4i8 
THE TERTIARY VOLCANOES 
BOOK VIII 
V. THE GRANITE OF ARRAN 
The northern half of the island of Arran is mainly occupied by one of 
the most compact and picturesque groups of granite mountains in Scotland. 1 
These heights, rising out of the Firth of Clyde to a height of 2866 feet, 
present, in their spiry and serrated crests, a contrast to the smoother contours 
of the older granitic elevations of this country. The granite is surrounded 
by a ring of schistose rocks, belonging to the metamorphic series of the 
Southern Highlands, save for a short distance on the eastern margin, where 
it comes in contact with and indurates the Lower Old Bed Sandstone. 
Macculloch long ago pointed out that no pebbles of the granite are to be 
found in the surrounding conglomerates and red sandstones of Carboniferous 
and younger age. 2 Geologists accordingly came to the conclusion that the 
protrusion of the granite took place after Carboniferous time, and hence that 
it had no connection with the appearance of the far older granites of the 
Highlands. In the year 1873 I gave reasons for believing the granite to 
be not only younger than the Carboniferous formations, but to be referable 
with most probability to the Tertiary volcanic series. 3 The progress of 
inquiry has tended to confirm this inference, though no direct proof of 
its correctness has been obtained. Two lines of investigation may be 
pursued, and each leads to the conclusion of the probability of the Tertiary 
age of the granite. One of these proceeds on a comparison of the petro- 
graphical characters of the Arran rocks with those of undoubted members of 
the Tertiary series among the Western Isles. The other inquiry deals with 
the relation of the rocks to each other in the general geological structure of 
Arran itself. 
Macculloch first remarked the strong lithological resemblance of the 
Arran granite to the “syenite,” or granopliyre, of Skye and St. Hilda. 4 
More recent petrographical investigation, as already stated, has furnished 
additional proofs of the connection between the acid rocks of these islands. So 
closely indeed are these rocks linked by megascopic and microscopic characters, 
that the petrologist has no hesitation in placing them together as probably 
products of the same period of igneous activity. 
From the general geological structure of Arran, a further strong argu- 
ment may be deduced in favour of the late date of the eruptions of granite. 
Good reasons have been given for classing as Permian the bright red sand- 
stones which occupy much of the central and southern parts of this island, 
and include the little volcanic group already referred to. These sand- 
stones have been invaded by a complex series of eruptive rocks which would 
thus be later than the Permian period. No igneous masses posterior to this 
1 The rooks of Arran have often been described. Besides the work of Macculloch above 
quoted, reference may he made to the paper by Sedgwick and Murchison, Trans. Geol. Soc. 2nd 
Ser. vol. iii. p. 21 ; A. C. Ramsay's Geology of the Island of Arran, 1841, the paper of Necker 
de Saussure quoted on p. 412 ; J. Bryce’s Geology of Clydesdale and Arran, 3rd edit. 1865. The 
island is at present being surveyed for the Geological Survey by Mr. \V. Gunn. 
2 Description of the Western Islands of Scotland, vol. ii. p. 388. 
2 Trans. Edin. Geol. Soc. vol. ii.- part iii. 4 Description, vol. ii. p. 352. 
