CHAP. VI 
99 
BOSSES AND SILLS 
undulations, one crossing the other, there would he limited dome-shaped 
elevations at the intersections of these waves, and if at the same time actual 
rupture of the crust should take place, the magma might find its way 
upward under such domes and give rise to the formation of laccolitic 
intrusions. Cessation of the earth -movements might allow the intruded 
material slowly to solidify without ever making an opening to the surface 
and forming a volcano. Doubtless many sills, laccolites and bosses repre- 
sent such early or arrested stages in volcanic history. 
Propelled into the crust at a high temperature, and endowed with great 
energy from the tension of its absorbed vapours and gases, the magma will 
avail itself of every rent which may be opened in the surrounding crust, 
and where it has succeeded in reaching tlie surface, its own explosive 
violence may enable it to rupture the crust still fui’ther, aud open for 
itself many new passages. Thus an eruptive laccolito or boss is often fringed 
with veins, dykes and sills which proceed from its mass into the rocks around. 
Tlie question how far an ascending mass of magma can melt down 
its walls is one to which no definite answer can yet be given. Recent 
observations show that where the difference in the silica percentage between 
the magma and the rock attacked is great, there may be considerable 
dissolution of material from this cause. Allusion has already been made 
to Mr. Harker’s computation that some of the acid granophyres of Skye have 
melted down about a fourth of their bulk of the basic gabbros. If such a 
reaction should take place between the magma of a boss, sill or laccolite and 
the rocks among wliich it has been intruded, great changes might result in 
the composition of the intruded rock. We are not yet, however, in 
possession of evidence to indicate that absorption of this kind really takes 
place on an extensive scale within the earth’s crust. If it did occur to a 
large extent, we shoirld expect much greater varieties in the composition of 
eruptive rocks than usually occur, and also some observable relation between 
the composition of the igneous material and that of the rocks into which it 
has been injected. But enough is not yet known of this subject to warrant 
any decided opinion regarding it. 
