98 



DR GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



sheets. In many places, indeed, bands of dirty-green tuff or basalt-conglomerate may 

 be observed, sometimes, as at Portree harbour, associated with lenticular seams of coal, 

 from a few inches to three feet in thickness. Sheets of finer tuff of brighter colours, 

 violet, bluish, and red, like those of Antrim, form conspicuous features in some of the 

 western sea-cliffs, as at Talisker. But compared with the enormous area and thickness 

 of the basalts, these fragmentary ejections are of the most trifling extent. 



In no part of the Tertiary volcanic area of Britain can the characters of the lavas and 

 the structure of the plateaux be so impressively seen as along the west side of Skye, 

 north of Loch Bracadale. The precipices rise sheer out of the sea, to heights of some- 

 times 1000 feet, and from base to summit every individual bed may be counted. As an 

 illustration of the general succession of beds, I give here a diagrammatic view of the 



Fig. 23. — Section of the largest of Macleod's Maidens. 



largest of M'Leod's Maidens — the three wierd sea-stacks that rise so grandly in front of the 

 storm-swept precipice at the mouth of Loch Bracadale. The height of the stack must be 

 at least 150 feet (figs. 22 and 23). About ten distinct sheets of igneous rock can be counted 

 in it, which gives an average thickness of 15 feet for the individual beds. It will be 

 observed that there is a kind of alternation between the compact, prismatic basalts and the 

 more earthy amygdaloids, but that the former are generally thickest.* These features, 



* A striking and illustrative contrast between the relative thickness of the beds of the two kinds of rock is supplied 

 by the fine sections of this district. The amygdaloids range from perhaps 6 or 8 to 25 or 30 feet; but the prismatic 

 basalts, while never so thin as the others, sometimes enormously exceed them in bulk. In the island of Wiay, for 

 example, a bed of compact black basalt, with the confused starch-like grouping of columns, reaches a thickness of no 

 less than 170 feet. Its bottom rests upon a red parting on the top of a dull greenish earthy amygdaloid. 



