70 
SUBTERRANEAN VOLCANIC ACTION 
BOOK I 
succeeded by a third (Kg. 29). . The three cones and craters of the little 
island of Volcanello supply a singularly perfect recent instance of this 
structure (Fig. 214). Here the funnel has twice shifted its position, each 
cone becoming successively smaller and partially effacing that which pre- 
ceded it. In Auvergne, the Puy de Pariou has long been celebrated as 
an example of a fresh cinder-cone partially effacing an earlier one. In the 
much denuded Paheozoic volcanic tracts of Britain, where the cones have 
long since disappeared and only the stumps of the volcanic cylinders are 
left, many illirstrations occur of a similar displacement of the funnel, especi- 
ally among the volcanoes of the Carboniferous system. 
Among the irregularities of necks that may indicate a connection with 
lines of fissure, reference may be made here to dykes or dyke-like masses 
of agglomerate which are sometimes to be seen among the volcanic districts 
of Britain. In these cases the fragmentary materials, instead of lying in a 
more or less cylindrical pipe, appear to fill up a long fissure. We may 
suppose that the explosions which produced them did actually occur in 
Fig. 29. — Successive sliiftiiigs of vents giving rise to douhle or triple cones. 
A, ground-plan ; B, vertical section. 
fissures instead of in ordinary vents. The remarkable Icelandic fissures 
with their long rows of cinder cones are doubtless, at least in their upper 
parts, largely filled up with slag and scorim. Some illustrations of this 
structure will be given in the account of the Carboniferous volcanic rocks 
of Scotland (see No. 1 in Fig. 22). 
There is yet another consideration in regard to the form and size of 
necks which deserves attention. Where the actual margin of a neck and 
its line of vertical junction with the rocks through which it has been 
drilled can be seen, there is no room for dispute as to the diameter of the 
original funnel, which must have been that of the actual neck. But in 
many cases it is impossible to observe the boundary ; not merely because 
of superficial soil or drift, but occasionally because the volcanic detritus 
extends beyond the actual limits of the funnel. In sucli cases the necks 
have retained some portion of the origmal volcanic cone which accumulated 
on the surface around the eruptive vent. It may even chance that what 
appears to he a large neck would be considerably reduced in diameter, and 
might be shown to mclude more than one pipe if all this outer casing could 
be removed from it. In Fig. 30, for example, a section is given of a neclc in) 
