96 DR GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



of later intrusive bosses and of prolonged denudation. That it once extended over the 

 site of the whole of Rum can hardly be doubted. The edges of the beds that form the 

 outliers would, if prolonged, cover the northern or lower half of the island, where the 

 ancient Palaeozoic and Archaean rocks form the surface. In the southern half, the 

 continuity of the basalts has been partly obscured and partly destroyed by the protrusion 

 of the great masses of gabbro that form the singularly picturesque mountain group to 

 which this island owes its prominence as a land-mark far and wide along the west coast 

 of Scotland. 



4. Skye. — This is the largest and geologically most important of all the Scottish 

 plateaux. Comprising the island of Skye, at least as far south as Loch Eishort, the west 

 side of Scalpa and the southern half of Raasay, and probably extending to the Shiant 

 Isles, it may be reckoned to embrace an area of not less than 800 square miles. The 

 evidence that its limits are now greatly less than they originally were is, like that of 

 Mull, abundant and impressive. Its truncated edges, rising here and there for a thousand 

 feet as a great sea-wall above the breakers at their base, and presenting everywhere their 

 succession of level or gently inclined bars of basalt-beds, are among the most stupendous 



Fig. 21.— Terraced Hills of Basalt Plateau (Macleod's Tables), Skye. 



monuments of denudation in this country. But still more striking to the geologist is the 

 proof, furnished along the eastern margin of the plateau, that the Jurassic and other 

 older rocks there visible were originally buried deep under the basalt-sheets, which have 

 thus been entirely stripped off that part of the country. 



Throughout most of the district, wherever the base of the basalts can be seen, it is 

 found to rest upon some member of the Jurassic series, but with a complete unconforma- 

 bility. The underlying sedimentary strata had been dislocated and extensively denuded 

 before the volcanic period began. On the southern margin, however, the red (Cambrian 

 or Torridon) sandstones emerge from under the basalts of Loch Scavaig, and extending 

 into the island of Soay are no doubt prolonged under the sea into Rum. This ridge 

 probably represents the range of the ancient high ground of the latter island already 

 referred to. 



Nowhere are the distinctive topographical features and geological structure of the 

 1>; ^alt-plateaux more impressively displayed than in the northern half of the island of 

 Skye. The green terraced slopes, with their parallel bands of brown rock formed by the 



