Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/427

 north end, it slopes gently along the length of the island to the south end. In the southern half of the island, however, the ground rises, owing to the preservation of an upper mass of beds, which denudation has removed from the northern half of the island. On this rising part of the plateau stands the distinguishing feature of the island, the strange fantastic ridge of the Scur of Eigg. Seen from the north or south, this portion of the island looks like a long steep hillcrest, ending in a sharp precipice on the east. But when we get to the east side, the precipice is seen to be the end of a huge mountain-wall, which rises vertically above the basalt plateau to a height of more than 350 feet. It will be seen from the accompanying map (fig. 1) that this ridge of the Scur corresponds with the area occupied by a mass of pitch-stone, and that while the basaltic rocks cover the whole of the rest of the southern half of the island, they gradually rise towards the north, and successive beds of the oolitic series make their appearance until, at the cliffs of Dunan Thalasgair, the latter cover the greater part of the surface, and leave the volcanic rocks as a mere stripe capping the cliffs.

In the accompanying section (fig. 2) the general structure of the island is represented.

B. Literature of the Subject.

Several geologists have published descriptions, more or less detailed, of the mineralogy and geology of this interesting island. In the year 1800 Professor Jameson gave a brief account of the different rocks noticed by him in Eigg ; but he did not at- tempt any description of its geological structure, further than to notice that one variety of rock occurred above or below another. He was then full

Fig. 2.-Section of the Geological Structure of the Island of Eigg.

P. Pitchstone of Scur. c. Ancient river-gravel. p p, Pitchstone veins. ff. Intrusive felstone &c. b b. Dolerite and basalt dykes and veins. B. Intrusive dolerite and basalt sheets. D. Bedded dolerites and basalts. F. Porphyrite bed. 1-4. Oolitic rocks, as shown in the Table at page 289.