CHAPTER XL VIII 
THE ACID SILLS, DYKES AND VEINS 
i. THE SILLS 
Not only have the acid rocks been protruded in small and large bosses, they 
have also been injected as sills between the bedding-planes of stratified rocks, 
between the surfaces of the basalt-beds, and between the bottom of the 
plateau-basalts or of the gabbros and the platform of older rock on which 
the volcanic series has been piled up. Every gradation of size may be 
observed, from mere partings not more than an inch or two in thickness, up 
to massive sheets, which now, owing to the removal of their original covering 
of rock by denudation, form minor groups and ranges of hills. Where the 
sheets are numerous, they are usually small in size ; where, on the other 
hand, they are few in number, they reach their greatest dimensions. 
It is not always possible to discriminate between bosses and large irregular 
sills. A good illustration of the connection between these two forms of 
intrusion will be cited from the island of Iiaasay, where a widespread intru- 
sive sheet is in part connected with a true boss. 
In Mull, sills of acid eruptive rocks are profusely abundant throughout 
the central mountainous tract between Loch na Keal and Loch Spelve. If we 
ascend the slopes from the Sound of Mull, for instance, we have not gone far 
before some of these sheets make their appearance. They are usually dull 
granular quartz-porphyries, or granophyres, often only two or three feet in 
thickness, and interposed between the beds of basalt that form the mass of 
the hills. Along the crest of the ridge that stretches through Beinn 
Chreagach Mhor to Mainnir nam Fiadh they take a prominent place among 
the ledges of basalt, basalt-conglomerate and dolerite. The largest sheet in 
Mull is probably that which has thrust itself between the base of the basalts 
and the underlying J urassic strata and crystalline-schists on the shore of the 
Sound of Mull at Craignure. The porphyry of this sheet is referred to by 
Professor Zirkel as only a finer-grained variety of the same quartziferous 
rock, with hornblende and orthoclase crystals, which in Skye breaks through 
the Lias . 1 On the south coast also, at the base of the thick basalt series, 
similar porphyries have been injected into the underlying strata ; and under 
1 Zeitseh, Deutsch. Geol. Gcsellsch. xxiii. p. 54. 
