Vol. 62.] STRUCTURE OF THE SGURR OF EIGG. 49 



with its projecting arms, seems at first to accord with the supposi- 

 tion that the pitchstone occupies an old valley with smaller tributary 

 glens ; and it is also very noticeable that, at the base of the escarp- 

 ment, the lower surface of the pitchstone has in most places an 

 inward slope. Even on a broad view, however, the actual courses 

 of the supposed valley and its tributaries are not easily laid down 

 on any plausible lines ; and the main valley must be supposed to 

 narrow to a gorge at Bidein Boidheach, although it is in this part of 

 its channel that the assumed river-gravel has been accumulated to 

 a depth of about 100 feet (see PL IV & fig. 4, p. 53). 



Closer examination of the actual sections raises more serious diffi- 

 culties in the way of this hypothesis. The junction of the pitchstone 

 with the underlying rocks is sometimes a steeply-inclined surface, 

 and in certain parts of the boundary it slopes outward. This is very 

 noticeable near Loch Beinn Tighe, where there is a sharp dip down 

 towards the tarn. In the south-western part of Beinn Tighe the 

 base of the pitchstone inclines towards the main body ; but in the 

 north-eastern part of the hill the dip is in the opposite direction, 

 implying that the supposed tributary at this place ran uphill. The 

 same is true of the smaller projecting tongue between Beinn Tighe 

 and Cornbheinn. Along Cornbheinn the base of the pitchstone has 

 apparently an undulating inclination, without any general slope 

 towards the west ; nor is it easy to see how the supposed tributary 

 glen at this place could enter the main valley. Similar difficulties 

 are encountered at places on the opposite side of the main ridge, 

 notably at the projecting spur north of the Grulin crofts. 



The detailed mapping of the ground thus leads to a different 

 interpretation of the structure of the Sgurr. The ridge and its 

 offshoots, as we now see them, seem to be rather the relics of a 

 more extensive sheet which had an irregularly-undulating lower 

 surface. The position of the escarpment in different places has 

 been determined by lateral erosion operating in relation to the 

 columnar jointing which is so striking a feature of the pitchstone- 

 complex. Where the columns leaned outward, they were easily 

 destroyed, and the escarpment was thus cut back to a line along 

 which the columns leaned inward. This corresponds as a rule with 

 an inward dip of the base of the sheet, since the columns tend 

 usually (but with exceptions) to be perpendicular to the lower 

 surface. The exceptions are seen at such places as Loch Beinn 

 Tighe, where the columns lean inward, although the base of the 

 pitchstone dips outward. 



The visible form of the lower surface of the pitchstone-complex, 

 as seen in detail along all parts of the base of the escarpment, is a 

 highly significant one. The pitchstone, as we have seen, truncates 

 an alternating succession of basalts and dolerites, the latter of which 

 are much more durable rocks than the former. The slope leading 

 up to the Sgurr, or any other hillside in the plateau-region, shows 

 accordingly a strongly terraced form which is very characteristic, 

 and is well seen in PI. III. Such should be the form of the base 



Q.J.G.S. No. 245. e 



