CHAP. V 
RELATION OF VENTS TO SURROUNDING ROCKS 
73 
they would permeate this material, and in the end produce in it a series of 
changes similar to, and possibly even more pronounced than, those traceable 
in the walls of the vent. Instances of this kind of metamorphism will be 
cited in the following chapters (see in particular p. 404). 
V. Invjcml Dip of Rochs toivards Necks 
One concluding observation requires to be made regarding the rela- 
tion of old volainic necks to the rocks which immediately surround them. 
Where a vent has been opened through massive rocks, such as granite, 
felsite, andesite or basalt, it is generally difficult or impossible to determine 
whether there has been any displacement of these rocks, beyond the 
disruption of them caused by the explosions that blew out the orifice. 
But where the pipe has been drilled through stratified roclcs, especi- 
ally when these still lie nearly flat, the planes of stratification usually 
supply a ready test and measure of any such movement. Investigation of 
the volcanic rocks of Britain has shown me that where any displacement 
can 1)6 detected at a neck, it is almost invariably in a downward direction. 
The strata immediately around the vent tend to dip towai-ds it, whatever may 
be their prevalent inclination in the ground beyond (Big. 24). This is the 
reverse of the position which might have been expected. It is so frequent, 
however, that it appears to indicate a general tendency to subsidence at the 
sites of volcanic vents. After copious eruptions, large cavernous spaces may 
conceivably be left at the roots of volcanoes, and the materials that have tilled 
the vents, losing support underneath, will tend to gravitate downwards, and 
if firmly welded to their surrounding walls may drag these irregularly down 
with them. Examples of such sagging structures are abundantly to be seen 
among the dissected vents of the Carboniferous and Permian volcanic series 
of Scotland. 
vi. Influence of Conteni'porancous DcnuclaMon upon Volcanic Cones 
It must be remembered that former vents, except those of the later geo- 
logical periods, are revealed at the surface now only after extensive denudation. 
As a rule, the volcanoes that formed them appeared and continued in erup- 
tion during periods of general subsidence, and were one by one submerged 
and buried beneath subaqueous deposits. We can conceive that, while a 
volcanic cone was sinking under water, it miglit be seriously altered in form 
and height by waves and currents. If it consisted of loose ashes and stones, 
it might be entirely levelled, and its material might be strewn over the floor 
of the sea or lake in which it stood. But, as has been already pointed out, 
the destruction of the cone would still leave the choked-up pipe or funnel 
from which the materials of that cone had been ejected. Though, during 
the subsidence, every outward vestige of the actual volcano might disappear, 
yet the agglomerate or lava that solidified in the funnel underneath would 
remain. And if these materials had risen some way within the cone or 
