CHAP. XLII 
THE BASIC SILLS OF SKYE 
3°7 
while the plateau-basalts rise above them further inland as a lofty escarpment, 
which includes the picturesque landslips of the Storr Rock and Quiraing (Figs. 
318, 319). Beneath the thick sills, the Jurassic sandstones form a range of 
pale yellow precipices, along which many thinner sheets of eruptive material 
have been intruded. As Maeculloch well showed, many of these sheets, if 
seen only at one point, might readily be taken for regularly interstratified 
beds, but perhaps only a few yards distant they may be found to break 
across the strata and to resume their course on a different level. 
The sills of this Trotternish coast may be distinguished even at some 
distance from the bedded basalts by the regular prismatic jointing, already 
referred to, and by their frequently greater thickness, while on closer 
inspection they are characterized by their much coarser texture. They are 
generally somewhat largely crystalline ophitic dolerites, gabbros or diabases, 
and exhibit the persistent uniformity of composition and structure so charac- 
teristic of intrusive sheets and dykes. These characters are well exhibited 
in the Kilt Rock, a columnar sill capping the cliffs to the south of Loch 
Staffin (Fig. 319). 
These massive sills are prolonged in a series of picturesque flat tabular 
islets beyond the most northerly headlands of Skye. They probably continue 
northwards under the sea at least 12 miles further, for sills of the same 
type rise there in the singularly striking group of the Shiant Isles (Fig. 320). 
These lonely islets, extending in an east and west direction for about three 
miles, display in great perfection most of the chief characters of the Skye sills. 
They are especially noteworthy for including the thickest intrusive sheet and 
the noblest columnar cliff in the whole of the Tertiary volcanic series of 
Britain. The larger of the two chief islands consists of two masses of rock 
connected by a strip of shingle-beach, and having a united length from north 
to south of about two miles. The northern half, or Garbh Eilean, presents 
towards the north a sheer precipice 500 feet high. This magnificent face of 
rock consists of one single sill, but as its original upper limit has been 
removed by denudation and its base, where it is thickest, is concealed under 
the sea, the sill may exceed 500 feet in thickness. The rock has the usual 
prismatic structure, which imparts to it an impressive appearance of 
regularity. The columns retain their individuality to a great height, and 
though none of them perhaps can be followed from base to crest of the cliff, 
many of them are evidently at lea'st 300 or 400 feet long. 
Maeculloch, who gave the first geological description of the Shiant Isles, 
showed the intrusive nature of the igneous rocks, and described the remark- 
able globular or botryoidal structure of the Jurassic shales between which 
they have been injected. 1 Professor Ileddle has published a brief account of 
the geology of the islands. 2 Professor Judd visited the group and brought 
away a series of specimens of their eruptive rocks, which he found to include 
basic and ultra-basic varieties. 3 
1 Western Islands, vol. i. p. 441. 2 Trans. Norfolk Nat. Hist. Soc. vol. iii. (1880) p. 61. 
3 Quart. Joum. Geol. Soc. vol. xxxiv. (1878) p. 677, and xli. (1886) p. 393. My description 
in the text is the result of three successive visits to the islands. 
