CHAP. XLVII 
THE ACID BOSSES OF MULL 
397 
certainly the most extraordinary. So little disturbed are the lavas, that 
one’s first impulse is to search for pebbles of the granophyre between the 
basalts, for it seems incredible that the inner rock should be anything but a 
central core of older eruptive material, against and round which the younger 
basic rocks have flowed. But, though the granophyre is so decomposing and 
covers its slopes with such “ screes ” of debris, that had the basalts been 
poured round it, they must infallibly have had some of its fragments washed 
down between their successive flows, not a single pebble of it is there to be 
found This might not be considered decisive evidence, but it is extended 
and confirmed by the fact that the acid rock gives off veins which ramify 
through the basalts. 
Before examining the actual contact of the two rocks, however, the 
geologist will not fail to observe here an admirable example of the gradual 
change which was described in the foregoing chapter as coming over the 
bedded basalts near the acid bosses. As he approaches the nucleus of white 
rock, the basalts assume the usual hard indurated character, not decaying 
into brown sand as on the plateaux, but often standing out as massive crags 
with vertical clean-cut joint-faces. This metamorphosed condition extends 
in some cases to a considerable distance from the main body of acid rock, 
especially where knobs of that material, protruding through the more basic 
lavas, show that it must extend in some mass underneath. Thus along the 
shore at Saline the bedded basalts succeed each other in well-defined sheets, 
some being solid, massive and non-amygdaloidal, others quite vesicular, and 
recalling the black seoriform surfaces of recent Vesuvian lavas; yet they 
are all more indurated than in the normal plateau-country, and they break 
with a hard splintery fracture. Immense numbers of dykes cut these rocks, 
and they are likewise pierced by occasional felsitic intrusions. 
If we cross to the other side of the island and trace the bedded basalts 
away from the central masses of acid rock we meet with so gradual a diminu- 
tion of the induration that no definite boundary-line for the metamorphism 
can be drawn. As we recede from the centre of alteration, the rocks 
insensibly begin to show brown weathered crusts, with spheroidal exfolia- 
tion, the reticulations of epidote and calcite become much less abundant, 
the amygdaloids gradually assume their .normal earthy character, and 
eventually we find ourselves on the familiar types of the plateau. This 
transition is well seen along the shores of Loch na Keal. 1 
These proofs of the alteration of the plateau-basalts are accompanied in 
Mull as in Skye by further abundant evidence that the acid rocks are of 
younger date than the basic. In particular, dykes and veins may be traced 
proceeding from the former and intersecting the latter. Thus, in the bed 
of the south fork of the Scarrisdale stream, a separate mass of granophyre 
(which under the microscope exhibits in perfection the characteristic struc- 
ture of this rock) protrudes through the basalts in advance of the main 
mass, and a little higher up on the outskirts of that mass narrow ribbons 
1 Some of the thick massive sheets of basic rock along the south side of this inlet may 
possibly be altered sills. 
