tier of beautiful columns seen in the cliff
above, and known as "The Organ."

[6] Sleamish and several other of the Antrim vents are described by Sir
A. Geikie in the monograph already referred to, _loc. cit._, p. 101, _et
seq._ Also in the _Expl. Memoirs of the Geological Survey of Ireland_.

[7] A diagrammatised section of the Carrick-a-raide volcanic neck is
given by Sir A. Geikie, _loc. cit._, p. 105.

[8] Geikie, _loc. cit._, p. 29, _et seq._

[9] P. 32. The view that the crust of the earth has been horizontally
extended by the intrusion of dykes is noticed by McCulloch in reference
to the dykes of Skye.

[10] Hopkins, _Cambridge Phil. Trans._, vol. vi. p. 1 (1836).

[11] As suggested in my Presidential Address to Section C. of the
British Association at Belfast, 1874.

CHAPTER III.

ISLAND OF MULL AND ADJOINING COAST.

The Island of Mull, with the adjoining districts of Morvern and
Ardnamurchan, forms the more southern of the two chief centres of
Tertiary volcanic eruptions in the West of Scotland, that of Skye being
the more northern. These districts have been the subject of critical and
detailed study by several geologists, from McCulloch down to the present
day; and amongst the more recent, Sir Archibald Geikie and Professor
Judd hold the chief place. Unfortunately, the interpretation of the
volcanic phenomena by these two accomplished observers has led them to
very different conclusions as regards several important points in the
volcanic history of these groups of islands; as, for example, regarding
the relative ages of the plateau-basalts and the acid rocks, such as the
trachytes and granophyres; again as regards the presence of distinct
centres of eruption; and also as regards the relations of the gabbros of
Skye to the basaltic sheets. Such being the case, it would appear the
height of rashness on the part of the writer, especially in the absence
of a detailed examination of the sections over the whole region, to
venture on a statement of opinion regarding the points at issue; and he
must, therefore, content himself with a brief account of the phenomena
as gathered from a perusal of the writings of these and other
observers,[1] guided also to some extent by the analogous phenomena
presented by the volcanic region of the North-east of Ireland.

(_a._) _General Features._--As in the case of the Antrim district, the
Island of Mull and adjoining tracts present us with the spectacle of a
vast