CHAP. XLI 
VENTS OF THE BASALT-PLATEAUX 
279 
Maclean’s Nose, at the mouth of Loch Sunart, and affords better evidence 
of its relation to the bedded basalts. It measures about 1000 yards m 
length by 300 in breadth, and its summit rises more that 900 feet above 
the sea, which washes the base of its southern front. It is filled with an 
agglomerate even coarser than that on the northern coast. 1 he blocks are 
of all sizes, up to eight or ten feet in diameter. Ly far the largest pro- 
portion of them consists of varieties of basalt and andesite, slaggy and 
vesicular structures being especially conspicuous. There are also large 
blocks of different andesitic porphyries and felsitic rocks like those just 
referred to, a porphyry with felspar crystals two inches long being particu- 
larly abundant. All the stones are more or less rounded, and are wrapped 
up in a dull-green compact matrix of basalt-debris. There is no stratifica- 
tion or structure of any kind in the mass. Numerous dykes or veins ol 
basalt, of andesite, and of a porphyry, resembling that of Craignure, in Mull, 
traverse the agglomerate. Some of the narrow basalt-dykes cut through 
the others. 
The position of the vent, with reference to the surrounding rocks, 
Fig 302. — Section of agglomerate Neck at Maclean’s Nose, Ardimrnurohau. 
inerate ; e, dolerite sheets of Ben Hiant. 
will be understood from the accompanying section (Fig. 302). On the 
eastern side, the agglomerate can be seen to abut against the truncated 
ends of the flat beds of the plateau-basalts, which are of the usual bedded 
compact and amygdaloidal character. There can be no doubt, therefore, 
that the vent has been opened through these basalts. But it will be 
observed that the latter belong to the lower paid of the volcanic series 
These lowest sheets are exposed on the slope, resting upon yellow is 1 am 
spotted grey sandstone, with seams of jet and a reddish breccia, which, lying 
in hollows of the quartzites, quartz-schists, and mica-schists, form no doubt the 
local base of the Jurassic rocks of the district. Hence, the vent, though 
younger than the older sheets of the plateau, may quite well lie contempor- 
aneous with some of the later sheets. 1 
structure of tins ground, with numerous details as to the petrography of the rocks. The 
geological structure of this area is more fully referred to on pp. 318 et seq. 
i It may here be remarked that there is evidence of great differences in the level of the base 
