378 SIR A. GEIKIE ON THE TERTIARY [May 1 896, 



less than 1 to 3 inches or more in width, which keep in a 

 general sense parallel to the top of the sill, but at a distance of a 

 few inches or feet from it. The sill becomes as usual fine-grained 

 towards the contact, the shales and sandstones being indurated and 

 the limestone marmorized. 



Still farther south the bottom of the basalt-plateau is again 

 reached in the Sound of Soa, where the volcanic pile has been 

 poured out over the upturned edges of the Torridon Sandstone. It 

 is hardly possible to exaggerate the wild confusion of sills, dykes, 

 and veins which have been injected among the rocks at and on 

 both sides of the unconformability. Endless sheets of basalt and 

 dolerite have forced their way between the bedded basalts and the 

 sandstones, while across the whole rise vast numbers of dykes and 

 veins. Narrow, black, wavy ribbons of basic material cross many of 

 these veins, while the later north-western dykes cut sharply through 

 everything older than themselves. As a natural section for the study 

 of the phenomena of intrusion in many of their most characteristic 

 phases, I know no locality equal to the northern coast-line of the 

 Sound of Soa, unless it be the cliffs of Ardnamurchan. But the 

 Skye cliffs, though less imposing than those of the great Argyle- 

 shire headland, have this great advantage, that instead of being 

 exposed to the full roll of the open Atlantic, they form the margin 

 of a comparatively sheltered strait, and can thus be conveniently 

 examined. 



There is one remaining locality in Skye to which I wish to direct 

 attention, since it displays certain phenomena of sills which I have 

 never seen so perfectly exhibited elsewhere. It lies on the western 

 side of the promontory of Sleat, about midway between the basalt- 

 plateau of Strathaird and that of Eigg, and about 8 or 9 miles 

 in a direct line from either. The basalts cannot be proved to have 

 once stretched continuously between Eigg and Strathaird, and to have 

 covered this part of Sleat ; but the position of the rocks which I am 

 about to describe makes it probable that this continuation did formerly 

 exist. The denudation of the West of Scotland since early Tertiary 

 time has been so stupendous that I am prepared for almost any 

 seemingly incredible evidence of its effects. There cannot be any 

 doubt, however, that the rocks of which I now speak belong to the 

 great platform of intrusive sheets, and that they were injected 

 under a pile of Secondary strata, if not also of Tertiary basalts, 

 which has here been entirely removed. 



In his map of Skye Maeculloch showed a small outlier of ' trap ? 

 on the western side of the promontory of Sleat. The locality was 

 visited by Prof. Judd, who called the rock a ' phonolite.' x During 

 an excursion last year with my colleague Mr. C. T. Clough, I was 

 able to examine the place and to obtain the facts which I now 

 describe. 



At Rudh' an Iasgaich, about 2 miles from the Point of Sleat, 

 a small outlier of conglomerate lies on the edges of the Torridon 



1 Quart. Journ. Geol. Soc. vol. xxxiv. (1878) p. 692. 



