lava-field, in which the group of sedimentary materials accumulated, was filled up and buried. This lava is about 20 feet thick, and consists of a coarsely-crystalline, jointed dolerite with highly amygdaloidal upper and under surface. Its slaggy bottom has caught up or pushed aside the layer of green sand, so as to lie directly on the coal, and has there been converted into the earthy modification so familiar under the name of "white trap" among our coal-fields. It is interesting to find that this kind of alteration, where molten rock comes in contact with carbonaceous materials, is not confined to subterranean sills, but may show itself in lavas that have flowed over a terrestrial surface. [Illustration: Fig. 285.--Intercalated group of strata between Basalts, An Ceannaich, western side of Skye. ] From the frequent intercalation of such local deposits of sedimentary material between the basalts, we may reasonably infer that during older Tertiary time the rainfall in North-Western Europe was copious enough to supply many little lakes and streams of water. As the surface of the lava-fields decayed into soil, vegetation spread over it, so that, perhaps for long intervals, some tracts remained green and forest-clad. But volcanic action still continued to show itself, now from one vent, now from another. These wooded tracts were buried under overflows of lava, and, the water-courses being filled up, their streams were driven into new channels, and other pools and lakes were formed. [Illustration: Fig. 286.--Escarpment of Plateau-basalts, Cliffs of Talisker, Skye.] In no part of the Tertiary volcanic area of Britain can the characters of the lavas and the structure of the plateaux be better seen than along the west side of Skye, north of Loch Bracadale. The precipices rise sheer out of the sea, to heights of sometimes 1000 feet, and from base to summit every individual bed may be counted. Some particulars have already been given (p. 192) regarding the average thickness of the basalt-sheets on this coast-line. The general aspect of these cliffs and the arrangement of their component lavas is shown in Fig. 286. As a further detailed illustration of the general succession of the basalts in the Skye plateau, I give a diagrammatic view of the largest of Macleod's Maidens--the three weird sea-stalks 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. 284 and 287). 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.[264]