408 
THE TERTIARY VOLCANOES 
BOOK VIII 
with the preceding, except that the olivine is rather more plentiful, and 
in some grains is partially serpentinized.” 
While the gabbros of St. Kilda are not a mere uniform boss, but a series 
of sills and irregular masses which have been successively injected into each 
other, they have subsequently been cut through by many basalt-dykes and 
veins. These, which are sometimes as abundant as in the gabbro of the 
Cuillin Hills, traverse the rocks both in the line of bedding and also at 
many different angles across it. As they generally weather faster than the 
gabbros, they give rise to deep narrow clefts which may be traced up the 
whole height of the precipices, occasioning sea-caves below and sharp notches 
on the crests above. 
These scenic features, so indicative of the geological structure that causes 
them, are specially well seen on the western face of the Dune or south- 
western promontory of the island, and likewise in the strangely rifted preci- 
pices further north and in Soay. They are, however, most impressively 
displayed around the naked walls of Borrera, which in their marvellous 
combination of spiry ridges, deep straight gullies, and splintered crests, 
remind one at every turn of the scenery of Blaven and the Cuillin Hills. 
2. The Granophyre Boss and its Apophyses .- — The eastern half of the 
island of St. Kilda consists of a pale rock which Macculloch long ago 
identified with the granophyre of Skye, and which, as he pointed out, has 
much resemblance to parts of the granite of Arran. 1 Hot only does it give 
rise to topographical forms like those of the Bed Hills, but it weathers, like 
the Skye granophyre and the Arran granite, into thick bed-like sheets 
divided by transverse joints into large quadrangular blocks. On closer 
inspection it is found to resemble still more precisely the acid rocks of the 
Inner Hebrides. It possesses the same drusy micropeg mati tic structure as 
the granophyres of Skye, Bum and Mull. The ferro-magnesian constituents 
are present in small quantity, hence the pale hue of the stone. The quartz 
and felspar project in well-terminated crystals into the drusy cavities, which 
are sometimes further adorned with delicate tufts of clear crystallized 
epidote. In these and other respects the rock displays the familiar external 
forms of the younger or Tertiary granites of Britain. 
Mr. Harker’s notes on the microscopic structure of this granophyre are 
as follows : — T The prevailing felspar is ortlioelase, often very turbid from 
secondary products. Even what appear to be distinct crystals are sometimes 
seen in the slices to be invaded on the margin by quartz in rough micro- 
graphic intergrowths, and much of the finer intergrowth occurs as a fringe 
to the crystals. In this case the felspar of the micropegmatite can often 
be verified to be in crystalline continuity with the crystal which has served 
as a nucleus [6624], Quartz occurs in distinct crystals and grains as well 
as in the micropegmatite. There is a more granitoid variety of the rock, in 
which only a very rude approach to micrographic intergrowths is seen 
[6623]. In both varieties there is but little trace of any ferro-magnesian 
mineral ; the more typical granophyre has what seems to be destroyed 
1 Description, vol. ii. p. 54. 
