388 SIR A. GEIXIE ON THE TERTIARY [May 1 896, 



sometimes an inch in diameter, so that when closely grouped along 

 particular layers they give rise to strikingly coarse-grained varieties 

 of rock. Magnetite, on the whole, is rather less conspicuous than in 

 the Cuillin gabbro ; at least, it is not so prominently aggregated in 

 special layers. 



So rude is the parallel structure in these rocks of Rum that, 

 although quite recognizable on a weathered surface where the con- 

 stituent minerals are revealed by the way in which they respec- 

 tively decay, it is often hardly to be detected on a freshly-broken 

 exposure. 



The western and more rugged part of the island of St. Kilda * is 

 built up of various gabbros, dolerites, and basalts, traversed by 

 dykes and veins of similar material. The gabbros include rocks of 

 coarse, medium, and fine grain, like those of Skye, which lie in 

 sheets or sills, but also apparently in large irregularly-shaped 

 masses. In one or two places I noticed a faint banding, but my 

 opportunities of studying these rocks were cut short by a change 

 of weather which necessitated an abrupt departure, there being no 

 safe anchorage at the island. I sailed round the coast, however, 

 near enough to form a good idea of the general structure of the 

 rock. Like the corresponding masses of the Cuillin Hills, the 

 St. Kilda gabbro arrests attention by its singular blackness of tone, 

 varied by its yellow coating of lichens and its grey crust of 

 weathering, while its occasional slopes of debris are covered with a 

 thick bright-green carpet of turf formed of matted sea-pink. 



While the gabbros of St. Kilda are not a mere uniform boss, but 

 rather a series of sills and irregular masses which have been 

 successively injected into each other, they have subsequently been 

 cut through by the basalt-dykes and veins already noticed. These, 

 which are sometimes as abundant as in the gabbro of the Cuillin 

 Hills, traverse the rock at all angles, and, as they generally weather 

 faster than it does, they give rise to deep clefts which ascend 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 precipices to the north. They are, however, 

 still more impressively displayed around the naked walls of the 

 neighbouring islet of Borrera (1000 feet high), which consists entirely 

 of gabbro pierced with dykes, and in its marvellous combination of 

 spiry ridges, deep straight gullies, and splintered crests, reminds one 

 at every turn of the scenery of Blaven and the Cuillin Hills. 



Nowhere in St. Kilda or its dependent islets can any certain 

 trace be obtained of a rock more ancient than the gabbros. So 

 great has been the denudation that the eruptive core of this volcanic 

 district has been reduced merely to a few scattered islets. If, as is 

 probable, this core was once surrounded and covered by a plateau 

 of basalt, no fragment of such a plateau remains, unless we may be 



1 For references to published information on the geology of this island 

 see pp. 389, 390. 



