CHAP. XLVII 
THE ACID BOSSES OF SMALL ISLES 
can be traced upward into the main body of coarse gabbro, forming the 
ridge of Ben Buy. Some of them are of the usual granular granophyric 
texture, others are dull and fine-grained (claystones of the older authors). 
Hence it is evident that the granophyres of Mull have been protruded 
not only after the accumulation of the plateau-basalts, but after these were 
traversed by the sheets and veins of gabbro. The amount of acid rock 
injected into these older rocks over the mountainous part of the island is 
enormous ; but I reserve further reference to it for the section on acid 
Dykes and Veins, for these are the forms in which it chiefly occurs in that 
region. It should be added, that in the localities here referred to basalt- 
veins and dykes are generally abundant, cutting through all the other 
rocks (Fig. 359). So numerous are they that the geologist ceases to take 
note of them when his thoughts are engaged upon the problems presented 
by the masses through which they rise. 
iii. THE ACID BOSSES OF SMALL ISLES 
In the island of Eigg three small bosses or sheets of acid rock occur. 
That at the northern end rises through the Jurassic sedimentary rocks, and 
forms a bold cliff from 150 to 200 feet high. It is a light grey grano- 
phyric porphyry, with rounded blebs of quartz in a micropegmatic base 
of quartz and felspar. The other two masses, of smaller size, cut throimh 
the bedded basalts 1 (Map VI.). 
In the opposite island of Eum, the acid protrusions play a much more' 
important part. On the east side, of the hills, they occur in sheets at the 
base of the gabbros ; on the west side, they form a large tract of hilly 
ground, which, stretching along the coast -line for about three and a 
half miles from the headland of A’ Bhrideanach to Harris, forms there a 
range of shattered sea-cliffs, that tower for 1000 feet above the Atlantic 
breakers that beat about their base. The area extends inland to the slopes 
on the west side of Loch Sgathaig, a distance of about three and a half 
miles, descending in a range of precipices along its northern front, and 
reaching in its culminating summit, Orval, a height of 1868 feet above the 
sea. The rocks of which this triangular area consists resemble those of the 
Mull bosses. They are chiefly quartz -porphyries, becoming felsitic in 
textuie towards their contact with adjacent rocks. In some places, as was 
noticed by Maceulloch on the sea-cliffs, 2 they have a rudely bedded structure. 
bus on the north-west front of Orval, this structure is shown by parallel 
planes that dip outwards or north-west at 30° to 40°, and which are made 
Aill more distinct by an occasional intrusive dyke or sheet of basalt between 
their surfaces. I have already alluded to indications of an internal arrange- 
ment in the granitoid bosses of Skye (p. 381). 
As in the other islands, the granophyres, porphyries and felsites of Bum 
lave been intruded at the base of the volcanic series. Over much, if not 
a11 , of their area they lie directly on the red Torridon sandstone. That the 
1 Quart. Jourtu Geol. Soc. xxvii. (1871) p. 294. 2 Western Islands., vol. i. p. 487. 
