306 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Apr. 5, 



into each other, that it is then difficult to regard the porphyry as 

 other than a peculiar and decomposed modification of the pitchstone. 

 I'his 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, con- 

 taining 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 quartz- 

 iferous 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 dif- 

 ferent 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 be- 

 tween 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 wiU 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 Icrffet 

 200 feet above its base on the south side. Here, as everywhere else, 

 the basalt- veins are abruptly cut ofli" along the denuded surface on 



* See ray ' Scenery of Scotland viewed m connexion with its Physical Geo- 

 logy,' p. 278. 



