcanoes. This is suggested by the simple fact 
that the cones are occasionally preserved—which could hardly have 
happened had the volcanoes erupted upon a land-surface. The volcanoes 
referred to obviously discharged their ejecta upon the gradually subsiding 
bed of sea, lagoon, oer lake, and thus the sheets of materials that accumu- 
lated round the vents passed outwards in all directions and became 
interstratified with sediments, charged with the organic remains of the 
period. When at last the volcanoes became extinct, they were finally 
covered up by successive deposits of sediment, and thus the cones 
escaped the denudation that ere long must have demolished them had 
they been formed upon dry land (see Fig. 72). 





Fic. 72.—CONE OF AGGLOMERATE, AND NECK OF CRYSTALLINE 
IGNEOUS ROCK. 
The inward dip of the strata surrounding a neck has been attributed 
to that sinking of surface which so frequently takes place near a volcanic 
centre. After prolonged activity the rocks surrounding a vent probably 
become undermined, and this must tend to bring about subsidence in its 
immediate neighbourhood. In the case of extensive necks, from which 
much material has been discharged, the inward dip of the surrounding 
strata may be due to some such cause. A large number of the necks, 
however, are too small and erupted for too short a period to have 
produced any marked subsidence of the surrounding rock-masses—and 
yet the abrupt inward dip of the strata surrounding such necks is quite as 

