354 
THE TERTIARY VOLCANOES 
BOOK VIII 
nut-like aggregates of pale green olivine. Examined under the microscope, 
flow-structure is admirably seen, the lath-shaped felspars being drawn out 
parallel to the planes of movement, and giving thereby the peculiarly 
schistose structure which is so deceptive. 
The massive and coarsely crystalline gabbros below and above this 
troctolite are all more or less affected by the same laminar structure. Some 
of those in higher parts of the mountain are quite massive in part, but also 
include bands of lamination. Banding like that of the Skye gabbros is 
generally developed among them, the individual bands varying from less than 
an inch to a foot or more in thickness. This structure, like the lamination, 
is parallel to the general bedding of the sheets. As in the Cuillin Hills, the 
bands differ from each other in the relative proportions of the constituent 
minerals, especially the predominent pyroxene and olivine. The crystals or 
crystalline aggregates are often from a quarter of an inch to an inch in 
diameter, and in these large forms are crowded together in certain bands. 
Magnetite, on the whole, is rather less conspicuous than in the Cuillin gabbro : 
at least, it is not so prominently aggregated in special layers. In one or 
two instances I have observed curvature of the handing, but no example so 
striking as that cited from the Cuillin area (Fig. 337). 
On weathered surfaces, where the felspars decay into a creamy white and 
the ferro-magnesian minerals assume tints of green, brown and red, the 
resemblance of the rocks to schists is striking. This external likeness is 
combined with a tendency to split into thin plates parallel to the lamina- 
tion, which still further increases their schistose appearance. Though less 
developed than in Skye, the banding appears to be of the same kind and 
origin ; but in Bum it is combined with the remarkable lamination above 
mentioned, produced by the arrangement of the component minerals with 
their longer axes parallel to the planes of bedding, as in flow-structure — a 
combination which I have not yet observed in Skye. 
The bedded arrangement of the gabbros of Bum, so conspicuous in the 
great eastern cones (Figs. 339 and 340), is emphasized by the fact that some 
sheets, of a more durable kind, stand out boldly as prominent ribs, while 
the softer crumble into a kind of sand, which forms talus-slopes between the 
others. Alternations of this nature are continued up to the very top of the 
mountains. The beds are nearly flat, but dip slightly into the interior or 
towards the south-west. On the west side of the island also, beyond Loch 
Sgathaig, a distinct bedding may be traced, the inclination being here once 
more inwards or to the east. But from Glen Harris and the base of Askival 
this structure becomes less marked, and gradually disappears. There is thus 
a central or southern more amorphous region, while round the margin to- 
wards the north and east the rock appears in frequent alternating beds. 
It is clear that in the broad features of their architecture the hills of 
Bum follow closely the plan shown in the Cuillin Hills of Skye. But, un- 
fortunately, in the former island denudation has gone so far that no con- 
nection can be traced on the ground between the gabbros and the plateau- 
basalts. As already stated, the latter rocks have been almost entirely 
