Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/446

 into each other, that it is then difficult to regard the porphyry as other than a peculiar and decomposed modification of the pitchstone. This is particularly to be observed under the precipice at the east end of the Scur. At that locality the pitchstone is underlain by a band of very hard flinty porphyry, varying in colour from white through various shades of flesh-colour and brown into black, containing a little free quartz and crystals of glassy felspar. Where it becomes black it passes into a rock like that of the main mass of the Scur. Such pitchstone parts of the bed look like kernels of less- decomposed rock. The lower six feet of the porphyry are white and still more decomposed. The relation of the mass is shown in fig. 9, where the basalt-rocks of the plateau (a) are shown to be cut through by basalt dykes (b, b), and overlain by the porphyry (c) and the pitchstone (d). In the porphyry are shown several pitchstone kernels (p, p). It is deserving of remark also that in different parts of the Scur, particularly along the north side, the bottom of the pitchstone beds passes into a dull grey earthy porphyry, like that now under description. Reference has already been made to the occurrence of the pitchstone vein at Laig road along with quartziferous porphyry, and also of similar porphyry and pitchstone filling the same vein at Rudh an Tangairt. Hence, between these two rocks there appears to be in Eigg a close relationship both as to origin and age.

Although the Scur of Eigg is thus evidently the product of different flows, subsequent to the eruption of the highest of the now visible basalt-beds, it was separated from these latter eruptions by an enormous lapse of time. This point, which is as yet a unique feature in Hebridean Geology, I was so fortunate as to ascertain during my survey ; and though I have elsewhere * announced the fact, I wish now to adduce the evidence upon which the conclusion is based. My observations show that what is now the great ridge of the Scur was formerly a river-valley, that this valley was filled with successive flows of pitchstone-lava, that this river-silt, gravel, and drift-wood were buried under the eruptions, and that after long subsequent denudation the surrounding hills have been worn away, and the river-valley, by virtue of the superior permanence of the vitreous lava which occupied its course, has been left standing now as the highest ridge of the district.

A little attention to the form of the bottom over which the rocks of the Scur have been erupted suffices to reveal the fact that between the basalt-beds of the plateau and the pitchstone sheets of the Scur there is a marked discordance, since the latter lie upon a denuded surface of the former. Let us take a section at any part of the ridge, and this feature will be made clear. At the little tarn of the Bhealaich, already referred to, a section may be seen, where the base of the pitchstone on the north side is at least 200 feet above its base on the south side. Here, as everywhere else, the basalt-veins are abruptly cut off along the denuded surface on


 * See ray ' Scenery of Scotland viewed in connexion with its Physical Geology,' p. 278.