

Parr VIL Secr.i.§1.] ERUPTIVE BOSSES. 539 
rock within the earth’s crust may be thus stated: Every fluid mass 
impelled upwards by pressure from below, or by the expansion of its ° 
own imprisoned vapour, has sought egress along the line of least 
resistance. That line has depended in each case upon the structure 
of the terrestrial crust and the energy of eruption. It may have been 
determined by an already existent dislocation ; by planes of strati- 
fication, by the surface of junction of two unconformable formations, 
by irregular contemporaneously formed cracks, or by other more com- 
plex lines of weakness. Sometimes the intruded mass has actually 
fused and obliterated some of the rock which it has invaded, incor- 
porating a portion into its own substance. ‘The shape of the channel 
of escape has thus determined the external form of the intrusive mass, 
as the mould regulates the form assumed by cast-iron. This re- 
lation offers a very convenient means of classifying the intrusive 
rocks. According to the shape of the mould in which they have 
solidified, they may be arranged as—(1) bosses or amorphous 
masses, (2) sheets, (3) veins and dykes, and (4) necks. 
§ 1. Bosses. 
Bosses or amorphous masses consist chiefly of crystalline coarse- 
textured rocks. Granite and syenite are the most conspicuous, but 
various quartz-porphyries, felsites, diorites, trachytes, dolerites, &e., 
also occur. Where rocks assume this form as well as that of sheets, 
dykes, and contemporaneous beds, it is commonly observed that 
they are more coarsely crystalline when in amorphous masses than 
in any other form. WDoleritic rocks afford many examples of this 
characteristic. In the basin of the Forth, for instance, while the 
outflows at the surface have been fine-grained basalts and anamesites, 
the masses consolidated underneath have generally been coarse 
dolerites and diabases.* 
Granite.—It was once a firmly-held tenet that granite is the 
oldest of rocks, the foundation on which all other rocks have been 
laid down. This idea no doubt originated in the fact that granite is 
found rising from beneath gneiss, schist, and other crystalline masses, 
which in their turn underlie very old stratified formations. The 
intrusive character of granite, shown by its numerous ramifying 
veins, proved it to be later than at least those rocks which it had 
invaded. Nevertheless the composition and structure of goneiss and 
mica-schist were believed to be best explained by supposing these 
rocks to have been derived from the waste of granite, and thus, 
though the existing intrusive granite had to be recognized as 
posterior in date, it was regarded as only a subsequent protrusion 
of the vast underlying granitic crust. In this way the idea of the 
primeval or fundamental nature of granite held its ground. From 
what is known regarding the fusion and consolidation of rocks 
(ante, p. 292, seq.) ; and from the evidence supplied by the microscopic 
1 Trans. Roy. Soc. Edin. xxix. p. 493 (1879). 
