E. B. Bailey — Tlte Sgkrr of Eigg. 303 



the foundations upon which he buikis, although to do so involves 

 a considerable digression. 



The plateau lavas of Eigg show the usual step, or trap, features. Harder 

 bands give r se to 'escarpments, softer bands to recesses or flat terraces. The 

 harder bands are generally more coarsely crystalline than the softer and are 

 often ophitic, while their exposures show vertical joints which give rise 

 occasionally to perfect columnar structure ; such harder bands are Barker's 

 dolerites. The softer bands are finer in crystallization, and typically granulitic 

 and amygdaloidal ; these are Harker's basalts (Fig. 1). 



Barker's contention is that the dolerites, as a class, are intrusive into the 

 basalts. Naturally, when several years ago Mr. Maufe, Mr. Wright, and I began 

 work for the Geological Survey in the Tertiary igneous district of Mull, we 

 considered this important suggestion very carefully indeed ; but neither in Mull, 

 nor Morvern, nor yet in Skye, were we able to adopt it. I make this statement 

 because I wish to disclaim at once any originality for my position in this 

 matter, since I merely hold it in common with most of those, old and young, 

 with whom I have had the privilege of discussion in the field. I also wish to 

 excuse myself for not handling the subject from the general standpoint of 

 Hebridean geology, although my friend Dr. Harker will doubtless feel that 

 a limited trciitment is inadequate. 



Restricting attention to Eigg, I give the following reason for regarding the 

 great majority of the dolerite sheets of the island as merely the solid cores of 

 lava-flows. In very many representative cases I have definitely followed 

 dolerite merging upward into slag. The vesicular structure starts before the 

 solidarity and the vertical jointing of the dolerite disappears. The transition 

 is perfect. The proof to my mind is rendered complete by finding, in not a few 

 such instances, that the slaggy top has weathered, in large measure, to a red 

 soil before being covered by the next succeeding lava. One may examine 

 successive illustrations of this phenomenon clearly exposed in the precipitous 

 face of Dunan Thalasgair at the northern end of the island. After having 

 seen this convincing section it was pleasant to read in Geikie's description 

 [2, p. 293] — 



" Here and there, indeed, between the beds, we not unfrequently meet with 

 a thin irregular seam of red earth, which, when fine, might be called bole. In 

 the cliff below Dunan Thalasgair, for example, several of the dolerite beds are 

 not only covered by this substance, but seem to pass into it." Careful scrutiny 

 left no doubt in my mind of the reality of this passage from dolerite to red 

 earth. The bottoms of the dolerite sheets are in like manner generally 

 vesicular and fairly often brecciated, though to a much less extent than their 

 upper portions. 



Now Harker writes [3, p. 41] : " It is to be remarked that sheets of dolerite, in 

 all respects identical with those in question, occur, not only among the basalts, 

 but also among the subjacent Jurassic strata along the eastern coast of Skye 

 and in the northern part of Eigg. Here their intrusive nature has long been 

 recognized. It seems an arbitrary interpretation to make the dolerites intrusive 

 when they occur below the base of the basalt group, and extrusive when they 

 occur above that line." The crux of this argument lies in the premise that the 

 dolerite sheets in the two cases are " in all respects identical ". I visited the 

 particular zone of Juras?ic rocks, which in Eigg is crowded with basalt and 

 dolerite sills, and found that these sills differed from the dolerite sheets which 

 have been described above in being clearly transgressive and in having compact 

 chilled uijpcr and loiucr S2(rfaces. Similar intrusions occur here and there 

 among the lavas and are quite conspicuous in good exposures ; one such may 

 be readily visited south of the pier on the east coast. In all this discussion 

 I find myself in essential agreement with Sir Archibald Geikie's early 

 descriptions of the island [2]. 



Now let us return once more to the sea-clifF (Fig. 3). Looking at 



the fragraental deposit it is clear that it includes harder and softer 



. elements, the former often conspicuous as rounded boulders, the latter 



