Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/447

 which the pitchstone rests. Again, at the east end of the Scur the pitchstone wall is placed not fairly on the crest of the dolerite- plateau, but on the south side of it. This cannot fail to arrest the notice of every observer, even from a distance (see Pl. XIV. fig. 1). It shows us not only that the rocks of the Scur were erupted along a hollow or valley, but that only the north or north-eastern side of that valley is now preserved.

Allusion has been already made to two minor tongues of pitchstone which project to the north-east from the main ridge of the Scur, and form small hills. Even in these offshoots the same evidence of want of sequence between the rock of which they are composed and the underlying basaltic sheets is clearly exposed. In Beinn Tighe, for instance, the northern projection, a section taken across the isle from east to west shows the basalts at a much higher level on the one side than on the other. These offshoots appear to have been originally either recesses of the main valley, or tributary valleys descending into it, and to have been buried and preserved under portions of the coulees of the pitchstone lava which overflowed from the main mass.

Underneath the eastern end of the precipice of the Scur, on its southern or lower side, a bed of fragmentary materials is found to intervene between the pitchstone and the dolerites. The base of the pitchstone dips into the hill, forming the roof of a small cave. The under surface of the pitchstone is tolerably smooth, but undulating, and shows the ends of the columns as a polygonal reticulation over the roof. The breccia is a pale-yellow or grey felspathic rock, like the more decomposing parts of the grey porphyry of the same cliff. Through its mass are dispersed great numbers of angular and subangular pieces of pitchstone, some of which have a striped texture. Fragments of basalt, red sandstone, and other rocks are rare ; and the bed suggests the idea that it is a kind of brecciated base or flow of the main pitchstone mass. A similar rock is found along the bottom of the pitchstone on both sides of the ridge (c, in fig. 9). At some points where this breccia is only a yard or two in thickness, and consists of subangular fragments of the various dolerites and basalts of the neighbourhood, along with pieces of red sandstone, quartz-rock, clay-slate, &c. The matrix is in some places a mass of hard basalt debris ; in others it becomes more calcareous, passing into a sandstone or grit in which chips and angular or irregular-shaped pieces of coniferous wood are abundant*. A little further east, beyond the base of the Scur, a patch of similar breccia is seen, but with the stones much more rounded and smoothed. This outlier rests against the denuded ends of the ba-

(Poss. Vegetables, p. 37), and two magnified representations were given to show its coniferous character. Lindley and Hutton further described it in their ' Fossil Flora,' naming it Pinites eiggensis, and regarding it as belonging to the Oolitic series of the Hebrides — an inference founded perhaps on the erroneous statement of Witham to that effect. William Nicol corrected that statement by showing that the wood-fragments occurred, not among the " lias rocks," but "among the debris of the pitchstone" (Edin. New Phil. Journal, xviii. p. 154);
 * The microscopic structure of this wood was briefly described by Witham