CHAPTEE VI 
Underground Phases of Volcanic Action — continued. II. Subterranean Movements of the 
Magma : i. Dykes and Veins; ii. Sills and Laccolites ; iii. Bosses (Stocks, Culots) — 
Conditions that govern the Intrusion of Molten Rock •within the Terrestrial Crust. 
II. SUBTERKANEAN MOVEMENTS OF THE MaGMA 
In the foregoing pages attention has been more specially directed to those 
aspects of volcanic energy which reveal themselves above ground and in 
eruptive vents. We have now to consider the various ways in which the 
molten magma is injected into the crust of the earth. 
Such injection must obviously take place during the expulsion of vol- 
canic materials to the surface. If the explosive violence of an eruption, 
or the concomitant movements of the earth’s crust, should lead to ruptures 
among the subterranean rocks, the molten magma will be forced into these 
rents. It is evident that this may happen either with or without any 
discharge of lava at the surface. It may be either entirely a plutonic, that 
is, a deep-seated phenomenon, or it may be part of a truly volcanic series 
of events. 
It is clear that, by the study of old volcanoes that have had their 
structure laid bare by denudation, we may hope to obtain fresh light in 
regard to some of the more deeply-seated features of volcanic energy, which 
in a modern volcano are entirely concealed from view. A little reflection 
will convince us that the conditions for consolidation within the crust are 
so different from those at the surface that we may expect them to make 
themselves visible in the internal characters of the rocks. 
An essential distinction between underground propulsions of molten 
rock and superficial outflows of the same material lies in the fact that 
while the latter are free to take any shape which the form and slope of 
the ground may permit, the subterranean injections, like metal poured into 
a mould, are always bounded by the walls of the aperture into which they 
are thnist. According, therefore, to the shape of this aperture a con- 
venient classification of such intrusions may be made. Where the molten 
material has risen up vertical fissures or irregular cracks, it has solidified 
as Dykes and A^eins. Where it has been thrust between the divisional 
planes either of stratified or unstratified rocks, so as to form beds, these are 
conveniently known as Sills, Laccolites or Intrusive Sheets. Where it has 
taken the form of large cylindrical masses, which, ascending through the 
