72 
SUBTERRANEAN VOLCANIC ACTION 
BOOK I 
from the time of Sir James Hall to that of Professor Daubree, how power- 
fully rocks are acted upon when exposed to superheated vapour of water 
under great pressure. But the steam of volcanoes often contains other 
vapours or mineralizing agents dissolved in it, which increase its meta- 
morphic influence. The mineral acids, for instance, must exert a powerful 
effect in corroding most minerals and rocks. At the Solfatara of Naples 
and at other volcanic orifices in different parts of Italy, considerable altera- 
tion is seen to be due to this cause. 
Be.'iring these well-known facts in mind, we may be prepared to find 
various proofs of metaraorphisrn around and wdthin old volcanic vents. 
The surrounding rocks are generally much hardened immediately con- 
tiguous to a neck, whether its materials be fragmental or massive. Sand- 
stones, for example, are often markedly bleached, acquire the vitreous lustre 
and texture of quartzite, lose their usual fissility, break irregularly into angular 
blocks, and on an exposed surface j^roject above the level of the unaltered 
parts beyond. Shales are baked into a kind of porcelain-like substance. 
Coal-seams are entirely destroyed for economic purposes, having been burnt 
into a kind of cinder or fused into a blistered slag-like mass. Limestones 
likewise lose their usual bluish -grey tint, become white and hard, and 
assume the saccaroid texture of marble. 
The distance to which this metamorphism extends from the wall is, 
among the exposed necks in Britain, smaller than might be anticipated. 
Thns I have seldom been able to trace it among those of Carboniferous or 
Permian age for more than 15 or ilO yards in ordinary arenaceous and 
argillaceous strata, even where every detail of a neck and its surroundings 
has been laid bare in plan upon a beach. The alteration seems to reach 
furthest in carbonaceous seams, such as coals. 
It is evident that the element of time must enter into the question of 
the amount of metaniorphism produced in the terrestrial crust immedi- 
ately surrounding a volcanic pipe. A volcano, of which the eruptions 
begin and end within an interval of a few days or honrs, cannot be 
expected to have had much metamorphic influence on the rocks through 
which its vent was opened. On the other hand, around a funnel which 
served for many centuries as a channel for the escape of hot vapours, ashes 
or lava to the surface, there could hardly fail to be a considerable amount of 
alteration. The absence or comparatively slight development of meta- 
morphism at the Carboniferous and Permian necks of Scotland may perhaps 
be regarded as some indication that these volcanoes were generally short- 
lived. On the other hand, more extensive alteration may be taken as 
pointing to a longer continuance of eruptive vigour. 
The same causes which have induced metaniorphism in the rocks sur- 
rounding a volcanic vent might obviously effect it also among the frag- 
mentary materials by which the vent may have been filled up. When the 
eruptions ceased and the funnel was left choked with volcanic debris, hot 
vapours and gases would no doubt still continue for a time to find their 
way upward through the loose or partially compacted mass. In their ascent 
