CHAP. XL1I 
THE BASIC SILLS OF THE SHIANT ISLES 
3°9 
Above these strata comes the great columnar sill, its hase gradually sinking 
towards the west until it passes under the sea, and the vertical columns 
then plunge abruptly into the water. The rock of which this massive sill 
consists is another large-grained gabbro or dolerite, with an ophitic structure. 
Owing to the form of the ground it cannot be so satisfactorily examined 
as the neighbouring island of Eilean Mhuire, which, though less lofty and 
rather smaller than Garbh Eilean, affords a succession of admirable and 
easily examined sections along its precipitous shores. 
Professor J udd found that while the rocks are mainly ophitic gabbros and 
dolerites, they include such highly basic compounds as dunite. An examina- 
tion of the Eilean Mhuire cliffs enables the observer to ascertain that the 
sills display considerable variety in texture and in the character and 
arrangement of their component minerals. They are marked by a persistent, 
more or less distinct disposition in rude beds, and these again often dis- 
play a banding of their constituents in lines parallel with the general 
bedding. Some of these bands are largely felspathic, and are thus paler in 
colour. Others, where the ferro-magnesian minerals and ores are more 
specially aggregated, are dark in colour. In some layers the long 
black prisms of augite are ranged in a general parallelism with the 
banding. 
A specimen selected as typical of the ordinary coarse-grained amorphous 
rock was sliced and placed in Mr. Harker’s for microscopic examination, 
and he has supplied the following observations regarding it : “ The gabbro 
from Eilean Mhuire [7110] is a crystalline rock showing to the eye lustrous 
black augites, half an inch long, and (predominating) felspar. The micro- 
scope reveals, in addition, irregular grains of black iron-ore and little 
hexagonal prisms of apatite, hlo olivine is to be detected. As regards 
structure, tire augite has tended to crystallise out in advance of the felspar, 
but this relation is not constant. 
“ The augite is of a light-brown tint in slices, and has an unusual kind 
of pleochroism. The colour for vibrations parallel to the /3-axis is of the 
purplish-brown tone seen in some soda-bearing augites ; parallel to 7 and a 
it has a yellow or citron tint. The colour and pleochroism are more marked 
in the interior of a crystal than towards the margin, but some crystals 
pass at the margin into a slightly pleochroic, pale-green, recalling a?gerine- 
augite. The felspar tends to build elongated crystals. It is a rather finely 
lamellated labradorite, sometimes showing pericline- as well as albite- 
lamellae.” 
Another specimen from one of the black bands in the same island, with 
a linear arrangement of its component minerals, is thus described by the 
same petrographer : “This rock [7111] is of darker appearance than the 
preceding, and contains abundant black iron-ore, besides some pyrites. It 
also differs in having a marked parallel disposition of its crystals. 
“ Except for the greater prominence of large irregular grains of iron-ore, 
tins rock under the microscope closely resembles the last described, the 
parallel structure not being conspicuous in the slice. The augite has the 
