114 
Proceedings of the Royal Society 
The boulder consists of a coarse-grained granite. But in Iona 
there is no granite rock of any kind. The prevailing rock is a fine- 
grained gneiss, approaching in many places to clay slate. 
Captain Stewart of Coll was with the Convener when he examined 
the boulder. On breaking off portions from it, and also from 
another small boulder lying below, exactly similar in composition, 
he at once said, “ This is Coll granite.” 
These Iona boulders, in respect both of situation and position, 
undoubtedly indicate, that they were lodged by some agent which 
brought them from the N. or N.W. That agent had stranded 
upon the hill and stuck there till the boulders dropped from it. 
From no eastern quarter could the boulders have reached their 
position. Their site is 250 feet above the sea. The hill on which 
they are, being 350 feet high, and forming a ridge of about a quarter 
of a mile running north and south, would preclude access from any 
eastern point. 
The granite in the Boss of Mull, situated to the east of Iona, is 
different in composition from that of the boulders now referred to. 
On the east side of Iona there are granite boulders, similar to the 
Mull granite, as mentioned in the Committee’s second Beport. But 
the boulders on the FT. W. shoulder of “ Dun-Ii ” are larger grained 
and of a different colour; and they occupy a level considerably 
above most of the granite rocks at the Boss of Mull. 
With reference to Captain Stewart’s remark, as to the large 
boulder above referred to being of the same kind of granite as in the 
island of Coll, the suggestion is so far favoured by the position of 
Coll, which hears about N.N.W. from Iona, and is distant about 
20 miles. But on the other hand, the Convener must state that 
when he visited the island of Coll a few days afterwards, he found 
that the rocks everywhere were gneiss, and with only occasional 
veins of granite. The boulders he saw on Coll were of granite. 
II. ISLAND OF TIREE. 
1. Heynish Hill, situated near the S.W. end of the island, 
reaches a height of about 600 feet above the sea-level. This hill 
consists chiefly of gneiss rock, though in some parts the ingredients 
become so coarse as to pass into granite. 
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