granites in one direction, and through
quartz-porphyry and felsite to rhyolite in another--probably depending
upon the conditions of cooling and consolidation. In their mode of
weathering and general appearance on a large scale, they present a
marked contrast to the basic lavas with which they are in contact from
the coast of L. na Keal to that of L. Buy. The nature of this contact,
whether indicating the priority of the granophyres to the
plateau-basalts or otherwise, is a matter of dispute between the two
observers above named; but the circumstantial account given by Sir A.
Geikie,[4] accompanied by drawings of special sections showing this
contact, appears to prove that the granophyre is the newer of the two
masses of volcanic rock, and that it has been intruded amongst the
basaltic-lavas at a late period in the volcanic history of these
islands. A copy of one of these sketches is here given (Fig. 33),
according to which the felsite is shown to penetrate the basaltic sheets
at Alt na Searmoin in Mull; other sections seen at Cruach Torr an
Lochain, and on the south side of Beinn Fada, appear to lead to similar
conclusions. These rocks are penetrated by numerous basaltic dykes.

[Illustration: Fig. 33.--Section at Alt na Searmoin, Mull, to show the
intrusion of felsite (or granophyre) (_b_) into basalt and dolerite
(_a_) of the plateau-basalt series.--(Geikie.)]

(_c._) _Representative Rocks of Mourne and Carlingford,
Ireland._--Assuming Sir A. Geikie's view to be correct, it is possible
that we may have in the granite and quartz-porphyries of Mourne and
Carlingford representatives of the granites, granophyres, and other acid
rocks of the later period of Mull. The granite of Mourne is peculiar in
structure, and differs from the ordinary type of that rock in which the
silica forms the ground mass. In the case of the granite of the Mourne
Mountains, the rock consists of a crystalline granular aggregate of
orthoclase, albite, smoke-quartz, and mica; it is also full of drusy
cavities, in which the various minerals crystallise out in very perfect
form. As far as regards direct evidence, the age of this rock can only
be stated to be post-Carboniferous, and earlier than certain Tertiary
basaltic dykes by which it is traversed. The granophyres of Mull are
traversed by similar dykes, which are representatives of the very latest
stage of volcanic action in the British Islands. The author is therefore
inclined to concur with Si