106 W. B. Wright — Preglacial Shoreline in the 



A well-marked platform, cut in these latter can be seen emerging from 

 beneath the scree. It probably extends south of the point for some 

 distance. Levelling showed its height to be about 110 feet, where it 

 can be seen passing beneath the scree, but its inner angle is probably 

 a little higher, say 115 feet. 



In Ion a remnants of the preglacial beach are poorly preserved. 

 It possibly never attained any regularity or perfection of development 

 here owing to the nature of the country rock. There seem, however, 

 to be traces of it on the south-east face of Drum Dhugaill and the hill 

 to the north, and also west of Dun Cul Bhuirg, where many of the 

 crags are reduced to a level of 115 to 130 feet, but very irregularly. 

 On Erraid and north of Eidden, on the adjoining coast of the Ross of 

 Mull, there are considerable areas at about the same level, which may 

 be portions of the old platform much modified by glaciation. 



V. The Treshnish Isles. 



The Treshnish Isles are a small group lying to the west of Mull 

 outside Gometra and Staffa. Only two of the islands, the Bac Mor 

 and Lunga, are of sufficient height to show the preglacial shoreline, 

 but on both of these it is remarkably developed. The Bac Mor, 

 commonly known as the Dutchman's Cap, is a striking object. 

 Visitors from the neighbourhood of Oban who have seen the beautiful 

 terraces of the 25-foot beach surrounding the islands of that district 

 cannot fail to recognize here a similar phenomenon on a much grander 

 scale. From the broad platform of the preglacial beach which forms 

 the rim of the hat, the central eminence rises steeply to a height of 

 284 feet above O.D. The platform, which forms more than two- 

 thirds of the area of the island, lies almost entirely below the 

 100-foot contour. The slope of the platform, which is higher at the 

 north end, is more or less controlled by the rock-structure, the slight 

 dip of the lavas (as apparent when viewed from the south-east) being 

 towards the south or south-west in the Bac Mor and towards the 

 north in the Bac Beag. The control is, however, not nearly so 

 marked as might at first sight be imagined, and there is no bed 

 sufficiently strong to lead to the production of the shelf through 

 atmospheric weathering or glacial erosion alone. The shelf, indeed, 

 shifts from one bed to another, and in this manner varies considerably 

 in level. 



Owing to the absence of any modern beach deposits and of the zone 

 of Fucus canalieulatus, which indicates the high-water mark on mo^t 

 rocky coasts, accurate determinations of level were rendered rather 

 difficult. As a datum it was necessary to take the upper limit of 

 Balanus, a much less reliable index of high water than the seaweed 

 zones. Estimating from this, however, the inner edge of the platform 

 round the south side of the central hill was found to be from 75 or 

 80 feet to 90 or possibly 95 feet above high-water mark of spring- 

 tides. The general level of the more outlying portions of the platform 

 at this south end was 70 to 75 feet, rising to near 80 feet on the 

 Bac Beag. 



At the north end of the central hill, with the same difficulty of 

 determining high-water mark, the inner angle was found to be at 



