CHAP. XLVII 
THE ROCKS OF ST. KILDA 
407 
diawn that at St. Kilda a basalt-plateau once existed which has been more 
completely destroyed than in the other regions. Not a fragment of such a 
plateau has survived, unless we may perhaps be allowed to recognize it in 
some of the basalts enclosed among the gabbro-sills. Placed far amid the 
melancholy main and exposed to the full fury of the Atlantic gales, these 
islets must lie regarded as the mere fragmentary cores of a once much more 
extensive volcanic area. The geologist who visits them is deeply impressed 
at every turn by the evidence of the active and unceasing destruction which 
their cliffs are undergoing. Nothing now remains save the deejt-seated 
nucleus of intrusive sills, bosses and dykes. 
1. The Gabbro Sills . — The rudely-bedded arrangement of these rocks is 
conspicuous along the west side of St. Kilda, in Soay and in Borrera. They 
consist of coarse and fine varieties disposed in successive sheets which dip at 
angles varying from as little as 15° up to as much as 60° or even more. In 
St. Kilda they form the picturesque promontory of the Dune, and extend 
thence along the western side of the island to its extreme northern end. 
Their escarpments face the ocean, and their dip-slopes descend towards the 
north-east in grassy declivities to the south bay and the long verdant glen 
which runs thence across to the north bay. The same strike is prolonged into 
Soay, but further east in Borrera the direction curves so as to present vast 
escarpments towards the west and shelving sheets of rock towards the east. 
None of the gabbros seen by me are as coarse as the large-grained 
varieties of Skye, nor does there appear ever to be such a marked banded 
structure among them as that displayed by the Cuillin rocks. Faint 
banding, however, may be noticed. A series of specimens which I collected 
Irom the west side of the island has been sliced for microscopic examination, 
and Mr. Harker has furnished me with the following notes regarding them. 
“An olivine-gabbro from the west side of St. Kilda [7107] is a dark, 
heavy, medium-grained rock, in which augite and felspar are conspicuous. 
The microscope shows, in addition, plentiful grains of olivine, with but 
little original iron-ore, and some apatite-needles. The structure is ophitic, 
the plates of pale-brown axtgite enveloping both olivine and felspar. A 
little brown hornblende and red-brown mica are probably original, the rock 
showing little sign of alteration. The felspar is labradorite, with albite- 
and Carlsbad-twinning, and forms elongated rectangular crystals. 
“Another specimen [7108] is a rock of similar appearance but some- 
what coarser texture, and structurally is a more typical gabbro than the 
preceding, the felspar having little of the ’ lath ’ shape, while the augite, 
though still moulded on the felspar, scarcely assumes an ophitic habit. A 
striking feature in this rock is the way in which the augite is crowded 
with 1 schiller ’-inclusions, in places so closely as to be almost opaque. A 
high magnification shows that these inclusions are dark, linear in form, and 
disposed along two directions intersecting at a high angle. The labradorite 
has unusually close twin-lamellation on both albite and pericline laws, and 
it is possible that this is a strain-effect. 
“A third specimen [7109] is from a rock in every respect identical 
