CHAP. V 
METAMORPHISM AROUND VENTS 
IT- 
from which on the right-hand side all the cone and surrounding tuffs (^) 
have been removed by denudation, the original form of the volcano being 
suggested by the dotted lines. On the left side, however, the tuffs which 
were interstratified with the contemporaneous sediments are still connected 
with the neck, denudation not having yet severed them from it. The over- 
lying strata Q., 1) which originally overspread the extinct volcano liave been 
Fig. 30. — Section to show the coimection of a uech with a cone and surrounding bedded tuffs. 
bent into an anticline, and the neck of the vent has thus been laid bare by 
the removal of the crest of the arch. 
The instances where a structure of this kind is concealed are probably 
fewer in number in proportion to their antiquity. But among Tertiary 
cones they may perhaps not be so rare. The possibility of their occurrence 
should be kept in view during the investigation of extinct volcanoes. The 
term IS'eck ought not properly to be applied to such degraded volcanic 
cones. The true neck still remains preserved in the inside of them. As 
illustrative of the structure here referred to, 1 may cite the example of the 
Saline Hill (Fig. 148) and of Largo Law (Fig. 226), both in Fife. 
iv. Metamorphism in and around Volcanic Vents — Solfataric Action 
The prolonged ascent of hot vapours, stones, dust and lava, in the 
funnel of a volcano must necessarily affect the rocks through which the 
funnel has been driven. We may therefore expect some signs of alteration 
in the material forming the walls of a volcanic neck. The nature of the 
metamorphism will no doubt depend, in the first place, on the character and 
duration of the agents producing it, and in the second, on the suscexitibility 
of the rocks to undergo change. Mere heat will indurate rocks, baking 
sandstone, for instance, into quartzite, and shales into porcellanite. But 
there will almost invariably be causes of alteration other than mere high 
temperature. Water- vapour, for mstailce, has probably always been one of 
the most abundant and most powerful of them. Tlie copious evolution of 
steam from volcanoes is one of their most characteristic features at the 
present day, and that it was equally so in past time seems to be put beyond 
question by the constantly recurring vesicular structure in ancient lavas 
and in the lapilli and ejected blocks of old agglomerates and tuffs. Direct 
experiment has demonstrated, in the hands of various skilful observers. 
