CHAP. XLVII 
THE ACID BOSSES OF MULL 
40 
basalt-debris, there were ejected many pieces of different felsitic or rhyolitic 
rocks, and that these eruptions of fragmentary material took place during 
the accumulation of the plateau-basalts. These volcanic funnels occasioned 
a series ol points or a line of weakness of which, in a long subsequent episode 
of the protracted volcanic period, the acid rocks took advantage, forcing 
themselves upwards therein, and leaving only slight traces of the vents 
which assisted their ascent. The mingling of acid and basic fragments in 
the material ejected from these vents is another proof of the existence of 
acid rocks in the volcanic reservoirs before the advent of the great grano- 
phyre intrusions. The evidence thus entirely confir m s the conclusions 
deduced from the Skye area. 
The second or Glen More boss, instead of rising into hilly ground, is 
confined to the bottom of the main and tributary valleys, and has only been 
revealed by the extensive denudation to which these hollows owe their 
i IG. 357. — Section of junction of south side of Locli li;i' granophyre boss, with 
the bedded basalts, Mull. 
a, bedded basalts ; b b, basalt-tuff and breccia ; c, granophyre ; il, black felsite ; e, coarse dolerite dvke 
SO or 40 feet wide. ’ 
origin. It begins nearly a mile below Torness and extends up to Loch 
Airdeglais — a distance ol almost four miles. Though singularly devoid of 
topographical feature, it exhibits with admirable clearness the relation of 
the granophyres to the gabbros, and thus deserves an important place among 
the tracts ol acid rocks in the Western Islands. Its petrographical char- 
acters change considerably from one part of its body to another. For the 
most part, it is a true granophyre, sometimes with orthoclase, sometimes 
with plagioclase as its predominant felspar. At Ishriff, as already stated, it 
is sprinkled with long acicular decayed crystals of hornblende; but at the. 
watershed the ferro-magnesian mineral is augite. The surrounding rocks 
are mainly the plateau-basalts, with their* sills of dolerite and gabbro. 
This strip of granophyre sends abundant apophyses from its mass into 
the dark basic rocks around it. Some of the best sections to show the 
nature of these offshoots are to be found on the steep hill-slope which 
mounts from the watershed in Glen More southward into the Creao- na 
vol. 11 2d 
