Vol. 65.] THE CAULDRON-SUBSIDENCE OF GLEN COE. 619 



The main structural features of this north-eastern portion of the 

 district are readily understood. Sharp marginal upturning exposes 

 the base of the group, resting, with the intervention of a variable 

 accumulation of breccia, sandstone, and shale, upon the old schist 

 floor. The base is seen in Coire Mhorair and Coire Odhar-mhor, 

 and then may be followed fairly continuously towards the south-east 

 across Glen Coe and Glen Etive into Sron na Creise. The average 

 dip along this line is probably not less than 50°, while the beds are 

 sometimes vertical or, as in Coire Mhorair, overturned (PI. XXXIII, 

 Sections III & IV). This marginal dip towards the south-west meets 

 the south-easterly marginal dip, which prevails in the western half of 

 Glen Coe, and thus gives rise to a shallow synclinal structure, the 

 centre of which is marked by the outcrop of andesite crossing the 

 River Coe at the foot of the Buachaille Etive Beag. 



In the Cam Ghleann, strangely enough, the rhyolites flatten, 

 and in the river-bed are found resting upon a seam of dark shale, 

 which in turn overlies a massive flow of augite-andesite. Probably 

 this latter is a representative of the augite-andesites of group (1). 

 Between it and the schists a conglomerate, exposed in the river, 

 contains blocks of schist and many boulders of granite. 



To the south, in the Dalness district, group (2) is apparently 

 represented by interbedded rhyolites and andesites in the Lairig 

 Gartain ; and by a rhyolitic group, including probably both lavas 

 and agglomerates, on the slopes of Beinn Ceitlein, south of Glen 

 Etivj. In the latter locality a variable intercalation of ashes and 

 ashy sandstones immediately underlies the rhyolites, and, in places, 

 exceeds 200 feet in thickness, but is very lenticular. 



The basic andesites of group (1), which figure so pro- 

 minently in the Coire nam Beith section, cross Glen Coe, and their 

 unconformable junction with the schists is well displayed on the 

 slopes north-west of Loch Achtriochtan. The basement beds are 

 here very different from the sandy shales, 2 feet thick, which 

 occur in like position in the type section, for they consist chiefly 

 of conglomerates containing well-rounded boulders of granite, 

 andesite, and schist. The andesitic boulders and pebbles include 

 both the augitic and hornblendic types found among the overlying 

 volcanic rocks. A few quartz-porphyry pebbles can be found, but 

 other dyke rocks appear to be wanting. The granite boulders 

 exactly resemble those found in the basement conglomerate of the 

 Cam Ghleann. With its associated sandstones and shales, the con- 

 glomerate series above Loch Achtriochtan is about 60 feet thick. 

 It is seen again, with like characters, in Coire Cam at the foot of 

 Meall Dearg, where it is overturned at an angle of 70° in close 

 proximity to the fault. 



The andesites overlying the conglomerate in the slopes of Aonach 

 Eagach are the continuation of those already described in Coire 

 nam Beith. The upper limit of the group is, however, very ill- 

 defined on this side of Glen Coe, owing to the change of character 

 in the succeeding division (PI. XXXIII, Section IV). There is a 



