CHAPTER XL VI I 
THE ACID BOSSES OF MULL, SMALL ISLES, ST. HILDA, ARRAN AND THE 
NORTH-EAST OF IRELAND 
ii. THE ACID BOSSES OF MULL 
Though of comparatively small extent, the granophyre bosses of the island 
of Mull afford to the geologist a large amount of instruction in regard to 
the relations of the different members of the volcanic series to each other. 
Especially important is the evidence which they contain of the connection 
between the acid and basic groups of rocks. They have been laid bare in 
many natural sections, some of which, forming entire hillsides, are among 
the most astonishing in the whole wonderful series which, dissected by denu- 
dation, reveal to us the structure of these volcanic regions. They lie in two 
chief areas. One of these extends along the northern Hanks of the moun- 
tainous tract from the western side of Beinn Fhada across Loch Ba’ to the 
west side of Glen Eorsa. The other occupies for over three miles the bottom 
of Glen More, the deep valley which, skirting the southern side of the chief 
group of hills, connects the east side of the island by road with the head of 
the great western inlet of Loch Scridain. There are other minor areas. One 
of these extends for about a mile along the declivities to the south of Salen, 
across the valley of the Allt na Searmoin ; another occurs at Salen ; a 
third runs along the shore at Craignure. In the interior also, many 
isolated areas of similar rocks, besides thousands of veins, occur in the 
central group of hills and valleys which form the basins of the Glencannel 
and Forsa rivers (Map VI.). 
The chief northern boss, which for the sake of convenience of reference 
may be called that of Loch Ba’, has a length of nearly six miles, with a 
breadth varying from a quarter of a mile to about a mile and a quarter. It 
descends to within 5 0 feet of the sea-level, and is exposed along the crest of 
Beinn Fhada at a height of more than 1800 feet. It chiefly consists of 
a grey crystalline rock which might readily be identified as a granite, but 
which when examined microscopically is found to possess the granophyric 
structure. With this distinctly granular-crystalline rock are associated 
various porphyritic and felsitic masses, which pass into it, and are more 
specially observable along its border. An exceedingly compact black 
