94 
SUBTERRANEAN VOLCANIC ACTION 
BOOK I 
more alteration contiguons to numerous true necks than around bosses, which 
there is no reason to suppose ever communicated directly with the surface, 
and which Avere probably the result of a single intrusion. We must always 
remember that the denudation which has revealed these bosses has generally 
removed the evidence of their upward termination and of their possible 
connection with any volcanic ejections. Many of them may mark the sites 
of true vents from which only single eruptions to(jk place. The opening of 
a volcanic A'ent does not necessarily imply a prolonged ascent of volcanic 
material. In a vast number of cases the original eruption Avas the first and 
last effort of the volcano, so that in such circumstances there seems no more 
reason for much alteration of the walls of the chimney than for the meta- 
morphism of the rocks round a boss, laccolite, sill or dyke. 
The metamorphism produced by intrusions of molten material upon the 
rocks Avith Avhieh they have come in contact has long been studied. Its 
amount varies so greatly in different cases that the conditions on which it has 
specially depended are not easily determined. Three factors have obAdously 
been of great importance — first, the bulk of the intruded material ; secondly, 
the chemical composition and lithological texture and structure of the 
rocks affected; and thirdly, the constitution and temperature of the in- 
vading magma. 
1. It is clear that a huge boss of eruptive material Avill be likely to 
effect much more alteration of the surrounding rocks than a small boss, sill 
or dyke. Its initial temperature will probably bo higher at the time of its 
assuming its final place than that of the same material after it has found 
its way into the narroAver space of a thin sill or dyke. It will likewise 
take much longer to cool. Hence the infiuence of its heat and its Aurpours 
Avill continue to act long after those of the dyke or sill have ceased to 
manifest themselves. 
2. It is equally evident that much of the resultant metamorphism Avill 
depend on the susceptibility of the rocks to change. An obdurate material 
such as pure quartz-sand, for example, Avill resist further alteration than 
mere hardening into quartzite. Shales and mudstones may be indurated 
into cherty substances of various textures. Limestones and dolomites, on 
the other hand, may become entirely crystalline, and may even have new 
minerals, such as garnet, tremolite, pyroxene, etc., developed in them. Hence 
in compai’ing the amount of metamorphism attendant on two separate bosses 
we must always take into account the nature of the rocks in which it 
has been induced. 
3. But perhaps the most effectiA''e cause of A'ariatiou in the nature and 
amount of contact metamorphisin has been the constitution of the eruptive 
magma. A broad distinction may be drawn between the alteration pro- 
duced by basic and by acid rocks. The intrusion of basic material has 
often produced singularly little change, even Avhen the eruptive mass has 
been of considerable size. The greatest amount of alteration is to be found 
where the basic boss has caught up and enveloped portions of the surrounding 
rocks. Thus where the gabbro of Carrock Fell has invaded the basic 
